Anti-Solar Radiative Cooling Power Calculator

What this anti-solar radiative cooling calculator estimates

Anti-solar radiative cooling describes nighttime heat rejection from a surface to the colder apparent sky through longwave infrared emission. In a practical arrangement, a high-emissivity radiator can cool relative to its surroundings. When a thermoelectric generator (TEG) connects that cooler radiator to a warmer source—such as ambient air, a thermal mass, or a controlled heat sink—the resulting temperature difference can generate electrical power. This calculator gives a transparent estimate of the upper-bound, radiation-driven heat flow and applies your selected conversion efficiency to estimate nighttime electrical output.

For an anti-solar radiator design, the calculator reports three main outputs:

• Net radiative cooling power from your radiator to the sky (W).
• Average electrical power after applying an overall conversion efficiency (W).
• Total nighttime energy over a chosen duration (Wh).

This anti-solar cooling model isolates radiative exchange between a surface at temperature T and an effective sky temperature T_sky. It does not explicitly model convection, conduction, wind, clouds, view factors, spectral selectivity, or the detailed thermodynamics of a specific TEG module. Use it for early radiator sizing, weather sensitivity checks, and comparisons such as adding sky-facing area versus improving conversion efficiency.

How anti-solar radiative cooling power is calculated (formula)

The anti-solar cooling estimate uses the Stefan–Boltzmann law for a gray radiator exchanging thermal radiation with the sky. Net radiative cooling power per unit area is:

For the entered anti-solar radiator area A, total radiative cooling power is: Prad = A · ε · σ · (T⁴ − Tsky⁴). The calculator estimates electrical power with the overall thermoelectric conversion efficiency η: Pelec = η · Prad. For a night lasting t hours, it reports: Enight = Pelec · t in Wh.

For this radiative-cooling calculation, T > T_sky is required. If the radiator is no warmer than the effective sky, the net radiative term is zero or negative and does not represent positive cooling power to the sky under this simplified definition.

Anti-solar radiator inputs: units, definitions, and practical meaning

The anti-solar cooling inputs match a simplified radiative energy balance. Each describes a physical part of the radiator-and-TEG scenario, so begin with conservative conditions and change one parameter at a time to identify the strongest driver of the estimate.

Anti-solar radiator area (m²)

This is the effective area that “sees” the sky and emits thermal infrared radiation. For a flat plate with an unobstructed view, the effective area is close to the physical area. For complex geometries (fins, corrugations, folded sheets), the true emitting area can be larger, but self-viewing and reduced sky view can offset that benefit. Because the calculator does not include view factors, treat the area input as the sky-facing effective area.

• Small prototypes: 0.1–2 m²
• Rooftop panels: 2–10+ m²
• Field arrays: tens to hundreds of m² (use the max limit only if you understand the assumptions)

Anti-solar surface emissivity (0–1)

For an anti-solar radiator, emissivity measures thermal-infrared emission relative to an ideal blackbody. The calculator treats it as a gray-surface value, so higher emissivity increases estimated radiative cooling in direct proportion. Many radiative-cooling materials are engineered for high emission in the atmospheric window (roughly 8–13 μm), but this calculator uses one emissivity value rather than a spectral model. If spectral data are available, select an emissivity that represents the material’s effective longwave emission in your expected conditions.

• Polished metals: ~0.05–0.2
• Painted/oxidized surfaces: ~0.8–0.95
• High-IR-emissivity coatings: ~0.95–0.99

Anti-solar radiator surface temperature (K)

Enter the anti-solar radiator surface temperature in kelvin. If you measure in °C, convert using K = °C + 273.15. Example: 25 °C ≈ 298 K. In a real device, the radiator temperature is not fixed; it results from a balance of radiation, convection, conduction, and any heat drawn through the TEG. Here, you provide a representative surface temperature for the nighttime period being evaluated.

Effective sky temperature for radiative cooling (K)

In this anti-solar cooling model, effective sky temperature condenses atmospheric longwave emission back toward the radiator into one value. It is not the air temperature and it is not the temperature of outer space; it represents how cold the sky appears in the thermal infrared. Clear, dry nights tend to have lower T_sky and stronger modeled cooling, while humid or cloudy nights raise T_sky and weaken it.

• Clear, dry night: ~220–270 K
• Typical clear conditions: ~250–290 K
• Humid/cloudy night: ~270–300 K

For an anti-solar radiator without a measured sky-temperature estimate, run several weather cases—for example, 240 K, 260 K, and 280 K—to see how atmospheric conditions affect the design.

Thermoelectric efficiency for radiative cooling (0–1)

In this anti-solar power estimate, efficiency is the overall factor that converts modeled radiative cooling power into electrical power. It combines TEG conversion efficiency with practical effects including thermal contact resistance, parasitic conduction, heat spreading, and power electronics. For small temperature differences, real-world system efficiency is often only a few percent or less. If you have a TEG datasheet, remember that its stated efficiency normally applies to particular hot- and cold-side temperatures and may exclude system losses.

• Simple lab setups: ~0.01–0.05
• Well-optimized prototypes: sometimes ~0.03–0.08 depending on ΔT and design
• Values above ~0.10 are usually optimistic for this application

Anti-solar cooling night duration (hours)

Use the number of hours over which you expect anti-solar cooling conditions to remain roughly steady. The calculator multiplies average electrical power by this duration to estimate energy. If clouds, wind, or humidity change substantially overnight, run separate early-night, midnight, and pre-dawn cases rather than treating the whole night as identical. For seasonal planning, short-night and long-night scenarios can bracket expected energy.

Worked example (step-by-step): nighttime anti-solar radiative cooling

This anti-solar cooling example uses a 2 m² radiator with emissivity 0.95, a 300 K radiator surface, a 260 K effective sky temperature, overall thermoelectric efficiency of 0.05, and a 12-hour night.

1. Compute the radiative temperature term: T⁴ − T_sky⁴. Because 300 K is higher than 260 K, the term is positive.
2. Multiply by σ (Stefan–Boltzmann constant), emissivity, and area to get Prad in watts.
3. Multiply by efficiency η to get average electrical power Pelec.
4. Multiply by 12 hours to get nighttime energy in watt-hours.

For this anti-solar cooling scenario, a surprisingly large result can reflect the model’s omission of convective heat gain from ambient air and conductive heat leaks. A very small electrical result can also be realistic: the available radiative cooling power is constrained by the fourth-power temperature difference, and only the entered fraction becomes electricity. The comparison table generated with the result shows how a 50% increase in radiator area compares with a 20% increase in conversion efficiency.

Assumptions and limitations for anti-solar radiative cooling power

This anti-solar radiative cooling estimate is intended for screening radiator concepts, not for predicting every heat flow in a finished device.

• Radiation-only exchange: convection and conduction losses can significantly reduce real-world performance, especially in windy conditions.
• Single effective sky temperature: a single number cannot capture clouds, humidity profiles, or spectral effects; treat it as a scenario parameter.
• View to sky: nearby buildings, trees, and tilt can reduce the effective radiating view; shading and obstructions matter.
• Gray emissivity: real materials have wavelength-dependent emissivity; selective emitters may behave differently than a gray surface.
• Efficiency is lumped: η is a system-level estimate, not a guaranteed TEG module efficiency; include wiring and conversion losses if relevant.
• Average-power assumption: the calculator treats inputs as steady over the chosen duration; real nights vary.

How to use: Applying anti-solar radiative cooling results

Use these anti-solar cooling outputs as a planning tool. Start with a baseline radiator case, then test sensitivity by changing area, emissivity, effective sky temperature, and conversion efficiency. The generated scenario table compares that baseline with a 50% area increase and a 20% efficiency increase.

When comparing anti-solar radiator concepts, prioritize relative changes over absolute numbers. If doubling area roughly doubles modeled energy, area is a strong lever in this radiation-only model. In a real installation, more area may also increase structural cost, mass, wind exposure, and potentially alter radiator temperature. Likewise, raising conversion efficiency may require improved thermal interfaces or a different TEG module, which can change the achievable temperature difference.

To compare the anti-solar model with measurements, measure or estimate radiator surface temperature and effective sky temperature—or longwave downwelling radiation—over the same period. You can then choose an η that aligns the simplified estimate with observed electrical output and use that calibrated value for comparable nighttime scenarios.

Common anti-solar radiative cooling questions and troubleshooting

Anti-solar cooling temperature validation: why do I get an error?

The anti-solar cooling calculator requires the radiator surface to be at least 1 K warmer than the effective sky temperature. When T ≤ T_sky, the net radiative term is non-positive. A physical radiator may still exchange energy with the sky in that situation, but this page defines useful net cooling to sky only for a warmer radiator and colder effective sky.

Is the anti-solar radiative flux value per square meter?

No. In the anti-solar cooling results and comparison table, “Radiative flux (W)” is the total radiative cooling power for the full radiator area entered. To obtain a per-area value, divide the reported radiative power by the area.

Why is estimated anti-solar electrical power so small?

Thermoelectric conversion from the modest temperature differences available in anti-solar cooling is inherently limited. Even when the radiator rejects tens of watts of heat, only a small fraction may become electricity. Real convection and conduction losses can further reduce the temperature difference across the TEG.

Can this estimate daytime anti-solar coating performance?

This anti-solar calculator is limited to nighttime longwave radiative exchange represented by an effective sky temperature. Daytime radiative cooling also requires solar absorption, atmospheric transmission, and often spectral-selectivity inputs. A daytime assessment needs a model that includes solar irradiance and the coating’s solar reflectance.