Optimization of two-stage transcritical carbon dioxide heat pump cycles

Optimization studies of two-stage transcritical carbon dioxide heat pump cycles, incorporating options such as flash gas bypass, flash intercooling and compressor intercooling, are presented based on cycle simulation. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide coded and then integrated with the simulation code for further analyses. Results exhibit improvement in performance by adopting optimal operating conditions. The optimum interstage pressure, thus obtained, deviate from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that the flash gas bypass system yields the best performance among the three two stage cycles analyzed. Internal heat exchanger effectiveness and compressor isentropic efficiency shows marginal influence on the system performance. Internal heat exchanger effectiveness shows marginal influence on the system performance while compressor isentropic efficiency shows an about 10% variation in COP. However, optimum gas cooler pressure and optimum intermediate pressure are only marginally affected. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler temperature and evaporator temperature are obtained. This would be useful as a guideline in design of such systems.