Next Generation Geothermal Power Plants

Abstract

A number of current and prospective power plant concepts were investigated to evaluate their potential to serve as the basis of the next generation geothermal power plant (NGGPP). The NGGPP has been envisaged as a power plant that would be more cost competitive (than current geothermal power plants) with fossil fuel power plants, would efficiently use resources and mitigate the risk of reservoir under-performance, and minimize or eliminate emission of pollutants and consumption of surface and ground water. Power plant concepts were analyzed using resource characteristics at ten different geothermal sites located in the western United States. Concepts were developed into viable power plant processes, capital costs were estimated and levelized busbar costs determined. Thus, the study results should be considered as useful indicators of the commercial viability of the various power plants concepts that were investigated. Broadly, the different power plant concepts that were analyzed in this study fall into the following categories: commercial binary and flash plants, advanced binary plants, advanced flash plants, flash/binary hybrid plants, and fossil/geothed hybrid plants. Commercial binary plants were evaluated using commercial isobutane as a working fluid; both air-cooling and water-cooling were considered. Advanced binary concepts included cycles using synchronous turbine-generators, cycles with metastable expansion, and cycles utilizing mixtures as working fluids. Dual flash steam plants were used as the model for the commercial flash cycle. The following advanced flash concepts were examined: dual flash with rotary separator turbine, dual flash with steam reheater, dual flash with hot water turbine, and subatmospheric flash. Both dual flash and binary cycles were combined with other cycles to develop a number of hybrid cycles: dual flash binary bottoming cycle, dual flash backpressure turbine binary cycle, dual flash gas turbine cycle, and binary gas turbine cycle. Results of this study indicate that dual flash type plants are preferred at resources with temperatures above 400 F. Closed loop (binary type) plants are preferred at resources with temperatures below 400 F. A rotary separator turbine upstream of a dual flash plant can be beneficial at Salton Sea, the hottest resource, or at high temperature resources where there is a significant variance in wellhead pressures from well to well. Full scale demonstration is required to verify cost and performance. Hot water turbines that recover energy from the spent brine in a dual flash cycle improve that cycle's brine efficiency. Prototype field tests of this technology have established its technical feasibility. If natural gas prices remain low, a combustion turbine/binary hybrid is an economic option for the lowest temperature sites. The use of mixed fluids appear to be an attractive low risk option. The synchronous turbine option as prepared by Barber-Nichols is attractive but requires a pilot test to prove cost and performance. Dual flash binary bottoming cycles appear promising provided that scaling of the brine/working fluid exchangers is controllable. Metastable expansion, reheater, Subatmospheric flash, dual flash backpressure turbine, and hot dry rock concepts do not seem to offer any cost advantage over the baseline technologies. If implemented, the next generation geothermal power plant concept may improve brine utilization but is unlikely to reduce the cost of power generation by much more than 10%. Colder resources will benefit more from the development of a next generation geothermal power plant than will hotter resources. All values presented in this study for plant cost and for busbar cost of power are relative numbers intended to allow an objective and meaningful comparison of technologies. The goal of this study is to assess various technologies on an common basis and, secondarily, to give an approximate idea of the current costs of the technologies at actual resource sites. Absolute costs at a given site will be determined by the specifics of a given project.