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?
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.
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.
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.