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A. Hesaraki et al. / Renewable and Sustainable Energy Reviews 43 (2015) 1199–1213 1201 There are different ways to store heat as seasonal thermal Fig. 1. Energy conservation for seasonal thermal energy storage with heat pump. heat pump, so as to upgrade the temperature to be suitable for domestic hot water (DHW) or space heating [22]. The two main factors that determine the efficiency of seasonal thermal energy storage with a heat pump are the solar fraction (SF) and coefficient of performance (COP) of the heat pump. These factors change with changing collector area and storage volume. The relation between SF, COP, collector area and storage volume can be calculated considering energy conservation principles, with energy in the storage calculated by Eq. (2). The left side shows total annual energy supplied to the system, i.e. solar energy and heat pump work. The right side indicates heat load in the building, heat loss into the surrounding earth and atmosphere, and the part remaining in the storage. Fig. 1 shows how these terms are applied to the system. qc þWhp 1⁄4 Qhd þQloss þQtank ð2Þ where qc is the collector output, Whp is the electricity input to the heat pump, Qhd is the heating demand for space heating and DHW if needed, Qloss is the heat loss from the system, and Qtank is the stored energy in the tank. Units in Eq. (2) are thus kW h. The purpose of this article was to review the previous studies regarding the combination of heat pump with different seasonal thermal energy storage methods in terms of SF and COP of heat pump and to provide a relation between those factors with collector area and storage volume based on past projects. 2. Seasonal thermal energy storage medium and methods An appropriate storage medium is expected to have a high specific heat storage capacity, long term stability under the thermal cycling, good compatibility with its containment and low cost. In seasonal storage systems there are mainly two types of storage medium [23], solid, e.g. soil or rock, and liquid, e.g. water. The capacity of the storage medium to absorb or release heat, depending on the thermal conductivity for solids and the convective heat transfer rates for liquids [24] plays an important role in a seasonal storage system. Both mediums have their own advantages and disadvantages. Thermal capacity of liquids is higher than that of solids and it is easier to exchange heat in liquids. However, solids can tolerate a higher range of tempera- tures since they will not freeze or boil [25] and solids cannot leak from the container. The maximum total storage capacity of a storage medium is calculated using Eq. (3). Qmax 1⁄4V ρcp ðθmaxθminÞ ð3Þ where Qmax is the maximum storage capacity, V is the volume (m3) of the thermal energy storage (TES), ρ is the medium density (kg m 3) and cp is the specific heat of the storage medium (J kg 1 K 1), θmax and θmin are the temperature (1C) of fully charged and fully discharged storages, respectively. energy storage (STES). The most common storage systems are: Hot water tank storage (HWTS): The storage tank, of stainless steel or reinforced concrete, is usually buried underground [26] in order to decrease the heat loss and increase the solar fraction. This system is also called water pit storage. In order to increase stratification and decrease heat loss, a high level of insulation should surround the storage tank [27]. The main problem with this storage system is high cost due to ground works, concrete construction, insulation, and liners to prevent leakage and protect against moisture. Water-gravel pit storage (WGPS): In this storage system both water and rock are used as storage mediums. The application of rock and water in such pits can overcome some problems such as high cost of hot water tank storage (HWTS) and the low thermal capacity of rock [17]. This type of storage is also called man-made or artificial aquifer [11]. Using this system the natural aquifers remain untouched. The high cost of this system is due to ground works, sealing of the pit, insulation and moisture protection. Duct thermal energy storage (DTES): In this storage method, vertical or horizontal ducts are inserted under the ground to store heat. The optimum depth of the DTES depends on the heat load, ground thermal conductivity, the natural tempera- ture in the ground, the ground water level, and the distance to other similar storage systems [28,29]. For DTES with channels deeper than 3 m the extracted heat from the ground during winter is higher than the natural heat supplied to the ground during summer [30]. Therefore, with borehole systems it is recommended to charge the ground artificially with heat, e.g. by solar collector, or exhaust air from the ventilation system [31]. The temperature of DTES [32] ranges from 2 to 20 1C and from 3 to 6 1C for charged and un-charged storage, respec- tively. Due to its low stored temperature, this system is usually combined with a heat pump. Hence, in addition to resulting in a higher COP for the heat pump, i.e. up to 4–5 [24,33], the combination of solar collector with DTES also allows for reducing the borehole depth 4.5 to 7.7 m per square meter of solar collector area [34]. However, for a single family house with a single borehole, the economical aspect of recharging should be considered. In addition, where there is a high water table recharging would not be helpful. In DTES the first three to five years of operation is the start-time [35] needed to obtain normal operating conditions, slowly heating the underground surroundings of the storage system and thereby decreasing heat loss. The efficiency of the system is therefore lower in the first years [36]. Aquifer thermal energy storage (ATES): In this storage system there are at least two wells, one warm and another cold, that are drilled into the aquifer to inject/extract groundwater. This system is equipped with pumps, and extraction and injection pipes. During the charging process in summer, the water is extracted from the cold well, heated by the chosen heat source and injected into the hot well. For the discharging process in the heating season the cycle is reversed, i.e. hot water is extracted from the warm well, cooled by a heat sink and injected into the cold well. Aquifer storage is usually used for cold storage in district cooling applications [37] and is not suitable for small loads such as single family houses [25] due to large site requirements. Advantages and disadvantages of each storage method are summed up in Table 1 [6,9,38,35,39–41]. In addition to the type of storage, another classification of STES is based on the stored temperature level. Different temperature ranges can be achievedPDF Image | Renewable and Sustainable Energy Reviews 43
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