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.

 

[IRstuff]

How small is this window? Maximum solar load is only ~1100 W/m^2. Why is the window and not the housing a problem?

TTFN

FAQ731-376

 

[EngAP]

The window is approximately 100 x 100mm.

The housing is a problem but we have several ways to minimise the effect of solar loading on the housing including a heat shield with optimised air gap, reflective coating etc.

The reason I ask specifically about the window is because I need to clarify the mechanisms behind the solar gain effect through a transparent medium as I am not entirely clear about it myself.

The figure of 1100W/m^2 is very close to what I have assumed (880W/m^2 direct and 120W/m^ diffuse). even given the small size of the window and the enclosure as a whole there is a significant (dominant in some cases) heating effect from the sun that needs to be reduced.

Any help or comments will be greatly appreciated.

 

[IRstuff]

Why is the window so large? What's the requirement that calls for a window? Without knowing the rationale for having a window, we can only talk in generalities.

TTFN

FAQ731-376

 

[davefitz]

just look up the fraction of blackbody radiation emitted as a function of wavelength * temp ( um* K). tabulated in most heat transfer books. Use sunlight T = 10,000 F = 5811 K.

 

[trashcanman]

Can you put a small roof over it?

 

[EngAP]

IRStuff - The window is as small as is physically possible and is a neccessary function of the product. My problem is this: The product HAS to have a window of the above dimensions hence I need to minimise the solar heat gain.

So am I correct in my understanding: The solar rdiation peaks in the visible light range. This is transmitted throught the window and heats the internals. These then emmit thermal radiation at a much longer wavelength.

Therfore I believe I need to do the following:
1) Reduce the transmittance of solar radiation in the short wave spectrum as much as possible,
2) Allow the long wave thermal radiation emmitted by the internals to be transmitted back out throught the window.
3) If possible allow thermal radiation to only be transmitted out of the enclosure, hence some one-way refelctive coating.

 

[IRstuff]

There is no practical one-way reflective coating.

Again, you did not give the relevant requirements for the window. What transmission does it need to have? Can you put lighting inside to compensate for the reduced transmission?

Why not have a reflective, hinged cover?

TTFN

FAQ731-376

 

[EngAP]

I think I may have made myself mis-understood slightly. I am asking this question mainly to (hopefully) get advice on the solar heating mechanism so that I have a good understanding as I tackle the problem. Therefore I am still interested in any good books on the subject and advice as to whether I understand the physics behind solar heating (whether the hole spectrum causes the heating or just the IR band etc.).

davefitz - thanks, good suggestion. The enclosure will have a sunshield that will act as a roof over the window.

IRstuff - I apologise if I have made myself mis-understood. One of the parts within the enclosure is a camera that needs the light transmitted through the window. Internal lighting would not help I am afraid. I am in the process of determining that minimum allowable visible light transmission.

I would be grateful for any input as to the physics behind solar heating, as discussed above.

 

[unclesyd]

You might want to look at the thin film approach. These people have several products that address heat rejection through glass. The Select series is probably the way to go.

http://www.huperoptikusa.com/products_klasse.cfm

If this approach is applicable you might look at a gold film.

 

[IRstuff]

OK, then you need to bandlimit to only 400 nm to 800 nm band, since that's the range of eye vision. However, many cameras are used in near infrared, so again, you need to be more specific about the requirements on the window. What percent transmission is acceptable to your camera? A purely bright daylight camera could accept even a 1% transmission, although that's not recommended. A low-light camera might not.

Spectral coatings are generally quite expensive.

A photochromic glass or coating would be another option.

A controllable LCD filter would be another.

You might consider just a simple sunshade or louver as well.

TTFN

FAQ731-376

 

[EngAP]

unclesyd - Thanks I will look into these.

If anyone has advice on the physics behind solar radiation heating I would be grateful for a brief explanation and/or recommended books.

Thanks again.

 

[davefitz]

The classic book is by Threlkeld, and ASHREA fundamentals does a good job and providing correlations for buildings.

Frank Kreith, Mike Modest, and Siegel and Howell have each produces excellent thermal radiationheat transfer texts, used in college courses.

 

[IRstuff]

You've already described the process, with the exception that letting longwave through a visible window is really not necessarily in your budget. Wien's displacement law also tells you the wavelength at peak for the infrared emission curve. For something around 50ºC, Wien's displacement law gives 9 microns. This would require a zinc selenide or zinc sulfide or Cleartran window, which is not cheap.

Additionally, since the window does not encompass the whole box, the amount of radiated emission through the opening would be low.

The more standard approach is to treat the solar gain as just another internal heat source and deal with it as you would any other internal heat source.

TTFN

FAQ731-376

 

[EngAP]

davefitz - Thanks, I will definitely look the books up.

IRstuff - Thanks for replying again. That is the answer I was looking for. So if I block as much IR as possible (above the longest wavelength required by the camera) I will be able to determine the effective heating load from solar gain. If I know the solar load (scaled by the angle of incident into the window) and the transmitance of the window as a function of the spectrum I can roughly determine the extra heat load by multiplying the solar load as a function of wavelength by the transmittance. Would this be correct?

The material we use for the window curently has a 90% transmittance between 300nm and 2100nm dropping sharply to 0% either side. However the transmittance band can be tightly controlled either side.

Thanks all for the replies.

 

[IRstuff]

A relatively cheap solution is to add a hot mirror as suggested by someone earlier. Edmund Optics makes one up to 100mm x 127mm:

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1492

If you add a sunshade, you can restrict most of the entering light to early morning or late afternoon, when the air temperatures are lower.

TTFN

FAQ731-376

 

[EngAP]

The hot mirror literature suggests that the only part of the spectrum that causes heating is the IR range.

This is what i am trying to understand - will the transmitted radiation in the visible light spectrum heat up the components that absorb it?

 

[IRstuff]

Yes, that's why that filter is specified to cut off above 700 nm

TTFN

FAQ731-376

 

[EngAP]

IRstuff - thanks again but could you clarify which statement you are saying yes to.

 

[IRstuff]

the transmitted radiation in the visible light spectrum heat up the components that absorb it?

Yes, but you haven't given enough information about the in-band requirements for the camera. What is the minimum transmission that the camera can tolerate?

TTFN

FAQ731-376