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1202 A. Hesaraki et al. / Renewable and Sustainable Energy Reviews 43 (2015) 1199–1213 Table 1 Summary of different types of seasonal storage systems [6,9,38,35,39,40,41,51]. Storage medium Maximum storage capacity Advantages Hot water tank storage, HWTS Water 60–80 kW h m 3 – Can be built at almost any location – Most common system – No special geological condition is needed – High stratification – High heat capacity – Easy to install – High cost in buried water tank – High thermal loss – Corrosion – Leakage Aquifer thermal energy storage, ATES Water—sand/gravel 30–40kWhm3 – Cost effective – Can be used for both heating and cooling – Ability to produce direct cooling without using any supporting device, e.g. heat pump – Low maintenance – Much more efficient heat transfer compared to DTES – Needs special geological conditions, e.g. water saturated sand layers with high permeability without natural groundwater flow – High thermal loss due to no thermal insulation – Needs 2–3 times larger storage volume compared to the HWTS – Clogging effects – Long initial process due to extensive geological investigation temperature, respectively (1C); L is the average monthly value of atmosphere lucidity. From the environmental contribution perspective, low tem- perature seasonal thermal energy storage (STES) is not harmful for environment since high storage temperature may cause geochem- ical, geotechnical, hydro-chemical and hydro-biological problems [46,47]. As mentioned earlier, although low temperature STES has many advantages, it cannot be used directly to meet heating demand. Therefore, assisting systems such as those utilizing heat pumps are required. 3. Heat pumps Heat pumps are an energy saving and energy efficient technol- ogy for supplying both heating and cooling demand [48,49]. Heat pumps usually deliver more useful energy than the required energy to operate them [50]. In heating mode the heat source of the heat pump normally uses renewable energy stored in ground, groundwater, ambient air, or exhaust air. In heat pumps this low grade energy is converted to high grade by putting in the required amount of work, e.g. by electrical energy. In cooling mode this cycle is reversed and the indoor air acts as an evaporator for the heat pump. The efficiency of a heat pump in heating mode is determined by the coefficient of performance (COP). The COP of a heat pump indicates the ratio of produced energy to used energy. Presently the average COP of an efficient heat pump can be up to 4. COP depends on many factors, e.g. the temperatures of heat source and heat sink, the efficiency of its compressor, and the type of its working medium. Above all, the temperature of the heat source and heat sink are very important factors influencing the COP value, Limitations – Can be built almost everywhere – No special geological condition is needed – More cost effective than the HWTS – Leaving natural aquifer untouched – High cost – Low stratification due to high thermal conductivity – Leakage – Needs 1.3–2 times larger storage volume compared to HWTS – – – – – – – – – Can be used for both heating and cooling In case of vertical borehole (30–200 m depth with the spacing of about 2–4 m) needs less surface area and it is less sensitive to outdoor climate due to constant ground temperature which is equal to the annual mean temperature In case of horizontal duct needs less excavation (at depth of 0.8 to 1.5 m) and have lower cost Feasible for very large and very small application Needs 3–5 times larger storage volume compared to the HWTS Not suitable for all locations with ground-water flow Needs drillable ground High initial cost 3–4 years needed to reach typical performance Water-gravel pit Duct thermal energy storage, DTES storage, WGPS, Artificial aquifer Water and gravel Soil/rock 30–50kWhm3 15–30kWhm3 according to types of storage, and solar collectors systems. Higher solar collector area and smaller storage volume allow for higher storage temperature. The temperature range as well as its application is shown in Table 2 [21,42]. In low temperature storage a heat pump is needed, as explained in the following section. 2.1. Low temperature seasonal thermal energy storage From the energy supply efficiency perspective, low tempera- ture seasonal thermal energy storage has many advantages. In addition to the lower heat loss mentioned earlier, the smaller size of the system [43] in terms of storage volume and collector area, allows cost reduction. Furthermore, as shown by Eq. (4) [44], the heat production by the collector (qc) depends on the temperature difference between heat carrier inlet temperature (Tin) into the collector, and ambient temperature (Ta). The lower outlet tem- perature from the low temperature storage, which assuming low heat losses from the pipes is approximately equal to inlet tem- perature to the collector, leads to higher solar collector efficiency [35]. This is due to reducing convective and radiative heat losses from the collectors to the ambient surrounding [9,45]. 2! 1a TinTa þb TinTa ð4Þ qc 1⁄4Ac Ec η where qc is the average amount of heat produced by a solar LL collector (kW h); Ac is the collector area (m2); Ec is the average amount of energy received by 1m2 of a solar collector (kW h m 2); η is the efficiency of the collector, a, b are the experimentally determined coefficients, Tin and Ta are the heat carrier inlet temperature into collector and surrounding air and size ofPDF Image | Renewable and Sustainable Energy Reviews 43
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