Power Production from a Moderate Temperature Geothermal Resource with Regenerative Organic Rankine Cycles

Much remains to be done in binary geothermal power plant technology, especially for exploiting low-enthalpy resources. Due to the great variability of available resources (temperature, pressure, chemical composition), it is really difficult to "standardize the technology".The problem involves many different variables: working fluid selection, heat recovery system definition, heat transfer surfaces sizing and auxiliary systems consumption. Electricity generation from geothermal resources is convenient if temperature of geothermal resources is higher than 130. °C. Extension of binary power technology to use low-temperature geothermal resources has received much attention in the last years. This paper analyzes and discusses the exploitation of low temperature, water-dominated geothermal fields with a specific attention to regenerative Organic Rankine Cycles (ORC). The geothermal fluid inlet temperatures considered are in the 100-130. °C range, while the return temperature of the brine is assumed to be between 70 and 100. °C. The performances of different configurations, two basic cycle configurations and two recuperated cycles are analyzed and compared using dry organic fluids as the working fluids. The dry organic fluids for this study are R134a, isobutane, n-pentane and R245fa. Effects of the operating parameters such as turbine inlet temperature and pressure on the thermal efficiency, exergy destruction rate and Second Law efficiency are evaluated. The possible advantages of recuperated configurations in comparison with basic configurations are analyzed, showing that in a lot of cases the advantage in terms of performance increase is minimal but significant reductions in cooling systems surface area can be obtained (up to 20%).