Summary

A biphase turbine applied to a geothermal well can produce power from both the steam and the brine flows. Biphase applications in topping cycles for medium and high pressure geothermal wells can provide as much as 40% more power from a given well, while continuing to deliver low-pressure steam to an existing central power plant. Douglas Energy Company has successfully tested a sub-scale biphase turbine on a geothermal well in Coso Hot Springs, California. This test verified the analytic approach and helped refine design improvements for a full-scale biphase turbine that will operate for two years in Cerro Prieto, Mexico. Power generated will be sold to the Comision Federal de Electricidad, Mexico’s national utility.

Project Background

Biphase turbine energy conversion technology uses nozzles to change the energy from pressure to kinetic (flow) energy. When a mixture of water and steam expands, the expanding steam accelerates the water so that both steam and water contain kinetic energy. Kinetic energy of the water is converted to power on the biphase rotating drum; the energy of the moving steam is converted to power by impulse blades on the web of the drum. In phase one of the project, a 0.3 m diameter biphase turbine operated at an existing California geothermal well. Three 250-hour tests were conducted to obtain performance, reliability and maintenance data and to verify analytical modeling and steam blade performance. Geothermal application of a low-alloy steel, HY80, was demonstrated successfully. The flexibility of the turbine was demonstrated by changing the nozzle sets to adjust for different flow conditions. Design improvements verified at this stage included the addition of a hydroblast scale-removal system and an upstream rock/sand catcher. Following phase one, a full-size 0.76 m diameter turbine was designed for service in Mexico. It features a simplified single-rotor design, impulse steam blades and a unique application of water-lubricated bearings. Phase two applies this larger design to a well with a relatively high inlet pressure of 52 bar to show the economic viability of biphase power production at commercial scale.

The Project

The well at Cerro Prieto, Mexico, normally operates at a flow rate of 140,000 kg/hour and a steam fraction of 45%. In a flash process, power is produced as the flow is expanded from 52 bar to 9 bar but some of the energy inherent in the pressure is wasted in the process. The steam is then delivered to the utility’s existing central steam turbine to produce 7.4 MW of electricity. In the biphase turbine demonstration, production is increased using the same pressure without additional flow. The rotating drum and steam blade combination of the biphase turbine expands the 52 bar fluid to 29 bar while delivering steam at this pressure, and produces an additional 1 MW. The 29 bar steam leaving the biphase turbine is then piped to a standard design back-pressure turbine to produce more than 3 MW of power at the wellhead. From here, steam is discharged to the existing utility plant at the facility’s required pressure of 26 bar.

Performance

The biphase and back pressure turbines have a total rated output of 4.18 MW. In the process, the central utility plant power is reduced slightly, to 6.61 MW. However, taken together, the combination of turbines produces 10.8 MW, far exceeding the 7.4 MW generated without the use of the biphase turbine. This extra output requires no change to well operations and has no adverse effect on the environment. The suitability of adding a backpressure component to the design depends on well conditions at various locations.

Economics

The project is being funded in part by the US Department of Energy and the California Energy Commission. The installed plant cost at Cerro Prieto is about $2.69 million (where $ is the US dollar), giving a capital cost of $678/kW. At the agreed purchase price of $0.042/kWh from the utility, the plant is expected to be profitable within one year. The capital costs compare very favorably with conventional flash steam plants, which produce power at between $1,000/kW and $1,500/kW. Douglas Energy is considering installing more plants operating under various well conditions in Mexico, as well as plants at other sites in the United States, the Philippines and Indonesia. The company estimates a range of capital costs between $500/kW and $700/kW. Biphase turbines can be applied to a single well or a group of wells. An analysis of near-term marketability from existing wells in Mexico suggests that a project producing 100 MW from 40 wells would be commercially viable at a selling price of $ 0.037/kWh. About half the plants connected to these wells would benefit from the addition of a back-pressure turbine.

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