Developing and Modelling 3D-printed Soft Active Elastocaloric Regenerators: An exploration for green cooling

Worldwide, heating and cooling account for the largest contribution to energy consumption and greenhouse gas emissions [1]. Decarbonizing heating and cooling is crucial for achieving net-zero carbon dioxide emissions. However, existing technologies continue to heavily rely on the combustion of natural gas for heating and the compression of volatile greenhouse gases for cooling. It is thus crucially important to explore the next generation of green cooling and heating to replace vapour compression.

Elastocaloric cooling has been recommended as the most promising non-vapor compression technology by the US Department of Energy and EU Commission, utilizing the temperature change of materials in response to uniaxial strain. In this
PhD thesis, low-stress regenerative elastocaloric cooling employing printable soft elastomers is explored. Soft elastocaloric elastomers exhibit a much lower applied stress requirement to induce the elastocaloric effect (eCE) compared to alloys. Implementing additive manufacturing (AM) facilitates the fabrication of fully-printed elastocaloric regenerators with customized heat-transfer microchannels. Moreover, AM allows the design of 3D-printed regenerators in a compact configuration. Prior to regenerator printing, five commercial thermoplastic filaments were screened as potential elastocaloric elastomers, suggesting thermoplastic polyurethane (TPU). NinjaFlex TPU demonstrates a notable material coefficient of performance (COP_mat) of 3.2 and a maximum adiabatic temperature change (ΔT_ad) of 12 K at 5.7 MPa.

Importantly, operating in an active elastocaloric cooling cycle, these regenerators display an asymmetric fluid exchange, due to the large required strains and associated volume change. A finite-element (FE) model is developed to qualitatively predict regenerator volume changes, and is shown to demonstrate a good agreement with experimental measurements. Furthermore, the regenerator volume changes and geometrical information obtained from FE simulations can be supplied to an improved 1D numerical model for modeling large-deformation elastocaloric regenerators. This combination facilitates numerical investigations for the active regenerator performance and parametric optimization. A theoretical maximum temperature span of 11.9 K is achieved at a Utilization of 0.7 in square-channel regenerators based on NinjaFlex TPU.

The experimental proof-of-concept was performed on a volume compensation flow system using three 3D printed regenerators with different microchannels. Due to the need for watertight regenerators, the regenerator fabrication is compromised to employing the FilaFlex TPU with a lower ΔT_ad of 2.5 K. The results show a REG3 regenerator with a low porosity ϵ of 19.4%, achieving a maximum temperature span of 4.7 K and a cooling power of 2.5W, while a REG2 regenerator (ϵ = 27.2%) attains a maximum COP of 1.7. The 3D printed regenerator achieves remarkable regeneration ratio of 1.84 and specific cooling power of 1850 W/kg, comparable to some polymerbased and NiTi-based prototypes. Overall, such 3D-printed eCE regenerators could inspire research in low-stress regenerative elastocaloric cooling using cost-efficient elastomers, paving the way for wide-ranging applications in cooling and heating systems.