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This paper presents an experimental performance investigation and exergy analysis using R1234yf in an automotive air conditioning system. The R1234yf system with an internal heat exchanger (IHX) was compared with an R1234yf system without the IHX and an R134a system. The R1234yf system had 4.0–7.0% smaller cooling capacity and 3.6–4.5% lower COP compared to the R134a system. With the IHX, the R1234yf system showed almost equivalent cooling capacity to the R134a system. The COP of the R1234yf system with the IHX is lower than that of the R134a system by 0.3–2.9% for a compressor speed of 800–1800 rpm. The COP increased by 0.9% at a compressor speed of 2500 rpm. The second law efficiency of the R1234yf system was 3.4–4.6% lower than that of the R134a system at all compressor speeds. The second law efficiency of R1234yf with the IHX was improved by 1.5–4.6% compared to the R1234yf system without the IHX. The EDR of the R1234yf system was 0.5–3.3% higher than that of the R134a system in the range of 800–1800 rpm. However, the R1234yf system with the IHX had 1.2% lower EDR than the R134a system at a compressor speed of 2500 rpm.

Using thermal energy balance, this paper analyzes and investigates the thermal performance of a U-tube solar collector whose temperature thermal energy is high due to solar radiation. A working fluid of 20% PG (propylene glycol)–water is used. Solar collector efficiency was calculated and energy savings predicted for various nanofluids, such as MWCNT, Al 2 O 3 , CuO, SiO 2 , and TiO 2 . As a result, thermal conductivity increased as the concentration of nanofluid increased. Solar collector efficiency increased in the following order from greatest to least: MWCNT, CuO, Al 2 O 3 , TiO 2 , and SiO 2 nanofluids. When the thermal loss value (( T i − T a )/ G ) was equal to 0, the solar collector using 0.2vol% MWCNT nanofluid showed the greatest efficiency (62.8%, a 10.5% improvement compared to 20% PG–water). By dispersing nanoparticles in the working fluid, the coal usage could be further reduced by approximately 39.5–131.3 kg per year when 50 solar collectors are used. Therefore, CO 2 generation could be reduced by 103.8–345.3 kg and SO 2 generation by 0.4–1.1 kg per year, compared to solar collectors using a base working fluid of 20% PG–water. These findings contribute to knowledge of solar energy technology, which has the potential to reduce electricity and energy consumption world-wide.

This paper presents a numerical study on the performance of a vapor compression cycle equipped with an ejectoras an expansion device to improve the COP by reducing the expansion loss and compressor work. The simulation is carriedout using a model based on the conservation of mass, energy and momentum in the ejector. From the results of the simulation,the vapor compression cycle equipped with an ejector showed a maximum COP improvement of 14.0% when using R134a refrigerant and 16.8% when using R1234yf. In addition, the performance of the system with an ejector represents the increasedperformance as the temperature difference between condensing and evaporating increased. Key words Ejector(이젝터), Entrainment ratio(유입비), Area ratio(면적비), R1234yf(R1234yf), GWP(지구온난화지수), COP(성능계수)†Corresponding author, E-mail: cspark@hoseo.edu 기호설명 A ： 단면적 [m 2 ] COP ： 성적계수 h ： 엔탈피 [kJ/kg]  ： 질량유량 [kg/s] P ： 압력 [kPa] Q ： 냉방용량 [kW] S ： 엔트로피 [kJ/kg ℃ ] V ： 속도 [m/s] W ： 압축기 소요동력 [kW] x ： 건도그리스 문자  ：

R1234yf has been developed as an alternative refrigerant to R134a, which has been associated with global warming. The capillary tubes as expansion valves control the mass flow rate and balance system pressure in the refrigeration cycle. The present numerical model used the governing equations including the law of conservation of mass, momentum, and energy in a capillary tube. The mass flow rate of R1234yf decreased by 47.0% as the capillary tube length was increased from 1 to 4 m. As the inner diameter of the capillary tubes was changed from 1.3 to 1.7 mm, the mass flow rate of R134a and R1234yf increased by 117.9% and 121.0%, respectively. The mass flow rate of the R134a and R1234yf increased by 28.3% and 29.1% with subcooling increasing from 0 to 7°C. In addition, when the inlet temperature of the capillary tubes was changed from 35 to 60°C, the mass flow rate of R134a and R1234yf increased by 31.0% and 45.4%, respectively.

The objective of this study is to investigate the influence of the cooling performance for a water-to-water ground source heat pump (GSHP) by using the counter flow and parallel flow methods. The GSHP uses R-410A as a refrigerant, and its main components are a scroll compressor, plate heat exchangers as a condenser, an evaporator, a thermostatic expansion valve, a receiver, and an inverter. Based on our modeling results, the heat transfer rate of the counter flow evaporator is higher than that of the parallel flow evaporator for a heat exchanger length greater than 0.42 m. The evaporator length of the GSHP used in this study was set to over 0.5 m. The performance of the water-to-water GSHP was measured by varying the compressor speed and source-side entering water temperature (EWT). The cooling capacity of the GSHP increased with increased compressor RPMs and source side EWT. Also, using the counter flow method, compared to the parallel flow method, improves the COP by approximately 5.9% for an ISO 13256-2 rated condition.

The performance characteristics of a two-stage CO2 system with two different evaporating temperatures have been analyzed through simulation with such variables as outdoor air temperature, outdoor air velocity, and 2nd-stage EEV opening. The simulation of performance of the two-stage compression cycle indicated that the cooling COP decreased by 22.9, 16.1, 18.9, and 19.4%, respectively, for frostless cooling capacities of 0, −10, −20, and −30%, when the outdoor temperature was increased. The system had a maximum cooling COP for the frostless cooling capacity of −10%, or −20% when the 2nd-stage EEV opening was 56%. The maximum cooling COPs for the non-frost and the frostless cooling capacity of −30% were 1.45 and 1.003, respectively, when the 2nd-stage EEV opening was 52%. The system performance was very sensitive to the variation of 2nd-stage EEV opening.

In this study, performance was assessed by charging the same automotive refrigeration systems with the refrigerants R134a and R1234yf, respectively, to compare the characteristics of the refrigeration cycle of the two refrigerants. The internal heat exchanger was installed in order to improve cooling performance of R1234yf and to investigate the level of performance improvement in comparison with conventional R134a system. Performance test by using R1234yf and R134a in the same system showed low power consumption and cooling capacity for using R1234yf, that is, up to 4% and 7%. In particular, performance comparison between the R1234yf and R134a for automotive air conditioning revealed that cooling capacity and COP of the 1234yf system without the IHX decreased by up to 7% and 4.5%, respectively, but those with the IHX decreased by up to 1.8% and 2.9%, respectively.

The present study investigated a two-stage compression heat pump system for district heating utilizing waste energy. An evaporator and condenser exhibit a large difference in temperature when hot water is produced for heating by a heat pump. With single-stage compression, this causes a dramatic drop in the compressor efficiency and lowers the system performance; so, in the present study, a two-stage compression heat pump system comprising an intercooler and flash tank was designed, and the performance characteristics under various operating conditions were tested. When the heat source temperature was raised from 10 °C to 30 °C, the COP (coefficient of performance) was improved by up to 22.6%. As the superheating at the low-stage compressor was increased from 2 °C to 11 °C, the refrigerant flow rate and heating capacity decreased by as much as 7.6% and 2.2%, respectively, but there was no major impact on the temperature of the hot water produced nor on the system performance. Controlling the frequency of the high-stage compressor to control the intermediate pressure resulted in the ability to improve performance by as much as 5.2% under identical heat source conditions.

The performance of automobile air conditioning systems of R1234yf was evaluated and compared with that of R134a. In particular, the performance evaluation was carried out by installing an internal heat exchanger in order to improve the performance of the system used in R1234yf. A performance comparison between the R1234yf and R134a for automotive air conditioning revealed that the cooling capacity and COP of the 1234yf system without the IHX decreased by up to 7% and 4.5%, respectively, but those with the IHX decreased by up to 1.8% and 2.9%, respectively.

In this study, the performance of mobile air conditioner(MAC) systems to which the refrigerants R134a and R1234yf were used was evaluated to compare the characteristic of automotive refrigeration cycles with refrigerant. The experimental setup of a MAC consists of an belt driven compressor, a condenser, an evaporator and a block type thermal expansion valve. The drop-in test on MAC were carried out under variable compressor speed from 800 to 2500 rpm. Performance test by using R1234yf and R134a in the same system revealed low the charge amount and mass flow rates for using R1234yf, that is, up to 10% and 17%, respectively. The compressor discharge temperature of R1234yf is 8℃ lower than that of R134a. The cooling capacity with R1234yf system decreased by 4~7% compared with R134a system. In addition, The COP of R1234yf system is lower 3~4% than that of R134a system.

The objective of this study is to investigate the performance of a two-stage com-pression heat pump system for district heating. The experimental setup of heat pump consists of compressor, condenser, evaporator, expansion device, intercooler, flash tank, oil separator and accumulator. The experimental evaluations on the two-stage compression cycle were carried out under various operating conditions which were heat source temperature, the degree of compressor inlet superheat, and intermediate pressure. The temperature ranges of unutilized energy as the heat source were used in the test conditions. As the heat source temperature increased from 10℃ to 30℃, the COP and heating capacity of the heat pump system increased by 22.6% and 45.8%, respectively. The performance of the two-stage heat pump system increased by 5.2% with the variation of the intermediate pressure in the same heat source temperature conditions.Keywords:District heating(지역난방), Intermediate pressure(중간압), Two-stage compression heat pump(2단 압축 히트펌프), Unutilized energy(미활용에너지)

A new analytic solution was derived for the diffusion into or from an immobile zone of a rectangular 2-D pore. For a long time, the new solution converges to a traditional mobile-immobile zone (MIM) model, but only if the latter is used with an apparent initial concentration that is smaller by almost 20% than the true one. This is the tradeoff for using a simple MIM model instead of an exact model based on the diffusion equation. The mass-transfer coefficient was found to be constant for a sufficiently long time; it was proportional to the molecular diffusion and inversely proportional to the square of the pore depth. The mass-transfer coefficient was time-dependent for a sufficiently short time and may be significantly larger than its asymptotic value.

In this study, the heat transfer and pressure drop characteristics of FC-72 in small channel heat sinks, which were designed for liquid cooling of electronic components, were measured by varying the mass flux, saturation temperature, and vapor quality. The small channels had circular cross-sections with diameters of 2 and 4 mm and length of 100 mm. The heat flux provided by the heaters in the copper block ranged from 0.5 to 3.0 W/cm2. Based on data comparisons, the existing pressure drop correlations were modified by introducing the effective viscosity including wall effects of the fluid in the small channel. The modified homogeneous pressure drop model yielded the best predictions with average and mean deviations of 0.9% and 27.0%, respectively. The measured heat transfer coefficient was also compared with the predictions obtained by using the existing heat transfer correlations. The Kandlikar correlation with the fluid surface parameter for water yielded consistent trends with the measured heat transfer coefficient. A new fluid surface parameter FFl = 0.747 for FC-72 was proposed based on the present data, and satisfactory predictions were obtained with average and mean deviations of 0.4% and 7.0%, respectively.

The applications of electronic expansion valves (EEVs) into multi-type heat pumps and inverter heat pumps in building air-conditioning systems have increased for comfort environmental control and energy conservation. However, test data and mass flow models of EEVs are very limited in open literature. The objectives of this study are to investigate the mass flow characteristics of R22 and R410A through EEVs and to develop an empirical correlation for the prediction of mass flow rates of R22 and R410A through EEVs. Mass flow rates through six EEVs were measured by varying the EEV opening, inlet and outlet pressures, and the subcooling. Mass flow rates of R410A were compared with those of R22 at the same test conditions. Based on the experimental data, an empirical correlation for mass flow predictions in EEVs was developed by modifying the orifice equation. The predictions of the present correlation showed good agreement with the measured data with average and standard deviations of 0.76% and 5.9%, respectively. Approximately 92% of the measured data were within ±10% of the predictions.

Air-conditioners use spirally coiled capillary tubes as an expansion device to enhance compactness of the unit. However, most empirical correlations for predicting refrigerant flow rate through capillary tubes were developed for straight capillary tubes without consideration of coiled effects. The objectives of this study are to investigate the flow characteristics of the coiled capillary tubes and to develop a generalized correlation for the mass flow rate through the coiled capillary tubes. The mass flow rate of R22 through the coiled capillary tubes and straight capillary tubes was measured for various operating conditions and tube geometries. The mass flow rates of the coiled capillary tubes decreased by 5–16% more than those of the straight capillary tubes at the same operating conditions. A generalized correlation for predicting refrigerant mass flow rate through coiled capillary tubes was developed by introducing the parameter of capillary equivalent length. The present correlation showed good predictions with the present database for R22, R407C and R410A in the straight and coiled capillary tubes, yielding average and standard deviations of 0.24% and 4.4%, respectively.

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A microchannel heat exchanger was numerically analyzed using the finite volume method. The air and refrigerant-side heat transfer coefficients and pressure drops were calculated using the existing correlations that were developed for microchannel heat exchangers. To verify the present model, performance tests of the microchannel heat exchanger were conducted at various test conditions with R134a. The present model yielded a good correlation with the measured heat transfer rate, demonstrating a mean deviation of 6.8%. The performance of the microchannel evaporator for CO2 systems can be improved by varying the refrigerant flow rate to each slab and changing fin space to increase the two-phase region in the microchannel. Based on the comparison of the performance of the microchannel heat exchanger with that of the fin-tube heat exchanger designed for CO2 systems, it was proposed that the arrangement of the slabs and inlet air velocity in the microchannel heat exchanger need to be optimized by considering heat exchanger size, air outlet conditions and required capacity.

An accumulator heat exchanger (AHX) consists of an accumulator and an inner heat exchanger (IHX) contained in a shell. The AHX has been used in multi-air-conditioners to obtain system reliability and high performance by providing liquid refrigerant into expansion devices and preventing wet-compression. Energy is exchanged between the evaporator exit and the condenser exit in the AHX. In this study, the heat transfer characteristics of the AHX were investigated experimentally, and the effects of the AHX on the performance of a refrigeration system using R22 were measured. The operating characteristics of the refrigeration system with the AHX were considerably different from those without the AHX. The AHX system showed higher refrigerant flow rate than the non-AHX system at a constant EEV (electronic expansion valve) opening because of higher subcooling, resulting in better performance and reliability of the refrigeration system. At 50% EEV opening, the cooling capacity and COP of the AHX system were higher than those of the non-AHX system by 7.5% and 3.2%, respectively.

A simulation study on the performance of the solar and geothermal hybrid heat pump system by using R22 was carried out with a variation of operating conditions. The system was consisted of solar system (concentric evacuated tube solar collector, heat storage tank) and geothermal heat pump system (double pipe heat exchanger, electric expansion valve and compressor). As a result, the heating capacity is linearly decreased from 13.2 kW to 11 kW as the heat pump operating temperature increases from 40 o C to 48 o C. Besides, the heating COP decreases by 13.6% from 4.4 to 3.8 when the ground temperature raises 13 o C to 17 o C. The heating capacity is increased by 4.7% from 11.5 kW to 12.2 kW and the heating COP rises by 19% from 4.7 to 5.6.