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42 Chapter 6. System-Level Considerations the Martian night. Thermal losses are modest with insulation; we estimate that 65 MJ or 3.3% per sol is achievable with R-30 insulation (5.3 K-m2/W). We have investigated thermal gains from rejected heat from cryopumps maintaining liquefied CH4 and O2. These operate at an efficiency of 5%. Assuming 7 t CH4 and 28 t O2 are being maintained at their boiling points, thermal losses from 5% emissivity spherical tanks are 3 W, requiring a cryopump energy of 60 W to operate. Thus, 56 W are available; we assume 80% of this quantity could be harvested for electrolyte thermal storage, or 45 W. However, this only adds 4 MJ/sol, a negligible amount. After these storage losses and gains, there is 1919 MJ of useful energy from the PEC system to use each night to keep it warm. Spread over 14.7 h, this is a flux of 227 W/m2, just slightly more than needed to make up for the nighttime losses of 214 W/m2. To increase this margin of safety, a lower PEC operating temperature, higher solar absorption, and/or lower thermal emissivities would be needed. 6.3.1 Storage Tank We assume 43 m3 of the PEC electrolyte (modeled as H2O) is stored in an underground tank of radius 2.2 m with a surface area of 60 m2. Assuming R-30 insulation (5.3 K-m2/W), thermal losses from tank to surrounding rock with an average temperature of 225 K (calculated from average of sunlit and nighttime temperatures) implies a temperature gradient of 65 K, or 732 W continuous thermal loss. Over a Martian sol, this is 65 MJ or 3.3% of stored energy, a reasonably small value. A temperature drop due to the electrolyte of 0.36 K/sol is further assumed. During sunlit operation, assuming a PEC device layer thickness of 1 cm, a total PEC electrolyte volume is 1.6 m3 or 3.7% of stored electrolyte volume. If the entire volume of electrolyte is circulated through the PEC system over the 10 h sunlit period, this implies a residence time of 22 min, during which time the electrolyte temperature increases by 11.0 K. During nighttime operation, the residence time is longer (33 min) and temperature decrease is 10.0 K due to smaller thermal losses. Subtracting the temperature drop during storage, the system is net positive by 0.6 K/sol, implying that a small margin is available. We have investigated the capacity of the system to store energy during a 30-sol dust storm when PEC output is reduced to essentially zero, and conclude that it is possible provided the system operates at a reduced temperature, allowing surplus thermal energy to be stored over long periods. Initial calculations indicate that lowering the operating point to 280 K would provide an excess of 57 W/m2 during sunlit hours or 331 MJ/sol of stored energy. Over a 30-sol dust storm, a total of 62,000 MJ would be needed to maintain PEC temperatures. However, over the 187 sols that would be required to store this thermal energy, losses would be considerable. Instead, we constrain total thermal energy loss to 50%, implying gross thermal energy of 124,000 MJ and aPDF Image | ISRU Challenge Production of O2 and Fuel from CO2
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