Studies on a two-stage transcritical carbon dioxide heat pump cycle with flash intercooling

Simulation studies on a two-stage flash intercooling transcritical carbon dioxide heat pump cycle are presented. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide are calculated employing an exclusive precision property code based on recently published correlations. Results exhibit that flash intercooling technique is not economical with CO2 refrigerant unlike NH3 as the refrigerant. COP is considerably lower than that of the single cycle for a given gas cooler and evaporator temperature. There is no optimum inter-stage pressure as well. However, a marginal increase in COP occurs as inter-stage pressure decreases from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that incorporation of desuperheating of vapour in the intercooler almost doubles the mass flow rate in the second stage which can be attributed to the large flashing that occurs in the intercooler; this increase depends on the discharge temperature from the first stage and mass flow rate of refrigerant flow in the evaporator. Compressor isentropic efficiency shows marginal influence on system performance.