Transcritical Co2 Mobile Heat Pump and A/c System Experimental and Model Results

This article presents the results of the experimental runs of a prototype of R744 (CO2) refrigeration system operating in a both air conditioning and heat pump mode when heat rejection is done in supercritical region. The prototype system is sized for a compact car. Data presented are in the limited range of operation. Further optimization and extension of operating range is underway. Test facilities for such experiments and systems are described. The prospect of extending the ability of a mobile a/c system to a mobile heat pump operation is very promising. INTRODUCTION Transcritical CO2 systems are attracting significant attention in last several years not only due their environmental impact, but also due to unexpectedly good performance. The performance of some such systems were presented by Pettersen et al. (1993, 1994, 1997a and b), University of Maryland CEEE, several companies as well as by our group in Yin et al. (1998), Boewe et al. (1999a,b), Beaver et al. (1999a, b), etc... In few earlier articles we have analyzed and compared performance of a prototype of a transcritical CO2 system with the same volume and air-side pressure drop of heat exchangers as in a typical, of-the-shelf R134a mobile system. Results showed slightly worse performance of CO2 system at very high ambient temperatures (above 45C) close between 35 and 45C and better performance at lower ambient temperatures. We continue to working in the same area. There are four such systems that we are exploring at this moment. These systems are indicative of air conditioning systems used in typical compact cars and sport utility/military vehicles in the USA. Heat exchangers used in the baseline systems are typical for the respective vehicle size. The R744 systems are designed to have similar or smaller heat exchanger core volumes, face areas, and air side pressure drops. In this article we will focus to and present results for heat pump operation of the system first designed R744 system (MAC1) as shown in Table 1 and in the Figure 3. One of the reasons to be focused to heat pump application are our modeling analysis that indicated great potential of transcritical CO2 system operation in the heat pump mode. Favorable heat pump operation could append additional reason for considering transcritical CO2 systems as a viable alternative to existing R134a systems. ∗ Author to whom correspondence should be addressed, [email protected] 2/10 EXPERIMENTAL FACILITIES The test facility is shown in Figure 1. Two environmental chambers have been constructed for each heat exchanger (outdoor and indoor), each containing wind tunnel with variable speed blower and different piping for the two refrigerants. Each chamber and heat exchanger can operate in both regimes: heat rejecting and absorbing. Third chamber in between is for the compressor. Figure 1. Test facility for R744 air conditioning system Figure 2. R744 heat pump system B – blower, C – compressor, Dp – differential pressure, Evap – Evaporator-indoor coil, FS – flow straightener, GC – gas cooler/outdoor coil, H – heater, Hu – humidifier, ICC – indoor cooling coil, mg & mg2 – glycol mass flow meter, mo – oil mass flow meter, mr – refrigerant mass flow meter, Mtr – motor, N – nozzle, OCC – outdoor cooling coil, P – pressure, RH – relative humidity, S – separator, SA – suction accumulator, Sc – condensate scale, SG – sight glass, SLHX – suction line (internal) heat exchanger, Sp – speed controller, Th tachometer, T – thermocouple, TC – temperature controller, TG – thermocouple grid, Tor – torque transducer, W – watt transducer, XV – expansion valve (any type). Indices: a – air, c – condenser/gas cooler/outdoor coil, cp – compressor, dp – dew point, e – evaporator/indoor coil, g & g2– glycol, i – inlet, n – nozzle, o – outlet, r – refrigerant, sh – suction line (internal) heat exchanger Tcri B H OCC