Program Description The transient behavior of the phase change material is described by the first order differential equation, equation 2. Euler’s method [4] is used to numerically integrate this equation to provide an updated value for the specific internal energy, u. Equation 3, then, determines the liquid fraction X. A time step of 1 min is used. If the PCM becomes liquid, X=l, all power is, then, diverted to the parasitic load. A constant 95 minute orbit is assumed; comprised of 36 minute and 59 minute eclipse and insolation periods, respectively. Mass Quantity of the Phase Change Material The amount of receiver mass required for the two phase mixture is related to the accumulated energy and its latent heat by [3]: In addition to the normal loads, the energy stored in the PCM must be capable of accommodating peak load demands and contingency requirements. A hypothetical worst case scenario is shown in Figure 3. This scenario is divided into three segments: first, an isolation period with peaking (49 min at 60 kW and 10 min at 112.5kW); second, an eclipse period with peaking (10 min at 112.5kW and 26 min at 75kW); and third, a contingency period (95 min at 37.5 kW) [5]. The contingency requirement is defined as one half power for one complete orbit. The accumulated energy is found by [6,7]: The value for Qa determines the maximum value of latent energy necessary (in the worst case) for the PCM with its associated power system. By inserting equation 5 into equation 4, the required mass is readily attained. The Brayton power cycle, using a PCM of 67 mole percent lithium fluoride, LiF, and 33 mole percent magnesium fluoride eutectic, MgF2, requires a mass of 1725 kg. The Organic Rankine cycle, using a PCM of lithium hydroxide, LiOH, requires a mass of 2320 kg. The respective latent heats are 913 kJ/kg for the Brayton PCM, and 872 kJ/kg for the Rankine PCM [8]. The efficiencies were 0.25 for the Brayton system and 0.2 for the Rankine system. Using the worst case scenario of Figure 3 (P. 205) the liquid fraction is tracked as shown in Figures 4 and 5 for Brayton and Rankine systems respectively. The liquid fraction, X, must be equal to 1 (one) entering the eclipse period. Figures 4 and 5 reveal the accumulated energy is being extracted at three different load rates
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