Wind Energy Storages - Possibilities

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1 1 Wind Energy Storages - Possibilities Ervin Spahić, Gerd Balzer, Britta Hellmich and Wolfram Münch Abstract-- The volatility of wind power can cause large problems for power systems operation. To remedy the disadvantages of wind power generation different storage technologies can be applied. In this paper technical and economical analysis of possible wind energy storages has been made. According to technical parameters the applicable technologies have been selected. Afterwards, for selected storages economical analyses which take into account different scenarios of investment and electricity prices have been made. Obtained results suggested compressed air energy storages, batteries and pump hydro storages as very good solutions for wind energy storage. Due to lack of suitable places in Germany a pump hydro storage solution is less feasible. The largest disadvantage of the wind power production is its strong weather dependence. Due to sudden and large changes of wind speed the power output from a wind farm can have large fluctuations (Fig. 2.). Power (MW) Index Terms wind energy, storage, electricity market, feedin tariffs, costs. O I. INTRODUCTION NLY in 26 was installed 15.2 GW of wind generators worldwide, making the total of 74.2 GW of installed wind generation. This is a continuing trend as it can be seen from Fig. 1 [1]. Installed power (GW) ,6 1,2 13,6 17,4 23,9 31,1 39,4 47,6 59,1 74, year Fig. 1. Installed wind generation capacity in [1]. In last 1 years the installed capacity became ten times larger (Fig. 1). It is to expect that this growing trend remains and that the wind power will play very important role in the future. E. Spahić is with the Institute of Electrical Power Systems, Darmstadt University of Technology, Landgraf-Georg-Str. 4, Darmstadt, Germany ( ervin.spahic@eev.tu-darmstadt.de). G. Balzer is with the Institute of Electrical Power Systems, Darmstadt University of Technology, Landgraf-Georg-Str. 4, Darmstadt, Germany ( gerd.balzer@eev.tu-darmstadt.de). B. Hellmich is with the Institute of Electrical Power Systems, Darmstadt University of Technology, Landgraf-Georg-Str. 4, Darmstadt, Germany. W. Münch is with EnBW Energie Baden-Württemberg AG, Forschung, Entwicklung und Demonstration, 7618 Karlsruhe, Germany ( w.muench@enbw.com). Fig. 2. Wind penetration in EoN grid [2]. With the increased wind power penetration and sizes of the wind farms (offshore over 1 MW) their impact on the power system operation (stability, control, load flow ) will increase too. In case of large offshore wind farms these sudden changes can lead to the power system instability. Moreover, planning of the load flows for the next day and avoiding of the congestion in the system and possible wind energy curtailment will become very important issue. The feasible solutions for these problems can be: better wind forecast or introduction of wind energy storage. The wind forecast is, although much improved, still very complex and not very exact. A storage used as a power and energy buffer can smooth the power output fluctuations from a wind farm and remedy the volatility of wind energy. As a possible solutions for wind energy storage following technologies have been analysed: flywheels, capacitors, superconducting magnetic energy storages, batteries, compressed air energy storages, hydro pump stations and hydrogen. A thorough analysis of their technical and economical parameters has been made. From technical point of view the parameters like storage capacity, energy density, access time, life time have been evaluated. The economical analysis includes the possible future development (scenarios) of investment costs of the technology, market electricity prices and feed-in tariffs for wind energy (German case) [3]. In the economical analysis the possible use of the storage at the electricity market has been detailed. II. STORAGE TECHNOLOGIES The storage of electrical energy can take place in different way, i.e. it can be stored as chemical, mechanical or electrical energy. In the following, the main characteristics of flywheels, capacitors, superconducting magnetic energy

2 2 storages, batteries, compressed air energy storages, hydro pump stations and hydrogen will be made. A. Flywheel The energy is being stored with the help of a rotating mass (rotor) in a form of rotating energy. This rotor is over one motor-generator unit connected to the power grid. Conventional flywheels are made of metal (steel or titanium). By introducing new materials (glass or carbon fibre reinforced plastics) substantially higher peripheral speeds can be achieved and thus higher energy quantities can be stored. The self discharge rate of a flywheel is high and depending on design lies between 1% and 1% per hour. Therefore, the use of flywheels for longer period can not be recommended. Flywheels are characterised by long life time, high energy density, large maximum power output, short access time, high efficiency and small environment impact. B. Capacitors The capacitors are particularly characterised by their high power density. Double-layer capacitors, which are here considered, are due to their, compared to conventional capacitors, higher storage capacity also known as super capacitors. They can charge or discharge a large amount of power within very short access time of a few milliseconds. However, this power is only for a short time at disposal. The self discharge rate of the double-layer capacitors, which amounts to about 1% per day, leads to the fact that they are less suitable for a long-term storage. Also they still can not store large amounts of energy. C. Superconducting Magnetic Energy Storage (SMES) SMES store energy in the magnetic field of a coil. Within few milliseconds a very high power can be released. However, this is possible only for few seconds, i.e. they are not suitable for the energy storage yet. Frequent charging and discharging have no influence on their life time. In case of large SMES the resulting magnetic field can have an impact on the environment. The disadvantages are small energy density and stability problems for large systems caused by the strong magnetic field. D. Batteries Due to their very good technical characteristics (large energy density, fast access time, pulse power ) the sodium sulphur batteries (NAS) have been chosen as battery storage of the large amount of wind energy [4]. They can supply the system with a large amount of the power in a short time, or large amount of energy for a longer period. A higher power capacity can be achieved by connecting more modules. E. Pump Hydro Stations During the off-peak periods (nights) and weekends is water pumped from lower into upper basin and stored as potential energy. In peak load periods this water is used to drive the generator and produce electricity which is being fed into the system. The efficiency is app. 75%. These stations have a life time of app. 5 years. The largest problems for building these stations are: the lack of suitable places and the impact in the nature environment. F. Compressed Air Energy Storage (CAES) CAES use the electricity during off-peak periods and weekends to store and compress the air into the underground cavities (salt cavern, abandon mines, rock structures, artificial reservoirs ). At peak load times the compressed air is used to produce electrical energy. The efficiency is app. 42% to 52% (exhaust gases). Very actual and interesting are the adiabatic CAES which have additional heat storage. In this case the waste heat is stored in this additional heat storage and later used to warm up the compressed air. Therewith the efficiency of the CAES can be increased up to 7%. G. Hydrogen Hydrogen is being gained with the help of electrolysis and, afterwards, stored into a gas tank. When required, the fuel cells can use stored hydrogen and produced electricity which is supplied into the system. During the transformation the fuel cell develops pure water and no emissions are set free. They can store large amount of the power, but with low efficiency (25%). All relevant technical characteristics of considered technologies are summarised in Table I. TABLE I SUMMARISED OVERVIEW OF TECHNICAL CHARACTERISTICS OF STORAGES Flywheel Capacitors SMES NAS battery Storage capacity 2,5 MWh small 3 kwh several 1 MWh Pump hydro station 5 8 MWh Power capacity 25 MW large 1 MW several 1 MW 1 1 MW Energy density (kwh/m 3 ) CAES 5 25 MWh several 1 MW Hydrogen several 1 MWh several 1 MW 1 5 2, Cycle life time several Life time (years) Access time ms ms ms ms 1 3 Minuten 1 Minuten - Self discharging 1-1% /h 1% /day cooling power no no - - Efficiency (%) < / 7* 25 Power/Energy Environment impact 25 MW for 5 min or 5 MW for 3 min rated power for sec up to several min high power for several sec rated power for rated power for rated power rated power hours, very high long time for long time for long time power for minutes small medium small mediem high medium medium *adiabat III. CRITERIONS FOR THE COMPARISON AND SELECTION Different forms of energy and conversion have the consequence that the storage technologies have different characteristics (Table I). These characteristics lead to their different application possibilities. An overview of the most important technical requirements of the storages for power and energy applications is given in Table II. Power application: supplying large amount of power in a short time (seconds and minutes). Energy application: supplying large amount of energy for longer period (hours and days). It can be seen that depending on the application different requirements are important for the storage. For example, fast access time is more important for smoothening of the power output fluctuations (power application), than for peak shaving (energy application).

3 3 TABLE II REQUIREMENTS FOR THE STORAGES AND THEIR WEIGHTING Large storage capacity Large power capacity Power Energy Power density kw/m Energy density kwh/m Power gradient Cycle life time Calender life time Access time Low self discharging very important + important - less important Taking into account the requirements that storages have to satisfy for either power or energy application (Table II) and the characteristics of the individual storage technologies (Table I), it can be concluded that not all of them can be used for both of application. On one side, flywheels, capacitors, SMES and batteries have possibility to supply large amount of power in a very short time. That means they are suitable for power application. On the other side, CAES, hydro pump storages, hydrogen and, again, batteries can be used to store large amount of energy, i.e. for a longer time, i.e. suitable for energy application.. The size of the wind farm in study case is rated at 1 MW. Therefore, the main criterion for the storage technology was: the storage has to be able to supply rated power of the wind farm into the system for at least 15 minutes (primary and secondary control according to the UCTE [5]). Under these defaults NAS batteries, pump hydro storage, CAES and hydrogen will be considered for possible use. IV. ECONOMICAL ANALYSIS - FUTURE DEVELOPMENT Firstly the future development of the investment costs of the chosen storage technologies (battery, hydro, CAES and hydrogen) will be detailed. Afterwards, the development of the electricity market prices at the European Energy Exchange Market in Leipzig (EEX) will be introduced. The main reason for it was the idea to use the electricity market and to store the wind power generation at low electricity prices and to sell it at high electricity prices. Finally, the development of the feed-in tariffs for wind energy in German case is presented. A. Investment Costs For each technology several factors, which can affect future development of the costs have been considered: state of the art of the technology, increase/decrease of the demand for the technology, technical improvements (efficiency, life time, lower production costs ), commercialisation of the technology, position of other storage technologies (competition), environmental impact etc. Accordingly, for all selected technologies three scenarios up to 23 were developed: optimistic, middle and conservative. Only for pump hydro stations two scenarios have been considered. The values of the initial investment costs in 26 have been taken from the manufacturers data or estimated from the costs of previous projects. At figures 3-6 the scenarios of future development of the investment costs for all, in previous chapter, selected storage technologies have been presented (NAS battery, pump hydro stations, CAES and hydrogen) scenario 1 scenario 2 scenario 3 % year 23 Fig. 3. Scenarios of investment costs for NAS batteries up to 23. 1% 8% 6% 4% 2% The investment costs of NAS batteries in 26 were around 22 /kw [6]. Scenario 1 (best case) assumes the increase of demand for NAS batteries, improvements in production technology and expansion of the production. Up to 215 is expected strong decrease of the costs (more than 5%). Due to expected maturity of the technology slower decrease of the costs after 22 can be foreseen. On the other side, for scenario 3 (worst case) is expected the increase of the production up to 21, but due to more competitiveness of other battery technologies (e.g. Vanadium-Redox, Zink-Brom ) the demand for NAS batteries will not increase. Therefore only small decrease of the costs is expected (~7 in 23). The development of scenarios for future pump hydro stations is very difficult because of very small number of projects, large dependence on the countries (geography, worker costs ) etc. Here, the initial investment costs for 26 have been extracted from 13 previous project which have been implemented (Goldisthal - Germany, Bad Creek - USA etc.). Two developed scenarios of investment costs for pump hydro station have been presented at Fig. 4. The costs in 26 have been set at 6 /kw scenario 1 scenario 2 2% % year 23 Fig. 4. Scenarios of investment costs for pump hydro station up to % 16% 14% 12% 1% 8% 6% 4% 2% Previous projects have shown that the investment costs have an increasing tendency and, thereat, both of scenarios continue this trend. This is mainly the result of lack of suitable places and high environment and social costs. Therefore, scenario 1 assumes the increase of the costs of 4% up to 23, and scenario 2 the increase up to 8%.

4 4 Due to low efficiency of conventional CAES (Table I) and probable breakthrough of adiabatic CAES in the near future here have been introduced scenarios for both conventional CAES (scenario 1) and adiabatic CAES (scenarios 2 and 3). All scenarios have been presented at Fig scenario 1 (conv. CAES) scenario 2 (adiab. CAES) scenario 3 (adiab. CAES) 2% % year 23 Fig. 5. Scenarios of investment costs for CAES up to % 16% 14% 12% 1% 8% 6% 4% 2% Although actual demand for CAES storage increases, the conventional CAES have reached their technological maturity. Therefore, only small decrease in investment costs has been foreseen in scenario 1 (app. 8 in 23). On the other side, the higher initial investment costs for adiabatic CAES (8 and 9 /kw in 21) are expected to decrease more significant. Depending on the scenario, these costs are estimated at app. 55 /kw (scenario 2) and 7 /kw (scenario 3) in 23. The use of hydrogen for storage of large energy amount is still not commercial and the costs are very high. The initial investment costs in 26 are estimated at 9 /kw. At Fig. 6 three developed scenarios of future development of hydrogen technology have been given scenario 1 scenario 2 scenario 3 % year 23 Fig. 6. Scenarios of investment costs for hydrogen up to 23. 1% 8% 6% 4% 2% The future development of the costs depends mainly on the research results and on the global political trends on the use of hydrogen. Intensive research and technical innovations can lead to large decrease in investment costs (scenario 1), however the large costs drop can be expected after 22. Under these circumstances the price in 23 has been estimated at 4 /kw. Scenario 3 gives the possible price development in case of slow technology improvement and slower implementation of hydrogen in energy sector. This scenario assumes the breakthrough of hydrogen for energy storages after 23. Therefore, the investment costs in 23 are estimated at app. 8 /kw. B. Market Electricity Price The electricity is being traded at hourly and block contracts at European Energy Exchange in Leipzig (EEX). The indices Phelix base (average value for a whole day - 24h) and Phelix peak (average value from 8-2h) are daily determined [7]. Based on average values in 25, three different scenarios of future electricity price development up to 23 have been determined. Scenario I [7] assumes the rise of the electricity prices like following: period ,1% p.a., period 21-22,5% p.a. and 22-23,8% p.a. Scenarios II and III assume an annual increase of 2,5% and 3,5% respectively. At Fig. 7 all three scenarios of the electricity prices for EEX Phelix peak up to 23 have been presented. Price ( /MWh) scenario I scenario II scenario III year 23 Fig. 7. Scenarios of electricity price up to 23 - EEX Phelix peak. The highest forecasted peak price in 23 is for scenario III and amounts 217 /MWh. On the other side, the lowest price is for scenario I and it amounts 116 /MWh. It is obvious that these values have large discrepancy and small percentile changes can have large impact on the forecasted electricity price, especially for a large forecasted period. C. Feed-in Tariffs - German Case In order to stimulate building of new renewable power generation, some countries have introduced legislation which guarantees the prices for electricity produced from renewable sources and their priority over conventional power plants. Germany adopted in 2 the Renewably Energy Sources Act (Erneuerbare Energien Gesetz - EEG) [3], which has been revised in 24. Among other renewable sources like hydro, solar, biomass etc., the EEG regulates the feed-in tariffs for power produced from wind. It guarantees the payment for every produced kwh from the wind generators at given feed-in tariffs for 2 years after commissioning of the wind generator. The feed-in tariffs, which consist of base and increased fee, are different for onshore and offshore wind generation. These fees depend on the year of commission of the wind farm. The actual EEG defines the feed-in tariffs for the wind farms erected up to 22 - Table III.

5 5 TABLE III FEED-IN TARIFFS FOR ONSHORE AND OFFSHORE WIND POWER GENERATION IN GERMANY [3] Year of commission increased fee Onshore base fee increased fee Offshore base fee 26 8,36 5,28 9,1 6, ,19 5,17 9,1 6, ,3 5,7 8,92 6,7 29 7,87 4,97 8,74 5, ,71 4,87 8,57 5, ,56 4,77-5, ,41 4,67-5, ,26 4,58-5, ,11 4,49-5, ,97 4,4-5, ,83 4,31-5, ,69 4,22-5, ,56 4,14-4, ,43 4,6-4, ,3 3,98-4,76 The base fee sinks around 2% per year from 25 for onshore and from 28 for offshore wind generators (WG). For the offshore WG built after 21 there is no increased, i.e. only the base fee will be paid for produced electricity. Furthermore, the EEG defines that the increased fees will be paid for five years for onshore WG (for the rest of fifteen years only the base fee will be paid) and twelve years for offshore (for the rest of eight years only the base fee will be paid). At Fig. 8 one example of the guaranteed fees for onshore and offshore WG commissioned in 21 and 215, respectively, has been shown. c /kwh Onshore 21 Offshore 21 Onshore 215 Offshore year 235 Fig. 8. Feed-in tariffs for onshore and offshore wind generators built in 21 and German case. According to EEG, the feed-in tariff for the onshore wind generation built in 21 is 7,71 c /kwh for the first five years, and 4,87 c /kwh in next fifteen years (up to 23), making an average price of 5,58 c /kwh for a observed period of 2 years. If this WG will be built in 215, then the feed-in tariff for the first five years will be 6,97 c /kwh and 4,4 c /kwh in the following fefteen years, making an average price of 5,4 c /kwh. For an offshore wind farm built in 21, for the first twelve years the price of 8,57 c /kwh will be guaranteed. In the following eight years it will be 5,83 c /kwh making an average price of 7,47 c /kwh in twenty years. As already mentioned, the feed-in tariffs for offshore WG commissioned after 21 are considerably lower, i.e. for the offshore WG commissioned in 215 only the base fee of 5,27 c /kwh will be paid. Therefore, it is financial much more attractive to built an offshore WG before 211. However, the feed-in tariffs are sinking nominally every year for 2% and, especially, for the offshore WG to be built after 21 these prices are relatively low. Given price scenarios assume that electricity price will very soon be higher than feed-in tariffs (compare values at Fig. 7. and Table III). Thereat it this case is more profitable to sell the electricity at market prices than at feed-in tariffs. For the wind generators to be commissioned after 22 there are no feed-in tariffs, yet. The EEG is being revised according to the actual situation: new political goals driven by EU or German Government, public opinion, reached share of the renewable sources in the total electricity production etc. V. RESULTS OF THE ANALYSIS The analysis give the answer, is it possible, beside the power application, to use these storages on the electricity market and try to return their investment. Can this be done by storing the wind energy during off-peaks of electricity prices (nights and weekends) and selling it at the electricity market at much higher prices (peak). Is it possible to achieve financial benefits comparing peak price with feed-in tariffs price given by the government (Germany case) or compared to the base market price? If the result is positive, than is the storage solution economically plausible. For each of selected technologies a detailed analysis including all parameters from Chapter IV (investment and life time costs, electricity price and feed-in tariffs) have been made. The commission of the system (wind farm and storage) is set for 21, 215 and 22 and the time horizon is 2 years. As already said, the study case is one offshore wind farm with rated power of 1 MW and with 42 full-load hours pro year. The costs of storage technologies are based on a fully backup of wind farm, i.e. storages are also projected at 1 MW of installed power. Their investment costs are calculated according different scenarios which were developed in previous chapter. It has been assumed that 8% (this value can be even much higher as shown in [9]) of the produced electrical energy from the wind farm is stored in storage and than sold at EEX peak price. The rest of 2% is fed into the system at the feed-in tariffs. At Fig. 9 the results for commissioning years 21, 215 and 22 have been shown. In all of presented cases the investment costs scenario 2 has been considered. In brackets are given the electricity price scenarios I, II and III. The results for all four storage technologies (NAS battery, pump hydro, CAES and hydrogen) are presented. Mio NAS battery pump hydro station CAES hydrogen 21 (I) 21 (II) 21 (III) 215 (I) 215 (II) 215 (III) 22 (I) 22 (II) 22 (III) Fig. 9. Scenario: investment cost 2 and electricity price I, II and III for 21, 215 and 22 - NAS battery, pump hydro station, CAES and hydrogen.

6 6 The application of batteries, CAES and pump hydro stations is almost for all case positive, i.e. the building of these storages is financial plausible. Only for commissioning year 21 for price scenarios I and II the results are negative, i.e. storages are not plausible. In best case (price scenario III) the financial benefits when using storages in commissioning year 22 can be from app. 75 M (CAES) up to more than 9 M (battery). On the other hand, for the conservative price scenario I, the results are negative for 21. This is due to high feed-in tariffs in 21 for offshore wind farms. It is obvious that the use of hydrogen as storage is still too expensive for every case under given considerations. At Fig. 1 all results of different cases have been given as a range between best and worst case for battery, CAES and pump hydro stations commissioning years 21, 215 and 22. Mio Batterie Pump CAES Batterie Pump CAES Batterie Pump CAES Fig. 1. All scenarios in range best-worst case for 21, 215 and 22. The spans are similar, whereby the largest differences are to be recognized in case of the battery. It is shown that all scenarios are economically meaningful starting from 215. It can be said that the financial benefits depend mostly on the electricity price scenarios. The influence of investment costs scenarios is smaller. The life time of CAES and pump hydro stations is much longer than the life time of wind generators. Therefore, these storages can be also used after given time horizon of 2 years and, therewith, bring extra financial benefits. VI. CONCLUSION Technical analysis of different storage solutions made in this paper suggest NAS batteries, pump hydro stations, CAES and hydrogen can be used as energy storages for wind energy. For power applications NAS batteries, flywheels, SMES and capacitors are suitable. Performed economical analysis which took different investment and electricity price scenarios as well as feed-in tariffs showed that it is in principle possible to operate energy storage economically. The difference between the peak market price and feed-in tariffs (German case) or base market price is high enough for most of the cases to make these solutions plausible. NAS batteries and CAES showed the most promising results under given considerations. Due to high environment impact and no adequate place near to the connection points of the large offshore wind farms, building a pump hydro station is less likely. Due to very high investment costs, hydrogen solution is for the near future not economical plausible. VII. REFERENCES [1] Global Wind Energy Council. [Online]. Available: [2] Eon Netz: Wind report 25. [3] German Ministry for the Environment, Nature Conservation and Nuclear Safety: Renewable Energy Sources Act, Germany, 26. [4] E. Spahić and G. Balzer: "The Application of Batteries as a Backup of Large Wind Farms ", 6th Conference Large Scale Integration of Wind Power and Transmission Systems for Offshore Wind Farms, Delft, The Netherlands, October 26. [5] Union for the Co-ordination of Transmission of Electricity - UCTE: UCTE Operation Handbook, Brussels, Belgium. [6] NGK Insulators Ltd. Japan, [7] European Energy Exchange, Leipzig, Germany. [8] German Ministry of Economics and Technology: Development of energy markets up to 23, Berlin, Germany, Mai 25. [9] E. Spahić, G. Balzer and W. Munch, Windenergiespeicherung von großen Offshore Wind Parks mit Batterien, Energiewirtschaftliche Tagesfragen, to be published in June 27. VIII. BIOGRAPHIES Ervin Spahić received Dipl. Ing. degree and Magister degree in Electrical Engineering, Power Systems, from the University of Montenegro, in 1997 and 21 respectively. He was from research scientist at Univ. of Montenegro, Electrical Power System Chair. In he was the Chief of Laboratory at Department of Electrical Engineering, Univ. of Montenegro. Since 23 he is doing his PhD thesis at Darmstadt Univ. of Technology, Institute of Electrical Power Systems in the area of offshore wind farms and their connection to the power systems. His research interests are wind energy, HVDC, power system stability and control, wind energy storage, wind and electricity market. Gerd Balzer received the Dr.-Ing. degree in 1977 from Darmstadt Univ. of Technology, Germany. He was employee of BBC/ABB and the Head of the Department of Electrical Consultancy for 17 years. He joined the Darmstadt Univ. of Technology in 1994 and got a full professorship in the Department of Electrical Engineering and Information Technology. His main research interests include asset management and network planning. Prof. Balzer is the Head of the Institute of Electric Power Systems, a senior consultant of ABB and the Chairman of the IEC Working Group "Short circuit calculations". He is member of the VDE and CIGRE. Britta Hellmich was born in Nienburg/Weser, Germany in She is currently pursuing a Joint Master s degree in Electrical Engineering and Business Administration at Darmstadt Univ. of Technology, Germany. She specialised in the field of Electrical Power Systems and is primarily interested in Renewable Energies. Wolfram Münch finished undergraduate studies in physics, astronomy and mathematics at Heidelberg University, Germany. He received PhD in theoretical physics on turbulence research at Cambridge in 199. He was from 199 to 1998 research scientist in the new materials department at the DaimlerChrysler-Research Centre in Ulm focussing on materials for energy conversion devices. From 1998 to 21 he was the coach of the DaimlerChrysler Exchange Group in Stuttgart. He did his habilitation at Ulm University in 2. W. Münch is since 21 the Head of the research, development and demonstration department of EnBW Energie Baden-Württemberg AG in Karlsruhe.