Abstract
In a solar dynamic power generation system for the space station, salts (phase change materials with high latent heat of fusion) will be used as a thermal storage medium to convert the time varying (on-off) energy from the sun to a constant power output to the engine during all phases of the orbit. The selected salts for testing encompassed fluoride salt and a hydroxide salt. These salts will be contained in sealed tubes (canisters) with the heat transfer fluid passing over the surface of the canisters. They will periodically cycle between a liquid state and a solid state during the "on" (sun shining) and the "off" (occult) periods, respectively. To characterize the behaviors of these salts, Rocketdyne, under its own funding, has conducted extensive testing in the last several years to determine the performance of the canisters.
Many alloys have been evaluated for their potential for long-term (30 yr) containment of the salts. For the LiF-CaF2 eutectic fluoride salt, two alloys were thermally cycled: Hastelloy B2 (an Ni-Mo-Fe corrosion-resistant alloy) and Haynes 1.88 (a Co-Cr-Ni-W alloy). Both alloys had been shown by laboratory tests to successfully contain the salt. A Hastelloy alloy canister had been cycled through a maximum of 7231 cycles (11,328 hr) without failure and a Haynes 188 canister has been cycled through a maximum of 3291 cycles (5156 hr) without failure. It has been shown that (1) control of potential contamination during the salt-filling process and (2) control of the salt purity are critical parameters in achieving a canister that can be successfully cycled. Metallurgical analysis was performed to understand the contamination and purity issues in order to understand the failure mechanisms and to establish an initial filling procedure. Based on these test results, subsequent tilling and thermal-cycle testing of subsize and full-size canisters (of HB2 loaded under improved salt contamination/purity control) did not show any cracking of the HB2 canisters.
