Low-Temperature Stirling Engine for Geothermal Electricity Generation

Up to 2700 terawatt-hours per year of geothermal electricity generation capacity has been shown to be available within North America, typically with wells drilled into geologically active regions of the earths crust where this energy is concentrated (Huttrer, 2001). Of this potential, about half is considered to have temperatures high enough for conventional (steam-based) power production, while the other half requires unconventional power conversion approaches, such as organic Rankine cycle systems or Stirling engines. If captured and converted effectively, geothermal power generation could replace up to 100 GW of fossil fuel electric power generation, leading to a significant reduction of US power sector pollution emissions. In addition, with the rapid growth of hydro-fracking in oil and gas production, there are smaller- scale distributed power generation opportunities in heated liquids that are co-produced with the main oil and gas products. Since 2006, Cool Energy, Inc. (CEI) has designed, fabricated and tested four generations of low-temperature (100 C to 300 C) Stirling engine power conversion equipment. The electric power output of these engines has been demonstrated at over 3 kWe and over 22% thermal conversion efficiency for an input temperature of 300 C. In Phase I, Cool Energy and CSU analyzed thousands of oil and gas wells for electric power generation potential, and modeled the performance of electricity generation in oil and gas production system using heat from co-produced liquids to drive a Stirling engine. It was estimated that 470MW of electrical generating potential exists from geothermal energy in producing oil and gas wells. In addition, CEI customized the design of a 20 kWe engine specifically for the low-temperature geothermal application by optimizing engine performance and customer value for the temperature ranges supplied by co-produced liquids. The 20 kWe design uses novel approaches of self- lubricating, low-wear-rate bearing surfaces, non-metallic regenerators, high-effectiveness, low-cost heat exchangers, and a rotary transmission to transfer the mechanical work of the engine to the electric generator built into the engine housing. A Stirling approach has several advantages over the current products used in these applications: smaller opportunities can be addressed, part-load performance is excellent, and Stirling engines do not use the environmentally hazardous organic fluids found in incumbent technologies. Phase I established clear economic and technical feasibility of the geothermal-Stirling approach, and Phase II will support the design completion and prototype build of the 20 kWe GeoHeart Engine, customized for geothermal power generation. This engine will be completed in mid-2014, and will be fully characterized in terms of thermodynamic, mechanical, and electrical power outputs, thermal-to-electrical conversion efficiency, heat draw, cooling requirements, parasitic loads, and performance under a range of operating conditions.