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INTRODUCTION

Recently, natural fluid CO₂ has been shown to be a promising alternative particularly in heat pump applications due to gliding temperature heat rejection in the gas cooler and performance related various advantages. Some of the current theoretical and experimental investigations on several heat pump applications of transcritical CO₂ cycle have been presented by Neksa (2002) and Kim et al. (2004).

Neksa et al. (1998) and White et al. (2002) experimentally investigated the effects of discharge pressure, water inlet and outlet temperatures on the CO₂ heat pump water heater performances. Hwang et al. (1999) experimentally showed that the actual CO₂ cycle performed similar to the R22 cycle when the same outside volume of the heat exchanger was employed for both refrigerants. Yarral et al. (1999) experimentally investigated to effect of discharge pressure on CO₂ heat pump performance for simultaneous production of refrigeration and water heating to 90oC for the food processing industry. Adriansyah (2004) experimentally studied the effect of compressor discharge pressure for simultaneous air-conditioning and water heating. Stene (2005) presented the effect of water inlet temperature on CO₂ heat pump performance for combination of space and water heating. Cho et al. (2005) studied the performance of the CO₂ heat pump by varying the refrigerant charge amount at standard cooling condition to show the importance of refrigerant charge to achieve better performance. Kim et al. (2005) have done experimental study on CO₂ heat pump to study the effect of internal heat exchanger using water as secondary fluid for both sides with emphasizes only on heating. Sarkar et al. (2006) numerically studied the effects of water inlet temperature, compressor speed and heat exchanger inventory on optimal performance for simultaneous water cooling and heating applications. Yokoyama (2007) experimentally studied the influence of ambient temperature on performance of a CO₂ heat pump water heating system. Cabello et al. (2008) experimentally evaluated the effect of operating temperatures on optimal gas-cooler pressures of a CO₂ refrigerating plant and showed that Sarkar et al. (2004) correlation matches best with the test data of optimal gascooler pressures. However, effects of water mass flow rate on heat pump behavior have not been reported in the open literature.

In the present investigation, both simulation and experimental results on the working prototype of a transcritical CO2 heat pump system for simultaneous water cooling and heating are presented. The cooling and heating capacities, system COP and water outlets temperatures have been studied for various water mass flow rates and water inlet temperatures of both evaporator and gas cooler. Comparisons of simulated and experimental results are presented as well.