Self-actuating liquid bridge thermal rectifier for phase change energy storage

Efficient thermal management in Phase Change Material (PCM) storage requires a system that allows rapid charging while minimizing passive heat leakage. Conventional temperature-gated thermal switches are limited by Thermal Contact Resistance (TCR) at dry solid-to-solid interfaces, while liquid-based rectifiers that avoid TCR rely on external pumps, electrowetting circuits or magnetic actuation. This study proposes a passive thermal rectifier in which the PCM serves as both storage medium and switching element. The ∼18 % volumetric expansion of an internal paraffin charge raises its own liquid level across an insulating air gap to submerge suspended heat collector pipes, replacing the gas gap with a conformable wet contact and inducing buoyancy-driven natural convection, without external pump, field or secondary fluid. A 45 g copper-walled prototype gave an effective heat transfer coefficient of 34.9 W/(m 2 · K) in the OFF state and 107.7 W/(m 2 ·K) in the ON state, a rectification ratio of γ = 3.1 ± 0.4, matching the numerical prediction of 3.0. The transient cycle gave τ ON ≈ 7500 s and τ OFF ≈ 11000 s, where the engagement was gated by source temperature, and disengagement by stored latent heat. Two optimised configurations were built and measured: a polycarbonate vessel gave γ = 10.9 ± 0.4, and a 0.25 wt% CNT–paraffin composite γ = 7.0 ± 0.5. Computational fluid dynamics projects γ = 53.1 as an idealized upper bound for the combined configuration. Overall, this work presents a proof of concept for a simple, scalable and contact-resistance-free mechanism of passive liquid thermal rectification for thermal energy storage.