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Fig. 5. Water-gravel pit storage with a heat pump and solar collectors. Fig. 6. Borehole thermal energy storage with a heat pump and solar thermal collectors. the campus of Stuttgart University. Initial measurements during the first two years showed a failure in heat pump performance with low COP. The heat pump was then changed to a better one with an average COP of 4. Based on 15 years monitoring, this system has worked satisfactorily with a COP of 4.5 and solar fraction of 60%. Another recent WGPS-HP was built in Eggenstein, Germany as the first system for providing heating for renovated buildings [70]. This system was designed to cover 35–40% of total heating demand for a school, gym, pool, and fire station, with total heated floor area of 12,000 m2. 4.3. Duct thermal energy storage with heat pump (DTES-HP) Fig. 6 shows a diagram of the DTES-HP system. To reduce heat loss and heat conduction from the pipes to the ground, the supply pipes were connected to the center of storage and the return pipes were at the boundaries. Different configurations of this system allow for charging the system by solar collectors. Kjellsson et al. [29] theoretically compared the following three alternatives: (a) all solar heating used for recharging the borehole during the whole year, (b) all solar heating used for DHW production with un- charged ground-coupled heat pump for heating, and (c) all solar heating from November through February used for charging the borehole and for the rest of the year all solar was fully used for DHW production. The study parameters were COP of heat pump and energy savings. The COP of case “a” was higher through the whole year due to higher evaporator temperatures. However, the case “c” showed a higher energy savings due to sufficient natural recharging of the borehole during summertime and more efficient use of solar heat for domestic hot water in this period. The application of DTES-HP for both heating and cooling has also received much interest. A beneficial aspect of this system is that in cooling mode, injected surplus heat from the building also can be used to heat the ground. The control of this DTES-HP system can be based on the temperatures produced by solar collectors and desirable for thermal comfort [71,72]. Wang et al. [71] experimentally investigated this system in a small residential house. In the control system three modes were defined. One was for charging the ground from April to October when the tempera- ture of carrying fluid in solar collectors is higher than 25 1C. The second mode was cooling by the half of the heat exchanger via radiant floor cooling from July to August when the indoor temperature was not between 24 and 26 1C. The cooling was supplied without heat pump interaction. The third mode was to heat the building using the solar collectors directly or with a heat pump using seasonal storage from October to April. The result showed that 88% of total demand was covered by DTES-HP and the average COP of the heat pump was 4.3. In addition, the performance of DTES-HP for large applications was investigated. The system in Kungsbacka [65] in Sweden was able to cover the 64% heating demand of a school building with 1500 m2 of collector area and 85,000 m3 of storage volume [73]. A recent DTES-HP in Crailsheim, Germany [13] was designed to supply 50% of DHW and space heating for 260 flats and a school building with solar energy [74]. This system consists of 37,500 m3 of storage volume and 7300 m2 collector area. 4.4. Aquifer thermal energy storage with heat pump (ATES-HP) A combination of aquifer thermal energy storage and heat pump is shown in Fig. 7. Paksoy et al. [75] found a 60% increase in COP of the ATES-HP, when compared to a COP of a conventional HP using ambient air. In ATES-HP, depending on the required temperature level, it is optional to artificially charge the aquifer using, for example, a solar collector or waste heat from industry. Due to high temperature of underground water, many projects, e.g. [75–77] investigated the performance of low temperature un- charged ATES-HP for heating and cooling of buildings. Ghaebi et al. [76] evaluated the performance of ATES for three configurations in a residential complex located in Tehran, Iran, using numerical simulation. The first one used ATES for cooling only, the second configuration consisted of ATES-HP for both heating and cooling, and the third one used charged ATES with solar collector for heating only. The investigation showed that the second possibility, i.e. ATES-HP was the best in terms of a high COP, i.e. 17.2 and 5 for cooling and heating, respectively. Andersson et al. [37] compared the energy savings, thermal capacity and payback time for different configurations of un- charged ATES-HP in Sweden. The systems investigated were for providing heating and cooling, and also heating only. The energy saving for the former system was 80–87% with 1–3 years payback Fig. 7. Aquifer thermal energy storage combination with a heat pump and solar thermal collectors. A. Hesaraki et al. / Renewable and Sustainable Energy Reviews 43 (2015) 1199–1213 1205PDF Image | Renewable and Sustainable Energy Reviews 43
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