Abstract
Geothermal energy is one of the more efficient renewable energy sources. It uses heat from the Earth's interior to produce electricity in geothermal power plants. In binary cycle power plants, geothermal water can often be produced naturally from high-pressure wells. But when reservoir pressure drops, these power plants need to add artificial lift to continue to produce needed quantities of hot water. The geothermal industry is looking at electrical submersible pumping (ESP) systems as a way to improve geothermal fluid production. But ESPs were designed for the conditions in oil wells and are subject to severe complicating factors in geothermal conditions that significantly reduce runlife, such as temperatures up to 200°C (390°F), highly corrosive fluid, and salt deposition (scale). At the same time, production rates need to be higher than those typical of oil production.
The most commonly used geothermal pumps are driven by a transmission shaft and drive on the surface, or they use a submersible asynchronous induction motor. Surface-driven pumps, commonly called line-shaft pumps, have significant depth limitations. Submersible asynchronous induction motors cannot provide a sufficient volume of fluid supply and tend to overheat in high-temperature conditions. To compensate for the heat, induction motors must operate underloaded. Even so, they are subject to frequent premature failures with operating times of between 30 and 100 days. To solve the problem of cost-effective exploitation of geothermal fields, Novomet used its expertise with permanent magnet motors and high-speed pumps to develop an electrical submersible pumping system that would offer more reliability and runlife in geothermal conditions.
A 254-mm (10-in.) geothermal submersible pumping (GSP) system was designed, manufactured, and tested with a production output of up to 12,000 m3/d (75,477 bbl/d, 139 l/s, 2201 gpm,). It featured new generation, high-efficiency pump stages and a permanent magnet motor with a capacity of up to 1.5 MW. The GSP system design was field tested in Turkey. Improvements to early system designs include the use of a heat-conducting filler in the materials used to compound the permanent magnet motor, the adoption of various high-temperature-rated components (AFLAS rubber elements, RYTON motor terminals, and heat-resistant motor oil), and the development of metal-to-metal sealing in the motor lead extension.
One of the early GSP systems installed in the field performed reliably for 470 days at a frequency of 90 Hz, significantly exceeding the target runtime. More than thirty units with a total flow rate of 190,000 m3/d (1,195,000 bbl/d, 2199 l/s, 34,856 gpm) are currently in operation in Turkey. The electrical consumption savings average 25% for each GSP system with a permanent magnet motor compared to systems using asynchronous induction motors.
While designed for geothermal applications, GSPs can also be used in oil and gas operations.
Experimental Verification on Achievement of High Efficiency Operation in Constant Power Region for Permanent Magnet Motor Driven using PWM Inverter with Voltage Booster