Smart Polymeric Composite Thermal Switch for Geothermal Energy Harvesting

Geothermal energy represents a promising sustainable power source that harnesses Earth's natural heat through water circulation systems. A critical challenge in geothermal power generation is controlling the water heating process through formation of cracks in hot zones. This requires an innovative thermal switch or reversible proppant that can regulate water flow and ensure optimal heat absorption before extraction for electricity generation. In this study, we designed and synthesized a composite thermal switch with a polymeric artificial muscle embedded in a fluoroelastomer matrix. In this design, the fluoroelastomer protects the artificial muscle from superhot water (200 °C or above) damage and also provides tensile stress to the muscle; the artificial muscle provides the desired reversible actuation. In this study, poly(vinylidene fluoride-co-bexafluoropropylene) (PVDF-HFP) was cross-linked by dicumyl peroxide (DCP) to form a fluoroelastomer PVDF-HFP/DCP. A Nylon fiber was twisted until it was used to coil as the artificial muscle. Under zero external loading, the composite thermal switch exhibited a maximum contraction upon heating (CUH) of about 10.76% and expansion upon cooling (EUC) of about 10.89% when the temperature cycled from 25 to 200 °C. Furthermore, the CUH and EUC were about 2.12 and 3.36%, respectively, when the temperature cycled from 150 to 200 °C. The composite thermal switch still exhibited excellent reversible actuation and chemical stability after soaking in 200 °C water within a pressure vessel for 4 weeks. A design-oriented structural mechanics model was also developed to evaluate the various design parameters on the reversible actuation of the smart thermal switch. To further highlight the performance of PVDF-HFP/DCP, we also prepared three other fluorinated rubbers as controls. This composite thermal switch shows promise as a smart proppant or valve for enhanced geothermal systems, offering the potential to improve heat extraction efficiency through controlled fracture conductivity.