An in-depth assessment of hybrid solar-geothermal power generation

A major problem faced by many standalone geothermal power plants, particularly in hot and arid climates such as Australia, is the adverse effects of diurnal temperature change on the operation of air-cooled condensers which typically leads to fluctuation in the power output and degradation of thermal efficiency. This study is concerned with the assessment of hybrid solar–geothermal power plants as a means of boosting the power output and where possible moderating the impact of diurnal temperature change. The ultimate goal is to explore the potential benefits from the synergies between the solar and geothermal energy sources. For this purpose the performances of the hybrid systems in terms of power output and the cost of electricity were compared with that of stand-alone solar and geothermal plants. Moreover, the influence of various controlling parameters including the ambient temperature, solar irradiance, geographical location, resource quality, and the operating mode of the power cycle on the performance of the hybrid system were investigated under steady-state conditions. Unsteady-state case studies were also performed to examine the dynamic behaviour of hybrid systems. These case studies were carried out for three different Australian geographic locations using raw hourly meteorological data of a typical year. The process simulation package Aspen-HYSYS was used to simulate plant configurations of interest. Thermodynamic analyses carried out for a reservoir temperature of 120 °C and a fixed brine flow rate of 50 kg/s revealed that under Australian climatic conditions (with a typical ambient temperature of 31 °C in summer) a hybrid plant would outperform stand-alone geothermal and solar power plants if at least 68% of its energy input is met by solar energy (i.e. a solar energy fraction of ≈68%). This figure drops to about 19% for reservoir temperatures greater than 170 °C. Case studies also showed that, for a mid-range reservoir temperature of 150 °C, the cost of electricity production can be reduced by 20% when a hybrid plant is used instead of the stand-alone Enhanced Geothermal System (EGS)