Vertical Planar Liquid-Vapour Thermal Diodes (PLVTD) and their application in building façade energy systems

Abstract

Buildings represent one-third of global energy consumption and corresponding CO2 emissions which can be reduced through enhanced insulation and building integrated renewables. Thermal diodes can potentially revolutionise passive heat collection/rejection devices such as Integrated Collector-Storage Solar Water Heaters (ICSSWH) and Climate-Control Building Envelopes (CCBE) employed for decarbonisation. We present novel theoretical and experimental validation work on a lumped parameter heat transfer model of Planar Liquid-Vapour Thermal Diodes (PLVTD) to support development of ICSSWH and CCBE components for building facades. This study augments our previous work on a passive horizontal PLVTD model, by introducing falling film evaporation, vapour convection in vertical rectangular enclosures, condensation on vertical plates, and a methodology for evaluating working fluid temperatures. Experimental validations are presented for vertical aluminium (Ap = 0.15 m2, x = 22 mm) and stainless steel (Ap = 0.98 m2, x = 70 mm) prototypes using two different laboratory test methodologies where temperature setpoints were controlled and measured. The model predicts measured steady state thermal conductances in reverse mode (Ur ≈ 12 W·m–2 K−1 and Ur ≈ 1.7 W·m–2 K−1 for x = 22 mm and x = 70 mm PLVTDs respectively) and forward mode (175 < Uf < 730 W m–2 K−1 and 50 < ULvL < 900 W·m–2·K−1 respectively) with reasonable accuracy across investigated ranges (15 < T2 < 65 °C condenser temperatures, 5 < –ΔT12 < 25 °C reverse mode plate-to-plate temperature differences, 50 < q/A < 1000 W·m−2 forward mode heat fluxes). Forward mode behaviour is determined by working fluid vapour mass flow driven by heat flux and influenced by temperature dependent vapour viscosity. Reverse mode behaviour is determined by vapour convection, plate-to-plate radiation, and envelope/structure conduction. Parametric design influences are theoretically examined and ς > 99% diodicity relevant to CCBE and ICSSWH applications is demonstrated experimentally. Study findings contribute towards global efforts tackling the climate crisis by enabling commercial R&D for new Net Zero Energy Building components.