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1210 A. Hesaraki et al. / Renewable and Sustainable Energy Reviews 43 (2015) 1199–1213 Fig. 11. COP of heat pump and solar fraction vs. ratio of storage volume to collector area. Based on the results of previous studies given in Table 4, correlation between storage volume, collector area, COP and SF is shown in Figs. 9–11. In Figs. 9 and 10, in order to be able to compare different systems with different heating demand regard- less of weather conditions and the building type, a ratio of solar collector area and storage volume to the annual energy demand was calculated. In addition, in Fig. 9 to make all storage methods comparable, the equivalent storage volume of water for all storages was considered. Equivalent storage volume of water corresponds to water volume that would store the same amount of heat. As can be seen in Fig. 9, as the ratio of storage volume to energy demand increased the COP and solar fraction increased. However, as mentioned earlier when the storage volume became large enough then there was no increase or little enhancement in COP and SF with increasing volume. It means that as storage was further increased the performance of the system in terms of COP and SF improved slowly. In Fig. 10 the relation between SF and COP with collector area is shown. As can be seen, both COP and SF increased linearly with expanding collector area. In addition, Fig. 11 shows how the COP and solar fraction varied as a ratio of storage volume to collector area changed. As can be seen, a higher ratio resulted in higher COP and higher solar fraction. 8. Future prospects The paper provides a basis for development of new intelligent energy storage methods for sustainable building. This approach could be defined as combining e.g. photovoltaic-thermal system (PV/T) and heat pump [135] with seasonal thermal (ST) and electrical energy storage (EES), called PV/T-ST/EES-HP. The bene- ficial aspects of this system could be: Higher PV efficiency due to cooling down of the PV cells by the collector. Seasonal electrical storage for heat pump. Higher SF due to providing the electrical energy required for a heat pump by solar energy through PV. High COP of a heat pump. However, the cost of this system for energy efficient buildings should be considered. 9. Conclusion The overall aim of this paper was to conduct an extensive literature review of early and recent applications of seasonal thermal energy storage with a heat pump, both in large and small scales. Seasonal thermal energy storage can contribute significantly to the needs of energy efficient and environmentally friendly heating and cooling systems, as the replacement of conventional systems with renewable energy considerably reduces CO2 emis- sions. Thermal loss from seasonal thermal energy storage has always been a consideration, however. Thermal loss from seasonal storage can be decreased by lowering the stored energy tempera- ture. The low temperature storage also favors collector efficiency. Nevertheless, this temperature should be sufficient for covering the energy demand in the building. Therefore, seasonal thermal energy storage can be combined with a heat pump as an efficient heating system to increase the stored energy temperature to the appro- priate level. Both heat pump and seasonal storage of solar energy are two promising methods of increasing the renewable energy consumption. In addition, the heat pump helps to make the storage stratified through decreasing the return temperature to the storage. Stratification is beneficial in terms of increasing the efficiency of the solar collector and increasing the exergy saving. In this review study some well-known existing methods for seasonal thermal storage were introduced. Those methods were: hot water tank storage, gravel-water pit storage, duct thermal energy storage and aquifer thermal energy storage. Then, the combination of heat pump with those seasonal energy storages was discussed. The selection of suitable STES depends on many factors, including geological conditions, heat demand, and cost. Each of the studied storage systems has its own advantages and disadvantages. For instance, a water tank is easy to install and no special geological condition is needed, but the cost is high. Then again, aquifer thermal energy storage is cheap but extensive geological investigation is needed. Nevertheless, with careful consideration of the application of the system, size requirements and heating or cooling demand, the appropriate system can be chosen. In large and small buildings with only heating demand hot water tank storage with heat pump and gravel-water pit storage with heat pump can be installed. For large buildings with only cooling demand aquifer thermal energy storage with heat pump can be an option. This would cause considerable savings in expensive electricity during peak hour for large applications with high cooling demand. For small applications with only cooling demand duct thermal energy storage with heat pump is suitable. For applications with both heating and cooling demand aquifer and duct thermal energy storage with heat pump would be appropriate. Two main factors influence the efficiency of seasonal thermal energy storage with a heat pump. These are the COP of the heat pump, and the solar fraction. Both factors are a function of solar collector area and storage volume. In this study a relation between these two factors based on past projects was found. The review showed that higher solar fraction and higher COP of a heat pump result from a higher energy storage volume and collector area. In addition, a higher ratio of storage volume to collector area causes a higher solar fraction and higher COP of a heat pump. It is difficult to generalize from this experience to other applications, however. All reviewed papers showed that seasonal storage is a promis- ing technology for energy saving, but its cost did not make itPDF Image | Renewable and Sustainable Energy Reviews 43
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