The encyclopedia · Engineering & Operations · Technical decision · 2014–2023
SkyCool cools below ambient by radiating heat through the sky window to deep space
A seven-layer film reflects 97% of sunlight and radiates heat to the cold sky, cooling below ambient with no electricity.
SkyCool Systems
the move
Air conditioning is a big slice of building energy and the main driver of peak electricity demand, so a way to cool without running a compressor would change the load curve.
A Stanford team built a seven-layer HfO2/SiO2 photonic structure that both reflects nearly all sunlight and emits heat through the atmospheric transparency window, so it loses energy to deep space even in the day.
On a rooftop under direct sun the cooler settled about 4.9C below ambient with a cooling power of 40.1 W/m2, a daytime result the field had not previously hit, and SkyCool Systems was spun out to make panels from it.
why it works
- It converts an unlimited heat sink — outer space — into a usable resource for buildings.
- The design is passive: no moving parts, no refrigerant, no daytime power input.
- It works at the hottest hour, when demand and prices peak, not just at night.
- The multilayer film is a manufacturing problem, not a resource problem, so it can scale.
what transfers
Find a free heat sink nobody is using — the cold night sky actually works at noon.
what came after
SkyCool Systems commercialized the panels for refrigeration and building sub-cooling, raising a $5.0M seed round in 2023 and drawing ARPA-E support; the underlying physics remains a reference point for daytime radiative cooling research.
references
- Passive radiative cooling below ambient air temperature under direct sunlight
- SkyCool Systems Raises $5.0M in Seed Financing
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