Solar cooker / greenhouse?

I had a dream recently about making a greenhouse that was somewhat like a solar cooker: the walls were covered in aluminium foil to reflect the light so it would be hot even on a cold winter day. Bermed with earth, this heat might stay in the greenhouse overnight and continue to radiate due to the mass.

The one thing is that a solar cooker is quite powerful. It would definitely need to be retired when spring arrived (or the aluminium removed) otherwise it would fry the plants. My earth bermed cold frame fried the chrysanthemum as soon as spring came—what more would this do! Nevertheless it may be a good way of getting enough heat and light to, say, ripen tomatoes on a cloudy day in the dead of winter. Thoughts?

I love the idea of function stacking, especially when it comes to purpose built structures.
You can use the normal heat of a greenhouse for lots of purposes anyhow. I am looking forwards to my polytunnel next year, not just for growing plants, but for the warmth for drying herbs and grains and seeds....Reasonably locally to me they use the heat in a polytunnel during the summer to dehydrate seawater to make sea salt.


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It's a good way of turning what would be an energy intensive process into a carbon neutral one.

I think I'd be somewhat wary of creating a death trap if the whole greenhouse became like an oven, and, as you say the plants and oven may not mix well. Maybe an oven within the structure though - you'd lose less heat to the surroundings for a start....

That is very interesting about the salt production! I like the idea of using the space for drying things. In summer things here are as likely to dry as to mould.

Maybe just having one wall (the north side) covered with aluminium would benefit. I am guessing it would effectively double the energy available to plants, reflecting it also onto the earthen walls and floor, which would retain the heat. So it wouldn’t be a cooker, but a reflector.

Or something different? I am not sure. The issue I see is that whatever surface has foil on it, is not going to be absorbing the heat from the sun. But perhaps the increased solar energy to the plants would validate it.

Theoretically, there will be no more energy collected with a solar concentrating collector than with greenhouse surface of the same size.
In practice I think heating a smaller mass to higher temperature could be very useful.

I immediately think of heating air in a tube and directing that air into a solid mass under the roots of the plants you are trying to grow.

Because water holds a lot of heat and we have lots of infrastructure for moving it, heating water and irrigating with it seems like good idea.
That could be very simple, down to leaving a big pot to boil and pouring into the irrigation reservoir.

No, that’s true—reflecting would not lead to more energy being collected. But if the foil (mirror?) is positioned correctly then it would reflect the light energy, as opposed to converting some of it into heat when it reaches the wall. So it would probably help a bit with strengthening the light that plants get, where photosynthesis rather than warmth is the issue.

I like the idea of using hot water! There is the issue that the soils in winter here are already extremely wet but maybe in other parts that would work.

M Ljin, you make a great point.
Just reflecting the light towards the plants or soil  could be an improvement.
Foil covered foam is common and cheap insulation used to block car windows.
If it were applied to the northside (or south, depending on where you live)of an east to west  greenhouse, it's liable to be a net gain in retained energy.

If we just put reflectors behind or to the right or left of a greenhouse, we could increase solar gain,but  outside the greenhouse these would be subject to wind and snow.

Picture a tall  A frame green house, 12' on the north side 10' on the other, 10' on the bottom.
The 10' side will be at about 73 degrees  facing south.
On the 10' sides we put clear glazing.
On the 12' side that we form a parabolic reflective trough, focused inward.

I think the focus point will be at around 54" up from grade, but we could make the parabola asymmetric, to focus it lower in the greenhouse, but 40 gallon water heaters are about 60" tall.
Plumb them together and run a loop through the planting beds.
Circulate your water with an inline pump.

Because the interior of the A frame is about 9 1/2' tall at its height, we can use clear plastic to make a smaller space inside of it, effectively giving us double glazing.
Normally the space high up in an A frame is hard to use/justify ,but in this design it would collect extra solar energy.

That is interesting. This also sounds as if it might be a potential design for making solar hot water, too? Quite function stacking if so. But it would remain to be tested whether that would actually heat up significantly in winter to work for a household. I suppose it depends on the size of the reflector compared with the water.