Constraints-free modeling and experimental validation of a transcritical CO₂ system for medium and large scale applications

A constraints-free transcritical CO₂ heat pump model for medium and large system applications is proposed and validated through experimental measurements. The model features five components, namely: compressor, chevron-type plate heat exchanger as gas cooler, expansion valve, fin-and-tube evaporator and chevron-type plate internal heat exchanger. Unlike existing models, the proposed model takes into account the detailed geometric characteristics of each component at a wide range of operating conditions without imposing any constraints, such as constant temperatures and pressures. Parametric studies regarding the influence of relevant operating and design parameters, such as ambient temperature, water inlet temperature and chevron inclination angle are conducted and a brief discussion on pressure optimization regarding the influence of compressor speed, expansion valve openness and number of plates at the internal heat exchanger is carried out. Comparison between the experiments and simulations showed that the model accurately reflects the actual system with a maximum error of 9.6% and 3.9% for the coefficient of performance and heating capacity, respectively. The parametric examinations are thoroughly compared to results published in literatures. In essence, the results showed that the COP and the heating capacity simultaneously increase with dry bulb temperature and chevron inclination angle, reaching a maximum at about 70°, while the effects of water inlet temperature are more significant and opposite to that of dry bulb temperature.