An Investigation into Subsea Hydrogen Production

AN INVESTIGATION INTO SUBSEA HYDROGEN PRODUCTION

Brian Clohissey Degree Stream : Electronic Engineering (Renewable Energy) Project Supervisor : Mr. Andrew Meehan

Submitted to the Department of Electronic Engineering in

Partial Fulfilment of the Requirements for the Degree of

Master of Engineering in Electronic Engineering at the

National University of Ireland Maynooth

August 2014

Department of Electronic Engineering National University of Ireland Maynooth

Clohissey_Brian_ME_Thesis.docx ii Brian Clohissey

ABSTRACT

An investigation into the feasibility of a modular seabed located hydrogen generation

system is described. Using the excess power generated by offshore tidal energy converters, the

device would produce hydrogen primarily, and potable water and oxygen as by-products. The

system consists of filtration elements, reverse osmosis water desalination pumping systems, and

an electrolyzer stack powered via a direct current generator. Gas compressors, hydraulic rotary

actuators including an energy recovery device are integral equipment to the process, controlled

and monitored via a localized control system. The produced water unused by the electrolyzer is

pumped ashore, and the oxygen if considered unusable, could be vented to the surrounding

water column for oxygenation. It is expected that the concentrate i.e. brine outfall would be

expelled in a manner not affecting local biota, potentially released into the current stream for

dispersion.

Clohissey_Brian_ME_Thesis.docx iii Brian Clohissey

DECLARATION

I herby certify that this report and the project it describes is my original work and has not been

taken from the work of others save and to the extent that such work has been cited and

acknowledged in the report.

The image on page 8 was taken from the following article: Denny, E., The economics of tidal

energy, Trinity College Dublin, 2009

The images on page 13 were taken from the following article: Dashtpour, R., Al-Zubaidy, S.,

N., Energy Efficient Reverse Osmosis Desalination Process, International Journal of

Environmental Science and Development, Vol. 3, No. 4, August 2012

The image on page 26 was by kind permission of Mr. Blair Marnie of DP Energy

The image on page 30 was taken from the following article: Bryans, A., G., Impacts of Tidal

Stream Devices on Electrical Power Systems, Queens University Belfast, p. 6,7, 2006

The image on page 41 was taken from the following article: Petipas, F., Design and control of

high temperature electrolyser systems fed with renewable energies, Paris Institute of

Technology, p.16, 2013

Assistance was gained with the Simulink application model of page 48 through consultation

with Mathworks.com

The images of the PMHH generator on page 55 are by kind permission from Polar power inc.

The circuit schematic of page 56 was modified from a circuit by Rucker, J., E., in his paper,

Design and Analysis of a Permanent Magnet Generator for Naval Applications, MIT, 2005

Signed: Student No: 13250082 Date: 28/08/2014

Clohissey_Brian_ME_Thesis.docx iv Brian Clohissey

ACKNOWLEDGEMENTS

I would like to thank the following people for their help throughout my project and during the

course of my master degree.

Mr. David Stevenson (Colleague, Canyon): For his advice on Tidal turbine

installation

Mr. David JJ Jeffrey (Colleague, Canyon): For his advice and recommendations on

hydraulic equipment

Mr. Blair Marnie (DP Energy): For allowing the use of an image of tidal turbine

components

Mr. Arthur D. Sams (Polar Power Inc.): For allowing the usage of data and images

on PMHH generators

Clohissey_Brian_ME_Thesis.docx v Brian Clohissey

TABLE OF CONTENTS

ABSTRACT…………………………………………….………….……………………ii

DECLARATION ............................................................................................................iii

ACKNOWLEDGEMENTS…………………………………………………………….iv

TABLE OF CONTENTS………………………………………………………………..v

LIST OF FIGURES AND TABLES………………………………………………..…viii

1. Introduction…………………………………………………………………………...1

1.1 Background on tidal Energy……………………………………………….…….......2

1.2 Background on hydrogen production…………………………………….………….3

1.2.1 Water electrolysis………………………………………………………………….4

1.3 Project aim…………………………………………………………….……………..5

2. Related work and literature review……………………………………………………6

2.1 Offshore power generation….….……………………………………………………6

2.1.1 Tidal energy……………………………….……………………………………….7

2.2 Storage of energy – The Irish case……….………………………………………...10

2.3 Review of desalination of seawater………..……......……………………………...12

2.3.1 Power consumption of desalination………………………………………………12

2.4 Review of water electrolysis…..………………………………..…………………..14

2.4.1 Electricity costs….………………………………………………………………..17

2.4.2 Efficiency of the hydrogen cycle…….…………………………………………...17

2.4.3 Developments in hydrogen implementation…….……………………………......20

2.4.4 Hydrogen combustion in gas turbines……………………………………………22

2.4.5 Storage and transport of hydrogen………………………………………………..23

3. Problem description and specification……………………………………………….24

3.1 Ocean energy implementation.……………………………………………………..24

3.1.1 Tidal energy………………………………………………………………………25

3.2 Operating principle of tidal turbine…………………………………………...……27

3.2.1 Energy in a tidal stream……………………………………………………….….27

3.2.2 Betz’s Law………………………………………………………………….…….28

3.2.3 Tidal turbine power curve……………………………………………...…………29

3.2.4 Electrical down-rating (EDR)…………………………………………………….30

3.2.5 Capacity factor…………………….…………………………….………………..31

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3.2.6 Survivability, reliability, MTBF………………………………………………….31

3.3 Desalination description………………………………………………………........32

3.3.1 Pre-treatment filtration…………………………………………………………...33

3.3.2 Membrane selection………………………………………………………………34

3.3.3 Operating principle of RO………………………………………………………..35

3.3.4 RO staging………………………………………………………………………..35

3.3.5 Power consumption of RO……………………………………………………….36

3.4 Description of water electrolysis…………………………………………………...36

3.4.1 Thermodynamics……………………………………………………………........37

3.4.2 Lower and higher heating value (LHV and HHV)……………………………….38

3.4.3 Equilibrium voltage………………………………………………………………39

3.4.4 Pressure and temperature effects on electrolysis…………………………………41

3.4.5 Current density…………………………………………………………….……..42

3.4.6 Efficiency of electrolyzers……………………………………………………….42

4. Detailed design and implementation………………………………………………...43

4.1 Tidal turbine systems……………………………………………………………….43

4.1.1 Pitch/ Yaw mechanisms…………………………………….………………...…..44

4.1.2 Structure and mooring……………………………………………………………45

4.1.3 Shore connection…………………………………………………………………45

4.2 Hydraulic system description……………………………………….......………….46

4.2.1 Hydraulic power……………………………………………………………….....47

4.2.2 Hydraulic pump description…………………….………………………………..49

4.2.3 Hydraulic motor description…….………………………………………………..51

4.2.4 Hydraulic intensifier description…………………………………………………52

4.3 RO high-pressure pump description………………………………………………..53

4.3.1 Energy recovery device…………………………………………………………..53

4.3.2 Booster pump…………………………………………………………………….54

4.4 Electrolyzer power supply………………………………………………………….55

4.4.1 Electrolyzer sizing considerations…………………………………………….….58

4.4.2 Electrolyzer energy analysis……………………………………………………...60

4.5 Battery and charging system………………………………………………………..61

4.6 Subsea Control Module…………………………………………………………….62

4.7 Communications……………………………………………………………………63

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4.8 Hydraulic containment/ contamination control…………………………………….64

5. Evaluation of design…………………………………………………………………65

5.1 Power extracted from the tidal stream………………………………….…………..65

5.1.1 Rated speed/ rated power…………………………………………………..……..66

5.2 Hydraulic characteristics…………………………………...………………………67

5.3 Context of the project and aims…………………………………………………….69

5.4 Water treatment…………………………………………………………………….71

5.5 Electrical power generation…………………………………………………….…..72

5.5.1 Electrolyzer considerations……………………………………………………….73

5.6 Hydrogen compression……………………………………………………………..74

5.7 Energy analysis……………………………………………………………………..75

5.8 Control system and other loads…………………………………………………….76

6. Conclusions..………………………………………………………………………...77

7. Further work…………………………………………………………………………78

8. References………………………………………………………………...…………79

Appendices

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LIST OF FIGURES

Figures 4.4-1 and 4.4-2: PMHH exploded view and cross sectional ……………………………1

Figure 2.1.1-1: Electrical Down-Rating of Tidal Output…………………………...……………8

Figure 2.1.1-2: Ireland’s Tidal Resource…………………………………………………………9

Figure 2.3.1-1: Submarine RO System………………………………………………………….13

Figure 2.3.1-2: Specific Energy Consumption vs. Distance from shore………………………..13

Figure 2.4-1: Typical PEM water electrolyser efficiency curve………………………………..16

Figure 2.4.3-1: Nexpel Project aims and research areas………………………………………..20

Figure 2.4.4-1: Enel Hydrogen Gas Turbine……………………………………………………22

Figure 3.1.1-1: Tides at Bay of Fundy………………………………………………………….25

Figure 3.1.1-2: Spring and Neap tides, Dublin Bay…………………………………………….25

Figure 3.1.1-3: Tidal Turbine Components…………………….……………………………….26

Figure 3.2-1: Velocity/cross sectional area……………………………………………………..27

Figure 3.2.3-1: Tidal device power curve………………………………………………………29

Figure 3.2.4-1: EDR of one turbine by 50% of the maximum energy extractable……………...30

Figure 3.2.4-2: Marine Current Turbines - Strangford Lough………………………………….31

Figure 3.3-1: Reverse Osmosis………………………………………………………………….33

Figure 3.3.2-1: Spiral- wound configuration RO membrane……………………...……………34

Figure 3.4-1: Proton exchange membrane……………………………………………………...37

Figure 3.4-2: Nexpel test piece…………………………………………………………………37

Figures 3.4.4-1 Typical polarization curves for water electrolysis cells……………..…………41

Figure 3.4.4-2: Typical polarization curves for water electrolysis cells………..………………41

Figure 4.1-1: Side view of the Tidal turbine/ Hydrogen manifold system……………………..43

Figure 4.1.3-1: Submarine cable and bend restrictor…………………………………………...46

Figure 4.1.3-2: Remotely Operated Vehicle removing Hydraulic Flying Lead………………..46

Figure 4.2.1-1: Simulink application model……………………………………………………48

Figure 4.2.1-2: Simulink graph application model showing changing variables……………….48

Figure 4.2.2-1: Radial piston pump cross section………………………………………………49

Figure 4.2.2-2: Radial piston pump operation…………………………………………………..49

Figure 4.2.2-3: Radial piston pump exploded view…………………………………………….49

Figure 4.2.3-1: Axial piston motor, engineering drawing.……………………………...………51

Figure 4.2.4-1: Stage 1 compression – hydrogen compressor………………………………….52

Figure 4.2.4-2: Stage 2 compression – hydrogen compressor………………………………….52

Figure 4.3-1: Axial piston, fixed-displacement pump, engineering drawing………..……….…53

Figure 4.3-2: Axial piston, fixed-displacement pump, cutaway view……..……………………53

Figure 4.3.2-1: Energy recovery device (ERD)…………………………………………………54

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Figure 4.4-1 PMHH exploded view…………………………………………………………….55

Figure 4.4-2: PMHH cross sectional view……………………………………………….……..55

Figure 4.4-3: Simplified layout of Electrolyser power supply………………………………….56

Figure 4.4-4 Average analysis of buck converter ………………………………………………57

Figure 4.4.1-1: Application model of buck converter circuit………………...…………………58

Figure 4.4.1-2: Power electronic system reference…………………………..…………………59

Figure 4.4.1-3: Simplified layout of electrolyzer power supply……………..…………………59

Figure 4.4.2-1: Graph of hydrogen production rate…………………………………………….60

Figure 4.5: Layout of subsea manifold system…………………………………………………61

Figure 4.6-1: Subsea Control Manifold without housing……………………………………….63

Figure 4.6-2: Subsea Control Manifold and base……………………………………………….63

Figure 4.6-3: Subsea Control Manifold deployed using running tool…………………………..63

Figure 4.8-1: Various spring-compensated subsea hydraulic reservoirs…………………..……64

Figure 4.8-2: Submersible water filtration system……………………………………………...64

Figure 5.1.3-1: EDR of spring tide while retaining rated power during neap cycle……………66

Figure 5.2-1: Hydraulic “rectifier” simplified operation……………………………………….68

Figure 5.3-1: Illustration of rated power maintenance through torque application…………….70

Figure 5.5-1: Simplified layout of hydraulic circuit……………………………………………73

Figure 5.7-1: Power consumption of subsystems as % of total power at 100 kW…………...…75

Figure 5.7-2: Power consumption of subsystems as % of total power at 200 kW……….……..75

LIST OF TABLES Table 1: Power extracted levels at two different blade lengths for a given tidal velocity…………65

Table 2: Varying conditions of operation, hydraulic parameters……………………………….68

Table 3: Torque values of the HPU to operate in producing rated horsepower………………...69

Table 4: Torque values of tidal turbine shaft at rated speed……………………………………69

Table 5: Power extracted from tidal stream at rated speed c/w coupled HPU torque…………..70

Table 6: Power requirements of High Pressure RO pumps……………………….…………….71

Table 7. Reverse Osmosis Circuit estimated power consumption……………………………...72

Table 8: Shaft speed and power generated proportional relationship – DC generator………….73

Table 9: High Pressure hydrogen generators……………………………………………………74

Clohissey_Brian_ME_Thesis.docx 1 Brian Clohissey

1. INTRODUCTION

With the current pace of advancements in subsea oil and gas engineering, a concerted

focus has appeared among Exploration & Production (E&P) contractors to locate entire subsea

fields on the seabed. This is due to technical challenges such as increased working depth

brought about by a shortage of viable sites, seasonal weather windows bringing inclement

conditions, and other contractual factors including economic and environmental constraints.

Numerous commercial research and development projects have been carried out on improving

technology to withstand the extreme environment of the ocean floor. It is on this basis, that the

subject of this project is based, i.e. the feasibility of modular submarine equipment.

Due to the fact that subsea located hydrogen production systems have not previously

been developed, this is an original investigation based on a logical step forward for the future

hydrogen economy as well as the design of subsea technologies. The project focus represents a

renewable incentive in the implementation of the hydrogen economy and has been conceived

both to promote the use of hydrogen in the long term, and tidal energy implementation in the

short to medium term. Presently, hydrogen is mostly produced via a fossil fuel intensive steam

reforming process; one that does not represent a step forward in emissions reduction and fossil

fuel independence. For this reason, hydrogen is not considered clean when produced via fossil

fuel means. To realize the benefits of using hydrogen, renewable energy devices coupled with

water electrolysis systems are required to perform with acceptable energy efficiency and capital

costs. By way of experience in subsea engineering, the author has contributed by adding an

original idea to a developing industry i.e. the immature tidal energy industry, with new

perspectives on the mature oil and gas industry. The feasibility of this concept is perceived as

valid and sound from an engineering standpoint.

Figure 1-1: Layout for Proposed Hydrogen Generation System


1.1 BACKGROUND ON TIDAL ENERGY

Designed to extract energy from fast flowing water in tidal streams, tidal stream energy

conversion devices are a fairly recent addition to the marine renewable energy industry. With

increased private investment in offshore tidal energy, tidal technologies are becoming

commercially viable, with progression from more advanced device developers beyond single

unit demonstration devices into the development of arrays of multi-megawatt projects [1].

Moreover, the evolution in offshore/subsea engineering technologies combined with the

drive to exploit renewable energy resources is enabling tidal stream energy to approach large-

scale technical feasibility. Compared to a highly rated wind site, often there is ten times or more

energy concentration available in a tidal site [1, page 14]. Utilization of this resource and the energy

availability represents a major advantage to be gained. Also, while not dependent on random

weather variations, tidal movements are accurately predictable. As this project focuses on

bidirectional tidal movement, locations in which the water flows continuously in one direction

only, i.e. those dependent on large thermal movements generally running from the equator to

cooler areas such as the Gulf Stream will not be mentioned.

Large bodies of water such as seas and oceans, when acted upon by the gravitational

forces of the sun and moon, and in combination with the rotation of the earth, exert movements

known as tides. The motions of the moon and sun relative to the earth cause a periodic variation

in the forces that generate the tides. Vertical motion (tidal range) can be seen in the difference in

water level at low and high tide and the horizontal movement of water is known as tidal current.

Tidal streaming occurs as a result of the propensity for continuity within the fluid flow: As

water flows through a constriction, the flow is accelerated [2]. Tidal power is unlike ocean wave

energy (another massive kinetic energy resource), in that it is variable but can be predicted into

the indefinite future. This characteristic is applicable to the integrated hydrogen generation

system of this project. Furthermore, predictability is imperative to electrical grid dispatchers,

who must balance the changing demand with the supply available. The most prevalent tidal

energy converter design is the horizontal axis turbine. This type, and conceptual designs similar

to it are at the forefront of the forthcoming industry, and have been selected as the prime mover

of this project’s focus. Horizontal axis turbines work much the same as conventional wind

turbines, and when placed in tidal streams, cause the turbine to rotate and produce power.

Secondary flow effects, i.e. creation of a concentrated flow, thus producing a pressure

difference, have been demonstrated through housing the turbine in a surrounding duct/cowling.


1.2 BACKGROUND ON HYDROGEN PRODUCTION

Hydrogen is an energy carrier; a vector capable of generating energy when burned in a

combustion chamber or when utilized in fuel cells (without any form of thermal combustion

process). In both cases the only waste produced by the process is water; therefore hydrogen has

been described as a worthwhile alternative to fossil fuel energy sources. As well as usage in

electrochemical fuel cells it has been shown that hydrogen, after a process of minor

modifications and raised to a moderately high pressure, can be used directly in internal

reciprocating combustion engines, process heaters, and turbines at combined cycle gas turbine

plants. Hydrogen, like electricity, is an energy carrier and not an energy source, and can store

and deliver energy in an easily usable form. Pure hydrogen must be produced from other

hydrogen-containing compounds, such as fossil fuels, biomass, or water, as it does not naturally

exist in its elemental form on Earth due to its physical and chemical properties. Production

methods include thermal (heat), electrolytic (electricity), or photolytic (light) energy intensive

processes [3]. It is the purpose of this project to investigate the electrolytic method of hydrogen

production through water electrolysis. This method represents a clean, environmentally benign

option to produce hydrogen when coupled with renewables.

Although hydrogen is an inherent component of conventional hydrocarbon fuels, such

as oil, natural gas and coal, little attention has been given to the energy carrier. It is not common

knowledge that hydrogen is used in large quantities every day as a component in the fuelling of

cars, in the heating and cooling of buildings, and in the fertilization of grass and crops.

Hydrogen is produced in large quantities as an industrial "intermediate"[4] in the production of

ammonia, fertilizers, methanol and other chemicals and in the refining of petroleum. Other uses

include food production and metals treatment. However, many are unaware of the significance

of hydrogen in daily life because it maintains a low profile in combination with other elements

as gasoline, diesel, natural gas or fertilizer rather than as a free substance. Currently, the main

methods of producing hydrogen are by reforming natural gas and dissociating hydrocarbons. A

smaller amount is produced by water electrolysis. In the Steam Methane Reforming (SMR)

process, hydrogen is chemically ‘split’ from natural gas, and is currently the least expensive

way to produce large quantities of hydrogen [5]. As the process is based on a non-renewable

fossil-fuel source, the reactions of the SMR process produce carbon dioxide (which is a risk to a

carbon-constrained global economy) with the resulting hydrogen product gas having high levels

of impurities.


The hydrocarbon feedstock price is also volatile, with natural gas being primarily a

nondomestic resource for many nations. Hydrogen is “made” as opposed to “found” like crude

oil or natural gas. As with the generation of electricity and the production of hydrogen from the

various processes available, their environmental impacts vary significantly by method and the

accompanying value or fuel chain. A description of this projects’ goal is “a merchant, on-

purpose Hydrogen production system.” Merchant hydrogen production (where the hydrogen is

produced for delivery to other locations as an industrial gas) and on-purpose (describing

hydrogen production as the main goal) are terms used within the hydrogen market structure.

1.2.1 WATER ELECTROLYSIS

Electrolysis describes the dissociation of water molecules into its constituents, hydrogen

and oxygen. The water-forming reaction is reversible, i.e. is the reverse of the fuel cell reaction.

In fact, many fuel cells based on Proton Exchange Membrane (PEM) technology can work both

as a fuel cell or a water electrolysis cell, depending on the direction of the electrical current.

Electrolysis uses Direct Current (DC) electricity to split water into its basic elements of

hydrogen and oxygen. Since this process uses only water as a source, it can produce up to

99.9995% pure hydrogen and oxygen. The requirements to split water are electricity and heat to

achieve the electrochemical process. The core investigation of this project is focused on the

electrolysis application only.

The electrolyzer industry is relatively mature, growing substantially during the 1920s

and 1930s [6]. Companies such as Oerlikon, Norsk Hydro, and Cominco were at that time

producing devices in multi Megawatt (MW) sizes. As these installations were near hydroelectric

plants, they were supplied with an inexpensive source of electricity. SMR gradually took over

as the prevalent production method as more hydrogen was needed for industries because it was

a less expensive process. Water electrolysis declined somewhat in the interim, but is still used in

places where low electricity prices are available or that have high hydrogen purity requirements,

such as when used in conjunction with conventional generation such as thermal and nuclear

power plants. Also, the technical feasibility and capability in confined-space environments has

been validated, such as in nuclear submarines and spacecraft where water electrolysis has

produced hydrogen for powering fuel cells and also oxygen for the crew to breathe. As the price

of natural gas increases, electrolysis again becomes a viable option for competition in the

hydrogen market.


It is envisaged that the integration of hydrogen production with tidal energy converters

can assist the redeployment of the electrolyser industry. Relative to this project in particular,

highly effective environmentally friendly PEM water electrolyzers have previously been

successfully integrated with plants producing electric power using renewable energy sources,

which is the only method in which the full benefits may be realized. The submarine capability is

of interest to this project in terms of survivability and maintenance scheduling.

1.3 PROJECT AIM

The aim of this project is to investigate the technical feasibility of utilizing renewable

energies to decompose water into hydrogen by integrating a modular subsea system of power

take-off for hydrogen generation with a tidal turbine with subsequent transport to shore of

products. Other aims are the research of hydrogen end uses, viewed with potential for merger

between the transportation and power sectors. Among the strategies proposed for exploiting

large amounts of renewable energy, using energy storage to absorb excess electricity generating

capacity is considered a viable option. During times of low demand and/or high rates of

generation, renewable energy can be stored in the form of hydrogen and then reconverted into

electricity during periods of high demand and/or low renewable generation. Tidal energy, due

its immaturity, has thus far not been reviewed as a possible participant in this concern.

As can be seen in fig.1-1, the aim is to make use of off-peak and/or excess renewable

electricity, in part-time or full-time cycles of operation, depending on the characteristics of the

tidal turbines involved, purposed for the electrolysis of water to produce hydrogen. This could

then be stored in shore–based compressed gas tanks or theoretically, offshore underground

geologic formations. The hydrogen may be time-shifted by being reconverted into electricity

using a PEM fuel cell or hydrogen expansion combustion turbine when needed. For all of the

intended end uses of the energy vector in the future economy, hydrogen must be cost-

competitive with conventional fuels and technologies to succeed in the commercial

marketplace, which includes the cost of production as well as delivery to the point of use. The

next chapter details some research by the author into relevant subjects and papers pertaining to

the entire subsea hydrogen production cycle.


2. RELATED WORK AND LITERATURE REVIEW

In this chapter will be a review of the current state of the use of tidal power,

desalination, and water electrolysis to produce hydrogen. The following review is based on past

and current research on the applicable subjects.

2.1 OFFSHORE POWER GENERATION

There are relatively few published and respected papers available on tidal generation.

Of those that were reviewed, the Irish case was predominantly discussed. Wave, wind, and solar

energies were considered as the possible power source for this project, but were disregarded at

this point in terms of economic and engineering factors. Further work is possible in this respect.

Tidal stream generation has been focused on as the more appropriate choice for this design.

However; It is not within the scope of this project to discuss tidal generation in itself in great

technical detail, or to argue its benefits to the energy mix solely as a renewable energy source,

but more so to discuss the adaptation of certain forms of tidal turbines in the effective

production of hydrogen in a novel manner. It is because of this that only applicable aspects of

tidal energy engineering have been reviewed, so as to avoid going too broad in the subject. Key

aspects of all the papers reviewed on tidal and offshore energy generation were the development

steps involved in reaching technological readiness, strategic technology challenges associated

with offshore energy converters, and key challenge areas in their implementation, which

provided an insight into the viability of the project in question.

In a 2007 Energy White Paper, the Irish government set a target of achieving 500 MW

of installed ocean energy capacity by the year 2020 [7]. This target is as yet uncertain of being

met, but the progress associated with reaching it is important in terms of the rapid development

of viable technologies, and as such, methods and practices involved in their implementation.

This target includes tidal and wave energy schemes, where, from an engineering standpoint,

wave energy has similar challenges to tidal generation. It was therefore reviewed as applicable.

In his thesis on Capacity Value of Wave Energy in Ireland [8], David Kavanagh points out the

evolution in recent years in the areas of device research and deployment, and the development

of ocean energy test centres off the west coast of Ireland. The availability of these facilities in

the local jurisdiction has made research and development of novel prototypes possible.


In order to gauge the potential impact of wave power in Ireland, an analysis was carried

out to determine the capacity factor of theoretical wave farms deployed at various locations

around the Irish coast. Comparisons were made on the system-wide capacity factor (average

power output divided by rated capacity) of wind power generation over the same period, widely

used to determine the worth of that unit. Common themes among the works of Bryans, Denny

and Kavanagh [8,9,11], issues such as generation system adequacy and capacity value were

discussed. Kavanagh has shown that wave power off the coast of Ireland experiences a high

seasonal variation compared to other variable energy resources such as wind power, but with a

lower average hourly variation. The capacity value of wave power at each site was then

referenced to the Irish government target of 500 MW of installed ocean energy capacity by 2020

[7].

Wave power has not been proposed as the immediate prime mover for the project, given

the technical difficulties and variability involved. The conclusion reached suggests that the

capacity value of wave power is comparable to that of wind power, given that both are

instantaneous variable generation sources, and have similar attributes from the perspective of

the power system operator. As such, this suggests an increased likelihood of large capacity (>10

MW) wave farms being deployed. This data is relevant to this project, based on cost balancing,

avoidance of wasted power, and CO2 reduction in the long term. Investment in wind and wave,

and thus tidal energy development could increase the commercial viability of such projects.

Kavanagh goes on to suggest other potential areas of future research in the connection of wave

farms and their impact on local electrical networks, and how they may interact with potential

offshore networks in the future.

2.1.1 TIDAL ENERGY

Bryans, in his work “The Impacts of Tidal Stream Devices on Electrical Power

Systems” [9], describes tidal generation’s predictability in comparison to wind generation, which

proved to offer benefits over wind in most aspects considered. Considerable study was carried

out on the impact of tidal energy on generation profiles and system operations. The impact of

the currently viable tidal generation on the system ramp rate was found to be manageable,

whilst its effect on the demand profile were described as beneficial during times of peak

demand, which also coincides with the times of maximum average tidal generation.


Fig 2.1.1-1: Electrical Down-Rating of Tidal Output [10]

Applicable to this project in terms of the power available to the hydrogen production

system from tidal turbines, the power output from a turbine or group of turbines reaches

maximum output during a spring tide, which occurs for a short time twice a month. Based on

these factors, including engineering feasibility, tidal energy has been put forward for the main

prime mover of this project. Relevant to the rated power and associated power capture available,

Bryans [9], and Denny [10], envisage that developers would not consider it economically viable to

rate the electrical equipment to harness all of the energy available at a spring tide. Instead, the

maximum power from the turbine would be down rated by altering the pitch of the blades. This

has been termed Electrical Down Rating (EDR).

Spilled energy is a term describing the energy that is generated, but cannot be used in

the current system setup, due to low demand or lack of transmission capacity. The economics

suggests that the benefits of tidal generation are such that the capital costs would have to be

dramatically lower than the cheapest conventional unit in order to be economically viable from

a societal perspective. Moreover, the conclusions were, that given the current conventional plant

mix, tidal generation might produce negative net benefits at all penetrations [10]. Denny and

Bryans came to the same conclusion that developers of tidal energy arrays will balance the costs

of grid connection and as such the rating of electrical components against the capacity saving

from spilling energy at higher tidal flow rates. It is on this basis that the additional capital costs

of a hydrogen production system could be balanced against this capacity saving, whilst avoiding

spilled energy to a certain extent.


In terms for work done on specific engineering aspects, A study by Araquistain [12]

concluded that although uni-directional turbines produce more power than bi-directional

devices, the total energy produced by bi-directional devices in one day is higher due to the fact

that generation of energy is occurring during both ebb and flood tides, which are geographically

and technically more applicable to this design. Also shown was that bi-directional devices

present a lower energy cost. Moreover, the extra-power generated by these devices compensates

the extra costs of the technology.

Economically, and for efficient generation, tidal energy technology in the early stages

of development require stream velocities at least 2.0 m/s. By 2015, and through further

development, stream velocities of 1.5 m/s are expected to produce efficient generation.

Sustainable Energy Ireland (SEI), describe in “Tidal & Current Energy Resources in Ireland” [1,

page 7], that the viable tidal energy resource (which refers to the applied technical, physical,

institutional and commercial viability constraints on the energy resource) stands at

approximately 0.915 TWh/year, representative of 2.18% of the electricity consumption for the

nation for the year 2010. As projected in the study, between 2010 and 2015, the viable resource

could rise to 6.27% of the predicted electricity consumption – fig.2.1.1-2.

Figure 2.1.1-2: Ireland’s Tidal Resource [13]


A previous study by Castro [14] showed that one of the key benefits associated with

increased tidal generation was the additional capacity added to the system. The capacity credit

of a generator can be considered as a gauge of the amount of conventional generation that

would be displaced by renewable generation without making the system less reliable.

Investment in tidal generation would thus defer investment in other forms of generation. This is

important in terms of emissions avoidance in future energy markets.

2.2 STORAGE OF ENERGY - THE IRISH CASE

Many research and working papers have been written concerned with increased wind

energy on the grid. In the context of this project, and due to the relative immaturity of tidal

energy on the grid, tidal generation can be compared to wind in its effects on the electrical

system. Even though wind energy is a mature technology, there are significant barriers and

technical challenges associated with increased grid integration. This factor is applicable to the

introduction of hydrogen from offshore renewable means. As described in the work by

McKeogh et al. [15], possible options to increase wind penetration without adverse effects to the

grid may include hydrogen production systems. However, even allowing for significant cost

reductions in electrolyser and associated balance-of-plant equipment, low average surplus wind

electricity cost plus a high hydrogen market price are also necessary to achieve the economic

viability of such technology. This directly correlates with the case of tidal generation coupled

with hydrogen production, suggesting only a low average surplus tidal electricity cost would

permit economic viability of a tidal/hydrogen scheme.

Analogous to an increase in wind capacity, with a further increase in tidal generation on

the electrical system, changes in the operation of the electric system would be necessary. In

saying that, McKeogh et al. describe how increased wind energy penetration without sufficient

grid improvement would result in the system eventually becoming incapable of absorbing all the

wind energy generated in some periods of time, forcing curtailments. Areas with high wind

concentration might demand costly grid upgrade to be fully exploited. However, as posed, the

generation of hydrogen from excess wind generated electricity could offset the drop of wind

energy values at high penetration levels in the following ways:

• Smoothing the power output and hence facilitating a more efficient operation of the

electrical system.


• Reducing the scale of the necessary grid reinforcements if hydrogen generation plants

are installed in areas with a high wind power concentration. (In this case the production

plant is subsea)

• Avoiding wind plant shedding. The substantial lost production in Ireland stresses the

potential of hydrogen generation.

It is on this basis that a novel tidal/hydrogen system, and the applicable engineering

knowledge gained as a consequence, could benefit the wind industry. The technical challenges

faced with wind turbine engineering are largely overcome in the maturity of the industry, but

the socio-economic barriers are comparable to the challenges that will face the immature tidal

generation industry. However, lessons learned in wind energy engineering are actively being

applied forward to tidal energy. Moreover, it is worth noting that the goal of this project is to

provide an additional economic incentive to the integration of tidal energy through hydrogen

production and potentially, water desalination. It is hoped that this factor could cross over into

the wind energy industry, thus leading to a bi-directional dissemination of knowledge and

experience.

Previous studies have assessed the economics of wind farms used entirely to generate

hydrogen [16]. However, It could be seen that the full dedication of wind farms to hydrogen

production would mean that the CO2 abatement potential would be smaller than if part of the

electricity displaced conventional thermal plant electricity generation. Based on this, and

because the capital costs of constructing hydrogen-only producing tidal turbines would from the

outset appear excessive, the concurrent generation of electricity and hydrogen is the goal of this

investigation.

In a study carried out by Kroposki et al. at the National Renewable Energy Laboratory

(USA) on the benefits of using off-peak electricity to make hydrogen [6, page 20], findings included

an improvement in the load factor of electric power distribution and transmission facilities,

while allowing intermediate generation to run at their optimal efficiencies. As proposed, by

producing hydrogen and storing it for later use, hydrogen systems were said to improve the

capacity factor of renewable energy systems, therefore assisting in keeping renewable energy

supply constant, or during peaking periods only. Other benefits of electrolysis found were the

effect of reduced intermittent electricity production from renewable resources, implying that

transmission-constrained areas could store hydrogen off peak and use it for grid support on

peak.


2.3 REVIEW OF DESALINATION OF SEAWATER

Within the scope of this project, tidal turbines are the main prime mover in providing

the power for hydrogen production. As this is a subsea action, papers on desalination of

seawater were consulted. It was found that under normal conditions of seawater electrolysis,

mass transfer limitations and reaction kinetics combine to make the cell produce hydrogen and

chlorine. As discussed by K.M. Zohdy et al., the evolution of chlorine as a major anode gas

product is unavoidable in the direct electrolysis of saline water containing chlorides [17]. It is

because of this that desalination processes were reviewed, with Reverse Osmosis (RO) decided

upon as the process best suited to this project.

2.3.1 POWER CONSUMPTION OF DESALINATION

The majority of the power consumed in a typical reverse osmosis plant is used to

pressurize the feed water. The energy required for pressuring feed water could theoretically be

reduced if hydrostatic pressure is used. In the shallow depths usually associated with the current

technical level of tidal stream devices, i.e. (< 40 meters) this effect would be less pronounced,

but still applicable.

A typical seawater reverse osmosis plant requires 3 to 10 kWh of electrical energy to

produce one cubic meter of freshwater. As reported by Al Zubaidi and Dashtpour [18], The

description of their novel RO submarine desalination system, involving only the product water

to be pumped to the surface in contrast to a conventional RO plant, resulted in a proposed

specific energy consumption of 2.46 kWh/m3 water – see fig. 2.3.1-1. As a result only produced

potable water would have to be pumped up to the sea level which, if the recovery ratio remained

the same, in theory suggested reduction of shaft power to ¼ of what is reportedly needed in

current conventional desalination processes. Exploiting the natural hydrostatic pressure of the

water, potable water would be produced offshore, pumped to an onshore storage tank using

submersible pumps and, if necessary, post treatment could take place prior to distribution. This

scheme proves that subsea RO is possible, if not more efficient than conventional systems due

to the proximity of feedstock. Such a system would have the ability to be raised to the surface

for maintenance work also.


Figure 2.3.1-1: Submarine RO System [19]

As was discussed by Professor Al Zubaidi, since higher pressure feedstock is abundant

with increasing depth, a submarine RO plant could be operated at a lower conversion ratio (ratio

of product water to feed water) implying improved RO membrane functionality and lifetime. Al

Zubaidi and Dashtpour showed in their design that pulsating pressure waves due to operation of

high pressure pumps decreases the overall performance and lifetime of membrane modules in

reverse osmosis plants. By utilizing natural hydrostatic pressure in the reverse osmosis process,

the need for high-pressure pumps would be effectively eliminated, and with it pulsating pressure

waves which would lead to longer membrane lifetimes.

For the purposes of this project however, this is beneficial only in lowering overall

energy requirements, thus improving overall energy efficiency. As will be seen in the technical

section, included high-pressure (HP) pumps are a major part of the design. With reference to

fig. 2.3.1-2, it can be seen that the lowest specific energy consumption when pumping ashore

can be obtained for schemes within range of 1km, while even for distances as far as 10km from

the shore, the energy consumption remains considerably less than those of typical sea water

reverse osmosis plants.

Figure 2.3.1-2: Specific Energy Consumption vs. Distance from shore [20]


2.4 REVIEW OF WATER ELECTROLYSIS

For the purposes of this project, Proton Exchange Membrane (PEM) electrolysis

technologies were chosen, having numerous advantages over existing alkaline water electrolysis

systems, which are commonly used in major water electrolysis processes. As found in various

works on electrolysis [21-26], the advantages of PEM over alkaline for hydrogen production

include:

• A lack of a circulating liquid in PEM systems, small mass and compact dimensions.

• Lower power consumption.

• Rapid start-up/shut-down rates – beneficial to variable renewable energy integration.

• Production of very high purity gases: more than 99.998% for hydrogen.

• Higher performance / efficiency.

• Much higher current densities.

• Wide range of current densities, resulting in flexible production rate.

• Greater safety (there is no circulating caustic electrolyte.)

• High differential pressure across membrane possible.

• Inherent gas separation by membrane electrolyte.

• The electrolyte membrane can be made very thin, allowing high conductivity without

risk of gas crossover. This is especially useful in submarine environments.

• The electrolyte is immobilized and cannot be leached out of the cell.

• Considerably smaller mass–volume characteristics.

• Lower internal resistance.

• Compressed gases can be obtained directly from the stack.

• There is an increased level of safety due to a lack of caustic electrolyte circulated in the

cell stack.

• An ability to sustain higher differential pressures.

• Significantly lower power consumption.

• The acid is contained in the PEM material resulting in much lower corrosion rates than

in the case of acidic or caustic solutions.

• Because the PEM is a solid it can be made very thin while still separating the H2 and O2

gas on the sides of the PEM.

• There is no need to add further electrolytes or chemicals – self contained and thus

applicable to a submarine/remotely located system


The disadvantages of the solid-polymer-electrolyte (SPE) cell [21,22] are that the

electrolyte costs more than conventional alkaline solutions and is corrosive, and there is a

requirement for expensive noble metal components to be used in the cell. For these reasons,

solid-polymer-electrolyte cells are usually operated at somewhat higher current densities than

cells that use a liquid alkaline electrolyte.

According to Barbir [27], the energy required for electrolysis is greater than the energy

which is released when the hydrogen is used e.g. in a fuel cell. This would imply a net energy

loss. Of concern to the overall efficiency calculation is the current density of the electrolysers,

since this is inversely proportional to the current density of critical importance to choosing the

size of the electrolyser is the capacity factor, describing how much the electrolyser is used

compared to its capacity. In the paper, two examples were described in which a renewable

energy source that produces varying power was connected to a small electrolyser and a larger

electrolyser. The small electrolyser was seen as likely to use most of its capacity most of the

time, and therefore would have a large capacity factor. A larger electrolyser connected to the

same variable renewable source would have a lower capacity factor. However a small

electrolyser cannot utilise all the energy provided by the renewable energy source when the

available energy is highest. Suggesting that factors such as relative sizing of the electrolyser and

the energy generating device, intermittent operation, efficiency, and output pressure need to be

considered in such schemes.

Of particular note to this project, Barbir also posed that pressurizing the hydrogen using

the electrolyser is the most efficient way to compress the gas, since the only loss arises from the

hydrogen permeation through the membrane. However, Larminie and Dicks [28] imply that the

work done by high-pressure electrolysers and compressors is the same, and are equally effective

at compressing the hydrogen. Moreover, there are more likely to be inefficiencies at the

compressor, suggesting it would be easier to use only an electrolyser instead of an electrolyser

and a compressor. However, high-pressure electrolysers are harder to construct. For this project,

high-pressure operation is carried out at the compressor stage, and the electrolysis process is not

designed for operation at very high pressures – 85 bar. It can be seen in later sections that the

efficiency lost in the compression stage against the possible maintenance penalties incurred with

having a higher-pressure electrolysis operation are justified. Fig. 2.4-1 indicates that low

hydrogen generation rates give higher efficiencies.


Figure 2.4-1: A typical PEM water electrolyser efficiency curve for various delivery pressures. The hydrogen generation rate is

proportional to the current density. HHV stands for higher heating value. [29]

As Barbir describes, since low hydrogen generation rates are given by lower current

densities, electrolyzers are most efficient when working at lower current densities, which is

achieved by using low cell potentials. It was stated that electrolyzer efficiency is inversely

proportional to the voltage. Furthermore, Larminie and Dicks mention that low current densities

give low hydrogen production rates, which means that a larger, and more expensive electrolyzer

would be needed to produce the same amount of hydrogen. It is envisaged that the hydrogen

production system of this project is not a high-flow rate device, but more based on functionality

for total utilization of all available power provided to it from excess tidal generation in a

uniform manner, thus avoiding spilled power, with priority placed on maximum energy

efficiency.

Other relevant findings to this project were in the paper by Laoun, “The

thermodynamics aspect of high-pressure hydrogen production by water electrolysis”. Laoun

describes how hydrogen produced by using pressurized water required less power than using a

hydrogen compressor, with a difference of about 15 % /kW [30]. It would appear advantageous

on this basis to use pressurized water via a High Pressure (HP) pump instead of a power-

consuming compressor. This suggestion relates to this project in that the subjected pressure on

the feed water via RO will potentially reduce power consumption of the electrolysis process and

increase overall energy efficiency. This is subject to further work but is outside the scope of this

project.


2.4.1 ELECTRICITY COSTS

As a result of an investigation of water electrolysis for hydrogen production in the

energy system, water consumption was found to be about one litre per normal cubic meter

(Nm3) of hydrogen, with approximately 4kWh (Kilowatt hour) per Nm3 of electricity required.

The Technical University of Denmark, the Fuel Cells and Solid State Chemistry Department,

Risø National Laboratory, and DONG Energy carried out the study jointly [22, page 130]. The

findings of the study were that, for the amount evaluated, the water price is minimal compared

to the price of the electricity. This is true even though the water needs to be purified in order to

remove traces of salts and organic residues that will otherwise accumulate in the electrolyzer.

As part of the aforementioned RO system, feedwater would be adequately conditioned. In

accordance with how electricity is traded at hour-to-hour prices with prices varying, the study

proved it obvious to use electricity for hydrogen production during the cheap hours. In relation

to this project, electricity is derived from a renewable source.

2.4.2 EFFICIENCY OF THE HYDROGEN CYCLE

Given that the electrochemical processes involved in electrolysers and in fuel cells do

not involve a carnot cycle, efficiency should be very high [31]. In contrast to this idealized

statement, realistically, primary energy is consumed in every step of all the projected routes to a

hydrogen economy chain: production, storage, distribution, delivery and end use. In a study to

prove the round-trip efficiencies of the hydrogen cycle, Bossel [32] (2006) performed an analysis

of the energy losses associated with these steps adopting water electrolysis for hydrogen

production from renewable sources such as hydroelectric, wind or solar energy. It must be borne

in mind however, that this study is concerned with the uses of hydrogen in transport only.

Starting from an assumed value of 100 kWh of AC renewable electricity, Bossel estimated and

compared the overall efficiency between the electricity source and wheel motion for two types

of cars: a hydrogen Fuel Cell Electric Vehicle (FCEV) and a Battery Electric Vehicle (BEV)

with regenerative braking.

The results for the FCEV case when hydrogen was distributed, stored and transferred to

the vehicle as compressed result in energy losses that began at the AC–DC conversion of

electricity to feed the water electrolyzer, which resulted in 23 – 27.7 kWh of final useful energy,

that is, an estimated overall efficiency of only 23 - 28%. The final result also depended on the

way hydrogen was to be stored and delivered.


At the time of the study, compressed and liquefied hydrogen were the only options

commercially available. In the case of the BEV, the losses correspond to the electricity

transmission through the grid (90%), AC–DC conversion and battery charging (85%) and the

electric powertrain (90%). The final overall efficiency of the BEV, 69%, is much higher than

that for the FCEV.

Bossel however, did not review tidal generation, having not been the focus of his study.

It is hoped that the focus of this project could improve upon the round trip efficiencies of

hydrogen delivered by renewable means. Also, it appears there has not been any concerted prior

research into subsea hydrogen production with possible marketable by-products of permeates of

fresh water or oxygen. Rather than simply venting these by-products, energy efficient transport

to conditioning stations ashore could offer an additional benefit to tidal generation

implementation. Integral to the design is correct balancing of EDR by controlled pitch of the

blades of tidal turbines, where it is projected that hydrogen production can utilize otherwise

spilled power without affecting capacity credit.

The production of hydrogen from electricity (and water) to reconvert it back to

electricity in a hydrogen fuel cell will be always much less efficient that the direct use of the

original electric resource regardless of the way hydrogen is stored and delivered. From Bossel’s

study, it was reported that about 1/4 of the electric power obtained from a renewable resource

would finally be put to service when using hydrogen produced from water electrolysis as the

energy carrier, with current technology at the time of the study (2006). It is outside of the scope

of this project to analyse and thus compare the round-trip efficiencies of direct electrical

transmission.

In continuing on the efficiency theme, Rand and Dell (2008) [33] suggest that it would be

very inefficient and uneconomic under most conditions to convert renewable electricity to a

chemical fuel to be used in an Internal Combustion Engine (ICE), or reconverted back to

electricity in a fuel cell. According to these authors, it would be generally better to utilize

directly renewable electricity since energy losses during electricity distribution are

comparatively small. Improvement and interconnection of national grids appears to be the

answer to that point. However, this does not remove the fluctuant nature of certain renewable

energy sources such as wind, which is expected to be between 35-40% of the renewable

electricity contribution to the energy mix by 2020 [7, page 30].


Rand and Dell go on to describe the case of islands or isolated communities with large

renewable resources but without storing means that could produce hydrogen with surplus

energy and reconvert it to electricity during periods of peak demand or insufficient renewable

resource. Countries such as Iceland or Norway that have abundant and cheap geothermal or

hydroelectricity could be used to produce hydrogen for use in transport. Expanding on that

point, countries like Ireland with abundant offshore tidal, wave and wind resources could

accommodate hydrogen production as part of the broader integration of renewables to the grid,

set by goals of the EU roadmap 2050 [34]. In spite of the obvious round trip efficiency limitations

at present on the future proposed hydrogen economy, much work is being done to improve this,

while it seems that energy storage will appear deployable before national grids are strong

enough to handle all the output of available renewable energy resources.

One example of hydrogen produced from water electrolysis describes the possibility of

storing renewable energy during over-generation periods. The Spanish case, reported by

Gutierrez and Hernandes [35], constitutes a good example of the potential of hydrogen to balance

grid loads. The study revealed that the power generation capacity for 2012 in Spain was over 90

GW while the peak demand reached about 45 GW, showing particular underutilization. The

study showed that part of that overcapacity, even that corresponding to non-renewable energy

sources might have been considered for hydrogen production.

An analysis of the data from the Spanish power operator showed that annual power

generation exceeded the electricity demand in 2009 that reached 3.25% (8.17 TWh), with a

daily average surplus of 22.4 GW. The study carried out was based on the analysis of the

deployment of 53 water electrolysis plants at a rated capacity of 50 MW each during the period

2011–2020. It was suggested that, by starting the operation in 2011 with 23 electrolysis units, a

positive cash flow could be obtained from 2014 with a net income of €1,863 million over the

whole period considered, described as possible to attain up to 42.2% share of renewable power

generation in Spain by 2020. Also mentioned was that total surplus energy could amount up to

126 TWh, being about 61% off-peak generated energy. The energy then generated with

hydrogen was projected to be 11.6 TWh or ≈ 1 Mtoe, (Million tonnes of oil equivalent) or the

energy content equivalent of about 297,000 tons of hydrogen. The study served to illustrate that

it is technically and economically feasible to improve the management of the electric grid using

the excess power during over-generation periods for producing hydrogen from water

electrolysis.


2.4.3 DEVELOPMENTS IN HYDROGEN IMPLEMENTATION

The NEXPEL project [36], (2010-2012) consisting of a European consortium of research

and development (R&D) organizations from 4 member states was a concerted effort to develop

new materials and stack design concepts, the aim of which was to increase the efficiency and

lifetime of PEM electrolysers while at the same time cut costs. Focus on improvement in the

market penetration of water electrolysis is seeing progress. As discussed in the context of the

project report, the main objectives i.e. the development of lower cost components resulted in

new hydrocarbon membranes and supported catalysts for the oxygen evolution reaction, which

potentially could lead to reduced overall capital cost of current electrolysers. The high expense

is due to the presence of expensive fluorinated membranes and noble metals. The main cost

reduction potential of the NEXPEL stack is the reduction in materials use and production cost of

the Titanium bipolar plates.

The NOVEL project [37], is an ongoing European Commission research continuation of the

NEXPEL project, taking advantage of the successes in the project and continuing the

development of technical solutions in the below objectives. During the NOVEL project a PEM

electrolyser based on the innovations in the NEXPEL project will be constructed to demonstrate

the functionality of improved materials and stack designs in view of hydrogen production

efficiency. The three main ongoing targets for the NEXPEL/NOVEL project are to achieve:

• Electrolyser efficiency greater than 75%

• A stack life time of 40,000 h

• A reduction in system costs to € 5,000/Nm3 production capacity

Figure 2.4.3-1: Nexpel Project aims and research areas [38]


As part of the objectives, the project was concerned with the following points:

• Higher performance, in particular of the Membrane Electrode Assembly (MEA)

• Longer lifetime of the most crucial PEM components, e.g. the membrane

• Higher efficiency in the power electronics

• Novel stack design for high-pressure operation and low assembly costs.

• Lower capital costs of the main stack components - membrane, electrodes and bipolar

plates, current collectors.

In the UK, one current project evaluating the potential and developing a rollout strategy for

hydrogen transport is UK H2Mobility [39]. Phase 1 consisted of a hydrogen rollout strategy

development plan; phase 2 being the business case development while phase 3 is full

Implementation. Phase 1 has been completed and the results have been released. The project is a

joint industry/government study to evaluate the potential for hydrogen as a transport fuel and

develop a rollout strategy that will contribute towards:

• Assembling a comprehensive fact base, specific to the UK, covering FCEVs, Hydrogen

Refuelling Systems (HRS) and the means of hydrogen production and distribution

• Gaining novel insights into consumer perceptions of FCEVs and the factors influencing

purchasing decisions through primary market research

• Developing a roll-out plan for a national network of HRS and quantifying the cost of

building the HRS network

• Investigation into a broad range of hydrogen production methods and quantifying, for

the first time, the benefits that hydrogen production by water electrolysis can have on

the broader UK energy system

• Combining all elements into a roadmap showing that hydrogen mobility can be a

credible solution to decarbonising road transport in the UK

• Identifying the challenges that must be overcome in order to deliver the roadmap

Findings of the study thus far include demand forecast showing that a cumulative fleet of ~1.6

million FCEVs could be reached by 2030 as ownership costs are expected to decline over time.

Analysis suggests that a financing need of GBP ~ 400 million would need to be covered to

establish a HRS infrastructure in the UK, also suggesting that new hydrogen stations could be

profitable from ~2020.


The project has stated the long-term benefits of a hydrogen infrastructure wherein

FCEV have CO2 emissions 75% less than that of diesel cars by 2030, with CO2 abatement

potential between 10 and 30 million tonnes per year possible by 2050 [39].

2.4.4 HYDROGEN COMBUSTION IN GAS TURBINES

Research available in the further uses of hydrogen has been found to include direct

hydrogen combustion in modified gas turbines. As part of their programme for the projected

hydrogen economy implementation, Enel Research [40] has constructed a demonstration project

in Fusina (Venice province). The plant consists of a combined cycle turbine conception burner

specifically fuelled by pure hydrogen in order to produce electricity and heat. Thermal energy

produced by the combustion is converted into electricity in the gas turbine, said to reach power

amounting to approximately 12 MW, with flue gases constituting exclusively hot air and water

vapour. Extremely low emissions of nitrogen oxide were reported. As stated in their findings,

the performance of the cycle was increased by exploiting the heat present in the flue gases in the

production of high temperature steam which, when sent to the existing onsite coal plant,

produced further energy for additional capacity amounting to approximately 4 MW. Overall, 16

MW were declared as produced starting from hydrogen, with a reported overall efficiency of

41.6%. Initially developed to be fuelled using natural gas, the Fusina plant utilizes a new

conception turbogas burner, modified ad hoc for hydrogen combustion. This experimental site

appears ideal for proof of concept, due to the proximity of the nearby Andrea Palladio thermal

coal power plant, as well as the nearby petrochemical hub of Marghera, which provides the

necessary raw materials and logistics.

Figure 2.4.4-1: Enel Hydrogen Gas Turbine [41]


2.4.5 STORAGE AND TRANSPORT OF HYDROGEN

The issue of transport of hydrogen after production has been a matter of much work in

the recent years. Issues of the characteristics of hydrogen, including its volumetric density,

energy density, and hydrogen embrittlement of some steels associated with current gas pipeline

transport schemes have been consulted, of which the National Renewable Energy Laboratory

(NREL), Boulder, Colorado, USA, is responsible for many papers, some having been reviewed.

A study conducted by Melaina et al. [42], showed that blending hydrogen into natural gas

pipeline networks at low concentrations had the potential to increase output from renewable

energy production facilities in the near term. In the longer term, blending may provide an

economic means of hydrogen delivery when the hydrogen is injected upstream and then

extracted downstream. This study was specifically aimed at hydrogen to be used in FCEVs or

stationary fuel cells.

A working paper, commissioned by the Division of Energy Conversion, University of

Leuven, Belgium [43], showed a transition towards a hydrogen-economy by means of hydrogen-

natural gas mixtures, transported and distributed through the existing natural-gas grid, to be

possible, both in a liberalized and regulated energy market. A possible stumbling block for the

transition towards the mass use of hydrogen was the gas transport grid, regardless of the market

conditions in this study.

Dodds and MacDowall, of the UCL Energy Institute, University College London, in

their working paper “A review of hydrogen delivery technologies for energy system models”

[44], concluded that delivering hydrogen to consumers is much more difficult than delivering

petrol and the delivery infrastructure investment costs form a significant part of the overall fuel

cost. Tube trailers (via heavy goods vehicle) appeared the most economical choice in the early

stages of a hydrogen economy when demand is low. For higher demand or delivery over long

distances, pipeline or liquid hydrogen pathways were suggested as more economical. The option

for pipeline transport would appear to be the only feasible choice in the case of transport of

output hydrogen from the submarine system of this project.


3. PROBLEM DESCRIPTION AND SPECIFICATION

It is important to specify that the project’s main focus is the rapid deployment of tidal

energy converters with integrated hydrogen production systems, thereby offering added

economic incentive to the problems involved in implementation of either system alone. A

further incentive, especially in drought-affected regions is the possibility of using the potable

water that is produced in the process as a commodity. Also possible is the oxygenation of the

surrounding water column by the vented oxygen from the process, leading to biota stimulation.

3.1 OCEAN ENERGY IMPLEMENTATION

According to European Commission Article 4 of Directive 2009/28/EC on Renewable

Energy, a requirement of all the member states was to submit detailed roadmaps of how to reach

their legally binding 2020 targets for the share of renewable energy in final energy

consumption. These were the National Renewable Energy Action Plans [45] (NREAP), submitted

in 2010. As of 2011, 3.4 MW of wave and tidal energy devices in advanced stages of

technology development had been tested across Europe. To reach the Europe-wide target of 1.9

GW of installed wave and tidal energy by 2020 – as outlined by NREAP, mass deployment is

expected. To achieve the 3.6 GW industry target – as outlined in the EU-OEA 2050 Ocean

Energy Roadmap, described in the Strategic Initiative for Ocean Energy project – an even

higher rate of deployment across the continent will be required. The SI Ocean project [46], a

European Commission funded study, focuses on wave and tidal technologies developed in the

Atlantic Arc area.

With reference to the Irish case, Sustainable Energy Authority of Ireland (SEAI),

assume that by 2017, ocean energy will start commercial production, propelled by an expansion

in the capacity of wave electricity-generation technology. Possible deviations from the NREAP

scenario were investigated, such as in the exploratory case [47], which included the impacts of

higher energy demand flowing from a faster economic recovery in Ireland. In that scenario,

renewable energy accounted for 50% of electricity and 18.3% of total energy use, with energy

harnessed from the ocean envisaged to contribute over 5% to Renewable Electricity (RES-E) by

2020. The projections indicated are positive, but the realities of the implementation of ocean

energy are yet to be seen. This section discusses the inherent challenges associated.


3.1.1 TIDAL ENERGY

The gravitational force of the moon is strongest on the side of the earth closest to the

moon. This causes the surface of the ocean to bulge towards the moon. The force is weakest on

the opposing side, resulting in the rotation of the earth causing a bulge away from the moon.

Were other factors not involved, the forces involved and the response of the ocean would result

in semi-diurnal (twice daily) tides that were perfectly aligned with the moon. The bulge of the

tides appears ahead of the moon’s actual position and is displaced by the spinning of the earth

on its own axis. The ocean’s response to this gravitational pull is slowed due to the

displacement as a result of frictional forces between the surface of the earth and the mass of the

oceans. The ocean is also subject to a weaker gravitational pull from the sun, roughly 2/5 of the

moon’s exertion force [48]. The monthly cycles of tidal amplitude are relative to the positions of

the sun and moon. When they are pulling together, the range of tides is greatest, resulting in the

higher spring tide, while lower neap tides are the result of the sun and moon’s relative positions

at right angles to each other. Most coastal areas experience semi-diurnal tides. In general, as the

tides rise there will be a flow from the oceans into the bays, harbors and estuaries known as the

flood current. As the tides fall there will be a flow towards the oceans known as the ebb current.

Using the flow of water created by the tides and accelerated by coastal topography, tidal stream

technologies generate electricity in a similar way to wind turbines, except that the density of

seawater is 800 times greater than air [49]. Seawater flow rates are also typically one fifth those

of air. The effects of gravity and the subsequent tidal range can be seen below.

Figure 3.1.1-1: Tides at Bay of Fundy [50]. Figure 3.1.1-2: Spring and Neap tides, Dublin Bay diving club [51]


Tidal stream devices use the kinetic energy of the currents flowing in and out of tidal

areas wherein the resource follows a sinusoidal curve with the largest currents generated during

the mid-tide, with the ebb tide often being slightly larger than the flood tide. Generally, marine

current strength is variable and is directly related to the tidal height of the location. In certain

places a combination of the seabed bathymetry and the shape of the coastline are such that tidal

flows are concentrated and reach velocities that can provide cost-effective power generation.

Tidal streams where the water depth is relatively shallow, where a tidal range exists, and where

funnelling effects of the local coastline and seabed amplify the speed of the currents are

generally the largest resources. A good in-stream tidal site is one that will allow a tidal stream

device to be sited, is close to load and grid interconnection, and has bathymetry and seabed

properties that have minimum or no conflicts with other uses of the designated area. Tidal

energy offers some advantages over other renewable resources such as wind and wave. The

fluid medium, sea water, is over 800 times denser than air, so tidal power offers a greater energy

density than wind for a given turbine rotor swept area [49, page 73]. Also, as the tides flow with a

predictable intermittency, (resulting from gravitational forcing), the variability is deterministic

(and not stochastic like wave or wind) [52]; this factor should ease the integration of tidal energy

into existing electricity networks. This is also applicable to hydrogen generated by tidal energy,

as the market could become concurrently predictable. The correlation to electricity demand with

patterns of generation differs significantly with tidal energy. Maximum power output is known

to at some times coincide with peak demand, but at other times minimum power output does.

However, the magnitude of tidal stream supply variations is small compared to demand

variability. The components can be seen in fig. 3.1.1-3

Figure 3.1.1-3: Tidal Turbine Components [53]


3.2 OPERATING PRINCIPLE OF TIDAL TURBINES

Tidal energy conversion devices extract kinetic energy from the flow and impart this

into mechanical motion of the rotor thus harnessing the energy contained within the moving

current. The device then converts the mechanical motion into electrical energy by means of a

power take-off system. On connection to the electricity grid, the electrical power output from

the device needs to be conditioned in order to make it compliant with grid code regulations. The

maximum power available to a turbine is the kinetic energy of the fluid in a stream tube whose

diameter is equal to the diameter of the turbine as defined by Eqn. 3-1.

Thus: (Eqn.3-1)

Where:

Figure 3.2-1: Velocity/cross sectional area [54]

3.2.1 ENERGY IN A TIDAL STREAM

As is commonly known, energy can be neither created nor destroyed. It can only be

converted to one form or another; therefore to convert the kinetic energy of tidal stream

currents, the water molecules have to be slowed down. It is not feasible to extract all of the tidal

movement’s energy, as this would slow the stream down completely. If a tidal turbine slowed

down the current so that it was static behind it, this immovable water would stop the current

flow. The water must be able to pass through the turbine blades and also be able to move on

behind the rotor, thus keeping some of its tidal stream velocity.

Pmax =12ρΑν 3


Where a horizontal axis tidal turbine is free to the surrounding water column, in a

conventional hydropower station water flow is led to a duct that surrounds the rotor, thus

allowing no water to escape except for past the turbine, thus using most of the kinetic energy in

the streaming water. The rate by which the tidal stream is retarded by the rotor determines how

efficiently the power in the current is utilized. It is possible to slow down the stream too much

or too little in this respect.

3.2.2 BETZ’S LAW

German scientist Albert Betz showed through his theory for wind turbines that when the

wind speed is retarded by one third just in front of the rotor, and by another third behind the

rotor, a wind turbine is most efficient. The undisturbed wind v is retarded by the rotor to 2/3v

and will decrease to 1/3v behind the rotor before it returns to original wind speed by influence

of surrounding wind. In this case, the power in the wind is used most efficiently – 16/27 or 59%

[55] power of the wind can be extracted, ignoring mechanical or aerodynamic losses. Thus, the

rotor of a wind turbine can only utilize 59% of the energy content of the wind. In the case of a

tidal turbine the Betz criterion applies to the theoretical maximum efficiency of frictionless

turbines. Power coefficient, CP, is the percentage of power that can be utilized by the turbine

extracted from the water flowing through it. This value varies for different technical

characteristics and current speeds, but the maximum attainable value, CPmax = 16/27

by the Betz limit [55 – page 66]. Therefore, The ideal power for a mass of water passing through the

rotor with a cross sectional area, A, as seen below in Eqn. 3-2 can be expressed as follows:

(Eqn. 3-2)

Where: Pext = Power extracted

In calculating the efficiency of a tidal turbine, the power being extracted by the turbine is also

given by:

(Eqn. 3-3)

(≈ 0.593)

Pext =CPPmax =CP12ρΑν 3

P =Tω


Where: = Angular velocity of turbine, rad/s

P = Extracted power, W

T = Torque, Nm (Newton meters)

3.2.3 TIDAL TURBINE POWER CURVE

The cut-in speed of a tidal turbine is the tidal current velocity at which the unit starts

generating power. Below this level, the generator field windings will not be supplied with

sufficient power, and/or friction in the bearings of the rotor will not be overcome. The cut�in

speed is the minimum needed to generate net power. Since no power is generated at tidal speeds

below the cut�in speed, that portion of tidal energy is essentially wasted. Typical cut-in speeds

are between 1 and 2 knots [56] (0.5-1 m/s) – fig.3.2.3-1.

As the tidal stream current velocity increases, power generated increases up to the rated

speed. The turbine extracts power in proportion to the kinetic power incident over its swept

area, as per Eqn. 3-2. For variable pitch turbines this is roughly constant over a range of

velocities. Above rated speed, constant power is extracted from the flow by changing blade

pitch with current speed. For safety reasons, the cut-out speed is the velocity at which the

turbine is stopped, which is a further operating regime wherein the turbine blades are feathered

to avoid damage during periods of excessively high currents.

Figure 3.2.3-1: Tidal device power curve [57]

ω


Since tidal currents are largely predictable, this measure is largely to compensate for

unusually strong currents. Feasibility studies by the Northwest National Marine Renewable

Energy Center (USA) [58] indicate that the lowest cost of energy for tidal turbines would be

achieved with capacity factors between 30 and 40% depending on the particulars of the tidal

regime. Therefore, the selection of the rated speed is largely an economic decision.

3.2.4 ELECTRICAL DOWN RATING (EDR)

As mentioned in chapter 2, maximum power is only achieved for a few days per month

during spring tide. For this reason, if a turbine were to be installed rated to capture the full

energy potential during the spring tide, it would have a low very load factor. Therefore,

developers have tended to install turbines that are rated at a lower level than the maximum

power available at a particular site [9 – page 67] to maintain economic viability. This is known as

Electrical Down Rating (EDR), which is shown below in fig. 3.2.4-1 below. Thus, even during

neap tides, an EDR of 50% ensures that the output can reach its peak; consequently, the turbine

presents a very high load factor. EDR spills excess energy during the hours of spring tides thus

increasing the overall average power output of the turbine [12-page 31].

Device developer Marine Current Turbines Ltd., with an installed system at Strangford

Lough, Co. Down, reaches its maximum output even during neap tide - fig. 3.2.4-2. This is an

example of EDR in practice. In this site, the neap tidal velocity is approximately 50% of spring

tidal velocity.

Figure 3.2.4-1: EDR of one turbine by 50% of the maximum energy extractable from the tidal steam resource [59]


Figure 3.2.4-2: The current system design for a ~1.2 MW system to be located in Strangford Lough [60]

3.2.5 CAPACITY FACTOR

Device utilization is quantified by the capacity factor, defined as the ratio of average

power extracted to power extracted at rated speed [61]. For a tidal power plant, the rated

(maximum) power can only be generated at times of maximum tidal current velocity. The

average power is considerably less than the rated power, typically between 30 and 40% of the

rated power. The term used to describe this “derating to average” is capacity factor, which is the

ratio of the mean generation to the peak generation on a renewable energy generator, and is

therefore the energy produced during a given period as a proportion of the energy that would

have been produced had the device been running continually and at maximum output expressed

as:

(Eqn. 3-4)

3.2.6 SURVIVABILITY, RELIABILITY, MTBF

A significant challenge for the tidal energy industry and to this project’s feasibility is

the survivability of devices in the marine environment, one in which is far harsher than that of

other onshore renewables locales with extremely high loading due to the density of water.

However, component development within the offshore wind sector is likely to spill over into the

ocean energy sector, particularly in the areas of Power Take-Off (PTO) and installation.

Capacity Factor = Electricity production during the period [kWh]Installed capacity [kW] × time in period [h]


Individual component Mean Time Between Failure (MTBF) [52 – page 69], and life

expectancy is unknown in terms of offshore ocean energy converters, therefore to achieve

significant periods of operation in between maintenance a greater understanding of these

parameters is essential. The oil and gas industry has significant offshore experience in

equipment placement in challenging sub-sea environments, with critical reliability constraints,

of which the opportunity to transfer data should bring about knowledge in reducing unplanned

maintenance requirements. Tidal energy converters must be able to survive both their expected

operational loading, and also the extreme loading presented by storm conditions. Seabed

mounted fully submerged Tidal Energy Converters (TECs) need to be designed with

consideration for wave loading due to the fatigue stresses and loads that will result.

3.3 DESALINATION DESCRIPTION

Although the oceans are an abundant source of possible feedstock, globally, only 2.5%

of the total resource is fresh water, of which only 0.3% is available in rivers and lakes with the

remainder being trapped in glaciers and in groundwater. In the direct electrolysis of seawater,

chlorine and hydrogen are produced. As this was perceived to be a possible barrier to the

feasibility of the project, i.e. being an environmental hazard to the surrounding biota, the

environmentally benign option in this case would be to subject the water to total desalination to

remove all dissolved salts and produce essentially distilled water. For this reason, seawater

desalination onsite for subsequent freshwater electrolysis is essential to this design. This

approach incurs additional capital cost of water treatment and desalination, however the product

or permeate water could theoretically be of use in certain drought-affected regions or as a

marketable commodity; thereby reducing the capital costs somewhat over the lifecycle of the

equipment. Reverse Osmosis was decided upon as the desalination process of this project.

Salinity is usually expressed in Parts Per Million (PPM). Therefore, a salinity of 1000

PPM corresponds to a salinity of 0.1%. Seawater is heavily saline, being a mixture of pure water

and salt. In a conventional desalination plant, incoming seawater is separated into two streams,

one of product water and the other of brine. The product water is low in concentrated dissolved

salts, whereas the brine is of a high concentration of dissolved salts and needs to be expelled - in

this case, via dispersion in the tidal stream. Total dissolved solids (TDS) is a parameter in

differentiating saline water quality and is defined as the sum of the inorganic salts and organic

materials that are dissolved [62].


Figure 3.3-1: Reverse Osmosis [63]

Seawater usually contains upwards of 20,000 mg/l of TDS, with fresh water usually

containing less than 1,000 mg/l TDS. Through the natural phenomenon of osmosis, water tends

to diffuse through a semi-permeable membrane that separates it from a salt solution – fig. 3.3-1.

In the non-thermal Reverse osmosis (RO) process, applied pressure in excess of osmotic

pressure natural to the salt solution causes water to move through a semi-permeable membrane

to the freshwater side. It is through this process of pressurized reverse osmosis that seawater can

be desalinated to a point where it is usable in an electrolysis process. The difference in osmotic

pressure between the permeate product water and feed water can be calculated using the

following:

(Eqn. 3-5)

Where: = osmotic pressure

Based on Eqn. 3-5, the osmotic pressure of seawater at a TDS concentration of 40,000

Milligram per Liter (mg/l) is approx. 452 Pounds per Square Inch (PSI) or 31 bar.

3.3.1 PRE-TREATMENT FILTRATION

Filtration is carried out by passing the in-feed through a granular media followed by

possible additional Microfiltration (MF) or Ultrafiltration (UF) membrane-based low-pressure

pre-treatments for the removal of non-ionic species such as suspended matter or viruses and

pathogens [63 – page 3]. It is expected that tertiary filtration may not be required, as the tidal stream

will have relatively low suspended matter of any significant mass. Furthermore, additional

chemical pretreatment should not be required. This also results in reduced power consumption

in operation of the pretreatment system.

Δπ = 0.078× TDSfeed( ) −TDS product( )( )

Δπ


However, as feed water salinity increases so does the requirement for an increase in

membrane feed pressure and associated energy. A general “rule of thumb” is that for each 1000

mg/L, the net driving pressure needed to produce an equivalent amount of permeate (product)

will increase or decrease by about 11 PSI (0.76 bar) [62 – page 23]. Salt-rejection, a capability of

certain ultrafiltration units, reduces both the particulate material and salt loading on membrane

desalination processes with characteristics of particulate removal at 0.005-micron levels, thus

reducing RO required energy further.

3.3.2 MEMBRANE SELECTION

As can be seen in fig. 3.3.2-1, a common reverse osmosis membrane configuration is

the spiral-wound configuration. These units are usually stacked in a parallel topology,

depending on volume required. A reverse osmosis membrane acts as a physical barrier to the

flow, allowing selective permeation of the solvent (mostly water) and partial or total retention of

the other dissolved substances (mostly salt). Separation basically takes place on a thin, selective

layer of the reverse osmosis membrane [64]. Usually, a suck-back tank, which maintains a

minimum volume and backpressure on the reverse osmosis membranes, supplies the required

volume of product water; in this case natural hydrostatic effects provide the backpressure. The

concentrate discharge can have possible environment impacts due to the high salt content and

high density. Therefore it is envisaged that sufficient kinetic energy within the tidal stream can

dispose of much of the brine outfall so as not to affect the local ecology.

Figure 3.3.2-1: Spiral-wound configuration RO membrane [65]


3.3.3 OPERATING PRINCIPLE OF RO

A HP pump pressurises the feed flow across the membrane of the RO system, part of

which passes through the membrane, with the majority of TDS being removed and remaining

salts and brine rejected. More than 70% of the intake water may be rejected as brine in the

reverse osmosis process. This brine outfall would then be directed to an area suitable in relation

to biota, but within mechanical proximity of the structure, such as directed through the tidal

stream. The effects of which are not known, and is a possible subject for further work. Reverse

osmosis requires a feed water temperature below 35 degrees Celsius [62 – page 7]. This is easily

achievable in a subsea environment. Typical pressures applied are between 50 - 80 bar in order

to have a sufficient amount of water pass through a unit area of membrane [66]. Theoretically, the

only energy requirement is to pump the feed water at a pressure above the osmotic pressure.

However, in practice, higher pressures must be used. The required pressure depends on the salt

concentration of the resource of saline solution; Approx. 70 bar is normally needed for seawater

desalination. In conventional seawater reverse osmosis plants with typical recovery ratios of

around 25%, for each unit of freshwater four units of feed water have to be pressurized. Even

though it is possible to operate reverse osmosis units at higher recovery ratio, this will result in

shorter membranes lifetime and increases overall operational cost. The system of this project is

a low flow, low demand device, so MTBF is crucial.

3.3.4 RO STAGING

Product staging is used when a single pass through one RO membrane does not bring a

constituent concentration down to required salinity by a single pass, rather operating in series

with two or more reverse osmosis membrane systems. The second stage would take suction

from a storage tank of the first stage RO array. In the case that the permeate is not suitable for

electrolysis, a post first-stage reservoir is possible, pressure compensated in order to contain the

output of the first pass through the RO membranes until such time as a second pass is required.

The desalinated water, produced at low pressure and collected in the submarine reservoir at the

same working depth is utilised in the electrolysis cell with an option of additional high-pressure

pumping of excess water up to the sea surface.


3.3.5 POWER CONSUMPTION OF RO

As described in work carried out on energy efficient reverse osmosis schemes covered

previously [18], conventional schemes require between 3 - 10 kWh of electric energy to produce

one m3 of freshwater. Seawater RO membrane energy consumption is related to site-specific

salinity and temperature (as previously discussed) and other design-specific characteristics such

as hydraulic throughput (flux) and the percentage of feed water recovered [63 – page 5]. In this case

only low flow is required. The primary power-consuming devices are the pumps required to

achieve the feed pressure needed to facilitate the reverse osmosis process. However, the energy

required for this action can be significantly reduced if hydrostatic pressure is exploited; in

addition, mitigating the energy somewhat for pretreatment of the inlet seawater. Also, in

relation to the subsea hydrogen generation device, due to the immediate proximity of the

electrolyzer to the RO system, the energy required pumping conditioned water into the

electrochemical cell/reservoir would be low. Any further usage of permeates over and above

electrolysis processes could be theoretically pumped ashore with excess tidal energy, but with

resultant energy requirements.

3.4 DESCRIPTION OF WATER ELECTROLYSIS

The basic principle of hydrogen production through electrolysis is as follows: By

adding energy that is slightly higher than the energy previously released, water, at a low energy

state can be converted to hydrogen and oxygen at a higher energy state. The extra energy is

required to cover losses. When an electric field is applied across the membrane of a PEM

electrolyser, hydrogen is produced as water is supplied to the anode, with hydrated protons

being transported through the proton conductive membrane to the negatively charged electrode -

the cathode. Electrons exit the cell via an external circuit, which supplies the driving force or

cell potential for the reaction. At the cathode, the electrons and protons recombine to give off

hydrogen gas. The membrane acts as a protonic conductor of electricity, since a moving charge

is identical with electric current.


Figure 3.4-1: Proton Exchange Membrane [67] Figure 3.4-2: Nexpel PEM test piece [68]

3.4.1 THERMODYNAMICS

In discussing the electrical efficiency of an electrolysis cell, thermodynamic principles

are discussed. As described in NREL (2010) [5], the total energy of a system is composed of

thermal and electrical energy known as enthalpy. The amount of electrical energy is known as

the Gibbs free energy, which is the net-work done by a system, and corresponds to the

maximum amount of useable electrical energy available when hydrogen recombines with

oxygen. Irreversible energy or entropy is a measure of the unavailable energy in a closed

thermodynamic system and is dependent on the temperature that the reaction takes place. A

change in these quantities from a standard set of conditions follows the form shown below:

(Eqn. 3-6)

Where: H = Enthalpy

G = Gibbs free energy

S = Entropy

T = Temperature

= Rate of change

ΔH = ΔG +TΔS

Δ


The term heat of reaction refers to the heat released in a chemical reaction, combustion

or otherwise. The heat of combustion (or heating value) is defined as the amount of heat

released when 1 gram molecular weight of a substance is burned in oxygen. The heat of

formation is defined as the heat released or required when chemicals are formed from their

elements in their standard state. The heat of formation of the elements in their standard state is,

by definition, zero [5 – page 1]. The free energy of formation of a compound is the work that could

be recovered from a reaction in which the compound is formed from its elements under standard

conditions. The heat of formation of water has the same values as the heat of combustion of

hydrogen. This is because in the combustion reaction, water is formed from its elements [6 –

appendix B]. The reaction of the formation of water is:

H2 +12O2 → H2O+ energy

(Eqn. 3-7)

3.4.2 LOWER HEATING VALUE AND HIGHER HEATING VALUE

According to Faraday’s law, the amount of chemical change during electrolysis is

proportional to the charge passed. In other words, the current passing through an electrolysis

cell defines the rate of hydrogen and oxygen being produced. The heat of reaction defines the

theoretical total energy needed to split the water molecule, which is the reverse of the heat of

combustion of hydrogen. Values are usually given as either Higher Heating Value (HHV) or

Lower Heating Value (LHV), based on the end product of combustion being either liquid water

or water vapour [69]. Throughout this project, HHV is used, as liquid water is to be electrolyzed.

The electrolysis reaction is the opposite of the formation of water reaction:

(Eqn. 3-8)

Specifically: H2O (liquid) + 237.2 kJ/mole (electricity) +48.6 kJ/mole (heat) H2+1/2 O2

Harrison et al. [5 – page 8] illustrate that the HHV is easily converted into more common forms of

the higher heating value:

H2O+ energy→ H2 +12O2

→


(Eqn.3-9)

Where:

As can be seen in Eqn. 3-9, splitting a mole of liquid water to produce a mole of

hydrogen at the heat of formation of liquid water, or the energy released when water is formed

in the reaction above is 39.4 kWh/kg of hydrogen, requires 285.8 kJ of energy (237.2 kJ as

electricity and 48.6 kJ as heat.) Therefore 285.8 kJ, not 237.2 kJ of electricity is the minimum

required to split water in these cells.

As found, an ideal 39.4 kWh of electricity and 8.9 liters of water are required to produce

1 kg of hydrogen at a standard condition of 25°C and 1 atmosphere pressure. As a result, the

amount of energy needed to create hydrogen from water using electrolysis is 39.4 kWh/kg. This

value is the higher heating value (HHV) of hydrogen. The heat of formation of steam is 33

kWh/kg of hydrogen, and is the lower heating value (LHV) of hydrogen [6 – appendix B]. Again, the

LHV is not referred to in this work. In order to determine the efficiency of the electrolysis

process, the theoretical amount of energy needed, 39.4 kWh/kg of hydrogen, needs to be divided

by the actual amount of energy used by the electrolysis unit to create hydrogen.

3.4.3 EQUILIBRIUM VOLTAGE

To split water and thus break the bonds between hydrogen and oxygen atoms, the

minimum thermodynamic electric voltage applied over the two electrodes in an electrolysis

system must exceed a minimum value: the minimum reversible voltage, which is determined by

Gibbs energy of water decomposition. Gibbs free energy is defined as: “energy available to do

external work, neglecting any work done by changes in pressure and/or volume, thereby being a

function of pressure and temperature. At standard conditions (25 ºC and 1 bar) the minimum

reversible voltage is 1.23 v [70]. (Where: Equilibrium voltage is known as Erev)

This voltage is also known as the equilibrium voltage of water and is a function of both

pressure and temperature. The equilibrium voltage is the cell voltage at no current load.

However, much higher voltage levels are used in industrial electrolysis cells, with the excess

voltage referred to as the “overpotential” [71] of the process reaction.

285,840 Jmol

×1molH2

2.0158g×1000g1kg

=141,799,781 Jkg

=141.8MJkg

141.8MJkg

×1watt − sec

J×1kW1000w

×1h

3600sec= 39.4 kWh

kg


As described in an independent review published for the U.S. Department of Energy

Hydrogen Program, NREL [69 – page 13], the electric energy consumption in electrolysis is directly

proportional to the cell voltage that must be applied. As mentioned, the minimum reversible

voltage is 1.23 V, equivalent to the change in Gibb’s free energy for the reaction. While this

reflects the minimum amount of energy that must be applied as electricity, the total energy is

greater. A more realistic number is 1.48 V/cell, which is the electrochemical potential (standard

potential) corresponding to the HHV of hydrogen. This is known as the thermoneutral voltage,

and it represents a 100% electric and thermal energy efficient electrolyzer that has electricity as

its only energy input (i.e. no waste heat produced from the reaction.)

The thermoneutral voltage is calculated using the enthalpy change of reaction divided

by the number of mole of electrons producing one mole of hydrogen multiplied by the electric

charge per mole of electrons [4 – page 16] (Faraday constant.) Specifically, this is determined using

Faraday’s Law, and dividing the HHV (285,840 J/mole) by the Faraday constant and the

number of electrons needed to create a molecule of hydrogen.

[5, page8] (Eqn. 3-10)

Where: = The thermodynamic voltage under standard conditions

= The enthalpy change of reaction (temperature dependent)

= 96,485 coulombs mole-1

Z = 2 (electrons needed to create a molecule of hydrogen)

1.48v/cell is a more reasonable value to use when calculating cell and stack voltage

efficiency. Practical electrolysis cells usually in the range of 1.6 to 2.0 volts [5 – page 6].

Eo =Δ f H

o

zF=285,840 J

mol

2×96,485 Cmol

=1.481Voltscell

Eo

Δ f Ho

F


The formula for calculating the voltage efficiency of a cell or cell stack is expressed as:

Voltage efficiency = Thermoneutral voltage (E) Cell operating voltage (V)

(Eqn. 3-11)

3.4.4 PRESSURE AND TEMPERATURE EFFECTS ON ELECTROLYSIS

As the design of this project is a submarine one, i.e. located on the seafloor, ambient

pressure and temperature effects as well as system level effects from other processes were

considered. As stated previously, the minimum voltage required varies with temperature and

pressure since the Gibbs free energy is a function of both variables. Common themes found

[6,30,72], indicated that an increasing temperature would reduce the stack operating voltage of the

electrolyzer, reducing the amount of energy needed to initiate the reaction (activation), therefore

reducing the reversible potential greatly. Other findings show that Erev is increased by an

increase in partial pressure. In other words, the reactions at the electrodes are increased, which

lowers the overpotential (loss) at the electrode. Therefore thermodynamics suggests that the best

conditions for operating in case of saving energy would be met at high temperatures and low

pressures if water remains in liquid state. This means that an overall increase in efficiency can

be realized by operating at higher temperatures, but with a subsequent increase in corrosion

rates of electrodes and separator membranes. It would appear that a fine balance between an

elevated temperature and lower range of operational pressure offer the most benefits in this

case. This would apply to this project by suggesting a need for a dedicated heating element

system or effective heat exchange system for the intake water for the electrolyser.

Figures 3.4.4-1 [73] & 3.4.4-2 [74]: Typical polarization curves for water electrolysis cells


3.4.5 CURRENT DENSITY

From technical data available, electrolyzers have been seen to usually perform their

process under low voltage and high current level conditions. Current density is the electric

current per unit area of cross section, measured in amperes per square meter.

With the aim of reducing capital costs, PEM water electrolyzers are typically operated

at around 2 V/cell allowing the achievement of high current densities [75] (better than 1A/cm2).

However, with large increases in current densities a noticeable value of unwanted ohmic voltage

drop occurs between the electrodes [71 2.3]. Formation of unwanted electrical power loss and less

process efficiency values results. The higher the cell voltage is increased, the higher the current

density and in turn the production rate – fig.3.4.4-1. A high efficiency is beneficial, but an

economically optimized production rate is usually more important in order to optimize the

production economy. Applicable to this project was that for electrolysis at increased pressure

(up to 30 Bar) the improvement of volt–amp characteristics was observed at operating current

density in comparison with electrolysis at atmospheric pressure thus reduction of power

consumption due to reduction of membrane resistance and decrease of an overvoltage takes

place [22 – page 70].

3.4.6 EFFICIENCY OF ELECTROLYZERS

The efficiency of an electrolysis system can be calculated as the heating value of the

hydrogen produced divided by the electrical energy input including heat used for the electrolysis

reaction plus energy losses of any kind, expressed as:

(Eqn.3-12)

An efficiency goal for electrolyzers in the future has been reported to be in the

50kWh/kg range, or a system efficiency of 78% [76]. However, this 78% includes compression of

the hydrogen gas to 6000 psi. Typical commercial electrolyzer system efficiencies are 56 - 73%

and corresponding to 70.1 - 53.4 kWh/kg

Electrical efficiencyHHV( ) =

HHV of H2 producedElectricity used


4. DETAILED DESIGN AND IMPLEMENTATION

The device of this project is designed to operate in the extreme environment of seabed tidal

stream channels and is governed by saltwater density, corrosion, temperature, pressure and

hydraulic loading effects. With respect to subsea engineering, the author contributed in

providing tacit knowledge, experience and a hands-on approach to the following considerations:

• The overall construction must have an acceptable cost, so the concept must be

economically viable and survivable.

• The hydrogen generation manifold must work in a submarine environment where

maintenance is onerous resulting in the need for machinery construction to be as simple

as possible.

• All marine operations for installation and maintenance must take into account the strong

currents prevailing in the potential sites.

• A compromise must be found between the capital cost and the yearly energy

production.

• By-products require transport i.e. potable water, oxygen venting. Also disposal of brine

outfall & subsequent environmental effects needs to be considered.

4.1 TIDAL TURBINE SYSTEMS

With reference to fig. 4.1-1, a horizontal axis turbine has been focused upon for the

prime mover of this project. This type of machine has a turbine mounted on a horizontal drive

shaft that is coupled to an electrical generator through a large reduction ratio gearbox.

Figure 4.1-1: Side view of the Tidal turbine/ Hydrogen manifold system [77]


The design of this project emphasizes that the rotor blades of the horizontal axis type

system are variable pitch so that the optimum angle of attack of the flow over the blades is

maintained to ensure maximum hydraulic conversion efficiency as the stream velocity varies

over the tidal cycle. Blade pitching is used to limit the peak power so that the installed capacity

of the generator is not exceeded.

In this device torque and power train rating is limited on economic grounds. The

hydrogen generation system of this project is designed to be powered by the mechanical motion

of the tidal stream over and above the electrical generation profile, and furthermore, through

tuning of the blade pitch by torque load management, will not significantly affect drag

performance of the tidal turbine in the process of providing rated electrical power output

throughout. As previously discussed, no power is generated at tidal speeds below the cut�in

speed, that portion of tidal energy is essentially wasted. However, if friction were to be

defeated, thus causing the tidal turbine drive shaft to rotate, energy could still be captured in the

form of hydraulic pressure as a product of the torque generated, albeit at a reduced rate. It is

envisaged that in certain conditions and with further tidal velocity and due to the presence of a

shaft-coupled Hydraulic Power Unit (HPU), hydraulic pressure could be generated, given

correct tuning of the turbine blades at all kinetic energy levels of the tidal stream spectrum. As

the hydrogen generation system of this project relies upon hydraulic pressure/flow to operate,

torque loading on the turbine shaft due to the hydraulic system operation would require a

corresponding pitch adjustment in the blades, which is thought to be feasible and not in excess

of the rating of the generator, i.e. within economic bounds. It is envisaged that hydrogen can be

produced in concurrence with electrical generation, as the HPU can create pressure whenever

the shaft is turning. EDR will function to maintain capacity credit, while the tuning of blade

pitch would compensate for the additional torque load associated with the HPU throughout the

upper generating regions.

4.1.1 PITCH/ YAW MECHANISMS

Thought has been given to the design parameters, with the resultant prime mover being

a bidirectional single-ended tidal turbine. As the applicable location for this system would be in

semi-diurnal tidal streams, the blades would pitch 180° to allow for the changing direction of

the tide, controlling the angle of attack of the blades to face both ebb and flood tidal flow

directions thus regulating the loading faced by the rotor.


This optimises the efficiency of the rotor over a wide range of flow speeds [52 – page 38].

Pitch regulation would be controlled local to the turbine, and would not contain systems that

allow orientation of the device to face the oncoming tidal flow, i.e. yaw mechanisms to rotate

the entire assembly both ways in the changing tidal direction. This eliminates the use of

complicated yaw systems and slip rings to transfer electrical power ashore. This characteristic is

also suitable for the hydraulic and fiber optic linkages between the systems, with a telemetry

link to the hydrogen generation manifold.

4.1.2 STRUCTURE AND MOORING

In most tidal devices, steel has been found to be the most robust material for foundation

and structural components, in terms of yield and tensile strength. Steel properties are well

known, and certain grades of steel have a proven track record in the marine environment, and

can be protected from corrosion through use of paints, or by using cathodic protection [52 – page 60].

Composite materials and anti-fouling coatings have been traditionally been used within the

shipping industry, and knowledge of these areas, together with corrosion prevention techniques,

can be applied to this project in particular. Turbine blades are often made from Glass Fibre

Reinforced Plastic (GFRP).

The monopile type foundation has been decided upon for the hydrogen manifold, as it is

similar to the installation of tidal turbines. Minimal preparation of the seabed necessary, and the

foundations consists of large diameter steel cylinders, typically 2m diameter, of wall thickness

between 40mm and 60mm, driven typically 20-30 meters into the seabed. Monopiles are often

drilled and then grouted (fixed in place with marine cement) into position depending upon

ground conditions at the site.

4.1.3 SHORE CONNECTION

Device interconnection can be carried out using dry-mate or wet-mate connectors, with

applicable technologies for sub-sea operation already present within the Oil and Gas sector.

Dry-mate connectors require the connector to be above the water surface when the connection is

made. The connected devices are then lowered into position.


Figures 4.1.3-1 and 4.1.3-2: Submarine cable and bend restrictor, ROV removing HFL [77,78]

Wet-mate connectors allow a connection to be made subsea, which would allow an

ocean energy device to be placed into position prior to the cable being connected. For this

system, a hydraulic and fiber optic link is required; therefore an Electrical Flying Lead (EFL)

would in this case carry optical fibers for the subsea-surface telemetry, and local

communications to the tidal turbine. A Hydraulic Flying Lead (HFL) would link the devices so

that hydraulic flow can reach the subsystems of the manifold. Submarine cable installation

techniques have been used extensively in the oil, gas and telecommunications industries

especially in the already mature Remotely Operated Vehicle (ROV) industry, and should be

applicable to the installation of the hydrogen pipeline from tidal current farms. It is most likely

that an output pipeline to shore would be buried for maximum protection.

4.2 HYDRAULIC SYSTEM DESCRIPTION

As this system is heavily reliant on hydraulic principles in its operation, a brief

description of some of the major components will follow. According to Pascal’s law: “Pressure

applied on a confined fluid is transmitted undiminished in all directions, acts with equal force

on equal areas, and at right angles to them.” In a hydraulic system, force is indicated by

pressure, while speed and distance are represented by flow. Pressure is created whenever the

flow of fluid is restricted. Hydraulics can be defined as a means of transmitting power by

pushing on a confined liquid. Similar to hydrogen, hydraulic systems are not sources of power,

but rather energy carriers. The power source is the prime mover; in this case a tidal turbine.

When compared to fixed-speed electrical motors, hydraulic systems can be versatile and

flexible, providing a high power to weight ratio in a small footprint.


Hydraulic actuators (linear or rotary) can operate throughout a broad range from

minimum to maximum speeds and can stop immediately without adverse effects. The pressure

relief valve in a hydraulic system protects the circuit from overload damage. When the load is in

excess of the valve setting, e.g. in the event of a stalled motor, the relief valve will divert

delivery from the pump to the reservoir, limiting the amount of torque or force applied to the

actuator. Hydraulic systems possess good sealing capabilities and anti-corrosion characteristics

in industrial enviroments, so hydraulic power has traditionally been used in subsea operations in

the oil and gas industry. Furthermore, the device of this project could be constructed of many

items that are currently being employed by work-class Remotely Operated Vehicles (ROVs) -

electro-hydraulic submersibles capable of withstanding hydrostatic pressure, saltwater ingress,

oxidation and corrosion. Hydrodynamic transmission of the output power of tidal turbines

coupled with hydrogen generation was chosen for this project because of these characteristcs, as

well as the elimination of local switchgear, transformers and distribution systems that would

add additional capital and maintenance costs.

4.2.1 HYDRAULIC POWER

Hydraulic power is the product of pressure and flow, and flow is the product of

rotational speed and pump displacement. As the prime mover i.e. the tidal turbine is directly

coupled to a hydraulic pump, pressure correlates directly with the torque established by the

shaft. Fig. 4.2.1-1, a modified application model [80] shows a typical power unit consisting of a

fixed-displacement pump driven by the tidal turbine through a flexible transmission, a pressure-

relief valve, and a variable orifice (simulating system fluid consumption.) The motor model is

represented as an ideal angular velocity source block, which is instantaneously rotating the shaft

at 188 rad/s (1800 Revolutions Per Minute.) This is chosen as nominal rated speed RPM.

The simulation starts with the variable orifice open, which results in a low output

pressure and the maximum flow rate going to the system. The orifice starts closing at 0.5

seconds and is closed completely at 3 seconds. The output pressure builds up and is maintained

at a level set by the pressure relief valve. At 3 seconds, the valve starts opening, thus returning

the system to its initial state.


Figure 4.2.1-1: Simulink application model of fixed-displacement pump coupled to a Tidal turbine [80]

Fig. 4.2.1-2 shows the effects of torque applied versus pressure/flow, and shaft velocity

can be seen i.e. with increased torque, shaft velocity decreases temporarily, with a correponding

increase in output pressure to a certain level until pressure relief occurs. There is a correlated

response in system flow rate i.e. increased pressure results in decreased flow.

Figure 4.2.1-2: Simulink Illustration of how changing loads and/or changing supply influence other variables [81]


4.2.2 HYDRAULIC PUMP DESCRIPTION

Hydraulic pumps convert mechanical energy into hydraulic energy (horsepower) by

pushing fluid into a system, operating by pushing an increasing volume on the intake side of the

rotating group, and a decreasing volume at the discharge side. Output hydraulic horsepower is

determined by the flow provided by the pump and the operating pressure. A common formula is

used:

(Eqn. 4-1)

Where: P = Pressure

Q = Flow

Displacement is a term for the flow capacity of a pump, and is the volume of liquid that is

transferred from inlet to outlet during one revolution. The pump delivery is proportional to drive

shaft speed, and will be variable based on a variably rotating prime mover. Expressed as:

(Eqn. 4-2)

Where: GPM = Gallons per minute

RPM = Revolutions per minute

Figures 4.2.2-1, 4.2.2-2, and 4.2.2-3: Radial Piston hydraulic pump various views [82, 83, 84]

HP = PQ1714

D =GPM ×231RPM


The HPU of this project consists of a fixed-displacement radial piston pump. Positive-

displacement pumps have a very small clearance between rotating and stationary parts, and

deliver a specific amount of fluid to the system for each revolution. Output of the pump is not

significantly affected by resistance to flow.

Fixed-delivery pumps provide a single, specific volume displacement per revolution.

They are also simpler in construction than variable-displacement types. Characterized by a

radial piston arrangement within a cylinder block, these types of pumps are used especially for

high pressure and relatively small flows [85]. As the pistons reciprocate in a radial piston pump,

rotary shaft motion is converted into radial motion. These types of pumps and cut-away views

can be seen in the above figures. Friction and turbulence effects within moving hydraulic fluid

generate heat with the passing of the medium through valves and orifices in the circuit. These

are mechanical losses, and result in a loss of power to the system. The product of the

mechanical and volumetric efficiencies is referred to as the overall efficiency, expressed as a

percentage:

(Eqn. 4-3)

Considering mechanical and volumetric losses, there will always be less power being delivered

by the pump than the power being used to drive it. The pump input power in horsepower

required is expressed as:

(Eqn. 4-4)

Where: n = Number of turns in RPM, pump torque in inch pounds is given by the following:

T = HP ×63025n

(Eqn. 4-5)

EffOv =HPoutHPin

×100

HPIn =PQ

EffOv ×1714


4.2.3 HYDRAULIC MOTOR DESCRIPTION

Closely resembling pumps, rotary actuators convert fluid power to rotary motion. Fluid

is pushed into the inlet of the motor, causing rotation of the shaft. The goal of using hydraulic

motors is to produce a constant rotational speed at a sufficient torque to move a given load, with

torque being the rotational force component of the motor’s output. Hydraulic motors have been

selected as the prime movers for the integrated water pumps of the desalination phase of this

project. With torque sufficient to exceed shaft resistance, rotary motion occurs. Torque, in inch

pounds is expressed as:

(Eqn. 4-6)

The output horsepower of a rotary actuator is based on torque and speed, and is expressed as:

HP = Tn63025

(Eqn. 4-7)

For this project, RO is achieved through fixed displacement axial piston type motors

coupled with water pumps. Piston motors generate torque through pressure on the ends of

reciprocating pistons in a cylinder block [86]. Pressure at the piston ends causes a reaction against

a fixed swash plate and thus a fixed speed, controlled by a change in inlet pressure or flow rate.

Figure 4.2.3-1: Axial Piston motor [87]

T = 63025×HPn


Piston motors work best in high torque, low speed applications, have high efficiency

including the ability to withstand stall torque easily with relatively high power to weight ratios.

Lastly, piston motors will have a case drain (motor housing) line to allow piston leakage to flow

to return. As the priorities of this system are minimal maintenance and simplicity of

construction, a fixed displacement system has been decided upon, meaning any variation in the

output of the main pump brought about by a variation in the tidal turbine shaft torque and speed

will correlate and the whole system will meter accordingly.

4.2.4 HYDRAULIC INTENSIFIER DESCRIPTION

Hydraulic Intensifiers are devices used to multiply pressure. A hydraulically operated

pressure-intensified gas compressor has been selected to compress and transport product

hydrogen gas of this design ashore. The arrangement in this case would be a hydraulic cylinder

in the center with a gas cylinder (for hydrogen compression) on each side of the double-acting

cylinder. A directional control valve ports hydraulic flow to the ends of the cylinder in an

alternating manner. As the hydraulic cylinder strokes, gas is compressed and displaced from one

gas cylinder while simultaneously filling the other gas cylinder.

In this way, a hydrogen compressor assembly can be functioned from the main hydraulic circuit.

By tuning the actuation of the directional control valve, control of both discharge pressure and

gas flow rate can be achieved through hydraulically driven intensification. According to

engineers at Hydro-Pac Inc. [88], multi-stage units boost hydrogen gas from inlet pressures as

low as 70 PSI (4.82 bar) to discharge pressures as high as 15,000 PSI (1032 bar), which is

within the transmission pressure range for hydrogen. This can be seen in the below figures

showing the compression stages.

Figures 4.2.4-1 and 4.2.4-2: Stages 1 & 2 of a 2-stage compression of hydrogen [89, 90]


4.3 RO HIGH PRESSURE PUMP DESCRIPTION

For the purposes of providing the pressurized water feed to the membrane assembly, an

axial piston, fixed displacement pump has been selected as the high-pressure component.

Figure 4.3-1 and 4.3-2: Axial-piston, fixed displacement pumps – RO system components [91, 92]

Figure 4.3-1 and 4.3-2 show the internal assembly of the axial piston pump. Suitable pumps

are available in the mature RO market that are constructed of non-corrosive materials such as

duplex stainless steels or carbon reinforced Polyether Ether Ketone (PEEK) [93] characteristics,

with the ability to supply low viscosity and corrosive fluids (seawater) under high pressure.

Priority in design is minimal maintenance/long service life.

Additional lubrication is not required as the seawater provides lubrication of the moving

parts in the pumps, with an integrated flushing valve that allows the seawater to flow from inlet

to the outlet when the pump is not running. Pumps selected are of the fixed displacement type,

wherein the flow is proportional to the number of revolutions of the input shaft and the pump

displacement. With this type of pump the flow is affected by a change in the rotational speed.

The flow/rpm ratio is constant.

4.3.1 ENERGY RECOVERY DEVICE

The energy recovery device (ERD) consists of a rotating isobaric pressure exchanger

and a positive displacement pump, also called a booster pump. The rotation speed of the

pressure exchanger and the pump are the same, as a common hydraulic motor drives them.


The role of the ERD is to capture otherwise wasted pressure from the membrane reject

flow and transfer it directly to the membrane feed flow, thus changing the HP concentrate into

HP seawater that is fed into the RO membranes. This technology thus significantly reduces flow

needed from the main HP pump. Overall energy consumption of the RO process depends on the

recovery rate, which in this case is relatively low given that the electrolyser needs 9 liters of

water per 1 kg hydrogen; As previously described, ideally 39.4 kWh of electricity and 8.9 liters

of water are required to produce 1 kg of hydrogen at standard conditions (25°C and 1 bar.) The

pressure exchanger transfers pressure from the HP concentrate (HP in) to the Low-Pressure (LP)

seawater coming from the surrounding water column at hydrostatic pressure (LP in). This

increases efficiency to 95 %, with net energy consumption reduced by up to 60 % and pumps

sizes comparatively smaller compared to systems not using ERD [94].

4.3.2 BOOSTER PUMP

The booster pump is a positive displacement pump, which means that the flow is

proportional to the rotational speed of the hydraulic motor; e.g. if the rotation speed of the

motor is raised by 10%, the flow will be 10% higher and vice versa. The booster pump

integrated to the ERD must only overcome the pressure drop from the high-pressure outlet to

the high-pressure inlet. The relative speed of the hydraulic motor controls both the pressure

exchanger and the high-pressure booster pump via the same shaft, therefore the whole system

operates corresponding to shaft rotation. This can be seen in figs. 4.3.2 -1, and 4.3.2-2.

Figure 4.3.2-1: Energy Recovery Device (ERD)/ booster pump pair [95]


Figure 4.3.2-2: Line diagram of the RO system [96]

4.4 ELECTROLYZER POWER SUPPLY

The design of this project originally focused on power for the electrolyser being derived

from the tidal turbine. As this would have required switchgear, transformers, rectifiers and

control systems (all needing a dry-nitrogen purged environment to operate in), this was deemed

inefficient. Also, by rectification, harmonic voltages and currents are generated on the AC side

and ripple on the DC side. A localised DC generator was opted for as the simplest option. The

generator, sufficiently protected from water ingress and hydrostatic pressure compensated, and

of the Permanent magnet hybrid homopolar type (PMHH) technology, uses a solid rotor with

magnets revolving around a stationary stator and field coil, and was found to offer the most

benefits in this case; wherein a Permanent Magnet (PM) rotor is combined with stationary field

windings located on the stator – as can be seen in the below figures.

Figures 4.4-1 and 4.4-2: PMHH exploded view and cross sectional view [97, 98]


The advantage of having the field winding in the stator is the elimination of slip rings

and greatly simplified rotor construction. This technology does not require brushes, rotating

fields, or excitation and are reported to have efficiencies of between 75 to 85% versus 45 to

70% for other DC alternators [99]. The permanent magnetic portion produces the majority of the

output power; the homopolar portion boosts the power produced by the permanent magnet and

provides voltage regulation in response to load transients, and due to their high DC output

capability and high power density [100] appear suitable for electrolyser applications.

The PMHH would be coupled to a hydraulic motor. A representation of the power

electronics can be seen in fig. 4.4-3. The input filter is needed to prevent the converter switching

fluctuations from affecting the voltage and current from the rectifier bridge. The filter selected

is a common two-stage filter with an L-C filter and an R-C damping leg [101] The buck converter

is considered a high-frequency DC-DC switching converter. The low-pass filters help to reduce

the output voltage ripple.

Given the fact that in this converter, the input voltage is always greater that the output voltage,

the PMHH generator voltage must always be greater than the rated voltage of the electrolyzer.

High frequency switching controls the level of voltage reduction. The switch opens and closes

at a specific frequency with the ratio of on time to the period defined as the duty ratio – Eqn. 4-8

[101 – page 64]. Duty ratio monitoring/ control using feedback would ensure efficient operation

throughout the variable output range experienced by the DC generator, which is converted to a

high frequency AC, using a switching or ‘chopper’ transistor, driven usually by Pulse Width

Modulated (PWM) square wave. This results in a high frequency AC wave, which is then re-

converted to DC. To maintain a continuous output, the circuit uses the energy stored in the

buck-integrated inductor during the on periods of the switching transistor, to continue supplying

the load during the off periods.

Figure 4.4-3: Simplified layout of Electrolyser power supply [102]


In many applications, Insulated Gate Bipolar Transistors (IGBTs) are used as the main

switching device in the converter circuit because they have the current density and low loss of

bipolar transistors with the high-speed and high input impedance of Metal Oxide Semiconductor

Field-Effect Transistor (MOSFETs) [101 – page 73]. The output voltage from the converter normally

fluctuates between 0 and Vout , so the filter, consisting of an inductor and capacitor, is used to

minimize this problem. For the buck converter, the voltage conversion ratio is dictated by:

VoutVin

= D IoutIin

=1D

(Eqn. 4-8)

Where: D = Duty Ratio

Figure 4.4-4 Average analysis of buck converter [103]

Figure 4.4-4, a multisim application model [103], illustrates the frequency response of the

buck converter to a disturbance in the controlling signal. As seen, a Switch Mode Power Supply

(SMPS) has been chosen as the power supply for the electrolyzer. These converters transfer

energy between two DC systems with different values. SMPS make it possible to supply a

constant, controlled voltage to DC loads from a DC power supply that is either unsuited to the

load or whose amplitude is uncontrolled.


4.4.1 ELECTROLYZER SIZING CONSIDERATIONS

Due to the fact that hydrogen production systems are characterized by being DC loads

that require high currents and low voltages, they are not of a conventional load for power

electronic systems, both with regard to the DC energy demand profile and the required voltage

and current values.

Moreover, the high current and low voltage required for the operation of electrolyzers,

in addition to the wide power range due to the intermittency of the renewable resources, could

lead to a significant reduction in the efficiency of the conversion stages. Therefore as

mentioned, a direct connection between the renewable systems and the electrolyzer has been

decided upon, with no conversion stage. The connection to the tidal turbine through DC/DC

power conversion stages makes it possible to condition the currents and voltages generated to

some suitable values for the electrolyzer and, furthermore, to obtain the maximum power from

the tidal turbine systems at all times. The reason for the DC/AC/DC process is that, by changing

the DC to a high frequency AC, the discrete components needed for conversion back to a

regulated DC supply can be much smaller and cheaper than those needed to do the same job at

turbine output frequency. The circuit schematic for the multisim application model [104] can be

seen below:

Figure 4.4.1-1: Application model of buck converter circuit connected to the output of the PMHH generator [104]


The high frequency AC produced during the conversion process is a square wave,

which provides a way to control the output voltage by means of PWM, allowing regulation of

the output to be more efficient than is possible in linear regulated supplies. In most switched

mode supplies, regulation of both line (input voltage) and load (output voltage) is normally

provided. In this case, the input voltage is variable, but the output can be controlled to a certain

degree.

This is achieved by altering the mark to space ratio, which is the ratio of the on time to

the off time. Control of the mark to space ratio is achieved by comparing voltage feedback from

the output of the supply with a stable reference voltage, hereby provided by the system battery.

By using this feedback to control the mark to space ratio of the oscillator, the duty cycle and

therefore the average DC output of the circuit can be controlled.

In this way, some level of protection for the electrolyzer from both over voltage and

over current may be provided. To provide a well-regulated output, a sample of the DC output

voltage would be fed back to the control circuitry and compared with the reference voltage. Any

error produced would be used to control the output voltage. To maintain electrical isolation

between input and output, feedback would usually be via a device such as an optoisolator. A

disadvantage of using such a high frequency square wave in a powerful circuit such as a SMPS

is that many powerful high frequency harmonics are created; therefore effective Radio

Frequency (RF) screening and filtering may be required. Additional solid-state current limiting

circuitry may be added to provide safety for the electrolyzer.

Figures 4.4.1-2 and 4.4.1-3: Power electronic system reference and simplified layout of electrolyzer power supply [105.106]


4.4.2 ELECTROLYZER ENERGY ANALYSIS

The amount of hydrogen generated in the electrolysis process during a certain time

interval corresponds to the mean value of the current flowing through the cell stack. Therefore,

the ideal hydrogen production rate in an electrolyzer is directly proportional to the transfer rate

of the charge, in other words, to the electric current, with hydrogen losses being greater in

percentage terms as the electrolyser current decreases. To carry out an energy analysis for the

complete hydrogen production system, the actual power supply needs to be included, given the

fact that all the energy supplied to the electrolyzer is derived from the power supply. Therefore,

the correct design of a hydrogen production system would not only aim to optimize the

electrolysis stage, but also the actual power supply. Any errors in the design of the supply could

cause considerable reduction in the overall efficiency of the system, resulting in a more

expensive process. In the case of electrolyzers, which require high currents, this could create

considerable losses in the passive components and semiconductors.

Figure 4.4.2-1: An example of Hydrogen production rate at 55° and 25 bar (ideal vs. losses) [107]

The energy efficiency of the system is obtained from the product between the efficiency

of the electrolyzer and the efficiency of the power supplies, or as the ratio between the power of

the useful hydrogen production and that of the input into the complete system – Eqn. 3-12. Also,

because electrolyzer cells are not, in fact, 100% efficient, a considerable amount of waste heat is

generated and would need to be removed from the cells. As the seawater is an excellent

conductor of heat, and surrounds the entire unit’s structure, this does not appear to be a problem,

requiring no further cooling equipment.


4.5 BATTERY AND CHARGING SYSTEM

An integrated battery and charging circuit is included in the control system of this

design. This is to account for periods of possible inactivity, requiring monitoring and alarm

reporting of the system. The battery would serve as a power supply for the control systems:

Fiber optic transceivers, Digital and Analog Input/output (I/O), and Central Processing Unit

(CPU), therefore a regulated DC low voltage supply would be essential. The battery would also

supply current to the hydraulic valves whereby operation of the RO system could continue in

times of lower tidal stream current: described later. The schematic of the system can be seen in

fig. 4.5.

Figure 4.5: Layout of subsea manifold systems

The hydraulic valves, requiring more current for operation would be supplied from the battery

and/or directly from the generator through a voltage divider module. From literature available

[99], PMHH type alternators are available in nominal voltages of between 12 and 500 Vdc with

variable power ratings from 2 kW to 40 kW based on variable tidal turbine speed.


4.6 SUBSEA CONTROL MODULE

The control component of the hydrogen generation manifold system is the Subsea Control

Module (SCM). The SCM contains electronics and hydraulics in sealed environments, with

equipment and instrumentation packaged in a retrievable unit/housing consisting of:

• Subsea Electronics Module (SEM)

• Electro-hydraulic Directional Control Valves (DCV) and other valves

• Wet – mate able connectors (electrical, fiber optic and hydraulic)

• Control Module base

• Lock/unlock mechanism for running tool (device for retrieval and deployment)

• Internal sensors and fiber optic transceivers

• Hydraulic filters/strainers

• Pressure reducers (optional)

• Pressure intensifiers (optional)

As the SEM is for all intents and purposes a subsea computer fig.4.6-1, all active electronic

circuits would be enclosed in a one atmosphere, dry-nitrogen-purged enclosure designed for the

water depth involved with a good safety margin. The SEM would be dual-redundant,

programmable and allow for operation from the surface while in place through fiber optic

telemetry. The subsea electrohydraulic components would be mounted in a separate dielectric

fluid filled and pressure compensated compartment of the SCM [75] complete with local driver

boards. The manifold would contain a pressure transducer and temperature probe. To minimise

the electrical power consumption and efficiency of solenoid operated valves these valves would

be pulse operated. The SCM housing would be made of structural steel fig.4.6-2, with all

hydraulic and electric couplings manufactured from stainless steel. Individual hydraulic-supply

filters would be mounted within the control module, maintaining fluid-cleanliness

specifications.


Figures 4.6-1, 4.6-2, and 4.6-3: SCM without housing, SCM and base, SCM deployed using running tool [108, 109, 110]

It is envisaged that by mounting as many consumable and replaceable parts as possible

within the SCM housing (which is ROV retrievable by running tool - fig.4.6-3) MTBF can be

planned for through preventative maintenance and upgrade.

4.7 COMMUNICATIONS

The telemetry for this system is in this case carried over single mode fiber optic cables,

providing a transparent link for the data interfaces. Each end would have

multiplexing/demultiplexing capabilities. The function of the multiplexer is to partition the total

system bandwidth into usable sub-bandwidths for use by the peripheral boards. The

bidirectional data communication between the surface and subsea computers is accomplished

using a full-duplex RS232C serial link. Fiber optic cables carry data over long distances, while

RS232 hardware permits communication at distances up to 50 feet, which is suitable for this

application i.e. local to the manifold or turbine. RS232 is a telecommunications protocol

standard for binary communications between devices. The devices commonly interfaced are

generally referred to as Data Terminal Equipment (DTE) and Data Communications Equipment

(DCE). The transmit/receive functions are not switched in full duplex systems, therefore

allowing data transfer in both directions simultaneously.

RS232 is used for many purposes, such as connecting computers to sensors and

modems, or for instrument control, but is limited to point-to-point connections between PC

serial ports and devices. It is also considered a very robust and simple protocol.


4.8 HYDRAULIC CONTAINMENT/ CONTAMINATION CONTROL

Fluid cleanliness is maintained by a low micron HP filter assembly connected to the

output of the main pump, and located within the SCM, featuring HP collapse rated elements

without a bypass valve, also a case drain filter for the possible leakage flows from the rotary

actuators. To help prevent water contamination of the hydraulic system, a water filtration

element would be installed in a bypass loop between the main HPU lines, which would cause oil

to be driven though the filter. Hydraulic fluid is supplied to the pump suction from a pressure

compensated, hydraulic reservoir. Also included is a large volume relief valve to protect the

compensator.

The total hydraulic system variable volume provided by the compensating system

allows for system compensation at depth. The compensators (spring-compensated reservoirs)

utilise either a roller diaphragm or bellow arrangement to transmit ambient changes in pressure.

The compensating mechanism is complemented with springs of various rates to ensure the

internal pressure remains above that of the surrounding water. Analog transducers would

monitor the oil levels in the compensators continually. Examples of various submarine

hydraulic reservoirs are below.

Figure 4.8-1 and 4.8-2: Various spring-compensated subsea hydraulic reservoirs, submersible water filtration system [111, 112]


5.0 EVALUATION OF DESIGN

With the excess of kinetic power over the nominal capacity of the generator being lost

in a typical tidal turbine design, this project focused on the extraction of energy at rated power

and above through pitching the blades accordingly for economic electricity generation; while

applying the extra energy to hydrogen production. As a result of the previous chapters, a design

has been formulated. Data available was limited, being mainly commercial proprietary research

material, and as this is a novel and theoretical concept at this stage, in the below examples,

various assumptions have been made in terms of the energy requirements. Differing conditions

are discussed, as contributors to differing hydrogen production levels.

5.1 POWER EXTRACTED FROM THE TIDAL STREAM

To evaluate the viability and feasibility of the design, two scenarios are presented. This

is to show varied conditions of operation. In both cases, the power coefficient CP of the tidal

turbine is 0.34. The examples are based on a horizontal axis turbine in a tidal stream.

Example 1: Turbine blade length is 11.5 meters and rotor sweep area A (πr2 ) is 415.47 m2.

Example 2: Turbine blade length is 12 meters and rotor sweep area A (πr2 ) is 452.39 m2.

The following table reflects conditions without blade pitch limitations or EDR, with power

extracted focused on tidal velocities at an arbitrary location between 1.5 and 4.5 m/s. The power

converted from the tidal stream into rotational energy in the turbine using Eqn. 3-2 results in an

instantaneous power output for a given V3, which can be seen in table 1, where r = blade length.

Tidal Velocity

in m/s (V3) Power Extracted in MW at r = 11.5m Power Extracted in MW at r = 12m

1.5 0.238 0.26

2.0 0.56 0.615

2.5 1.1 1.20

3.0 1.91 2.07

3.5 3.028 3.29

4.0 4.52 4.92

4.5 6.43 7 Table 1: Power extracted levels at two different blade lengths for a given tidal velocity


Table 1 illustrates the difference in theoretical power extracted from the tidal stream for

a variation or redesign of blade length. In terms of design considerations, increasing the size of

the generator or the turbine diameter can increase the power generated by a tidal turbine. In the

case of the blade diameter, rated power is reached at a lower tidal speed. The result is an

increase of the output power for lower velocity tidal currents. In the case of generator size being

increased, there is an improvement in the output power at higher velocities.

5.1.1 RATED SPEED/ RATED POWER

This device’s design, and that of many functional tidal energy converters use blade

pitching, which limits the maximum thrust and torque to give controllability over the tidal

turbine. However, when used to maintain rated electrical output and economic capacity factor,

power is in effect wasted over and above rated power levels. Starting with the assumptions of

table 1: Twice during the monthly lunar cycle, the lowest peak neap tide occurs at 2.5 m/s tidal

stream velocity at an arbitrary location. As this is the minimum averaged-out peak tidal velocity

this value was chosen as the rated speed for the tidal turbine of this project as can be seen in fig.

5.1.3-1 below. The red line represents the minimum peak at neap tide, and is also the rated

power for the turbine. The green line represents the spring peak. EDR would reduce the spring

peak by 41% to maintain rated power levels.

Figure 5.1.3-1. EDR of Springs while retaining rated power during neap cycle [113]


This power rating would maintain economic generation of electrical power throughout

tidal fluctuations. Rated power is thus achieved in tidal currents > 2.5 m/s. In this way, the

device captures rated power throughout the monthly tidal cycle. The cut-in speed in this case is

1.8 m/s.

Using example 1 and table 1: From a power extracted level of 1.1 MW, by rating the

electrical generator at 1 MW (rated output) this results in a remaining 100 kW (134.1 HP) that

can be purposed for hydrostatic transmission of mechanical torque in the operation of the

hydrogen generation system.

Using example 2 and table 1: From a power extracted level of 1.2 MW, by changing the

blade length to 12 meters, and still rating the electrical generator at 1 MW (rated output) this

results in a remaining 200 kW (268.2 HP) that can be purposed for the same function. The usage

of which would not affect the rated power level of the tidal turbine. Which can be seen:

Prated = Pext − Phyd (Eqn. 5-1)

Where: Prated = Rated power

Pext = Power extracted from the tidal stream

Phyd = Power used by the hydrogen generation system

*This will be proved, but firstly a discussion on hydraulics is necessary.

5.2 HYDRAULIC CHARACTERISTICS

The hydraulic output of the pump is fixed in its displacement, but the prime mover is

variable in both amplitude and direction, leading to bidirectional flow - unacceptable for this

design. All of the subsystems hydraulic pressure and flow is derived from the HPU, thus are to

be nominally rated within this power range. As the HPU will be bidirectional in accordance

with the tidal turbine’s rotation, power is sinusoidal with the changing direction and amplitude

of the tides.


Fig. 5.2-1 Hydraulic “Rectifier” simplified schematic [114]

The hydraulic “rectifier” analogy has been designed to accommodate for this alternating

type of operation, for the purpose of providing direct hydraulic flow rather than alternating

hydraulic flow to the subsystems. This is achieved by in-line or pilot-operated low pressure

check valves.

Tidal turbines typically rotate at 10 -20 RPM, therefore a reduction ratio gearbox

(1:100) is required to provide a nominal 1800 RPM (188.5 rad/sec) at rated speed for the HPU

to operate. Using hydraulic affinity laws (see appendices), which are derived from analysis of

three important parameters that describe hydraulic pump performance: flow, pressure and

power, the correlation between tidal velocity and hydraulic output can be seen in table 2.

Tidal velocity in

m/s HPU shaft RPM Flow Rate in GPM Pressure in PSI

HPU Power

in kW

1.5 1080 46.2 1080 21.6

1.8 1296 55.4 1555 37.3

2.0 1440 61.6 1920 51.2

2.5 1800 77 3000 100

3.0 2160 92.4 4320 172.8

3.5 2520 107.8 5880 274.4

4.0 2880 123.2 7680 409.6

4.5 3240 138.6 9720 583.2 Table 2: Varying conditions of operation, and effects on hydraulic parameters without limiting output


The values of table 2 represent those at a blade length of 11.5 meters as in example 1.

Again, the table reflects conditions without blade pitch limitations or EDR, but serve to

illustrate the proportional relationship between the variables.

5.3 CONTEXT OF THE PROJECT AND AIMS

In the context of this project, it is necessary to use hydrostatic transmission as a means

to transfer shaft torque into mechanical and electrical energy to produce hydrogen. Therefore

the theoretical power required for the HPU to transmit power to the subsystems should be

extracted from the tidal stream while the turbine maintains enough thrust to generate rated

electrical power. Most importantly, this functionality must not slow the turbine shaft down

where a condition exists that affects the generator’s ability to generate rated power. The torque

load experienced by the turbine shaft when coupled to the HPU (when loaded) determines this

factor. On this basis, a torque calculation needed to be done to ascertain the total torque present

within the designed system when running. A sufficient torque load that slows the rotation of the

shaft enough to maintain economic electrical generation was the goal.

Using the assumed values of examples 1 and 2, a torque load analysis was done. As can

be seen, for known horsepower, torques required can be found, using Eqn. 4-5. The values were

then converted from inch pounds to newton meters [115]. The values following are all based on a

tidal velocity of 2.5 m/s. This is shown in table 3:

kW of Hydraulic power unit 100 200

HP of Hydraulic power unit 134 268

Pump Torque in In. lbs. 4691.86 9383.72

Pump Torque in NM 530 1061 Table 3: Torque values of the HPU to operate in producing rated horsepower

The torque values associated with the turbine reduction ratio gearbox (i.e. at the PTO where the

pump would derive torque) were found using Eqn. 3-3. Shown in table 4:

Turbine Power at 1800 RPM 1.1 MW 1.2 MW

Turbine Torque at 1800 RPM 5851 NM 6383 NM Table 4: Torque values of tidal turbine shaft at rated speed


Subtracting the pump torque from the torque produced at the turbine PTO, which using

Eqn. 3-3 again, and Eqn. 5-1, results in a power extracted in MW by the turbine in table 5:

Initial condition - Turbine Power at 1800 RPM 1.1 MW 1.2 MW

Initial condition - Turbine Torque at 1800 RPM 5851 NM 6383 NM

New condition – Turbine torque + HPU load at 1800 RPM 5321 NM 5321 NM

Power extracted + HPU load at 1800 RPM 1 MW 1 MW Table 5: Power extracted from tidal stream at rated speed with coupled HPU torque load

It can be seen that by adding a hydraulic system to a tidal turbine, the torque load acts

as a resistance, and would suggest that 100% operation is necessary to maintain a reasonable

capacity factor, given that partial operation would result in partial torque load. However, a

suitable control algorithm is required to maintain rated power throughout fluctuations of the

hydrogen production system via blade pitching. This also suggests that any size of hydrogen

production system can be added to a tidal turbine, with proper design consideration. In this case

torque is a major factor in deciding the capacity of such a system. Also seen is that an integrated

HPU and hydrogen production system can operate without affecting capacity credit of a tidal

turbine, on the grounds that the torque load imposed upon the shaft by the HPU cancels out the

extra power extracted from the tidal stream. This is a fundamental concept in the feasibility of

this project.

Figure 5.3-1: Illustration of rated power maintenance through torque application [116]


5.4 WATER TREATMENT

*From this point, all of the subsystems have been rated within a 100 kW power

consumption limit, as per example 1, to size the system accordingly. An assumed depth for this

project is 40 meters, where the hydrostatic pressure would be 5.03 bar absolute (including

atmospheric pressure of 1 bar.) This factor should attribute to reduce energy consumption of

passing the in-feed water through the pretreatment systems, but has not been considered in

lowering the HP pump systems power consumption. From the literature, [93] the selected in-feed

HP pump provides 80 bar pressure over the RO membranes at around 1500 RPM. This is a

typical RO pressure for high TDS seawater. As envisaged, one HP pump moves water across

the membranes, while the other HP pump is an optional device to transport water back to shore.

All of the hydraulic pumps of this design are of the fixed displacement type, wherein the flow is

proportional to the number of turns of the input shaft and the pump displacement. As the pump

displacement remains the same, as can be seen in table 2, with an RPM of 1460, there is a

corresponding flow rate and specific energy requirement.

Flow Rate in m3/h Power Requirement at 80 bar RPM Specific energy

5.0 14 kW 1460 2.8 kWh/m3

Table 6: Power requirements of High Pressure RO pumps

In calculating the power consumption and specific energy per amount of product for an

electrolyzer or RO pump (as in table 6), the following equation applies:

Or: (Eqn. 5-2)

Where: = Power consumed in kW

= Production rate in amount/hr.

= Production efficiency/ Specific energy in kWhr/amount

Productioneff =Prequired

Product flowratePrequired = Productioneff ×Product flowrate

Prequired

Product flowrate

Productioneff


A hydraulic motor couples the ERD system (booster and HP pump). The combination

of the pair of HP pumps and the ERD bring an estimate of energy requirements as can be seen

in table 7. This based on literature on the RO components (see appendices).

Equipment Specific Energy Power consumed at 1460 RPM

HP Pump (input) 2.8kWh/m3 14 kW

HP Pump (transport) 2.8kWh/m3 14 kW

ERD & Booster pump pair 5.5 kW

Total Power Consumption 33.5 kW Table 7. Reverse Osmosis Circuit estimated power consumption

A pressure compensated reservoir - fig. 4.8-1, would be sized to house 10 times the

needed volume at all times regardless of pump output, resulting in a 90 liter tank; as previously

discussed in the literature review, the system requires roughly 9 liters of water per kilogram of

hydrogen. The product water would exit the membranes at low pressure. As such, a pressure

compensated reservoir would contain the product until required by the electrolyzer.

5.5 ELECTRICAL POWER GENERATION

Rotary actuation of the DC generator is provided by hydraulic motor connected to the

PMHH rotor. As the assembly is a large mass, the moment of inertia, i.e. the amount of energy

required to start the mass of the armature rotating up to the operating speed, contributes to the

start-up torque. Literature on the DC generator (appendices) shows a required 51 HP (38 kW) to

achieve 3000 RPM, implying an 81% efficient transfer of kinetic to electrical power. A soft -

start hydraulic valve would minimize this element of start-up torque and shock loading, by

limiting the flow rate on start up until a fixed interval. DC generator operating characteristics

are shown in table 6. The value of 26 kW (2600 RPM of PMHH rotor) corresponds to a HPU

shaft RPM of 1800, which is indicative of electrical generation at rated speed. This value is

especially important to the electrical power available to the electrolyzer and the associated

subsystems. It is from this value that all of the electrical components have been sized.


Shaft speed in RPM Electrical power output in kW

2200 21

2400 23

2600 26

2800 29

3000 31 Table 8: Shaft speed and power generated proportional relationship – DC generator

A simplified schematic of the entire fluid power circuit is shown in fig. 5.5-1. After

power is generated, the output is fed to the DC-DC converter, whereupon the DC bus feeds the

electrolyzer, battery charging circuit and control system.

Fig 5.5-1: Simplified layout of hydraulic circuit [116]

5.5.1 ELECTROLYZER CONSIDERATIONS

As data on electrolyzers is mostly proprietary, the conditions of table 9 have been used

in relation to sizing the PEM electrolyzer power requirements. Costs have not been considered.

This is data based on Giner inc. high-pressure electrolyzers [117]. Maximum current density is

given in kilo amps per meter squared.


Variables Current technology Future technology

Max current density (kA/m2) 20 25

Hydrogen pressure (bar) 85 50

Cell voltage 2.0 1.7

Maximum Production rate (kg/hr) 0.333 0 – 5.0

Max stack power (kW) 20 228

Production efficiency (kWh/kg) 60 45.7 Table 9: High Pressure hydrogen generators [117]

As can be seen in table 8 and 9, the rated power for the electrolyser implies a required

minimum 2200 RPM for operation. This also implies that hydrogen can start to be produced at

this number of rotations of the generator. However, the electrolyzer can operate over a dynamic

range: 5–100 % of rated capacity has previously been achieved and typically PEM electrolysers

will have a response time from 5-100 % in less than a second. The buck converter compensates

for the variation in the input voltage. It is on this basis that the integral PEM electrolyser is

selected in terms of control range and regulation time. Under the above assumptions, a

modification on Eqn. 3-12 below produces a 65% efficient electrolyzer stack, which is a typical

value for electrolyzer stack efficiencies. The electrolyzer is rated at 20 kW at 100% capacity.

= 65% efficient

5.6 HYDROGEN COMPRESSION

Once the hydrogen gas is produced, a storage tank or intermediary is necessary in order

to provide a constant feed to the compression stage. This could be accomplished using a suitable

low-pressure reservoir of a similar type described that would house product volume for

subsequent compression and transport. A gas dryer stage could also be integrated at this point to

remove moisture. As mentioned, the SCM could house consumable items like element type

filters for this purpose. As per the literature [88], a hydrogen compressor would need to increase

the pressure up to approx. 6000 PSI (413.7 bar) to be transmitted by subsea pipeline.

Electrical efficiencyelectrolyzer( ) =

HHV of H2 producedElectricity used + heat supplied

=39kWh

60kWh / kg


A directional control valve, functioned via the control system would create the

oscillatory motion associated with intensifying hydraulic fluid to achieve gas compression – fig.

4.2.4-1 As this system is not a rotary actuator, rather a double cylinder controlled via oscillating

directional control (controlled via software) it is not envisaged to operate over varying

conditions of input, as the electrolyzer is rated to provide hydrogen at 85 bar. To reach output

pressure of 6000 PSI, the following table illustrates the power required for a given flow rate,

and that the compression capacity increases in proportion to power consumed.

Power Required - 6000 PSI output (413.68 bar) Capacity at 85 bar hydrogen inlet pressure

20 HP (14.7 kW) 34.4 standard cubic feet/ minute (58.44 m3/h)

40 HP (29.4 kW) 68.8 standard cubic feet/ minute (116.9 m3/h) Table 10: Compression rates for hydrogen multi-stage compressor

5.7 ENERGY ANALYSIS

The share of power consumption can be seen in the below figures. At 100 kW as per

example 1, the components are seen in fig. 5.7-1. As the electrolyzer, control system, battery

charging circuit and ancillaries are electrical loads; these values are included in the DC

generator sector. These figures indicate 100% operation, therefore utilising full available power

at rated speed. In fig. 5.7-2, as per example 2, a 200 kW system was sized therefore requiring a

200 kW HPU, and doubling of the subsystem components.

Figures 5.7-1, and 5.7-2: Power consumption of subsystems as percentages of total transmitted power [118. 119]


To show figures associated with hydrogen production rates, table 2: “Varying

conditions of operation, and effects on hydraulic parameters without limiting output” is

referenced. It can be seen that at cut-in speed of 1.8 m/s, there is sufficient power to function the

RO process, but not hydrogen production at this point. Moreover, as the power required for RO

(table 7) was 33.7 kW at rated speed, any speed below that would have RO power consumption

that was even lower. This implies that potable water can be produced between 1.8 m/s and 2.5

m/s, including pump power consumption to transport potable water ashore. The following table

shows the hydrogen production rates at various RPM (of the HPU). Above rated speed is not

considered as the electrolyzer is already operating at 100% capacity.

Tidal

Velocity in

m/s

Power

Extracted in

MW

HPU

RPM

DC generator power

output in kW

Hydrogen

Produced in

kg/h

Water

Desalinated/

transported

in m3/h

1.8 0.41 1296 18.72 0 5

2.0 0.56 1440 20.8 0 5

2.5 1.1 1800 26 0.333 5 Table 11: Hydrogen and potable water production rates for a given tidal velocity

It is important to point out the conditions on which hydrogen is produced. Hydrogen

production occurs between 2.0 m/s and 2.5 m/s, which is considered the optimal operating band

for the system as a whole. However, below rated speed, the torque at the HPU is insufficient to

provide hydraulic pressure to all the systems. Also of note, is that economic electrical

generation will not occur before rated speed. Through the control system the RO plant and DC

generator plant may still operate as low as 2.0 m/s, but hydrogen compression will not be

available. As can be seen in table. 11, below 2.0 m/s, the power extracted is insufficient to drive

the DC generator (and thus drive the electrolysis process) in addition to the RO plant.

5.8 CONTROL SYSTEM AND OTHER LOADS

Other systems, including the control system, hydraulic valves, battery chargers, and

heating elements operate via the DC-DC converter that is in direct connection to the DC bus and

the battery at low voltage (24VDC).


An optional safety measure could be included such as an electromagnetic clutch:

(appendices) normally-on and fail-safe to disengage the HPU in a danger condition. This would

operate via an electric actuation, but transmit torque mechanically. The control system is

localized, and 24 VDC input is required to power the SCM. The Subsea Electronics Module

would derive its operating power from the trickle-charged battery, requiring less than 1 kW,

with an additional DC-DC converter and/or voltage divider required to bring the voltage to

adequate levels for battery charging. Hydraulic solenoids would be of the low power

consumption type. A heating element could also be integrated to raise the permeate water

temperature, and thus theoretically, lower the power consumption of the electrolyzer. However,

this element is only required to elevate the water temperature to ( ≈ 25°C ), with a compensated

reservoir insulated from ambient temperature. This adds approx. 2 kW to electrical power

required. These loads are envisaged not to exceed 5 kW, therefore need to be sized to operate

from the battery system, with additional current available direct from the DC bus.

6. CONCLUSION

To conclude this project, an evaluation of the returns provided was carried out. Starting

with a figure of electrolyzer-only consumption at 60 kWh/kg, (a typical production efficiency

rate for electrolyzers) and given that the specific energy consumption of hydrogen production

(seen in section 3.4.2) is 39.4 kWh/kg, an analysis of round trip efficiency was carried out. The

efficiency of the stack is 65%, as can be seen in section 5.5.1.

As this system’s production and generation capabilities are determined by the tides, a figure of

18 hours per day (a typical rated speed timeframe) was decided upon for hours of operation at

rated speed. Using the figures of table 11 results in a production rate of 6 kg over 18 hours.

Specific energy for the electrolyzer is then given as:

20 kW over 18 hours = 360kWh / day6kg

= 60 kWh/kg

The entire hydrogen generation system is rated at 100 kW as per example 1. Using the entire

power rating to find total specific energy:

100 kW over 18 hours = 1800kWh / day6kg

= 300 kWh/kg


It can be seen this value is 5 times the specific energy of the electrolyzer alone, and

seems excessive. However, it must be stressed that this figure is for the entire system, including

the power consumption of the electrolyzer, hydrogen compression, water treatment, and

transport processes of water and hydrogen gas to shore. The by-product potable water is also

useful, having undergone a reverse osmosis process. It is envisaged that once Capital costs and

Operation and Maintenance (O & M) costs have been considered, the hydrogen and water

product is then essentially free, being produced without harmful emissions. This consideration

results in energy consumption (when viewed as producing hydrogen and water concurrently)

that is reasonable, given that the process is completely renewable. With a combination of blade

pitching and increased hydraulic loading, it is possible to extract more energy from the tidal

stream in the form of hydrogen production. However, it does not appear to be feasible to operate

the system over a broad range. Rather, the device must be rated prior to installation of the

system, such that for a given rated power, hydrogen generation is consistent.

The goal of this project was to prove the feasibility of a subsea hydrogen production

system. As can be seen in the previous sections, this is possible, and would appear to be a good

use of tidal energy, given that the hydrogen and fresh water produced are derived from

renewable energy. The capital costs of the entire system have not been quantified, as this is

outside the scope of this project, but hydrogen production rates and thus the energy used have

been assessed.

7. FURTHER WORK

This has been an initial investigation into the subject matter. As such, application of the

engineering concerned has been mainly theoretical in nature. Further design and testing is

applicable on the practicalities of the working characteristics and conditions involved. It is

acknowledged that the end result of this project is a high specific energy for the amount of

hydrogen produced, but the implementation of a subsea hydrogen production system could be

beneficial in two ways – in the desalination of seawater and production of hydrogen from

otherwise wasted tidal power. Advancements in PEM technology could increase the hydrogen

production rates of water electrolysis, therefore lowering the entire specific energy of the

hydrogen process cycle. This is on going and the subject of many fields of research. It is the

intention of the author to create interest in this type of technology, and its potential uses.


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Transport In A Changing Market Structure, KULeuven Energy Institute, 2008

[44] Modisette, J., Physics of pipeline flow, Pipeline simulation interest group, 2003

[45] European Commission Article 4 of Directive 2009/28/EC on Renewable Energy, National

Renewable Energy Action Plan, 2010

[46] SI Ocean, Project brief, available at: www.oceanenergy-europe.eu Retrieved on 20/07/14

[47] SEAI, Energy Forecasts for Ireland to 2020, Sustainable Energy Authority Ireland, 2010


[48] Pernetta, Dr. J., Guide to the oceans, Bounty, London, 2007, ISBN 13:978-0-753715-31-5

[49] Aquaret, Tidal Energy Description, Available at: www.aquaret.com Retrieved on 25/07/14

[50] Figure used: Bay of Fundy tides, available at www.bayoffundy.com Retrieved on 28/07/14

[51] Figure used: Dublin bay tides, Dublin University Sub-Aqua Club, available at:

www.dusac.org, Retrieved on 28/07/14

[52] SI Ocean Project, Ocean energy state of the art, Strategic Initiative for Ocean Energy, p.

38, 69, 2010

[53] Figure used: DP energy turbine, www.westislaytidal.com Retrieved on 15/07/14

[54] Figure: Cross sectional area - Tidal stream

[55] Wizelius, T., Developing wind energy projects, Earthscan, London, p. 66, 2007

[56] Georgia Tech Research Corporation, Assessment of Energy Production Potential from

Tidal Streams in the United States, Georgia Tech Research Corporation, 2011

[57] Figure: Power curve of Tidal Turbine, Ocean Energy Lectures, NUIM, 2014

[58] Northwest National Marine Renewable Energy Center, Project Description, available at:

http://depts.washington.edu/nnmrec/overview.html Retrieved on 17/07/14

[59] Figure used: Bryans, A., G., Impacts of Tidal Stream Devices on Electrical Power Systems,

Queens University Belfast, p. 6,7, 2006

[60] Figure used: Wright, M. (2004) SeaFlow tidal current turbine. WATTS Conference,

London, 16th March

[61] [2] Bedard, R., Power and Energy from the Ocean Energy Waves and Tides, Electric power

research institute, 2007

[62] Guyer, P., J., et al., Introduction to Pretreatment Considerations for Water Desalination,

CED engineering, p. 23, 7, 2012

[63] Water reuse association, Seawater Desalination Power Consumption, white paper, 2011

[64] Fumatech, Brief on Reverse Osmosis, available at: www.fumatech.com Retrieved on

18/07/14

[65] Figure used: Spiral wound RO membrane, U.S Bureau of reclamation water quality

improvement center, 2007

[66] Kalogirou, S., A., Seawater desalination using renewable energy sources, Progress in

Energy and Combustion Science 31 (2005) 242–281

[67] Figure: Proton Exchange Membrane


[68] Figure used: PEM – Nexpel Test Piece: Nexpel, Final Publishable Summary, Available at:

www.sintef.no/nexpel Retrieved on 20/06/14

[69] NREL, Current (2009) State-of-the-Art Hydrogen Production Cost Estimate Using Water

Electrolysis, NREL/BK-6A1-46676, 2009

[70] Henriques, P., Hydrogen Storage for Intermittent Power Sources, Universidad de Lisboa,

2008

[71] Mazloomi, K., et al., Electrical Efficiency of Electrolytic Hydrogen Production, Int. J.

Electrochem. Sci., 7 (2012) 3314 – 3326, 2012

[72] Green, R., et al. Turning the wind into hydrogen: The long-run impact on electricity prices

and generating capacity, University of Birmingham

[73] Figure used: Zavieh, A., H., Optimization and Modelling of electrode structure and

composition for novel PEM water electrolyser MEAs, Norwegian University of Science and

Technology, 2011

[74] Figure used: Petipas, F., Design and control of high temperature electrolyser systems fed

with renewable energies, Paris Institute of Technology, p.16, 2013

[75] Siracusano, S., et al., Investigation of Composite Nafion/Sulfated Zirconia Membrane for

Solid Polymer Electrolyte Electrolyzer Applications, Int. J. Electrochem. Sci., 7 (2012) 1532 –

1542, 2011

[76] Ivy, J., Summary of Electrolytic Hydrogen Production, NREL/MP-560-36734, 2004

[77] Figure: Side View of Tidal Turbine

[78] Figure used: Tekmar bend restrictor, available at: http://www.tekmar.co.uk/oil-

gas/item/bend-restrictors-oil-gas Retrieved 15/08/14

[79] Figure used: GE Oil and Gas business, Mecon WM1, EFL Brochure, GE Oil and Gas 2013

[80] Simulink application model, available at:

http://www.mathworks.com/help/physmod/hydro/examples/index.html Retrieved on 10/08/14

[81] Simulink graph model, available at:

http://www.mathworks.com/help/physmod/hydro/examples/index.html Retrieved on 10/08/14

[82] Figure used: Radial piston pump, available at www.directindustry.com Retrieved on

16/08/14

[83] Figure used: Radial piston pump, available at: www.roymech.co.uk Retrieved on 16/08/14

[84] Figure used: Moog, Radial piston pump brochure, Moog industrial group, 2010

[85] Mobile hydraulics; What are hydraulic pumps? Available at:

http://www.mobilehydraulictips.com/what-are-hydraulic-pumps/ Retrieved on 19/08/14


[86] Eaton, Industrial Hydraulics manual, Minnesota, 2001, ISBN 0-9634162-0-0

[87] Daerospace, Motors, hydraulic description, available at:

http://www.daerospace.com/HydraulicSystems/MotorDesc.php Retrieved on 20/08/14

[88] Hydropac.com, Description of hydraulic intensifiers, available at:

http://www.hydropac.com/HTML/hydrogen-compressor.html Retrieved on 20/08/14

[89] Figure used: Hydropac.com, Description of hydraulic intensifiers, available at:


[90] Figure used: Hydropac.com, Description of hydraulic intensifiers, available at:


[91] Figure used: APP pump, http://ro-solutions.danfoss.com/media/1123/521b0851_data-sheet-

app-51-102_uk.pdf

[92] Figure used: APP pump, http://ro-solutions.danfoss.com/media/1123/521b0851_data-sheet-

app-51-102_uk.pdf

[93] Danfoss RO Solutions, APP manual, available at:

http://ro-solutions.danfoss.com/media/1123/521b0851_data-sheet-app-51-102_uk.pdf

Retrieved on 18/08/14

[94] Danfoss RO Solutions, ERD manual, available at:

http://ro-solutions.danfoss.com/media/1224/180r9221_iom-isave-21_uk.pdf Retrieved on

18/08/14

[95] Figure used: ERD cutaway, http://ro-solutions.danfoss.com/media/1224/180r9221_iom-

isave-21_uk.pdf

[96] Figure used: Line diagram ERD,

http://ro-solutions.danfoss.com/media/1224/180r9221_iom-isave-21_uk.pdf

[97] Figure used: 8340P-40422, 26 kW Diesel DC Generator, specification sheet

[98] Figure used: 8340P-40422, 26 kW Diesel DC Generator, manual

[99] Polarinc DC generator brochure, available at:

http://www.polarpowerinc.com/8340P_40422.pdf Retrieved on 16/08/14

[100] Cherus, D., Modelling, simulation and performance analysis of a hybrid power system,

Kassel University press, 2004

[101] Rucker, J., E., Design and Analysis of a Permanent Magnet Generator for Naval

Applications, MIT, 2005

[102] Figure used: modified from: Rucker, J., E., Design and Analysis of a Permanent Magnet

Generator for Naval Applications, MIT, 2005

[103] Figure: multisim application model – Buck analysis


[104] Figure: multisim circuit – buck analysis

[105] Figure used: Power Electronics Manual, Rashid, M., third edition, Elsevier, 2007

[106] Figure: Electrolyzer drawing

[107] Figure – hydrogen production rate - Google book

[108] Figure used: SCM, Weatherford Subsea control module brochure, Weatherford, 2012

[109] Figure used: SCM, FMC technologies, Subsea control system, FMC, 2014

[110] Figure used: SCM, Cameron subsea technology brochure, Cameron 2013

[111] Figure used: Compensated reservoirs, Soil Machine Dynamics, SMD, 2010

[112] Figure used: Hydraulic water contamination system, Cardev ltd. 2009

[113] Figure: EDR imposed on spring tides

[114] Figure: Hydraulic rectifier

[115] Conversion program: available at: www.convert-me.com Retrieved 22/08/14

[116] Figure: Fluid power circuit

[117] Giner, High pressure electrolyzers, current vs. future technology, available at:

http://www.ginerinc.com/products.php?a=HPH2 Retrieved on 23/08/14

[118] Figure: Pie chart of 100 kW % of power consumption

[119] Figure: Pie chart of 200 kW % of power consumption

9. APPENDICES 9.1 Hydraulic Affinity Laws:

1) Flow is proportional to shaft speed = V1V2

=N1N2

!

"##

$

%&&

2) Pressure is proportional to the square of the shaft speed = H1H2

=N1N2

!

"##

$

%&&

2

3) Power is proportional to the cube of the shaft speed = P1P2=N1N2

!

"##

$

%&&

3

Where:

V = Volumetric flow rate in GPM

N = Shaft rotational speed in RPM

H = Pressure developed by the pump

P = Shaft Power


Appendix 1. Affinity law relationship – www.engineeringtoolbox.com

Table 2 of section 5.2 was calculated using the following start point:

Tidal velocity in

m/s

HPU shaft RPM Flow Rate in GPM Pressure in PSI HPU Power

in kW

2.5 1800 77 3000 100

*From these values, the rest of the table was made up using the affinity law relationship, i.e for

the above variables, a series of variables could be found.

APP 6.5 type pumps (RO components) with flow, pressure, RPM and kW values shown [93]

Also shown below is the proportional relationship of flow rate with RPM [93]


The above table is of Data from Giner inc. [117] on their high-pressure electrolyzers. These data

were used to build the system of this project

The above shows compressor capacity as a function of motor HP, i.e. for a given required

increase in output pressure and capacity, a required increase HP is necessary [88]

This figure from Polar power shows the relationship between shaft RPM and output power [99]

Documents

An Investigation into Subsea Hydrogen Production