Evaluation of available electricity storage technologies and the possible economic gain for Växjö Energi

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Evaluation of available electricity storage technologies and the possible economic gain for Växjö

Energi

Rasam Sheibeh

KTH

SKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT KTH Industrial Engineering

and Management

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Evaluation of available

electricity storage technologies and the possible economic gain

for Växjö Energi

Rasam Sheibeh

EXAMENSARBETE INOM HÅLLBAR ENERGITEKNIK PÅ PROGRAMMET CIVILINGENJÖR MASKINTEKNIK

Titel på Svenska: Analys av tillgängliga energilagringsteknologier och

lönsamhetsmöjligheten för Växjö Energi genom ellagring

Titel på engelska: Evaluation of available electricity storage technologies

and the possible economic gain for Växjö Energi

Handledare: Ning-Wei Chiu, KTH TRITA-ITM-EX 2021: 16

Handledare: Henrik Larsson, Växjö Energi Ev. Uppdragsgivare: Växjö Energi

Examinator: Ning-Wei Chiu, KTH

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Sammanfattning

Förnybara energikällor får en större andel av energiproduktionsteknikerna samtidigt som den svenska energimarknaden genomgår förändringar. Förnybara energikällor i energiproduktionssystemet ger mer volatilitet och prisfluktuationer som kan innebära både utmaningar och möjligheter. Svenska Kraftnät, den ansvariga myndigheten för säkerhet och stabilitet i det svenska överföringssystemet, hanterar utmaningarna relaterade till högre andel förnybara energikällor med mer frekvensstabiliserande lösningar men samtidigt styrs elpriserna av den fria marknaden som leds av NordPool.

Växjö Energi är ett statligt företag med energiproduktionsanläggning för kraftvärme som verkar inom SE4-området på elmarknaden. Eftersom SE4 är den region som drabbas mest av prisfluktuationerna, är Växjö Energi intresserad av att analysera möjligheten att öka deras vinst genom att använda tillgängliga energilagringsteknologier på marknaden för energibitrageapplikationer.

De tillgängliga energilagringslösningarna och de som är under utveckling har alla sina egna fördelar och nackdelar som detta projekt analyserar ur ett ekonomiskt-, tekniskt- och hållbarhetsperspektiv. Teknik som tryckluft, energilagring och vattenkraft är mer mogna och det finns mer information om dem samt mindre osäkerhet. Däremot, energilagringsystem såsom gravitationskraftmodul är ny vilket gör att den tillgängliga informationen är begränsad och följaktligen mer osäker.

Detta projekt har utvecklat en modell utifrån tidigare forskning i området, för att mäta högsta möjliga vinst för varje energilagringsteknik under en specifik tid genom ellagring.

Resultatet antyder att lagring av tryckluft, tyngdkraftsmodul och pumpad termisk ellagring är de intressanta teknikerna för vidare studier. Genom detta arbete visar vi att deras kostnader och eventuella intäkter är jämförbara. Vidare studier utifrån detta projekt är att studera de föreslagna teknikerna djupare med hänsyn till Växjö Energis förhållanden för mer detaljerade och tillförlitliga resultat.

Nyckelord: Energilagring, Energilagring, Växjö energi, Svenska Kraftnät, Monte Carlo, Tryckluftslagring, Gravity power-modul, Pumpad värmelagring.

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Abstract

As the renewable energy sources are finding more place in the energy generation technologies, the Swedish energy market is also undergoing transformations. Renewable energy sources in the energy generation system brings more volatility and price fluctuations which can mean challenges and opportunities. Svenska Kraftnät, as the authority responsible for safety and stability of Swedish transmission system, addresses the challenges with higher shares of renewable energy sources to some extent with more frequency stabilizing solutions but the electricity prices are controlled by free market which is led by NordPool.

Växjö Energi is a state-owned company with energy generation facility of combined heat and power, operating in SE4 area of electricity market. As SE4 is the region affected the most with the price fluctuations, Växjö Energi is interested in analyzing the possibility of increasing their profit by utilizing the available energy storage technologies in the market in long term energy storage applications.

The available energy storage solutions and the ones under development have each, their own pros and cons that this project attempts to go through from economical, technical, and sustainability perspective. Technologies such as compressed air energy storage and pumped hydro are more mature and there are more data available about them with less uncertainty.

However, technologies such as gravity power module are new and there is not much information so the uncertainty of data is higher.

A model has been developed in this project from earlier work of other researchers, to measure the highest possible profit for each energy storage technology in a specific price time series through electricity storage.

The result suggests the compressed air energy storage, gravity power module, and pumped thermal electricity storage are the interesting technologies for further study. We show through this work that their costs and possible revenues are comparable. The future work on this subject is to include the suggested technologies with more details and adaptation to Växjö Energi conditions for more detailed and reliable results.

Keywords: Energy storage, Energy storage, Växjö energi, Svenska Kraftnät, Monte Carlo, Compressed air storage, Gravity power module, Pumped thermal electricity storage.

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Foreword

This thesis report constitutes a final degree project of 30 ECTS, completing a master’s degree in Sustainable Energy Engineering and a five-year degree program in Mechanical engineering. The thesis was a task by Växjö Energi which is conducted at the department of Energy Technology, KTH.

First and foremost, I would like to express my gratitude to my supervisor at Växjö Energi, Henrik Larsson, for his invaluable guidance and help throughout the thesis process, and for giving me the opportunity to take part of such an interesting research project remotely.

I would like to gratefully acknowledge my examiner and supervisor Assistant professor Justin NingWei Chiu, for realizing this thesis project and helping through the project with his guidance, constant support.

Finally, from the bottom of my heart, I am grateful to everyone and organizations that helped me go through these 5 years program despite unique challenges such as distance classes and projects because of covid19 pandemic.

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Nomenclature Abbreviations

$

$/kWh ... US Dollar per kilo Watt hour A

aFRR ... Automated Frequency restoration reserves

AL-TES ... Aquiferous low temperature TES B

BOP ... Balance of plant C

CAES ... Compressed air energy storage CES ... Cryogenic energy storage E

e.g. ... For Example EES ... Electrical energy storage ESS ... energy storage systems F

FCR ... Frequency containment reserves FCR-D . are automatic products - Disturbance FCR-N ... are automatic products - Normal FES Flywheel Energy System, flywheel energy

storage

FRR ... Frequency restoration reserves G

GES ... 2.6.1.4 Gravity energy storage GPM ... Gravity power module H

h Hour(s) L

LAES ... Liquid air energy storage

LCOE ... levelized cost of electricity LCOS ... levelized cost of storage M

mFRR Manual frequency restoration reserves MW ... Mega Watt N

nn_bias ... Nearest neighbor bias O

O&M ... Operations and maintenance P

PCM ... phase Change Materials PCS ... the power conversion system PTES ... pumped thermal electricity storage R

rpm ... rounds per minute RR ... Replacement reserves RTIL ... Room Temperature Ionic Liquids S

SCES ... Super capacitor energy storage SU... Storage Unit T

T&D ... Transmission and distribution TEA ... techno-economic assessments TES ... Thermal energy storage V,W

V Volts, Volts

VEAB ... Växjö Energi AB Wh/kg ...Watt hour per kilogram

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Table of Contents

Sammanfattning ... v

Abstract ... vi

Foreword ... vii

Nomenclature ... viii

Abbreviations ... viii

Table of Contents ... ix

List of Figures ... xi

List of Tables ... xi

1 Introduction ... 13

1.1 Problem statement ... 13

1.2 Aims and objectives ... 13

1.3 Delimitation ... 13

1.4 Thesis outline ... 14

2 Literature review ... 14

2.1 Energy Policy of Sweden ... 14

2.2 Swedish electricity market and supply security ... 15

2.3 Electricity markets ... 17

2.4 Energy storage market ... 19

2.5 Växjö Energi AB (VEAB) ... 19

2.6 Energy storage technologies ... 19

3 Methodology ... 42

3.1 Original Models ... 43

3.2 The final model ... 46

3.3 Software ... 47

4 Results ... 48

4.1 Electricity price schemes ... 48

4.2 Comparison of the technologies ... 50

4.3 Result of the simulation ... 53

5 Discussion ... 55

5.1 Suitable technologies ... 55

5.2 Other factors ... 56

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5.3 Sensibility analysis ... 57

6 Conclusions ... 58

7 References ... 59

8 Appendix ... 63

8.1 The final model in MATLAB code ... 63

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List of Figures

Figure 1 Electricity generation by source, Sweden 1990 – 2018 (IEA, 2019) ...15

Figure 2 Reserve products in the Nordic power system (Svenska kraftnät, 2016) ... 17

Figure 3 Nordic bidding areas (Nord Pool, u.d.) ... 18

Figure 4 The market deman for energy storage in the US, by technology, 2016 – 2027 (Grand view research, 2020) ... 19

Figure 5 Categories of energy storage technologies with examples ... 20

Figure 6 Energy storage technologies comparison (Zhibin Zhou, 2013)... 21

Figure 7 Energy storage worldwide installed capacity (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019)... 22

Figure 8 Number of projects in operation by storage type for different services in the world (Md Mustafizur Rahman, 2020) ... 23

Figure 9 Comparison of EES technologies according to energy and power density (S. Kalaiselvam, 2014) ... 23

Figure 10 Generalized total life cycle cost for ESS’s. ... 24

Figure 11 An example of hydro energy storage ... 25

Figure 12 Flywheel energy storage system (S. Kalaiselvam, 2014) ... 26

Figure 13 Schematic of CAES plant (S. Kalaiselvam, 2014) ... 29

Figure 14 illustration of gravity storage system (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019)... 30

Figure 15 Schematic view of a capacitor storage system (S. Kalaiselvam, 2014) ... 32

Figure 16 Schematic illustration of the SMES system (S. Kalaiselvam, 2014) ... 33

Figure 17 Schematic diagram of a NaS battery system (S. Kalaiselvam, 2014) ... 34

Figure 18 Schematic overview of a flow battery energy storage (S. Kalaiselvam, 2014) ... 35

Figure 19 Hydrogen fuel cell (H. Chen, 2009) ... 38

Figure 20 Schematic diagram of CES (H. Chen, 2009) ... 41

Figure 21 state of charge of a device with self-discharge constant of 168 hours ... 44

Figure 22 flowchart of the original Monte-Carlo model (E. Barbour, 2012) ... 45

Figure 23 The original flowchart of the linear optimization model (D. Connolly, 2011) ... 46

Figure 24 The average of hourly electricity prices in each day of a week (2013 to 2020) ... 48

Figure 25 The average of hourly electricity prices in a day (2013 to 2019) ... 48

Figure 26 the differences between the max and min electricity prices in each day of the period of 2013 to 2019 ... 49

Figure 27 Average differences between high and lows in one day ... 49

Figure 28 The profitability of each technology (Profit per energy output) ... 54

Figure 29 Comparison of the cost of each technology with their average profit per energy output ... 55

Figure 30 Comparison of the cost of CAES, GPM, and PTES with its average profit per energy output ... 55

List of Tables

Table 1 Rated installed capacity (GW) for different categories of energy storage around the world (Md Mustafizur Rahman, 2020) ... 22

Table 2 Technical parameters of flywheel technology (Md Mustafizur Rahman, 2020) ... 27

Table 3 Detailed cost items of FES technology (Md Mustafizur Rahman, 2020) ... 27

Table 4 Technical parameters of CAES system (Md Mustafizur Rahman, 2020) ... 29

Table 5 Detailed cost items of CAES technology (Md Mustafizur Rahman, 2020) ... 30

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Table 6 GPM technical characteristics (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019)

... 31

Table 7 Technical characteristics of CES and SCES (B. Zakeri, 2015) ... 32

Table 8 Comparison between different technologies of flow batteries (S. Kalaiselvam, 2014) ... 35

Table 9 Comparison of some well-known integrated batteries (S. Kalaiselvam, 2014) ... 36

Table 10 Technical characteristics of some integrated batteries (S. Kalaiselvam, 2014) ... 37

Table 11 price component of some batteries (Md Mustafizur Rahman, 2020) ... 37

Table 12 Cost components of power-to-power hydrogen energy storage ... 39

Table 13 characteristics of some thermal electricity storage technologies (A. Benato, 2018) 42 Table 14 critical parameters to make combinations of and run the final model on ... 42

Table 15 examples of nn_bias and their effects ... 43

Table 16 Comparison between the Monte-Carlo and Linear optimization models ... 47

Table 17 Comparison of different energy storage systems ...51

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1 Introduction

Without storage, electricity has to be used right away when it is produced. Energy storage technologies, with different characteristics have abilities to store the electricity in different forms for later use. As the renewable energy sources are increasingly being used, the uncertainty in the electricity generation is becoming concerning. Energy storage as a solution can help stabilize the energy production and the energy demand.

In addition, the uncertainty in the electricity generation affects the electricity prices and leads to more volatility in the market. Växjö energy, as an energy producer, looks after increasing profit through taking advantage of volatile prices.

1.1 Problem statement

Without any storage possibility, the produced electricity is delivered right away. As the prices of electricity fluctuates, there is an idea to store electricity when the prices are lower to be able to sell that more expensive in peak times. Such an idea leads to higher profitability, especially for electricity producers such as Växjö Energi (VEAB). At the moment, VEAB has a warm water accumulator to regulate its energy production to some extent. This means that they are able to produce more electricity when the prices are high and store the extra produced heat in the accumulator and vice versa.

By finding an energy storage solution for VEAB case, it would be possible to take advantage of the electricity price fluctuations. We study here the possibility of gaining economic profit from electricity storage in electricity market in south of Sweden (SE4) by applying commercialized energy storage technologies.

The results are studied for both the economic and environmental sustainability considering the energy market future forecasts.

1.2 Aims and objectives

The aim of this project is to evaluate different energy storage technologies with focus on cost and profit estimations, capacity, efficiency, and future risk and possibilities. Likewise, to find out the possibility for profit out of storage of electricity in the current market for VEAB with the available energy storage technologies. The studied technologies are evaluated in following cases:

• 8 hours production, to remove the night production

• 58 hours production, to remove weekend and night production.

The base electricity production power rate of VEAB through the year is at 6MW at a minimum and is up to 60 MW in winter. Considering the on-site consumption and loss, this project analyzes the production power rate of 5 and 30MW.

1.3 Delimitation

The focus of this project is on electrical energy storage (EES) or other energy storage carriers that can be converted to electricity economically and conveniently. Accordingly, if any energy storage works with other medium than electricity, the process of converting electricity to that medium is included in the cost analysis.

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VEAB has already claimed that hydro energy storage is not feasible considering the geographical conditions and municipality rules, therefore, this solution is not analyzed in detail.

The application for the energy storage is bulk energy storage which means less frequent charge and discharge and higher power rates (in MW scale).

1.3.1 Contributions to research

Different energy storage solutions have been studied through the years for different applications. Furthermore, there are research that compare the energy storage technologies to each other in different aspects. This project gives a summary on these projects focusing on the cost, technicality, and sustainability of each technology.

The main application for the energy storages in this project is bulk energy storage which put limitations on characteristics of the energy storage systems. There have been few case studies to apply energy arbitrage considering special energy storage solutions in specific markets.

However, this thesis project, contributes with more holistic simulation over more energy storage solutions with focus on Swedish market (SE4) and recent price data. The conditions, then, are discussed for Växjö Energi case.

Even though the model used in this project is based on other projects, but the model has been developed for more abilities such as running for more capacities and more comparison applications.

1.4 Thesis outline

The thesis starts with reviewing the relevant literature. The focus on the literature review is on introducing the Swedish energy market and commercialized energy storage solutions. In the following, the thesis explains the methodology and how the analysis is conducted. Under methodology, the development of the simulation model, and how it works is explained.

The result of the model and comparison of different technologies is presented in the result.

In the discussion section, the writer discusses the results and the observations. Moreover, the sensibility of the result is mentioned and discussed.

In the end, the conclusion is formulated and suggestions for future work.

2 Literature review

In this section, the relevant concepts and organizations are introduced and explained. The electricity market in Sweden and its security is addressed as well.

The energy storage market is explained later, while several energy storage technologies are introduced with focus on techno-economic assessments.

Techno-economic assessments (TEAs) of energy storage technologies refer to evaluating the performance of different technologies in terms of capital cost, life cycle cost, and levelized cost of energy (Md Mustafizur Rahman, 2020).

2.1 Energy Policy of Sweden

According to Swedish government, Sweden is targeting a 100 percent renewable electricity production by 2040 with an emphasis on a controlled transition (Government Offices of Sweden, 2019). In the same report, Swedish government adds that it is important to have a robust electricity system with a secure supply and low environmental impact.

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As IEA reports, Sweden is doing great in low carbon economy being the second-lowest CO2

emissions per capita and GDP among IEA member countries in 2017 (IEA, 2020).

Furthermore, Sweden is well on track with their energy agreement and climate framework, which is being a net-zero carbon economy by 2045. Sweden’s policy has been to get advantage of energy efficiency and switching to domestic renewable energy in order to meet the targets.

In the latest “energy in Sweden” report, the targets for energy supply in Sweden are (Swedish energy agency, 2019):

• 20 % more efficient energy consumption by 2020 compared to 2008

• At least 50 % renewable energy in the total energy use by 2020

• At least 10 % of energy use in transport sector shall be renewable by 2020

• 50 % more efficient energy consumption by 2030 compared to 2005

• 100 % renewable electricity production by 2040. This is not supposed to be an absolute deadline for nuclear.

2.2 Swedish electricity market and supply security

Electricity generation in Sweden, as Figure 1 shows, has mainly come from Hydro power and Nuclear power but the important trend is the rise of renewable sources excluding hydro power. Wind power and Biofuels has gone through a huge grow in Sweden in the last 10 years.

Swedish energy production transition to renewable alternatives, is serving the environment with less impacts compared to fossil fuels, even though each renewable energy technology may have some environmental impacts (Union of Concerned Scientists, 2013). This addresses the first concern of the government, meaning the lower environment impact.

The other concern of this energy transition is the secure supply of energy.

In this project, the focus is on electricity supply in Sweden. To understand the electricity supply in Sweden, the electricity supply chain has been studied in the following.

2.2.1 Electricity market as a system

A basic rule for an electricity system to function is that the production and consumption must have a constant balance (Svenska Kraftnät, 2017). The biggest challenge for a balanced electricity system is all the factors that can influence the consumption and production of

Figure 1 Electricity generation by source, Sweden 1990 – 2018 (IEA, 2019)

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electricity such as weather and unpredictable malfunctions. Also, as wind and solar are growing as sources of electricity, the production of electricity is becoming more variable and decentralized which requires a more flexible grid for a balance between supply and demand (Swedish energy agency, 2019).

Svenska Kraftnät as the system operator for electricity in Sweden has an overall responsibility keep the system in balance (Svenska Kraftnät, 2017). As Svenska Kraftnät explains, an electricity supplier has the obligation to produce the exact amount of electricity that their customers consume. The electricity supplier may take the responsibility themselves or transfer the responsibility to another company. The companies which take on the responsibility in the end are called balance responsibility parties (Svenska Kraftnät, 2017).

There must be balance in the system in each hour of the day and Svenska Kraftnät controls that in retrospect and all the over and underproductions of the balance responsibility parties will be economically regulated. This process is called balance settlement (Svenska Kraftnät, 2017).

As the frequency of the grid is an indicator of the balance in the grid, the highest permissible variation in normal situations is between 49.90 to 50.10 Hz and the goal is to keep it on 50 Hz (Svenska kraftnät, 2016). The goal is that the duration of frequency deviation outside the normal situation interval in a year be less than 10 000 minutes. Another factor to be controlled is the time deviation meaning the constant time the frequency has been lower or higher than the 50 Hz. The frequency target is higher prioritized than the time deviation (Svenska kraftnät, 2016).

To keep the balance of the system, different types of reserves are defined in the Nordic power system as it is illustrated in Figure 2. Frequency containment reserves (FCR) are automatic products planned to keep the frequency in the normal frequency zone and help in case of disturbances (Svenska kraftnät, 2016). The normal FCR (FCR-N) is supposed to balance the system in the normal zone with the target of 50 Hz and The disturbance FCR (FCR-D) balance the system In the time of disturbances when the frequency drops lower than the normal frequency zone. Frequency restoration reserves (FRR) which exist both in automatic and manual are supposed to restore the frequency to 50 Hz and the activated FCR. The automated FRR (aFRR) is different from FCRs as it is controlled remotely while FCRs are locally controlled, also aFRR is supposed to bring the frequency back to 50 Hz while the FCRs stabilize the frequency. The benefit of aFRR over the manual FRR (mFRR) is that it has a faster response in addition to considering the congestions in the grid. On the other hand mFRR are the main balancing resources meaning that when they are active, they will replace all other reserves type (Svenska kraftnät, 2016). Replacement reserves (RR) restore the activated reserves to the state of readiness in case of new disturbances which are not used in the Nordic power system (Fingrid, u.d.) .

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2.3 Electricity markets

The electricity market in the Nordic countries is liberalized meaning that the power can be traded freely on the Nordic power exchange Nord Pool which is owned jointly by Swedish Svenska Kraftnät and Nordic and Baltic counterparts (Svenska Kraftnät, 2017). Nord Pool is the marketplace for trading electricity in the Nordic market. It delivers both day-ahead and intraday trading to enable for longer production planning and shorter ones (Svenska Kraftnät, 2017).

The prices are determined by the availability of electricity and the amount of electricity used at a particular time. The higher the electricity supply is the lower the prices would be and vice versa. This relation would be the other way around for demand meaning that a higher demand would result in higher prices. Svenska Kraftnät adds that there are 15 Geographically divided areas in the Nordic system called bidding areas. The electricity is priced differently in each bidding area depending on the production and demand in that specific area and transition capacity between areas (Svenska Kraftnät, 2017). 4 out of these 15 bidding areas are in Sweden which are indicates by SE1, SE2, SE3, and SE4. The areas and their relative place are visible in Figure 3. In SE1 and SE2, north of Sweden, there is more electricity produced than needed resulting to cheaper electricity prices. In the contrary, in the south, the demand is higher than production capacity and therefore electricity is usually imported to the area (Svenska Kraftnät, 2017).

Figure 2 Reserve products in the Nordic power system (Svenska kraftnät, 2016)

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Nord Pool offers both Day-ahead trading and Intraday trading for electricity in Nordic market (Nord Pool, u.d.). The day-ahead trading is open all the hours of the year and it is possible to bid in different ways depending on the region such as single hourly blocks, block order, and minimum acceptance ration (Nord Pool, u.d.). The unit of the trades are MWh per hour. The day-ahead market is the main platform for trading physical energy which is also called Elspot (E. Lundin, 2020). More than 80 % of all electricity produced in Nordic region is traded on Elspot.

The intraday trading market is also open all the time and offers 15 minutes, 30 minutes, and hourly products in addition to block products in order to meet the flexibility needed in the market (Nord Pool, u.d.)

As mentioned earlier the Nordic market has been divided to different bidding areas and each bidding area has their own price calculated (Nord Pool, u.d.). However, there is a system price calculated as well as a reference for an unconstrained market clearing for Nordic region. In System price there is no consideration about the congestion restrictions by setting the capacities to infinity (Nord Pool, u.d.).

As a means for risk management, a financial market has also been defined for financial contracts which have a time horizon up to ten years with different covering period, for instance, hourly or quarterly (Nord Pool, u.d.) . For these contracts, the system price is being used as the reference. These contracts are settled with no physical delivery but cash (Nord Pool, u.d.). The financial market is hosted on Nasdaq commodities and not Nord Pool (Energiföretagen, 2018). Beyond all these means, it is also possible to issue bilateral contracts through Nasdaq Commodities (Energiföretagen, 2018).

To understand the price schemes for both daily and hourly, hourly prices data from the day- ahead market of Nord-Pool has been chosen (Nord Pool). SE4 bidding area is in focus however, other areas and system prices are available in the data for further studies. The data has been studied between 2013 to 2020.

Figure 3 Nordic bidding areas (Nord Pool, u.d.)

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2.4 Energy storage market

The increasing trend of renewable energy sources in the grid, and the mismatch of demand and supply is becoming a huge challenge due to intermittent nature of renewable sources (Fayaz Hussain, 2020). An article in “energy for sustainable development” journal claims that this issue can be overcome by energy storage concepts (Fayaz Hussain, 2020). The article adds that the integration of storage technologies has been proven to be useful for different part of electricity supply chain.

For instance, as Swedish National Grid (Svenska Kraftnät) estimates there is a need of 50 MW of constantly available control power for each GW of installed wind power which is planned to be supplied mainly by hydro storage (Abrahamsson, 2014).

The demand in global energy storage system market was 194 GW in 2019 with a growth rate expectation of 7.8 % from 2020 to 2027 (Grand view research, 2020). This demand growth is due to rapid industrialization, infrastructure development, and ongoing population growth.

Furthermore, the supportive policies from governments is estimated to favor the growth of the market.

2.5 Växjö Energi AB (VEAB)

Växjö Energi is an energy and communication company which is owned by Växjö municipality (VEAB, u.d.). They are responsible to provide district heating and electricity in their combined heat and power facility, Sandviksverket, which uses biofuel as the primary energy. VEAB is also responsible for development of district heating and through its affiliated company VEAB Elnät AB the accessibility of electricity through their electricity grid. VEAB also delivers fiber network (VEAB, u.d.).

In district heating area, VEAB’s advantages are for instance, offering renewable district heating locally for the customers with high delivery accuracy and a low environmental footprint (VEAB, u.d.). In 2019 became VEAB completely fossil-free in all their facilities (VEAB, u.d.). Sandviksverket is VEAB’s combined heat and power facility in the Växjö and there are local heating systems in Braås, Ingelstad, and Rottne.

In Sandviksverket, there are three main parts of Sandvik 1 to 3 (VEAB, u.d.). Sandvik 1 produces only hot water (25 MW heat) while Sandvik 2 and 3 produce both heat and electricity. Sandvik 3 has a capacity of 65 MW heat and 39 MW electricity while Sandvik 2 has a capacity of 65 MW heat and 35 MW electricity combined with 25 MW heat that comes from flue gas condensation. There are also reserve facilities with capacity of 45 MW heat in Sandvik 1, 40 MW heat in Täljstenen, and 45 MW heat in Teleborg.

Sandviksverket produced 232.5 GWh electricity in 2019 and 223.7 GWh in 2018 while heat production was 602.9 GWh and 615.3 GWh in the same order (VEAB, 2020).

2.6 Energy storage technologies

Energy storage means the conversion of energy to other forms for future use in an economically and convenient way (M.M. Islam, 2020). The output of energy storage solutions is mostly either thermal energy or electric energy (S. Kalaiselvam, 2014).

The energy for sustainable development journal divides the energy storage technologies by the form of energy stored, into 5 main categories as follows, chemical, electrochemical, electrical, mechanical, and thermal energy storage (TES) (M.M. Islam, 2020). These 5 categories have been illustrated in Figure 5 with the examples which will be studied in more detail in the following chapter of this project.

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Figure 5 Categories of energy storage technologies with examples

Another way of categorizing energy storage is by their applications. In the book “Thermal Energy Storage Technologies for sustainability” the technologies have been divided in 4 application areas (S. Kalaiselvam, 2014):

• Applications with low power requirement in isolated regions meant for urgent situations.

• Applications with medium power requirement in isolated regions for instance for individual electrical systems and town supply

• Applications with peak-load leveling

• Applications with power quality control.

Storage technologies

Mechanical

Hydropump power

Compressed air

Flywheel

Gravity power modul

Electrical

SMES

Condensers

Electrochemical

Integrated batteries

Flow batteries

Fuel cells

Chemical

Hydrogen

Synthetic natural gas

Other Chemicals

Thermal

Low- temperature TES

High- temperature TES

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Solutions for energy storage systems (ESS) are either focused on high power or high capacity, or both at the same time which are supposed to match these applications. The first two applications, small scale solutions, are well studied and solutions such as compressed air and hydrogen fuel cells have been optimized for such conditions. However, for the applications with peak-load leveling or power control in large scale solutions include technologies with higher capacities such as flywheel or hydraulic systems (gravitation energy) etc. (S.

Kalaiselvam, 2014). Figure 6 shows the power rating and storage capacity of some of the most known storage technologies.

The assessment of energy storage technologies article has collected data about the status of different types of energy storage technology categories in the world which is indicated in Table 1. The largest installed capacity of EES is in China with 32 GW followed by Japan (29 GW) and the US (24 GW) (Md Mustafizur Rahman, 2020).

Figure 6 Energy storage technologies comparison (Zhibin Zhou, 2013)

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Table 1 Rated installed capacity (GW) for different categories of energy storage around the world (Md Mustafizur Rahman, 2020)

Technology Operational Offline/under

repair Under construction Mechanical

storage1 166.20 0.28 5.95

Electrochemical 2.03 0.05 0.7

Chemical

storage2 0.01 - -

Thermal storage 3.21 0.21 0.12

To get a more detailed view on energy storage market status, Figure 7 shows the status of installed capacity of most popular ESS and how Pumped hydro storage stands for 98% of the amount of power stored (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019).

The application of energy storage systems in the power grid is huge for instance, electric energy time-shift, electric supply capacity, frequency and voltage support, and electricity bill management. Figure 8 has shown the number of different storage system operating for different purpose (Md Mustafizur Rahman, 2020). The purposes mentioned in Figure 8 can be summarized in three categories of (Zhibin Zhou, 2013)

1. Power quality which needs charge/discharge time of seconds to minutes to make sure of power quality such as frequency regulation

2. Bridging power which refers to ensuring the continuity of power supply specially during the switching sources or during black out; A storage of several minutes to hours is needed here

3. Energy management, meaning the decoupling of energy supply and consumption. In this application, the storage need is huge, often hours or days. The idea is to store cheaper energy and sell it during high-price peak hours. (Zhibin Zhou, 2013)

1 In this category it is only hydro pump, compressed air and flywheel storage systems being considered.

2 Only hydrogen storage systems have been considered.

Figure 7 Energy storage worldwide installed capacity (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019)

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Figure 8 Number of projects in operation by storage type for different services in the world (Md Mustafizur Rahman, 2020)

Another aspect of choosing the right energy storage technology is the size of the system or the energy/power density of the facility (S. Kalaiselvam, 2014). Figure 9 illustrate where each ESS technology belong to where it comes to energy and power density.

0 100 200 300 400 500 600 700

Electric energy timeshift Renewables capacity firming Electricity bill management Voltage support Electric supply reserve capacityspinning Grid-connected commercial Transportation services Grid-connected residential (reliability) Load following Black start Transmission support

Mechanical Electro-chemical Chemical Thermal

Figure 9 Comparison of EES technologies according to energy and power density (S. Kalaiselvam, 2014)

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The developed cost models in TEAs consist mainly of three components of the power conversion system (PCS), storage unit (SU) and balance of plant (BOP). PCS includes the power devices that regulate voltage, current and frequency based on the load pattern while SU is about the storage medium which for instance, is the storage tank in compressed air energy storage system. BOP, in the end, covers all the remaining parts from SU and PCS such as research and development, transportation and installation (Md Mustafizur Rahman, 2020).

The total life cycle cost of an ESS, Beyond the capital cost which consists of PCS, SU and BOP, includes replacement cost, operation, and maintenance (O&M and end-of-life cost as it is shown in Figure 10.

Figure 10 Generalized total life cycle cost for ESS’s.

As the writer in the “Assessment of energy storage technology” cover article claims, different studies choose different approaches to a TEA study of ESS´s. This means that some may focus on only some parts of the life cost cycle and not cover all the component (Md Mustafizur Rahman, 2020). There are examples of studies which ignore for instance, the replacement cost, and the writer’s point is that for batteries it is extra important to include this component.

Batteries have a limited number of cycles and must be replaced more often compared to other EES (Md Mustafizur Rahman, 2020).

Another way of reporting the cost of each technology is the levelized cost of electricity (LCOE) which refers to the price at which the electricity should be sold to cover the cost elements over an ESS’s lifetime. This method has also been called the levelized cost of storage (LCOS) for specifically storage technologies (Julch, 2016). LCOS or LCOE for energy storage technologies, is the net present value of all costs of the technology over the lifetime of the technology which is presented per annual energy output. The costs are mostly consisting of the capital expenditure, operation and maintenance and cost of input electricity (Julch, 2016).

B. Zakeri has assumed 250 to 300 cycles per year for the EES systems in purpose of bulk energy storage or energy arbitrage (B. Zakeri, 2015).

Concerning the comparison of potential profit of electricity energy storage systems through arbitrage, there are several models mentioned in the journal of energy storage (I.A.G. Wilson, 2018). The basic approaches are either a nondeterministic or a linear optimization. The writer claims that there is a computational speed benefit to the nondeterministic model, but further studies are needed to conclude which one is the most optimal (I.A.G. Wilson, 2018). The

Life cost cycle

Total capital cost

PCS

SU

BOP Replacement cost

Fixed and variable O&M

costs

End-of-life cost

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Monte Carlo model has been introduced thoroughly in the energy & environment science journal (E. Barbour, 2012) and the other approach, the linear optimization, is introduced in the energy policy journal (D. Connolly, 2011).

Both models can be found on GitHub3 under the name of “MonteCarlo-optimisation.m” and

“Find-optimisation.m” as codes for MATLAB.

2.6.1 Mechanical energy storage

In the book “Energy Storage”, mechanical energy has been defined as follows:

“The concepts of enabling kinetic and potential energies energy storage through a mechanical medium” (S. Kalaiselvam, 2014)

Mechanical energy storage is currently the most used category specially as pumped hydro storage systems. This category also includes flywheel and compressed air energy storage (M.M. Islam, 2020).

2.6.1.1 Hydro energy storage

The basic principle of Hydro energy storage is the conversion of kinetic energy of falling water to electricity. Water is pumped to higher heights when there is excess of power and when energy is needed the water would let go on turbines which is connected to a generator to produce electricity, this is indicated in Figure 11. The important factors in the amount of generated electricity are the amount of stored water and the height difference between the reservoirs, which is indicated by ΔH in Figure 11 (Fayaz Hussain, 2020).

Hydro energy storage or as it is described in the energy storage article, pump hydro energy storage (PHES) is the most mature large-scale energy storage technology available in the world (Fayaz Hussain, 2020).

The LCOE of PHES has been calculated for a real plant to about 0.02 – 0.27 $/kWh (M. Obi, 2017). The efficiency of plants being studies in the same paper has been around 80% (M. Obi, 2017).

3 https://github.com/EdwardBarbour/ArbitrageOptimisation Retrieved November 03, 2020 Figure 11 An example of hydro energy storage

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In an article, the power rate of PHES has been mentioned in between 100 to 5000 MW with a very negligible self-discharge which makes it suitable for longer periods of energy storage (H. Chen, 2009).

2.6.1.2 Flywheels (kinetic energy storage)

Flywheel energy system (FES), as the name suggests, stores energy as kinetic energy, rotational energy, to be retrieved as friction energy which is supposed to turn a generator (S.

Kalaiselvam, 2014).

The main component of the flywheel is the massive rotating cylinder which is supported using magnetic levitation to reduce mechanical wear. The flywheel is connected to an electric generator/motor to convert the mechanical energy to electricity or vice versa. The whole system keeps in a low pressure (or vacuum) atmosphere to reduce friction losses (S.

Kalaiselvam, 2014). A flywheel system is illustrated in Figure 12.

In times of cheaper electricity or existing excess of energy the motor charges the flywheel meaning that the cylinder starts turning faster and getting higher energy stored in it. When the energy is needed, the flywheel discharges by utilizing its kinetic energy and activating the electric generator (S. Kalaiselvam, 2014). The writer in the book “Energy Storage” claims that with proper power controls and power converters, it is possible to meet the needed energy effectively. The reason is that the amount of energy being stored in the system is directly proportional to the mass and square of the velocity (rotating speed) of the cylinder. The maximum storage capacity depends on the tensile strength of the flywheel material and shape and inertial effects of rotating components (S. Kalaiselvam, 2014). The maximum specific energy for flywheel system is defined as the energy density divided by the density of the rotating disk material (S. Kalaiselvam, 2014). Some advantages of FES include the independency to temperature and precise state of charge determination (A. Buchroithner, 2019). In the same article it has been mentioned that the most obvious disadvantage of FES’s is the self-discharging as it is also obvious in Table 2.

The available commercialized flywheels are divided to two types of the conventional steel rotor flywheel that are being used mostly for low speed cycling (<6000rpm) and the advanced composite flywheels which are for high speed operations (104 - 105 rpm) (S. Kalaiselvam, 2014). The conventional flywheels can achieve energy density of 5 – 30 Wh/kg while the high- speed flywheels can achieve higher much higher energy density up to 100 Wh/kg (Zhibin Zhou, 2013). In the book “Renewable and sustainable energy reviews” some of the reasons that the high-speed flywheels can be so effective are mentioned to be the low friction bearings (e.g. superconducting magnetic bearings) and composite rims which are lighter and much stronger than steel (Zhibin Zhou, 2013).

Figure 12 Flywheel energy storage system (S. Kalaiselvam, 2014)

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The best area to take advantage of flywheel technology in the grid is the frequency regulation, meaning where a large number of charge/discharge cycles are needed. In addition, it provides voltage regulation and general smoothing of short-term variations in demand and supply (Abrahamsson, 2014).

Table 1 shows some technical characteristics of flywheel technologies. The specific energy varies quite much as mentioned before, the most differences are between the two categories of the flywheels meaning the conventional and high-speed flywheels. The response time of less than 1 cycle comes from the application areas of flywheels, in the other word, transition of load. Transition happens both in small scale like heavy cars and big scale like the load in the grid (Schoenung, 2001).

Table 2 Technical parameters of flywheel technology (Md Mustafizur Rahman, 2020)

Rated power (MW) <10

Specific energy (Wh/kg) 5-100

Energy efficiency (%) 80-90

Discharge at rated capacity (h) 0.000-0.01

Response time <1 cycle

Lifetime (cycles) 104-107

self-discharge/day (%) 100

The flywheel energy storage (FES) is already applicable for short duration grid applications as MD Mustafizur Rahman claims (Md Mustafizur Rahman, 2020). FES is suitable for operations with over 5000 cycles per year with a storage duration of less than 30 minutes (Oliver Schmidt, 2019). The reported LCOEs in Rahman’s article are between $0.14-

$0.64/kWh (Md Mustafizur Rahman, 2020). He continues that the material cost of carbon fiber composite for high-speed flywheels is 20 times the cost of steel. In Table 3, the more detailed cost items of a FES system have been shown.

Table 3 Detailed cost items of FES technology (Md Mustafizur Rahman, 2020) Storage section

cost ($/kWh)

PCS cost

($/kW)

BOP cost

($/kW)

O&M cost ($/kW-year.)

FES 216-162 000 32-756 54-300 5-6

The USA is the only region in the world with a significant flywheel system (C. Doetsch, 2015).

It has 5 plants using FES. Hazel project is one of the biggest flywheel projects in the world with 20MW capacity (Beacon power, 2014). The project included 200 flywheels at 2.1 m tall and 0,9 m in diameter. A more than a ton rotor mass is supposed to spin up to 15 500 rpm in each of them. Max power rating for each entity is 100 kW, at 25 kWh charge and discharge.

2.6.1.3 Compressed air energy storage (CAES)

CAES system takes advantage of compressibility of air by compressing air to 40-70 bar and store it in a reservoir (usually an underground cavern) to be released, heated and expand later (discharge) through turbines for generating electricity (Zhibin Zhou, 2013). Figure 13 shows an example of CAES plant there, off-peak electricity is being used to compress the air so that it can be expanded through turbines in peak-demands. Compression of air creates heat in contrary to expansion where heat is needed. The needed heat is usually coming from primary energy sources for CAES, but another commercialized solution is to keep the heat from the compression process which is called adiabatic CAES (Seamus D. Garvey, 2016).

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There are different solutions for the storage tanks of CAES. Underground caverns are the cheapest solution while they put the geological limitation on the project; on the other hand, the compressed air can be stored in high pressure pipelines or above ground reservoir which even make the system to operate easier (Zhibin Zhou, 2013). An example of above ground solution is to store the compressed air in fabricated high-pressure tanks made of carbon-fiber (up to 300 bar). These solutions can be good for small- medium scale projects when the costs are economical for the project (Zhibin Zhou, 2013).

The underground storage for the CAES can be found in different ways such as (S. Kalaiselvam, 2014)

• Formation of excavating hard and impervious rocks

• Using dry mining or other technologies to make salt caverns

• Caverns made up of porous media reservoirs for instance, oil or depleted gas fields.

One of the oldest plants of CAES is in Huntorf, Germany with a capacity of 290 MW and a cavern of salt dome beneath the ground with capacity of 310 000 m3. The plant is operated on a daily cycle basis of 8 h charging and 2 h of power generation up to 290 MW. The plant has been functioned since 1978 with an availability of 90% (S. Kalaiselvam, 2014).

CAES technology can be under an adiabatic process as well. There are very few adiabatic plants functioning in the world at the moment (Md Mustafizur Rahman, 2020). In the article from M. Rahman, it is mentioned that a 660-kW adiabatic CAES plant in Toronto, Canada was the only adiabatic plant and another one with 5 MW/10 MWh was planned to be constructed and operate on 2020 (Md Mustafizur Rahman, 2020). Another article mentions the ADELE project on 2003 in Germany as an adiabatic CAES project with 70% cycle efficiency and output power of 200 MW (G. Caralis, 2019).

Some of the most important limitations of CAES are listed as (S. Kalaiselvam, 2014)

• Geographical requirements for the cavern

• the need for gas turbine plant as an undependable system for an effective functioning

• not being suitable to couple with other major power plants such as nuclear and wind turbine.

• The need for combustion of other primary energy sources or even fossil fuels which leads to larger amount emissions.

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Figure 13 Schematic of CAES plant (S. Kalaiselvam, 2014)

CAES is a proven technology and a cost-effective solution for large quantity of energy storage (Zhibin Zhou, 2013). In another article, CAES as the same as Hydro pump storage has been named as the best solutions for centered large-scale storage as they offer high power and energy capacity and geographical situations (Sérgio Faias, 2008). Table 4 shows some data on technical parameters of a CAES system. The numbers showed in the table are closer to the reality of the existing projects in 2020.

Table 4 Technical parameters of CAES system (Md Mustafizur Rahman, 2020)

Rated power (MW) 50 - 350

Specific energy (kW/kg) 30 - 60

Energy efficiency (%) 41 - 75

Discharge at rated capacity (h) 1 – 24+

Response time seconds - minutes

Lifetime (cycles) 10 000

self-discharge/day (%) small

CAES has relatively lower cost per unit energy than other ESSs (Md Mustafizur Rahman, 2020). About the choice of cavern solution, as it is mentioned earlier, the underground solution is usually cheaper but still the above ground tanks are easier to construct and implement. The underground facilities usually provide around 8 – 26 h of discharge while the number for above ground ones is about 2-4 h. In several studies, the adiabatic CAES has turned out to be around $0.05 more expensive per kWh in LCOE model, that is while the most expensive part of the adiabatic technology is the capital cost while for the conventional one it is the fuel cost (Md Mustafizur Rahman, 2020). Another study has listed several commercialized underground CAES projects, which shows higher efficiencies of more than 70% with LCOE of 30 to 133 $/MWh and lower efficiencies with LCOE of 16 to 61 $/MWh (M.

Obi, 2017). In another cover study the levelized price of CAES for bulk energy storage or

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arbitrage has been reported, in average, 134 €/MWh for underground and 159 €/MWh for above ground (B. Zakeri, 2015). The exchange rate of euro to Swedish “kronor” was 9.5 in 2015 which is the date for the last-mentioned study4.

A more detailed cost study of both on the ground and underground CAES has been shown in Table 5.

Table 5 Detailed cost items of CAES technology (Md Mustafizur Rahman, 2020) Storage section

cost ($/kWh)

PCS cost

($/kW)

BOP cost

($/kW)

O&M cost ($/kW-year.) CAES above the

ground

93 - 141 868 - 960 3 - 30 2 - 4

CAES under the ground

2 - 130 432 - 1674 3 - 30 2 - 5

2.6.1.4 Gravity power module energy storage (GPM)

GES is one of the modern solutions which are still under research and development (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019). The storage utilizes the gravity to store energy as PHES does. The advantages of this solution compared to PHES is, for instance, less water requirement and no geographical limitation. Figure 14 illustrates how the system looks like schematically. During the existence of excess power, the power house pumps the water in order to lift the pistons and during the peak hours the gravity cause the piston to fall and the turbine generates electricity (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019).

4 The exchange rate has been read from google finance service.

Figure 14 illustration of gravity storage system (Asmae Berrada PhD, Chapter 1 -

Energy storage, 2019)

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Table 6 shows some technical characteristics of GPM. It is important to mention that GPM is at concept stage (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019). In the chapter 2 of

“Gravity energy storage” book, the writer suggests a LCOE of 123 €/MWh which with exchange rate of 1.16 will be 143.5 $/MWh (Asmae Berrada PhD, Economic evolution and risk analysis of gravity energy storage, 2019).

In another study, GPM energy storage has been reported to have a discharge time of 1 to 4 hours with a low self-discharge rate (A. C. Ruoso, 2019). In the same study, GPM has suggested for large scale applications because of their favorable features.

Table 6 GPM technical characteristics (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019) Energy

density (Wh/kg)

Power density (Wh/kg)

Power rating (MW)

Storage duration

Lifetime (years)

Efficiency (%) 1.06 3.13 40 - 150 h - month 30+ 75 - 80

2.6.2 Electrical energy storage

Electrical energy storage systems refer to capacitors, supercapacitors, and superconducting magnetic energy storage (M.M. Islam, 2020).

2.6.2.1 Electrostatic energy storage

The principle of electrostatic energy storage is the stored electric charge between two metal or conductive plates which are separated by a insulation (S. Kalaiselvam, 2014). The storage capacity in this type is mainly dependent on the size of the metal plates, distance between the plates, and the material type of insulation. The stored energy is thus related to the capacity and the square of the applied voltage on the capacitor (S. Kalaiselvam, 2014). A schematic view of a capacitor is showed in Figure 15. In this figure it is indicated how different cells are built in parallel and how in each cell the plates stand and the insulator separates them (S.

Kalaiselvam, 2014).

The advantages of electrostatic storage are the instantaneous recharge capability and long cycle lifetime (S. Kalaiselvam, 2014). Therefore, capacitors are suitable for utility small-scale power control applications. On the other hand, the lower energy density makes it less applicable for large-scale usages.

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in supercapacitors (SCES), the insulation is replaced with an electrolyte ionic conductor in order to raise the energy density of the capacitor. In supercapacitors, despite of the high energy density, the voltage is per cell is limited to 2.7 V (S. Kalaiselvam, 2014).

Supercapacitors can reach approximately 50 – 100 kW of power output with possibility for higher energy storage as they can be interconnected. They have around 500 000 cycles per lifetime with a life expectancy of 12 years (S. Kalaiselvam, 2014). An application of supercapacitors can be to absorb the frequency fluctuations of renewable energy sources and improving the quality of energy generation (Zhibin Zhou, 2013). Also, a hybrid ESS such as battery and supercapacitor, there the supercapacitor controls the high-frequency fluctuations and let the battery deal with low-frequency fluctuations (Zhibin Zhou, 2013).

The huge drawback of supercapacitors is their high cost which is around five times that of conventional lead-acid batteries. In addition to that they have high self-discharge rate and very low energy density (5 Wh/kg) (S. Kalaiselvam, 2014).

The PCS cost of supercapacitors is between 108 – 864 ($/kW) and the BOP cost is between 11 – 108 ($/kW). The operation and maintenance costs are between 1 – 6 ($/kW-years) (Md Mustafizur Rahman, 2020)

Some technical features of capacitor energy storage and SCES are shown in Table 7 Table 7 Technical characteristics of CES and SCES (B. Zakeri, 2015)

Power range (kW)

Discharge time Overall efficiency

Storage durability

Self- discharge Capacitors <50 ms – 60 m 60 – 65% Seconds -

hours

40%

SCES <300 ms – 60 m 85 – 95% Seconds -

hours

20 – 40%

Figure 15 Schematic view of a capacitor storage system (S. Kalaiselvam, 2014)

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2.6.2.2 Superconducting magnetic energy storage (SMES)

In SMES, the electrical energy is stored in a magnetic field by inducting the DC into superconducting coil cables. Superconducting coil cables are usually made of niobiumtitane (NbTi) which is subjected to very low temperature (-270 °C) and has almost zero resistance.

The biggest losses of the system are associated with the refrigeration and the resistive losses occurring in solid-state; but still the efficiency of SMES is considered very high in commercial applications of MW capacity (S. Kalaiselvam, 2014). The energy stored in SMES systems is proportional to the inductance of the coil and square of the current (H. Chen, 2009)

Figure 16 illustrates a SMES system including the refrigeration unit. SMES systems have rapid response for charge/discharge requests and the available energy is independent of the discharge rate (Schoenung, 2001). Their energy efficiency is around 97% (Schoenung, 2001).

The response time of SMES systems is limited only by the energy release rate and the switching operation of the power electronics (S. Kalaiselvam, 2014). These features make SMES suitable for constant full cycling and continues mode of operation (Schoenung, 2001).

SMES is a good solution for voltage stability and power quality problems for large-scale customers. The power rating of SMES is 1 – 10 MW but in storage time of seconds even though there are researches ongoing for higher power ratings and longer storage time (minutes) (Schoenung, 2001). Commercially available SMES systems have an energy content up to 1 kWh (S. Kalaiselvam, 2014). Although SMES systems, in comparison to other ESS, have high life expectancy and cycling time, because of the high-cost factor, their development for utility power/energy management and load leveling applications is limited (S. Kalaiselvam, 2014).

Schoenung refers even to the environmental problem of SMES considering the strong magnetic field (Schoenung, 2001).

In the energy storage article, it has been mentioned that SMES also have a very low self- discharge in addition to less than 100 ms in respond time but the technology is still expensive and applicable only for short time storages (Asmae Berrada PhD, Chapter 1 - Energy storage, 2019)

Figure 16 Schematic illustration of the SMES system (S. Kalaiselvam, 2014)

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2.6.3 Electrochemical and chemical energy storage

In chemical energy storage, a secondary energy carrier is being used to store the energy for instance, hydrogen or synthetic natural gas (M.M. Islam, 2020). In electrochemical category, the chemical energy is converted to electrical energy through some chemical reactions.

Examples of electrochemical energy storage includes batteries, electrochemical capacitors, fuel cells, etc. (M.M. Islam, 2020).

2.6.3.1 Batteries

A battery always consists of two electrochemical cells which represent the positive (anode) and negative (cathode) electrodes. Each electrochemical cell can be filled with liquid, paste or solid electrolyte. During the discharging of the battery, the chemical reactions lead to a current of electrons through external circuit and in order to charge the batteries, an external extra voltage applies to the battery which enables the charging (S. Kalaiselvam, 2014).

Batteries are divided to two major categories of integrated energy storage which includes for instance, Pb-acid batteries, NiCd batteries, and Li-ion batteries and the external energy storage systems or flow batteries. Figure 17 shows a NaS battery as an example of an integrated battery system. In integrated systems, the charging and discharging of the battery takes place within the active material of the battery system. In the integrated battery systems, changes to the capacity specifications are impossible as it is always depending on the charge/discharge power ability (S. Kalaiselvam, 2014). Integrated batteries are used mostly in regulating frequency and managing fluctuation demand. On the other hand, flow batteries are more applicable for long term energy storing. Flow batteries are consisting of two different liquid electrolytes which are contained in separate containers. The function by pumping the electrolytes to an electrochemical reactor to generate electricity through a redox reaction.

Separated electrolyte containers lead to flexibility of power and energy storage capacity.

Figure 18 shows a schematic overview of a flow battery (S. Kalaiselvam, 2014).

Figure 17 Schematic diagram of a NaS battery system (S. Kalaiselvam, 2014)

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Some advantages of flow batteries are (S. Kalaiselvam, 2014):

• High discharge rate up to 10 h.

• Flexible energy storage capacity

• Possible to combine with a larger system for a lower installation cost

• Possibility of complete discharging without any damage

• Relatively low self-discharging

• Long life and low O&M over a longer period of energy storage

Two major commercialized types of flow batteries are vanadium redox battery (VRB), and ZnBr flow batteries which their specifications can be read in Table 8. In the Energy storage article, it has been claimed that VRB is the most efficient of them (S. Kalaiselvam, 2014). Both types of fluid batteries are suitable for hours to months of storage period (B. Zakeri, 2015).

Some advantage of VRB batteries are low operating cost, high life expectancy, and safety.

Table 8 Comparison between different technologies of flow batteries (S. Kalaiselvam, 2014)

Technology VRB ZnBr

Efficiency (%) 85 75

Cycle life charge/discharge 13 000 2500

Capacity (MW) 0.5 – 100 0.05 - 1

Operation temp. (°C) 0 – 40 50

Energy density (Wh/kg) 30 50

Self-discharge Small Small

in general flow batteries have a medium energy density (40 – 70 Wh/kg) while they offer independent energy and power ratings, long service life and negligible self-discharge (S.

Figure 18 Schematic overview of a flow battery energy storage (S. Kalaiselvam, 2014)

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Kalaiselvam, 2014). Flow batteries are more suitable for large energy capacity applications (Zhibin Zhou, 2013).

Some of more known integrated batteries are Lead-acid, Nickel-based batteries, Lithium-ion and Sodium sulfur (NaS) batteries which are compared in Table 9 (S. Kalaiselvam, 2014).

Lead-acid batteries are the oldest version of rechargeable batteries (Zhibin Zhou, 2013). The characteristics mentioned in Table 9 make Lead-acid batteries an ideal candidate for long- term storage applications. In Nickel-based batteries, Nickel hydroxide functions as the positive electrode and other materials can be used for the negative electrode. As expected, there are different types of nickel-based batteries with nickel-cadmium (NiCd), nickel-metal hydride (NiMH) and nickel-zinc (NiZn) being the most popular ones. NiMH batteries, having twice energy density of Lead-acid batteries, are recyclable and their components are harmless to the environment. They are also flexible with the operating temperatures and high voltage operations. Lithium-ion batteries which because of the light weight and high energy density have become popular in portable electronic devices and medical devices. Sodium-sulfur batteries are considered the new promising high temperature battery technology meaning that they operate at over 300 °C. The structure of NaS batteries are quite simple as it is shown in Figure 17. These batteries should always be heated to keep the electrodes in molten state (Zhibin Zhou, 2013). Some other characteristics of these integrated batteries can be found in Table 10.

Table 9 Comparison of some well-known integrated batteries (S. Kalaiselvam, 2014)

Battery type Advantages Disadvantages

Lead-acid Low cost

Low self-discharge

Short cycle life (1200 – 1800 cycles)

Cycle life affected by depth of charge

Low energy density (about 40 Wh/kg)

Nickel-based Can be fully charged (3000 cycles)

Higher energy density (50 – 80 Wh/kg)

High cost, 10 times of Lead- acid battery

Lithium-ion High energy density (80-190 Wh/kg)

Very high efficiency Low self-discharge

Very high cost (900 – 1300

$/kWh)

Life cycle severely shortened by deep discharge

Require special overcharge protection circuit

Sodium sulfur (NaS) High efficiency

High energy density (100 Wh/kg)

No degradation for deep charge

Negligible self-discharge

Be heated in stand-by mode at 325 °C

Figure

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References

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