text: PDRC coatings or paints tend to be advantageous for their direct application to surfaces, simplifying preparation processes and reducing costs, although not all PDRC coatings are inexpensive. Coatings generally offer "strong operability, convenient processing, and low cost, which have the prospect of large-scale utilization," as per Dong et al. PDRC coatings have been developed in colors other than white while still demonstrating high solar reflectance and heat emissivity.
  Coatings must be durable and resistant to soiling, which can be achieved with porous PDRCs or hydrophobic topcoats that can withstand cleaning, although hydrophic coatings use polytetrafluoroethylene or other similar compounds to be water-resistant. Negative environmental impacts can be mitigated by limiting use of other toxic solvents common in paints, such as acetone. Non-toxic or water-based paints have been developed. More research and development is needed.
  Porous Polymers Coating (PPC) exhibit excellent PDRC performance. These polymers have a high concentration of tiny pores, which scatter light effectively at the boundary between the polymer and the air. This scattering enhances both solar reflectance (reflecting more than 96% of sunlight) and thermal emittance (emitting 97% of heat), resulting in a surface temperature that is six degrees cooler than the surroundings at noon in Phoenix. Additionally, this process is solution-based and can be easily scaled up. A new design for PPC coloring is discovered. The dye of desired color is coated on the porous polymer. Comparing to traditional dye in porous polymer, which the dye is mixed in the polymer, the new design can have a higher refractive index of infrared and heat.
  The cost of PDRC coatings was significantly lowered with a 2018 study by Atiganyanun et al. which demonstrated how "photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a black substrate and reduce its temperature by radiative cooling." This coating could "outperform commercially available solar-reflective white paint for daytime cooling" without using expensive manufacturing steps or materials.
  PDRC coatings that are described as scalable and low-cost include:
  * Li et al. (2019), aluminum phosphate coating, solar reflectance 97%, heat emittance 90%, daytime air temperature ~4.2 °C lower than ambient temperature (~4.8 °C lower than commercial heat insulation coating), predicted estimated cost by Dong et al. at $1.2/m2, tested in Guangzhou (daytime humidity 41%), selective emitter (SE).
  * Li et al. (2021), ultrawhite Barium sulfate| paint with 60% volume concentration, solar reflectance 98.1%, heat emittance 95%, daytime air temperature ~4.5 °C lower than ambient, "providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process."
  * Weng et al. (2021), porous PDMS (Polydimethylsiloxane) sponge emitter template method for coatings, solar reflectance 95%, heat emittance 96.5%, daytime air temperature ~8 °C lower than ambient, avoids hazardous etching agents (e.g., hydrofluoric acid, hydrogen peroxide, acetic acid) or VOCs (e.g., acetone, dimethylformamide, tetrahydrofuran, hexane), "compatibility with large-scale production," tested in Hangzhou (daytime humidity ~61%).
  * Wang et al. (2022), waterborne thermochromic coating free of ecotoxic and carcinogenic titanium dioxide, solar reflectance 96%, heat emittance 94%, daytime air temperature ~7.1 °C lower than ambient, and "can be produced at a large scale and conveniently coated on various substrates through traditional drop casting, spraying, roller painting, or spin-coating methods" and "switchable [between] solar heating and radiative cooling," tested in Shanghai (daytime humidity ~28%).
  * Dong et al. (2022), Barium sulphate|, Calcium carbonate|, and Silicon dioxide| particles coating, solar reflectance 97.6%, heat emittance 89%, daytime air temperature ~8.3 °C lower than ambient (~5.5 °C lower than commercial white paints), described "for large-scale commercial production" with a predicted estimated cost of $0.5/m2, tested in Weihai (daytime humidity 40%).
  * Zhai et al. (2022), α-Bismuth(III) oxide| colored coating, solar reflectance 99%, heat emittance 97%, daytime air temperature ~2.31 °C (average cooling power 68 Wm-2), uses "low cost of raw oxide materials, and simple preparation process," tested in Nanjing (daytime humidity 54%).