Search

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  ：

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, 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.

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.

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.