Experiments and modeling of a shape memory alloy tension spring heat engine

The operation and performance of a benchtop shape memory alloy (SMA) tension-spring heat engine is explored through experiments and modeling. The heat engine has a continuous loop of SMA spring mounted to geared pulleys which automatically revolve the spring between regions of hot and cold air to produce mechanical work against a torque brake. Several parameters, including the output moment, hot reservoir temperature, gear ratio, and spring prestretch, are varied to access their in sequence on the steady state engine performance. The heat transfer environment is characterized by thermocouples located throughout the engine, and the distributions of spring stretch ratio and spring temperature are monitored by optical and infrared imaging. This heat engine's small footprint, solid state, low cost, and simple operation make it suitable for harvesting waste heat from a variety of industrial or commercial sources. A system-level mathematical model is developed to capture the steady-state behavior of the SMA heat engine. The model uses a reduced-order constitutive model for the SMA tension spring that is accurate and computationally efficient. Considerations of periodicity, compatibility, mechanical equilibrium, and energy conservation leads to a few nonlinear integral equations that can be solved numerically. This provides the distributions of temperature, stretch ratio, and tensile force in the SMA spring that can be validated against detailed experimental results. It provides, for the first time, a rigorous analysis and design tool for quick parametric studies of this type of heat engine.