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  • A 25 kW Solar-Stirling Concept for Planetary Surface Exploration

    Paper ID

    2322

    author

    • Henry Brandhorst

    company

    Auburn University

    country

    United States

    year

    2005

    abstract

    In the 1990’s, NASA developed a 25 kW free piston Stirling convertor for testing under the SP-100 space nuclear power program. This convertor was successfully tested before the program ended. The 25 kW convertor consisted of two 12.5 kW convertors connected through the central heat source, resulting in a dumbbell-shaped system. Later the system was disassembled into two separate convertor units. The goal was a system operating at TH of 1050 K and TC of 525 K and a ratio of 2. The convertor was built from Inconel 718 and was operated at a hot end temperature of 650 K and a cold end of 325 K to save time and costs. It was operated for 1500 hrs of essentially unattended operation with efficiency above 25%. This program laid a solid foundation that demonstrated the feasibility of the free piston Stirling convertor system for space power systems. Mass estimates based on known technology advances projected a convertor mass over 200 W/kg. Since that time, new developments in free piston Stirling convertors have produced smaller and lighter versions, albeit at much smaller sizes. These convertors operate as high as TH of 925 K and TC as low as 325 K for a ratio of 2.8 and efficiencies above 30%. Thus we expect convertor specific power to increase further with more materials advances. The purpose of this paper is to propose a new lightweight solar-powered system concept that uses the updated 25 kW convertor, an inflatable Fresnel lens solar concentrator and an updated liquid sheet radiator. Inflatable Fresnel lens concentrators have been produced in a 5 m diameter at a specific mass of 0.5 kg/m2. The new version of the liquid sheet radiator adapted for planetary surfaces is essentially a fountain enclosed in a transparent envelope. The liquid that flows down the inside of this envelope is thick enough to have high optical emissivity for the system. Past studies using silicone oil have shown that a liquid thickness of only 300 micrometers is sufficient to achieve an optical emissivity of 0.85 at a temperature of 373 K. Theoretical calculations indicate further increases in emissivity with temperature up to at least 400 K. One additional characteristic of the liquid sheet radiator concept is that it is exceptionally stable and does not require special machining to achieve its performance. Additional advantages of the Stirling system compared to solar arrays is that it produces alternating current and provides waste heat that can be used for habitat heating. This heat can also supply part of the thermal energy needed for in-situ resource processing. Based on existing data, it appears likely that this system could achieve a specific power equivalent to or better than the best solar arrays of today. This system represents a novel, updated concept for planetary surface power systems in the 25 kW range. The system can also accept other forms of energy from a laser or a radioisotope heat source for night time use. From the use of lightweight Fresnel lenses and enclosed liquid sheet radiators coupled with recent advances in Stirling convertor technology, a new, space tolerant planetary power system may emerge.