Thermodynamic performance analysis of a R245fa organic Rankine cycle (ORC) with different kinds of heat sources at evaporator

The first and the second law analyses of a 20 kW(e)R245fa organic Rankine cycle (ORC) with low grade temperature heat sources were numerically studied. There were three different heat sources such as hot water, saturated steam, and combined hot water/saturated steam used to supply heat at the ORC evaporator. The heat source temperature was varied in a range of 80-110 degrees C and the pinch temperature difference between the heat source temperature and the evaporating temperature was in a range of 1-10 degrees C. The ORC condensing temperature was kept constant at 40 degrees C. The results revealed that the combined hot water/saturated steam heat source provided a highest second law or exergy efficiency due to less irreversibility generated at the evaporator followed by the saturated steam and the hot water heat sources, respectively. The increase of heat source temperature at a specific pinch could increase the second law efficiency of the cycle. However, the increase of pinch value resulted in lower second law efficiency. In addition, for the below pinch zone, with the hot water/saturated steam heat source, the reduction of the hot water flow rate slightly enhanced the ORC second law efficiency as the exergy destruction below pinch was reduced. However, there was a limit that the flow rate should not be reduced below a specific value otherwise a very high heat exchanger area was needed.