International Journal of Hydrogen Energy 32 (2007) 1537–1541www.elsevier.com/locate/ijhydene

Optimization of a solar hydrogen storage system: Exergetic considerations

E. López∗, F. Isorna, F. RosaInstituto Nacional de Técnica Aeroespacial, Ctra. S. Juan-Matalascañas, km.34, 21130 Mazagón (Huelva), Spain

Available online 28 November 2006

Abstract

From production to end-users, the choice of suitable hydrogen delivery and storage systems will be essential to assure the adequate introductionand development of these facilities. This article describes the main options for hydrogen storage when produced from renewable energy, andexplains different criteria to be considered in the design and building-up of stationary hydrogen storage systems, with special attention toexergy issues. An example of exergy analysis is done using data from the solar hydrogen storage facility of the Spanish Instituto Nacional deTécnica Aeroespacial (INTA).

As expected, the main conclusions of this analysis show the advantage of low pressure hydrogen in comparison with other available methodsto store hydrogen. Another interesting option, from the exergy efficiency point of view, is the storage of hydrogen in metal hydride systems.The last option, and the most inefficient, is the high pressure hydrogen storage.� 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen; Storage; Exergy; Renewable energy

1. Introduction

The intermittent nature of renewable energy, mainly solar andwind, supposes an important barrier to the objective of achiev-ing a high penetration level of such energies in the short andmedium term energy scenario. Hydrogen is a promising optionfor energy storage, mainly due to its flexibility in productionand its potential use in stationary fuel cells and vehicles [1–3].

Nevertheless, several technical, economic and societal bar-riers must be overcome prior to widespread hydrogen utiliza-tion. One of the most important barriers is the development ofsuitable hydrogen storage and delivery systems, produced fromdiverse sources and intended for diverse uses. These are key el-ements of the hydrogen economy. Flexible use of hydrogen asan energy carrier needs proper means of storage for later use,and transportation from the point of production to the usagepoint [4].

Hydrogen production systems from renewable energy havebeen mentioned and described in many articles, but compar-atively less attention has been received by hydrogen storage

∗ Corresponding author.E-mail address: lopezge@inta.es (E. López).

0360-3199/$ - see front matter � 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2006.10.032

facilities [5–7]. Hydrogen production from renewable energyrequires suitable hydrogen storage systems, and it must followseveral criteria in order to fulfill hydrogen production and userequirements [8]. This article presents a general vision of suchdesign criteria, paying special attention to energy and exergyefficiency parameters, and showing an example of exergy anal-ysis of a real hydrogen production and storage facility.

2. Hydrogen storage systems for renewable energiesapplications

Hydrogen storage system are conditioned by the final use ofhydrogen. Two types of functional requirements are needed forthe hydrogen economy. Hydrogen storage systems used for sta-tionary applications usually need large areas. Hydrogen storagefor transportation, in contrast, must operate within minimumvolume and weight specifications, supply enough hydrogen toenable required driving range, charge/recharge near room tem-perature, and provide hydrogen at rates fast enough to fuel celllocomotion of cars, trucks and buses. Hydrogen storage re-quirements for transportation applications are more stringentand difficult to achieve than those for stationary applications.

In general, several parameters and criteria are taken into ac-count concerning the design and build-up of a hydrogen storage

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system. Some of these general requirements are:

• high energy density, both in mass and volume;• technical availability;• low requirements in auxiliary systems and ancillary;• reliable and tested technology;• low possible cost (initial, operation and maintenance);• low energy consumption;• high storage capacity during long periods without hydrogen

loss or with minimum loss;• low charging/discharging time;• availability to operate in different environmental conditions;• long lifetime;• safe use in normal operations and acceptable risk under ab-

normal conditions.

Other specific requirements would depend on particular hydro-gen production and utilization facilities.

At present, only three methods of hydrogen storage fulfillthese requirements at different levels: pressurized gas (at lowor high pressure), metal hydrides and liquid hydrogen.

In the specific case of hydrogen produced from renewableenergy (solar or wind), the size of the plant is a very impor-tant constraint, because the hydrogen rate production dependson this size. Up to now, the usual size for renewable hydrogenproduction plants is lower than 300 kW. For this hydrogen pro-duction rate, liquid hydrogen is not an option to be considered,the remaining options from a technical viewpoint are pressur-ized gas and/or metal hydrides.

Once the hydrogen storage method has been chosen, thedesign of a particular hydrogen storage system will dependon the size of the production facility, the operation modes,the availability of auxiliary systems, the legal restrictions, therequirements for hydrogen at the usage or delivery point, etc.

Traditionally, the evaluation of these systems has been basedon conventional parameters such as technical performance, en-ergetic efficiency, availability, economic and technical viability,safety considerations, etc. In recent years, exergy considera-tions have also been incorporated to the design and evaluationof the merits of hydrogen storage systems. Exergy analysis canplay an important role in choosing the most adequate solutionfor each project.

Exergy analysis of energy systems integrates the first andsecond laws of thermodynamics and specified reference envi-ronmental conditions. Exergy is defined as the maximum workthat may be achieved by bringing a system into equilibriumwith its environment [9].

An exergy analysis is similar to an energy analysis, takinginto consideration inputs and outputs of exergy, and quantify-ing the locations, types and magnitudes of wastes and losses,and yields meaningful efficiencies that are always a measureof the approach to the ideal. The exergy method considersnot only the quantity of the energy but also its quality. Ex-ergy is a measure for the quality of energy, and, therefore,exergy loss is a measure for the loss in energy quality in aprocess.

3. INTA solar hydrogen storage facility

This facility is coupled with a solar hydrogen productionplant and was built in the mid 90’s. The hydrogen pro-duction plant includes an 8.5 kWp PV field connected to a5.2 kW alkaline electrolyzer. Table 1 shows the main char-acteristics of this production plant. To store the hydrogenproduced, two different storage systems have been chosen:metal hydride and pressurized gas. Both systems are inter-connected, so the gas stored in the hydride container can becompressed and stored in bottles at 200 bar. Both systemshave common devices like a purification unit, an intermediatebuffer, etc.

The hydride container can be used as another intermediatebuffer of the pressurized gas system, and it can be consideredas one single storage system that integrates the pressurized gassubsystem and the metal hydride subsystem. Hydrogen can bestored in pressurized gas form or in metal hydride, dependingon the production rate, availability of cold water or hot water,availability of compressed air, etc. Both systems will be evalu-ated from an economic and energetic point of view in order tofind the best option of hydrogen storage for small solar elec-trolytic facilities.

Main design parameters for the INTA solar hydrogen storagefacility were as follows:

• hydrogen production rate: 1.2 N m3/h;• hydrogen storage capacity: enough for an operation week

(25–30 N m3);• 48 operation weeks per year;• number of charging cycles higher than number of discharging

cycles;• reasonable cost and availability for small facilities;• other requirements: availability, auxiliary systems, etc.

At present, a new approach for the evaluation of the systemhas been done, using previous results and new data, and tak-ing into consideration exergy analysis of the system. The goalof this exergy analysis will be to improve new designs, findingcritical points where a “low” quality energy could be used in-stead of “high” quality energy produced by PV field, helpingto increase global efficiency of the existing system and includ-ing exergetic considerations in the development of simulationmodels.

Table 1Solar hydrogen production plant characteristics

Photovoltaic field8.5 kWp at 1000 W/m2 and 25 ◦C cell temperature144 BP solar modules (mod. BP 260S)Flexible topology

Electrolyzer5.2 kW at nominal 108A and 48 vH2 production: 1.2 N m3/hH2 purity: 99.7% ± 0.1% vvOperational conditions: 6 bar, 80 ◦C, 30% KOH

E. López et al. / International Journal of Hydrogen Energy 32 (2007) 1537–1541 1539

Three different sections can be considered in the INTA solarhydrogen storage system:

• low pressure storage area;• metal hydride area;• compression and high pressure storage area.

The low pressure storage area consists of a 1000 l of nominalcapacity intermediate buffer located behind the purification unitand the electrolyzer. Maximum operation pressure at the elec-trolyzer is 6 bar and, in consequence, maximum hydrogen ca-pacity of the intermediate buffer will be approximately 6 N m3.

Table 2Metal hydride container characteristics

Hydride type: based on TiMn2

Nominal capacity: 24 N m3 hydrogenDesigned pressure: max. 10 bar (80 ◦C)Discharge pressure: min. 2 bar (70 ◦C)Charge pressure: min. 2 bar (15 ◦C)Container weight: approx. 210 kgHydride weight: approx. 130 kgDimensions: approx. 1600 × 300 mm (H/D)

Table 3Compression and high pressure area

Compressed air driven gas booster compressorType: two units three stage Gas Booster compressor systemInlet hydrogen pressure: min. 2.6 barOutlet hydrogen pressure: max. 240 barAir driven pressure: min. 6.2 bar

Compressed air supplyTwo cylinders, two stages air compressor10 H.P. power

AC CurrentAir

Compressor

AirFrom electrolyzer

H2

H2

Intermediate

Buffer

H2

Warm water

Warm water

Production

System

AC Current AC Current

Cool water

Production

System

Cool water

Metal hydride

container

H2

H2

High pressure

storage

Hydrogen

Compressor

H2

To fuel cells

H2

Fig. 1. General layout of INTA solar hydrogen storage system.

Solar hydrogen nominal production is approx. 1 N m3/h, andafter a day of operation from sunrise to sunset the intermediatebuffer is full. The hydrogen then passes to the metal hydridearea, the compression and high pressure storage area, or it isused directly in the fuel cell facilities existing in the laboratory.

The metal hydride area includes the metal hydride container,the cooling/heating water supply system and several sensorsfor data acquisition. The hydride storage container comprisesa pressurized tank filled with the alloy, a cooling/heating shell,water supply and hydrogen supply provided with safety andshut-off valves. Table 2 summarizes the main characteristics ofthis metal hydride container.

The hydrogen supplied by the metal hydride container duringthe discharging process can be compressed at high pressure orused directly in fuel cells.

In the compression and high pressure storage area, the hydro-gen gas coming from intermediate buffer or metal hydride con-tainer is compressed and bottled in metallic cylinders at 200 bar.Table 3 summarizes the main characteristics of the componentsof this area. These components are a two-stage compressed airdriven gas booster compressor, an air compressor and a filling-bottles device.

Fig. 1 shows the general layout for the hydrogen storagesystem, and the relationship between the different componentsin term of species (hydrogen, cool water, warm water) orenergy.

4. Exergy analysis of INTA solar hydrogen storage system

There are no previous experiences for the exergy analysisof solar hydrogen storage systems, and a suitable methodologyand approach has to be developed for that kind of analysis.

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Table 4Specific exergy destruction per system operation mode

System operation mode Steps Specific exergy destruction (MJ/kg)

Hydrogen to fuel cells from intermediate buffer (No. 1) 1, 2 1.88Hydrogen to fuel cells from high pressure storage and intermediate buffer (No. 2) 1, 2, 5, 6 113.57Hydrogen to fuel cells from high pressure storage and metal hydride (No. 3) 1, 2, 3, 4, 5, 6 136.00Hydrogen to fuel cells from metal hydride (No. 4) 1, 2, 3, 4 24.31

Every possible process for each main device of the systemhas been considered as an elemental step, taking into accountan initial state, a final state, and the total inputs and outputs ofexergy, both in form of hydrogen or energy, needed to achievethe final state from the initial state. Final and initial state rep-resent when the device is full or empty. In fact, no device iscompletely empty, but some quantity of hydrogen not usableremains at the device.

For each elemental step, the total exergy has been calcu-lated for initial state and final state (full/empty) and the totalinput/output exergy from one state to the other one. From theseexergy data, a value of specific exergy destruction, per unit ofmass of hydrogen, has been calculated for each step. This pa-rameter is related with irreversibilities and the thermodynamicinefficiencies of the system.

Traditional definition of exergy efficiency is 1 −(exergy loss/total exergy input), where exergy loss includesusually waste exergy emission and exergy destruction. Inthe present study, there is no exergy associated to wasteemissions, and the exergy efficiency can be calculated as1 − (exergy destruction/total exergy input) [10].

The specified reference environmental conditions are as-sumed as atmospheric air at SPT condition (298 K and 1 atm).The specific exergy destruction, in terms of MJ/kg of hydro-gen, and the steps involved for each system operation modeare presented in Table 4. In general, operation modes moreinefficient consist in the use of solar hydrogen in fuel cellscoming from the high pressure storage, either directly fromthe intermediate buffer (113.57 MJ/kg of hydrogen) or fromthe metal hydrides (136.00 MJ/kg of hydrogen). This is duemainly to the high exergy consumption at the air compressor.As a second more efficient option appears the use of hydrogendirectly from the metal hydrides (24.31 MJ/kg of hydrogen).This is an interesting option for high storage capacity if thereare no space constraints and cooling/heating water is available.Clearly, the most efficient option is the use of hydrogen infuel cells directly from the intermediate buffer at low pressure(1.88 MJ/kg of hydrogen). These results are also shown in amore graphic way in Fig. 2.

5. Conclusions

Pressurized gas and metal hydrides, separated or combined,are the ways used to store hydrogen systems in solar hydrogenfacilities.

Additionally, several criteria have to be taken into consider-ation during the designing and building of the most adequate

Sp

ecif

ic E

xerg

y

Destr

ucti

on

(M

J/k

g)

Exergy destruction vs. System operation mode

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

System operation mode

No. 2

No. 4

No. 3

No. 1

Fig. 2. Specific exergy destruction vs. system operation mode.

hydrogen system storage facilities, with special considerationfor the final use of the hydrogen.

Exergy considerations can help to choose suitable hydrogenstorage system solution for each project related with renewablehydrogen production, optimising the design of the system, find-ing the location and type of wastes and losses, and reducingthe inefficiencies in existing systems.

From an exergetic point of view, direct use of hydrogen fromlow pressure storage is the most efficient and simple option. Thepressure storage is higher than the hydrogen pressure needed forfuel cells, and exergy destruction during the process is mainlyassociated with the pressure regulation.

Hydrogen storage in metal hydride is an interesting optionfor high storage capacity. For this storage option, exergy de-struction is mainly associated to the use of AC current for heat-ing and cooling purposes, and can be reduced using a “low”quality energy like thermal energy from solar energy.

High pressure hydrogen storage is the most inefficient option,due mainly to electric energy consumption in the compressor.If high pressure is needed (e.g. vehicles refuelling), options likehigh pressure electrolyzers, low consumption compressors, ormetal hydride compressors could be considered.

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