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788 measured by a device called differential scanning calorimeter. When testing with differential scanning calorimeter, a net dynamic heating power will be input to the tested sample to make its temperature increase at a constant rate, and the value of this net dynamic heating power is recorded and finally plotted in the form of differential scanning calorimeter curves. In order to get the correct “net” dynamic heating power input to the tested sample, a reference material should be used. The reference materials normally used during differential scanning calorimeter testing’s are those with a constant heating or cooling rate, i.e. with the constant specific heat capacity, such as Alumina and indium metal [7],[19]. The latent heat thermal energy storage method that is suitable for solar heating and air conditioning has received considerable attention due to its advantages of storing a large amount of energy as a phase transition at a constant temperature. The latent heat of melting is the large quantity of energy that needs to be absorbed or released when a material changes phase from a solid state to a liquid state or vice versa [12]. The storage temperature or phase change can be improved by choosing the phase change material in such a way that its phase change temperature optimizes the thermal gradient with respect to the substance with which the heat is being exchanged. For example, with paraffins and alkanes it is possible to vary the number of carbon atoms or form different molecular alloys, which allows a practically continuous variation of the phase change temperature with certain ranges [10],[20]. Figure 5 shows the increase of internal energy when energy in the form of heat when it is added to a substance [20]. The well known consequence is an increase in temperature (sensible heating) or change of phase (latent heating). Starting with an initial solid state at point O, a heat addition to the substance first causes sensible heating of the solid (region O-A) followed by a solid to liquid phase change (region A-B), a sensible heating of the liquid (region B-C), a liquid to vapor phase change (region C-D), and a sensible heating of the vapor (region D-E). Fig. 5. Temperature (T) -time (t) diagram for the heating of a substance The total amount of heat can be written in the following formula: [6] ⎡TA Q=m⋅⎢∫Cps(T)dT+qt + ⎣TO (4) TC TE ⎤ +∫Cpl(T)dT+qi +∫Cpv(T)dT⎥ TB TD ⎦ Some of the important properties required for phase change material are: •High latent heat of fusion per unit mass, so that a lesser amount of material stores a given amount of energy; •High specific heat that provides additional sensible heat storage effect and also avoid sub cooling; • High thermal conductivity so that the temperature gradient required for charging the storage material is small; •High density, so that a smaller container volume holds the material; •A melting point in the desired operating temperature range; •The phase change material should be non- poisonous, non-flammable and non-explosive; •No chemical decomposition, so that the latent heat storage system life is assured; • No corrosiveness to construction material; •Phase change material should exhibit little or no super cooling during freezing [16]. Phase change materials are either packaged in specialized containers such as tubes, shallow panels, plastic bags, etc., or contained in conventional building elements (wall board, ceiling) or encapsulated as self-contained grains. Because the chemicals in some phase change materials separate and stratify when inPDF Image | Thermal energy storage overview
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