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the material, m is the material total mass, and cp is the specific heat capacity of the material. As cp is a function of temperature, we can use equation (1) to calculate the total amount of stored thermal energy. However, if the temperature range is too small to consider the variation of cp, equation (1) can be rewritten as: Q=m⋅c ⋅ΔT=m⋅c ⋅(T −T) p.avg p.avg 2 1 (2) where cp.avg is the average specific heat capacity between temperature T1 and T2 [11]. The high specific heat capacity cp can have direct impact on the amount of stored thermal energy based on equation (1) and (2). The long term stability assures the low degradation of the heat storage material after thousands of thermal cycling. From the foregoing definition as well as equations (1) and (2), we can see that desirable sensible heat storage requires the energy storage material to have four characteristics: • High specific heat capacity, • Long term stability under the thermal cycling, • Good compatibility with its containment, • Low cost [9]. Sensible heat storage systems are simpler in design than latent heat or thermo chemical storage systems. However they suffer from the disadvantage of being bigger in size and cannot store or deliver energy at a constant temperature. The main factors of the total cost of the storage system depends on compatibility with its containment is requirement for both the heat storage material and the containment. The cost of the sensible heat storage solution mainly depends on the characteristics of the storage material. It is very common to utilize very cheap materials such as water, rocks, pebbles, sands, etc., as the storage medium [18]. There are five types of sensible seasonal thermal storage: hot water thermal energy storage, aquifer thermal energy storage, gravel- water thermal energy storage, borehole thermal energy storage, and cavern thermal energy storage. Latent heat storage The main principle in latent heat storage is that when heat is applied to the material it changes its phase from solid to liquid by storing the heat as latent heat of fusion or from liquid to vapor as latent heat of vaporization. Theoretically, the phase change material has a phase change point when the phase transition happens, but in practice the phase change process happens in a certain temperature range instead of one exact point. Figure 3 and 4 shows the specific heat capacity (cp) – temperature (T) curve, respectively specific enthalpy (h) – temperature (T) curve of certain phase change material. 787 Fig. 3. The specific heat capacity (cp) – temperature (T) curve of certain phase change material Fig. 4. The specific enthalpy (h) – temperature (T) curve of certain phase change material We can see that on temperature range [T1,T2], cp-T curve experiences a peak interval, while the h-T curve presents a corresponding slope change in the same temperature range due to the relations between h-T and cp-T functions: cp (T)= dh(T) (3) dT In practice, the phase change temperatures and enthalpy changes of a certain phase change material during the phase change process arePDF Image | Thermal energy storage overview
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