Theoretical and Experimental Foundations for the Development of an Electric Power Generation Unit with a Solid-State Heat Engine Based on a Ni-Ti-Cu Alloy for the Utilization of Low-Grade Thermal Energy

The main objectives of the study are to develop a generalized thermo-electromechanical model of a solid-state heat engine based on a Ni-Ti-Cu shape memory alloy for converting low-grade thermal energy (30-100 °C) into electrical energy, establish physically justified relationships governing reactive force generation and determine conditions for energy-efficient operation of the system as a part of an electric power generation unit. To achieve the stated objectives, the following tasks were performed: experimental studies of the thermomechanical characteristics of thermosensitive springs; derivation of a generalized relationship between maximum force, pre-deformation and temperature; development of a torque generation model, accounting for the phase position of active elements, construction of an electromechanical model of the "heat engine-generator-load" system; introduction of a system of dimensionless criteria for thermal, mechanical, electromechanical and energy consistency. Key results include analytical laws for scaling torque and mechanical power, a thermal consistency criterion defining the limiting rotational speed under complete phase transformation, the condition for a steady operating point and a material energy criterion setting the fundamental limit on the engine's thermal efficiency. The significance of these results lies in enabling a transition from the analysis of an individual prototype to a generalized description of a class of SMA engines, providing a basis for performance prediction, parameter scaling, design optimization and improvement of the efficiency of low-grade thermal energy utilization systems. The findings can be applied in designing low-power autonomous energy modules, industrial waste heat recovery systems and the development of functional alloys with narrow thermomechanical hysteresis for further applications in energy systems, including distributed and renewable energy sources of the future.