Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves

A capillary tube based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. In the present study, a steady state simulation model has been developed to evaluate the performance of a capillary tube based transcritical CO₂ heat pump system for simultaneous heating and cooling at 73 °C and 4 °C, respectively against optimized expansion valve systems. Capillary tubes of various configurations having diameters of 1.4, 1.5 and 1.6 mm along with internal surface roughness of 0.001–0.003 mm have been tested to obtain the optimum design and operating conditions. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in-house property code.

It is observed that the capillary tube system is quite flexible in response to changes in ambient temperature, almost behaving to offer an optimal pressure control. System performance is marginally better with a capillary tube at higher gas cooler exit temperature. Capillary tube length turns out to be the critical parameter that influences system optimum conditions. A novel nomogram has been developed that can be employed as a guideline to select the optimum capillary tube.