Experimental study on automotive cooling and heating air conditioning system using CO2 as a refrigerant

Recently, as one of the countermeasures against the global warming and energy conservation problems, natural refrigerants such as CO₂ 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 CO₂-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 CO₂ as a refrigerant.

As the result, a prototype CO₂ 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.     

Residential CO2 heat pump system for combined space heating and hot water heating

A theoretical and experimental study has been carried out for a residential brine-to-water CO₂ heat pump system for combined space heating and hot water heating. A 6.5 kW prototype heat pump unit was constructed and extensively tested in order to document the performance and to study component and system behaviour over a wide range of operating conditions. The CO₂ heat pump was equipped with a unique counter-flow tripartite gas cooler for preheating of domestic hot water (DHW), low-temperature space heating and reheating of DHW.

The CO₂ heat pump was tested in three different modes: space heating only, DHW heating only and simultaneous space heating and DHW heating. The heat pump unit gave off heat to a floor heating system at supply/return temperatures of 33/28, 35/30 or 40/35 °C, and the set-point temperature for the DHW was 60, 70 or 80 °C. Most tests were carried out at an evaporation temperature of −5 °C, and the average city water temperature was 6.5 °C. The experimental results proved that a brine-to-water CO₂ heat pump system may achieve the same or higher seasonal performance factor (SPF) than the most energy efficient state-of-the-art brine-to-water heat pump systems as long as: (1) the heating demand for hot water production constitutes at least 25% of the total annual heating demand of the residence, (2) the return temperature in the space heating system is about 30 °C or lower, (3) the city water temperature is about 10 °C or lower and (4) the exergy losses in the DHW tank are small.     

Integration of a three-stage expander into a CO2 refrigeration system

The inclusion of an expander with work recovery provides two advantages for transcritical CO₂ refrigeration cycles: the COP is improved and the exhaust pressure of the main compressor is lowered. Several designs of expanders have been proposed for this application and some prototypes have been tested already. In our laboratory a three-stage expander has been developed, which replaces the throttle valve of the normal refrigeration cycle and expands into the two-phase region. For optimum integration into the overall system it is proposed to install a vapour-liquid separator between the second and third stage of expansion. The vapour is guided back to the third expander stage whereas the liquid is supplied to the cooling stations via thermostatic or electronic expansion valves.     

Effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump

A typical transcritical CO₂ system shows lower performance than conventional air conditioners in cooling mode operation. In addition, the CO₂ system shows a large variation of the performance according to refrigerant charge whereas the conventional systems do not show large variation. In this study, the performance of the CO₂ heat pump was measured and analyzed by varying the refrigerant charge amount at standard cooling condition. In addition, the performance sensitivity of the CO₂ system as a function of refrigerant charge was compared to those for the R22, R410A, and R407C systems. The cooling COP of the CO₂ system was reduced more significantly at undercharged conditions than at overcharged conditions as the deviation from the optimal charge increased. The expansion loss was the dominant factor affecting system performance at undercharged conditions, while the gascooler loss became the major parameter at overcharged conditions. Among the systems investigated and compared in this study, the CO₂ system showed the most reduction in performance at undercharged conditions.     

Fan-less heat exchanger concept for CO2 heat pump systems

A novel system for space heating has been developed taking advantage of the favourable characteristics of the transcritical CO₂ cycle, where heat is rejected by cooling of supercritical gas at gliding temperature. By a proper design of a counter flow heat exchanger it is possible to heat air to high temperatures and thereby giving the driving force for circulation of air through the heat exchanger, in consequence without using a fan. A concept without a fan, here called a fan-less concept, would give several advantages; no noise, no power consumption for the fan and increased comfort with reduced air draft in the room. The concept may also be used for heat rejection in systems for light commercial applications or other applications where fan assisted heat rejection concepts are used today.

An experimental study of a CO₂ to air heat exchanger has been performed. The heat exchanger was made of a vertically finned aluminium profile. Tubes for CO₂ were mounted in the base of the profile. CO₂ at supercritical pressure flowing downwards through the profile was heating air flowing in the channels formed by the fins of the profile. In this way a perfect counter flow heat exchange was obtained. The prototype heat exchanger was 2000 mm high and 190 mm wide, with 45 mm deep fins.

A simulation model was developed and verified to give good accordance with the experimental data. The model was then used to study how different design parameters influence the efficiency of the heat exchanger. By altering the number of fins and the fin thickness of the tested profile, the heat output at a given condition could be increased to almost double, meaning that the initial design was relatively far from optimal.

With the original heat exchanger profile design concept a heat exchanger with height, width and depth of, respectively 2000, 750 and 200 mm, would be required in order to achieve a heat output of 2500 W if the constraints for assumed acceptable efficiency was applied. If a heat exchanger with less height is preferred, the width will have to be increased in order to maintain about the same front area, width times height. Ideas have also been introduced for how to improve both the compactness and efficiency of the heat exchanger by introducing a compact counter flow heat exchanger in the lower part of the air flow channel. It is concluded that the new concept looks promising for use as the indoor heat exchanger in an air-to-air heat pump or as a gascooler for heat rejection in small commercial equipment, when using CO₂ as refrigerant.     

Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO₂ and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO₂ system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO₂ with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO₂ was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO₂.     

Modeling and experimental investigation of accumulators for automotive air conditioning systems

A stream analysis model was developed to simulate the behavior of accumulators and their influence on the automotive air conditioning (A/C) systems. It allows a comprehensive steady state simulation with a set of input conditions such as refrigerant vapor mass flow rate and pressure at the inlet of an accumulator. In this study, the refrigerant/oil mixture is R134a/PAG oil which are totally miscible, but could be any air conditioning refrigerant/oil, including carbon dioxide (CO₂)/oil. The model accounts for all major effects inside the accumulator, such as friction, bends, sudden expansion, sudden contraction and heat exchange. The outputs are vapor quality, pressure and temperature at various positions of accumulator. In order to verify the mathematical model, experiments are performed in an experimental setup made up of real size automotive air conditioning components. The simulated results agree well with the experimental data. The simulation and experimental results show an important function of accumulators that is to determine the vapor quality into compressor, and thus has influence on the performance of whole automotive A/C systems.     

Study of capillary tubes in a transcritical CO2 refrigeration system

Capillary tubes have been used in refrigeration systems for many years, but not with a transcritical CO₂ system. In this article, the effects of capillary tubes in a transcritical CO₂ refrigeration system have been investigated experimentally and theoretically. Different types of capillary tubes with different lengths (0.5–4 m) and diameters (1–2 mm) have been tested. The result of this work is a static model, which is used in the further work to make a simulation model (static) of a complete refrigeration system. The model is based on Friedel's and Colebrook's pressure drop correlations.

The behaviour of an adiabatic capillary tube in a refrigeration cycle has been investigated theoretically. The conclusion is that the COP of a system with capillary tubes generally is better than when a fixed high pressure is used, but not as good as when variable optimal high pressure is used. Capillary tubes are especially interesting in applications where the evaporation pressure is constant and the temperature out of the gas cooler varies no more than ±10 K from the design condition. The reduction in COP is more significant at low temperatures out of the gas cooler.     

Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO₂ cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11 MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated.

A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle.     

Testing and modelling of an automotive wobble plate compressor

Wobble plate compressors are well used in air conditioning for high-class automobiles. They allow continuous control by automatic adjustment of the piston stroke, to keep the low pressure above a certain limit. Here an externally controlled wobble plate compressor is analyzed experimentally through its isentropic and volumetric effectivenesses and control characteristics. Compressor effectivenesses depend mainly on the compressor speed and pressure ratio: there is obtained, for example, isentropic and volumetric effectivenesses of 0.65 and 0.8 for a pressure ratio of 4 at 1000 rpm and 0.4 and 0.35 for the same pressure ratio at 4000 rpm. This degradation is attributed to the increasing of the supply pressure drop. The “lubricant” (oil + dissolved refrigerant) mass flow rate is obtained by minimization of the residuals of the thermal balances on the compressor, condenser and evaporator. Here an important oil-flow circulation is obtained: between 9.5% and 12.5% of the refrigerant flow rate. A special displacement sensor is used to measure instantaneous piston stroke and to relate it to overall compressor performance. This measurement is then compared with the results obtained with a semi-empirical model, which is able to predict, in part load, the compressor displacement. The model predicts the displacement ratio with deviations that vary between −14.5% and +8.1%.     

Cooling performance of a variable speed CO2 cycle with an electronic expansion valve and internal heat exchanger

The cooling performance of a CO₂ cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO₂ cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO₂ system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO₂ cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO₂ cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.     

Modeling and experimental evaluation of an automotive air conditioning system with a variable capacity compressor

A steady state computer simulation model has been developed for refrigeration circuits of automobile air conditioning systems. The simulation model includes a variable capacity compressor and a thermostatic expansion valve in addition to the evaporator and micro channel parallel flow condenser. An experimental bench made up of original components from the air conditioning system of a compact passenger vehicle has been developed in order to check results from the model. The refrigeration circuit was equipped with a variable capacity compressor run by an electric motor controlled by a frequency converter. Effects on system performance of such operational parameters as compressor speed, return air in the evaporator and condensing air temperatures have been experimentally evaluated and simulated by means of developed model. Model results deviate from the experimentally obtained within a 20% range though most of them are within a 10% range. Effects of the refrigerant inventory have also been experimentally evaluated with results showing no effects on system performance over a wide range of refrigerant charges.     

The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers

The system performance of a CO₂ refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO₂ systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure P_k,opt and other systematic parameters was performed. Evaporating temperature has little influence on P_k,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of P_k,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t₁=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.     

Optimization of a CO2–C3H8 cascade system for refrigeration and heating

Conventional working fluids (refrigerants) are being phased out worldwide to combat with the twin menace of ozone layer depletion and global warming and natural refrigerants are fast gaining favour lately. Single stage and multi stage refrigeration systems fail to widen the gap between heat source and heat sink temperatures required in many industrial applications requiring simultaneous heating and cooling and cascaded systems appear to be the best alternative. Modest research has been done in cascaded systems based on natural refrigerants thereby offering good potential for research. In this paper, a cascaded system for simultaneous heating and cooling (refrigeration and heat pump system) with a carbon dioxide based HT cycle and propane based LT cycle for simultaneous refrigeration and heating applications has been analyzed. To facilitate prediction of optimum performance parameters, performance trends with variation in the design parameters and operating variables have been presented in this article. Relevant expressions have been developed to serve as guidelines to the user for selecting appropriate design parameters like intermediate temperature so that the system yields optimum performance. Independently developed property codes have been employed for both carbon dioxide and propane for higher accuracy.     

CO2双缸滚动活塞膨胀机的实验研究

通过实验研究分析了CO2双缸滚动活塞膨胀机的中间通道对其效率及膨胀比的影响。对CO2双缸滚动活塞膨胀机的中间通道进行了改进,同时减小了中间通道的长度和直径,并进行了相关实验,结果表明:膨胀机的入口最优工况为26℃,6.5MPa,回收功率最高达到200W,效率最高达到42.3%。在各种实验工况下,测得的膨胀比已经达到了2.2左右,已经比较接近CO2双缸滚动活塞膨胀机的设计膨胀比2.5,但仍有改进的空间。     

Numerical simulation and experimental validation of vapour compression refrigeration systems. Special emphasis on CO2 trans-critical cycles

A numerical and experimental comparative study of a carbon dioxide trans-critical refrigerating system and a conventional sub-critical refrigerating cycle is presented. Attention is focussed not only on the whole refrigeration cycle, but also on the behaviour of the hermetic reciprocating compressors used in these systems. The comparative cases presented have been specially designed for small cooling capacity units with an evaporation temperature around 0 °C. A detailed numerical simulation model for hermetic reciprocating compressors performance, widely validated under conventional fluid refrigerants, has been extended to numerically obtain the CO₂ compressor prototypes behaviour. Two CO₂ compressor prototypes working with CO₂ have been experimentally tested in a specific unit, specially designed and built to analyse high-pressure single stage vapour compression trans-critical refrigerating equipments. This set-up has allowed validating a detailed numerical simulation code for the thermal and fluid-dynamic behaviour of single stage vapour compression refrigeration system working with CO₂ as fluid refrigerant. The numerical results and the experimental data obtained to validate compressors, heat exchangers and whole cycle behaviour have shown a really good agreement. Finally, the numerical and experimental comparison between the carbon dioxide system and the sub-critical conventional cycle has shown the possibility of CO₂ as fluid refrigerant under the studied working conditions.     

Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation

This study presents experimental results obtained from a transcritical R744 system using a refrigerant ejector. The results were compared to that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Experiments were analyzed to quantitatively assess the effects on system performance as a result of changes in basic ejector dimensions such as motive nozzle and diffuser sizing. Small angles of 5° yielded best results for the static pressure recovery of the high-speed two-phase flow entering the diffuser. Experiments confirmed that like in a conventional transcritical R744 system with expansion valve, the high-side pressure control integrated into the ejector could be used to maximize the system performance. Numerical simulation results helped identifying this basic trend. Due to difficulties in the ejector throat pressure measurements, a more practical performance metric was introduced in order to quantify overall ejector efficiencies. According to this definition, the prototype ejector was able to recover up to 14.5% of the throttling losses.     

CO2 heat pump systems

After the CFCs and the HCFCs were deemed unfit as working fluids in refrigeration, air conditioning, and heat pump applications, there has been a renaissance for carbon dioxide technology. Heat pumps is one of the application areas where theoretical and experimental investigations are now performed by an increasing number of research institutions and manufacturers. This paper gives an overview of some of the current activities in the CO₂ heat pump field. Discussed are the important characteristics of the transcritical CO₂ process applied to heat pumps, and also discussed are theoretical and experimental results from several heat pump applications. Provided that calculations and system designs are performed on the premises of the working fluid, and that test plants are constructed and operated to fully exploit the specific characteristics of both the fluid and the transcritical process, the results show that CO₂ is an attractive alternative to the synthetic fluids. Competitive products may be launched in the near future.     

Experimental study on the improvement of CO2 air conditioning system performance using an ejector

Several preceding researches have evidenced that the transcritical air conditioning system using CO₂ as a refrigerant has an inherent inefficiency resulting in degraded steady-state system performance of a CO₂ air conditioning system compared with that of a conventional air conditioning system. As a practical improvement, two-phase ejector was considered in place of expansion device in this study. The two-phase ejector for CO₂ air conditioning system was designed and developed considering the non-equilibrium state for evaluating the sonic velocity and the critical mass flux. The experiments of performance with respect to variation of ejector geometry such as the motive nozzle throat diameter, mixing section diameter and the distance between motive nozzle and diffuser were carried out. There exist optimum design parameters in each test. Experiments showed that the coefficient of performance of the system using an ejector was about 15% higher than that of the conventional system.     

The performance of a transcritical CO2 cycle with an internal heat exchanger for hot water heating

The main purpose of this study is to investigate the performance of a transcritical CO₂ cycle with an internal heat exchanger for hot water heating. Performance test and simulation have been carried out for a transcritical CO₂ cycle by varying secondary heat transfer fluid temperatures at evaporator and gas-cooler inlets as well as the discharge pressure. Variations of mass flow rate of refrigerant, compressor power, heating capacity, and co-efficient of performance (COP) with respect to the length of an internal heat exchanger are presented at various operating conditions. Good quantitative agreement between model predictions and experimental results has been found; most parameters have absolute average deviations of less than 4%. As the length of the internal heat exchanger increases, COP is enhanced but heating capacity tends to decrease due to the trade-offs between the effectiveness and pressure drop in the internal heat exchanger.