Pumped Hydroelectric Storage

Chapter 2Pumped Hydroelectric StorageChi-Jen YangDuke University, Durham, North Carolina 27708, USA. Tel. +1 919 9459075. E-mail address: cj.y@ContentsIntroductionPros and ConsHistorical DevelopmentsProspects4.1 Revival of Conventional Pumped Hydroelectricity Storage4.2 Alternative and Novel Pumped Hydroelectricity Storage designs4.3 Retrofits of existing PHS and conventional hydropower stationsReferencesAbstractPumped hydroelectric storage (PHS) is the most established technology for utility-scale electricity storage and has been commercially deployed since the 1890s. Since the 2000s, there have been revived interests in developing PHS facilities worldwide. Because most low-carbon electricity resources (for example, wind, solar, and nuclear) cannot flexibly adjust their output to match fluctuating power demands, there is an increasing need for bulk electricity storage due to increasing adoption of renewable energy. This chapter introduces the PHS technology, the pros and cons, its historical developments, and the prospect.Key Words:Pumped Hydroelectric Storage; Closed-loop; Pump-back; Peak Shaving1. Introduction:Pumped hydroelectric storage (PHS) is the most widely adopted utility-scale electricity storage technology. Furthermore, PHS provides the most mature and commercially1available solution to bulk electricity storage. It serves to stabilize the electricity grid through peak shaving, load balancing, frequency regulation, and reserve generation. Japan currently has the largest installed PHS capacity in the world [1], followed by China [2] and the United States [3]. China currently has the most aggressive plan to expand PHS installation, with 14 GW under construction and many more planned. China is expected to surpass Japan in installed PHS capacity by 2018. Table 1 shows the installed PHS capacities in major countries [2, 3, 4, 5].Table 1. Installed PHS capacities.2A PHS facility is typically equipped with pumps and generators connecting an upper and a lower reservoir (Figure 1). The pumps utilize relatively cheap electricity from the power grid during off-peak hours to move water from the lower reservoir to the upper one to store energy. During periods of high electricity demand (peak-hours), water is released from the upper reservoir to generate power at a higher price.3Figure 1. PHS Diagram.There are two main types of PHS facilities: (1) pure or off-stream PHS, which rely entirely on water that were previously pumped into an upper reservoir as the source of energy; (2) combined, hybrid, or pump-back PHS, which use both pumped water and natural stream flow water to generate power [4]. Off-stream PHS is sometimes also referred to as closed-loop systems. However, some may define closed-loop systems more strictly as being entirely isolated from natural ecosystems. The U.S. Federal Energy Regulatory Commission defines closed-loop pumped storage as projects that are not continuously connected to a naturally-flowing water feature [5].The efficiency of PHS varies quite significantly due to the long history of the technology and the long life of a facility. The round-trip efficiency (electricity generated divided by the electricity used to pump water) of facilities with older designs may be lower than60%, while a state-of-the-art PHS system may achieve over 80% efficiency.42. Pros and ConsBy storing electricity, PHS facilities can protect the power system from outages. Coupled with advanced power electronics, PHS systems can also reduce harmonic distortions, and eliminate voltage sags and surges. Among all kinds of power generators, those peak-load generators typically produce electricity at much higher costs than the base-load ones. PHS provides an alternative to peaking power by storing cheap base-load electricity and releasing it during peak hours.PHS facilities provide very large capacities of electricity, with low operation and maintenance cost, and high reliability. The levelized storage cost for electricity using PHS is usually much lower than other electricity storage technologies.There are several drawbacks to PHS technology. The deployment of PHS requires suitable terrains with significant elevation difference between the two reservoirs and significant amount of water resource. The construction of a PHS station typically takes many years, sometimes over a decade. Although the operation and maintenance cost is very low, there is a high upfront capital investment in civil construction, which can only be recouped over decades.Environmental impacts are also serious concerns and have caused many cancellations of proposed PHS projects. Conventional PHS construction sometimes involves damming a river to create a reservoir. Blocking natural water flows disrupt the aquatic ecosystem and the flooding of previously dry areas may destroy terrestrial wildlife habitats and significantly change the landscape. Pumping may also increase the water temperature and stir up sediments at the bottom of the reservoirs and deteriorate water quality. PHS operation may also trap and kill fish. There are technologies to mitigate the ecological impacts. Fish deterrent systems could be installed to minimize fish entrapment and reduce fish kill. The water intake and outlet could be designed to minimize the turbulence. An oxygen injection system could also compensate for the potential oxygen loss due to warming of the water because of pumping.In some cases, the PHS system may serve to stabilize water level and maintain water quality [6]. The potential impacts of PHS projects are site-specific and must be evaluated on a case-by-case basis. Governments usually require an environmental impact assessment before approving a PHS project. Most PHS facilities have good safety records. Nevertheless, the upper reservoir failure of Taum Sauk PHS station in the United States should be heeded as a reminder of its potential danger. On December 14, 2005, the upper reservoir of Taum Suak was overfilled, and the embankment was then overtopped and breached. The suddenly leased water washed away over one square kilometer of forest, uprooted all the trees and obliterated a house in its path. Although the Taum Sauk PHS station was later repaired5and brought back to operation in 2010, the reservoir failure incident should provide important lessons for future PHS design, construction and operation.3. Historical DevelopmentThe earliest PHS in the world appeared in the Alpine regions of Switzerland, Austria, and Italy in the 1890s. The earliest designs use separate pump impellers and turbine generators. Since the 1950s, a single reversible pump-turbine has become the dominant design for PHS [7]. The development of PHS remained relatively slow until the 1960s, when utilities in many countries began to envision a dominant role for nuclear power. Many PHS facilities were intended to complement nuclear power in providing peaking power.In the 1990s, the development of PHS significantly declined in many countries. Many factors may have contributed to the decline. Low natural gas prices during this period make gas turbines more competitive in providing peaking power than PHS.The earliest PHS facilities were built in Italy and Switzerland in the 1890s [7]. Before the 1950s, most of the PHS facilities were located in Europe. The United States completed its first PHS station in 1928. Japan built its first PHS in 1934 and China in 1968. Since the 1950s, the adoption of PHS has gradually spread all over the world. As of 2014, the U.S. DOE Global Energy Storage Database recorded over three hundred operating PHS stations with total capacity of 142 GW in 41 countries [8].The design and site selection for PHS facilities are greatly influenced by national policies. Japan, China, and the United States have the largest PHS capacities in the world. Table 2, 3, and 4 list the PHS facilities in Japan, China, and the United States. The distinctive policies and regulatory regimes for PHS in Japan, China, and the United States offer interesting contrasts and may reveal useful policy insights.The buildup of PHS capacities in Japan has been relatively steady over several decades. The Japanese power sector is mainly composed of vertically-integrated regional electric power utilities, which build, own and operate the PHS facilities. The vertically integrated power sector structure has provided a stable and predictable business environment that is favorable to the investments in PHS. The path of PHS development PHS in Japan is an epitome of PHS development worldwide. Before the early 1960s, PHS facilities were rare and small, mostly of hybrid design. The deployment started to accelerate since the 1960s and continued throughout the 1990s. Since the 1970s, pure/off-stream PHS has become the dominant design, which is likely a result of increased concerns for ecological impacts of the hybrid systems. In addition to having the world’s largest PHS capacities, Japan is also the world leader in employing seawater PHS and variable-speed PHS.6Table 2 PHS stations in Japan.Plant Name Rating/ CommissionPlant Name諸塚Morotsuka Miyazaki Prefecture Hybrid 50 1961畑薙第一Hatakenagi No.1 Shizuoka Prefecture Hybrid 137 1962三尾Mio Nagano Prefecture Hybrid 36 1963池原Ikehara Nara Prefecture Hybrid 350 1964穴内川Ananaigawa Kochi Prefecture Hybrid 13 1964城山Shiroyama Kanagawa Prefecture Pure 250 1965矢木沢Yagisawa Gunma Prefecture Hybrid 240 1965新成羽川Shinnaruhagawa Okayama Prefecture Hybrid 303 1968長野Nagano Fukui Prefecture Hybrid 220 1968蔭平Kagetaira Tokushima Prefecture Hybrid 47 1968安曇Azumi Nagano Prefecture Hybrid 623 1969高根第一Takane No.1 Gifu Prefecture Hybrid 340 1969水殿Midono Nagano Prefecture Hybrid 245 1969喜撰山Kisen'yama Kyoto Pure 466 1970新豊根Shintoyone Aichi Prefecture Hybrid 1 125 1972沼原Numappara Tochigi Prefecture Pure 675 1973奥多々良木Okutataragi Hyogo Prefecture Pure 1 932 1974新冠Niikappu Hokkaido Hybrid 200 1974大平Ohira Kumamoto Prefecture Pure 500 1975南原Namwon Hiroshima Prefecture Pure 620 1976馬瀬川第一Mazekawa No.1 Gifu Prefecture Hybrid 288 1976奥清津Futai Dam Niigata Prefecture Pure 1 000 1978新高瀬川Shin-Takasegawa Nagano Prefecture Hybrid 1 280 1979奥吉野Okuyoshino Nara Prefecture Pure 1 206 1980奥矢作第二Okuyahagi No.2 Aichi Prefecture Pure 780 1980奥矢作第一Okuyahagi No.1 Aichi Prefecture Pure 323 1980玉原Tamahara Gunma Prefecture Pure 1 200 1981本川Motokawa Kochi Prefecture Pure 615 1982第二沼沢Daini Numazawa Fukushima Prefecture Pure 460 1982高見Takami Hokkaido Hybrid 200 19837俣野川Matanoagawa Tottori Prefecture Pure 1 200 1986 天山Tenzan Saga Prefecture Pure 600 1986 今市Imaichi Tochigi Prefecture Pure 1 050 1988 下郷Shimogo Fukushima Prefecture Pure 1 000 1988 大河内Okawachi Hyogo Prefecture Pure 1 280 1992 奥美濃Okumino Gifu Prefecture Pure 1 500 1994 塩原Shiobara Tochigi Prefecture Pure 900 1994 奥清津第二Futai Dam No.2 Niigata Prefecture Pure 600 1996 葛野川Kazunogawa Yamanashi Prefecture Pure 1 200 1999沖縄やんばる海水揚水Okinawa SeawaterPumped Hydro Okinawa Prefecture Pure 30 1999神流川Kannagawa Gunma Prefecture Pure 940 2005 小丸川Omarugawa Miyazaki Prefecture Pure 1 200 2007 朱鞠内Shumarinai Hokkaido Hybrid 1 2013 京極Kyogoku Hokkaido Pure 200 2014China is a latecomer in the worldwide PHS deployment, but it is catching up quickly. With the largest PHS capacities planned and under construction, China will soon overtake Japan as the host of the largest PHS capacities in the world. Most of the PHS facilities in China are relative new, with large capacity and off-steam design.China’s regulatory regime for PHS has been through great changes in the past two decades. Before 2004, most of the PHS facilities in China were built by local governments and local grid companies with diverse pricing models. In 2002, China restructured its power sector by separating them into 2 state-owned grid companies and 5 power generation corporations. In 2004, the National Development and Reform Commission promulgated a regulation which specified that PHS stations are transmission facilities and should be constructed and managed by the grid companies, and that the construction and operation costs of PHS should be incorporated into the operation costs of the grid companies [9]. The decision to treat PHS as transmission facilities has contributed to the rapid expansion of PHS in China.Table 3 PHS stations in China.Plant Name (Chinese) Plant Name(English) Province TypeRating/(MW)CommissionYear8岗南Gangnan Hebei Hybrid 11 1968 密云Miyun Beijing Hybrid 22 1975 潘家口Panjiakou Hebei Hybrid 270 1992 寸塘口Cuntangkou Sichuan Pure 2 1992 广州一期Guangzhou Phase 1 Guangdong Pure 1,200 1994 十三陵Shisanling Beijing Pure 800 1997 羊卓雍湖Yangzhuoyong Tibet Pure 90 1997 溪口Xikou Zhejiang Pure 80 1998 天荒坪Tianhuangping Zhejiang Pure 1 800 2000 广州二期Guangzhou Phase 2 Guangdong Pure 1 200 2000 响洪甸Xianghongdian Anhui Hybrid 80 2000 天堂Tiantang Hubei Pure 70 2001 沙河Shahe Jiangsu Pure 100 2002 回龙Huilong Henan Pure 120 2005 桐柏TONGBAI Zhejiang Pure 1 200 2006 白山Hakusan Jilin Hybrid 300 2006 泰安Taian Shandong Pure 1 000 2007 琅琊山Langyashan Anhui Pure 600 2007 西龙池Xilongchi Shanxi Pure 1 200 2008 宜兴Yixing Jiangsu Pure 1 000 2008 张河湾Zhanghewan Hebei Pure 1 000 2008 惠州Huizhou Guangdong Pure 2 400 2009 黑麋峰一期Heimifeng Phase 1 Hunan Pure 1 200 2009 白莲河Bailianhe Hubei Pure 1 200 2010 宝泉Baoquan Henan Pure 1 200 2011 蒲石河Pushihe Liaoning Pure 1 200 2012 响水涧Xiangshuijian Anhui Pure 1 000 2012 仙游Xianyou Fujian Pure 1 200 2013 9Most of the PHS facilities in the United States were built in the 1970s and 1980s. Sincethe 1990s, the construction of PHS slowed down in the United States. Environmental concerns caused the cancellation of several PHS projects and significantly prolonged the permitting process. Power sector restructure also contributed to this slowdown. Duringthe 1990s, the United States started to restructure the power sector by separating generation from transmission. The nature of energy storage falls into the gray area between generation and transmission [10]. Because the net electricity output of PHS operation is negative, a PHS facility usually cannot qualify as a power generator.Although their crucial load-balancing and ancillary services to the grid and reduces the needs for transmission upgrades, PHS facilities are not recognized as parts of the transmission infrastructure [11]. This confusion in business models has deterred the development of PHS in the United States.Table 4 PHS stations in the United States.Rating/ CommissionFlatiron Larimer CO 9 1954 Hiwassee Dam Cherokee NC Hybrid 95 1956 Lewiston Niagara Niagara NY Hybrid 240 1962 Taum Sauk Reynolds MO Pure 408 1963 Yards Creek Warren NJ Pure 453 1965 Cabin Creek Clear Creek CO Pure 300 1967W R Gianelli Merced CA Pure 424 1967 Muddy Run Lancaster PA Pure 1 072 1967 ONeill Merced CA Pure 25 1968 Thermalito Butte CA Hybrid 83 1968 Edward C Hyatt Butte CA Hybrid 293 1968 Salina Mayes OK Pure 288 1970 FirstEnergy Seneca Warren PA Pure 469 1970 Smith Mountain Franklin VA Hybrid 247 1970 Mormon Flat Maricopa AZ Hybrid 54 1971 Horse Mesa Maricopa AZ Hybrid 100 1972 Degray Clark AR Hybrid 28 1972 Northfield Mountain Franklin MA Pure 940 1973 Ludington Mason MI Pure 1 979 1973 Blenheim Gilboa Schoharie NY Pure 1 000 1973 Jocassee Pickens SC Pure 612 197410Bear Swamp Berkshire MA Pure 600 1974 Castaic Los Angeles CA Hybrid 1 275 1976 Carters Murray GA Hybrid 250 1977 Fairfield Pumped Storage Fairfield SC Pure 511 1978 Raccoon Mountain Hamilton TN Pure 1 714 1979 Wallace Dam Hancock GA Hybrid 209 1980 Grand Coulee Grant WA Hybrid 314 1980 Harry Truman Benton MO Pure 161 1981 Mount Elbert Lake CO Pure 200 1983 Helms Pumped Storage Fresno CA Pure 1 053 1984 Clarence Cannon Ralls MO Hybrid 31 1984 Bath County Bath VA Pure 2 862 1985J S Eastwood Fresno CA Pure 200 1987 Bad Creek Oconee SC Pure 1 065 1991 Waddell Maricopa AZ Pure 40 1993 North Hollywood Los Angeles CA Hybrid 5 1993 Rocky Mountain Floyd GA Pure 848 1995 Richard B Russell Elbert GA Hybrid 328 2002 Lake Hodges San Diego CA Pure 42 2012 The diverged paths of PHS development in Japan, China, and the United States have shown that the national regulatory and institutional environments have tremendous impacts on the deployment of PHS. PHS facilities are large facilities that require huge upfront capital investments, and the paybacks are spread over many decades. If a government wishes to facilitate the development of PHS, it needs to provide a stable and predictable regulatory environment, and a reasonable pricing scheme that allow the PHS facilities to be compensated for their services to the transmission grid.4. ProspectsIn recent years, due to increasing concern for global warming and the call to de-carbonize electricity, there has been increasing commercial interest in PHS [12]. Developers are actively pursuing new PHS projects around the world.4.1 Revival of conventional PHSMore than 100 new PHS plants with about 74 GW capacities worldwide are expected to be in operation by 2020 [13]. China has the most aggressive plan. In 2014, the Chinese government announced its plan to more than quadruple its current PHS installations to a total capacity of 100 GW by 2025 [14]. Driven by the need to accommodating rapid11growth of intermittent renewable electricity, Europe is also witnessing a renaissance of PHS, particularly in Spain, Switzerland, and Austria, with 27 GW new PHS capacity expected by 2020 [15]. Although Japan already has the highest density of PHS installation in the world, Japanese power companies are continuing to develop more PHS plants. The United States is also experiencing a revival of PHS development. In 2014, the U.S. Federal Energy Regulatory Commission issued licenses to construct and operate two new PHS facilities (1.3 GW Eagle Mountain PHS and 400MW Iowa River PHS). Another application for construction and operation license (for the 1 GW Parker Noll PHS) is currently under review. In addition, there are over 40 proposed PHS projects currently conducting feasibility studies with issued preliminary permits. With the mature technology and high volume of commercial development activities, PHS will certainly remain the most dominant energy storage technology in the foreseeable future.4.2 Alternative and novel PHS designsVariable Speed PHS: Most existing PHS facilities are equipped with fixed-speed pump turbine. While those fixed-speed PHS facilities may provide economical bulk electricity storage, they can only provide frequency regulation during its generating mode, but not in pumping mode. With the increasing adoption of variable power sources such as wind and solar, there is an increasing demand for frequency regulation. New variable speed technology allows PHS facilities to regulate frequency at both pumping and generating modes. Japan has pioneered the variable-speed PHS technology and has successfully operated such systems at the Okawachi PHS station for over twenty years [16, 17]. European countries are actively introducing variable speed PHS in recent years in order to accommodate more variable renewable electricity in their power portfolios [18]. Seawater PHS: In addition to the worldwide revived interests in developing conventional PHS projects, many developers are also proposing new approaches. Japan has pioneered seawater PHS. The Okinawa seawater PHS station, which has commenced operation in 1999, is the world’s first seawater PHS system [19]. The Okinawa PHS station uses the open sea as the lower reservoir together with a constructed upper reservoir at 150 meters above sea level. New seawater projects have been proposed in Ireland, Greece, Belgium, and the Netherlands [20, 21, 22, 23]. The energy islands concept proposed by the Dutch consulting company DNV KEMA has an unusual approach; they plan to use the open sea as the upper reservoir, and construct the lower reservoir by dredging and building a ring of dikes at a depth of 50 m below sea level.Underground PHS: Researchers since the 1970s [24], have proposed the possibility of utilizing underground caverns as lower reservoirs for PHS projects but so far none have been built [25]. The commercial interests in developing underground PHS have resurfaced in recent years in the United States. Several U.S. developers have received12preliminary permits to study the feasibility of building underground PHS facilities at their identified sites. A British company Quarry Battery is also working on developing underground PHS facilities with abandoned quarries [26].Compressed Air PHS: A promising innovative design (Figure 2) is to replace the upper reservoir in PHS with a pressurized water container [27]. The air within the pressure vessel becomes pressurized when water is pumped into the vessel. Instead of storing potential energy in elevated water, the proposed compressed air pumped hydro system stores the energy in compressed air. This innovative design could potentially free PHS from the geographic requirements and make it feasible at almost any location with flexible and scalable capacity. This concept is discussed in more detail in chapter 7.Figure 2. Diagram of compressed air PHSUndersea PHS: Another innovative concept (Figure 3) is to utilize the water pressure at the bottom of the sea to store electricity from off-shore wind turbines [28]. The system places submerged pressure vessels (hollow concrete tank) on the sea floor. It uses electricity to pump water out of the tank to store energy, and generate electricity when seawater is filling into the tank through the generator. This concept is further discussed in chapter 6.13Figure 3. Diagram of undersea PHS4.3 Retrofits of existing PHS and conventional hydropower stationsMany existing PHS facilities were built many decades ago and therefore were equipped with outdated and inefficient technology. There is a significant potential in increasing PHS capacity simply by renovating and upgrading the existing PHS facilities. Upgrades to old PHS facilities typically include replacing outdated pumps/turbines, impellers, and control systems with new advanced equipment. Many existing PHS station may increase the capacity by 15% to 20% and efficiency by 5% to 10% [7]. In addition, many existing conventional hydropower stations could be re-engineered to add a lower reservoir and pump-back units to pump the water back to the upper reservoir during off-peak hours, and become combined PHS stations for use with intermittent energy from renewable sources such as wind turbines and solar panels.Although PHS may be an essential enabling technology for de-carbonizing electricity, the political will to mitigate carbon dioxide and to remove regulatory barriers for PHS is far from certain. The price of natural gas is also a key determinant in the future of PHS.Because PHS is essentially a peak-load technology, which competes directly with gas-14fired power, low natural gas price may render PHS uncompetitive. The vision of de-carbonizing electricity and how PHS fits into it, will likely vary from country to country.References[1] Electrical Japan. 日本全国の揚水発電所ランキング.http://agora.ex.nii.ac.jp/earthquake/201103-eastjapan/energy/electrical-japan/type/5.html.ja (accessed October 20, 2014)[2] W. Peng, D. Chen. Some considerations on the development of pumped hydroelectric storage power station in China (In Chinese: 对我国抽水蓄能电站发展的几点思考). State Electricity Regulatory Commission, People’s Republic of China. 2010./jgyj/ztbg/201006/t20100621_13195.htm (assessed Nov. 24, 2010) [3] EIA. Electricity: Form EIA-860 detailed data 2012. Energy Information Administration, U.S. Department of Energy. Washington DC. 2012./electricity/data/eia860/[4] Army Corps. Engineering and design - hydropower. No. 1110-2-1701. Army Corps of Engineers, Department of the Army. Washington, DC. 1985.[5] U.S. Federal Energy Regulatory Commission./industries/hydropower/gen-info/licensing/pump-storage.asp (accessed October 21, 2014)[6] C.-J. Yang, R. Jackson. Opportunities and barriers to pumped-hydro energy storage in the United States. Renewable and Sustainable Energy Reviews 15 (2011) 839–844.[7] R. Baxter. Energy Storage: A Nontechnical Guide, PennWell, Tulsa, OK, 2006.[8] Available from: /[9] M. Zeng, K. Zhang, D. Liu. Overall review of pumped-hydro energy storage in China: Status quo, operation mechanism and policy barriers. Renewable and Sustainable Energy Reviews 17 (2013) 35–43.[10] APS Panel on Public Affairs Committee on Energy Environment. Challenges of Electricity Storage Technologies. American Physical Society, 2007.[11] FERC Encourages Transmission Grid Investment. Docket No. ER06-278-000. Federal Energy Regulatory Commission, Washington, DC; March 20, 2008.15[12] J.P. Deane, B.P. Gallachόir, E.J. McKeogh. Techno-economic review of existing and new pumped hydro energy storage plant. Renewable and Sustainable Energy Reviews 14 (2010) 1293–1302.[13] Ecoprog GmbH. 2013. The World Market for Pumped-Storage Power Plants. Köln, Germany.[14] Xinhuanet, 2014. /energy/2014-11/19/c_127223035.htm[15] M. Zuber. Renaissance for Pumped Storage in Europe, Hydro Review Worldwide 19 (2011). /articles/print/volume-19/issue-3/articles/new-development/renaissance-for-pumped-storage-in-europe.html[16] O. Nagura, M. Higuchi, K. Tani, T. Oyake. Hitachi’s adjustable-speed pumped-stroage system contributing to prevention of global warming. Hitachi Review 59 (2010) 99–105.[17] J.M.Henry, F. Maurer, J-L Drommi, T. Sautereau. Converting to variable speed at a pumped-storage plant. Hydro Review Worldwide 21 (2013)./articles/print/volume-21/issue-5/articles/pumped-storage/converting-to-variable-speed-at-a-pumped-storage-plant.html[18] N. Lefebvre, M. Tabarin, O. Teller. A solution to intermittent renewable using pumped hydropower. Renewable Energy World, March/April 2015. 49–57.[19] Available from: http://www.kankeiren.or.jp/kankyou/en/pdf/en108.pdf[20] E. McLean, D. Kearney. An evaluation of seawater pumped hydro storage for regulating the export of renewable energy to the national grid. Energy Procedia 46 (2014) 152–160.[21] D.A, Katsaprakakis, D.G. Christakis, I. Stefanakis, P. Spanos, N. Stefanakis. 2013. Technical details regarding the design, the construction and the operation of seawater pumped storage systems. Energy 55, 619–630.[22] M. LaMonica. A manmade island to store wind energy. MIT Technology Review, February 5, 2013.[23] Available from: /2013/01/energy-island-for-wind-power-storage-draws-belgiums-interest/[24] S.W. Tam, C.A. Blomquist, G.T. Kartsounes. Underground pumped hydro storage–an overview. Energy Sources 4 (1979) 329–351.[25] W.F. Pickard. 2012. The History, Present State, and Future Prospects of Underground Pumped Hydro for Massive Energy Storage. Proceedings of the IEEE 100, 473–483.16[26] Available from: /[27] H. Wang, L.Wang, X. Wang, E. Yao. A Novel Pumped Hydro Combined with Compressed Air Energy Storage System. Energies 6 (2013) 1554–1567.[28] A.A. Slocum, G.E. Fennell, G. Dundar B.G. Hodder J.D.C. Meredith, M.A. Sager. Ocean renewable energy storage (ORES) system: analysis of an undersea energy storage concept. Proceedings of the IEEE 101 (2013) 906–924.17。