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Mirrors to increase solar exposure?  RSS feed

 
Paul Roue
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Would I get an added benefit if I put a wall of mirror, either outside or build it as a lean-to structure with the mirrors on the inside? I've not seen it anywhere and I can't be the first person to consider the idea.

The details: I am in a northern climate, at winter solstice the sun is 19 degrees above the horizon at noon and above the horizon for about 8.5 hours. That isn't a lot of daylight if I want a year-round greenhouse but my thought is that if I put a mirror wall on the north side I would dramatically increase the amount of solar radiation the plants would get and they would be more productive. I am thinking an external wall would be better as there is less volume to heat.

Paul
 
Peter Ellis
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Location: Central New Jersey
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Considering the recommendations about using ponds for their reflective ability, sure, mirrors would work at least as well.

Questions arise in terms of cost/benefit and exactly where to place the mirror wall for best benefit.


You said "an external wall would be better as there is less volume to heat", and I don't follow your thinking on that.

I can see where placing a mirror wall outside the greenhouse, on the north side but out of the shadow of the greenhouse, might allow you to reflect a significant additional quantity of solar radiation into the greenhouse. Could probably also set it on the south side, more as a floor than as a wall, and bounce additional sunlight in that way. In the second case it would be very similar to capturing light off of a pond, but without the added benefits the pond provides.

If I happened to have a bunch of mirrored mylar, or even opaque white plastic, I might try using that to set up a trial experiment and see what happened.

I have difficulty imagining that the return on investment would justify purchasing mirrored glass or plexi.
 
R Scott
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Location: Kansas Zone 6a
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Silver mylar "space blankets" are a cost effective way to pull it off. Real mirrors would be too expensive unless you luck into a pile of them for free (could happen).

I have seen mirrors on the top of skiylight shafts to bring light down into the buildings in northern climes, even tracking models to maximize the light.

The question is whether enough usable light (by the plants) would be left. The light is already filtered to the point it is marginal, after the mirror loss it might not actually help the plant.
 
Peter Ellis
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Location: Central New Jersey
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R Scott wrote:

I have seen mirrors on the top of skiylight shafts to bring light down into the buildings in northern climes, even tracking models to maximize the light.

.


"Sun tubes" are utilized in a number of facilities to bring light from the rooftop down into a work space. They are just flexible tubes with a mirrored lining on the inside and hook up to a clear dome on the rooftop.

I don't know what is lost in the reflective process, so no input as to whether it would matter to plants.
 
Paul Roue
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From what I've read you want your glass roof 90 degrees to noon sun at winter solstice. That would seem to be an incredible amount of volume to heat. I was thinking about sloping the glass roof at 19 degrees... oh an important detail, I would excavate 5' below grade to capture ground heat so the roof would start at ground level. The exterior mirror wall would also start at ground level.

Yeah if I were considering this for as a financial investment to sell produce... It would suck. I was thinking more along the lines of being more sustainable by having the ability to have fresh produce year round and also quality of life improvement. Today our high temp was -17F. To be able to go in where it is warm and moist and exploding with green on deary winter days... hard to put a $$ value on that.

Paul
 
Rebecca Norman
gardener
Posts: 1255
Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
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food preservation greening the desert solar trees
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We recently tried this with a biogas digester. It worked well. Too well: it melted a hole in the black film tank that it was supposed to keep warm! Oops! Then we replaced it with a clear film tank, hoping the light will pass through the film and into the manure before it changes into heat.

The cow shed is home to two or three cows, is south facing and solar heated, with the dry fodder stored on the north side. The tank is a methane digester to get cooking gas while being able to use all the manure in the fields afterwards. The micro-organisms that create the methane work best a nice warm temperatures and when cooler, they just stop working and wait. We don't want to make a tank large enough to store all the winter's manure till the spring, so we tried this.


Biogas1-SECMOL-frame.jpg
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Empty frame for tank made of flexible film
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Tank with reflector above. In front is rigid 5-ply polycarbonate, but not yet reflector
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Insulating the reflector panel with waste goat hair from pashmina factory
 
Rebecca Norman
gardener
Posts: 1255
Location: Ladakh, Indian Himalayas at 10,500 feet, zone 5
126
food preservation greening the desert solar trees
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After the excitement of the first gas reaching the kitchen after about a week, the next day the reflectors melted a little hole in the black plastic tank. We started over with clear plastic, but then the winter set in so it was hard to heat the manure up very quickly.

Biogas4-not-full-black-film.jpg
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The black plastic tank not yet full
Biogas5-putting-manure-in.jpg
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Loading the raw ingredient from the cow shed
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First gas reaching the kitchen
 
Erica Worhatch
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great question! I've pondered using mirrors also as I'm limited with sun exposure also (north and overcast a lot).
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