International Journal of Air-Conditioning and Refrigeration

Miniature Stirling coolers are preferred to provide cryogenic cooling for infra-red (IR) sensors used for communication, military, and space applications. They provide 0.25–1.5W of cooling effect at 60–80K. Miniature Stirling coolers used for space applications are time tested, reliable, and have the maximum COP compared to other types of coolers. Helium is used as the working fluid because of its low boiling point, high thermal conductivity, high ratio of specific heat, and inert gas properties. A regenerator is the primary heat exchanger in the system, which periodically exchanges heat with the cold and hot gases passing through the regenerator material. The effectiveness of the regenerator is the most important parameter influencing the cooling effect produced by the system. For the optimum performance of the cryocooler, the regenerator should have maximum heat transfer area, minimum void volume, minimum pressure drop, large heat capacity ratio between the matrix material and gas, and minimum longitudinal conduction. Since some of these requirements are conflicting in nature, the design of the regenerator becomes a challenge in the overall design of the cooler. A state-of-the-art review of regenerator materials, designs, and operation is presented in this study. The different sources of regenerator losses and the issues related to regenerator design and optimization are discussed in detail. Results of various experimental and numerical investigations conducted on a Stirling regenerator are discussed and the recent developments in material selection and design are highlighted.

Although thermoacoustic devices comprise simple components, the design of these machines is very challenging. In order to predict the behavior and optimize the performance of a thermoacoustic refrigerator driven by a standing-wave thermoacoustic engine, considering the changes in geometrical parameters, two analogies have been presented in this paper. The first analogy is based on CFD analysis where a 2D model is implemented to investigate the influence of stack parameters on the refrigerator performance, to analyze the time variation of the temperature gradient across the stack, and to examine the refrigerator performance in terms of refrigeration temperature. The second analogy is based on the use of an optimization algorithm based on the simplified linear thermoacoustic theory applied for designing thermoacoustic refrigerators with different stack parameters and operating conditions. Simulation results show that the engine produced a high-powered acoustic wave with a pressure amplitude of 23kPa and a frequency of 584Hz and this wave applies a temperature difference across the refrigeration stack with a cooling temperature of 292.8K when the stacks are positioned next to the pressure antinode. The results from the algorithm give the ability to design any thermoacoustic refrigerator with high performance by picking the appropriate parameters.

This research presents a system to use natural gas to meet electricity, freshwater and cooling needs for a residential building in Bandar Abbas. The system includes a gas turbine, absorption chiller and multi-effect desalination (MED) plant. The energy produced in the gas turbine is used to generate electricity, and the excess energy is used to produce cooling and freshwater. Finally, an exergoeconomic evaluation of the system is performed. The effects of ambient temperature on the output power as well as the exergy current have been investigated. The COP of the absorption cycle has been investigated, and the results show that at an operating temperature of 150^∘C compared to 90^∘C, the efficiency rate increases to 20%. The highest exergoeconomic cost rate is related to absorption chiller, and the lowest is related to heat recovery steam generation. The results show that if the ambient temperature increases, the production capacity decreases. Increasing the fuel flow rate increases the power. Evaluation of two different solutions to reduce the ambient temperature and increase the fuel flow shows that increasing the fuel flow is a better solution, considering the exergy cost of the absorption chiller, which is 10 times higher than that of the gas turbine.

Traditional air conditioning systems run on the vapor compression cycle, which utilizes electricity generated from fossil fuels, the reserves of which are fast depleting. Moreover, the refrigerants used in such systems have exacerbated ozone layer depletion and climate change. Liquid desiccant air conditioning (LDAC) systems appear as a favorable option in light of these drawbacks. This paper has developed the numerical model of an LDAC system using a dehumidifier that is internally cooled and has a finned coil. The study of this particular model has been limited in the past. The model has been validated against a reference study available in the literature. Moreover, the commonly used desiccant materials tend to be corrosive and detrimental to the air quality. Therefore, the feasibility of an ionic liquid (65% [Emim]OAc) and an organic salt solution (73% HCOOK) as desiccant materials has also been investigated in this paper. With the proposed scheme for room air conditioning, the air temperature and humidity levels within the range of comfort (21^∘C, 53% R.H.) could be supplied to the conditioned space. Performance analysis revealed that the eco-friendly desiccants could achieve dehumidifier and regeneration effectiveness similar to that of the traditionally used corrosive salt solution of calcium chloride (CaCl₂). Furthermore, the estimated dehumidifier efficiency of about 76% is found to be higher than that available in the previous studies. Economic analysis provides a comparison of total cost between the conventional and the novel desiccant air conditioning systems and also reveals HCOOK to be a more economical choice as desiccant material.

To explore the application of the human body exergy analysis method in assessing the indoor thermal environment, strictly according to the definition of metabolic exergy, based on the two existing exergy analysis models, a more reasonable calculation method of metabolic exergy was first proposed and established, which is a new two-node exergy analysis model. Then, the experimental data in the ASHRAE database were used to verify the reliability of the established model. Finally, the human body exergy exchange rate and exergy loss rate with indoor and outdoor environmental parameters were revealed. The research results show that the new metabolic calculation method can more accurately analyze the human body’s exergy; the exergy loss rate is larger than the exergy exchange rate in the metabolic exergy rate; when the operating temperature is 25^∘C, the exergy exchange rate mainly includes the radiation exergy rate and the convective exergy rate; operation at a temperature of 32^∘C, the evaporation rate and respiration rate are the main components of the exergy exchange rate; the human body’s exergy loss rate has extreme values at low or high operating temperatures, and it is used separately for the evaluation of human thermal comfort. Combining the exergy loss rate and the exergy exchange rate can better evaluate the indoor thermal environment, the minimum exergy loss rate and the minimum exergy exchange rate under the given indoor conditions, both occur under outdoor high-temperature and low humidity conditions; outdoor temperature affects the human body exergy loss rate and human body exergy exchange rate more strongly than outdoor relative humidity.

A Miniature Pulse Tube Cryocooler (MPTC) is the first selection for the cooling of IR sensors, infrared detectors, etc. in space technology. The regenerator is one of the key components to operate an MPTC at high efficiency. The objective of this study is to explore the possibilities of the different matrices as regenerator filler materials for MPTC operating at ultra-high frequencies. REGEN 3.3 is one of the best software available for the design and optimization of cryocooler regenerators. We have used REGEN 3.3 for numerical simulations of the three different regenerator matrix materials viz. stainless steel wire mesh screen (SS 635#), brass wire mesh screen (500#) and copper wire mesh screen (500#) at the hot end and cold end temperatures of 300K and 80K for COP, cooling power, total power losses and pressure losses, at an ultra-high frequency of 100Hz and 200Hz. The simulation results depict that the regenerator using stainless steel mesh screen shows better results than that of the brass mesh screen and copper mesh screen at 100Hz. However, the performance of brass mesh screen and copper mesh screen performs better than the stainless steel at 200Hz. Therefore, the proposed matrix materials can be used as regenerator materials for the MPTC at ultra-high frequencies with better performances.

In zones separated by doors with many entrances and exits, it is crucial in terms of energy consumption to protect the conditioned air in the indoor environment from the effects of outside air. The increase in door dimensions and opening frequency make this effect even more evident. Various methods are used to prevent leakage of outside air into the indoor environment. In this study, we investigated the effects of using air curtains on energy consumption during the cooling season in a factory building. The door width and height in this building, which has a high story height, were also large. These 26 doors must remain open all the time because of the production process. With a transient model developed on Transient System Simulation Tool (TRNSYS) coupled with Contaminant Transport Analysis Software (CONTAM), we showed the effects of using the air curtains in different climate conditions. As a result, we demonstrated that using air curtains will provide great savings, especially in cities with high cooling requirements.

In this study, the performance of ejector-expansion refrigeration cycle (EERC) with R134a alternative refrigerants (R152a, R1234yf, R404A, R407C, R507A and R600a) for automobile air-conditioning application is investigated numerically. The ejector is modeled with a constant mixing-pressure assumption taking into consideration the friction effect in the ejector mixing section. The studied refrigerants are compared based on the optimum area ratio, discharge temperature, compressor input power, volumetric cooling capacity, exergy destruction, COP, exergy efficiency and COP improvement. The results show that R152a and R1234yf have the closest performance to R134a and can be considered the most suitable alternative refrigerants for R134a. The COP and exergy efficiency are improved by 2.26% and 2.27%, respectively, using R152a compared to the use of R134a, whereas they are reduced by 2.89% and 2.88% using R1234yf. The volumetric cooling capacity is reduced for both R152a and R1234yf by 6.14% and 6.8%, respectively. In addition, the effect of compressor rotational speed on the performances is reported.

In this work, R1234yf is tested in an air-conditioning system as a proposed alternative to R134a. The system is tested at 30, 35, 40, 45 and 50Hz evaporator fan frequency to cool the air entering into the evaporator section at a fixed temperature of 35^∘C. The system is charged with 600, 700, 800, 1000 and 1200g of each refrigerant. The charge mass of 800 grams gives the best performance for both refrigerants. Also, R1234yf has higher COP, exergetic efficiency, second law efficiency, and lower compressor power consumption. Both refrigerants operate at a moderate range of compressor discharge temperature of 85^∘C.

This paper deals with the advanced exergetic analysis of a horizontal direct-expansion ground sourced CO₂ heat pump operating in a transcritical cycle. The cycle is thermodynamically modeled in Engineering Equation Solver (EES) considering the pressure drops in both high and low temperature heat exchangers, and the system is to provide a fixed heating load. Conventional exergy analysis orderly suggests a compressor, expansion valve, gas cooler and ground heat exchanger to be considered for system improvement, while tracing exergy destruction of all components in detail demonstrates true improvement potential of each and all components and the system as a whole and offers a different order. Advanced exergy analysis points out that the compressor is directly and indirectly responsible for 56% of the overall exergy destruction generated in the cycle, confirming the detrimental role of this component in the system. The second influential component is recognized to be a ground heat exchanger accounting for 20% exergy destruction of the compressor as well as submitting 89% avoidability in its own exergy destruction, and expansion valve proves to be the last option for system improvement according to this analysis.

The desiccant cooling mechanism is one of the alternate methods to control the air humidity compared to the conventional air conditioning system. An experimental investigation has been carried out in the current research using a hybrid desiccant in a liquid desiccant dehumidification system. Triethylene glycol (TEG) mixed with different proportions of magnesium chloride (MgCl₂) has been used as the hybrid desiccants. The performance of the dehumidifier was measured using the parameter moisture removal rate and enthalpy effectiveness. The blends of TEG and MgCl₂ gave a better moisture removal rate (MRR) as compared to 100% TEG. The MRR and moisture effectiveness increased with the increasing desiccant flow rate and air flow rate for all of the blend ratios. The blend with 25% MgCl₂ and 75% TEG concentration had relatively high MRR and dehumidification effectiveness. The study reveals that if an optimized blend of TEG and MgCl₂ is used as the desiccant instead of a neat TEG, it will overcome the high viscosity problems of TEG and become one of the promising candidates for sustainable energy sources.