Working fluid selection for an increased efficiency hybridized geothermal-solar thermal power plant in Newcastle, Utah

Renewable sources of energy are of extreme importance to reduce greenhouse gas emissions from traditional power plants. Such renewable sources include geothermal and solar thermal energy. These involve harnessing the heat from underground water sources or from solar irradiance. The heat energy is used to vaporize a working fluid which turns a turbine and generator system to produce electricity. The working fluid is then condensed and reused in the cycle. The working fluid needs to be carefully selected based on the geothermal brine temperature or the amount of solar irradiation available and the fluid's thermodynamic properties. There are many low temperature geothermal resources that have the potential to generate commercial electricity, but due to their low temperatures cannot do so efficiently. The efficiency of geothermal power plants can be increased by adding solar energy. A commercially successful method of concentrating solar power is the solar trough. A solar trough is a parabolic mirror that focuses solar energy onto a collector tube. A heat transfer fluid is circulated within the tube to collect the energy. This heat transfer fluid would be heated and passed through a heat exchanger with the working fluid in a binary power plant to increase the working fluid's enthalpy. There are vast quantities of solar irradiance data available for many locations throughout the United States. The data of interest are the direct normal irradiance which is measured by a pyrheliometer and a solar tracking device. Published data have shown that the direct normal radiation can be as high as 1,000 W/m2 during peak hours of the day with a daily average, during sunlight hours, of 850 W/m2. With these energy rates it could be advantageous to use this solar energy to increase the enthalpy of a power system. Specific measurements should be made at a geothermal site in order to know if such a project is feasible. If the project is feasible, a working fluid for the power cycle should be selected and an economic analysis should be performed in order to make project recommendations.