A Transcritical Refrigeration Cycle with Carbon Dioxide for Bus Air Conditioning and Transport Refrigeration

The frrst part of this paper presents carbon dioxide air conditioning units which were installed in two city buses. The two buses were tested "on-the-road" during summer 1997 in Bad Hersfeld, Germany. By the end of 1997, the air condition systems of the two buses had been running together for more than 860 hours proving that C02 systems fulfill important on-the-road requirements. In the second part of this paper a laboratory prototype is discussed which used carbon dioxide as refrigerant for transport refrigeration. An internal heat exchanger improves the cooling capacity but has no significant effect on the COP for ambient air temperatures of 30 °C (86 F). The last part of the paper describes the design steps of the compressor used for both bus air conditioning and transport refrigeration. The comparison of compressor versions 1 and 2 show significant improvements differences in the measured indicator diagrams mainly due to the modified discharge valves. INTRODUCTION Carbon dioxide is an environmentally benign refrigerant. Unlike synthetically produced HFC-134a, carbon dioxide has a negligible global warming potential and it has no harmful breakdown products. Carbon dioxide is a natural substance and does not have to be produced especially as refrigerant. It is available from natural wells or as a byproduct from industrial processes. Hence, recycling of carbon dioxide is not necessary. That makes service and maintenance for C02 systems easier than for conventional systems, which use synthetically produced refrigerants. This is an important advantage of carbon dioxide compared to HFC's. ON-THE-ROAD TESTS WITH CARBON DIOXIDE AIR CONDITIONING SYSTEMS In August 1996 the frrst carbon dioxide air conditioning system was installed in a bus (Bus A in Fig. 1, 0 405). This bus, a five year old city bus is owned and operated by the public transportation company in Bad Hersfeld, Germany. A detailed description of the carbon dioxide air conditioning system was given by Sonnekalb and Kohler [1]. In May 1997 two new buses ( 0 405 N) were bought by the same public transportation company in Bad Hersfeld. These two buses are totally similar, except that one bus (Bus C in Fig. 1) is equipped with a standard HFC-134a air conditioning system, whereas the second bus (Bus B in Fig. 1) is airconditioned by a C02 air conditioning system. The carbon dioxide system is identical with the HFC-134a system with respect to weight, overall size, blowers and fans, and (air-side) dimensions of the heat exchangers. The co2 compressor is smaller than the HFC-134a compressor. Fig. 2 shows the C02 air conditioning system of Bus B. With the help of the Buses Band C the "on-the-road"-behavior of the COz system and the standard HFC-134a system can be ideally compared. All three buses (Buses A, B, and C) are used for public transportation in Bad Hersfe1d, Germany, all year round. By the end of 1997 the air conditioning system of Bus A had been running for more than 550 hours. At the same time the air conditioning system of Bus B bad been operating for more than 310 hours. Both C02 air conditioning systems operated during summer 1997 without significant problems. The carbon dioxide air conditioning systems were equipped with commercially available electronic expansion valves (see Fig. 3). These expansion valves were modified for the special carbon dioxide bus air conditioning application. Measurement data collected during 1997 showed that this type of expansion valve is able to maintain an almost constant evaporator superheat in spite of the significant variation of the compressor speed, and of the suction and discharge pressure. The expansion valve is well suited for highly transient operating conditions. Fig. 4 shows the variation of the interior air temperatures of the two identical buses (Bus B and Bus C) for a typical sununer day (August 5, 1997} in Bad Hersfeld, Gennany. In addition, the variation of the ambient air temperature is also depicted in Fig. 4. At that day the maximum ambient air temperature reached about 28-30 °C (85 F). The