International Journal of Air-Conditioning and Refrigeration

Energy consumption and its associated consequences can be reduced by implementing district cooling strategies that supply low temperature water to a wide range of end users through chillers and distribution networks. Adequate understanding, performance prediction and further optimization of vapor compression chillers used widely in district cooling plants have been a subject of intense research through model-based approaches. In this context, we perform an extensive review of different modeling techniques used for predicting steady-state or dynamic performance of vapor compression liquid chillers. The explored modeling techniques include physical and empirical models. Different physical models used for vapor compression chillers, based on physics laws, are discussed in detail. Furthermore, empirical models (based on artificial neural networks, regression analysis) are elaborated along with their advantages and drawbacks. The physical models can depict both steady- and unsteady-state performance of the vapor compression chiller; however, their accuracy and physical realism can be enhanced by considering the geometrical arrangement of the condenser and evaporator and validating them for various ecofriendly refrigerants and large system size (i.e., cooling capacity). Apparently, empirical models are easy to develop but do not provide the necessary physical realism of the process of vapor compression chiller. It is further observed that DC plants/networks have been modeled from the point of view of optimization or integration but no efforts have been made to model the chillers with multiple VCR cycles. The development of such models will facilitate to optimize the DC plant and provide improved control strategies for effective and efficient operation.

The objective of this paper is to study the effect of nanoparticles on Coefficient of Performance (COP) and thermal load of main components (generator, condenser, evaporator and absorber) of absorption refrigeration system using H₂O–LiBr. Al₂O₃ nanoparticles have been added into the H₂O–LiBr solution to make the binary nanofluid, and Polyvinyl Alcohol (PVA) has been used as a stabilizer. Experimentation part involves the preparation of nanofluid and tests on H₂O–LiBr and H₂O–LiBr with 0.05% Al₂O₃. The effect of nanoparticles on COP and thermal load of components has been studied by varying the operating temperatures (generator, condenser and absorber temperature) and volume fraction of nanoparticles. The results show that COP and thermal loads of system with nanoparticles at different operating temperatures are following the same trend as base fluid system. The COP of system with nanoparticles at each operating temperature is higher than base fluid system. The heat supplied to generator with nanoparticles is less than without nanoparticles while heat rejected in condenser and absorber with nanoparticles is higher than without nanoparticles at each operating temperature. The heat exchanged in evaporator with nanoparticles is also higher than that without nanoparticles at each operating temperature. We also obtained maximum COP with 0.2% volume fraction of nanoparticles.

In hot and humid climates, evaporative cooler is coupled with desiccant dehumidifier to obtain effective cooling. To realize the M-cycle-based cooler which combines liquid desiccant regeneration and evaporative cooling in a single apparatus, the effect of different feed water temperature on cooling performance is investigated in this paper. It was observed that wetting water film layer of conventional regenerative evaporative cooler (REC) carries a significant amount of sensible heat from the hot water/solution which reduced the wet-bulb effectiveness of the cooler. To enhance its effectiveness, an ultrasonic atomization mist REC was proposed. The influence of different intake conditions on cooling performance was studied and found that mist REC performed better than conventional cooler for higher feed water temperature. The wet-bulb effectiveness of the cooler ranged from 0.56 to 1.15 with maximum cooling capacity of 580W, which is comparable to the previous studies. This prototype can be further developed to an ultrasonic liquid desiccant waterless evaporative cooler.

The low performance of the thermoacoustic refrigerator has made it uncompetitive to currently available refrigeration systems and hence its path towards commercialization is being restricted. Recently, evolutionary algorithm such as genetic algorithm has become popular among researchers in optimizing the performance of the thermoacoustic refrigerator due to its capability to provide a solution with a global maximum or minimum through simultaneous optimization of several objectives. The purpose of this study was to maximize the performance of the thermoacoustic refrigerator using the Multi-Objective Particle Swarm Optimization (MOPSO), an evolutionary optimization tool that has not been tried in this field before. By optimizing the two conflicting objectives which are maximizing the cooling power and minimizing the acoustic power required, simultaneous optimization of inter-dependent controlling parameters has been performed for two, three and four parameters. Comparing with the results of past studies, MOPSO has improved the stack COP by 6.92% compared to the parametric optimization approach and 2.96% higher than the maximum COP achieved by multi-objective genetic algorithm (MOGA) with an optimum COP of 1.39. Also, a maximum cooling power of 10.8 W was obtained. This study has highlighted the potential of MOPSO in providing optimized conditions for conflicting objectives desired for a thermoacoustic system.

A steady-state mathematical model of a small-capacity Automotive Air Conditioning System (AACS) working with the refrigerants R134a (1,1,1,2-tetrafluoroethane) and R1234yf (2,3,3,3-tetrafluoropropene) is developed. The model considers the major components of the system, namely, the swash plate compressor with fixed stroke, thermostatic expansion valve (TXV) and the compact heat exchangers (flat-tube, louvered-fin evaporator and condenser). In order to verify the mathematical model, an experimental facility has been developed for testing the AACS and experiments are conducted with R134a. The experimental results are also validated with published data. The effects of various operating conditions, namely, the condensing air temperature, condensing air velocity and the compressor speed, on the performance of the AACS are predicted for both R134a and R1234yf. The results of the simulations with R134a and R1234yf are compared with the experimental results obtained with R134a. The results show good agreement with the experimental results, with most of them showing less than 10–15% of variations from the experiment results. The uncertainty analysis is performed and showed that the most of validations follow below 10% difference.

Frost crystal type and distribution were characterized in the initial periods of frost growth on an aluminum surface. Experiments were carried out for a range of wall temperatures from $-25^{\circ }$C to $-5^{\circ }$C, air temperatures from $6^{\circ }$C to $22^{\circ }$C, relative humidities from 15% to 85%, and air velocities from 0.5 to 5.0m/s. The results showed that frost crystal type was strongly dependent on the wall temperature and humidity. Changing the air temperature shifted the region of some frost crystal types. Decreasing the air temperature from 22 down to either $12^{\circ }$C or $6^{\circ }$C led to the decrease of feather crystals but increased the region of scroll crystals. Air velocity had smaller impacts on frost crystal type but had a strong influence on the distance between the crystals, particularly at lower air velocities. The results were compared to prior researchers. The results should provide a better understanding of frost morphology during the early stages of frost growth on metal surfaces.

In this paper, a simulation of a photovoltaic–thermal (PV/T) hybrid solar system with longitudinal fins absorbers was developed to determine optimal geometry parameters for conventional design of this system. In this case, we used a dynamic model based on the principle of the airflow rate to determine the optimum design of the PV/T system for each airflow velocity. In this regard, the influences of the geometric parameters (number, height) on the collector performance of each model with different flow velocities were investigated. The mathematical model is developed using the energy balance equations of the PV/T air collector. A good agreement is obtained between the simulation results and the reference data from the literature after the system PV/T air collector is evaluated by calculating the root mean square error. The studied cases indicate that for the four simulated models, the best highest performance rate is providing with the first model at 2m/s airflow velocity. The thermal and electric efficiencies were reached 83% and 12.85%, respectively, with the maximum temperature value on the photovoltaic cell as $56.8^{\circ }$C. However, a combined efficiency for the hybrid PV/T solar collector, which is the sum of the electrical efficiency and thermal efficiency, is equal to 95.98%.

Recently, renewable sources of energy, particularly, solar thermal energy, have gained significant attention for developing heating and cooling mechanisms for buildings. This work aims at developing a theoretical model for space heating based on phase change material (PCM) using solar energy in winter conditions for the Northern region of India. The system has two PCM containers placed inside the room on the two opposite walls, which receives heat from concentrating solar system during the daytime. After achieving the temperature of PCM above its melting point using the solar system, the stored heat of PCM is released to the room ambient during the nighttime when the room ambient is at lower temperature. OM-37, having melting temperature of $37^{\circ }$C, has been used as a PCM for the current research work. The numerical investigation of the system shows that temperature of the room reaches to 26–$27^{\circ }$C from $15^{\circ }$C in 4–5h of operation and thus ensuring thermal comfort of occupants.

Lately, absorption of carbon dioxide using nanofluids has gained more attention as this acidic gas creates global warming effect. The absorption test was conducted in a custom designed high-pressure vessel made up of stainless steel 316 L, where CO₂ and nanofluid are in direct contact at static state. The type of nanoparticles and influence of its concentration on absorption of carbon dioxide are analyzed. TiO₂ and Al₂O₃ nanofluids at 0.02–0.14wt.% concentrations are prepared by dispersing in DI water. The CO₂ absorption tests were carried out for the above-mentioned nanofluids at said concentrations with operating conditions being an initial pressure of 3 bar and initial temperature of 302K. The results show that relative absorption index (RAI) of CO₂ absorption has increased to a maximum and then decreased with increase in nanoparticle concentration. The aqueous-based TiO₂, Al₂O₃ nanofluids are found to be most effective at 0.1 and 0.14wt.%, respectively, with RAI showing 39.81% and 22.3% increase in CO₂ absorption as compared to basefluid, respectively. The absorption test has also been conducted for saline-based TiO₂ and Al₂O₃ nanofluids at 1, 2, 3 and 3.1wt.% of salt concentration. The stability of saline-based nanofluids was analyzed using turbidity meter. It was found that increase in salt concentration decreases the stability of nanofluids and also decreases the CO₂ absorption rate because of unstability of nanoparticles in salt solutions. Absorption decreased by 11.93% for TiO₂, and 5.68% for Al₂O₃, when salt concentration was increased from 1 to 3.1wt.%.

This study aimed to analyze the difference in operation of the vapor compression refrigeration (VCR) system with vortex tube cooling. By using varied loads, experiments were conducted on the evaporator section of a vapor compression refrigeration system. In an attempt to improve the use of subcooling for the refrigeration, the effect of subcooling of refrigerant by vortex tube cooling was likewise examined. The test conditions included various loads (25%, 50%, 75% and 100%) and cold mass fractions (25%, 50% and 75%). This research described coefficient of performance (COP) as one of the significant parameters, in addition to heat rejection and refrigerating effect. The ideal efficiency appeared to be with the cold mass fraction of 25% and load of 100%, as identified by the results. Consequently, the COP could be enhanced by 5.16% along with an approximately 4.36% decline in average power use. Improved guidelines for vapor compression refrigeration systems to enhance the operation of the system are an expected benefit of this study.

Recent concerns raised by the World Health Organization over the Coronavirus raised a worldwide reaction. Governments are racing to contain and stop the Coronavirus from reaching an epidemic/pandemic status. This research presents a way in tracking such a virus or any contagious germ capable of transferring through air specifically where such a transfer can be assisted by a mechanical room ventilation system. Tracking the spread of such a virus is a complicated process, as they can exist in a variety of forms, shapes, sizes, and can change with time. However, a beginning has to be made at some point. Assumptions had to be made based on published scientific data, and standards. The tracking of airborne viruses was carried out on the following assumption (for illustrative purposes); one person with one sneeze in a period of 600 s. The presence of viruses was tracked with curves plotted indicating how long it could take to remove the sneezed viruses from the mechanically ventilated room space. Results gave an indication of what time span is required to remove airborne viruses. Thus, we propose the following: (a) utilizing CFD software as a possible tool in optimizing a mechanical ventilation system in removing contagious viruses. This will track the dispersion of viruses and their removal. The numerical solution revealed that with one typical adult human sneeze, it can take approximately 640 s to reduce an average sneeze of 20,000 droplets to a fifth; (b) upscaling the status of human comfort to a “must have” with regards to the 50% relative humidity, and the use of Ultraviolet germicidal irradiation (UVGI) air disinfection in an epidemic/pandemic condition. A recommendation can be presented to the local authorities of jurisdiction in enforcing the above proposals partially/fully as seen fit as “prevention is better than cure”. This will preclude the spread of highly infectious viruses in mechanically ventilated buildings.