IDEA: Solar air curtain

[Minnow]

Hopefully this is an orriginal idea .... I've performed a few google searches and can't find anything similar.

Solar air to air heating seems to have a fair following and generally entails a glazed box with aluminium cans painted black, air fed through the cans which are warmed through infra red heat from the sun, directly heating the air passing through them. Some are mounted externally on the sides of houses with ducting into the home others are placed indoors in front of south facing windows.

This got me thinking, why bother with an ugly box indoors in front of the windows when a similar result could be attained from a secondary curtain in front of the window and air passed through fabric ducting sewn into the curtains themselves ?

This would mean the curtains could easily by opened or closed when needed and wouldn't be unsightely like a big box with Ali cans inside would be. Probably easier to build and of course would have a higher 'wife acceptance factor'. Assuming a feed fan of sufficient size / sufficient start up speed, the curtains could even 'auto close' when activated !!

Anyone know if this is an orriginal idea ? or any thoughts on why this is a good / bad idea ?

thanks,

Minnow

[biff]

sounds an exellent idea minnow,
             it could get a bit akward if you were in a hurry through the french doors,likewise you would depend on luck to get the best siting whereas the solar heat box can be sited externally to take advantage of the best position.
     just so long as you dont start on mirrors,
                                                                      biff

[mpooley]

Im probably being thick but I cant see any benefit in having a solar collector of any sort inside the house as the radiated heat is already through the glass into your house.
You are just moving it from say the carpet or the back of a chair (whatever its hitting) to a collector that moves the hot air to somewhere else.

maybe i'm missing something?   Brain cells probably lol 

Mike

[billi]

...    this idea was in my head quite a while , but my curtain was water-filled  and stored the low temp say 30 °  in a thermal store or underfloor  for the night

Billi

[EccentricAnomaly]

...which are warmed through infra red heat from the sun...

Actually, more of the heat comes from visible light than infrared.

...others are placed indoors in front of south facing windows.

Really, why would anybody sensible do that?

Im probably being thick but I cant see any benefit in having a solar collector of any sort inside the house as the radiated heat is already through the glass into your house.
You are just moving it from say the carpet or the back of a chair (whatever its hitting) to a collector that moves the hot air to somewhere else.

maybe i'm missing something?   Brain cells probably lol 

No, I think you're not being thick. Once the light's come in through the window most of it will be absorbed by the room contents anyway. Putting an absorber immediately behind the window would, if anything, increase the losses.

In general, the point of a solar warm air collector is to accept heat when the sunlight is available but not lose it at night or on dull days. The ideal is to have the minimum amount of window area needed for internal lighting, a nice view, fire escape, etc, so as to minimise heat loss then use external solar collectors to increase the available area for heat gain when it's needed in a manner which can be dumped easily when it's not.

[skyewright]

...I cant see any benefit in having a solar collector of any sort inside the house...maybe i'm missing something?

I've been looking at a number of solar air heaters recently and I don't recall noticing any that were for positioning inside windows. Maybe I missed some things too?

Generally speaking I rather like the idea of being able to see out of windows.  

Yes, we do sometimes draw a blind in the SW facing rooms if it gets too hot in the afternoon, but that usually only happens outside or on the edge of the heating season and when that room and one ones off it are up at a nice temp already, from the heat that's been coming in through the window and soaking into the carpet, walls, etc,...

So, interesting idea but I'm not clear where the benefit is. Perhaps what I'm missing is where the heat is being taken too?

[skyewright]

In general, the point of a solar warm air collector is to accept heat when the sunlight is available but not lose it at night or on dull days. The ideal is to have the minimum amount of window area needed for internal lighting, a nice view, fire escape, etc, so as to minimise heat loss then use external solar collectors to increase the available area for heat gain when it's needed in a manner which can be dumped easily when it's not.

Or, as it (sort of) said on a site I was reading recently, the aim with solar air heaters is to get the heat gain associated with big windows that let in the light, but without the heat loss associated with big windows that let out the heat, and in such a way that you can dump the heat if it is not needed.

[dhaslam]

Some buildings in the US use  wire mesh, painted black  on the outside of a south facing wall. Warm air is drawn through the mesh into the building.  It might be possible to  make something like that  look reasonably tidy.    

[Minnow]

Generally i think everyone is right ... probably a non starter although the reasons I think this are a little different ... modern window technology (low e glass).

my orriginal thinking was along the lines that generally black objects have high emmissity (sp?) meaning they can convert more light into heat where as a light coloured object tends to reflect the light without converting to radiant heat, hence why I thought blacking out the full window area would increase emmissity and reduce reflection of light that could be converted to heat. Of course if your decor is very dark / gothic then there probably isn't anything to gain.

Comming at this as a 9 to 5 worker, blacking out southern windows during work time would not have any impact on the view, I'd simply open the curtains when I return home ... obviously not suitable for everyone. Another (small) benefit would be that air would be drawn into the curtain at the top (where it is generally warmer) and expelled at the bottom so that it would be more noticable to the inhabitants ( abit like comparing UFH vs radiators ).

Where I think this really falls over is that low e glass witch is designed to either trap IR heat or reflect it (dependant on climate) so kinda does a lot of what I was thinking of achieving.

For those that hadn't seen window mounted solar air heaters, heres a couple of links:

http://cgi.ebay.co.uk/Solar-Hot-Air-Heater-Heating-Double-Hung-Window-32X24X4-/260661969130?pt=LH_DefaultDomain_0&hash=item3cb0aa00ea

http://cgi.ebay.co.uk/Indoor-window-Solar-Heater-Panel-Free-hot-air-/260784426865?pt=LH_DefaultDomain_0&hash=item3cb7f68f71

[Ivan]

Indoor solar heaters are a scam.

One of the most important aspects of solar panels is the fact that glass itself is a selective absorber/emitter ie greenhouse effect. Blacking out the windows simply concentrates the solar heat gain into a relatively small area, and many people will interpret that as gaining heat that they didn't have before. In fact if you lined up magnifying glasses you could start and sustain a small fire with the same energy, but you wouldn't be gaining anything you didn't have before.

[skyewright]

my orriginal thinking was along the lines that generally black objects have high emmissity (sp?) meaning they can convert more light into heat where as a light coloured object tends to reflect the light without converting to radiant heat

A good general point, but I don't think indoor surfaces like carpets and (fabric) soft furnishings probably reflect very much unless they are very smooth and very white.

Another (small) benefit would be that air would be drawn into the curtain at the top (where it is generally warmer) and expelled at the bottom

If you are going to pump in that direction you'll need a bigger fan as you'll be going against the (thermosyphon) grain. Most solar heaters I've seen work with the natural flow, i.e. extract at the top; some are even go so far as to be passive, doing away with the fan and relying just on the thermosyphon effect.

For those that hadn't seen window mounted solar air heaters, heres a couple of links:

Thank you.

[EccentricAnomaly]

my orriginal thinking was along the lines that generally black objects have high emmissity (sp?) meaning they can convert more light into heat where as a light coloured object tends to reflect the light without converting to radiant heat, hence why I thought blacking out the full window area would increase emmissity and reduce reflection of light that could be converted to heat. Of course if your decor is very dark / gothic then there probably isn't anything to gain.

It's worth pointing out that most of the light which is reflected by one object in the room will hit another object. Even if, say, only half the energy is being absorbed at each reflection a large proportion will be absorbed before the ray happens to hit the window and go out again. If you look at a house from a distance the windows appear black (apart from direct glints off the glass, of course) for this reason.

The classic laboratory method of producing a good approximation to a black-body radiator/absorber is similar to this: a cavity with a relatively small hole to the outside.

[Minnow]

If you look at a house from a distance the windows appear black (apart from direct glints off the glass, of course) for this reason.

I think the amount reflected back out through the window is much higher than this method of observation would appear .... e.g. sunny day, turn a light on indoors and you probably wouldn't notice it from outside, at night you would see the quantity of light that is actually reflected outwards ..... I guess because the ambient light levels out doors are so much higher than that reflected out from inside a building our eyes have trouble detecting it ..... hope that makes sense.

I detect a small experiment in the making .....

[EccentricAnomaly]

I guess because the ambient light levels out doors are so much higher than that reflected out from inside a building our eyes have trouble detecting it .....

Precisely, that's the point I'm making.

I detect a small experiment in the making .....

Something like a light meter outside with either a normal room or black sheet behind the window? Need to compare against light level going into the window.

[Minnow]

I guess because the ambient light levels out doors are so much higher than that reflected out from inside a building our eyes have trouble detecting it .....

Precisely, that's the point I'm making.

I detect a small experiment in the making .....

Something like a light meter outside with either a normal room or black sheet behind the window? Need to compare against light level going into the window.

Ok thats two experiments to try ... my orriginal idea was :

2 tin foil covered boxes, with identical sized glass covered aperatures. both boxes have the interiors painted a nice magnolia . One box has a black material placed in front of the aperature (pretend black curtain). Both have circulation fans inside and both have temp sensors. Place outside with the aperatures facing the sun, leave for an hour and see what the difference in the temperature of the interior of the two boxes.

If i get the ceiling plasterboarded at home tonight then this could be a project for the weekend.