Efficiency of the solid-state engine made with Nitinol memory material

Abstract

The thermal efficiency of the solid‐state engine (SSE) based on the shape‐recovery phenomenon accompanied by the martensite‐parent‐phase (M→P) transformation has been evaluated. Particular application is made for the memory components made with the Nitinol alloys. The result is expressed in terms of the hot and cold reservoir temperatures (T and T₀), the latent heat of the M→P transformation, ΔH, the fraction of the M→P transformation, α, and a coefficient β that depends on the geometry and type of the deformation of the memory component. The relationship between the efficiencies of the SS and the Carnot engines is discussed. The factor α is expressed in terms of T_c, the critical temperature under an applied load, the degree of prestraining of the component, and the volume change involved in the M→P transformation. It is seen that the maximum of the thermal efficiency approaches that of the Carnot engine at an x=T−T₀ value that depends on T₀, ΔH, and α. Choosing T₀=297 K and a Nitinol alloy with ΔH=2 cal/g, the maximum efficiency, about 20%, occurs at x=75 K. Moreover, for a steady‐state heat‐mechanical energy conversion, the critical temperature of the memory component is found in terms of the T and T₀ temperatures. The engine efficiency in terms of the energy loss due to friction and the heat‐transfer coefficient is analyzed and estimated to be about 16%. It is seen that higher thermal efficiency can be obtained if a Nitinol alloy with a larger H value than given above can be found.