Leave a transit case in the sun and the interior can get hot enough to kill electronic components. Inlets and outlets for convection mitigate the problem if the ambient air is cool enough. But, they may draw in dust and moisture. Active cooling – thermoelectric or air conditioner – is an alternative but incurs installation, weight and extra costs.
Alternatively, consider the impact of the case color and ways to reflect the incident solar radiation. Depending on where the case is going, this
can reduce the size of air conditioner needed, possibly even eliminating
Sizing the problem
Sunlight is intense near the equator and weaker at the poles. There’s also a coastal/moisture effect: cloudy regions experience less solar radiation. Within the United States, heat loads can be estimated from National Renewable Energy laboratory (NREL) charts showing the level of incident solar radiation. This varies from under 4.0 kWh/m2/day in Michigan and Ohio to more than 7.5 kWh/m2/day in the desert southwest.
These numbers don’t tell the whole story, though. Sunlight and heat can reflect off surrounding surfaces. This can increase the solar loading. So, even a shaded case might experience significant heating.
The case finish plays a part, too. Older and rougher finishes absorb more radiation. And, there’s the impact of color.
Absorption and reflection
When light falls on an opaque surface, it’s either reflected or absorbed. Reflection is what gives objects their color. Sunlight contains all wavelengths of visible light. But, an object appears a particular color because only that light wavelength is reflected off the surface. Red paint reflects red light, blue paint blue light and so on. Something that looks white is reflecting all the light falling on it. For a black surface, the opposite is true; it’s absorbing all that energy.
Logically then, the best way to minimize the solar load on a case is to paint it white. The effect is substantial. For example, consider a case the size of a desktop PC placed in the sun. If it’s black, the inside will get around 40oF warmer than if it was white.
White isn’t always the most practical color, especially in military applications. That’s one reason the US Navy prefers a light gray color. The Army goes for tan or olive colors. The ability of these colors to reflect solar radiation is not much lower than that of white.