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Project Title: Integrating cryogenic power conversion and hydraulic action
Principle Investigator(s): Raymond Sedwick (Aerospace Eng.)
Abstract: Future NASA missions are planned to explore and excavate craters located at the lunar poles. These permanantly shadowed areas typically remain at temperatures around 50-80 K. Power for these missions will most likely be provided by RTGs, which currently use thermoelectric power conversion at an efficiency of 10-15%. As an alternative, because the operating temperature of an RTG can be as high as 900K, sinking the waste heat of a thermodynamic cycle at the temperature of the surrounding environment would allow for nearly 90% theoretical conversion efficiency. At the same time, terrestrial excavation has been optimized over the years to use hydraulic actuation devices. These devices have very high mechanical advantages and are very efficient at transmission of mchanical power. Typical hydraulic fluids would, however, become prohibitively viscous (or solid) at the cryogenic temperatures of these missions. A solution is to integrate the hydraulic system with a Rankine power system, both sharing a common cryogenic working fluid such as LN2. The greater flow volume of LN2 through the hydraulic system allows it to be used as a heat sink for the working fluid of the Rankine cycle, while at the same time being kept from freezing (below 65 K). Additional heat can be taken from the high temperature turbine inlet if necessary to keep the LN2 tank warm, or radiated away at the temperature of the RTG (using a very small radiator) to keep it from boiling.
Figure 1. Schematic of integrated hydraulic actuator with Rankine power cycle (left) and cryogenic (LN2) Rankine cycle (right)
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