Minimising Solar Heat Gain
Minimising Solar Heat Gain
(OP)
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.
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.





RE: Minimising Solar Heat Gain
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RE: Minimising Solar Heat Gain
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.
RE: Minimising Solar Heat Gain
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RE: Minimising Solar Heat Gain
RE: Minimising Solar Heat Gain
RE: Minimising 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.
RE: Minimising Solar Heat Gain
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?
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RE: Minimising Solar Heat Gain
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.
RE: Minimising Solar Heat Gain
http://www.huperoptikusa.com/products_klasse.cfm
If this approach is applicable you might look at a gold film.
RE: Minimising Solar Heat Gain
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.
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RE: Minimising Solar Heat Gain
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.
RE: Minimising Solar Heat Gain
Frank Kreith, Mike Modest, and Siegel and Howell have each produces excellent thermal radiationheat transfer texts, used in college courses.
RE: Minimising Solar Heat Gain
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.
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RE: Minimising Solar Heat Gain
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.
RE: Minimising Solar Heat Gain
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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.
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RE: Minimising Solar Heat Gain
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?
RE: Minimising Solar Heat Gain
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RE: Minimising Solar Heat Gain
IRstuff - thanks again but could you clarify which statement you are saying yes to.
RE: Minimising Solar Heat Gain
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?
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RE: Minimising Solar Heat Gain
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.
RE: Minimising Solar Heat Gain
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RE: Minimising Solar Heat Gain
Yes, thanks for the advice.
RE: Minimising Solar Heat Gain
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
RE: Minimising Solar Heat Gain