Passive energy conservation through building envelopes is a key strategy to cut energy consumption and carbon emissions, yet traditional technologies face hurdles like cumbersome controls and narrow adjustment scopes. Researchers from Tsinghua University, Sichuan University, and other institutions have developed a novel phase-change thermal diode, offering enhanced unidirectional heat-transfer capacity for dynamic building envelopes. Their findings, published in Engineering, demonstrate significant energy-saving potential across China’s climate zones.

The thermal diode comprises two metal plates (with hydrophilic and hydrophobic surfaces) and a thermal insulating gasket, operating in forward and reverse modes. In forward mode, it leverages high-conductivity phase-change heat transfer of water, while reverse mode relies on low-heat-transfer-coefficient insulation gaskets. Aluminum was selected for metal plates considering building bearing capacity, with polytetrafluoroethylene (PTFE) as the insulation gasket material. Three hydrophobic surface fabrication methods—laser fabrication, fluorosilane coating, and PTFE coating—were tested, with laser-fabricated surfaces showing superior performance due to uniform micro-nanostructures and a contact angle exceeding 160°.

Experimental investigations revealed that vacuum conditions significantly impact the diode’s performance. At 25°C, optimizing the vacuum increased forward thermal conductivity 6.06-fold, from 0.17 to 1.20 W·m⁻¹·K⁻¹. The reverse thermal conductivity ranged from 0.09 to 0.11 W·m⁻¹·K⁻¹ at 2.1 kPa absolute pressure, meeting thermal insulation material standards. After material and structural optimization, the thermal rectification ratio of the diode ranged from 8.72 to 23.62, with a maximum unidirectional heat-transfer multiplier of 18 times within the 25–40 °C range suitable for buildings.

Simulation studies compared the diode-integrated dynamic envelope with conventional dynamic methods. The new envelope achieved a dynamic adjustment capacity of 11.21, outperforming traditional systems (1.97–5.40). Across seven climate zones, cooling energy savings ranged from 11.83% (Guangzhou) to 21.36% (Kunming). Nighttime passive heat removal reduced indoor temperatures by 3–4 °C compared to static envelopes, while daytime insulation minimized external heat ingress.

This research adapts jumping-droplet thermal diodes—previously used in electronics cooling—to building applications, fostering cross-innovation between building and material sciences. The diode’s simple structure and large-area manufacturability make it practical for public buildings with high internal heat loads. Future work will focus on optimizing hydrophobic surfaces via chemical etching and modifying the working medium to enhance winter performance. By providing a passive, high-adjustment-capacity solution, this technology offers a foundational reference for developing next-generation dynamic building envelopes and advancing low-carbon building goals.

The paper “Material and Structural Optimization of Novel Phase-Change Thermal Diode for Dynamic Building Envelope,” is authored by Hengxin Zhao, Yifan Wu, Guochen Jiang, Minlin Zhong, Hongli Sun, Borong Lin. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.07.008. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.