The principle of PHES is that hydraulic potential energy is stored by pumping water from a lower reservoir to an elevated reservoir. During periods of high electricity demand, water is extracted through a turbine generator in a manner similar to traditional hydroelectric facilities. The amount of stored energy is proportional to the height difference between the two reservoirs and the volume of water stored. Some high-dam hydro plants have a storage capability and can be used as a PHES facility. Underground pumped storage, using flooded mineshafts or other cavities, are also technically possible. Open sea can also be used as the lower reservoir. A seawater pumped hydro plant was first built in Japan in 1999 (Yanbaru, 30 MW) [10].

PHES is a mature technology with large-volume, long storage period, high–efficiency, and a relatively low capital cost per unit energy. Owing to relatively small evaporation and penetration the storage period of PHES can be very long—typically hours to days and even up to years. Taking into account evaporation losses from the exposed water surface and conversion losses, c. (71–85)% of the electrical energy used to pump water into the elevated reservoir can be regained. The typical rating of PHES is about 1 GW [(100–3000) MW] and facilities continue to be installed worldwide at a rate of up to 5 GW a^–1 (5000 MW per year). The rating of PHES is the highest of all the available EES systems. As a consequence, PHES is generally used for energy management, frequency control, and provision of reserve. According to data from an International Energy Agency (IEA) report in 2014 [54] the global installed capacity of PHES is about 140 GW which amounts to 99% of the total capacity of EES.

The major drawback of PHES is the scarcity of available sites for two large reservoirs and one or two dams. A long lead time (typically about 10 years); high costs (typically hundred millions to billions of US dollars) for construction; and environmental issues, (e.g., removing trees and vegetation from the large amounts of land prior to the reservoir being flooded) [45,46], are three other major constraints in the deployment of PHES.

Pump hydro is also widely used in China for peak shaving, peak loading, energy management, and renewable energy electricity. It now has the largest installed capacity in China over the other EES systems. The first commercial pump hydro in China was the Gangnan plant in Hebei province which was in operation in 1968 with a capacity of 11 MW. Up to 2014 the amount of stored energy in Chinese pumped hydro stations was about 22.1 GW, which is 99.2% of total installed capacity of EES. This amounts to about 1.7% of total generation capacity in China. Table 24.1 lists all the pumped hydro stations in operation in China. Total installed capacity is now third in the world (behind Japan and the United States). However, most of the stations are located in the southeast of China due to geological restrictions [1]. As a result other EES technologies are urgently needed in China.

The major components of a CAES installation include five aboveground and one underground components:

The principle of CAES is based on conventional gas turbine generation. CAES decouples the compression and expansion cycle of a conventional gas turbine into two separate processes and stores the energy in the form of elastic potential energy of compressed air. At times of low demand, energy is stored by compressing air in an air-tight space. To extract the stored energy, compressed air is drawn from the storage vessel, heated, and then expanded through a high-pressure turbine, which captures some of the energy in the compressed air. The air is then mixed with fuel and combusted with the exhaust gas expanded through a low-pressure turbine. Both the high- and low-pressure turbines are connected to a generator to produce electricity.

CAES has a relatively long storage period, low capital costs, and high efficiency. Typical ratings for a CAES system are in the range (50–300) MW, and currently manufacturers can create CAES machinery for facilities ranging from (1–350) MW. The rating is much higher than for storage technologies other than PHES. The storage period is also longer than for other storage methods since the losses are very small; a CAES system can be used to store energy for more than a year. A typical value of storage efficiency of CAES is in the range (40–75)%. Capital costs for CAES facilities vary depending on the type of underground storage but are typically in the range ($300–660) kW^–1.

Similar to the situation with PHES, the major barrier to implementation of CAES is the reliance on favorable geological structures, and CAES is only economically feasible for power plants that are near to rock mines, salt caverns, aquifers, or depleted gas fields. In addition, in comparison with PHES facilities and other currently available energy storage systems, CAES is not an independent system, and each facility must be linked to a gas turbine plant. It cannot be used in conjunction with other types of power plants such as coal-fired, nuclear, wind turbine, or solar photovoltaic plants. More importantly, the combustion of fossil fuel leads to emission of contaminates such as nitrogen oxides and carbon oxides which render CAES less attractive [19,45,46]. Many improved CAES systems are proposed or under investigation, eg, small-scale CAES systems with fabricated small vessels; advanced adiabatic CAES (AA-CAES) systems with thermal energy storage (TES) [19,21]; and compressed air storage with humidification (CASH) [13,20].

Although CAES is a mature, commercially available energy storage technology, there are only two operating CAES systems in the world. One is in Huntorf (Germany), the other is in McIntosh, Alabama (United States). Currently in China, CAES is still under research and development and there is no CAES station in commercial operation. The largest CAES station is the 1.5 W demonstration project at the Institute of Engineering Thermophysics (Chinese Academy of Sciences) (IET-CAS), Beijing, and a 10 W project which is under construction at the same institute. Table 24.2 lists the CAES projects in China. The institutions working on CAES include Zhejiang University, Shandong University, Tsinghua University, IET-CAS, and the Datang Power Company. Among the projects, Shandong University, IET-CAS, and Datang Power Company have been supported by national high-tech programs. The project conducted by Datang Power Company has now been terminated due to lack of an air-tight cavern. Scientists at IET-CAS are also working on two 1.5 MW scale projects for industrial users; these two projects are not listed in Table 24.2 as they are only at the design stage.

A flywheel storage device consists of the following components:

The major advantage of the flywheel is the capability of several hundred thousand full charge–discharge cycles [1] which provides a much better lifecycle than many other EES systems. The efficiency of flywheels is high and typically in the range (90–95)%. Their application is principally high power/short duration, (e.g., 100 s of a kilowatt every 10 s). The most common power quality application is to provide a ride through of interruptions up to 15 s long or to bridge the shift from one power source to another. Such systems may often be implemented in a hybrid configuration with standby generators (e.g., diesel generators). Flywheels have also been used for demand reduction and energy recovery in electrically powered mass transit systems. Megawatt flywheels can also be used for reactive power support, spinning reserve, and voltage regulation by power quality–sensitive customers such as communications facilities and computer server centers; the duration could be up to tens of minutes using a magnetic levitation bearing. Urenco Power Technologies (UPT) has recently demonstrated the application of flywheels to the smoothing of the output of wind turbine systems and the associated stabilization of small-scale island power supply networks. The rail traction industry represents another significant and high added value application for flywheel storage, particularly for trackside voltage support. Such an application could well represent a significant growth area in the years ahead, with the prevalence of increasing numbers of larger and heavier trains being imposed on existing infrastructures.

Compared with other EES systems the major disadvantages of the flywheel system are short duration, relatively high frictional loss (windage), and low energy density which restrict the use of flywheel systems. Much of the current research and developmental effort in relation to flywheel energy storage systems is directed toward high-speed composite machines, running at 10 000 rpm and utilizing fabric composite materials technology [39]. Units have already been supplied on a commercial basis by UPT, and further systems are being developed by AFS-Trinity, Beacon Power, Piller, and others. The largest project in the world is the Stephentown Advanced Energy Storage project located in Stephentown, New York (United States). The project was constructed by Beacon Power and its scale is 20 MW/5 MW h with an efficiency of 97% for frequency regulation or a roundtrip efficiency of 85% for an overall charge/discharge cycle.

China started research and development into flywheels during the 1980s when the Institute of Electrical Engineering (Chinese Academy of Sciences) investigated a 10 kW prototype with a capacity of 10 W h and rotating speeds of (66–133) Hz (4000–8000) rpm. In China work on flywheels is currently being conducted at the Institute of Electrical Engineering (Chinese Academy of Sciences); Tsinghua University; Harbin Engineering University; Beihang University; China Electric Power Research Institute; and the Ying-Li Company. In 2008 the first flywheel system for industrial application was supplied by the China Electric Power Research Institute, which was installed in a hospital in Beijing with a capacity of 250 kW/15 s. Zhengjiang University and Harbin Engineering University have announced prototypes with power capacity in the region of 100 kW. Tsinghua University is now working on a high-speed system with rotation speeds of 300 Hz (18 000 rpm) and a capacity of 100 kW per unit. A 1 MW array system is under investigation by Tsinghua University which expects to install a demonstration system within 3 years. Table 24.3 gives a summary of current flywheel projects in China.

Anode (oxidation): $\text{Pb}(\text{s})+{\text{SO}}_4^{2-}(\text{aq})\leftrightarrow {\text{PbSO}}_4(\text{s})+2{\text{e}}^{-}$

Cathode (reduction): ${\text{PbO}}_2(\text{s})+{\text{SO}}_4^{2-}(\text{aq})+4{\text{H}}^{+}+2{\text{e}}^{-}\leftrightarrow {\text{PbSO}}_4(\text{s})+2{\text{H}}_2\text{O}(\text{l})$

The electrolyte is dilute sulfuric acid, which provides the sulfate ions for discharge reactions. There are several types of lead–acid battery: the flooded battery, which requires regular topping up with distilled water; the sealed maintenance-free battery, which has a gelled or absorbed electrolyte; and the valve-regulated lead–acid battery.

A lead–acid battery is a low-cost, ($300–600) kW h^–1, highly reliable, efficient (70–90%), and popular storage choice for power quality, uninterrupted power supplies (UPS), and some spinning reserve applications. Its application for energy management, however, has been very limited due to its short lifecycle of between 500–1500 cycles and low energy density of (35–50) W h kg^–1 due to the inherent high density of lead, which results in a high total mass for large energy storage requirements. The lead–acid battery also has poor low-temperature performance and therefore requires a thermal management system. Nevertheless, lead–acid batteries have been used in many commercial and large-scale energy management applications such as the 8.5 MW h h^–1 system in the BEWAG Plant (Berlin, Germany), the 4 MW h h^–1 ESCAR system at the Iberdrola Technology Demonstration Center (Madrid, Spain), and the 14 MW h 1.5 h system in PREPA (Puerto Rico). The largest is the 40 MW h system in Chino, California (United States) which can work with a rated power of 10 MW for 4 h.

There are many companies and research institutions in China working on lead–acid batteries, such as Nandu, Fengfan, Suangdeng, Institute of Electrical Engineering, and the Chinese Academy of Sciences. There were 14 projects in operation in 2014 with a total capacity of 12.1 MW. Major projects include:

During discharge, positive Na⁺ ions flow through the electrolyte and electrons flow in the external circuit of the battery; a potential of about 2.0 V is produced. This process is reversible as charging causes sodium polysulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. The battery must be kept at c. (300–350) °C to allow the reactions to process. NaS batteries have a typical lifecycle of about 2500 cycles. Their typical energy and power density are in the range (150–240) W h kg^–1 and (90–230) W kg^–1, respectively.

NaS battery cells are efficient (75–90%) and have pulse power capability over six times their continuous rating (for 30 s). This attribute enables the NaS battery to be economically useful in combined power quality and peak-shaving applications. NaS battery technology has been installed at over 30 sites in China with a capacity of more than 316 MW/1896 MW h. The largest NaS installation is a 6 MW, 8 h unit for Tokyo Electric Power Company (TEPCO). Recently, Japan’s NGK Insulators Ltd. has commissioned a NaS energy storage system of 8 MW/58 MW h at a Hitachi plant in Japan.

The major drawback of the NaS battery is that a heat source is required which uses the stored energy of the battery, partially reducing battery performance. Initial capital cost remains another issue (c. $2000 kW^–1 and c. $250 kW h^–1), but it is expected to fall as manufacturing capacity expands.

The Shanghai Institute of Ceramics (Chinese Academy of Sciences) first started research on the NaS battery in the 1960s and the first 6 kW prototype for vans was in operation in 1977. In 2007 a unit with a capacity of 650 A h was successfully used for an EES operation. In 2011 the ShangHai NaS Company was established and its operation was based on the NaS technology developed by the Shanghai Institute of Ceramics (Chinese Academy of Sciences). The ShangHai NaS Company is now the leading company in China working on NaS technology. One 100 kW/800 kW h demonstration project, developed by the company, is in operation and a 1 MW project is under investigation. A demonstration project of capacity 200 kW is being operated by the Narui Company in Nanjing, Jiangsu province; it was supplied by the NGK Insulators Company (Table 24.5).

The first commercial lithium–ion batteries were produced by Sony in 1990. Since then, improved material developments have led to vast improvements in energy density terms from figures of (100–175) W h kg^–1 and increased lifecycles as high as 20 000 cycles. The efficiency of Li-ion batteries is almost 100% which is another advantage over other batteries. While Li-ion batteries took over 50% of the small portable market in just a few years, there are some challenges for making large-scale Li-ion batteries. The main hurdle is the high cost (>$600 kW h^–1) due to special packaging and internal overcharge protection circuits. Many companies are working to reduce the manufacturing cost of Li-ion batteries to capture large-scale energy markets. The leading companies are A123 System, NEC, LG, Samsung, Tesla, and BYD. The first megawatt-scale Li-ion battery project was supplied by the A123 System Company and developed by AES in the United States. The largest project announced recently is the Elkins project in West Virginia in the United States with a scale of 32 MW.

China started research into Li-ion batteries in the 1980s, and during the period 2000–10 products for mobile phones and laptops experienced a rapid increase in development. The development of Li-ion batteries for large-scale stationary applications started in 2009 when BYD tested its 1 MW system. In 2013 the Chinese company Wanxiang Group purchased A123 System and became one of the leading companies in the area of Li-ion battery development. As of 2015, more than 30 projects have been announced using Li-ion batteries over the past 5 years with a total capacity of about 62.3 MW. Table 24.6 lists the major projects in China with scales greater than 1 MW. Of them the 20 MW/40 MW h system developed by BYD in 2014 is the largest project in China.

In contrast with conventional batteries, flow batteries store energy in the electrolyte solutions. Therefore, the power and energy ratings are independent, the storage capacity being determined by the quantity of electrolyte used and the power rating determined by the active area of the cell stack. Flow batteries can release energy continuously at a high rate of discharge for up to 10 h.

Three different electrolytes form the basis of existing designs of flow batteries currently in demonstration or in large-scale project development. These electrolytes are sodium bromide (NaBr) by Regenesys in the United Kingdom, vanadium bromide (VBr) by VRB Power Systems, Inc. In Canada, and zinc bromide (ZnBr) by ZBB Energy Corporation. In China only the vanadium redox battery (VRB) has been extensively developed. The following discussion will focus on this kind of flow battery.

The principle of VRB is that it stores energy by employing vanadium redox couples (V²⁺/V³⁺ in the negative and V⁴⁺/V⁵⁺ in the positive half-cells). These are stored in mild sulfuric acid solutions (electrolytes). During charge/discharge cycles, H⁺ ions are exchanged between the two electrolyte tanks through the hydrogen–ion permeable polymer membrane. The reactions that occur in the battery during charging and discharging can be expressed simply by:

Positive electrode:${\text{V}}^{\text{4+}}\leftrightarrow {\text{V}}^{\text{5+}}{\text{e}}^{-}$

Negative electrode:${\text{V}}^{\text{3+}}+\text{e}\leftrightarrow {\text{V}}^{\text{2+}}$

Cell voltage is between 1.4 and 1.6 V. The net efficiency of this battery can be as high as 85%. Like other flow batteries the power and energy ratings of VRB are independent of each other.

VRBs are suitable for a wide range of energy storage applications for electricity utilities and industrial end-users. These include enhanced power quality, uninterruptible power supplies, peak shaving, increased security of supply and integration with renewable energy systems. The majority of development work has focused on stationary applications due to the relatively low energy density of VRBs.

The VRB was pioneered in the Australian University of New South Wales (UNSW) in the early 1980s. Australia’s Pinnacle VRB bought the basic patents in 1998 and licensed them to Sumitomo Electric Industries (SEI) and VRB Power Systems. VRB storage of up to 500 kW, 10 h (5 MW h) has been installed in Japan by SEI for Kwansei Gakuin University and other institutions. VRBs have also been applied for power quality applications (3 MW, 1.5 s, solid–electrolyte interphase) for Tottori Sanyo Electric.

China started research and development on VRBs in 2000; the major institutions involved are Dalian Institute of Chemical Physics (Chinese Academy of Sciences), Tsinghua University, and China University of Geosciences. In 2008, Beijing Prudent Energy purchased VRB Power Systems, resulting in China becoming the leading country in VRB battery technology. In 2009 the first VRB demonstration project was installed in Tibet. The system was supplied by Dalian Rongke Power Company, used for the PV power system, and had a capacity of 5 kW/50 kW h. In 2012 Prudent provided a 2 MW system to China Electric Power Research Institute for the National Wind–Solar Energy Storage Demonstration Project which is the first megawatt-scale VRB system in China. In 2013 Dalian Rongke Power Company provided a 5 MW VRB system to the Wo-Niu-Shi Wind Power Farm in Liaoning province, which is the largest VRB system up to now. Overall, 8.2 MW of VRB flow battery systems have been installed in China up to 2014. The major flow battery projects in China are summarized in Table 24.7.