Mathematical modelling and thermodynamic optimization of a CO2 heat pump cycle

Abstract

Due to the harmful environmental effects of HCFC’s (hydrochlorofluorocarbons) and CFC’s (chlorofluorocarbons) discovered in the late 20th century, interest has increased for more environmentally-friendly refrigerant alternatives in cooling systems. One of the alternatives is carbon dioxide due to its unique properties, low cost, and low environmental impact compared to conventional refrigerants. The aim of this report is to study the use of CO2 in a one-stage and two-stage heat pump cycle for transportation purposes using an existing HCFC-based heat pump modified to work with CO2. Tests were conducted on this existing experimental CO2 heat pump setup at SP Technical Research Institute of Sweden in Borås, Sweden. A model was developed with MatLab software primarily for the two-stage compression configuration, and the results of the model were validated against measurements conducted with the experimental setup. Overall, the measurement and model results showed that the gas cooler unit is over-dimensioned and modifications in future systems could be considered. It was found that the air side within each heat exchanger limits the heat transfer in the existing setup due to the ratio of outside to inside surface area and the low air heat transfer coefficient. Measurements results also indicate that there is significant uneven distribution at the entrance to the evaporator unit. This creates uncertainty when modelling as it is too complicated with the existing measurements and equipment to model anything other than perfect distribution at the inlet to the evaporator unit. Results also indicate that the 2nd stage compressor operates at a much lower efficiency than expected by manufacturer and theoretical data. Tests on compressor operating speed changes showed significant system performance improvements when decreasing the 2nd stage compressor speed from the manufacturer suggested value. It was also found that there are possibilities for improving the system performance such as installing an internal heat exchanger.