Equation 15 clearly reveals that the Brayton system stores 37% more energy per orbit than the Organic Rankine system before achieving the steady periodic profile. Conclusion This program simulates the phase change energy stored in a proposed PCM for a Space Station Freedom solar dynamic power system. The power system is modeled using a single node receiver and uses an overall efficiency for the receiver, heat engine and power distribution systems. The thermal energy storage material is assumed to remain in the two- phase region at all times. This simulation assumed a 95 minute orbit; comprised of 36 minute eclipse and 59 minute insolation periods. Given, (1) a load profile, (2) a net power to the receiver, (3) PCM properties [8], and (4) an overall efficiency, this transient model will compute the liquid fraction of the PCM and any excess power dissipated through the parasitic load. Using the representative peaking and contingency requirement [6,7], it is found that only about one-half of the available latent heat is used under normal load requirements. The rate of change of the liquid fraction, dX/dt, was also computed for each system. Using this in combination withXmin, the number of orbits required for the liquid fraction to reach a steady periodic profile was calculated. The analytical results compared well with the numerical results in this regard. This program can be used to conduct trade studies on the performance of a specific SD power system using different storage materials. It can also be used to analyze different power systems (i.e. heat engines) with their respective phase change materials as was done in this report. In addition, this simulation can be used as a tool to develop load management control strategies. As an example of the latter, for a given load profile, if the liquid fraction, X, is greater than one at the end of sun period, then the controller would add more loads to force X back to unity at the onset of eclipse. The goal is to minimize the use of the parasitic load, illustrated in Figure 2. REFERENCES [1] Calogeras, J.E., Dustin, M.W., and R. R. Secunde (1991) “Solar Dynamic Power for Earth Orbital and Lunar Application,” presented at the 26 Intersociety Energy Conversion Engineering Conference, Boston, MA, August 4-9, 1991, Vol. 1, pp. 274-283. [2] Kerslake, T.W., (1991) “Experiments with Phase Change Thermal Energy Storage Canisters for Space Station Freedom,” presented at the 26 th Intersociety Energy Conversion Engineering Conference, Boston, MA, August 4-9,1991, Vol. 1, pp. 248-261.
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