Greenhouses exhibit the greenhouse effect. They are warmer on the inside when the sun shines on them. However, their windows are flat surfaces and do not focus sunlight, so they could be hotter or brighter.
I am fully aware that it's impossible to gather more heat or light without increasing the collecting surface. However, it is possible to increase the surface area and concentrate heat and light in different regions of the greenhouse, in a couple of ways.
Firstly, the entire greenhouse could be placed within a large parabolic reflector whose focus was within it. This would lead to a brighter and hotter greenhouse than would previously have been possible at that site, meaning that plants needing more light or which thrive in warmer climates would do better within it.
Secondly, the windows of the greenhouse could consist of convex lenses of such a length that plants could be placed near their focal point, again allowing small bright and hot regions for the plants which need those conditions.
The two approaches could be combined to amplify the effect.
The entire assembly could move around throughout the day to the advantage of the plants.
Meanwhile, water could flow through the foci themselves in transparent pipes (the plants being merely near them) and be warmed by the heat, then piped into radiators for the rest of the greenhouse to prevent it from cooling too much during the night.
I'm sure i could make this less rambling if i wasn't knackered.-- nineteenthly, Feb 27 2008 I thought of this a while ago while toying with ideas for greenhouses on Mars as the sun is dimmer out there. The problem here is you'd have to have sun tracking mirrors and a bunch of them. At the very least some kind of a clockwork drive. The mirrors would be expensive too, as they would have to be strong enough to withstand the wind, etc. Sounds very pricey.-- MisterQED, Feb 27 2008 If the greenhouse rotated and was set in a depression lined by mirrors, this might help with the wind problem.-- nineteenthly, Feb 28 2008 Fresnel lenses might be the right sort of thing to use here.-- hippo, Feb 28 2008 Yes indeed. I had thought about those magnifying things on the back of buses actually, but i didn't know what they were called.-- nineteenthly, Feb 28 2008 // the sun is dimmer out there //
No it isn't. The sun's output is constant - it's just more attenuated on Mars because of inverse square law.
"Non-optical" fresnel lenses can be made quite easily by casting.
Curved mirrors would be the thing .... add some photovoltaic units to make the whole greenhouse self-powered (unless it's not sunny, in which case there's no point anyway).-- 8th of 7, Feb 28 2008 I haven't calculated it, but i think a mass as large as a greenhouse couldn't be rotated by the amount of energy that could be generated by a relatively small area of solar cells. I suppose the wind could blow it round or it could be set in motion on a low-friction surface in advance, and it could be inside an evacuated dome, but i don't think solar power would be enough to keep it rotating. Then again, the wind is ultimately solar powered as well.-- nineteenthly, Feb 28 2008 with ya right up to the "fresnel windows" bit: that would restrict your crops to a certain size, or at least spacing.-- FlyingToaster, Feb 28 2008 The advantage of the fresnel lens is that you could stand outside and look at the aphids on the inside.-- Ling, Feb 28 2008 I often wonder how the climate of the area people live in influences their view of solar power, because in England we see it as a pale, wimpy and watery thing we occasionally see in the sky, whereas in Australia you presumably see it as a huge macho thing that kills you if you stay out in it for too long. However, your statement, [UB], that the wattage from the sun is small is congruent with my perception of it before i do any calculations, so yes, i think you're right.
How about water-filled fresnel lenses? They could change shape more easily and it would heat the water into the bargain.-- nineteenthly, Feb 28 2008 No matter what you do, you're essentially increasing the volume of the greenhouse so why not just do that?-- phoenix, Feb 28 2008 This would be more compact and it would also allow microclimates within the greenhouse. The plants which need more light and heat could be placed near the foci and those needing less further away.-- nineteenthly, Feb 28 2008 "This would be more compact..." No it wouldn't. That is, the greenhouse itself might be smaller, but you've put it on a giant reflector.
If you're after zones of climate, I'd think you'd be better off with a big greenhouse and mirrors or lenses (or just blinds) that direct light/heat where you want it and away from where you don't. A lot depends on where you expect this greenhouse to be built.-- phoenix, Feb 28 2008 On reflection, maybe it could actually be a dwelling as well.-- nineteenthly, Feb 28 2008 How does collecting exactly the same amount of light and focusing the light increase the total amount of energy retained?-- WcW, Feb 28 2008 It wouldn't necessarily. However, the contents of the greenhouse might. If the building was completely full of water, it would end up storing a lot of heat and take longer to cool down at night. Individual species of plant are suited to different climates, for instance heather as opposed to pineapples. Processes within the plant such as photosynthesis use the energy of the light and heat to synthesise the substance of the plant. Enzymes are not going to work as well outside a particular temperature range and presumably there are similar problems with photosynthesis depending on how close the individual species is to its optimum environment. A greenhouse in the tropics containing heather would retain relatively little energy compared to the same area of moor in, say, Glencoe, because it's not working at optimum efficiency. It isn't the focusing that makes the difference, it's the plants and the water.
Which makes me think, actually. Some plants might end up radiating heat more than others in other conditions. Or, is it that in sunnier and warmer conditions, less hardy species are able to do different things which allow them to crowd others out? What would happen to heather in the tropics?-- nineteenthly, Feb 28 2008 I suspect with species that different two seperate houses or sections would be beter than trying to deliver two zones in the same room. (humidity being one reason)-- WcW, Feb 29 2008 The humidity isn't necessarily going to be different.-- nineteenthly, Feb 29 2008 random, halfbakery