A Comparative Study of Different Cooling/Dehumidification Systems Based on Compressor, Ejector, and Membrane Technologies
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Air conditioning and refrigeration systems for cooling and dehumidification are some of the largest consumers of energy with most of the systems using electricity or fossil fuels to operate. Additionally, refrigeration systems typically use refrigerants, which can deplete the ozone layer and contribute to global warming, as the working fluid during operations. Therefore, alternative cooling and dehumidification systems need to be developed and implemented as substitutes to conventional HVAC systems in order to reduce the destruction of the environment. In addition, it is important that these new non-refrigerant systems provide the same or better energy performance when compared to conventional system. The application of ejector and membrane technologies can provide an alternative approach to conventional systems; therefore, the performance characteristics of these systems are investigated herein by modelling and simulating. Four systems were modelled, evaluated, and analyzed in this study with the simulations for each system being performed by using the Engineering Equation Solver (EES). The first major system investigated was a conventional cooling system, commonly referred to as a vapor-compression refrigeration system. The model inputs for this system are hot region temperatures of 27 to 33°C and cold region temperatures of 6 to18°C, with these regions forming the heat sinks and sources, respectively. Additionally, the working fluids to the vapor-compression system were assumed to be either refrigerant R-22, which is still widely used for HVAC applications, or an ozone-safe replacement, namely (R-410A). The second major system investigated was a steam-ejector refrigeration system, which has the same inputs as the vapor-compression refrigeration system. This system has been operated for some time but has seen only limited applications because of its lack of optimization. Therefore, a particular focus herein for this system was the development of an ejector model capable of investigating optimum performance characteristics. The third major system was the membrane-ejector dehumidifier that uses a steam ejector for the purpose of creating a vacuum on the low-pressure side of a membrane, this low-pressure region promotes the removal of water vapor from the ambient air that is dehumidified as it flows on the other side of the membrane surface. A major difference between the compressor in the vapor-compression refrigeration system and the ejector in the membrane-ejector dehumidification system is that the compressor operates with high-cost mechanical and electrical energy while the ejector operates with low-cost thermal energy, which is used to produce driving steam in a 90-150°C boiler. The fourth system was also evaluated with this system being similar to the third system except that a condenser was installed between ejectors. As noted before, all four systems were simulated with idealized conditions in order to facilitate modelling. As such, the true value of the investigation reported herein is knowledge gained regarding the performance of each system as a function of various parameters, rather than a system to system performance comparison. For the given conditions and assumptions, the coefficient of performance of the vapor-compression systems with either refrigerant R-22 or R410A was found to range from 8 to 30, which is higher than the COP found in real-world operating conditions because of the idealized model. The steam-ejector refrigeration system which operated at the same heat source and sink temperature conditions, had COP’s ranging from 2.2 to 6.5. However, a direct comparison of COP’s for the two technologies is not possible because the ejector system used low-cost thermal energy, and the idealized assumption. The membrane-ejector dehumidifier gave COP’s of 0.12 to 0.19 but add in a condenser between the two ejectors doubled the COP to 0.44-0.5. Again, because the membrane-ejector system is the most innovative and complicated of the four systems, many opportunities exist for improvement and optimization. Of special note, the COP’s of the first two system is based on cooling while these last two system COP’s are based on dehumidification, precluding COP comparison. Another consideration is that non-idealized assumptions in this study is the significant air leakage through the membrane, and as membrane improvement are made significant increase in COP’s are expected. Furthermore, the small COP of the ejector system can be drastically increased if the thermal energy input comes from a renewable heat sources such as solar or geothermal.
Hendranata, Yongki (2018). A Comparative Study of Different Cooling/Dehumidification Systems Based on Compressor, Ejector, and Membrane Technologies. Master's thesis, Texas A & M University. Available electronically from