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

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.