Solar gain, please don’t forget the science
Solar gain, please don’t forget the science;
Just Googled ‘how do solar tubes work’. I quote from the very first search result that came up,
‘and if you just loathe the heat that accumulates inside the room, (brand name) is perfect for you because it effectively eliminates heat collection from the sun. It mainly gathers light and blocks the heat…’
Wrong, wrong, wrong, wrong, in my humble opinion, wrong!
Unfortunately this is not an isolated instance, another solar pipe report,
‘But energy efficiency is an issue with glass, whereas with the (brand name) it’s pretty neutral, there’s no solar gain….’
and that’s from a Marketing Director !
They, as so many in the glazing field have been misled and misinformed as to how the coatings on glazing work that they in turn incorrectly advise.
Plain glass, untreated, is a pretty good conductor of solar energy across its whole spectrum (range of wavelengths). Coatings and tints are added to glazing to absorb or reflect particular wavelengths of solar energy dependant on the proposed use for the glass.
Light (visible light) is short wave energy and glazing is usually ‘engineered’ to allow as much of this as possible to pass through. Heat (radiant energy) is long wave and as much of this as possible is usually blocked.
So, so far, so good, if it finished there the opening statements would be good, unfortunately it doesn’t. What they ignore is what happens to the Visible Light once it has entered the room.
Having passed through the glazing the short wave visible light will encounter the room’s contents (the flooring, wall coverings, furniture, curtains etc.) and depending on the colour and other properties of the content, all of this solar energy must be reflected, transmitted or absorbed.
This solar energy reflected back at the glass, being still at the same wave length as permitted to enter, can escape. The remainder of this solar energy however ends up being absorbed.
So what happens next?
The short wave energy having been absorbed is now slowly released as long wave radiant energy – heat.
The room is insulated, the windows coated to prevent this heat escaping and so the heat builds up.
If the Visible Light represented only a small portion of the solar energy spectrum this might not be too large a problem – unfortunately it represents approaching 50 %, so the possibilities for overheating are considerable.
Once these basics of solar energy wave lengths are understood then how solar shading for heat control can best work falls into place.
External shading can use both reflectance and absorbance to prevent the solar gain. Internal shading must use reflectance to get this solar energy back out through the glass before it changes wavelength.
One case study a couple of years ago illustrates the point well – A client had installed blinds manufactured from an open weave screen fabric in a mid brown colour. The blinds were doing the job for which they were originally specified, giving good light control and external vision but ‘they were acting as a radiator’ and ‘heating up the room’.
It took a site visit and a bit of ‘hand holding’ before the room’s occupants understood that the blinds were not ‘plugged in’ to some mystery power source but were in fact releasing solar energy that had entered the room as light, now converted into radiant energy.
Remove the blinds? No point, the same solar energy would still be released inside the room, only deeper in and a lot of it lower down at floor level. Where it could be even worse, as the warm air will rise up, warming the occupants as it passes.
Change the blinds colour to white? That would certainly have reduced the heat gain, but to the detriment of the initial objectives of light control and external through vision.
The use of colour and the openness factor of a blind fabric for light control….now that’s another story.
Tags: energy, heat control, heat gain, light control, radiant energy, solar energy, solar shading, solar tubes, sun pipes