3 ms·

I actually built a similar system as an experiment, basically a 1 x 2 x ¼ meters (multi-layered) cardboard box painted all black inside and covered with a shee

I actually built a similar system as an experiment, basically a 1 x 2 x ¼ meters (multi-layered) cardboard box painted all black inside and covered with a sheet of transparent plastic on one of the larger sides. It was really cheap, around $10-15 in materials. Then I added two holes (in and out), and put some cheap flexible air duct hoses with some old CPU fan, which was another $8. On a sunny day it heats up the air incredibly well, where it actually could be beneficial during the winter season. Unfortunately there are big problems that I encountered that I couldn't solve cheaply. 1) Mounted on the roof, it has to be sturdy enough to resist heavy rains and strong winds. We get winter windstorms, and one winter it got up to 55 knots with 70 knot gusts. I have no idea how to cheaply mount this box on the roof so it doesn't fly off and kill someone. 2) Intake and exhaust hoses would have to go either through the walls or through the windows. And since I don't own the house, my only real option would be to cut circular holes in the windows, and then replace the glass when we move out. Again, not cheap. 3) For the winter months I would have to also insulate these hoses, or they would lose most of the heat on the way to the house. EDIT: I also think if you don't care about what your house looks like, you can either paint it black or wrap it in black plastic.

heisenbit 10y ago
> I also think if you don't care about what your house looks like, you can either paint it black or wrap it in black plastic. Such heating will work during the day but transporting the energy from the heated plastic into the house is another matter. The hot surface also would radiate a lot of heat immediately away. And when the night comes you heat the sky with what you have stored up. The article is fascinating as it shows what can be achieved on a system engineering level. As an engineer however I would also like to see some performance measurements like air flow and temperature deltas going beyond the anecdotal story keeping the build warm in the northern US. Did not see anything on his homepage. There have been related posts on HN that showed advances on material science level where the absorption and radiation wavelengths of surfaces have been optimized. > Unfortunately there are big problems that I encountered that I couldn't solve cheaply. PV system cost these days is dominated by other costs than the solar cells. Engineering something that can withstand weather for a sustained period is both non-trival and not cheap. There may be some overlap in what PV systems require and what a passive heating system requires. Maybe the massive investments in the PV space make passive heating systems these days more practical?
anotheryou 10y ago
maybe you can build a "plug" for the half-open window?
- imaginenore 10y ago
  That would be challenging, we try to keep windows air tight for the winter to avoid any leaks.
  - deleted 10y ago
    [deleted]
rzzzt 10y ago
Some people build these with painted beer can columns on the inside serving as air ducts, see this video for an example: https://youtu.be/nuxanLdtwZQ https://youtu.be/nuxanLdtwZQ They also tend to install it on the side wall; it doesn't get that much sunshine in the summer, but it becomes more efficient in the winter when the Sun stays at lower angles. External circulation of air is still required and is usually done by using an electric fan.
- imaginenore 10y ago
  Yeah, I'm fully aware of that design. I've seen tests of it against a much simple design with the sheet of fabric in the middle, and there's basically no difference. Cutting so many cans' tops and bottoms is actually not an easy task, I had tried.
  - redtexture 10y ago
    A couple of pieces of plywood, for each window, and some foam strips for air-flow sealing would do. This is not a big effort, just more than your (exceedingly short-lived) cardboard design contemplates. The gain in creature-comfort in the apartment, might be more than the money saved in avoided heating, as a renter of the apartment.

heisenbit 10y ago

> I also think if you don't care about what your house looks like, you can either paint it black or wrap it in black plastic. Such heating will work during the day but transporting the energy from the heated plastic into the house is another matter. The hot surface also would radiate a lot of heat immediately away. And when the night comes you heat the sky with what you have stored up. The article is fascinating as it shows what can be achieved on a system engineering level. As an engineer however I would also like to see some performance measurements like air flow and temperature deltas going beyond the anecdotal story keeping the build warm in the northern US. Did not see anything on his homepage. There have been related posts on HN that showed advances on material science level where the absorption and radiation wavelengths of surfaces have been optimized. > Unfortunately there are big problems that I encountered that I couldn't solve cheaply. PV system cost these days is dominated by other costs than the solar cells. Engineering something that can withstand weather for a sustained period is both non-trival and not cheap. There may be some overlap in what PV systems require and what a passive heating system requires. Maybe the massive investments in the PV space make passive heating systems these days more practical?

anotheryou 10y ago

maybe you can build a "plug" for the half-open window?

imaginenore 10y ago
That would be challenging, we try to keep windows air tight for the winter to avoid any leaks.
- deleted 10y ago
  [deleted]

rzzzt 10y ago

Some people build these with painted beer can columns on the inside serving as air ducts, see this video for an example: https://youtu.be/nuxanLdtwZQ https://youtu.be/nuxanLdtwZQ They also tend to install it on the side wall; it doesn't get that much sunshine in the summer, but it becomes more efficient in the winter when the Sun stays at lower angles. External circulation of air is still required and is usually done by using an electric fan.

imaginenore 10y ago
Yeah, I'm fully aware of that design. I've seen tests of it against a much simple design with the sheet of fabric in the middle, and there's basically no difference. Cutting so many cans' tops and bottoms is actually not an easy task, I had tried.
- redtexture 10y ago
  A couple of pieces of plywood, for each window, and some foam strips for air-flow sealing would do. This is not a big effort, just more than your (exceedingly short-lived) cardboard design contemplates. The gain in creature-comfort in the apartment, might be more than the money saved in avoided heating, as a renter of the apartment.