International Journal of Refrigeration 28 (2005) 1302 1307 www.elsevier.com/locate/ijrefrig Experimental study on automotive cooling and heating air conditioning system using CO 2 as a refrigerant Tomoichiro Tamura*, Yuuichi Yakumaru, Fumitoshi Nishiwaki Living Environment Development Center, Matsushita Electric Industrial Co., Ltd, 3-1-1, Yagumo-Nakamachi, Moriguchi, Osaka 570-8501, Japan Received 1 February 2005; received in revised form 26 August 2005; accepted 7 September 2005 Available online 14 November 2005 Abstract Recently, as one of the countermeasures against the global warming and energy conservation problems, natural refrigerants such as CO 2 are now paid attention as substitutes for HFCs in automotive air conditioning systems. Also, in recent years because the heat release from the eco-car s engine decreases, there is a problem that the present automotive heating air conditioning system cannot provide sufficient heating capacity. As an alternative approach, we focused on a solution utilizing a CO 2 -based heat pump, whereby the waste heat from the heat pump cycle during dehumidification of the incoming air (referred to as the dehumidifying condition) is recovered and used as an auxiliary heat source instead of an electric heater. Based on this concept, we aimed to develop an effective automotive cooling and heating air conditioning system using CO 2 as a refrigerant. As the result, a prototype CO 2 automotive cooling and heating air conditioning system for medium-sized cars was successfully developed. With this system, performance superior to that of the present HFC134a system can be achieved. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Air conditioning; Automobile; Design; Experiment; Heat pump; CO 2 Etude expérimentale sur un système de refroidissement et de chauffage automobile utilisant le CO 2 entant que frigorigène Mots clés :Conditionnement d air ; Automobile ; Conception ; Expérimentation ; Pompe à chaleur ; CO 2 1. Introduction As one of the countermeasures against global warming * Corresponding author. Tel.: C81 6 6906 2821; fax: C81 6 6904 5163. E-mail address: tamura.tomoichiro@jp.panasonic.com (T. Tamura). and energy conservation problems, natural refrigerants such as CO 2 are now being investigated to substitute for HFCs in refrigeration and air conditioning systems [1 3]. Among natural refrigerants, CO 2 has the advantage of having none of the flammability and toxicity problems found in hydrocarbons and ammonia, and has received considerable attention in the field of automotive air conditioners. In addition, with automotive air conditioners in vehicles with low engine heat release, such as highly efficient 0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.09.010
T. Tamura et al. / International Journal of Refrigeration 28 (2005) 1302 1307 1303 Fig. 1. Examination of the optimal system at the heating and dehumidifying condition. automobiles, because heating capacity from the engine alone is insufficient, auxiliary heating is carried out using an electric heater. However, because the energy efficiency of electric heaters is poor, this leads to a lowering of the energy efficiency of the automotive air conditioning system as a whole. To counter this, we have developed a highly efficient automotive cooling and heating air conditioning system with the heat pump method, using CO 2 as a refrigerant. this, by using CO 2 as the refrigerant in an automotive air conditioning system, it becomes possible to use the heat released from the refrigerant (waste heat of the heat pump cycle during dehumidification) in the high-pressure side heat exchanger, as a method to increase the efficiency of the automotive heating air conditioning system. In this way, we developed the CO 2 automotive cooling and heating air conditioning system to make effective use of the heat released by the heat pump cycle during dehumidification. 2. Refrigerant properties of CO 2 Table 1 shows a comparison between the major refrigerant properties of CO 2 and HFC134a [4]. CO 2 refrigerant has a small GWP value, actually 1/1300 of that of HFC134a, and the direct effect on the environment is extremely small. In addition, because the critical temperature of CO 2 is low, the refrigerant in the high-pressure side of the refrigeration cycle exists in a supercritical state. In the supercritical state, the condensing process, the vapor liquid state, does not exist and the refrigerant temperature constantly changes in the high-pressure side heat exchanger. Therefore, when CO 2 refrigerant is used, it is possible to maintain a low temperature difference between the refrigerant and the heated fluid in the high-pressure side heat exchanger, in comparison with refrigerants that function within the sub-critical regime. In other words, it can be said that CO 2 refrigerant is a better refrigerant for heating the heated fluid to higher temperatures. Because of Table 1 Comparison of refrigerant property HFC134a CO 2 Ozone depleting potential (ODP) 0 0 Grobal warming potential (GWP) 1300 1 Critical temperature (8C)/critical pressure (MPa) 101.1/4. 06 31.1/7.38 3. Studies on making the CO 2 automotive cooling and heating air conditioning system highly efficient 3.1. Evaluation method In this report, we evaluate the performance for heating/dehumidifying and cooling conditions, with the constraints that compressor s discharge pressure is below 12.0 MPa, the discharge temperature is below 120 8C, and with the amount of refrigerant, expansion valve s aperture and compressor rotational speed set to give the maximum COP value. The definition of COP for the CO 2 automotive cooling and heating air conditioning system is as follows: cooling capacity Cooling COP Z compressor input Heating dehumidifying COP auxiliary heating capacity Cdehumidifying capacity Z compressor input auxiliary heating capacity: heating capacity of the heat released by the heat pump cycle during dehumidification.
1304 T. Tamura et al. / International Journal of Refrigeration 28 (2005) 1302 1307 Table 2 Analytical conditions Compressor efficiency ( ) 0.7 Evaporator temperature (8C) 0.0 Superheat (K) 0.0 Minimum temperature difference between 10.0 water and refrigerant a (K) Water flow rate a (kg/min) 6.0 a Water-refrigerant Hex. 3.2. Study of the CO 2 heating system construction Initially, we carried out studies on a new CO 2 air conditioning system with heating/dehumidifying performance equal to or exceeding that of a current HFC134a system. In an automotive air conditioning system, dehumidification is needed even when using heating in order to prevent condensation in the automobile. In addition, as reported in [5], aco 2 cooling system with the same performance as an HFC134a system has already been developed. A current HFC134a system with auxiliary heating using an electric heater is shown in Fig. 1. In the HFC134a system, the heat released during dehumidification is dissipated outside the car from the outdoor heat exchanger (highpressure side heat exchanger) and is not used. Here, we used that CO 2 refrigerant can heat the fluid in the high-pressure side heat exchanger to higher temperature than with an HFC134a refrigerant. We carried out a theoretical study of a system construction for a CO 2 air conditioning system, replacing the auxiliary electric heater with a heating method utilizing the heat released in the heat pump cycle during dehumidification. The CO 2 systems we studied are the two systems shown in Fig. 1: (1) the direct expansion method, (2) the hot water method. (1) Direct expansion method: Air is warmed by a sub heat exchanger using the heat released during dehumidification. Engine coolant is used as the heat source for the refrigerant circuit. Fig. 2. Heating/dehumidifying performance comparison and dehumidifying condition. (2) Hot water method: The heat released during dehumidification is transferred to engine coolant in a waterrefrigerant heat exchanger. Air is warmed by the heated engine coolant. Based on the analytical conditions shown in Table 2, the results of predicting the capabilities of the CO 2 heat pump cycle when the air conditioner (heating/dehumidifying) is in operation are shown in Table 3. It can be seen that, with the direct expansion method (1), input increases greatly compared to a current HFC134a system (for a mediumsized automobile). However, with the hot water method (2), input can be reduced by approximately 20% compared to a current HFC134a system. Therefore, in this research, we chose the hot water method (2) as a CO 2 air conditioning system. 3.3. Study on method for controlling optimum amount of refrigerant Next, we carried out a study on a system construction and control method to avoid the unbalance between refrigerant charge for cooling and heating/dehumidifying. Because the outside temperature when the heating is in use is generally lower than when the cooling is in use, the amount of refrigerant held in the outdoor heat exchanger during the heating operation is greater than the amount during the cooling operation. Therefore, if the refrigerant Table 3 Results of study on system construction (theoretical study) HFC134a system using the electric heater CO 2 system (1) Direct expansion method Output Heating capacity of engine waste heat (W) 2600 0 2600 Auxiliary heating capacity (W) 1000 3600 1000 Dehumidifying capacity (W) 350 350 350 Input Electric heater (W) 1000 0 0 Compressor (W) 175 1911 964 Total (W) 1175 1911 964 Input ratio ( ) 100 163 82 (2) Hot water method
T. Tamura et al. / International Journal of Refrigeration 28 (2005) 1302 1307 1305 W Z V!r Fig. 3. Illustration of control method concept. amount is the optimum amount for cooling, the amount of refrigerant circulating through the cycle during heating/ dehumidifying will be too little. In this case, problems such as insufficient dehumidifying capacity or excessive discharge temperatures will occur and the efficiency of heating/dehumidifying will decrease as shown in Fig. 2. Therefore, in order to avoid the unbalance between optimum refrigerant amount for cooling and heating (the optimum refrigerant amount for heatingzoptimum amount for coolingc120 g), two expansion valves, Exp. A and Exp. B, were used before and after the outdoor heat exchanger as shown in Fig. 3. By adjusting the apertures of these valves and controlling the refrigerant pressure in the outdoor heat exchanger to an intermediate pressure, we studied a method for controlling the amount of refrigerant held in the outdoor heat exchanger. Under the heating/dehumidifying condition, if the amount of refrigerant held in the outdoor heat exchanger is reduced by 120 g in comparison with the amount of refrigerant held for cooling by adjusting the apertures of two expansion valves, even if the amount of refrigerant held in the system is the optimum amount for cooling, the amount of refrigerant circulating in the system can be identical to the optimum amount of refrigerant for heating/dehumidifying. In addition, when the cooling is in use, because the refrigerant must dissipate heat in the outdoor heat exchanger, only expansion valve B is controlled (expansion valve A is opened fully). Fig. 4 shows the relation between pressure and amount of refrigerant held in the outdoor heat exchanger. The amount of refrigerant held in the outdoor heat exchanger is calculated using the following equation: W, refrigerant amount held in the outdoor heat exchanger; V, volume of the outdoor heat exchanger; r, average refrigerant density at intermediate pressure. (1) When controlled with Exp. B only: The outdoor heat exchanger becomes the high-pressure side and the amount of refrigerant held in the outdoor heat exchanger becomes 210 g. (2) When controlled with Exp.A only: The outdoor heat exchanger becomes the low-pressure side and the amount of refrigerant held in the outdoor heat exchanger becomes 70 g. (3) When controlled with Exp.A and Exp.B: It is possible to optionally adjust the pressure in the outdoor heat exchanger between 4 and 12 MPa and as a result it is possible to control the amount of refrigerant held in the outdoor heat exchanger between 70 and 210 g. In other words, it is possible to reduce the amount of refrigerant held in the outdoor heat exchanger by a maximum of 140 g, compared to when control is carried out with Exp B only. The results of the above studies show that, by controlling Exp.A and Exp.B to adjust the pressure in the outdoor heat exchanger to the optimum intermediate pressure (5.5 MPa), it becomes possible to reduce the amount of refrigerant held in the outdoor heat exchanger to 120 g less than when the cooling is in use. This means that by using this control method, even if the amount of refrigerant held in the system is the optimum refrigerant amount for cooling, the same heating/dehumidifying performance can be achieved as when the amount of refrigerant is the optimum amount for heating/dehumidifying. 3.4. Development of elemental devices for high efficiency Next, development of the component device making up the heat pump cycle was carried out to achieve high efficiency. Table 4 shows the specifications of each development elemental device. Taking advantage of the Exp compressor Fig. 4. Amount of refrigerant held in outdoor Hex. Fig. 5. CO 2 cooling and heating/dehumidifying system.
1306 T. Tamura et al. / International Journal of Refrigeration 28 (2005) 1302 1307 Table 4 Specifications of component devices of cooling and heating air conditioning system Component Specification Compressor A scroll compressor (hermetic type), cylinder volumez4.0 cm 3 Gas cooler (outdoor Hex) Microtube type, W580!H390!D38 (mm) Evaporater (indoor Hex) Microtube type, W220!H220!D70 (mm) Water-refrigerant Hex Double microtubes type, W255!H20!D20 Internal Hex Double tube type, outer diameter fz12.7 mm, inner diameter fz7.9 mm, LZ1.5 m Table 5 Results of experimental study for cooling/heating performance Target Result Cooling COP ratio to HFC134a More than 1.00 1.00 system Heating/dehumidifying Amount of the refrigerants Same amount with cooling ) (1670 g) condition Temperature of evaporator Less than 2.0 8C K0.2 8C Dehumidifying capacity 350 W 560 W Auxiliary heating capacity 1100 W 1100 W Temperature of heated air 40.0 8C 40.2 8C COP ratio to HFC134a system More than 1.00 1.31 properties of the CO 2 refrigerant, microtubes were used for both the indoor and outdoor heat exchangers, and double microtubes were used for the water-refrigerant heat exchanger, which is the auxiliary heating method. In addition, a scroll compressor (hermetic type) was used. The gas cooler (outdoor heat exchanger) and evaporator (indoor heat exchanger) were both enlarged to a size installable in medium-sized cars and the heat transfer tubes were arranged in two rows in front and behind so that counterflow heat exchange could take place between the air and refrigerant. The fins in the gas cooler were also separated into two rows in front and behind to prevent heat transfer between low temperature refrigerant and high temperature refrigerant in heat transfer tubes or fins near the exit of the gas cooler. Finally, high efficiency was aimed for by using the internal heat exchanger to reduce refrigerant enthalpy at the entrance to the evaporator. 4. Evaluation of system performance A structural diagram of the CO 2 cooling and heating air conditioning system is shown in Fig. 5. In summer, by absorbing heat from the indoor heat exchanger, and dissipating it from the outdoor heat exchanger, the automobile s interior is cooled. In winter, by reducing the humidity in the automobile and adding heat released at the dehumidifying condition to that released from the engine, and dissipating it inside the automobile from the sub heat exchanger, the automobile s interior is heated. When doing so, the optimum amount of refrigerant is controlled by adjusting the intermediate pressure of the outdoor heat exchanger. Table 5 shows the results of experimental study for the cooling and heating air conditioning system. This system achieves the same cooling performance as a current HFC134a system, and is able to achieve heating performance equal to or exceeding that of a current HFC134a system with the same amount of refrigerant held as for cooling. 5. Conclusion We have developed a highly efficient automotive heating and cooling air conditioning system using CO 2 as refrigerant, and obtained the following results: (1) We constructed a CO 2 cooling and heating air conditioning prototype system (for medium-sized cars) with performance equal to or exceeding that of a system using HFC134a refrigerant. The heat released during dehumidification was used effectively as the source for auxiliary heating for improvements in energy saving. The relative heating/dehumidifying COPZ1.31 (in comparison with current HFC134a). (2) We established an intermediate pressure control method for adjusting optimum refrigerant amount An intermediate pressure was maintained in the outdoor heat exchanger to avoid an unbalance in the optimum amounts of refrigerant for cooling and heating.
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