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Nickie Koskinen

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Aug 2, 2024, 8:08:20 PM8/2/24

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At the end of the course, the student should be able to differentiate between different types of refrigeration systems with respect to application as well as conventional and unconventional refrigeration systems. Thermodynamically analyse refrigeration and air conditioning systems and evaluate performance parameters. Apply the principles of Psychometrics to design the air conditioning loads for the industrial applications.

Conventional and not-in-kind refrigerators require heat exchangers for their operation. Yet, most magnetic cooling studies do not take full account of those components despite their importance in defining the cooling capacity and temperature span. To investigate the influence of heat exchanger design parameters on the performance of magnetic refrigerators, a model was developed to integrate the heat exchangers, regenerators and thermal reservoirs. The results were compared with data generated in an apparatus that emulates the conditions of the thermal fluid supplied by the regenerators to a cold heat exchanger positioned inside the cabinet of a retrofitted 130-liter wine cooler. Six tube-fin heat exchangers were evaluated to identify the most suitable geometry (number of tube rows and fin density) for the compact magnetic refrigerator. Numerical simulations described the influence of the heat exchanger on the regenerator performance in terms of the liquid stream effectiveness. For a temperature span of 20C between the external environment and the refrigerated compartment, the best heat exchanger/fan assembly resulted in a cooling capacity reduction of 37\% and a temperature span increase of 32\%, in comparison with an idealized system. The expected system coefficient of performance (COP) and second-law efficiency were 1.8\% and 13\%, respectively.

Due to the reversibility of the MCE in some types of materials and the use of regenerative thermodynamic cycles, magnetic refrigerators have the potential to develop high efficiencies. The absence of harmful gases and high pressures is also seen as an advantage compared to some cooling and heat pumping applications (e.g., air conditioning). Nevertheless, several challenges still prevent magnetic refrigerators from becoming commercially available, particularly those associated with cost (mainly of magnetocaloric materials and the magnetic circuits) and mechanical losses in ancillary systems, such as the hydraulic circuit (Yu et al. 2010YU B, LIU M, EGOLF PW & KITANOVSKI A. 2010. A review of magnetic refrigerator and heat pump prototypes built before the year 2010. Int J Refrig 33: 1029-1060., Kitanovski et al. 2015KITANOVSKI A, TUSEK J, TOMC U, PLAZNIK U, OBOLT M & POREDOS A. 2015. Magnetocaloric Energy Conversion, From Theory to Applications. Springer, 477 p., Trevizoli et al. 2016aTREVIZOLI PV, CHRISTIAANSE TV, GOVINDAPPA P, NIKNIA I, TEYBER R, BARBOSA JR & ROWE A. 2016a. Magnetic heat pumps: An overview of design principles and challenges. Sci Technol Built En 22: 507-519., Greco et al. 2019)GRECO A, APREA C, MAIORINO A & MASSELLI C. 2019. A review of the state of the art of solid-state caloric cooling processes at room-temperature before 2019. Int J Refrig 106: 66-88..

Heat exchangers are responsible for a substantial share of the overall losses in refrigerators and heat pumps. The efficiency breakdown of a domestic refrigerator performed by Gonalves et al. (2011)GONALVES JM, MELO C, HERMES CJL & BARBOSA JR JR. 2011. Experimental mapping of the thermodynamic losses in vapor compression refrigeration systems. J Braz Soc Mech Sci Eng 33: 159-165. indicated that the heat exchangers are the major source of irreversibility in household and light commercial applications. In their exergetic evaluation of vapor compression refrigerators, Morosuk & Tsatsaronis (2009)MOROSUK T & TSATSARONIS G. 2009. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy 34: 2248-2258. concluded that the evaporator should be the first component to be upgraded for the benefit of improving the overall system performance. Analogously, through an exergy destruction analysis, Arora & Kaushik (2008)ARORA A & KAUSHIK S. 2008. Theoretical analysis of a vapour compression refrigeration system with R-502, R-404A and R-507A. Int J Refrig 31: 998-1005. found that the major sources of irreversibility are located in the condenser.

Even though the importance of the heat exchangers to the overall performance of conventional cooling systems has been widely explored and documented in the literature (Klein & Reindl 1998KLEIN SA & REINDL DT. 1998. The relationship of optimum heat exchanger allocation and minimum entropy generation rate for refrigeration cycles. J Energ Resour-ASME 120: 172-178., Gholap & Khan 2007GHOLAP A & KHAN J. 2007. Design and multi-objective optimization of heat exchangers for refrigerators. Appl Energ 84: 1226-1239., Waltrich et al. 2011WALTRICH PJ, BARBOSA JR JR & HERMES CJL. 2011. COP-based optimization of accelerated flow evaporators for household refrigeration applications. Appl Therm Eng 31: 129-135.), to date, most magnetic cooling prototypes used Joule-effect heaters in contact with the heat transfer fluid on the cold side to generate the thermal load (Kitanovski et al. 2015KITANOVSKI A, TUSEK J, TOMC U, PLAZNIK U, OBOLT M & POREDOS A. 2015. Magnetocaloric Energy Conversion, From Theory to Applications. Springer, 477 p.). Despite the convenience of this technique, it is clearly not capable of fully reproducing the real thermal interaction between the cooling system and the refrigerated environment. In actual magnetic cooling systems, heat exchangers will have finite overall thermal conductances which will affect the operating conditions of the AMR and the thermodynamic performance of the device (Trevizoli et al. 2016aTREVIZOLI PV, CHRISTIAANSE TV, GOVINDAPPA P, NIKNIA I, TEYBER R, BARBOSA JR & ROWE A. 2016a. Magnetic heat pumps: An overview of design principles and challenges. Sci Technol Built En 22: 507-519.).

The refrigerated cabinet, another fundamental component of cooling devices, has not been considered or evaluated in the open literature as regards the performance of magnetic refrigerators. Thermodynamically speaking, the insulated cabinet links the cooling capacity to the temperature span, defining the operating point of the cooling device. Moreover, the power consumption of a refrigeration system is dictated principally by the heat load through the cabinet walls (Melo et al. 2000MELO C, DA SILVA LW & PEREIRA RH. 2000. Experimental evaluation of the heat transfer through the walls of household refrigerators. In: Proc. International Refrigeration and Air Conditioning Conference at Purdue, number 1502.), thus making the evaluation of this component fundamental to the design of magnetic refrigerators.

The experimental apparatus consists of two similar hydraulic circuits, the hot and cold loops, which are responsible for emulating the temperature and flow rate of the fluid flow streams produced by a hypothetical AMR during the cold and hot blows, respectively. The circuits, shown schematically in Fig. 1, are integrated with the cabinet of the wine cooler by the cold and hot tube-fin heat exchangers. Fig. 2 shows a photograph of the apparatus connected to the wine cooler cabinet.

Each hydraulic circuit is composed of a rotary vane pump powered by an electric motor to provide continuous changes of the mass flow rate in each circuit. The thermal fluid is a 20% vol. solution of automotive antifreeze (ethylene glycol with corrosion inhibitors) in deionized water. The operating frequency of the pumps are controlled by frequency inverters, while needle valves help to regulate the mass flow rate through the circuits. Check valves guarantee unidirectional fluid flow in the by-pass lines.

Using the cold circuit as an example, the temperature of the fluid entering the cold heat exchanger (CHEx), which corresponds to the temperature of the fluid exiting the AMR in the hot blow, is set by an auxiliary brazed-plate heat exchanger (AHEx) connected to a temperature controlled bath. An axial fan (see Section Heat exchanger samples) powered by an external source drives the cabinet air through the tubes and the fins of the CHEx. The air flow rate (at the operating point) is determined by matching the fan static pressure curve and the CHEx pressure drop curve. A similar setup was implemented in the hot circuit.

As shown in Fig. 2, a commercially available 130-l (31-bottle) wine cooler cabinet has been used in this study. Before removing the original vapor compression refrigeration system (compressor, roll-bond evaporators, wire-on-tube condenser and capillary tubes), power consumption and temperature pull-down tests have been carried out in a climatic chamber according to the methodology proposed by Hermes et al. (2013)HERMES CJ, MELO C & KNABBEN FT. 2013. Alternative test method to assess the energy performance of frost-free refrigerating appliances. Appl Therm Eng 50: 1029-1034. to serve as a baseline for future comparisons with real AMR systems.

The location of the three thermocouples used for measuring the internal air temperature during the wine cooler performance tests (after retrofitting) are shown in Fig. 3 together with the position of the cold heat exchanger/fan assembly and some internal cabinet dimensions. The mean surrounding ambient temperature, Tamb, is computed from three thermocouples placed around the cabinet (two of them are visible in Fig. 2, sticking out of the glass door and side wall).

The cold heat exchanger of the original refrigeration system was a natural draft roll-bond evaporator, which typically operates with a temperature driving potential higher than 10C in vapor compression systems. For the operation of magnetic cooling devices it would be more advantageous to work with smaller temperature spans, due to the high magnetization power consumption associated with larger spans, as will be discussed later. Therefore, compact herringbone wave-type tube-fin heat exchangers have been selected for the present analysis. Fig. 3 presents the geometric configuration and main design parameters of a typical heat exchanger of such configuration.