Power Generation From Geothermal Energy

Abstract

Geothermal reservoirs may be liquid- or vapor- dominated. Two types of power cycle will be considered in this paper for liquid-dominated reservoirs, the vapor flashing cycle and the binary cycle using isobutane as the working fluid. In vapor flashing cycle, flashing tanks are used to separate steam from brine, and the steam expands in turbine to generate power. In binary cycle, the working fluid is superheated by geothermal fluid and then expands in turbine for power generation. Addition of a regenerator to binary cycle improves the thermal efficiency. For identical conditions of geothermal fluid, the power generated by a regenerative binary cycle is slightly higher than that of basic binary cycle. Another advantage of the regenerative binary cycle is that the heat rejection rate of the system is lowered by using the regenerator. Geothermal fluid from well may exist in two phases, vapor and liquid, if the fluid in well is self-flowing. To avoid scaling, submersible pump may be used to force the fluid out of a well. When pump is used, there will be no vapor at the wellhead. Effects of pumping on power output and heat rejection of binary cycle have been evaluated in Section III. For geothermal fluids with high content of non-condensable gases, a non-condensing plant may be used. Condensing plants have much higher thermal efficiencies than non-condensing plants. If cooling water to condenser is not accessible, a cooling tower can be used to recycle the cooling water. At the end of this paper is an evaluation of multi-effect evaporator used to produce fresh water from the waste brine of regenerative binary plant.