Kinematic Stirling Engine Status for Solar Thermal Electric Systems

ABSTRACT:
"The Department of Energy's (DOE) Solar Thermal Program has as one of its program tasks the development and evaluation of conversion devices that are applicable to solar thermal electric systems. To date, the primary research and development activities have involved the dish-electric

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concept, which consists of a heat engine, solar receiver, and generator combined to form a power conversion assembly (PCA), which is then mounted at the focal plane of a parabolic dish concentrator. The solar thermal program has identified the Stirling cycle heat engine as the conversion device with the most potential for meeting the program goals for annual engine efficiency (41%), reliability (50,000 hour life), and cost ($300/kWe installed, 1984 $). Of the available Stirling heat engines the United Stirling 4-95 MKII automotive engine has received the most extensive development for dish-electric systems. The 4-95 was modified for the solar application by incorporating a direct-absorbing solar receiver with the engine heater heads, adapting the power control scheme, and redesigning the lubrication system because of the inverted operation. With DOE funding, the Advanco Corporation used this modified engine in a dish-electric demonstration project called Vanguard. This system demonstrated an overall net system conversion efficiency, solar-to-electric, of 29. 4%. This is the highest conversion efficiency achieved by any solar system technology. However, because this engine was originally designed as an automotive engine, its design life is limited to 3500 hours (100,000 miles) before major overhauls. The directly illuminated heater heads/solar receiver and complex power control system were the cause of problems during the demonstration. McDonnell Douglas Corporation utilized a similar United Stirling engine for a dish-electric system that it developed. The system demonstrated conversion efficiencies of 30%. However, again, the limitations of the automotive engine design caused problems for a reliable solar application. To advance the Stirling technology toward longer life and commercialization, Sandia recently acquired a Stirling Thermal Motors, Inc. (STM) kinematic Stirling engine, STM4-120 for evaluation. This is a four-cylinder engine with a double-acting piston design. It is nominally rated for 25 kW of shaft power output while operating at a heater head temperature of 800°C. STM has demonstrated 23 kW of shaft power with a conversion efficiency of 40% at a heater temperature of 785°C. This engine was designed from the outset to be a long-life machine --50,000 hours before major overhauls. To obtain this life STM has incorporated a pressurized crankcase (increase rod seal life), variable swashplate (efficient and reliable power control), and heat pipe technology (isothermal operation for better efficiency) into its engine. These features give the STM4-120 the potential to achieve the solar thermal program's operation and maintenance goals and increased system efficiencies."

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