Binary Systems with Geothermal Fluid Pressurization to Avoid Flashing: Energy Evaluation of Down-Hole Pump Cooling Below Geothermal Fluid Temperature

Abstract

The pressurization of the liquid geothermal fluid at a suitable depth in the well, by means of an electric powered pump, is an established technique to avoid flashing in the well. However this technique is limited by the maximum temperature which can be sustained by the windings of the pump motor. In the present paper, the basic feasibility and energy cost of cooling the windings are evaluated. The proposed solution consists of a cooling loop made by an evaporator fed with water, which removes the heat generated in the motor windings, a downward water pipe, which feeds the evaporator, an upward steam pipe which removes the evaporated steam and brings it at well head and a condenser which rejects the removed heat into the ambient; a pump is also needed in order to pressurize the cooling loop and avoid evaporation in the downward water pipe. The size of the pipes to be put into the well for the cooling system is preliminarily dealt with: when feasible, this technique can broaden the application range of the binary cycle technology. Concerns about the environmental impact and reservoir exploitation point out that the adoption of the binary cycle is particularly advantageous in many aspects: it is well known that emissions in the atmosphere of harmful or undesired components (for example: H2S, CO2) are to be avoided: in binary power plants the all-liquid geothermal brine can be easily re-injected after use. This solution therefore gives the chance to treat the noncondensables gases as dissolved in the liquid brine instead of as gasified in the vapor phase. Re-injection has also a beneficial effect on the reservoir exploitation.