Wind: The Free Coolant That Refunds Part of the Heat Penalty

The panel cools by convection, and wind runs the fan

A solar module in sunlight is a hot plate. It absorbs far more energy than it converts, and the surplus leaves as heat by three routes: radiation to the sky, conduction through the mount, and convection into the passing air. Convection is the big adjustable term, and wind is what drives it. Still air lets a boundary layer of heat sit against the glass; moving air scrubs it away, and the faster the wind, the cooler the cell and the smaller the temperature penalty that part 01 measured. The relationship is captured by the industry-standard Faiman model, in which module overheating above air temperature falls steeply as wind rises: from about 32 degrees in dead-still conditions toward the low teens in a steady breeze.

The wind resource itself, measured the same way across all 40 markets, varies more than threefold:

Selected from the 40-market wind pillar; full table in the public dataset. Wind is the NASA POWER 20-year mean at 10 m. Module overheat is the Faiman model at 800 W/m² reference irradiance; output recovered is that market's cooling relative to a still-air (zero-wind) baseline, via the 0.34%/°C PERC coefficient. Set mean wind 3.81 m/s; mean recovery 5.5%.

Wind does not follow the heat, which is the whole point

The temptation is to assume hot places are calm and cool places are breezy, so that climate roughly cancels out. It does not. Across the 40 markets the correlation between mean wind and mean air temperature is essentially zero (r = -0.09), which means wind is an independent lottery laid over the heat map. The consequence is visible in two markets at almost the same air temperature. Bogota sits at 18.7 degrees of air and barely moving 1.8 m/s wind, so its modules overheat by more than 21 degrees; Cape Town sits at 17.0 degrees but a brisk 6.3 m/s, so its modules overheat by under 12. Two near-identical air temperatures, a nine-degree difference in how hot the panel actually runs, decided entirely by wind.

This is where the story turns from curiosity to warning. Because wind is independent of heat, the hottest markets are not compensated with extra breeze, and some of the hottest are among the stillest. The humid tropics are the worst case: Jakarta, Singapore and Lagos are all above 26 degrees of air yet sit below the wind average, so they take the full heat penalty with little convective relief. The hot deserts fare somewhat better on average but unevenly, with windy coastal Perth (6.3 m/s) cooled hard while inland Bamako and Jodhpur, the two hottest-running markets in the whole baseline, get only middling wind. Wind cooling helps most where a sea breeze happens to coincide with the sun, and that coincidence is a matter of geography, not thermodynamics.

What the wind is worth, and how to use it

Translated into output, every market gains from its wind, but the gains are unequal:

Read against a dead-still baseline, wind refunds between 3.6% of output in calm Bogota and 6.9% in windy Cape Town, with a 40-market mean near 5.5%. That baseline is a deliberate illustration, not a real alternative site, since no location has zero wind; the honest, observable quantity is the spread, the roughly three percentage points that separate the windiest markets from the stillest at otherwise similar conditions. It is real money in the same currency as the heat penalty it offsets, and it is the reason a coastal site can outproduce an inland one at the same latitude and irradiance.

The buyer actions are practical. Do not fight the wind: leave the ventilation gap behind roof-mounted panels open, because a module clamped tight to hot tiles loses exactly the convective cooling this report is about, and a well-ventilated rack in a breezy site is quietly worth several percent a year. Expect a coastal or exposed site to run cooler, and therefore yield slightly better, than an inland site with the same sun and air temperature; that difference is wind, and it is durable. And in the still, humid tropics, respect the heat: with little wind to lean on, the temperature coefficient of the module matters more, which strengthens the same case part 01 made for paying up for a low-coefficient HJT panel where the air is hot and the air is still. The one caution running the other way is the sea breeze that cools also carries salt and, on exposed coasts, drives the cleaning and corrosion questions of part 05: the wind is not a free lunch, only a free coolant.

Method and limits

Wind speed is the NASA POWER 20-year climatology (2001 to 2020) at 10 metres and at 2 metres, the same MERRA-2-derived source family as the irradiance profiles, pulled for each market's coordinates. The cooling effect uses the Faiman module-temperature model (IEC 61853-2) with standard free-standing coefficients (U0 = 25, U1 = 6.84 W/m²K) at a reference plane-of-array irradiance of 800 W/m², and the output figure applies a 0.34%/°C PERC temperature coefficient. Three limits are worth naming. The clawback is quoted against a still-air baseline, an illustrative bound rather than a real site, so the meaningful comparison is between markets, not against zero. The Faiman coefficients assume a well-ventilated, free-standing module; a roof-hugging or building-integrated array cools far less, which is a caveat for the buyer, not a flaw in the resource. And these are annual means: wind, like the sun it cools under, has a daily and seasonal rhythm, and the cooling matters most precisely when it blows during the hot, high-irradiance hours. Every value is public and recomputable from the named NASA product.