Effect of the working fluid on the optimum work of binary-flashing geothermal power plants

Geothermal energy is an excellent form of renewable energy, which is continuously available for the production of electric power. At present, a very high percentage of geothermal power is generated by power systems that directly use the geofluid from a geothermal reservoir to produce electricity, such as dry steam and flashing power systems. These power plants operate at higher temperatures, typically greater than 160 °C. It appears that most of these high temperature geothermal reservoirs have already been developed and this leaves only the lower temperature resources available for the expansion and for the next generation of geothermal power plants. This paper examines the operation of a new system, the binary-flashing power plant, which may be used to harness more efficiently the available energy of geothermal resources at the lower range of resource temperatures. The paper compares the operation of the binary-flashing systems with the typical binary systems using the following substances as working fluids: normal butane, isobutane, hexane, pentane, refrigerant-114, and ammonia. It is observed that when both systems are optimized, the binary-flashing units would produce 25% more work than the typical binary units and that hexane and pentane appear to be better working fluids for these units.

► New geothermal power plants will utilize low temperature resources.
► Efficiencies of these plants must be as high as possible for the plants to become commercially successful.
► Binary-flashing geothermal plants produce up to 25% higher power than simple binary plants.
► In binary-flashing units, a higher amount of the enthalpy is transferred to the secondary fluid than in conventional units.
► Among secondary fluids pentane and hexane show the best performances for application in binary flashing power plants.