window ledge solar collectors

I just moved into an apartment in the concrete jungle called Beijing.  My apartment has an east facing balcony and is about 3 meters wide (10 feet) i have seen a lot of 4 liter water bottles in trash bins and was wondering...if i fill each bottle with water and some kind of antifreeze...cover half of the bottle with foil and put the bottles on the window ledge with the foil side opposite of the window....do you think I could capture enough heat to be useful? In the dead of winter we only get 5-6 hours of sunshine. Its not much but they are clear days. When the sun starts to go down i was thinking of putting some recaptured styrofoam between the window and the bottle, after turning the bottles around so that the foil faces outward. Any ideas?

Maybe you could use the foil to reflect more light in the window

You might like to try something like this:

http://www.instructables.com/id/How-to-Build-a-Soda-Can-Heater/

http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

Tom Connolly wrote:I just moved into an apartment in the concrete jungle called Beijing.  My apartment has an east facing balcony and is about 3 meters wide (10 feet) i have seen a lot of 4 liter water bottles in trash bins and was wondering...if i fill each bottle with water and some kind of antifreeze...cover half of the bottle with foil and put the bottles on the window ledge with the foil side opposite of the window....do you think I could capture enough heat to be useful? In the dead of winter we only get 5-6 hours of sunshine. Its not much but they are clear days. When the sun starts to go down i was thinking of putting some recaptured styrofoam between the window and the bottle, after turning the bottles around so that the foil faces outward. Any ideas?

You won't capture any more heat than you would if you just let the light come in the window and then block it from escaping after the light (heat) stops coming in.  Kume curtains or your recaptured styrofoam put into the window just as the sun moves from the window will be the most effective.

Your choices for getting more heat than would come into the window are to reflect more light into the window somehow, like P Mike said, or use one of the solar heaters like Michael posted, with the caveat that those heaters only increase the amount of heat you get if you make them larger than the window itself.  That is fairly easy to do if you have a way to fasten it to the side of the building, or to rest one end of it on the ground.  Bigger is better when it comes to these solar heaters.  That Builditsolar page that Michael posted is a great source of info for all these type projects.  The instructables site gives step-by-step instructions for making kume curtains if you go that direction.

Pretty sure they don't need to be bigger than the window itself. As I understand it they don't usually replace the window. The window remains as is, and the solar collector is mounted beneath the window on the outside, with the air flowing in. Solar gain from the window itself is normal, the solar collector supplements it.

Michael Cox wrote:Pretty sure they don't need to be bigger than the window itself. As I understand it they don't usually replace the window. The window remains as is, and the solar collector is mounted beneath the window on the outside, with the air flowing in. Solar gain from the window itself is normal, the solar collector supplements it.

I wasn't clear.  The type you show in your post aren't meant to go on or under a window, they are meant to be mounted on the wall and vents cut into the wall at the top and bottom of the solar collectors.  Some people make them and mount them over their windows in an attempt to make solar heaters this way.  Here is an example of one of those: Wrong way to do this  That doesn't work unless you make the collector bigger than the window.  If you make it bigger than the window (on the outside of your house), you can bring your intake and outflow vents thru the window opening (with framing around the venting) and you will get more gain than you would from the window itself.  There is another type that sits on the ground directly beneath the window.  In this type, the intake and outflow are separated by a single board inside the collector and the entire top of the collector fits into the bottom of the window thru the opening created by opening the window.  These are effective as well.

My background is that I took undergrad engineering courses in thermo and electromagnetics/optics.  (My degree is in EE).  Anyway, a lot of people seem to believe you need to put something black in the window to absorb sunlight.  I don't agree.  In theory the light that comes in the window will be absorbed simply by bouncing around the room.  Also I think the "emissivity" coefficient that re-radiates infrared thermal heat back out might be lower (and better) with nothing in the window.  (If you're really worried about it you can install "low-e" window film).  Even a black curtain might make the room colder, since you're making the window area warmer and it's easier for that heat to leak out the window.  A layer of clear plastic film can help if it's not already a triple-pane window, since it adds about +1 to the insulating R-value of the window.  Although it will reflect a few percent of the light back out, over 24 hours the improved insulation at night will make up for it.   Easiest thing you can do is just wash the window!

If it's like in the US where aluminum cans can be redeemed to collect the deposit, then I wouldn't use them since any dark-colored material will work just as well.  Surprisingly, even thin plastic film has sufficient thermal conductivity through its narrow thickness.  Even though plastic is a rather poor conductor, air is even less.  This illustrates how slow heat-transfer is with natural-convection, and how fast it is with conduction over short distances.  

I think to have an effect you really have to go beyond the window area to add more area as a reflector or a collector.  

P Mike wrote:My background is that I took undergrad engineering courses in thermo and electromagnetics/optics.  (My degree is in EE).  Anyway, a lot of people seem to believe you need to put something black in the window to absorb sunlight.  I don't agree.  In theory the light that comes in the window will be absorbed simply by bouncing around the room.  Also I think the "emissivity" coefficient that re-radiates infrared thermal heat back out might be lower (and better) with nothing in the window.  (If you're really worried about it you can install "low-e" window film).  Even a black curtain might make the room colder, since you're making the window area warmer and it's easier for that heat to leak out the window.  A layer of clear plastic film can help if it's not already a triple-pane window, since it adds about +1 to the insulating R-value of the window.  Although it will reflect a few percent of the light back out, over 24 hours the improved insulation at night will make up for it.   Easiest thing you can do is just wash the window!

If it's like in the US where aluminum cans can be redeemed to collect the deposit, then I wouldn't use them since any dark-colored material will work just as well.  Surprisingly, even thin plastic film has sufficient thermal conductivity through its narrow thickness.  Even though plastic is a rather poor conductor, air is even less.  This illustrates how slow heat-transfer is with natural-convection, and how fast it is with conduction over short distances.  

I think to have an effect you really have to go beyond the window area to add more area as a reflector or a collector.  

We were posting at the same time :)  

I agree with your post if that wasn't clear.  Rather than using aluminum cans, I think plain black window screen is the easiest and best to use.

the foil is just going to bounce the light back out and make your place colder.

The window is causing a greenhouse effect, it is after all made of the same material greenhouses are made out of. The incoming photons come from a very hot thing - the sun, so are high frequency. The frequency of the photons is outside the absorption spectrum of glass so they pass thru the window into your room. When the room then radiates heat, its photons are lower frequency, and the glass is able to adsorb the photons and reradiate them (pretty much all of them) back into the room.

Put your foil there, and the photons from the sun simply bounce off and are the same frequency as when they came thru the window so just go straight back out. Liquid water in the bottles (unlike water vapour in the atmosphere) also lets the high frequency photons pass. Liquid water is not good at trapping heat from the sun.


Put some dark coloured bricks or buckets of sand inside the room where the sunlight falls, and you might get a heating effect.

Thanks for the good discussion!  I really appreciate the level of knowledge and talent found on this forum. It helps me formulate my  thoughts.  In one sense, maybe it is easier to describe what I am doing as saving the heat for later use when it enters the balcony.  There is a radiator that keeps the house at around 60 F. during the day - the balcony creeps up past 65.  At night time, when the sun goes down, the balcony is a bit colder than the rest of the house because of the heat loss through the windows.  I am hoping to make a small greenhouse on the balcony to grow tomatoes hydroponically.  It will be my first attempt and I know that I need to provide some additional heat to the tomatoes because of the drop in temperature.  It will also be my first attempt at using solar energy.  

Tom Connolly wrote:Thanks for the good discussion!  I really appreciate the level of knowledge and talent found on this forum. It helps me formulate my  thoughts.  In one sense, maybe it is easier to describe what I am doing as saving the heat for later use when it enters the balcony.  There is a radiator that keeps the house at around 60 F. during the day - the balcony creeps up past 65.  At night time, when the sun goes down, the balcony is a bit colder than the rest of the house because of the heat loss through the windows.  I am hoping to make a small greenhouse on the balcony to grow tomatoes hydroponically.  It will be my first attempt and I know that I need to provide some additional heat to the tomatoes because of the drop in temperature.  It will also be my first attempt at using solar energy.  

If I were trying to do that, I would get a bunch of those water bottles and paint them black and put them in the sunniest spot.  I would make kume curtains for all the windows.  They are cheap and easy to make.  You could roll the curtains open in the morning as soon as the sun is hitting the windows, and roll them back down as soon as the sun isn't hitting the windows anymore.  If you are there during the day, you can roll the curtains down as the sun passes each window.  At my house, that would mean opening the east curtains first to let the morning light in, and as the sun moves to the south windows, closing the curtains on the east.  Otherwise, just open them in the morning and close them in the evening as soon as possible after the sun stops shining in.  You will probably need extra lighting.