Abstract—Surplus electricity can be converted into potential energy via pumped hydroelectric storage for future usage. Similarly, thermo-electric energy storage (TEES) uses heat pumps equipped with thermal storage to convert electrical energy into thermal energy; the stored energy is then converted back into electrical energy when necessary using a heat engine. The greatest advantage of this method is that, unlike pumped hydroelectric storage and compressed air energy storage, TEES is not restricted by geographical constraints. In this study, performance variation of the TEES according to the changes in cold-side storage temperature was investigated by simulation method. Keywords—Energy Storage System, Heat Pump. I. INTRODUCTION EW and renewable energy sources such as wind and photovoltaic has been developed. However, because of the intrinsic characteristics of these energy sources, a time discrepancy between supply and demand exists. In order to overcome this problem, electrical energy storage technologies have been developed, including pumped hydroelectric storage, batteries, flywheels, capacitors, and compressed air energy storage. Several researchers have recently proposed thermo-electric energy storage (TEES). Surplus electricity can be converted into potential energy via pumped hydroelectric storage for future usage. Similarly, TEES uses heat pumps equipped with thermal storage to convert electrical energy into thermal energy; the stored energy is then converted back into electrical energy when necessary using a heat engine. The greatest advantage of this method is that, unlike pumped hydroelectric storage and compressed air energy storage, TEES is not restricted by geographical constraints. Desrues et al. [1] proposed a thermal energy storage process based on the Brayton cycle with argon as the working fluid. Peterson [2] proposed a scheme that uses latent heat storage at Young-Jin Baik is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (phone: +82-42-860-3226; fax: +82-42-367-5067; e-mail: twinjin@kier.re.kr). Minsung Kim is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (Corresponding author, phone: +82-42-860-3062; fax: +82-42-367-5067; e-mail: minsungk@kier.re.kr). Junhyun Cho is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (phone: +82-42-860-3581; fax: +82-42-367-5067; e-mail: jhcho@kier.re.kr). Ho-Sang Ra is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (phone: +82-42-860-3167; fax: +82-42-367-5067; e-mail: hsra@kier.re.kr). Young-Soo Lee is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (phone: +82-42-860-3161; fax: +82-42-367-5067; e-mail: yslee@kier.re.kr). Ki-Chang Chang is with the Korea Institute of Energy Research, Daejeon 305-343, Korea (phone: +82-42-860-3163; fax: +82-42-367-5067; e-mail:kcchang@kier.re.kr). sub-ambient temperatures and demonstrated that his charging and discharging processes are favorable in the nighttime and daytime, respectively, owing to the differences in atmospheric temperature. Henchoz et al. [3] also noted the effectiveness of sub-ambient temperature TEES. Some studies have recently attempted to use CO 2 as the working fluid for TEES. Mercangöz et al. [4] proposed an energy storage scheme called electrothermal energy storage that uses a transcritical CO 2 cycle. Morandin et al. [5], [6] used pinch analysis tools to optimize various forms of transcritical-CO 2 -cycle-based TEES with multiple water tanks. More recently, Kim et al. [7] proposed a unique scheme in which an isothermal process was added to the conventional transcritical-CO 2 -cycle-based scheme for increased efficiency. In this study, performance variation of the TEES according to the changes in cold-side storage temperature was investigated by simulation method. II. MODELING Figs. 1 and 2 show schematics of the charging and discharging processes, respectively, of the TEES. This can be expressed as a combination of the charging process based on heat pump and Rankine power cycle. The operation principle of each process is as follows. During charging, CO 2 working fluid in a low-temperature, low-pressure state (state point 1 (SP1) in Fig. 1) passes through the compressor, to which power is supplied from outside of the system, and goes into a high-temperature, high-pressure state (SP2). Then, the fluid passes through the high-temperature-side heat exchanger and cools while transferring heat to water to go into a low-temperature, high-pressure state (SP3). The water from hot storage tank 1 passes through the high-temperature-side heat exchanger; here, its temperature increases owing to heat received from the working fluid, following which it is stored in hot storage tank 2. The low-temperature, high-pressure working fluid (SP3) goes into a low-temperature, low-pressure state (SP4) as it passes through the expansion valve. The working fluid in a low-temperature, low-pressure state (SP4) is introduced into the low-temperature-side heat exchanger receives heat from water, becomes vapor (SP1), and re-enters the compressor. The water from the cold storage tank 1transfers heat to the working fluid as it passes through the low-temperature-side heat exchanger, cools, and is stored in the cold storage tank 2. After charging, discharging can take place if necessary. Discharging can be explained as the reverse process of charging. First, CO 2 working fluid from the low-temperature-side heat exchanger in a low-temperature, low-pressure state (SP5) passes through the Performance Variation of the TEES According to the Changes in Cold-Side Storage Temperature Young-Jin Baik, Minsung Kim, Junhyun Cho, Ho-Sang Ra, Young-Soo Lee, Ki-Chang Chang N World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:8, No:8, 2014 1413 International Scholarly and Scientific Research & Innovation 8(8) 2014 scholar.waset.org/1307-6892/9999030 International Science Index, Mechanical and Mechatronics Engineering Vol:8, No:8, 2014 waset.org/Publication/9999030