Space cooling consumes large amounts of electricity, and the market-dominant vapor-compression refrigeration relies on high global-warming-potential refrigerants. Elastocaloric cooling using shape memory alloys (SMA) is a promising alternative because of its high energy efficiency and greenhouse-gas-free properties. However, the limited temperature span (record of 31 K) and cooling power (record of 260 W) in existing elastocaloric devices restrict the commercialization of this technology. This thesis addresses the challenge by designing and fabricating tubular NiTi SMA with large specific heat transfer area (spiral cross-section of 7.8 cm² g^-1 and multi-cell cross-section of 12.5 cm² g^-1), and developing cascade-unit architectures to increase the system driving efficiency without exceed...[ Read more ]

Space cooling consumes large amounts of electricity, and the market-dominant vapor-compression refrigeration relies on high global-warming-potential refrigerants. Elastocaloric cooling using shape memory alloys (SMA) is a promising alternative because of its high energy efficiency and greenhouse-gas-free properties. However, the limited temperature span (record of 31 K) and cooling power (record of 260 W) in existing elastocaloric devices restrict the commercialization of this technology. This thesis addresses the challenge by designing and fabricating tubular NiTi SMA with large specific heat transfer area (spiral cross-section of 7.8 cm² g^-1 and multi-cell cross-section of 12.5 cm² g^-1), and developing cascade-unit architectures to increase the system driving efficiency without exceeding the limit of Euler buckling under compression. Large specific heat transfer area of refrigerants enable fast heat exchange between NiTi and fluid, while a sufficient regenerative length facilitates a large temperature span along the fluid-flow direction. The effects of material austenite finish (Af) temperatures on system temperature span was investigated through constructing a multi-material layered elastocaloric heat pump. Through matching the working temperatures of NiTi units with their Af temperatures, a giant system temperature span of 75 K was achieved. The effects of operating frequency and heat exchange fluids on cooling density of NiTi was analyzed. Heat-exchange-enhanced NiTi and graphene nanofluid enable a large specific cooling power at high operating frequencies (12.3 W g^-1 at 3.5 Hz operation), while the ‘SMA in series-fluid in parallel’ architecture ensures sufficient elastocaloric mass without increasing system fluid pressure. This device achieves a cooling power of 1284 W on the fluid side and endures over 10 million compressive phase-transition cycles. This thesis reveals the great importance of topology and material design of shape memory alloys and the importance of heat exchange enhanced nanofluids, paving the way for the large-scale commercialization of elastocaloric cooling.