The importance of surface finish to energy performance

Abstract

Abstract. Power generation in solar energy systems, thermal control in buildings and mitigation of the Urban Heat Island problem, are all sensitive to directional response to incoming radiation. The radiation absorption and emission profile also plays a crucial role in each system's response and depends strongly on surface finish. This important sensitivity needs wider recognition in materials data sheets, system modeling, plus in materials and environmental engineering. The impact of surface roughness on thermal response of natural and man-made external environments is examined. Important examples will be given of the role of surface finish within each class. Total emittance links to the way surface finish influences directional emittance E(u). Smooth surface thermal emittance on PV module covers, many solar absorbers, some roof paints, polished concrete, and glass windows can be up to 15% different from insulator results based on fully diffuse models of the same material. Widespread evidence indicates smooth metals and low-E solar absorber surfaces cool faster, and smooth insulators slower than previously thought. Matt paint is cooler than low sheen paint under the same solar heating impacts and normal concrete cooler than polished. Emittance for water is the prime environmental example of oblique impacts as it reflects strongly at oblique incidence, which leads to a significant drop in E(u). Ripples or waves however raise water's average emittance. A surprise in this work was the high sensitivity of total E and its angular components to roughness in the depth range of 0.1-0.8 mm, which are well under ambient thermal IR wavelengths of 3-30 mm but common in metal finishing. Parallel energy flows such as evaporation and convective cooling vary if emittance varies. Thermal image analysis can provide insights into angular radiative effects.

Increasing Solar Reflectivity of Building Envelope Materials to Mitigate Urban Heat Islands: State-of-the-Art Review

“…In the context of building applications, the cooling performance of radiative cooling coatings on roofs is often assessed using the solar reflectance index (SRI). SRI is determined by considering solar reflectivity and the rate of infrared thermal emission, with higher SRI values denoting more efficient radiative cooling [43]. Advancements in nano-patterning techniques and designer materials are expected to provide a broader knowledge base regarding the radiative properties of different materials [44].…”

Heat-shedding with photonic structures: radiative cooling and its potential

“…Several studies have demonstrated effective radiative cooling and energy saving in buildings. [43][44][45][46] Raman et al fabricated cooling material consisting of seven layers of HfO 2 and SiO 2 , which reflects 97% of incident solar radiation while strongly emitting heat in the ATW 3 Radiative cooling for building exterior. (a) A schematic for a passive radiative cooling system in buildings.…”

A Scalable Microstructure Photonic Coating Fabricated by Roll-to-Roll “Defects” for Daytime Subambient Passive Radiative Cooling

“…The ideal radiative cooling materials should simultaneously possess high cooling power, low cost, scalable manufacturing, and contamination resistance characteristics. Previous research showed that rough microstructures could achieve high cooling power and anticontamination because the rough surfaces elevate the thermal emissivity by creating a gradual refractive index change and generating a superhydrophobic surface. ,− However, nontriviality arises when fabricating the desired microstructures with low cost and scalability because high mid-infrared (mid-IR) emissivity requires high-precision control of the photonic microstructures, where costly and low-yield microfabrication techniques are generally employed (such as photolithography and imprinting). ,− Thus, innovative manufacturing methods need to be developed to fabricate these microstructures for large-scale applications, such as building energy savings.…”