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6 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 TH Q =m∫CpdT =mCp (TH −TL )whenCp ~constant (1) TL where m is the mass of storage material, Cp is specific heat at con- stant pressure of the SHS materials, and TL and TH are the low and high temperatures of SHS use. Cp is assumed independent of tem- perature within the narrow temperature range of the SHS application. In applications below 100 °C, water is the best SHS liquid, because of its wide availability, low cost and relatively high specific heat. Above 100 °C, synthetic oils, molten salts, liquid metals, or powders can be used. For air heating applications, rock bed type storage ma- terials are more suitable. Besides physical properties, such as density and specific heat of the storage materials, there are other proper- ties that are important for the efficiency of SHS, including operational temperatures, thermal conductivity and diffusivity, vapour pres- sure, compatibility among materials and their stability, heat losses as a function of the surface area to volume ratio, and cost of the materials and systems. A SHS system usually consists of a storage medium, a container tank, and inlet/outlet connections. To prevent any potential loss of thermal energy, thermal insulation is re- quired. The storage medium can be solid or liquid. Solid media, such as concrete and cast ceramics, are commonly used in the form of packed beds, which require a fluid for effective heat exchanging and as heat carrier. If the fluid is a liquid, the system is called a dual storage system, because the heat capacity of the solid in the packed bed cannot be neglected. One of the most significant advantages of the dual system is the use of inexpensive solids, such as rock, sand, or concrete as storage materials. Liquid media mainly include molten salts or synthetic oils. They are naturally stratified in the storage vessel due to the difference in density between hot and cold layers: the hot fluid is supplied to the upper part of storage during charg- ing, whereas the cold fluid is extracted from the bottom part during discharging. Latent heat storage (LHS) makes use of the heat absorbed or re- leased by the storage material when it experiences a phase change between solid and liquid or liquid and gas. The storage capacity of a LHS system with phase change materials is given by: Fig.4. Totalheatstoredvs.temperatureforsensibleandlatentheatstoragemate- rials. 1: KNO3= NaNO3, 2: NaOH= NaCl, 3: SiO2, 4: SiC, 5: Sb2O3 [27]. HTF: Heat transfer fluid. Sb2O3–PCM: the total heat stored is lower than the sensible heat stored in SiC and/or SiO2. Molten salts KNO3—NaNO3 and NaOH—NaCl, on the contrary, offer a significant advantage. Due to the latent heat of fusion, there is a steep jump in energy stored at the temperature of the PCM melting point. Thermo-chemical storage (TCS) uses the heat absorbed and re- leased by a reversible chemical reaction. The amount of heat stored is proportional to the amount of storage material (m), the endo- thermic heat of the reaction (ΔHr) and the conversion (α), given by: Q=mαΔHr withα≤1 (4) It should be remembered that the external heat supply will also need to cater for the sensible heat of the initial reactant between the starting (ambient) and the reaction temperature. For an operation between 750 °C and 250 °C, with specific heat value around 1 kJ/kg, the heat storage capacity of sensible heat storage is ~500 kJ/kg, between 700 and 1000 kJ/kg in latent heat storage due to the contribution of the heat of phase transition, and in excess of 1000 kJ/kg for thermo-chemical storage, respectively. 1.5. Classificationsofthermalenergystorage TES systems can be classified according to different param- eters, being the temperature range of application, the mechanism of energy storage, and the integration within the energy storage concept. 1.5.1. Classification according to temperature range and associated re-use technology Essential to be able to capture, store and re-use thermal energy is the use of a heat transfer fluid (HTF): the ranges of applicable thermal energy storage are hence a function of the HTF selected. According to the temperature range at which the system works, TES can be classified as low temperature thermal energy storage and high temperature thermal energy storage. In this study the divi- sion temperature is considered at about 200 °C. The classification according to the working temperatures of the HTF, TES and thermal block is illustrated in Fig. 5. The increased technological risk reflects the impact of safety of operations at extremely high temperature and the use of high value construction materials. Traditional HTF include gas, water/steam, thermal fluids and molten salts. The use of particle suspension as Tm TH Q=m∫CpsdT+mΔHm +m∫CpldT TL Tm (2) where Tm is the melting point of the PCM, Cps and Cpl are the spe- cific heat of the PCM in solid and liquid state respectively, and ΔHm is the phase change enthalpy. If Cps and Cpl are not a function of tem- perature, the heat storage capability can be calculated by integration into the following equation: Q =m[Cps (Tm −TL )+ΔHm +Cpl (TH −Tm )] (3) LHS with PCMs is at present very promising, because it exhib- its high energy storage density and can store heat at constant temperatures, i.e. the phase transition temperatures of the mate- rials [22–26]. Phase transition may occur in forms of solid–solid, solid–liquid, solid–gas, and liquid–gas. Among these phase changes, only the solid–liquid phase change has been used for LHS. Liquid– gas phase changes are not practical for use as thermal storage due to the large volumes involved or high pressures required to store the energy when the compounds are in their gaseous or vapour phase, although liquid–gas transitions have a higher latent heat of transformation than solid–liquid transitions. Solid–solid phase changes are usually too slow and have a very low value of latent heat of phase change. Although PCMs are expected to have a po- tential advantage towards energy storage in comparison with sole sensible heat storage, Fig. 4 illustrates the temperature range where the efficacy of sensible heat storage of the heat carrier (e.g. sand) can exceed the latent heat storage capacity, as illustrated for e.g.PDF Image | Thermal energy storage: Recent developments
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