A case study of electricity generation from low quality geothermal energy sources

This study investigates whether the low efficiency of electricity generation from low quality geothermal energy sources can be improved by using night sky radiation. Useful energy or work can be generated from a heat source when there is a temperature difference ΔT, e.g., between the heat source (T > T0) and a heat sink T0. It is also possible to have a low temperature heat "source" (T < T0) as long as the Carnot efficiency factor (T0-T)/T0 is sufficiently large. By way of example we use a geothermal energy plant from which a maximum of useful energy (electricity) is to be extracted. We only give a schematic exergy analysis of geothermal energy extraction, which includes the exergy necessary for circulating water and material costs for a 20-year project. The useful energy produced is calculated using a device that contains series of Peltier elements that are exposed on one side to the produced hot water and on the other side to a vessel loosing heat to the night sky. The result is compared to a similar device where the cold side assumes an ambient temperature of 10o C. As expected, the process using night cooling is the more efficient process, but the present efficiency of Peltier elements is insufficient for an exergetically viable electricity production. It turns out that the use of night sky radiation leads to about the same efficiency as groundwater cooling.