Paul Maina

Effects of Refrigerant Charge in the Output of a CO2 Heat Pump

Many of the refrigerants currently being used in heating, ventilation, air conditioning and refrigeration systems have high global warming potential. One potential, environmentally friendly replacement refrigerant is carbon dioxide (CO2). In this study, a CO2 trans-critical water to water test bed was used to study the performance of a typical heat pump by varying the amount of refrigerant charge. Initial experimental results and thermo-physical properties were analysed by NIST REFROP and plotted in a temperature-entropy (T-S). The heat energy output in the gas cooler was compared to the mechanical work input in the compressor to obtain the efficiency of the system in terms of coefficient of performance (COP). It was observed that the refrigerant charge has a great effect on the heat output and efficiency of the system. Furthermore, the maximum amount of charge the system can take is not necessarily the optimum amount of charge. This was related to the effect of the charge amount on the equipment involved and flow process variables like pressure loss and friction which affect the heat transfer process.

Effects of various parameters on the efficiency of a CO2 heat pump: a statistical approach

The influence of variables such as; refrigerant amount, chilling and cooling water temperature, throttle valve opening, cooling and chilling water flow rates, on the efficiency (coefficient of performance – COP) of a water to water carbon dioxide heat pump was investigated. Design of experiments was done using design-expert® 6 software for regression analysis. A response surface method known as central cubic design was used to provide optimum results with minimum experiments. Through multiple regression analysis, an empirical equation relating the COP to the variables was derived. Analysis of variance revealed that these regressions are statistically significant at 95% confidence level compounded with a very low standard deviation and a high adequate precision. The close relationship between the predicted COP values and the actual values further proves the worthiness of the empirical equation. It was observed that cooling water temperature had the highest influence followed by the chilling water temperature. Surprisingly, the amount of the refrigerant was third followed by the throttle valve opening. Understandably, chilling water flow rate had the least effect on the COP. Through response surface diagrams, the interactive influence of the variables were also observed. The COP values arrived at varied from 1.545 to 6.914 although if the variables were optimized fully within the scope of this study, a value of up to 11.8 could be achieved. Still, if the variables range is increased further, higher COP could be achieved. Finally, a discussion was done in a bid to explain these results.

Performance of a CO 2 water to water heat pump with less charge: Inlet temperature effects

A CO2 water to water heat pump system was used to investigate the effects of cooling and chilling water inlet temperature at reduced charge capacity (approximately 75% of the full charge). The temperature were adjusted between 10 °C to 30 °C in nine steps while the throttle valve opening adjusted between 100%, 50% and 25%. It was observed that the as the cooling water inlet temperature increased, the heat output and general performance of the system diminished but as the chilling water temperature increased, there was an improvement of the system performance. Therefore, the best temperature combination was cooling - chilling water temperature of 10 °C - 30 °C. The opposite, i.e. cooling - chilling water temperature of 30 °C - 10 °C was the worst. Nevertheless, the effects of the cooling water temperature were higher on the system performance than the effect of the chilling water temperature. These observations were true in all throttle valve settings, where, the smaller the throttle valve opening, the higher the performance.