Low Temperature Difference Electricity Generation Using ORC

Abstract

Many geothermal sites in Australia have low temperature water as an available heat source. Often this temperature is under 75°C, in conditions where ambient temperatures can climb into the mid 30°s on a regular basis. These heat sources have often been considered to have an insufficient temperature difference, with respect to the atmospheric heat sink, to be worthwhile for driving a heat engine. As the temperature difference reduces, the maximum achievable efficiency must also reduce as demonstrated by classical thermodynamics. Thus, as the temperature difference drops, it is a matter of improving component efficiencies while reducing costs to make such systems viable. Lower temperature systems frequently use an Organic Rankine Cycle (ORC) heat engine. As the temperatures drop, the key parameters in the design of such systems become cost and efficiency. Of prime importance is the cost of the expander, efficiency of the heat exchangers, and the precision of the control system.