Analysis of some power plant options for enhanced geothermal systems (EGS)

Enhanced geothermal systems (EGS) extract heat from underground hot dry rocks (HDR) by first fracturing that hot underground zone and then circulating geofluid (typically water) into it and bringing the heated geofluid to a power plant as the heat source. This study first presents a brief review of flash and expansion geothermal power plants and proposals, and focuses on analysis, examination, and comparison of leading geothermal power plant configurations focused on mid-high grade deep geothermal HDR resources where the geofluid temperature ranges from 200-800°C, and also analyzes the embodied energy in EGS surface power plant construction. The power generation analysis is focused on single-flash and double-flash cycles for using subcritical geofluid (< 374 °C) and triple-expansion, double-expansion and single-expansion cycles for using supercritical geofluid (> 374 °C). Key findings of this study include: (i) double-flash plants have 27∼30% higher geofluid effectiveness than single-flash ones and 3∼10% lower specific embodied energy, (ii) The plants using supercritical geofluid have geofluid effectiveness > 750 kJ/kg, significantly higher than those using subcritical geofluid (geofluid effectiveness < 300 kJ/kg). The specific embodied energy is similar for subcritical and supercritical plants. (iii) We also proposed and investigated a plant configuration that includes superheating by geofluid for avoiding turbine outlet vapor fraction drop below 0.90, and compared it with the non-superheated one. The results indicated that superheating will indeed increase the turbine outlet vapor fraction from 0.75 to 0.90 but at the same time reduce the geofluid effectiveness by 7%. (iv) For geofluid temperatures above 650°C, it was found that double-expansion plants have a 2% higher geofluid effectiveness and 5∼8% lower specific embodied energy than single-expansion ones.