Compressive, Elastocaloric Cooling, martensitic phase transformation
Elastocaloric cooling, or thermoelastic cooling, has the potential to substitute the state-of-the-art vapor compression cooling systems. The elastocaloric effect refers to the latent heat associated with the stress-induced martensitic phase transformation process in shape memory alloys. In this study, we demonstrated our latest testing results of worldâ€™s first of-its-kind prototype based on elastocaloric effect. Two beds with Ni-Ti alloy tubes were implemented in the system to generate cooling and heating. Water was used as the heat transfer fluid to reject heat to an air-cooled heat sink and deliver cooling to an electric heater. The system was driven by a linear actuator under compression mode. Operating under a single stage reverse Brayton cycle, the two beds design also enabled the heat recovery and work recovery feature to enhance the systemâ€™s performance. The initial effort led to a successful elastocaloric cooling system prototype with useful cooling capacity of 38 W at a water-to-water temperature lift of 1.5 K. To enhance the system performance, a series of modifications were applied to the system. By better aligning the linear actuator and the two Ni-Ti tube beds, the system temperature lift was increased up to 1.8 K due to increased strain and latent heat. The plastic insulation tube design significantly reduced the heat loss from heat transfer fluid to the metal parts, which successfully increased the system temperature lift to 2.8 K. Furthermore, by modifying the motor layout and making it compress more Ni-Ti tubes per bed the system achieved a 4.2 K temperature lift with 65 W cooling capacity. Plastic insertions to block partial heat transfer fluid inside each Ni-Ti tube reduced the cyclic loss associated with periodic heating and cooling of the heat transfer fluid, which boosted the system temperature lift to 4.7 K.Â Â Unfortunately, the active thermal mass of Ni-Ti tubes was too small when compared to the big losses in the prototype, which indicated that the system mass should be reduced significantly. The system temperature lift of 6.1 K was predicted based on the test results, when assuming no pump parasitic heat generation and no heat conduction loss to the metal supporting frame in each bed. This study built the step stone for elastocaloric cooling technology by successfully achieving an effective cooling for the first time in the world. However, the elastocaloric cooling technology needs a substantial following research to enhance its performance.