A Thermo-Mechanical Model for SMA-Based Crank Heat Engines

Shape memory alloys (SMA) provide unique functional features and several SMA-based heat engines have been proposed in recent years, exploiting the thermally induced recovery capabilities of such class of materials. These engines represent simple and environmentally-friendly solutions to obtain mechanical energy from low-grade energy sources, such as warm wastewater, geothermal and solar sources. However, despite the scientific interest in last years, no commercial devices have been developed. One of the reasons of these unsuccessful commercial development is the lack of robust design tools. In the proposed work a thermo-mechanical model has been developed, which describes the mechanical response of a SMA-based crank heat engine, as a function of several geometrical configurations, such as the dimension and total number of cranks, as well as for different thermo-physical properties of the heating and cooling thermal sources. In particular, the engine is made of two parallel crankshafts and uses nickel-titanium helical springs operating between warm water and cold air. The model has been developed within the MatLab® Simulink software platform, and several simulations have been carried out to analyze the mechanical response of the engine, in terms of output characteristics (torque, specific power, thermodynamic efficiency) as a function of the aforementioned geometrical and physical parameters.