Theoretical and experimental analysis of a horizontal planar Liquid-Vapour Thermal Diode (PLVTD)

Thermal diodes are unidirectional heat transfer devices, analogous to electrical diodes, which offer low resistance (thermal conductance) in one direction and high resistance (thermal insulation) in the other. Thermal diodicity has significant potential to improve the efficacy of a wide variety of heating and cooling devices. This paper presents pioneering work to experimentally measure the heat transfer characteristics of a 0.15 m ² passive Planar Liquid-Vapour Thermal Diode (PLVTD). Prior work has typically examined much smaller devices aimed at micro-electronics cooling applications whereas the present work aims to improve understanding of larger scale devices for incorporation in solar collectors and multi-function climate control building envelopes. Such applications can facilitate local renewable energy generation (solar and ambient heat collection) and improve cooling system energy efficiency (passive heat rejection) in order to address the climate crisis by decarbonising built environment energy use. Experimental work involved a horizontally oriented PLVTD formed of two parallel isothermal plates with integral serpentine heat exchangers and external insulation. Plate, fluid, and ambient temperatures were controlled and measured to determine heat transfer coefficients under various temperature difference and heat flux operating scenarios in order to validate a simple one-dimensional lumped parameter model. Measured forward mode heat transfer 150 < U _f < 500 W·m ⁻² K ⁻¹ combined with reverse mode insulation U _r = 10 W·m ⁻² K ⁻¹ corresponds ς ≈ 88% diodicity at low condenser temperatures and fluxes (T ₂ ≈ 15 °C and q/A ≈ 120 W∙m ⁻²) and ς ≈ 96% at high condenser temperatures and heat fluxes (T ₂ ≈ 60 °C and q/A ≈ 2800 W∙m ⁻²). Forward mode performance increases with increasing heat flux and (to a lesser extent) with increasing operating temperature but is largely independent of PLVTD dimensions. Reverse mode performance is largely independent of heat flux and temperature but reduces (improves thermal insulation) with increasing cavity depth. The model has been used to show that a stainless steel PLVTD with x = 70 mm cavity can achieve U _r = 2 W·m ⁻² K ⁻¹ with similar forward mode performance to the experimental prototype. Such a device would achieve diodicity of ς > 97% and be suitable for application in solar collectors and climate control building envelopes.