Carbon neutrality before 2050 is becoming a global consensus, and more than 110 countries have committed to do so. It is crucial to improve energy efficiency to realize a carbon-neutral society. Heating, cooling, and air conditioning consume around half of the energy usage. Natural heat source (sunlight) and heat sink (outer space or ambient environment) are potential to be utilized to relieve the energy consumption from active heating and cooling. Spectra modification through nano/micro structures is the key for solar and thermal management. On the basis of spectra modification, passive radiative cooling, thermochromic smart window, and transparent photothermal coating are three important technologies being widely studied for energy saving and heat transfer applications. However, most...[ Read more ]

Carbon neutrality before 2050 is becoming a global consensus, and more than 110 countries have committed to do so. It is crucial to improve energy efficiency to realize a carbon-neutral society. Heating, cooling, and air conditioning consume around half of the energy usage. Natural heat source (sunlight) and heat sink (outer space or ambient environment) are potential to be utilized to relieve the energy consumption from active heating and cooling. Spectra modification through nano/micro structures is the key for solar and thermal management. On the basis of spectra modification, passive radiative cooling, thermochromic smart window, and transparent photothermal coating are three important technologies being widely studied for energy saving and heat transfer applications. However, most of the state-of-the-art structures have poor spectral selectivity with low efficiency for heat conversion. Also, their long-term durability and manufacturing scalability are still a concern. The major goal of the thesis is to fill the technology gaps and experimentally demonstrate three highly selective, durable, and scalable structures for passive radiative cooling, thermochromic smart windows and transparent photothermal coating.

For passive radiative cooling, we developed a solution-processed inorganic emitter with high spectral selectivity for efficient daytime cooling in hot humid climates. It shows a high and diffusive solar reflectance (96.4%) and strong infrared-selective concentrated emittance (94.6%) at low zenith angles with superior thermal spectral selectivity (1.46). Remarkable subambient cooling of up to 5 °C in autumn and 2.5 °C in summer was achieved under high humidity without any solar shading or convection cover at noontime in a subtropical coastal city, Hong Kong. Owing to the all-inorganic hydrophobic structure, the emitter showed outstanding resistance to ultraviolet and water in the long-term durability tests. The scalable solution-based fabrication renders this stable high-performance emitter promising for large-scale deployment in various climates.

For thermochromic smart windows, we reported a solar and thermal-regulative thermochromic window (STR smart window) suitable for all-weather applications. The excellent solar modulation (ΔT_sol=54.8%) and thermal modulation (Δε=57.1%) of the STR window realize indoor temperature regulation in the daytime and the nighttime. Compared with commercial Low-E glass and hydrogel windows, the STR window demonstrated better heat preservation in winter while better heat dissipation in summer. The assessed annual energy saving from the STR window is 6.8 times more than the Low-E window and 2.1 times more than Hydrogel windows in Beijing. This first combination of solar and thermal regulation may inspire new developments in thermochromic materials.

For transparent photothermal coating, we developed a highly transparent, photothermally selective coating based on solution-processed cesium-doped tungsten trioxide nanoparticles. The transparent selective photothermal coating provides a visible transmittance up to 82% and high absorption of ultraviolet and near-infrared light over 90%, and achieves a temperature rise by 38 ℃ under 1-sun illumination, resulting in remarkable anti-fogging and defogging abilities under extremely frigid or humid conditions. Large-scale complex surfaces with photothermal coatings were deployed in field anti-fogging tests as well as long-term durability tests, showing its great potential in antifogging applications.