Production of electricity from geothermal heat ?? efficiency calculation and ideal cycles

Abstract

Production of electricity from heat requires a heat engine working between two heat reservoirs, a heat source and a heat sink. The Carnot efficiency is defined for the reversible heat engine working between two reservoirs of infinite heat capacity and constant temperature. Geothermal heat source is a stream of water, either in the liquid phase or as liquid-steam mixture of any quality. The heat capacity of the heat source is thus finite, and the source temperature is not constant in the heat addition process of the
heat engine. Same applies for the heat rejection of the heat engine, usually the cooling fluid available has some strict limitation on the possible temperatures, and rejection to a sink at a constant temperature is practically almost impossible. This paper analyzes electricity production when the heat source and the heat sink do not have a constant
temperature. The maximum production efficiency is defined and calculated, both for pure electricity production (E) and Combined Heat and Power (CHP) generation. The maximum obtainable efficiencies are calculated for typical values for geothermal application. It is stated, that the Carnot efficiency should not be used as a reference for such power production, but the real maximum efficiency based on the process.