Getting the Most Out of Geothermal Energy: Economics of Power Generation, Heating, and Cooling

Abstract

Abstract Geothermal energy is mostly used for electric power generation, and there is the perception that power generation is the best use of high temperature geothermal energy since electricity is a more valuable commodity than heat, and the price of electrical energy is about 4 times the price of heat energy. However, the conversion efficiencies of geothermal power plants are typically about 10%, and thus the generation of 1 unit of electricity requires the consumption of about 10 units of geothermal energy. Also, the temperature of the geothermal fluid leaving the heat exchanger of a binary power plant can be as high as 100°C, which is very suitable for low-temperature uses such as space heating and preheating of process water, but is unsuitable for economical power generation, and is discarded. So it is no surprise that the electricity generated by a geothermal power plant is only about one-fifteenth of the heat that can be harvested and sold to prospective users for space, water, and process heating. As a result, the revenues from a geothermal site can be increased by 4 times by selling geothermal heat directly instead of producing and selling electricity. Therefore, for a specified geothermal resource, an initial investment of up to 4 times (even more, depending on the relative prices of electricity and natural gas) can be justified if the geothermal heat plant (including the circulation loop) is to be built at that site instead of a geothermal power plant. Typical coefficient of performance of heat driven absorption cooling systems is about 0.7, and the revenues from a geothermal resource can be increased by 2.5 times by using high-temperature geothermal energy for cooling. Of course a good compromise is to use the portion of geothermal energy above 115°C for power generation, the portion between 115°C and 90°C for cooling, and the remaining part for heating.