Evaporation Cold Energy Storage and Thermoelectric Harvesting via Hydrogel‐Phase Change Material‐Thermoelectric Generator Tandem Structure

This article presents a tandem hydrogel‐phase change material (PCM)‐thermoelectric generator (TEG) structure for evaporation‐driven cold energy storage and electricity harvesting. The hydrogel layer facilitates water evaporation under ambient conditions, generating a cooling effect that produces “cold energy”. This energy is stored in a PCM layer as latent energy, maintaining a stable temperature gradient without volume changes or leakage. The integrated TEG converts this gradient into electricity during both charging (evaporation‐induced cooling and PCM solidification) and discharging (PCM melting and heat release) phases, enabling dual‐phase power generation. Experimental results demonstrate a maximum power output of 1.6 mW and a power density of 100 μW/cm 2 at ambient condition without energy input. In addition, we demonstrate an average power output of 0.3 mW and a power density of 19 μW/cm 2 for a fully charged PCM under ambient conditions, with a cooling efficiency that reduces surface temperatures of TEG by up to 5°C. This article represents the first demonstration of simultaneous storage and utilization of evaporation thermal energy for electricity generation. This innovative design combines evaporative cooling, latent energy storage, and thermoelectric conversion, advancing sustainable energy solutions for low‐grade heat environments and efficiently powering portable electronics.