Photonic Radiative Cooling

Basics

• Radiative Cooling that uses Technology within the Scope of Photonics to optimize their function
• Thus rather than a conventional reflective coating (or lack thereof), PRC coatings are added to enhance not just reflectivity, but also produce significant Black Body Radiation in the Midinfrared, which due to the Atmospheric Infrared Window goes through most of Earth's Atmosphere, essentially using Outer Space as the Heatsink

Methods

Research Level

• Multi-Layered Coatings and Photonic Crystals
  • Complex to Fabricate/Apply; Typically similar to Semiconductor Fabrication
  • May be of use for applications where reflectivity is inherently not possible, such as in solar cells according to This Paper ( "Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody" )
• Barium Sulfate

High TRL Methods

Novel Paint by High Density Pigment using CaCO₃ Microspheres

• This was Pioneered by NightHawkinLight (YouTube Channel)
• Acrylic Paint in Acetone (To add novel drying feature that causes backscattering, akin to why transparent solids (Salt, Sugar, Snow, Plastics, etc) can appear reflective white)
• Sodium Citrate was originally used as a delaying agent, although Citric Acid forms this, yet also prevents the formation of impurities so Citric Acid is used in the latest methodology
• 4-5kg can be made with a Paddle Mixer Drill Attachment and a Commercial Plastic Trash Can
  • Timecode to the Section of the Video Discussing+ (Partially) Demonstrating This
• Typical Yield as per their testing: 8g of Microspheres per 10g of Calcium Chloride Initially used
  • Timecode to the Section of the Video Discussing This
• Timecode to the Part of their Video Where the Screen Shows the Methodolgy Written Down
  • Loose Notes Derived From the Overall Video + That:
  • Calcium Carbonate is Synthesized via a reaction of Calcium Chlorate (Sometimes sold as Deicing Solid or as a Desiccant) and Sodium Carbonate (often sold as Washing Soda)
• The Paint Itself is Foam Like due to the Water Evaporating Leaving behind Air Pockets

Microsphere Pigment Synthesis+Production

• • Steps:
    • Dissolve Each in Water, then Combining, this Forms the Pigment, byproduct of Sodium Chloride in Water Solution
  • 5-30*c temp
  • CO2 + Calcium Chloride = Calcium Carbonate (But in non-microsphere form); this complicates getting just microspheres from the reaction
    • Citric Acid can be used to alleviate this, it produces Sodium Citrate when it reacts with the Sodium Carbonate, is easier to find, and
• 1 to 10 ratio for all solutions:
  • For Example:
    • 20g Sodium Carbonate + 200ml Water
    • 10g Calcium Chloride + 100ml Water
    • 3g Citric Acid + 30ml Water
  • Citric Acid Solution is then Combined with the Calcium Chloride Solution
  • 10-20*C Pre-mixing
  • Sodium Carbonate is poured into the Calcium Chloride-Citric Acid Solution While Mixed in a Magnetic Stirrer or in a Blender
  • Timer is Started at the moment of Pouring
  • Containers can be cleaned with a mild acid such as food grade White Vinegar etc
  • More Stirring = Smaller Particle Size (In Theory, Upper Limit + QC Needs to be Determined)
  • Allow to Settle, Pour off Waste
  • Multiple Batches of Varying Stirring can be made then combined to get that High Density Pigment. (In the video, they did 3x 1 Minute in Blender, 1x 5 Minutes in Blender, 1/3x 8-10 Minutes)
  • Water Washes are done to Remove any Salts etc
  • They made an odd Disposable Metal Baking Pan - Paper Towel - Printer Paper sandwich for the filter/drying step, investigating various other forms of drying that may scale better may be of interest (although kiss / if it ain't broke don't fix it, i guess)
  • Contaminants can Supposedly Crystalize when drying, "locking in" microspheres into larger clumps
    • Timecode to the Section of the Video Discussing This

Paint Production

• 110ml (Dry) Acetone+12ml Water +10g Acrylic
  • Acrylic Dissolves over days at stp, hours at ~43*c
  • 7g of Pigment for Every 12ml of Paint Solution
    • Acetone Evaporation Complicates Things (This wasn't addressed too much in the video besides a brief mention
  • The "Dry Thickness" tends to be around a half of the wet/just applied coating thickness
  • 0.5mm is the optimal end result

Internal Links

• Cool Roof (One of the more Direct Applications of this Technology)
• Hydronic Cooling (A more complex, yet controllable utilization of this technology)
• Air Cooled Condenser / Dry Steam Condenser

External Links

• The Wikipedia Page On Passive daytime radiative cooling (One of the many names for this technology)
• A Video by the YouTube Channel "School of Energy and Environment City University of Hong Kong" Titled "Passive Radiative Cooling Technology" ( 3 Minute Watch)

Temporary Trashcan

• Misc Papers:
  • https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202305734 - Flexible Photonic Radiative Cooling Films: Fundamentals, Fabrication and Applications
  • https://www.pnas.org/doi/10.1073/pnas.1509453112 - Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody
  • https://www.nature.com/articles/s41377-023-01119-0 - Photonic structures in radiative cooling
  • https://www.sciencedirect.com/science/article/abs/pii/S0960148117310236 - Performance assessment of a photonic radiative cooling system for office building