Upscaling of SMA film-based elastocaloric cooling

Abstract

A new concept of upscaling a shape memory alloy (SMA) film based elastocaloric cooling device is presented by arranging SMA films in parallel to increase the specific cooling capacity at low actuation force, while maintaining the large surface-to-volume ratio needed for rapid heat transfer. Selected materials are cold-rolled TiNiFe films that exhibit maximum adiabatic temperature changes of 27.3 and -18.1 K upon loading and unloading, respectively. Demonstrators are designed, fabricated and characterized consisting of five free-standing TiNiFe film bridges that are coupled antagonistically for work recovery. Thermomechanical cycling is performed by out-of-plane deflection of the SMA bridges, while heat transfer is established through mechanical contact with solid heat sink/source elements. The cooling capacity of the demonstrators scales with the number of active SMA films, which confirms the concept of parallelization for upscaling. Investigated demonstrators reach a maximum cooling capacity of about 900 mW compared to a maximum of about 200 mW achieved for reference devices consisting of a single TiNiFe film. The investigation also reveals a number of open issues related to narrow fabrication tolerances upon upscaling, which may cause different plastic straining and varying inhomogeneous stress accumulation among the individual SMA films.