Thermo-economic analysis and optimization of a novel carbon dioxide based combined cooling and power system

Abstract

Supercritical carbon dioxide is widely used in power plant and refrigeration system as working fluid. In this paper, a novel combined cooling and power system consists of supercritical carbon dioxide recompression Brayton cycle and transcritical carbon dioxide refrigeration cycle is proposed. In the novel system, the working fluid through the recompressor in supercritical carbon dioxide recompression Brayton cycle is partially replaced by the refrigeration compressor outlet working fluid in transcritical carbon dioxide refrigeration cycle. Mathematical models are established to simulate the combined system under steady-state conditions and the simulated results show that the reactor dominates the exergy destruction followed by the cooler and high temperature recuperator. Effects of five parameters, including turbine discharge pressure, cooler outlet temperature, evaporation temperature, cooling capacity and mass flow fraction are evaluated. The performances of the proposed combined system and the separation system are optimized and then compared from the perspective of thermodynamics. The calculated results show that the combined system’s comparative advantage over separation system will increase with increasing cooling capacity and decreasing evaporation temperature. The exergy efficiency of combined system is up to 2.45% and 5.87% higher than separation system when evaporation temperature is 273.15 K and 253.15 K, respectively. In order to balance the contradiction between the investment and system performance, the multi-objective optimization is conducted with exergy efficiency (to be maximized) and annual cost per heat consumption (to be minimized) as objective functions at different evaporation temperature. Non-dominated sorting generic algorithm-II is employed and Pareto frontier solution for the proposed system is given. Suggestions for engineering practice are provided based on the distributions of some key parameters on Pareto frontier solution.