A benchmark natural rubber-based elastocaloric refrigerator

Abstract

Natural rubber (NR) is a cheap and accessible polymer that presents a giant caloric response to applied tensions due to stress-driven crystallisation. The reversibility of the process can be exploited to implement a refrigeration device that uses the material’s symmetric caloric response under periodically applied/released tension to cool it below room temperature, acting as a heat sink. In this thesis, NR’s adiabatic temperature change ΔTad under applied stress is studied subjecting samples to different tensions within an 80-120 N range and observing their temperature evolution via infrared imaging. The mechanical work required to induce the crystallisation is also calculated in order to derive NR’s coefficient of performance COPNR with and without energy recovery conditions, for comparison. The results indicated that a 90 N tension (3.35 MPa in strain) provided optimised caloric response without compromising the sample’s resistance. A simple device was then designed to test the effectiveness of NR as a refrigerant for cooling a water heat exchanger circuit, aimed to serve as a model for comparison for upgraded cooling prototypes. Temperature evolution during the cooler’s operation was registered using thermocouples for 20 consecutive cycles, yielding an effective cooling of ΔTspan = 0.65 ± 0.05 K after 400 seconds, resulting in a performance coefficient for the device of COPp = 0.36±0.02, and an effective cooling power per refrigerant mass of Pp = 4.1±0.2 W kg-1.