Energy efficient solid-state cooling nanomaterials for next-generation refrigeration: A comprehensive review, challenges, and future perspectives

The current need to identify energy saving cooling technologies and green technologies in cooling has seen the birth of solid-state cooling as a new and innovative alternative to the conventional vapor-compression cooling. Contrary to conventional systems, solid-state refrigeration uses functional materials which have reversible thermal response to external stimuli, such as electric, magnetic and mechanical fields. This review presents a wide and fundamental materials-based account of major groups of solid-state cooling materials, which include thermoelectric, magnetocaloric, electrocaloric and elastocaloric systems, unified by the underlying thermodynamic and caloric concepts. Specific attention is paid to the explanation of the structure-property-performance relationships that govern such important measures as cooling capacity, entropy change, coefficient of performance and cyclic stability. Nano structuring, band and defect engineering, doping, interface optimization is some of the new material design methods that are surveyed and analyzed systematically as far as they are leading to enhanced thermal and functional performance. It is also indicated that the invention of hybrid and composite materials, and multicaloric coupling techniques is a direction to look at in order to overcome inherent weaknesses of single material systems. Both in parallel, fundamental issues related to thermal hysteresis, mechanical fatigue, reliance on rare-earths, and scale manufacturability are also discussed in some depth, as well as recent developments in devices-level integration and compatibility of systems. This review gives a logical structure of the rational design and implementation of the next-generation solid-state cooling materials by interconnecting basic materials science to engineering concerns. It also defines the key research gaps and sets a direction towards high-efficiency and long-lasting as well as commercially scalable refrigeration methods and positions advanced materials in the forefront of sustainable thermal handling. This review does not simply list thermoelectric and caloric materials but critically considers the current paradoxes that hinder the transfer of laboratory performance to commercially reliable refrigeration systems. The balance between high entropy changes and thermal hysteresis, large temperature lift and cyclic fatigue, high figures of merit at the material level and low figures of merit at the device level, and the absence of a standard durability metric under realistic operating conditions are all given special attention. This important contextualization is employed to pinpoint missing areas of knowledge and to establish material-design priorities for next generation solid-state refrigeration.