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Heriot-Watt University
Institute of Mechanical, Process & Energy Engineering
School of Engineering and Physical Sciences
MSc in Energy
Project / Dissretation 2014-2015
Title: Comparative Analysis of Energy Storage Methods in Smart
Grids with Distributed Energy Production. An Approach for
Micro Grids to Medium Size Grids.
Author: Mr. Kokkotis Panagiotis – H00177171
Supervisor: Dr. C.S. Psomopoulos
F L A M E
Flexible Learning Advanced Masters in Energy
i
F L A M E
MSc in Energy
Declaration of Authorship
I, Mr. Kokkotis Panagiotis – H00177171 - Cohort 16
confirm that the report entitled
Comparative Analysis of Energy Storage Methods in Smart Grids with Distributed
Energy Production. An Approach for Micro Grids to Medium Size Grids.
is part of my assessment for module B51MD (or Masters Dissertation)
I declare that the report is my own work. I have not copied other material verbatim
except in explicit quotes, and I have identified the sources of the material clearly
Panagiotis Kokkotis Piraeus, September 2015
ii
Abstract
This dissertation introduces the reader to the background of the study field, which is the Energy
Storage Systems, and then analyzes the aims and objectives of this research. After analyzing the
practical and theoretical problems, the research methodology is presented.
Regarding the core research, the Energy Storage Systems are analyzed by their definition of the
energy form in which electrical energy is stored. Therefore, the main categories include the
electrochemical energy storage methods (batteries, SMES, Super capacitors), mechanical energy
storage methods (CAES, LAES, PHES, Flywheel), chemical energy storage methods (Hydrogen)
and thermal energy storage methods. The comparison showed significant differences between the
methods studied; from huge energy providers like Pumped Hydro to high power grid stabilizing
flywheels and super capacitors.
But can all methods be applied into a smart grid? A smart grid is defined by its scale, the smart
production of energy in order to flexibly meet the load with bidirectional communication of the
producer and the consumer, and finally, the fragility of the system due to grid issues and faults.
Rendering a real island (Tilos) as the “subject” in this dissertation, we tried to profile the load with
the limited measurements we had and concluded that a Hybrid Energy Storage System is best
suited for a small smart grid, as different energy storage methods can differently contribute into
the gird in terms of stability, frequency regulation and safety of supply.
iii
Acknowledgements
I would like to thank my supervisor Assoc. Prof. Dr. C.S. Psomopoulos for his guidance and help
and the Soft Energy Applications Laboratory of Piraeus’ University of Applied Sciences and
especially Prof. Dr. J. Kaldellis for their support and sharing of the information they have gathered.
A thank you is in order for the Hellenic Navy Hydrographic Service, and especially to Mrs.
Pandermaraki, as they provided me with valuable cartographic issues helping this dissertation.
I would also like to thank my colleagues for these two years of brainstorming and fun times.
Special thanks go to my family, whose, without their financial and emotional support, I could not
have made it this far.
Finally, I would like to thank my wife, Dionysia, for her support in all aspects, her patience and
her help towards finishing this thesis.
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Table of Contents
Abstract ........................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
Table of Contents ........................................................................................................................... iv
List of Figures .............................................................................................................................. viii
List of Tables ................................................................................................................................. ix
Glossary of terms and acronyms ..................................................................................................... x
Chapter 1 - Introduction .................................................................................................................. 1
1.1 Background and motivation for the work ........................................................................ 1
1.2 Aims and objectives ......................................................................................................... 3
1.3 The practical problem....................................................................................................... 4
1.3.1 The problem environment: ............................................................................................. 4
1.3.2 The problem context ....................................................................................................... 5
1.3.3 The problem of interest ................................................................................................... 6
1.4 The theoretical problem ........................................................................................................ 8
1.4.1 The subject ...................................................................................................................... 8
1.4.2 The area ........................................................................................................................ 12
1.4.3 The gap in knowledge ................................................................................................... 15
1.5 Outline of the dissertation ................................................................................................... 17
Chapter 2 - Research methodology ............................................................................................... 18
2.1 Description of the data sources used ................................................................................... 18
2.2 Background theory .............................................................................................................. 20
2.3 What was done to get the data ............................................................................................. 21
2.5 Problems encountered ......................................................................................................... 22
Chapter 3 - Energy storage systems applied in small and medium scale power grids ................. 23
3.1 Energy Storage Systems Overview ..................................................................................... 23
3.2 Electrochemical Energy Storage Methods .......................................................................... 24
3.2.1 Battery Energy Storage ................................................................................................. 24
3.2.2 Flow Batteries ............................................................................................................... 29
3.2.3 Superconducting Magnetic Energy Storage ................................................................. 30
v
3.2.4 Super capacitors Energy Storage .................................................................................. 32
3.3 Mechanical Energy Storage Methods ................................................................................. 33
3.3.1 Compressed Air Energy Storage .................................................................................. 33
3.3.2 Liquid Air Energy Storage or Cryogenic Energy Storage ............................................ 34
3.3.3 Pumped Hydro Energy Storage .................................................................................... 34
3.3.4 Flywheel ....................................................................................................................... 35
3.4 Chemical Energy Storage Methods ..................................................................................... 37
3.4.1 Hydrogen ...................................................................................................................... 37
3.5 Thermal Energy Storage...................................................................................................... 38
3.6 Environmental Aspects of ESS ........................................................................................... 39
Chapter 4 - Electric power systems .............................................................................................. 41
4.1 Introduction ......................................................................................................................... 41
4.2 Structure of Electric Power Systems ................................................................................... 42
4.3 Electricity Generation ......................................................................................................... 43
4.4 Transmission ....................................................................................................................... 44
4.5 Distribution.......................................................................................................................... 45
4.6 Loads ................................................................................................................................... 46
4.7 Analysis of Electric Power System ..................................................................................... 47
4.8 Operation and control of electric power systems in non-interconnected networks ............ 48
4.9 Grid Issues ........................................................................................................................... 49
4.10 Faults and Protection in Distribution networks ................................................................. 52
4.11 Switchgear Selection ......................................................................................................... 54
4.12 Connection to MV grid ..................................................................................................... 57
Chapter 5 – Energy Storage Methods Comparison ....................................................................... 59
5.1 Introduction ......................................................................................................................... 59
5.2 Rated Power and Discharge Time ....................................................................................... 60
5.3 Energy Density .................................................................................................................... 61
5.4 Cost per cycle ...................................................................................................................... 62
5.5 Lifetime - Efficiency ........................................................................................................... 63
5.6 ESS and Smart Grids ........................................................................................................... 65
5.7 Selection Guide ................................................................................................................... 66
vi
5.7.1 Scenario for ESS selection steps .................................................................................. 67
Chapter 6 – Case Study; Tilos ...................................................................................................... 68
6.1 Introduction ......................................................................................................................... 68
6.2 Background of TILOS Project ............................................................................................ 69
6.3 Selection of the energy storage method .............................................................................. 71
6.4 Electric design of Tilos ....................................................................................................... 73
6.5 Load Profile ......................................................................................................................... 75
6.5.1 General Load Profile .................................................................................................... 75
6.5.2 Load at April 16th, 2015 ................................................................................................ 76
6.5.3 Load at April 25th – 26th, 2015 ..................................................................................... 77
6.5.4 Load at April 30th, 2015 ................................................................................................ 77
6.5.5 Load at May 12th, 2015 ................................................................................................. 78
6.5.6 Load at May 24th, 2015 ................................................................................................. 79
6.5.7 Load at May 29th, 2015 ................................................................................................. 79
6.5.8 Load Summary ............................................................................................................. 80
6.6 Profile of the HESS ............................................................................................................. 81
6.6.1 Introduction .................................................................................................................. 81
6.6.2 Flywheel ....................................................................................................................... 81
6.6.3 Flow Battery ................................................................................................................. 81
6.6.4 Transformers and equipment ........................................................................................ 81
Chapter 7 - Discussion .................................................................................................................. 83
Chapter 8 - Conclusions ................................................................................................................ 85
8.1 Future Work ........................................................................................................................ 86
APPENDIX A ............................................................................................................................... 87
APPENDIX B ............................................................................................................................... 89
April 16th, 2015 ......................................................................................................................... 89
April 25th - 26th, 2015 ................................................................................................................ 90
April 30th, 2015 ......................................................................................................................... 91
May 12th, 2015 .......................................................................................................................... 92
May 24th, 2015 .......................................................................................................................... 93
May 29th, 2015 .......................................................................................................................... 94
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APPENDIX C ............................................................................................................................... 95
APPENDIX D ............................................................................................................................... 98
Switchgear ................................................................................................................................. 98
Circuit Breaker .......................................................................................................................... 99
Transformer ............................................................................................................................. 100
REFERENCES ........................................................................................................................... 102
viii
List of Figures
FIGURE 1: DIFFERENT USES OF EES IN GRIDS DEPENDING ON THE FREQUENCY AND DURATION OF USE (SOURCE:
INTERNATIONAL ELECTROTECHNICAL COMMISSION, 2011) .................................................................................. 2 FIGURE 2: WORLDWIDE INSTALLED STORAGE CAPACITY FOR ELECTRICAL ENERGY (SOURCE: ELECTRIC POWER
RESEARCH INSTITUTE, 2010) ................................................................................................................................. 8 FIGURE 3. BENEFITS OF EES ALONG THE ELECTRICITY VALUE CHAIN (SOURCE: MAKANSI ET AL., 2002) ...................... 9 FIGURE 4: THE OUTLINE OF THIS DISSERTATION (SOURCE: THE AUTHOR) ................................................................... 17 FIGURE 5: SCHEME OF PRISMATIC AND SPIRAL WOUND CONSTRUCTION OF LEAD-ACID BATTERY (KRIVIK AND BACA,
2013) ................................................................................................................................................................... 24 FIGURE 6: SCHEME OF SPIRAL WOUND AND PRISMATIC CONSTRUCTION OF NI-CD BATTERY (KRIVIK AND BACA, 2013)
............................................................................................................................................................................ 26 FIGURE 7: SCHEMATIC CROSS-SECTION OF NA-S CELL (KRIVIK AND BACA, 2013) ...................................................... 27 FIGURE 8: PRISMATIC AND CYLINDRICAL LI-ION CELL CONSTRUCTION (KRIVIK AND BACA, 2013) ............................. 28 FIGURE 9: TYPICAL ZINC AIR BUTTON CELL BATTERY (MICROPOWER BATTERY COMPANY, 2015) ............................. 29 FIGURE 10: SCHEME OF VANADIUM REDOX BATTERY (KRIVIK AND BACA, 2013) ........................................................ 30 FIGURE 11: ELEMENTS OF A SMES SYSTEM (TIXADOR, 2013) ..................................................................................... 31 FIGURE 12: TYPICAL DOUBLE LAYER CAPACITOR SCHEME (WWW.MPOWERUK.COM/SUPERCAPS.HTM) ...................... 32 FIGURE 13: WIND FARM WITH CAES SYSTEM (ZAFIRAKIS, 2010) ................................................................................ 33 FIGURE 14: AA-CAES CONCEPT (BULLOGH ET AL, 2004) ........................................................................................... 34 FIGURE 15: PUMPED HYDRO ELEMENTS (SOURCE: ALSTOM) ....................................................................................... 35 FIGURE 16: A FLYWHEEL PARTS SCHEME
(HTTP://WWW.PE.EEE.NTU.EDU.SG/RESEARCH/RESEARCHAREAS/PAGES/IEDS.ASPX) ........................................ 36 FIGURE 17: BASIC STRUCTURE OF THE ELECTRIC SYSTEM (SOURCE: UNIVERSITY OF IDAHO) ..................................... 41 FIGURE 18: METHODS OF CONNECTION TO MV GRID ................................................................................................... 58 FIGURE 19: POWER VS DISCHARGE TIME (ESA) ......................................................................................................... 60 FIGURE 20: VOLUME ENERGY DENSITY VS WEIGHT ENERGY DENSITY (ESA) ........................................................... 61 FIGURE 21: CAPITAL COST PER CYCLE IN ¢/KWH PRODUCED (ESA) ............................................................................ 62 FIGURE 22: EFFICIENCY VS LIFETIME AT 80% DOD..................................................................................................... 63 FIGURE 23: THE ISLAND OF TILOS (LEFT) AND ITS POSITION IN THE AEGEAN SEA (RIGHT) (SOURCES: GOOGLE MAPS
AND WIKIPEDIA) .................................................................................................................................................. 68 FIGURE 24: PROPOSED LOCATION FOR THE ESS ........................................................................................................... 73 FIGURE 25: LOAD MEASUREMENTS OF TILOS ISLAND (SEALAB,2015) ........................................................................ 75 FIGURE 26: ENERGY LOST AT M1 AT 16 APRIL 2015 .................................................................................................... 76 FIGURE 27: TECHNOLOGY MATURITY MAP (IEA, 2014) .............................................................................................. 83 FIGURE 28: ELECTRIC SINGLE LINE DIAGRAM OF TILOS ISLAND ................................................................................... 87 FIGURE 29: UNDERWATER CABLE ROUTE ENDING IN TILOS ISLAND (SOURCE: HELLENIC NAVY HYDROGRAPHIC
SERVICES, 2015) .................................................................................................................................................. 88
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List of Tables
TABLE 1: ENVIRONMENTAL ASPECTS OF ELECTROCHEMICAL MEANS OF ENERGY STORAGE (KOKKOTIS ET AL., 2015) 39 TABLE 2: ENVIRONMENTAL ASPECTS OF MECHANICAL MEANS OF ENERGY STORAGE (KOKKOTIS ET AL., 2015) .......... 40 TABLE 3: ENVIRONMENTAL ASPECTS OF CHEMICAL MEANS OF ENERGY STORAGE (KOKKOTIS ET AL., 2015) .............. 40 TABLE 4: GRID ISSUES AND ENERGY STORAGE SYSTEMS’ SUGGESTED ATTRIBUTES (INTERNATIONAL ENERGY AGENCY,
2014) ................................................................................................................................................................... 51 TABLE 5: ADVANTAGES AND DISADVATAGES OF VARIOUS BUS BAR CONFIGURATIONS (SOURCE: ABB, 2012) ........... 56 TABLE 6: SUMMARY OF TECHNOLOGIES USED ACCORDING TO GRID ISSUES ................................................................. 59 TABLE 7: SUMMARY OF ESS CHARACTERISTICS........................................................................................................... 66 TABLE 8: LEGEND OF TECHNOLOGY CATEGORIES FOR TABLE 7 ................................................................................... 66 TABLE 9: COMMERCIAL AND ACADEMIC PARTNERS IN TILOS PROJECT (SOURCE: HEDNO, 2015) ............................ 70 TABLE 10: MEASUREMENTS AT M1 AND M2 AT 16 APRIL 2015 ................................................................................... 76 TABLE 11: MEASUREMENTS AT M1 AND M2 AT 25-26 APRIL 2015 ............................................................................. 77 TABLE 12: MEASUREMENTS AT M1 AND M2 AT 30 APRIL 2015 ................................................................................... 78 TABLE 13: MEASUREMENTS AT M1 AND M2 AT 12 MAY 2015 .................................................................................... 78 TABLE 14: MEASUREMENTS AT M1 AND M2 AT 24 MAY 2015 .................................................................................... 79 TABLE 15: MEASUREMENTS AT M1 AND M2 AT 24 MAY 2015 .................................................................................... 79 TABLE 16: ENERGY NEEDS DURING MEASURED PERIOD .............................................................................................. 80
x
Glossary of terms and acronyms
AA-CAES Advanced Adiabatic CAES
AC Alternating Current
BES Battery Energy Storage
CHP Combined Heat and Power
CES Community Energy Storage
CAES Compressed Air Energy Storage
CES Cryogenic Energy Storage
DoD Depth of Discharge
DC Direct Current
EES Electrical Energy Storage
ESS Electrical Storage System
EU European Union
FBES Flow Battery Energy Storage
FES Flywheel Energy Storage
HESS Hybrid Energy Storage System
ICT Information and Communication Technologies
LHTES Latent Heat Thermal Energy Storage
LAES Liquid Air Energy Storage
LV / MV / HV Low Voltage / Medium Voltage / High Voltage
PHEV Plug-in Hybrid Electric Vehicle
PHES Pumped Hydro Energy Storage
RES Renewable Energy Sources
R&D Research and Development
SES Smart Electricity Systems
SCES Super Capacitor Energy Storage
SMES Superconducting Magnetic Energy Storage
TES Thermal Energy Storage
3P Three-phase systems
T&D Transmission and Distribution
UPS Uninterruptable Power Supply
1
Chapter 1 - Introduction
1.1 Background and motivation for the work
Electrical Energy Storage (EES) refers to a process where electrical energy from a power network
is stored and then converted back to electrical energy when needed. Such a process enables
electricity to be stored at times of low demand, low generation cost or from stochastic energy
sources such as wind and solar and be used at times of high demand, high generation cost or when
no other generation method is available (Chen et al., 2009).
Inevitably, the European electric power industry’s generation mix will change in the next years as
more renewable energy technologies and co-generation units will be introduced. Regarding these
two major factors, we should take into consideration the noted stochastic nature of wind and sun
resources along with the distribution of possible electric surplus coming from co-generation units
that have been primarily designed to cover heating loads. In that case, EES may help in the
mitigation of these problems while offering new solutions in amplification of smarter grids’
development. The economic evaluation of these systems have largely impended their usage (Wals,
2004) along with the understanding of the benefits of energy storage systems (Tsikalakis, 2010).
Smart Electricity Systems (SES) or Smart Grids is a joint effort towards the complicated challenge
of reducing modern societies’ impact on the environment and the climate while improving citizens’
quality of life. The SES concept actually refers to changes in the generation, transmission,
distribution and consumption of electricity due to the massive integration of renewable energy
sources. It also refers to integration of sensing, monitoring, control, automation and other
Information and Communication Technologies (ICT). Last but not least, SES refers to improved
metering, protection capabilities and communication between all parts involved. SES aim to
transform the grid in order to better exploit the energy provided by Renewable Energy Sources
(RES) mainly available at the distribution level, improve the service quality, mitigate grid losses
and moderate and make the consumption more efficient. The traditional operating principles of the
distribution grids were based on the passive transfer of electricity from core power plants and
2
through the transmission level distributed to the consumption points. Nowadays, due to the
distributed nature of RES, the flow of energy is bi-directional, leading to the change of the
traditional principles by which the grids are planned and controlled. Most of the projects carried
out in the European Union (EU) on advanced smart grids are currently at the Research and
Development (R&D) and demonstration stage (European Commision, 2013)
In general, the role of on-grid EES systems can be described by the number of cycles (uses) and
the duration of operation. For maintaining the voltage quality, ESS with high cycle stability and
short duration at high output power is required, where, on the other hand, longer storage duration
and fewer cycles are needed for time shifting. The different uses of electrical energy storage in
grids depending on the frequency and duration of use are shown in Figure 1 below (International
Electrotechnical Commision, 2011).
Figure 1: Different uses of EES in grids depending on the frequency and duration of use
(Source: International Electrotechnical Commission, 2011)
3
1.2 Aims and objectives
The main aim of the investigation is to analyse and compare the various electric energy storage
methods that can be used in smart grids with distributed energy production and also approach the
matter of storage in micro and medium grids. The key point is to be able to choose the correct EES
system regarding the grid types analysed.
In order to achieve the above mentioned aim, the characteristics of the EES have to be presented.
Amongst the characteristics under inspection are the following:
Capacity
Discharge time
Quality of discharged energy
Cost per kWh
Physical size incl. geographical restrictions
Operational characteristics
Another issue under investigation is the ability of the grid to absorb the given energy by the storage
system. Thus, a case study will be considered.
The results that are going to be obtained could be useful in the application of energy storage
methods.
4
1.3 The practical problem
1.3.1 The problem environment
In developing a more intelligent electricity network, also known as a smart grid, the challenge is
to balance all the dynamic load control variables that emerge from an ever increasing stochastic
nature of RES. If small amounts of energy is stored throughout the grid, the challenge mentioned
earlier can be made simpler. A true smart grid design will have to co-op the demand response
methods in homes, the dynamic loading of T&D lines, along with the temperature and the wind
speed.
Installing small energy storage systems (maintaining autonomy for 1-2 hours) on the feeders of a
residential area is a specific example of storage in a smart grid. The concept of the Community
Energy Storage (CES) deploys small (25kW), low-voltage units to protect small groups of houses
as mentioned by Bjelovuk et al. (2009). Moreover, each CES’ power electronic converter is
capable of generating 25kVArs for usage in voltage control.
The CES units are connected on the low-voltage side of the utility transformer and protect the final
120/240V circuits to individual customers. Placing a utility controlled device at the very edge of
the grid allows for the ultimate in voltage control and service reliability. Meeting this challenge of
even greater control of voltage at the point of customer use is a major departure for traditional
utility system control philosophy but is needed to deal with a rapidly changing customer load
profile. Customers who add more and more high technology appliances (personal computer,
appliances, etc.) demand greater grid reliability. Moreover, new, even larger loads (like plug-in
hybrid electric vehicles (PHEV) charging units) will be added randomly to the grid. On the other
hand, and in the context of load pattern changes, even more and more solar arrays on rooftops will
introduce an increasing amount of energy flowing back into the grid, when the generation will be
greater than the consumption of the producer. Nowadays, a neighborhood with enough rooftop
solar arrays can generate a fair amount of electrical energy that turns back into the grid during high
solar periods of time. During this solar peak, the customer’s load is lower than his generation by
two to three hours each day and therefore it is best to store this excess energy and use it when the
customer’s load is greater later in the day. With distributed CES units, excess energy can be
captured locally with less losses and then re injected into the same customers when needed. CES
5
can also contribute to voltage regulation when a cloud passes over the solar array. The more
customers are generating in this CES, the greater the problem is with the voltage stability regarding
the stochastic nature of the sun (clouds). As clouds cast a shadow over a large area and a large
number of solar arrays, the power output drops significantly leading to sudden voltage dips. The
power electronic devices can act as compensators during these dips to maintain nominal voltage
and frequency in the local area. On the other hand, after a clouding moment, the sun can reappear
and the voltage will tend to rise rapidly. The power electronic devices can also act and prevent
voltage sags. (Roberts, 2011)
As mentioned earlier, load demands due to the addition of more PHEV will be seriously affected.
The charging should take place during the night, although slower, in order to maintain the load
pattern. If there is a need for spontaneous “quick charging”, the local distribution hardware (e.g.
transformer) could be seriously stressed (Roberts, 2011).
1.3.2 The problem context
In order to be fully utilizing the possibilities RES can offer, and the fact that RES are considered
distributed generation methods, studying an “isolated” micro grid or an islandic grid is the main
approach of this study. As mentioned by Etxeberria (2010), a micro grid is a weak grid that consists
of many loads, storage systems, small sources and power converters. The micro grid can operate
both connected in an isolated islandic mode and to the main grid. This type of grid can overcome
the generation uncertainty of RES and their stochastic nature, thus creating an equally reliable
system compared to the main electricity network. In this context, the use of ESS is a welcomed
idea, as we can observe power smoothness, avoidance of power quality issues and the control of
both grid voltage and frequency.
The micro grid is sensitive to changes in load and/or generation, thus an ESS that can provide high
energy and power density simultaneously must be introduced. The lack of a system that can offer
these characteristics leads us to the need to combine two or more ES in order to create a Hybrid
Energy Storage System (HESS) (Hall and Bain, 2008). The HESS is usually formed by two
supplementary storage devices; one of high power and one of high energy. The usage of a high
energy but low power ESS enhances the power control issues because of their slow response time.
A high power demand affects the lifecycle of the ESS negatively. By adding a short storage system,
6
the operation of the main storage system is smoothed and prolonged, leading to the satisfaction of
the power requirements from the network, as they operate simultaneously. Finally, the parallel use
of a short storage system along with a long storage system lead to the reduction of power losses of
the main system (Wei et al, 2008).
1.3.3 The problem of interest
For low power applications we have to focus on the lowest possible self-discharge, and this
criterion is best covered by the lithium-ion batteries.
For small systems of a few kWh, in isolated areas that are based on stochastic RES, the criterion
that must be met is autonomy. Thus, the lead-acid batteries remain the best choice between
performance and cost. Regarding lithium-ion batteries, they have better performance but their cost
is still high.
For larger systems of a few hundredths kWh, there are some alternatives as solutions. One solution
points on lead-acid batteries, which are preferred over lithium-ion batteries, and the alternatives
are either less efficient or more expensive. The latter can be identified as the CAES, which has
self-discharge issues, fuel cells, which are expensive and have low energy efficiency, and flow
batteries, which suffer from high maintenance costs.
As for peak-hour load levelling, a high energy storage (many MWh) is required. The best
candidates are the CAES and the flow batteries, with CAES being more cost effective than flow
batteries. These technologies, though, have not been tested in the field (European Commission,
2001).
Concerning power quality, cycling capacity and energy release capacity are the key points. Hence,
flywheels and super capacitors can be better adapted than lithium-ion batteries. Also, lead-acid
batteries, although they have limited durability and reliability, they satisfy the criteria mentioned
before. The low performance and high cost of Nickel and metal-air batteries, exclude them from
this category. As for fuel cells, this technology has to mature more (Kaldellis, 2007).
Finally, the magnitude of the application under investigation and the storage method to be used
can be defined as large scale applications, in which hydraulic and thermal energy storage can be
7
used, and small scale applications, in which Superconducting Magnetic Energy Storage (SMES)
can be used.
To successfully counter the future needs of distributed electricity generation, energy storage
systems need to be technologically advanced regarding short and mid-term time frames. For
example, lithium-ion batteries are performing very well, but their cost might render them infeasible
in using them in remote areas. Although their recycling and waste management issues are being
addressed, there is still work to be done. Lead-acid batteries have the best cost/performance rating
but their life expectancy has to be strengthened in order to be able to provide the best link and
answer the future needs of the grid (Ibrahim et al., 2008).
8
1.4 The theoretical problem
1.4.1 The subject
The drive of becoming the world leader in the clean energy industry has seen some competitive
efforts between the researchers to increase energy efficiency, reduce greenhouse gases and
promote a cleaner and more sustainable energy generation. Certain types of energy storage such
as pumped-storage hydro-electricity are one of the oldest EES technologies that have been
employed in the electricity grid. To gain a better view of the world's energy storage scenario, a
comparative estimation of current installed capacity of worldwide energy storage plants is shown
in Figure 2.
Figure 2: Worldwide installed storage capacity for electrical energy (Source: Electric Power
Research Institute, 2010)
Electricity transmission and distribution sector (power quality and energy management) and
transport sector are the potential areas where energy storage systems can be fully utilized. EES
enhances the existing power plants and at the same time prevents expensive upgrades. ESS could
act as a regulator that manages the fluctuations of electricity from RES (Mahlia et al., 2014).
9
The traditional electricity value chain has been considered to consist of five links: fuel/energy
source, generation, transmission, distribution and customer-side energy service as shown in Figure
3 below.
Figure 3. Benefits of EES along the electricity value chain (Source: Makansi et al., 2002)
ESS could soon become the “sixth link” by supplying power whenever and wherever needed and
by integrating the existing segments and creating a market that is more responsive. It can be said
that potential applications of EES are numerous and various and could cover the full spectrum
ranging from larger scale, generation and transmission-related systems, to those primarily related
to the distribution network and even “beyond the meter”, into the customer/end-user site (Chen et
al., 2009).
Some important applications of the EES are described below by Dobie (1998):
(1) Generation :
a. Commodity storage: Storing bulk energy generated at low cost times of the day for
use during peak demand periods during the day allows for simultaneous buying and
selling of the production price of the two periods and a more uniform load factor
for the generation and T&D systems.
b. Contingency service: If and when a power plant falls off-line, the Contingency
reserve refers to the power capacity capable of providing power to serve customers’
needs. Spinning reserves are instantly available, while non-spinning reserves along
with long-term reserves are usually available within 10 minutes or longer.
10
c. Area control: Refers to the prevention of unplanned power transfer between two
utilities.
d. Frequency regulation: Refers to the maintenance of a steady frequency of the grid
during normal and abnormal grid operation. Large sags or dips could harm both the
generator and the customers’ hardware.
e. Black-Start: It is the ability of a power plant to start up on their own (without
external fuel or triggering) after a blackout has occurred. This could help other
facilities to start up and synchronise with the grid.
(2) Transmission and distribution:
a. System stability: It is the ability to provide stability to the grid, maintain all system
components in harmony and prevent the collapse of the whole system.
b. Voltage and frequency regulation: Stable voltage and frequency throughout the grid
can be achieved by this regulation.
c. Asset deferral: Refers to the deferral of the need for additional grid facilities, like
T&D infrastructure, in order to spare money that otherwise could go underutilized
for long periods of time.
(3) Energy service:
a. Energy Management allows customers to peak shave by shifting their loads from
one period of the day to another and lead to reduction of their charges.
b. Power Quality provides customers with a clean sinus waveform of electricity
without any secondary disruptions like sags, spikes, or harmonics.
c. Power Reliability provides a UPS concept to costumers in order to deal with power
disruption issues. It also allows remote power operation if it is paired with ESS
management systems.
11
(4) Renewable energy:
Inevitably, the cost of RES will be driven down due to future developments on renewable
energy technologies. This can be already seen in wind and solar applications for power
generation. Moreover, the future extensive usage of distributed generation through solar,
wind and wave power generating systems, will be faced with the basic and fundamental
difficulty of stochastic production, which can be overcome by customers’ demand
flexibility, secondary power sources and enough electrical energy storage systems that
could provide electricity for hours, days and up to a week. Applications of energy storage
in order to enhance RES generation as reported by Mears (2004), where single-function
applications were identified as
a. Transmission Curtailment: Addressing the power delivery limitation due to
insufficient transmission capacity.
b. Time-Shifting: Storing excess energy produced be RES during off-peak periods of
the day, and returning it to the grid during peak periods.
c. Forecast Hedge: Addressing the shortfalls in RES bids into the market prior to the
deliverance time requirements. This can reduce the volatility of prices and lower
the risk of consumers facing this volatility.
d. Grid Frequency Support: ESS can support the frequency of the grid during sudden
decreases of energy production from RES, over a short discharge interval.
e. Fluctuation Suppression: Like Grid Frequency, Fluctuation Suppression can
stabilise the grid by absorbing and discharging amounts of energy during short
variations in output. This can be extended to all forms of the stochastic nature of
RES.
12
1.4.2 The area
The area of this research highlights the need to store energy in order to strengthen power networks
and maintain load levels. There are various types of storage methods, some of which are already
in use, while others are still in development. A look has been taken at the main characteristics of
the different electricity storage techniques and their field of application (permanent or portable,
long or short-term storage, maximum power required, etc.). These characteristics will serve to
make comparisons in order to determine the most appropriate technique for each type of
application.
According to Ibrahim (2008), energy storage techniques can be classified according to three
criteria:
The type of application: permanent or portable.
Storage duration: short or long term.
Type of production: maximum power needed.
Therefore, it is imperative to analyze the technical and economic characteristics of ESS in order
to be able to establish the comparison criteria for the selecting the best technology. The main
characteristics analyzed are:
I. Storage capacity
This defines the energy available in the ESS after a complete charging. Because
discharging is often incomplete, Storage Capacity is defined on the available energy stored
which is the actually operational energy. Depth of Discharge limits the usable energy of
the system.
II. Available power
This parameter determines the constitution and size of the motor-generator in the stored
energy conversion chain. It is generally expressed as an average value, as well as a peak
value often used to represent maximum power of charge or discharge.
13
III. Depth of discharge or power transmission rate
This is closely related to the available power and the DoD. Although storing energy can
take some time, the ESS must be able to discharge faster and release energy on demand.
The rate at which the ESS can discharge its stored energy is called power transmission rate.
This rate defines the time that is needed to supply the grid with the stored energy.
IV. Discharge time
Refers to the duration that the ESS can provide its maximum power. It closely related to
DoD and the operational conditions of the ESS. For specific applications, specific
discharge time is considered.
V. Efficiency
Fundamental efficiency refers to the ratio between stored and released energy. Because
ESS systems can be described by three types of efficiency (charging efficiency, no load
efficiency, self-discharge losses), it is vital to base the efficiency on one or more realistic
values for the given application. A defining factor of efficiency is the instantaneous power
capability.
VI. Durability (cycling capacity)
Taking into consideration that a full cycle consists of one full charge and one full discharge,
durability can be defined as the number of times the ESS can provide the stored energy it
was designed for. All ESS inevitably show fatigue or wear by usage over the time. This is
caused by the aging of the materials, possible thermal degradation etc. It is an important
factor when choosing an ESS because it defines the endurance of the system.
VII. Autonomy
This refers to the maximum amount of time the system can continuously release energy. It
is defined by the ratio between the energy capacity (restorable energy) and maximum
discharge power. The autonomy of a system depends on the type of storage and the type of
application. For small systems (a few kWh) in an isolated area relying on intermittent
renewable energy, autonomy is a crucial criterion.
14
VIII. Costs
A storage system is an interesting venture when total gains exceed total expenses. The
capital invested and operational costs (maintenance, energy lost during cycling, aging) are
the most important factors to consider for the entire life of the system.
IX. Feasibility and adaptation to the generating source
In order for an ESS to be efficient, it has to be adapted correctly to the type of application
and production to be used to. For the application issue, one has to take into account the
power needed in the isolated area and for the production type, one should take into account
the portability, renewability and the permanent factor. Anyhow, it needs to be implemented
in harmony with the grid.
X. Self-discharge rate
This defines the rate at which the ESS losses its stored energy over time because of no
usage.
XI. Mass and volume densities of energy
These characteristics are vital when the application is considered portable. They represent
the actual maximum amount of energy available per unit of mass or volume of the ESS,
and as mentioned before, demonstrate the importance of volume and mass for certain
applications.
XII. Monitoring and control equipment
This kind of equipment guarantees the quality and the safety of the ESS and must be readily
accessible and available in order for errors to be diagnosed in time.
XIII. Operational constraints
Not all ESS can be used everywhere. Certain ESS have operational constraints regarding
the given environment to be implemented. The safety of the system (e.g. explosions risk,
waste management) and other operational conditions (e.g. ambient and equipment
temperature and/or pressure) can influence the choice of the ESS as a function of energy
needs.
15
XIV. Reliability
A vital characteristic, closely related to durability, which guarantees the demand needs.
XV. Environmental aspect
Although not an operational characteristic, an investor should take in mind the
environmental parameters in order to avoid the NIMBY syndrome and prepare the local
community with reliable and recyclable materials.
XVI. Other characteristics
How easy the maintenance can be, how simple the device is, how flexible the operation is,
the response time (fast or slow), etc. Finally, we should note that these characteristics must
not only be met by the ESS but by the supporting equipment (power converters, switchgear,
etc.) as well.
1.4.3 The gap in knowledge
The power system is undergoing rapid changes. On the generation side, renewable energy
mandates, according to U.S. Energy Information Administration, 2014, are accelerating the
replacement of large-scale, slow-ramping, dispatch able power plants with smaller non-dispatch
able RES such as solar and wind power plants. Similarly, electric vehicles, demand response and
advanced smart metering systems are altering usage patterns. Both supply and demand-side
changes are introducing uncertainty regarding the resource requirements for maintaining power
balance on the electricity grid (Ela et al., 2011). An example of this, as we also stated before, is
the ability of inherent variability and stochastic nature of many popular RES that can result in
fluctuating generation patterns along with sudden or unexpected changes in the power that is
available. On the demand-side, electrical vehicles can suddenly decrease or increase grid loads.
Till now, such added risk is managed through operating reserves or other auxiliary services that
can immediately address short-term imbalances. However, as the grid changes the size and
capacity requirements for dealing with new challenges are also uncertain and can vary dramatically
with regional, seasonal and real-time weather patterns; therefore it is difficult το make an accurate
estimate or even define resource adequacy. Ongoing renewable integration studies indicate that
the power grid can accommodate up to 20% of energy production from RES without EES.
16
However, grid operating paradigms and market designs need to be modified even at this level of
penetration (Denholm et al., 2010).
As the system incorporates an even larger number of non-dispatch able RES and encounters less
predictable, rapidly changing load patterns, current grid infrastructure and operational strategies
will be unable to maintain reliable function. New tools, technologies and additional grid services
will be required in order to maintain the current system stability and reliability (Varaiya et al.,
2011).
17
1.5 Outline of the dissertation
This dissertation will analyze the various energy storage methods currently being used worldwide
with the parameters mentioned earlier.
In order to properly compare the various ESS in their implementation phase, the structure of
electric power systems will be analyzed along with the grid issues commonly found in the grid and
the faults and protection measures that should be taken into account.
The actual comparison of the various ESS follows and finally an investigation will be made on
what method can be efficiently used regarding the small island of Tilos. The aging of the
infrastructure poses a serious threat towards certain energy storage methods.
Analysis of ESS
ESS Parameters
Electric Power Systems
ESS Comparison
Implementation on Tilos Island
Faults Issues
Discussion Conclusions
Figure 4: The outline of this dissertation (Source: The Author)
18
Chapter 2 - Research methodology
2.1 Description of the data sources used
The sources used in this dissertation include scientific work from journals, symposiums,
conferences and users’ manuals. More analytically, the main sources used are:
i. Applied Energy
ii. Electric Power Systems Research
iii. Energy
iv. Energy Conversion and Management
v. Energy Policy
vi. Renewable and Sustainable Energy Reviews
vii. IEEE Conference Proceedings
viii. ABB Switchgear Manual
ix. Schneider Electrical Installations Guide
x. SEA Lab, Piraeus University of Applied Sciences
xi. Hellenic Navy Hydrographic Service
The journals (i-vi) selected are globally reputable and present parts of this dissertation with
understandable detail and clarity.
Institute of Electrical and Electronics Engineers (IEEE) pioneers in advancing technological
innovation and excellence for the benefit of humanity. And since embedding energy storage
methods for promoting green energy solutions and smart grids is for the benefit of humanity,
certain conference proceedings will be referenced.
The switchgear manuals will be used for qualitative reasons and there is no certain promotion of
one company over the other.
Soft Energy Applications Laboratory of Piraeus’ University of Applied Sciences provided us with
the relevant data and grid topology in order to carry out our case study. Also, their extensive
experience in energy systems and storage methods, provided useful sources.
19
Finally, the Hellenic Navy Hydrographic Service provided us with a thematic map of the
underwater cable passage that ends at Tilos.
20
2.2 Background theory
It is mandatory to define the boundaries on the research method of this specific scientific proposal
in order to be able for the reader to identify the key elements that will be used in order to carry out
this research. The roadmap is provided by Skittides and Koilliari (2006) and explained specifically
for the context of this research proposal.
According to Skittides and Koilliari (2006), the unified framework of research design is divided
into two major categories: the strategic choices and the tactical choices. The strategic choice for
this specific research is a theoretical review. More analytically, Skittides and Koilliari (2006)
define the review as: “Non-empirical research that has the goal of synthesizing or re-interpreting
existing theory”. Also the theoretical part can also be defined as: “The aim of theoretical reviews
is to make sense of the research work that has been undertaken in a topic, comparing and
contrasting the work of different authorities. The outcome may be new insights, the identification
of inconsistencies or gaps that need to be tackled, and so on”.
In this context and as already mentioned in previous sections of this research, the main aspect is
to preview the available literature and after analyzing all the available Energy Storage Methods,
conclude on which method can and/or may be used according to its specifications for any given
grid size and utilization. A special report will be given on the island of Tilos, where, with the actual
imprint of this island’s grid and the load demand, we will study the most suitable energy storage
method.
The main tactical choice that will be used is that of secondary materials. These can be defined as
sources that are not directly connected to the subject studied and cannot be described as original.
We should also clarify that the obtained descriptions are not the author’s but from another person
or source, although these in return might or might not have been primary sources either. Cohen et
al. (2007) continues the definition as: “… Other instances of secondary sources used in historical
research include: quoted material, textbooks, encyclopedias, other reproductions of material or
information, prints of paintings or replicas of art objects. Best (1970) points out that secondary
sources of data are usually of limited worth because of the errors that result when information is
passed on from one person to another”.
21
2.3 What was done to get the data
Extensive use of University’s Information Services Guides granted access to databases and other
electronic resources in order to be able to gather the relevant journals and articles used in this
dissertation.
University’s library archives provided valuable access to books and publications.
Soft Energy Applications Laboratory of Piraeus’ University of Applied Sciences provided us with
the measurements and the single line grid schematic of Tilos; the island in which a case study will
be performed..
Finally, regarding the figures of the underwater cable connecting Tilos to Kos and Nisyros, were
provided by the Hellenic Navy Hydrographic Service.
22
2.5 Problems encountered
One of the problems encountered was the wide divergence of the characteristics of ESS along the
literature reviewed. These had to be estimated and evaluated in order for the comparison to be
solid.
Another problem regarding the case study was the lack of span of measurements that could guide
us to a more concrete load profile of the island. The best practice would have been the studying of
measurements in an annual manner. This study is confined in the availability of measurements.
Finally, the grid operator could not provide us with technical information regarding the current
grid infrastructure of the island in which the case study is performed, thus an estimation of the
efficiency of the grid has been made.
23
Chapter 3 - Energy storage systems applied in small and medium scale power
grids
3.1 Energy Storage Systems Overview
The current energy storage systems (Mahlia et al., 2014) are divided in the following categories
according to the means of energy storage:
Electrochemical energy
o Battery Energy Storage (BES)
Lead-Acid
Nickel battery
Sodium-sulfur
Lithium battery
Metal-air battery
o Flow Battery Energy Storage (FBES)
o Superconducting Magnetic Energy Storage (SMES)
o Super Capacitor Energy Storage (SCES)
Mechanical Energy
o Compressed Air Energy Storage (CAES)
o Liquid Air Energy Storage (LAES) or Cryogenic Energy Storage (CES)
o Pumped Hydro Energy Storage (PHES)
o Flywheel Energy Storage (FES)
Chemical Energy Storage
o Hydrogen
Thermal Energy Storage (TES)
o Sensible heat storage systems
o Latent Heat Thermal Energy Storage (LHTES)
o Thermochemical energy storage
24
3.2 Electrochemical Energy Storage Methods
3.2.1 Battery Energy Storage
Lead-Acid
According to Rand (2015), Lead-Acid batteries are electrochemical cells based upon chemical
reactions involving lead and sulfuric acid. Lead-Acid is one of the oldest and most developed
battery technologies. They comprise two electrodes were the negative is made of lead and the
positive is made from lead dioxide, separated by an electrolyte (diluted H2SO4) which electrically
isolate the two electrodes so that the sulfate ions for the discharge reaction can be provided.
Figure 5: Scheme of prismatic and spiral wound construction of Lead-Acid battery (Krivik and
Baca, 2013)
25
The main chemical reaction is shown below:
At the positive plate we have: Discharge
2 4 4 2ChargePbO 3H HSO 2e PbSO 2H O
At the negative plate we have: Discharge
4 4ChargePb HSO 2e PbSO H 2e
And the overall reaction is: Discharge
2 2 4 4 2ChargePbO Pb 2H SO 2PbSO 2H O
There are two main types of Lead-Acid batteries: Flooded and valve regulated. Flooded batteries
require periodic water refilling and thus demand periodic maintenance and present moderate
energy density of ~25Wh/kg. Valve regulated are maintenance free and are characterized by their
higher energy density of up to 50Wh/kg and deeper discharges. On the other hand, the life
expectancy of flooded batteries is almost three times greater than the life expectancy of valve
regulated batteries (3000 to 1000 cycles). Although Lead-Acid batteries can be described as a very
mature technology with known performance characteristics, they are also described as low energy
density energy storage method, limited service period, environmentally unfriendly content if not
recycled properly and, finally, the recommended low depth of discharge in order to maximize the
service period can be described as drawbacks (Zafirakis, 2010).
Nickel Batteries
Nickel batteries are electrochemical cells. There are a number of Nickel based batteries currently
available or under development, including Nickel-Cadmium (Ni-Cd), Nickel-Zinc (Ni-Zn),
Nickel-Metal Hydride (Ni-MH) and Sodium-Nickel Chloride (Na-NiCl2). Ni-Cd and Ni-MH are
the most developed of the Ni batteries.
26
Figure 6: Scheme of spiral wound and prismatic construction of Ni-Cd battery (Krivik and Baca,
2013)
For Ni-Cd, the chemical reactions are:
For Nickel: 2 2Ni OH OH NiOOH+H O+e
For Cadmium: 2Cd OH 2e Cd 2OH
Overall reaction: 2 2 22Ni OH +Cd OH 2NiOOH+Cd+2H O
Above we saw a typical chemical reaction of the Ni-Cd battery. Other Nickel batteries have
different chemical reactions, not covered in this dissertation.
The Ni-Cd and Ni-MH cells display a very flat discharge plateau with a mid-discharge voltage
around 1.25V. The service period of Ni-Cd batteries can reach 20 years because of their ability to
sustain very long cycles and their self-discharge rate is affected by the temperature, electrolyte
type and cell design and is not linear vs. time, as explained by Bernard and Lippert (2015).
These batteries can also be described as a very mature technology and although their energy
density is higher than Lead-Acid batteries, self-discharge is more significant along with deep
27
discharges and a considerable service period, which can be described as the drawbacks of these
batteries along with their environmental aspects regarding cadmium toxicity.
Sodium-Sulfur Batteries
Kaldellis et al (2007) describe Sodium-Sulfur (NaS) batteries as electrochemical cells. In the high
temperature batteries category, the most mature and developed technology is the NaS batteries,
thus a temperature of 300 oC must be maintained with a heat source. A NaS battery consists of
liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode
as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows
only the positive sodium ions to pass through it and combine with the sulfur to form sodium
polysulfides.
Figure 7: Schematic cross-section of Na-S cell (Krivik and Baca, 2013)
The materials’ high energy potential, high efficiency and high depth of discharge lead to no self-
discharge of these systems. More specifically, NaS batteries have a specific energy of 150-240
Wh/Kg and power density of 150-230 W/kg, according to Moseley and Rand (2015).
Lithium Batteries
Lithium batteries are electrochemical cells. Lithium-Ion (Li-ion) and Lithium-Polymer (Li-pol)
types are both available. Its negative electrode is made of graphite, while the positive electrode is
a “lithiated” metal oxide. The electrolyte is made up of a lithium salt such as LiPF6 or LiClO4,
28
which has been dissolved in organic carbonate solvents. The chemical reactions are described
below:
Charging: xC+xLi xe Li C
Discharging: 2 1 x 2LiMO Li MO xLi xe
Figure 8: Prismatic and cylindrical Li-ion cell construction (Krivik and Baca, 2013)
Li-ion batteries have high energy-to-weight ratio, low self-discharge losses, around 10000 cycles
at its life and an efficiency of ~100% compared to other battery types (Akinyele and Rayudu,
2014).
Metal-Air Batteries
Metal-air batteries are electrochemical cells and are the most compact batteries available. They
actually are a type of fuel cell that uses “metal” as the fuel and “air” as the oxidizing agent. The
anodes in these batteries are metals that are commonly found and have high energy density, such
as aluminum or zinc that release electrons when oxidized. The cathodes are often made of a porous
carbon material or a metal mesh covered with proper catalyst. The electrolyte may be in liquid
form or solid polymer membrane. The typical chemical reactions of Zinc-Air are shown below:
29
Anode: 2
4Zn+4OH Zn OH 2e
Fluid: 2
24Zn OH ZnO+H O+2OH
Cathode: 2 2O +2H O+4e 4OH
Overall: 22Zn O 2ZnO
Figure 9: Typical Zinc Air button cell battery (Micropower Battery Company, 2015)
The major challenge with these batteries is their poor recharging capacity (Chen et al, 2009).
3.2.2 Flow Batteries
Flow batteries store and release energy through a reversible electrochemical reaction between two
electrolytes. There are four types of flow battery currently being produced or in the late stages of
development; zinc bromine, vanadium redox (VRB), polysulphide bromide and cerium zinc.
30
Figure 10: Scheme of vanadium redox battery (Krivik and Baca, 2013)
An American corporation developed a hybrid flow battery, which has a zinc-bromine system as a
base and a Canadian corporation developed a leading form of the VRB system. Charging and
discharging take place along with the separation of chemicals that react from the electrochemical
cells. The electrolyte tank size define the storage capacity, while the size of the fuel cell define the
power output. The ability to vary the discharge time at full power, provides an advantage of the
VRB over the hybrid system (Alotto et al, 2014).
3.2.3 Superconducting Magnetic Energy Storage
SMES is a technology that utilizes a super-conducting coil in order to store electrical energy in its
magnetic field. In order for the coil to become superconducting, it has to be cryogenically cooled
in the temperature of -269oC. When the material achieves this temperature, it has no resistance to
electric currents and that results in allowing very high efficiency of up to 97%. A plus to that is
the ability to immediately release power which allows the system to be useful to consumers that
need high quality power output (Buckles and Hassenzahl, 2000).
When a Direct Current (DC) passes through a coil, the resistance of the coil will make the current
to dissipate quickly. However, when a DC flows through a superconducting coil, the electrical
energy will not dissipate and that way, the energy is stored in a magnetic form until needed.
31
Tixador (2013) describes a SMES consisting of:
1. A superconducting magnet; electric connections between the superconducting magnet and
the room temperature circuit
2. A cryogenic system; cryostat, vacuum pumps, cryocooler
3. A power conditioning system; interface between the superconducting magnet and the load
4. A primary source
5. A control and management system; electronics, cryogenics, magnet protection
An illustration of the above numbered elements is given in Figure 11 below
Figure 11: Elements of a SMES system (Tixador, 2013)
SMES is more a power source than an energy source such as a battery and this is the main reason
why SMES is suitable for high power (up to 100MW) and short duration (under a few seconds).
A hybrid model for covering a broader range of applications can be implemented. For example, a
SMES for short durations and batteries for long durations (Zafirakis, 2010).
32
3.2.4 Super capacitors Energy Storage
Super capacitors can store energy in the electric field between a pair of charged plates. Super
capacitors, ultra-capacitors or double-layer capacitors (DLCs). Comparing these types of
capacitors to conventional capacitors, we can see that they have a larger electrode surface area,
liquid electrolyte and polymer membrane (Zafirakis, 2010).
Figure 12: Typical Double Layer Capacitor scheme (www.mpoweruk.com/supercaps.htm)
Super capacitors have the highest power density of 5000+W/kg, fast charge and discharge
capabilities, low current dissipation, thousand cycles per year (106+) and high energy efficiency
of 99.9+% (Kurzweil, 2015).
33
3.3 Mechanical Energy Storage Methods
3.3.1 Compressed Air Energy Storage
Zafirakis (2010) describes CAES as projects that would use excess off-peak energy to
compress air and inject it into a depleted natural gas reservoir and then use the compressed
air to power a generator during peak periods when the energy is needed most. Traditional
CAES essentially dumps the heat into the atmosphere, therefore requiring a second injection
of heat prior to re-expansion. The entire power of the gas turbine is readily available for
consumption. During a charge/discharge cycle, for the generation of 1kWh, 0.75kWh of
electricity for the compressor and 4500kJ is required. CAES require sites and geological
formations that can be used as underground storage. The most common medium are the rock
and salt caverns, and buried pipes for small underground applications.
As seen in Figure 13, a wind park with CAES consists of:
1 Wind park 2 Motor 3 Air Compressor 4 Air storage
cavern
5 Preheater 6 Combustion
Chamber 7 Gas Turbine 8 Generator
9 Natural Gas
Tank 10 Electricity Grid 11
Electricity Consumption
Figure 13: Wind farm with CAES system (Zafirakis, 2010)
34
Advanced Adiabatic CAES (AA-CAES) instead, aims to remove the heat and store it
separately, then re-eject the heat at the expansion stage, thereby removing the need to reheat
with natural gas (Jubeh and Najjar, 2012).
Figure 14: AA-CAES concept (Bullogh et al, 2004)
3.3.2 Liquid Air Energy Storage or Cryogenic Energy Storage
Liquid Air Energy Storage or Cryogenic Energy Storage works similarly as a CAES system but
the difference is that the air is liquefied and stored in over ground tanks. One pilot plant operates
at present in Slough Trading Estate, UK (www.highview-power.com).
3.3.3 Pumped Hydro Energy Storage
It is considered to be the oldest and largest of all of the commercially available energy storage
technologies. Conventional pumped hydro facilities consist of two large reservoirs: one is located
at a low level and the other is situated at a higher elevation. During off-peak hours, water is pumped
from the lower to the upper reservoir, where it is stored. To generate electricity, the water is then
released back down to the lower reservoir, passing through hydraulic turbines and generating
electrical power.
35
Figure 15: Pumped Hydro Elements (Source: Alstom)
Prof. Dr. John Kaldellis (Kaldellis, 2008; Kaldellis, 2015; Kaldellis et al, 2006; Kaldellis et al,
2005) has extensively analyzed the benefits of hydro and pumped hydro plants in Greece. He
suggests that even in small grids, the implementation of pumped hydro systems is feasible and in
cooperation with RES, it is a very sustainable solution. Specifically in small grids, the volume of
the reservoirs can be relatively small and in the majority of islands, the lower reservoir can be the
sea itself.
3.3.4 Flywheel
The traditional flywheel is a mechanical form of storing energy through the kinetic energy of a
fast spinning cylinder. Modern flywheels’ cylinder is supported by a stator with magnetically
levitated bearings. These bearings minimize wear and lengthen the life of the system. The
efficiency can be increased when the flywheel operates in a low pressure environment, thus
minimizing friction with the air. The flywheel ESS draws energy from a primary source to spin
the high density cylinder at speeds greater than 100,000 rpm (Zafirakis, 2010).
36
Figure 16: A flywheel parts scheme (Source: Nanyang Technological University, 2011)
Flywheels have efficiency up to 99%, great durability (more than 106 cycles), but their daily self-
discharge is rated at 100%. These make flywheels ideal for power quality issues on the grid and
perfect devices for Uninterruptable Power Supply (UPS) needs.
37
3.4 Chemical Energy Storage Methods
3.4.1 Hydrogen
The hydrogen production pathways, according to Herzog and Tatsutani, 2005, include Steam
Methane Reforming using natural gas as feedstock, Gasification of Coal and other hydrocarbons,
Electrolysis using conventional grid or renewable power, Gasification of biomass and Nuclear
Power. Hydrogen can be then be stored in underground caverns, salt domes and depleted oil and
gas fields. Stored hydrogen can then be used in fuel cells or injected directly to natural gas pipes
to boost the calorific value and thus lead to better combustion in gas turbines (Correas, 2013).
From the methods mentioned above, it is clear that not all methods can be used in small islandic
grids, but could be used in mainland smart grids.
Although Hydrogen is not an energy storage method itself, the methods that it can be created is
actually energy conversion, leading to hydrogen being a method of stored energy.
38
3.5 Thermal Energy Storage
Thermal Energy Storage refers to the conversion and storage of energy to heat or cold. This can
be achieved through heat exchangers and heat pumps and the storage vessel can be either
underground tanks and cavities or over ground tanks. Guideline VDI 4640 Part 1 (Reuss, 2015)
also analyses the various underground minerals that can store thermal energy that can be later
retrieved. Finally, thermal energy storage is widely used in the forms of buildings’ air conditioning
systems and solar heating for domestic water usage. The combination of photovoltaic panels along
with solar heating from the same panel can greatly reduce greenhouse gases (Gholami et al, 2015)
thus making it an ideal solution for small scale heat storage solution with minimum to non-existent
environmental impacts.
39
3.6 Environmental Aspects of ESS
As engineers, we wish that every new technology is for the greater good and with zero impacts
either to the environment or humans. Unfortunately, every technology has its drawbacks regarding
the environmental aspects, and ESS could not but follow this as well. According to the author
(Kokkotis, 2015) all ESS have some environmental issues. Because the article is about small scale
ESS, the pumped hydro has been excluded but will be mentioned in this section separately. The
overview of the environmental aspects are summarized in the following tables and according to
the means of storage:
Technology Environmental Issues
Ele
ctro
chem
ical
Batteries
Lead-Acid
Lead is toxic
Sulfuric acid is corrosive
When overcharged it generates hydrogen leading to explosion risk
Nickel Nickel is corrosive
For Ni-Cd, cadmium is highly toxic
NaS Caution with the high temperature at which the battery must be
operated in order to maintain the sulfur in molten form
Lithium Resource depletion
Human Toxicity and Eco Toxicity associated with some elements
Metal-Air Zinc or Aluminum issues
Flow Batteries Are determined by the extent of plant
SMES Extremely Low Temperatures
Require protection against magnetic radiation
Supercaps Impacts from materials and compounds used within their
construction
Table 1: Environmental aspects of electrochemical means of energy storage (Kokkotis et al.,
2015)
40
Technology Environmental Issues M
ech
an
ical
Flywheel Safety safeguards should be applied to the operation of
heavy, rapidly rotating objects
CAES GHG, but lower than those from NG plants
AA-CAES
Lower impacts than CAES
Thermal energy storage must have thermal mass with
high heat transfer capabilitites
Possible heat leakage might affect local microclima
LAES A serious concern is the stratification of the liquid air in
the storage tank
PHES
(Intelligence
Energy Europe,
2013)
Ecology and
Natural
Systems
High Biodiversity impacts
Medium-High Fisheries issues
Medium Landscape and Visuals issues
Low to High Air and climate issues
Medium-High Water resources and quality issues
Physical
Environment
Medium to High Soil and Geology issues
High Hydrology and Hydrogeology issues
Table 2: Environmental aspects of mechanical means of energy storage (Kokkotis et al., 2015)
Technology Environmental Issues
Ch
emic
al
Hydrogen
Steam Methane Reforming
Burning NG contributes towards global warming
Extracting and transporting NG could harm sensitive landscapes
Gasification of Coal Making H2 from coal or heavy oil would generate large
amounts of carbon emissions
Electrolysis
Use of conventional grid power would generate more global warming pollution than steam methane reforming
with NG
Near term benefits of using RES may be greter if used to displace other sources of electricity
Gasification of Biomass Large scale production of feedstock and collection and transport of crops and residues may arise air, land and
ecosystem concerns
Nuclear Power Issues of waste management and disposal and extraction
and processing of uranium
Table 3: Environmental aspects of chemical means of energy storage (Kokkotis et al., 2015)
41
Chapter 4 - Electric power systems
4.1 Introduction
According to Vournas and Kontaksis (2001), an electric power system is a system of installations
and means used in providing electrical energy in serviced areas of power consumption. The electric
power system must provide electrical energy wherever the load is with the least cost and the least
environmental impacts while providing constant frequency, constant voltage and high feeding
reliability.
An electric power system can be broken down into three operation modes: generation, transmission
and distribution. The electrical energy, just from the point it is generated and until reaching the
final load, is in a constant motion and because it cannot be stored in the distribution lines, it has to
be generated exactly the moment it is needed or stored. Generation of electrical energy takes place
in power plants, such as lignite or natural gas fired power plants, diesel oil power plants, nuclear
power plants and renewable energy sources. Transportation in bulk takes place with high voltage
(HV) lines which transfer the electrical energy in central hubs of the network and into substations
where medium voltage (MV) lines start towards substations for low voltage (LV) consumers
(Figure 17).
Figure 17: Basic Structure of the Electric System (Source: University of Idaho)
42
4.2 Structure of Electric Power Systems
The structure of the system is of outmost importance for the geographical availability of electrical
energy. The structure and synthesis of an electrical power system are mostly dependent on their
size. The installations of generation and transmission are usually financially dependent and for that
reason the technical and financial design of power plants, main transmission lines and central
substation must be even, aiming in the energy needs of the load with the least expected cost and
the highest feeding availability. Distribution is another mode that is designed and developed
separately and is highly dependent in the local area and the final consumer.
This structure is affected by the load demand, its daily and seasonal variations and its land
planning. Current systems are three-phase (3P) alternating current (AC) of 50 or 60 Hz, but also
DC might be used. The voltage remains constant. Transmission and MV distribution lines have
three phase lines, where LV distribution lines also have a Neutral line. In 3P systems, the flux of
energy is constant and this makes its operation much smoother and more efficient than in one phase
systems.
Customers connected in HV and MV systems are usually industrial consumers, whereas in LV
systems the customers are usually domestic and commercial.
The total consumption defines the usage and the fuel used in power plants, and the demand curve
describes the usage of transmission and distribution in a timely manner. Load demand shapes the
functional cost of a power company.
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4.3 Electricity Generation
In order to generate electricity, a primary form of energy must be converted into electricity.
Nowadays, this primary energy is transformed into mechanical energy and then into electricity
through a generator. The main sources of electricity are:
Lignite fired power plants
Natural Gas power plants
Hydro plants
Nuclear plants
Diesel oil plants
Renewable Energy Sources
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4.4 Transmission
The transmission of electricity includes the sum of operational and installations control processes
that are used for the transmission of electricity from power plants to substations that feed the
central hubs where the distribution begins. They also feed big HV consumers, where the latter are
obliged in constructing their own step down substation and internal LV network.
The transmission network includes the HV transmission lines, the couplings and the step down
substations. This network must provide constant power and the voltage must be seamless in the
three phases and thus the efficiency must meet the lowest annual cost.
It operates in HV because this leads to lower electricity losses and increased capabilities of
transmitting power. There are variations in the voltage in transmissions lines depending on the
distance and the power that has to be transmitted. The topology of the transmission can be either
longitudinal or ring depending on the relevant position towards the consumption centers. The set-
up is looping in conjunction with the radial set-up of distribution networks.
The power that can be transmitted from a transmission line is proportionate to the square of the
voltage, and thus high voltage is used for the transmission of high power. This also leads to lower
losses and lower cost.
45
4.5 Distribution
The distribution of electricity includes the sum of operational and installations’ control processes
that are used for the distribution of electricity to the final consumers. These networks include the
power lines that are lead into step-down substations through which the consumers are connected
to the grid. The constant rise of electricity consumption and the technological advance of materials
have led to using higher and higher voltages for the transmission of electricity leading to older
networks used for transmission to be used as distribution ones nowadays.
Currently, the value of distribution in electric power systems is about 30% of the total installations
value. Another characteristic is the extent of this network. Losses are about the double than in the
transmission network.
The construction planning of a distribution network is directly dependent on the urban planning
characteristics of a city. It is also distinguished by their aerial or underground routing. Aerial
networks are cheaper and the faults are treated faster. On the other hand, in dense populated areas,
the network is underground because of the limited space in order to keep the minimum safety
distances and aesthetics.
46
4.6 Loads
Load is any device that draws power from the grid. Typical load categories are:
Motors
Heating devices
Electronic devices
Luminance devices
These loads can have an impact on the grid because of their different characteristics (symmetry,
magnitude, stability and usage period). For an electric power system to operate smoothly, these
characteristics must be known and extrapolated according to their power and frequency. There are
two major categories in the loads. The loads with constant resistance Z=R+jωL or loads of constant
power S=P+jQ. Mixed loads are a more realistic approach and are changing according to voltage
and frequency. The mean load is 60% inductive, 20% synchronous motors and 20% various loads.
47
4.7 Analysis of Electric Power System
Analysis of electric power systems have two branches:
Steady state
Transient state
Steady state studies include analysis of load flows and load financial distribution, while transitional
state studies include short circuit analysis, transient electric phenomena and stability issues.
Load flow analysis is the actual calculation of voltage, current and active and reactive power flows
in various spots in the electric power system under real conditions or simulated load and operation.
These studies are mandatory for the smooth daily operation but also for studying future expansions
of the network. Load flow analysis is also needed to determine the impacts in case of
interconnections with other systems, insertion of new consumers, and installation of new power
plants and the construction of new transmission lines.
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4.8 Operation and control of electric power systems in non-interconnected
networks
In Greece, due to its many islands, there are many networks that are isolated and not interconnected
to the main land. The main power plants of these stand-alone islands are the diesel oil generators.
The instability of these networks regarding the voltage and the frequency is characteristic
especially in the summer period where there is a substantial increase in demand mostly because of
increased tourism. The networks of Crete, Rhodes and Lesvos are considered medium
autonomous. Other islands are considered small magnitude networks. The uprising issue of non-
interconnected islands is the restriction of new RES installations, thus not being able to fully
exploit the wind and solar potential of the islands. Networks are unstable and weak as we will see
later on. Nevertheless, there are some moves towards sufficient RES until reaching the safety
threshold of the network. This issue can be mitigated by connecting the island to the mainland,
like Andros Island, but the cost in other cases might be high.
49
4.9 Grid Issues
In order to define the most suitable energy storage method, it is required to examine the
applications that must be covered.
Several energy storage methods offer both thermal and electricity output capabilities, whereas
other offer only thermal or only electricity. The discharge duration defines the time that this
particular energy storage method must operate in order to fulfill the grid needs depending on the
application. The typical cycles define the statistical need of this energy storage method; how many
interruptions are expected. Finally, the response time of any system is the time in which the system
must be fully operational.
Table 4 below summarizes the grid issues and what characteristics the energy storage system must
have in order to mitigate these issues.
Regarding the applications, more analytically, we have:
Seasonal Storage: Refers to the ability to store energy for a long period that can expand
from a day to several months in order to counter any disruption in the supply or the
variability of seasonal needs.
Arbitrage: Refers to the storing of energy during low priced electricity generation and
selling this amount of energy during periods of high priced demand.
Frequency Regulation: Refers to the equalization of an ever changing supply and demand
balance within the area of control.
Load Following: The second continuous electricity balancing mechanism for operation
under normal conditions, following frequency regulation. It manages system fluctuations
on a time frame that can range from 15 minutes to 24 hours.
Voltage Support: Can be maintained by either injecting or absorbing reactive power in
order to maintain stable voltage levels in the T&D system.
Black Start: If and when a power system falls off line and all other secondary safeties and
ancillary mechanisms fail, a system with black start capabilities is able to restart without
the need of external triggering.
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Transmission and Distribution (T&D) Congestion Relief and T&D infrastructure
investment deferral: ESS are used provisionally or in a specific area in order to shift energy
demand or supply from congested junctions in the T&D grid. ESS are also used to defer
the need for investments in the current grid.
Demand Shifting and Peak Reduction: Refers to the shifting of energy demand so that
supply and demand are in an equilibrium and to further integrate the variable nature of
RES. This shifting can happen by changing the time at which certain activities take place
and can be directly facilitated in order to reduce the peak demand.
Off-Grid: Consumers off the main grid mainly rely on diesel generators and small scale
RES in order to self-provide electricity and/or heat. ESS can provide reliable energy for
off-grid consumers and can support an increase to local resources usage. Finally, ESS can
be used to smooth the RES’ production stochastic nature.
Variable Supply Source Integration: ESS can optimize and change the output of the
intermittent nature of RES, minimizing fast and seasonal output and bridging both local
and temporal gaps between demand and supply in order to increase the value and the
quality of the supply.
Waste Heat Utilization: Refers to the ability to store heat that would otherwise be wasted,
thus disengaging temporary and local heat demand and supply.
CHP: Cogeneration can utilize thermal and electricity ESS in order to smooth the demand
and the supply gaps.
Spinning and Non-Spinning Reserve: For a system, to be kept in balance and overcome
fast and unexpected losses in generation, the reserve capacity is used. This reserve can
either be classified as spinning (with a response less than 15 minutes) and as non-spinning
(with a response time greater than 15 minutes). The faster the response time, the more
valuable it is to the system.
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Application Output (electricity
/ thermal) Size
(MW) Discharge Duration
Cycles (typical)
Response time
Seasonal storage Electricity /
Thermal 500-2000
days to months
1-5/year day
Arbitrage Electricity 100-2000 8-24hrs 0.25-1/day >1h
Frequency regulation Electricity 1-2000 1-15min 20-40/day 1m
Load following Electricity /
Thermal 1-2000 15m-24hr 1-29/day <15min
Voltage Support Electricity 1-40 1s-1m 10-100/day ms-s
Black Start Electricity 0.1-400 1-4hrs <1/year <1h
T&D Congestion Relief Electricity /
Thermal 10-500 2-4hrs 0.14-1.25,day >1h
T&D Infrastructure Investment Deferral
Electricity / Thermal
1-500 2-5hrs 0.75-1.25/day >1hr
Demand Shifting and Peak Reduction
Electricity / Thermal
0.001-1 min-hrs 1-29/day <15min
Off-grid Electricity /
Thermal 0.001-0.01
3-5hrs 0.75-1.5/day <1hr
Variable Supply Source Integration
Electricity / Thermal
1-400 1min-hrs 0.5-2/day <15mins
Waste Heat Utilization Thermal 1-10 1-24hrs 1-20/day <10mins
CHP Thermal 1-5 min-hrs 1-10/day <15mins
Spinning reserve Electricity 10-2000 15m-2hrs 0.5-2/day <15mins
Non-spinning reserve Electricity 10-2000 15m-2hrs 0.5-2/day >15mins
Table 4: Grid issues and energy storage systems’ suggested attributes (International Energy
Agency, 2014)
52
4.10 Faults and Protection in Distribution networks
According to Prévé (2006), the main faults occurring in networks and machines are the following:
Short Circuits
o Their Origin:
Mechanical: Connection between two conductors by accident (via a foreign
object like a tree branch) or breakdown of the conductors.
Electrical: Caused by (i) insulation failure between phases, (ii) insulation
failure between phase and earth (or frame), (iii) internal or atmospheric
overvoltage.
Operating error: Grounded phase, mistaken closing of a switching device,
contact among two aberrant voltage supplies or different phases.
o Their Location:
Inside equipment that often lead to equipment wear.
Outside equipment where the aftereffects are confined to disturbances,
which may lead to equipment wear and lead to internal faults.
o Their Duration:
Self-extinguishing: the fault disappears on its own
Fugitive: protective devices mitigate the fault and is not appeared when the
equipment restarts.
Permanent: these faults require the “draining” of the cable or the machine
and the hands-on care of the technical personnel.
On motors
o Too many successive start-ups could lead to mechanical and overheating shocks.
o Start-up times over the designed may lead to the same results.
53
On generators
o Fault in the rotor due to loss of excitation could lead to the overheating of the stator
and rotor resulting in discord with the grid.
o Faulty operation or overload of regulator’s frequency could lead in variations in
frequency.
Generators connected to the grid with opposed phases or different sources of the grid
coming from two parts of the network.
Over voltages due to lighting strikes
Surges in the switching equipment
Overloads on transformers, cables, generators or motors.
Energy flow direction inversion with absent electrical faults. An internal electricity
generator may supply the utility if a power cut or voltage dip occurs due to utility’s
malfunction.
Voltage variations due to erroneous operation of the on-load tap changers of a transformer,
or the network is under or overload.
The presence of a negative-phase component due to a non-symmetrical voltage source, a
large single-phase consumer, a connection error or phase cutting leads to overheating of
the motors or generators, and a loss in generator synchronism.
As seen above, a grid is not always stable and certain requirements must be met in order for the
stability and security of supply towards the consumer. Typical grid feeding monitoring devices
help automate the tripping the section switch which is connected between the distributed
generation and the public grid in order to disconnect the distributed generation in case of problems,
such as unstable grid, faults or maintenance on the grid (ABB, 2014)
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4.11 Switchgear Selection
In order to implement an Energy Storage System into the grid, certain measures should be taken
into account. As in generators, ESS switchgear must meet certain criteria. Switchgear is the
combination of any switching and interrupting devices combined with associated control,
regulating, metering and protective devices, used primarily in connection with the generation,
T&D and conversion of electric power. Switchgear is used to deaden equipment and allow the
personnel to have hands on the equipment, and to clear possible faults in the line. Switchgear
defines the reliability of the electricity supply. It may be a simple open-air switch that isolates the
load, or other substances might be used to insulate the switch. The most effective and costly form
of switchgear is the Gas-Insulated Switchgear (GIS), where the contacts and the conductors are
insulated by pressurized Sulfur Hexafluoride (SF6) gas. Other common types are the insulator to
be oil or vacuum.
The selective combination of equipment within the switchgear cabinet allows them to be able to
interrupt currents of 103 amperes originating from faults. Within the cabinet, the circuit breaker is
the primary component that interrupts fault current. Special design issues must be taken into
consideration for the dampening of the arc when the circuit breaker opens the circuit. Circuit
breakers fall into these types according to ABB Switchgear Manual:
Oil; The vaporization, leading to release of Hydrogen, of some of the oil leads to a jet blast
of oil along the path of the arc.
Air; they may use compressed air (puff) or the magnetic force of the arc itself to elongate
the arc. As the length of the sustainable arc is dependent on the available voltage, the
elongated arc will eventually exhaust itself. Alternatively, the contacts are rapidly swung
into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.
Circuit breakers are usually able to terminate all current flow very quickly: typically
between 30ms and 150ms depending upon the age and construction of the device.
Gas; Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then
rely upon the dielectric strength of the SF6 gas to quench the stretched arc.
Hybrid; they are a type which incorporates the components of two technologies: the
traditional air-insulated switchgear (AIS) and the SF6 gas-insulated switchgear (GIS). It is
55
characterized by a modular and compact design, which includes a variety of functions in
one module.
Vacuum; This type of circuit breaker offers minimum arcing characteristics as the vacuum
provides nothing to ionize other than the contact material. This lack of air suppresses the
arc. When the current is near to zero, the plasma cannot be maintained because the arc is
not of sufficient temperature, leading to the ability of the gap to withstand the rise of
voltage. Vacuum circuit breakers are usually used in MV switchgear equipment but are not
suitable for interrupting DC loads.
Carbon Dioxide (CO2); This type of circuit breaker has the same working principal as the
sulfur hexafluoride (SF6), as the CO2 is used as the insulating medium. CO2 circuit breakers
are more environmentally friendly than SF6.
On the other hand, a bus bar is a metallic strip or bar, usually copper, brass or aluminum, which
conducts electricity within electrical installations and substations. Its main purpose is to conduct a
substantial current of electricity and not to function as structural elements. A bus bar must be able
to withstand its own weight (rigid construction), the forces imposed by mechanical vibrations, and
the accumulation of moisture in outdoor environments. Another consideration is their ability to
withstand thermal expansion from temperature changes (originating from ohmic heating and the
variability of ambient temperatures), and magnetic forces caused by large currents. A switchgear
cabinet usually contains a bus bar system. A bus bar may either be supported on insulators or
insulation may completely surround it. They are protected from accidental contact either by a metal
earthed enclosure or by elevation out of normal reach. Earthing bus bars are typically bare and
bolted directly onto any metal chassis of their enclosure. Some concept configurations of bus bars
along with their advantages, disadvantages and their topology are shown in the next page.
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Concept
configuration Advantages (ABB Switchgear Manual 12th ed) Disadvantages
Single busbar least cost
BB fault causes complete station outage
maintenance difficult
no station extensions without disconnecting
the installation
for use only where loads can be disconnected
or supplied from elsewhere
Single busbar with
Bypass
low cost BB fault or any breaker fault causes complete
station outage
each breaker accessible for maintenance without
disconnecting extra breaker for bypass tie and coupling
Double busbar with
one Circuit Breaker
per Feeder
high changeover flexibility with two busbars of
equal merit
fault at tie breaker causes complete station
outage
each busbar can be isolated for maintenance BB protection disconnects all feeders
connected with faulty bus
each feeder can be connected to each bus with tie
breaker and BB disconnector without interruption
fault at branch breaker disconnects all feeders
on the affected busbar
2-breaker System
each branch has two circuit breakers most expensive method
connection possible to either busbar breaker defect causes half the feeders to drop
out if they are not connected to both bus bars
each breaker can be serviced without
disconnecting the feeder feeder circuits to be considered in protection
system; applies also to other multiple-breaker
concepts high availability
Ring Bus
low cost breaker maintenance and any faults interrupt
the ring each breaker can be maintained without
disconnecting load
only one breaker needed per feeder potential draw-off necessary in all feeders
no main busbar required
each feeder connected to network by two breakers
little scope for changeover switching all changeover switching done with circuit-
breakers
1 ½
great operational flexibility
three circuit-breakers required for two
feeders
high availability
breaker fault on the busbar side disconnects only
one feeder
each bus can be isolated at any time
all switching operations executed with circuit-
breakers greater outlay for protection and auto-
reclosure, as the middle breaker must
respond independantly in the direction of
both feeders
changeover switching is easy, without using
disconnectors
BB fault does not lead to feeder disconnections
Table 5: Advantages and disadvatages of various bus bar configurations (Source: ABB, 2012)
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4.12 Connection to MV grid
Papathanasiou, 2003, has issued a comprehensive guide that has to do with the connection of power
plants into the distribution grid and is in accordance with the directives given by PPC when
connecting these producers into the distribution network. Among the strict connection guides of
PPC, we should take into account the following issues:
Voltage difference must be between ±10% of the nominal
Frequency difference must be between ±0.5Hz of the nominal
Polar angle difference must be between ±10o
The figure in the following page describe the various methods that can be used when connecting
to a MV grid.
a. One generator without remote decoupling capabilities.
b. One generator with remote decoupling capabilities.
c. More than one generators without decoupling capabilities.
d. More than one generators with remote decoupling capabilities.
e. Production plant with more than one generators without central switch
f. Production plant with more than one generators with central switch
g. Typical installation with internal MV grid
The coupling and decoupling mechanisms must ensure the following:
i. Manual coupling-decoupling ability of the installation from the grid or parts of the grid
ii. The automatic grid isolation of the installation or parts of the installation, in case of short
circuit fault caused by the grid or the installation
iii. The prevention of abnormal operation modes and faults of the installation, in case of grid
disturbances (voltage dips and restoration)
iv. The avoidance of isolated operation of the installation and the part of the grid that is isolated
from the rest of the system (islanding), or of the installation alone if this is not refereed to
its design
v. The limitation of the unnecessary disconnects of the installation from the grid, which
disconnect, except from financial impacts in the producer, it can also create stability issues
in the system if the distributed generation is extensive.
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Figure 18: Methods of connection to MV grid
59
Chapter 5 – Energy Storage Methods Comparison
5.1 Introduction
Having analyzed the various energy storage systems and the electric power systems principles in
the previous chapters, we will attempt to sum up and compare the energy storage systems’
characteristics according to the parameters described in chapters 1.2, 1.4.1 and 1.4.2 and the needs
of a grid according to chapter 6.9.
Table 6 below summarizes the energy storage technologies used in various grid issues.
Category Applications Mature
Tech Potential Future
Tech
Bulk Storage
Load Leveling Hydro Flow Batteries
Spinning Reserve CAES
Hydrogen Peak Shaving/Valley Filling TES
Contingency Service Ni-Cd
Area Control Lead-Acid
Distributed Storage
Peak Shaving/Valley Filling CAES fuel cells
Investment Deferral Flywheels metal air
Load Following Lead-Acid SMES
DSM NaS Flow Batteries
Loss Reduction Ni-Cd Surface-CAES
Contingency Service TES
Black Start
Area Control
Power Quality
Power Quality Supercaps Li-ion
Intermittency Mitigation Lead-Acid NiMH
End-User Applications NaS SMES
Black Start Flywheels Zebra
Table 6: Summary of technologies used according to grid issues
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5.2 Rated Power and Discharge Time
The basic figure comparing the various ESS cannot be other than the actual power and energy an
ESS can provide. Given that energy is tied to time and power and following the demand a grid has,
one can decide of the ESS he can use. Figure 19 shows schematically the contents of Table 6 in
the previous paragraph.
Figure 19: Power VS Discharge Time (ESA)
In the power quality area, we see that high power supercapacitors, SMES, high power flywheels
and batteries are prevailing. Li-Ion, Ni-Cd and Lead-Acid batteries also suit for power quality but
could extend to the bridging power sector along with high energy supercapacitors and metal-air
batteries. Finally, regarding the energy management sector, NaS batteries and flow batteries are in
the mid-range. A special note can be given to PHES and CAES as they are considered bulk ESS
and can provide great power for long time.
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5.3 Energy Density
Figure 20 below shows the volume of energy density of various energy storage systems versus the
weight energy density. As we see, flywheels are the bulkiest ESS as they have the lowest weight
energy density at about 10-12 kWh / ton and are the means of storage that takes up the most space
as they can only provide 10-20 kWh / m3. From flywheels being the bulkiest and less dense ESS,
Metal-Air batteries are characterized the most energy dense ESS, with values ranging from
150kWh/ton to 600kWh/ton and 200kWh/m3 to 800kWh/m3.
Figure 20: Volume Energy Density VS Weight Energy Density (ESA)
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5.4 Cost per cycle
Figure 21 below shows the cost per cycle of the various ESS. As we see, the cost of PHES is the
lowest rating from 0.1 to 2 ¢/kWh. This can be explained by the maturity of this technology and
the medium used in order to store energy. CAES is following with a capital cost of 4 to 7 ¢/kWh
and is mostly associated with the gas turbines and gas itself in order to operate. Flow batteries need
a capital cost of 7 to 90 ¢/kWh, but the cost might be reduced when they are partially refurbished
to extend the life of the ESS. The majority of batteries like NaS, Li-Ion, Ni-Cd and Lead-Acid
have relatively high capital cost ranging from 9 to 40 ¢/kWh for NaS and 30 to 100 ¢/kWh for
Lead-Acid. Zinc-Air batteries are the most capital intensive with the cost being between 90 and
100 ¢/kWh mostly because their recharging abilities are limited.
Figure 21: Capital cost per Cycle in ¢/kWh produced (ESA)
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5.5 Lifetime - Efficiency
As seen in Figure 22, supercapacitors are the “pinnacle” of efficiency in terms of lifetime at 80%
Depth of Discharge. High efficiency is also present in SMES and Flywheels.
Figure 22: Efficiency VS lifetime at 80% DoD
In the mid-range efficiency of 65%-90%, is the majority of batteries with low to medium lifetime
and one can see that Li-ion batteries have the best efficiency with a lifetime of 1000 to 10000
cycles at 80% DoD. Sodium Nickel Chloride (Zebra) batteries have the highest efficiency but their
lifetime is limited between 2500 and 3000 cycles at 80% DoD. PHES usually have an efficiency
of 70% to 87% but their lifetime is pretty good between 12000 and 30000+ cycles at 80% DoD.
Similarly, Vanadium Redox Batteries have good lifetime that start at 10000 cycles and can go
beyond 13000 at 80% DoD.
In the low-medium efficiency range, one can see the Ni-MH and Ni-Cd batteries with 60%-66%
and 60%-70% efficiency respectively. But, Ni-Cd batteries seem to have longer lifetime than Ni-
MH; 1000-2500 cycles for the first and 200-1500 for the latter.
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Metal-Air batteries have the lowest efficiency (40%-60%) and the poorer lifetime (100-300
cycles).
Fuel Cells can have an efficiency of 40% to 70% and a lifetime of 1000 to 10000 cycles. This
range is mostly to cover the majority of fuels that can be used. Similarly, CAES can have an
efficiency of 40% to almost 80% with a lifetime of more than 30000 cycles. The efficiency of the
CAES, as in fuel cells, is closely related to the calorific value of the natural gas used and in the
temperature of the air in the turbine.
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5.6 ESS and Smart Grids
Almost every ESS can be used in smart grids. As seen in 1.3.2, the best solution seems to be a
HESS. Smart grids are continuously monitored and load is always fluctuating. On the other hand,
generation is either stable and used for base loads, or stochastic and used for variations in the load
but cannot be used more than 30% in the grid without means of energy storage.
An ideal HESS must be comprised with ESSs that each one can meet at least one of the
characteristics mentioned in Table 6. For example, a HESS containing a PHES, Lead-Acid
Batteries and Flywheels is a pretty stable hybrid system. The PHES can be used for bulk storage
since its discharge duration can be from 10 to 100 or even more hours. Lead-Acid batteries can
sustain the grid for some hours with faster response than a PHES and finally, flywheels can act at
μsec in order to provide the instantaneous stability the grid needs.
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5.7 Selection Guide
The tables below summarize the various ESS and their characteristics.
Power Rating
(MW)
Discharge
Duration
(h)
Self-
Discharge
per day
Suitable
Storage
Duration
Energy
Density
(Wh/kg)
Energy
Density
(Wh/L)
Power
Density
(W/kg)
Efficiency
(%)
Durability
(years)
Durability
(Cycles)
Capital cost
($/kW)
capital cost
($/kWh)
Tech
Maturity
(1-lower
to 5-
higher)
Availability
(%)
CAES 1-400 2-100 Small Hours-Months 30-60 3-6 40-80 20-100 30000+ 400-800 2-50 5 65-96
Flywheel 0.002-20 s-15m 100% seconds-minutes 5-130 20-80 400-1600 80-99 15-20 1000000 250-350 1000-5000 4 99.9+
Fuel Cell 0.000001-50 s-24+ Almost Zero Hours-Months 600-1200 500-3000 5-500 20-70 5-15 1000-10000 10000+ 6000-20000 2 90
Lead-Acid 0.001-50 h 0.1-0.3% Minutes-Days 30-50 50-80 75-300 70-92 5-15 500-1200 300-600 200-400 5 99.997
Li-ion 0.1-50 0.1-5 0.1-0.3% Minutes-Days 75-250 200-600 100-5000 85-90 5-20 1000-10000 1200-4000 600-2500 4 97+
Metal-Air 0.02-10 3-4 Very Small Hours-Months 110-3000 500-10000 40-60 100-300 100-250 Οκτ-60 1
NaS 0.05-34 5-8 ~20% Seconds-hours 150-240 150-240 150-230 75-90 15 2000-5000 1000-3000 300-500 4 99.98
Ni-Cd 0-46 s-h 0.2-0.6% Minutes-Days 50-75 60-150 150-230 60-70 5-20 1000-2500 500-1500 800-1500 4 99+
NiMH 0.01-Several MW s-h 30-110 140-435 250-2000 60-66 3-15 200-1500 4 99+
PHES 100-5000 10-100 Very Small Hours-Months 0.5-15 0.5-1.5 70-87 40-100 12000-30000+ 600-2000 5-100 5 95+
SMES 0.01-10 s Almost Zero Hours-Months 0.5-5 0.2-2.5 500-2000 85-99 20+ 100000+ 200-300 1000-10000 3 99.9+
Sodium Nickel
Chloride (Zebra) 0.001-1 min-8h ~15% Seconds-hours 100-140 150-280 130-245 ~90 8-14 2500-3000 150-300 100-200 4 99.9+
SuperCaps 0.001-10 s 20-40% Seconds-hours 0.05-30 10000+ 50-5000+ 97+ 20+ 1000000+ 100-300 300-2000 3 99.9+
TES 0.1-300 1-24+ 0.5-1% Minutes-Days 80-250 50-500 10-30 30-60 10-40 2000-14600 200-300 3-60 3-Απρ 90
VRB 0.005-1,5 s-8h Οκτ-75 15-33 65-85 10-20 13000+ 600-1500 150-1000 3 96-99
ZnBr 0.025-1 s-4h Small Hours-Months 60-85 30-60 50-150 75-80 5-20 ~2000 700-2500 150-1000 2 94
Table 7: Summary of ESS characteristics (Source: Compiled by the Author)
Mechanical Energy Storage
Chemical Energy Storage
Electrochemical Energy Storage
Batteries
Thermal Energy Storage
Table 8: Legend of technology categories for Table 7
67
The power rating column is not restrictive in the actual power of a plant/facility. It is mentioned
in MW in order to give a magnitude classification of the ESS.
How one should choose an ESS in order to be implemented in a grid should follow the steps
mentioned below:
1. Define the time that the ESS must cover the demand of the grid.
2. Define whether there is a geological formation that favors certain technologies.
3. Define the self-discharge margin of the ESS in terms of grid needs.
4. Define the durability that must be met in cycles and/or years.
5. Define the capital cost available for the investment.
In some cases the order of the selection is blurred mostly because of reasons closely tied with one
parameter. For example, if the budget is tight, one may choose a cheaper ESS while sacrificing the
discharge time. That way, the ESS selected will be cheaper but will not be able to meet the needs
of the grid.
5.7.1 Scenario for ESS selection steps
1. After extensive metering of the isolated area for over a year, the blackouts that are happening
have an average duration of 6 hours daily, thus the ESS must be able to cover this time frame.
From this first step, and always according to Table 7 above, we can safely exclude Flywheels, Li-
ion batteries, Metal-Air batteries, SMES, Super capacitors and ZnBr batteries with discharge
durations less than 6 hours.
2. There is no geological formation like caverns or elevation, so the PHES and the CAES are
excluded.
3. Because the blackouts are happening almost every day, the ESS must be able to maintain its
charging abilities for at least a day. Thus, NaS and Sodium Nickel Chloride batteries are excluded
because they have 15%-20% self-discharge per day, which is higher than the remaining
technologies.
4. Although Fuel Cells and Lead-Acid batteries have the same durability in years, Fuel Cells have
much higher durability in cycles; 1000-10000 instead of 500-1200 for Lead-Acid. Ni-Cd batteries
and VRB both have a maximum service period of 20 years but the cycle durability of VRB is
almost 5 times greater than Ni-Cd; max 2500 cycles for Ni-Cd and 13000+ for VRB. Thus, the
best candidates are the VRB and the Fuel Cells.
5. Between the VRB and the Fuel Cells, the most economical is the VRB, according to Table 7.
As seen, one must take into account the variables that define the area and the characteristics of the
ESS in order to be able to suitably install and operate an ESS.
68
Chapter 6 – Case Study; Tilos
6.1 Introduction
For the purposes of approaching a micro grid and try to implement a storage method, we will study
the island of Tilos. According to the Hellenic Statistical Authority (EL. STAT., 2015), by 2011,
Tilos has 780 permanent citizens and about 900 buildings. Tilos is part of the autonomous system
of Kos-Kalymnos that also include the islands of Leipsoi, Nisyros, Pserimos and Telendos. The
power plants are located in Kos and Kalymnos. The annual electricity need and peak for 2008 was
3.2GWh and 0.79MW or 790kW respectively (Karalis and Emmanouilidis, 2008) and the peak for
the measured period of 2015 is 600kW. The installed transformers capacity of Public Power
Company (PPC) is 3795kVA with a uniformed summer and winter load of 1500kVA. The single
line diagram (Figure 28) and the underwater cable route1 (Figure 29) can be seen in Appendix A.
The island of Tilos along with its position in the Aegean Sea are illustrated below in Figure 23.
Figure 23: The island of Tilos (left) and its position in the Aegean Sea (right) (Sources: Google
Maps and Wikipedia)
1 Courtesy of the Hellenic Navy Hydrographic Service. Do not replicate without their written permit.
69
6.2 Background of TILOS Project
TILOS stands for Technology Innovation for the Local Scale Optimum Integration of Battery
Energy Storage and it is a Horizon 2020 project.
According to the Hellenic Electricity Distribution Network Operator (HEDNO, 2015), the main
objective of TILOS will be the operation and development of a prototype battery system based on
NaNiCl2 batteries, provided with real time smart grid control system and will be able to co-op in:
Microgrid management
Maximum RES penetration
Grid Stability
Ancillary services to the main grid of Kos.
The battery system will support both grid connected operation and stand-alone operation, while
proving its flexibility to operate with the rest of the micro grid elements, such as Demand Side
Management and distributed generation.
TILOS project is also a multinational European demonstration and research project with 15
participating enterprises and Institutes from 7 European countries. The enterprises and institutions
are briefly mentioned in Table 9 below.
INDUSTRIAL / COMMERCIAL PARTNERS
FIAMM Energy Storage Solutions SRL (Italy)
SMA Solar Technologies AG (Germany)
Younicos AG (Germany)
EUNICE Laboratories SA (Greece)
Open Energi (United Kingdom)
RESEARCH / ACADEMIC PARTNERS
Commissariat a l’ Energie Atomique et aux Energies Alternatives (France)
Instituto Tecnologico de Canarias S.A. (Spain)
70
Piraeus University of Applied Sciences (T.E.I. of Peiraeus) (Greece)
University of East Anglia – Business School (United Kingdom)
Universite de Corse (France)
Rheinisch-Westfaelische Technische Hochschule Aachen (Germany)
Kungliga Technica Hogskolan (Sweden)
ISLAND GRID OPERATORS
Hellenic Electricity Distribution Network Operator S.A (Greece)
Schleswig-Holstein Netz AG (Germany)
NGOs
World Wide Fund for Nature – Greece (Greece)
Table 9: Commercial and Academic partners in TILOS project (Source: HEDNO, 2015)
71
6.3 Selection of the energy storage method
An energy storage system, in order to be successful, it must meet the requirements of the grid
design. Do we want the ESS to supply uninterruptable power for days or do we want it to regulate
the voltage and frequency failures in small interments of time? For the scenario under study, we
will try to implement a fast responding, high energy, low power ESS and a slower responding, low
energy, high power ESS. For the purposes of this study, the cost and other characteristics will not
be taken into account. These criteria are met by a flywheel and a flow battery system.
Dogo Island in Japan (Energy Central, 2005) is a small island just off the coast of Japan and can
be used as an example of how a flywheel EES can provide the stabilizing capability that is lacking
on many island power grids. In 2003, Fuji Electric installed a 200kW flywheel from Urenco Power
Technology in conjunction with an 1800kW installation of wind turbines to evaluate how wind
generators can be a viable source of power on remote islands with weak links to the mainland
power grid by smoothing their irregular power output. Four goals were met with the
implementation of a flywheel into this grid:
Stabilization of frequency stemming from the wind turbines
Capturing excess energy from short-term wind gusts
Optimization (or even eliminating the need) of operation of diesel generators on island
Elimination of the need for additional spinning reserve due to the introduction of WT.
The system operated from August, 2003 to June, 2004 and the results have been promising. By
acting both as a dynamic sink and source of energy, the flywheel improved the island’s power grid
efficiency and increased the penetration rate of the WTs. The flywheel’s ability to provide a
stabilizing capability to the highly variable WT power was found to be essential in allowing Fuji
Electric to connect WT to the island’s relatively weak electrical transmission system.
On the other hand, and according to IRENA, 2015, regarding a case study on flow batteries,
Prudent Energy supplied China’s Wind Power Research and Testing Center at Zhangbei with a
500kW/1MWh vanadium redox flow battery. It was commissioned in 2011 and is currently
operational. The system is used for ancillary services and time shifting of 78MW wind power and
640kW PV. The vanadium redox flow battery of this specific installation is given in the table
below:
72
Cycl
e L
ife
(full
-dep
th
char
ge
/
dis
char
ge)
Cycl
e L
ife
(par
tial
ch
arge
/
dis
char
ge)
Cal
endar
Lif
e
Fas
test
Res
ponse
Tim
e
Eff
icie
ncy
(D
C-
DC
)
Dim
ensi
ons
(250kW
Module
)
Dry
Wei
ght
(250kW
Module
)
Ele
ctro
lyte
Req
uir
ed p
er
Hour
of
Rat
ed
Dis
char
ge
10000 100000 10 years <50ms Up to
85%
9.3x2x2.8
(m) 13900kg 15.4m3
73
6.4 Electric design of Tilos
In order for one to proceed in installing an EES in a grid and especially in a weak islandic grid,
the current electric design must be taken into account. The Laboratory of Soft Energy Applications
and Environmental Protection of Piraeus University of Applied Sciences, as mentioned before is
an Academic / Research partner of TILOS project and has kindly provided the electric design of
Tilos island along with the measurements of power and energy of the island. The design can be
found in Appendix A and the measurements in Appendix B.
Until 1989, the energy needs of the islands were covered by a diesel plant located in the middle of
the island. This was replaced by a MV (20kV) underwater voltage cable running from the thermal
plant of Kos through Nisyros. The cable terminates at Plaka in NW Tilos from where electricity is
distributed throughout the island via wooden transmission poles and MV/LV transformers
(Vakkas, 2006).
The proposed location for our HESS is the new loop that has to be constructed in the area shown
in Figure 24 below. As seen in the single line diagram of the island in Appendix A, Figure 28, the
current system is radial. The loop to be created does not seem to interfere with the current system,
because it will only work if the island is in a blackout mode. The roles of the three transformers
will be mentioned later.
Figure 24: Proposed location for the ESS
74
This topology allows the island to operate smoothly even when switch 72 is open. The system
consists of two MV lines each one connected to the main island grid through a transformer for
each line and then connected to the HESS. The two main transformers proposed (XFMR 1 & 2)
are not the same because different parts of the island have different loads. The third transformer
(XFMR 3) could be installed if the HESS is to be supplying Nisiros and Kos with energy. Because
this requires a different approach and is not part of this dissertation, XFMR 3 is just illustrated for
showing that if a study is made regarding the demand needs of the other two islands, this particular
transformer could be installed in this specific location in order to minimize losses and deliver MV
current to the other islands.
75
6.5 Load Profile
The Laboratory of Soft Energy Applications and Environmental Protection (SEALab) of Piraeus
University of Applied Sciences also provided the load measurements of the island in two locations.
The first location is mentioned as M1 and the second is mentioned as M2. The time series of the
measurements is shown in Figure 25 below. Red shows the measurements in M1 and blue shows
the measurements in M2. Measurements have been recorded every ten minutes.
Figure 25: Load Measurements of Tilos island (SEAlab,2015)
As we see, there have been some blackouts in the island. In Appendix B, these days are magnified.
6.5.1 General Load Profile
Vakkas, 2006 states that the load peak was 400kW in August 1998 and in 2006 was 1MW for the
summer and 600kW for the winter. Karalis and Emmanouilidis, 2008 state that the peak load was
790kW. SEALab, estimated the peak load at 600kW.
0
60
120
180
240
300
360
420
480
540
600
04
.04
.15
06
.04
.15
08
.04
.15
10
.04
.15
12
.04
.15
14
.04
.15
16
.04
.15
18
.04
.15
20
.04
.15
22
.04
.15
24
.04
.15
26
.04
.15
28
.04
.15
30
.04
.15
02
.05
.15
04
.05
.15
06
.05
.15
08
.05
.15
10
.05
.15
12
.05
.15
14
.05
.15
16
.05
.15
18
.05
.15
20
.05
.15
22
.05
.15
24
.05
.15
26
.05
.15
28
.05
.15
30
.05
.15
01
.06
.15
03
.06
.15
05
.06
.15
07
.06
.15
09
.06
.15
11
.06
.15
12
.06
.15
14
.06
.15
16
.06
.15
Lo
ad
De
ma
nd
(k
W)
Date
Load Measurements_Tilos (4/4/2015 to 17/6/2015)
76
6.5.2 Load at April 16th, 2015
As it can be seen in Appendix B, during April 16th, 2015, the data recorder “lost” a measurement
at 12:00. Worst case scenario is the blackout occurred in 11:50:01 and power restored in 12:09:59,
supposing the measurement is taken exactly in time.
Time kVA kW
kVA kW
87% 87%
11:50
M1
245,1104 213,246
M2
99,24493 86,34309
12:00 0 0 0 0
12:10 353,4735 307,5219 132,752 115,4942
Table 10: Measurements at M1 and M2 at 16 April 2015
For the worst case scenario, the energy “lost” during the blackout time is calculated as the area of
the following figure:
Figure 26: Energy lost at M1 at 16 April 2015
The area is: lost
Power before blackout+Power after blackoutE *Time
2
Therefore, for M1 we have:
M1 M1 M1
lost lost lost
213.246 307.522 1198secE * E 260.384kW*0.33h E 85.93kWh
2 3600sec/ h
77
And for M2, accordingly:
M2 M2
lost lost
86.343 115.494 1198secE * E 33.3kWh
2 3600sec/ h
6.5.3 Load at April 25th – 26th, 2015
As it can be seen in Appendix B, in April 25th – 26th, 2015, the data recorder “lost” measurements
at 23:30 to 23:50 for the 25th of April and the first measurement of the 26th April at 00:00. Worst
case scenario is the blackout occurred in 23:20:01 and power restored in 00:09:59, supposing the
measurement is taken exactly in time.
Time kVA kW
kVA kW
87% 87%
23:20
M1
326,9494 284,446
M2
117.6295 102.3377
23:30 0 0 0 0
23:40 0 0 0 0
23:50 0 0 0 0
00:00 0 0 0 0
00:10 351,5389 305,8388 66.6609 57.99498
Table 11: Measurements at M1 and M2 at 25-26 April 2015
Following the steps at 6.5.2, the energy lost can be estimated as:
For M1: Energy lost= 245.55kWh and for M2: Energy lost= 66.7kWh
6.5.4 Load at April 30th, 2015
As it can be seen in Appendix B, in April 30th, 2015, the data recorder “lost” measurements at
06:20 and 10:50 for the 30th of April. Worst case scenario is the blackout occurred in 06:10:01 and
10:40:01 and power restored in 06:29:59 and 10:59:59 respectively, supposing the measurement
is taken exactly in time.
78
Time kVA kW
kVA kW
87% 87%
06:10
M1
217,88 189,56
M2
87,09 75,77
06:20 0 0 0 0
06:30 260,65 226,76 105,80 92,05
10:40 251,06 218,42 91,68 79,77
10:50 0 0 0 0
11:00 362,44 315,32 128,98 112,22
Table 12: Measurements at M1 and M2 at 30 April 2015
Following the steps at 6.5.2, the energy lost can be estimated as:
For M1: Energy lost= 68.69 kWh and 88.06kWh and for M2: Energy lost= 27.69kWh and
31.67kWh.
6.5.5 Load at May 12th, 2015
As it can be seen in Appendix B, for May 12th, 2015, the data recorder “lost” measurements at
11:40 for the 30th of April. Worst case scenario is the blackout occurred in 11:40:01 and power
restored in 12:09:59, supposing the measurement is taken exactly in time.
Time kVA kW
kVA kW
87% 87%
11:40
M1
351,14 305,50
M2
144,29 125,53
11:50 0 0 0 0
12:00 0 0 0 0
12:10 452,77 393,91 184,23 160,28
Table 13: Measurements at M1 and M2 at 12 May 2015
Following the steps at 6.5.2, the energy lost can be estimated as:
For M1: Energy lost= 171.35kWh and for M2: Energy lost= 70.02kWh.
79
6.5.6 Load at May 24th, 2015
As it can be seen in Appendix B, for May 24th, 2015, the data recorder “lost” measurements from
12:10 to 13:00 for the 24th of May. Worst case scenario is the blackout occurred in 12:00:01 and
power restored in 13:09:59, supposing the measurement is taken exactly in time.
Time kVA kW
kVA kW
87% 87%
12:00
M1
422,20 367,32
M2
157,20 136,77
12:10 0 0 0 0
12:20 0 0 0 0
12:30 0 0 0 0
12:40 0 0 0 0
12:50 0 0 0 0
13:00 0 0 0 0
13:10 528,289 459,6114 209,0938 181,9116
Table 14: Measurements at M1 and M2 at 24 May 2015
Following the steps at 6.5.2, the energy lost can be estimated as:
For M1: Energy lost= 479.62kWh and for M2: Energy lost= 184.83kWh.
6.5.7 Load at May 29th, 2015
As it can be seen in Appendix B, for May 29th, 2015, the data recorder “lost” measurements from
02:50 to 07:20 for the 29th of May. Worst case scenario is the blackout occurred in 02:50:01 and
power restored in 07:19:59, supposing the measurement is taken exactly in time.
Time kVA kW
kVA kW
87% 87%
02:50
M1
289,58 251,94
M2
121,90 106,05
03:00 0 0 0 0
… . . . .
07:10 0 0 0 0
07:20 488,26 424,79 200,88 174,76
Table 15: Measurements at M1 and M2 at 24 May 2015
Following the steps at 6.5.2, the energy lost can be estimated as:
For M1: Energy lost= 1522.64kWh and for M2: Energy lost= 631.82kWh.
80
6.5.8 Load Summary
Summarizing the results in Table 16, we see that the maximum energy need was almost 1.5MWh
and the minimum was about 69kWh for the whole island and the maximum energy need was about
632kWh and the minimum was about 28kWh for half the island.
According to Figure 25 and the measurements provided, the average load demand of the island
during the measured period was 276.89kW and the peak was 550.16kW. For half the island, the
average load demand was 110.52kW and the peak was 236.31kW.
Load Needs during Measured Period
M1 (kWh) M2 (kWh)
16.04.2015 85,93 33,3
25-26.04.2015 245,55 66,7
30.04.15 (1) 68,69 27,69
30.04.15 (2) 88,06 31,67
12.05.15 171,35 70,02
24.05.15 479,62 184,83
29.05.15 1522,64 631,82
Maximum 1522,64 631,82
Average 380,26 149,43
Minimum 68,69 27,69
Table 16: Energy Needs during Measured Period
81
6.6 Profile of the HESS
6.6.1 Introduction
The hybrid energy storage system will comprise of a flywheel and a flow battery system, as stated
in 6.3. The average time the island was not provided with energy was 80 minutes and the average
power provided, as we saw in 6.5.8 was 276.89kW for the island as a whole and 110.52kW for
half the island (leading to power needs of 166.37kW for the first half of the island), thus the energy
that the HESS must provide must be:
Energy=Time*Power Energy=80mins*276.89kW Energy=1.33 hrs*276.89kW
Energy 370kWh
6.6.2 Flywheel
The proposed flywheel can be a battery of flywheels. The proposed flywheel data sheet is shown
in Appendix C. This flywheel, according to its specifications, can provide a usable energy of
30kWh or 50kW for 35 minutes, configurable to 160kW for 5 minutes.
6.6.3 Flow Battery
The proposed flow battery is a VRB system providing, according to its specifications,
60kW / 300kWh. The data sheet can be found in Appendix C.
6.6.4 Transformers and equipment
As seen in chapter 4.12, certain measures should be taken into account when connecting a power
plant into the MV grid.
As for the transformers, they play a crucial part in any grid. As seen in 6.6.1, the proposed two
transformers must be able to cover 166.37kW and 110.52kW respectively in order to fully stabilize
the total power need of 276.89kW of the island. These transformers must step-up the voltage of
the HESS to 20kV in order to follow the MV principles of the current grid. The proposed
transformers are one of 160kVA and one of 250kVA. These ratings have been selected from the
commercially available transformer ratings of Schneider Electric. They have been slightly over
dimensioned in order not to operate in their nominal limits and because we must also bear in mind
82
that future expansions might be an issue. The technical leaflet can be found in Appendix D,
Transformer.
The proposed switchgear is a pair of Schneider Electric’s Gas Insulated Switchgear CBGS-O, and
the technical leaflet can be found in Appendix D, Switchgear. The rated voltage of the switchgear
is chosen to be 24kV.
For the circuit breakers, the proposed breaker is Schneider Electric’s Vacuum Circuit Breaker
Evolis with a rated voltage of 24kV. The technical leaflet can be found in Appendix D, Circuit
Breaker.
83
Chapter 7 - Discussion
The purpose of this dissertation is to compare the various ESS that can be used in smart grids. The
definition of smart grids is solid in the scientific community and is described as an intelligent
electricity network that is balancing all of the variables associated with dynamic load control
powered from an ever increasing variable of RES. This can be achieved by a bidirectional
communication between the consumer and the producer, thus making the transportation and
distribution network an active component in these types of grids. The conventional grid type only
utilizes the transportation and distribution grid as a passive element in order to provide consumers
with a fixed amount of energy that has to be consumed or otherwise there are going to be some
problems in the grid.
In order for a “balancing act” to be made simpler, small amounts of energy stored throughout the
grid should be introduced. Also, demand response techniques in households to dynamic loading
of transmission and distribution lines will come to a realization in a true smart grid design.
Having the context of small amounts of energy that can be stored throughout the grid, the ESS
have been presented with the majority, if not all, of their characteristics. International Energy
Agency (2014), provided the maturity map found below in Figure 27.
Figure 27: Technology Maturity Map (IEA, 2014)
84
This figure shows that the very mature technologies, like the PHES, can be used for bulk energy
storage and be able to provide great amounts of energy with great stability. The possible
malleability of new or upcoming technologies has led to various technical characteristics for each
technology. For example, SMES, which are still in the R&D phase, have an energy density ranging
from 0.5Wh/kg to 5Wh/kg while PHES have 0.5Wh/kg to 15Wh/kg. The difference is the storage
medium, where for the SMES is DC and it is closely related to the coil and cryogenics efficiency
of the system, while PHES is closely related to the volume of the medium and the available head.
Moreover, one can say that water is more easily manipulated, stored and used than DC.
In this context, the comparison made in this dissertation follows the international scientific reports
of many sources and summarizes the characteristics mentioned earlier.
Regarding the case study and the selection of this particular island, Tilos, we should mention again
that the island is to be autonomous by the TILOS Project, thus an ESS is feasible. Soft Energy
Applications Laboratory of Piraeus’ University of Applied Sciences, has already studied and
scheduled the implementation of a battery system in order to provide stability in the island. Our
proposal consists of a HESS including technologies that are not as mature as batteries, but it was
considered that this hybrid system will be able to further help the island both in voltage regulation
and system stability, as it seems to cover the high energy-low power and low energy-high power
aspects of the ESS. In order for this to functionally operate, it is imperative for the grid to be able
to support the needs of customers and simultaneously protect the grid from dips, surges and other
faults. The successfulness of the project will mark Tilos as the first, fully autonomous
Mediterranean island and pave the way for the implementation of these measures, technologies
and public opinion in other islands or small isolated communities in the mainland. The
implementation of the third transformer, as seen in Figure 24, Chapter 6.4, along with the
appropriate hardware, including the circuit breakers and the switchgear, and the feasible study of
an expansion of the currently presented HESS, might render the network of Kos-Nisiros-Tilos
truly independent and transform this case study of a micro grid to a smart medium grid. The HESS
must be coupled with more RES in this network in order to take advantage of the technologies
provided towards a sustainable future.
85
Chapter 8 - Conclusions
This dissertation carried out a comparison of ESS suitable for use in small and medium grids.
Table 7 summarizes the work that has been done in this dissertation and the selection guide
provides a step-by-step approach in order to define the limits or the capabilities or the needs that
must be met by any grid. Regarding smart grids, the best practice seems to be the implementation
of HESS along with smart metering and advanced electronics.
The current infrastructure of a grid plays a major role in the procedure towards implementing an
ESS. Fragility of these grids require a meticulous study of the current infrastructure so as to avoid
faults in this particular grid. Hardware accompanying the ESS have been studied in a broader
matter but they are in harmonization with the case study’s grid.
Concluding, an ESS has to be chosen taking in mind the needs of the grid (e.g. load) in the small
grid we studied along with the equipment mentioned, which equipment is not restrictive to the
ones mentioned, but serve the purposes of the wide commercial availability.
86
8.1 Future Work
Having seen the complexity of a smart grid, including advanced electronics and the bidirectional
flow of energy, possible future work would be the studying of these systems in order to take full
advantage of the current technologies to promote RES and energy autonomy of islands or regions.
Chapter 4 and Chapter 6.6 must be intertwined in order to make a full report on how an ESS can
be implemented into smart grids. Further analysis of the electric power system of the area under
investigation must be carried and the hardware that comprise this particular grid and the
installation must be also further analyzed.
Mathematical models and simulations along with flow analysis can be carried out in future work
to further investigate the stability of a smart grid. Financial reports can also be a field for future
work, along with public opinion’s outcomes regarding the installation of gear that, in some cases,
may seem “hostile”.
Summarizing and having covered the theoretical principles of ESS in smart grids, this dissertation
can be expanded in the actual controls and automations that are needed to fully, securely and
actively incorporate energy storage means to a smart grid.
Finally, an extensive study on the ageing of the grid can be carried out as a future work, with this
dissertation as a base, and analyze the factors that affect the lines and the junctions on the grid.
Having in mind that various climatological conditions play a role in the stability and the integrity
of a grid, along with frequent switching, the local network is burdened and is prone to faults. These
faults can be confronted with the rigid study of the current infrastructure and with certain steps to
be taken in order to lengthen the life of the grid and the equipment that tag along.
87
APPENDIX A
Figure 28: Electric single line diagram of Tilos Island
88
Figure 29: Underwater cable route ending in Tilos Island (Source: Hellenic Navy Hydrographic
Services, 2015)
89
APPENDIX B
April 16th, 2015
0
50
100
150
200
250
300
350
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
16.04.15 (M1)
0
20
40
60
80
100
120
140
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
16.04.15 (M2)
90
April 25th - 26th, 2015
0
50
100
150
200
250
300
350
400
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
25-26.04.15 (M1)
25.04.15 26.04.15
0
20
40
60
80
100
120
140
160
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
25-26.04.15 (M2)
25.04.15 26.04.15
91
April 30th, 2015
0
50
100
150
200
250
300
350
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
30.04.15 (M1)
0
20
40
60
80
100
120
140
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
30.04.15 (M2)
92
May 12th, 2015
0
50
100
150
200
250
300
350
400
450
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
12.05.12 (M1)
0
20
40
60
80
100
120
140
160
180
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
12.05.15 (M2)
93
May 24th, 2015
0
100
200
300
400
500
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
24.05.15 (M1)
0
20
40
60
80
100
120
140
160
180
200
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
24.05.15 (M2)
94
May 29th, 2015
0
50
100
150
200
250
300
350
400
450
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
29.05.15 (M1)
0
50
100
150
200
00:00 02:24 04:48 07:12 09:36 12:00 14:24 16:48 19:12 21:36 00:00
Po
we
r (k
W)
Time (HH:MM)
29.05.15 (M2)
95
APPENDIX C
96
97
98
APPENDIX D
Switchgear
99
Circuit Breaker
100
Transformer
101
102
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