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A. Hesaraki et al. / Renewable and Sustainable Energy Reviews 43 (2015) 1199–1213 1203 as indicated by Eq. (5). Lowering the temperature difference between the heat source and the heat sink for a heat pump results in a higher COP value, as shown in Eq. (5). A low temperature heating system and high temperature heat source is therefore beneficial [18]. COP improves by 1–2% [52] for every degree reduction in heat sink temperature. In addition, COP improves by 2–4% [53] for every degree enhancement in heat source tempera- ture. A high COP requires less work by a compressor, as shown by Eq.(6). where COP is the coefficient of performance of the heat pump, ηc is Carnot efficiency (the relation between the efficiency under real conditions and the theoretically maximum reachable efficiency [54]), Tsin and Tsor are the heat sink and heat source temperatures (1C), W is work done by the compressor, pump and fan (kW h), and Qhd is heating demand in the building (kW h). For a high temperature heat source heat pump, combined with STES, COP can be up to 5–6, as will be shown in Table 4 of this paper. The system in which the solar collector contributes as a heat source of a heat pump is called a solar-assisted heat pump. This system has been investigated both theoretically and experimen- tally [21,55,56]. The COP of a heat pump fluctuates greatly with varying climate conditions and heating demands. To measure the overall heating efficiency of a heat pump (mean COP) over an entire heating season the seasonal performance factor (SPF) is used. 4. Combination of seasonal thermal energy storage and heat pump (STES-HP) Combining a heat pump with seasonal thermal energy storage (STES-HP) had many advantages [57] for both large and small applications. In this study, a full range of single to multi-family houses was covered. Using a heat pump causes considerable reduction in the discharging temperature [35]. Due to this reduc- tion, the heat pump helped keep the storage system stratified [58]. Thermal stratification is recommended [45] to reduce thermal losses to the ground, and to increase the collector efficiency as the lower temperature strata return to the solar collector. Experimen- tal and numerical models by Ghaddar [59] showed an increase of 15–20% in storage efficiency with a stratified water tank compared to a mixed tank. This enhancement was also due to lowering the use of an auxiliary heating system [60] since in stratified tanks, the stored temperature is sometimes high enough to be used directly for heating. Mixing cold and hot water in non-stratified storage, however, caused uniformity of temperature through the whole system which decreased the useful quality of energy, exergy [61]. Moreover, the stratified tank caused an increase in COP of the heat pump as the temperature to the evaporator of the heat pump was supplied through the upper part of the stratified tank, with its higher temperature. In addition, the heat pump in cooling mode can support charging of energy storage by extracting the heat from the building during summer and transferring it to storage. There were different configurations for combining heat pump, solar collector and seasonal storage system. Based on the interaction between solar collector and heat pump there are three main configurations- series, parallel and series-parallel connections. Series and parallel configurations are shown in Figs. 2 and 3, respectively. As can be seen, in the case of series configuration, the collector is a source T sin ðtÞ T sin ðtÞTsorðtÞ COP 1⁄4 ηc Wcompressor þ ∑n Wi; pump and fan 1⁄4 Qhd ð6Þ ð5Þ i1⁄41 COP Table 2 Temperature range for thermal energy storage [21,42]. Type Cold temperature Temperature range Description Application Additional information – Antifreeze mixture is used to avoid freezing in heat exchanger fluid Low temperature Between 10 and 30 1C Consist of unglazed flat plate collector connected to the vertical ground heat exchanger and then to the heat pump For low temperature heating system in residential apartments, and attached houses Group of single family houses or group of apartments, commercial buildings, schools, offices – Artificial charging is required with solar collector Medium temperature Between 30 and 50 1C Consist of evacuated tube solar collector and a heat pump, the heat is transferred either directly to the heating system (if the temperature is high enough) or via a heat pump (if the temperature is not enough) – Artificial charging is required with solar collector or waste heat – The inner region of the storage can be designed to store the higher temperature and the outer region for keeping lower temperature (benefits: lower heat loss to the ground, gaining heat losses from inner region by outer region, outer region High temperature More than 50 1C High temperature evacuated tube collector District heating, group of apartments or group of commercial buildings, not applicable for small systems due to large heat losses connected to the heat pump and inner region are used directly) – No need for heat pump Less than 10 1C Consist of a vertical or horizontal heat exchanger in the ground coupled with heat pump (ground source heat pump) For low temperature heating system in single family house – No auxiliary conventional heat source is needed – No artificial charging to soil is applied – Auxiliary burner is necessary – Special duct technology is needed – Need a good thermal contact between heat carrier fluid and ground – Small heat loss and good thermal conductivity in the groundPDF Image | Renewable and Sustainable Energy Reviews 43
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