The shape factor is calculated by a sub-program of the calculating routine [15], The program can calculate the shape factors of flat plates only, so the cross section of the model storage unit is simulated as a regular octagon and surface of it is simulated eight flat plate around the model storage unit. The radiation coefficients are selected from references [16,11], The radiation coefficients are 0.6 on the surface of the model storage unit and 0.7 on the reflector respectively. Slits occupy half of the whole area of the heater, therefore half of the radiation coefficient, 0.4, is applied to the heaters instead of reducing the radiation area of the heater. The heat loss is calculated on the assumption of radiation from the reflectors to an infinite point. Effective radiation coefficient of the refractors for the heat loss is evaluated to be 0.12 from the experimental results. Radiated heat to the surfaces of the model storage units is conducted through the heat storage materials. The heat conduction equation is as follows. where, r is the effective density of the storage material, Cp is the effective specific heat of the storage material, Ac is the conduction area, 1 is the effective thermal conductivity of the storage material, Tim is the temperature at the calculating point, Tj is the temperature at the adjoining point and t is the time. Thermal conductivity of the spongy matrix of porous carbon was measured by a steady method using the C-Matic Tester completed by Dynatech R/D Co. Obtained values of the thermal conductivity are 13.6 and 11.0 W/mK respectively at 320 K and 380 K. Temperature dependence of the thermal conductivity is determined from the references [11,16], Latent heat of the LiF is 956 kJ/kg [10] measured by heat flux differential scanning calorimetry method using the Perkin-Elmer DTA 1700. The latent heat of LiF is simulated as increasing of specific heat around the melting temperature, i.e. from 1115 K to 1118 K. Other physical properties were selected from the references. [16,11]
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