Since the load demand influences the energy accumulation rate, the heat engine output power will vary with these changing load requirements. One technique to vary engine output power is to adjust the amount of working fluid in the system by using a sophisticated fluid inventory system. Engine output power would be tailored to follow coarse load changes. Rapidly changing load demands would be accommodated by using a parasitic load to dissipate excess power. During the insolation (sunlit) portion of the orbit 0 s is assumed to be a constant and is calculated from knowing the solar constant and the concentrator efficiency. Also, in equation (1) is assumed constant and includes the receiver, the heat engine and the power management and distribution (PMAD) efficiencies. £),, as previously mentioned, can vary and is a user input to the program. Defining in terms of its specific internal energy [3], equation (1) can be rewritten as: Assuming the PCM will remain in the two-phase region (and therefore isothermal), the liquid fraction, X, can be calculated and is a useful method of monitoring the energy level of the salt. It is related to u in the following manner [31: Equations 2 and 3 are used together to determine the liquid fraction of the salt at any instant in time. In the event that the salt has attained its maximum amount of latent energy, X=l, while in the sun portion of the orbit, the excess energy, Qd, is dissipated through the parasitic load. The salt, then, remains at T*. This scenario may occur if the load demand is less than the power available or if there exists a system failure downstream of the SD power unit. A block diagram illustrating this energy balance is shown in Figure 2 (P. 204). Referring to equation 1, the accumulated energy rate, , will be positive or zero provided that 0/ri does not exceed Qs during the sunlit period. Hence, u and X increase over time and the PCM is storing energy. When X attains the value of 1, the PCM is fully liquid (u is constant), and 0a will be zero. From Figure 2 it is then assumed that the PMAD control logic monitoring the accumulated power level,a, would begin to dump power, , whenX=l. Conversely, during the eclipse period with 0s=0, the specific internal energy, u, and the liquid fraction, X, decrease over time depleting the stored energy. It is not expected that the PCM will be fully solid (X=0) for any length of time.
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