Experimental Comparison of Mobile A/C Systems When Operated With Transcritical CO2 Versus Conventional R134A

This paper describes the experimental comparison of performance of prototype transcritical C02 system for mobile ale to it's Rl34a counterpart, unit for Ford Escort that was chosen as a representative for the US, European, and Japanese markets. The paper presents the design of experiments and facilities for such comparisons and preliminary results. INTRODUCTION One objective of this project is to provide an experimentally verified basis for calculation of TEWI (Total Equivalent Warming Impactsee Sand at al. 1997), particularly for R744 that are very sparse like Pettersen et al. (1997a) and (1997b). Test matrices are also defined for the purpose of developing component and system simulation models, as well as supporting data-to-data comparisons at normal, seasonal and extreme operating conditions. The comparison of Rl34a and C02 system is based on equal heat exchanger core volumes because cost data were unavailable. Component volumes were constrained to be equal; the weight was assumed be similar due to same materials used for the design. The face areas are kept equal. More details could be found in Table 1. Volumetric flow rates of air and air side pressure drops are maintained almost the same and are shown in Table 2. EXPERIMENTAL FACILITIES Separate environmental chambers have been constructed for each heat exchanger, each containing wind tunnel with separate piping for the two refrigerants. Three energy balances are obtained for each heat exchanger: air-side, refrigerant-side and room calorimetry. Special care was taken to develop test facilities that will produce accurate data in wide operating ranges, primarily in steady-state but also in transient (mostly cycling) conditions. Three independent methods are used instead of the two required by all applicable standards, not only to facilitate determincrtion of system capacities, but also to have two whenever refrigerant calorimetry is not reliable due to two-phase exit. This occurs mostly during transients, and with constant area expansion devices. Three independent procedures also improved our ability to troubleshoot early tests. The test facility consists of two environmental chambers (see Figure 1) that can maintain outdoor and indoor temperature within ±0.5°C and absolute humidity ±2%. A variable speed wind tunnel in each chamber simulates the range of operating conditions encountered in real applications, and allows measurement of air-flow rates within ±I%. Coriolis mass flow meters, together with immersion thermocouples and electronic pressure transducers upstream and downstream of every component yield refrigerant-side capacity determinations repeatable within ±I%. Room calorimetry is the most accurate: the walls are made of 30cm thick polyurethane. There are five thermocouples on both sides of each wall, floor, and ceiling of each environmental chamber. Transmission losses are carefully calibrated so that error is within ±0 .1 %, all dry energy inputs (electric) are measured within ±0 .2%. Special care is taken to ensure uniformity of the temperature and velocity profiles at the inlet to the heat exchangers, and representcrtive reading of the exit air temperatures and humidities. Test results show agreement between the independently determined capacities to be within ±5%, primarily due to uncertainties in air-side calorimetry. Figures 2 and 3 show the differences in heat balances determined on air and refrigerant side and for the environmental chamber.