Second, heat gain from the environment is minimized due to lower AT. At 10 mbar vapor pressure the saturation temperature for carbon dioxide is 150 K [21]. Hence, in order to avoid heat loss due to condensation of atmospheric carbon dioxide, storage at a temperature above 150 K is preferred. Third, storage above 151 K is desired so that trace amounts of nitrogen and argon can be vented from the methane storage vessel, as previously discussed. In addition, the small quantities of propellant that are vaporized can be reliquefied, eliminating propellant losses. Clearly, near critical storage tanks have a higher specific mass than low temperature storage tanks, but the lower energy requirements and the possibility of venting trace gases make them more desirable. The cooling power required of the refrigerator includes the sensible heat of the propellant gases from the heat rejection temperature of 280 K to their storage temperature, and the liquefaction energy. Systems which could provide the cooling power required for all the propellants, including hydrogen, are discussed in the literature [1,3,4], and are not included here. For the present analysis, the energy requirements for the refrigeration unit are based on 12% of the ideal Carnot coefficient of performance. This is consistent with the literature [1,4], which reports values ranging from 8% to as high as 30%. The assumed storage conditions and the values for the heat of vaporization (AHV) near the critical point used for calculating the liquefaction energies required are listed in Table 5. It is worth noting that AHv decreases rapidly as the critical temperature is approached, so storage near critical conditions results in far more material being evaporated and recondensed for a given heat input than does storage at temperatures corresponding to pressures around 1 atm.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==