Minimising Solar Heat Gain

[EngAP]

I need to miminise solar heat gain through a small transparent plastic window of an electronics enclosure.

I see two approaches: (1) Mimise the solar radiation transmitted into the enclosure and (2) maximise the thermal radiation transmitted out of the enclosure.

Several questions:

- The majority of solar radiation is in the visible light range, but what part of the spectrum causes the heating effect? (Is it the whole spectrum or specifically the IR portion?)

- Would an IR filter help by blocking the IR portion of the solar radiation or would it make the situation worse by preventing the thermal radiation from being transmitted out of the enclosure?

- If the whole solar band causes the heating effect then I would assume that minimising the transmittance of the whole solar spectrum would reduce the heating effect. Is this correct?

I have the option to tightly control the part of the spectrum that is transmitted throught the window and would like to understand this in depth to make the correct decision. Can anyone recommend any books on this subject?

As I understand it a glass window would transmit the short wavelength solar radiation into the enclosure. This is absorbed by the internals which then emmit this energy as longer wavelength thermal radiation which the glass absorbs. Therefore half of this long wavelength radiation is then re-emmited back into the enclosure and the other half emmited to the environment - hence the greenhouse effect.

 

[EngAP]

So, lets say the camera requires a minimum transmission of 40% up to 800nm.

Theortically, if I then limit the transmission through the window to this range then, as I see it, the band of radiation transmitted through the window (due to the solar loading) will heat the internals. The internals will then emmit IR radation which is unable to transmit out through the window. Hence a net gain of heat energy inside the enclosure.

But if I allow IR to be transmitted through the window then the components will be able to transmit out through the window, but at the expense of increased solar energy transmittance into the enclosure, which is likely to be greater than that transmitted out by the internal components.

Hence I need to calculate the best case scenario, which i suspect will be the former.

 

[IRstuff]

We've been through this already. An IR transmissive window will run you at least 10 times the cost of a glass window. The emitted IR radiates Lambertianly, so less than 30% will actually make it through the window.

TTFN

FAQ731-376

 

[EngAP]

Yes, thanks for the advice.

 

[Warpspeed]

Architectural glass used in most high rise office buildings these days does pretty much what you want. It reflects IR, transmits visible with some slight loss, and absorbs UV.

From the outside it looks like a silvered mirror, from the inside the glass appears slightly tinted, and it blocks the incoming UV glare. I have this glass fitted in my home, and it VASTLY cuts down direct solar gain, which is it's main arcitectural purpose.

I am sure you could get a sample to experiment with. This is one brand I am familiar with, but there are definitely alternatives.

http://www.inter-sky.com/glazing_data.htm

 

[davefitz]

For architectural retro-fits of windows, a much more cost effective ( and techically effective) solution is to use solar screens. They are now commonly used in Nevada, Arizona, and Kaleefornia.