Thermo-economic analysis of transcritical CO2 power cycle and comparison with Kalina cycle and ORC for a low-temperature heat source

Abstract

The utilization of low-temperature heat energy can improve energy conservation and protect the environment effectively. A transcritical CO2 power cycle is normally considered not competent with an organic Rankine cycle (ORC) for low-temperature applications. In this study, the advantages of transcritical CO2 power cycle for low-temperature heat sources are explored from a thermo-economic viewpoint. The performances of four different transcritical CO2 power cycles are evaluated theoretically and then compared with an ORC and a Kalina cycle. First, a mathematical model is established to estimate the thermodynamic and economic performances of the power cycles including a basic cycle, a recuperated cycle, a regenerative cycle with an open-feed heater, and a cycle with a reheater. Printed circuit heat exchangers are used for the evaporators and air-cooled heat exchangers are adopted for the condensers. Aspen EDR is used to obtain the areas of the air-cooled condensers and the overall thermo-economic performance is determined by Matlab. Then, one suitable cycle is selected and compared with the ORC and the Kalina cycle for a low-temperature heat source. The results indicate that the cycle with a reheater is preferable among the four transcritical CO2 cycles. Compared with the ORC and the Kalina cycle, the net power output of the transcritical CO2 cycle is the largest and the economic performance is between the Kalina cycle and the ORC. However, CO2 is cheaper, more environmentally-friendly, and safer than organic fluids. Therefore, from a thermo-economic viewpoint, transcritical CO2 power cycle is competent with ORC for low-temperature heat sources.