Thermal Management System for Long-Lived Venus Landers

Long-lived Venus landers require power and cooling. Heat from the roughly 64 General Purpose Heat Source (GPHS) modules must be delivered to the convertor with minimal ÄT. The cooling system must be shutoff during the transit to Venus without overheating the GPHS modules. This program will develop an alkali metal Variable Conductance Heat Pipe (VCHP) integrated with a two-phase heat collection/transport package (HTP) between the GPHS stack and the convertor. The VCHP allows the Stirling converter to be shutoff during transit to Venus. The two-phase HTP minimizes the temperature drop between the multi-GPHS stack and the heater head. The HTP is required due to the large number of modules that must be interfaced, and the low allowable ÄT between the heater head temperature of 1200oC and the maximum allowable iridium cladding temperature in the GPHS (1266oC). The HTP also improves the convertor efficiency by decreasing the temperature non-uniformities at the high heat flux interface of the hot end of the heater head. It is superior to pumped liquid systems for transferring heat, because it eliminates the low efficiency liquid metal pump that they require. Other advantages of the system include low mass and volume, and a high degree of redundancy.

One commercial application is VCHP heat exchangers in fuel cell reformers. In a fuel cell reformer, diesel fuel and air pass through a series of high temperature reactors to generate hydrogen. The operating temperature of the reactors must be closely controlled to maintain their chemical equilibrium. The current scheme uses a bypass valve, which has several drawbacks: it requires active control, requires power, and has a large pressure drop. A VCHP heat exchanger can replace the current heat exchanger and control system with a passive system that automatically maintains the output stream from the heat exchanger at a constant temperature.

The immediate application is for a long-lived Venus lander that contains a Stirling system integrated with a large number of GPHS modules. The thermal management system will efficiently collect the heat from the GPHS modules and deliver it to the Stirling engine. The thermal management system will allow the Stirling convertors to be shutoff during the transit to Venus. More generally, the systems developed on this program are applicable to all NASA missions with high powered radioisotope systems that require a large number of GPHS modules. Beside the Venus mission applications, the technology is applicable to lunar missions for both long (habitation modules) and short duration (International Lunar Network).

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