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Sustainability 2018, 10, 191 6 of 32 where Qs is the total heat capacity for a cycle operating through the temperature range ∆ts, and m and cp are the mass and the specific heat, respectively, of water in the unit. The temperature range over which such a unit can operate is limited at the lower extreme for most applications by the requirements of the process. The upper limit may be determined by the process, the vapor pressure of the liquid, or the collector heat loss. An energy balance on the no stratified tank is mc dts = Q − Q − U A (t − t ) (3) pdτ u L ssi a where Qu and QL are rates of addition or removal of energy from the collector and to the load; Us is the heat loss coefficient of storage tank; As is the storage tank surface area; tf is the final temperature, in ◦C; ta is the ambient temperature for the tank; τ is the time. Equation (3) is to be integrated over time to determine the long-term performance of the storage unit and the solar process. Useful long-term analytical solutions are not possible due to the complex time dependence of some of the terms. There are many possible numerical integration methods. Using simple Euler integration is usually satisfactory (i.e., rewriting the temperature derivative as (ts − ti)/∆τ and solving for the tank temperature at the end of a time increment): ts=ti+ ∆τ[Qu−QL−UsAs(ti−ta)]. (4) mcp Equation (4) can be used to predict water storage temperature as a function of time. Once the tank temperature is known, other temperature-dependent quantities can be estimated. Hot water storage systems used as buffer storage for DHW supply are usually in the range of 500 L to several cubic meters (m3). This technology is also used in solar thermal installations for DHW combined with building heating systems (comb-systems). Large hot-water tanks are used for seasonal storage of solar thermal heat in combination with small district heating systems. These systems can have a volume up to several thousand cubic meters. Charging temperatures are in the range of 80–90 ◦C. The usable temperature difference can be enhanced by the use of heat pumps for discharging (down to temperatures around 10 ◦C) [4]. A more complex system with tank storage is shown in Figure 4; a solar combisystem where water store is the central part. The so-called combistore is charged with solar collectors and a second heating source, such as a biofuel or gas boiler, and heat is extracted to two heat sinks of very different characteristics: domestic hot water and space heating [28]. Solar combisystems including combistores were also the topic of the European project Combisol, whose goal was the promotion and standardization of solar combisystems in Europe [29]. High specific heat capacity, wide availability, chemically stability, and low cost make water a good storage media suitable for low temperature solar cooling applications (e.g., single stage absorption chillers and desiccant systems). Due to the boiling point constraint (100 ◦C at 1 bar), the use of water as sensible heat storage medium for high temperature application (double effect and triple effect chillers) requires increasing the system pressure [16].PDF Image | Comprehensive Review of Thermal Energy Storage
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