William Bronson wrote: > How about steam? A solar collector...
William, yeah that could work. I think if folks make or buy a higher-concentrating collector they may want to generate electricity too. I was thinking that if there was a way to use a solar collector with (higher-temperature) thermal storage ( say 300-500 deg.F), and if this (when backed up by fuel-based heat sources if necessary) was used to power a heat engine, then the electricity could provide artificial lighting! This seems like it could be a significant benefit on cloudy days, summer, spring, autumn and winter nights, and winter days.
First I'll try to examine some of the fundamentals. Plants need a surprisingly narrow temperature range for good growth.
example. Also some plants (broccoli, potatoes, arugula and rosemary) do better in cool temperatures than others, (corn, tomatoes). So a greenhouse design needs to ensure there is
enough heat and insulation to keep it warm (at least during the lighted growth times), and also ensure the temperature is correct for the plants and very even and constant. Fortunately this seems possible, and in theory might be affordable too, with sufficient thermal mass, and a sufficient rate of heat exchange between the thermal mass and the air around the plant (assuming it's not practical to heat the plant directly).
In the off-season, when the sun is low in the sky, it hits the ground more obliquely, so the light intensity is less. The plant's optimal light intensity requirements are less than in summer but more than in winter.
light requirements. This suggests that ideally, summer sun should be diffused, while spring and autumn light is near optimal, and ideally winter light ought to be concentrated (if used at all).
So the main question seems to be: What's the most optimal, (and economical) way to maximize both the plant area and length of time in a year where the plants are at room temperature and well-lit? Assuming that economic resources are more of a design constraint than
land area, this suggests that the place to start is with "season extension", since it could be easier to build and operate and get more growth out of two 3-season greenhouses than one 4-season greenhouse. (And easier to store food in the off-season and/or import food from a warmer climate).
I think maybe the biggest challenge is what to do about window glazing. It seems like building materials are either: translucent, insulating or affordable. Pick two (as they say). One approach is to have a movable insulation that is installed each evening and removed each morning. This can be anything from a curtain to blown-in insulation like foam beads, or even
soap bubbles! Another approach is to use reflectors to make solar concentrators. This way, less window area is needed so more money can be spent per unit area to make it more insulating. Also the window can be put on the bottom (of all things!) so that there is
much less thermal-convection and much lower heat losses. Another approach is to use 100% artificial lighting so no translucent materials are needed.
I like the idea of using a foamed semi-translucent material, whether plastic (like "bubble wrap") or soap bubbles or a combination. This technique is the closest I've seen to achieving all 3 goals of being: translucent, insulating and (potentially) affordable. Due to reflection losses on so many surfaces, the light transmittance will be reduced somewhat, which could be good in summer, but not good in winter. Perhaps in winter this could be made up for with solar concentrators. It's fairly easy to concentrate sunlight by a factor of 3. The flexibility of other design considerations seems to allow that factor-of-3 design advantage to be applied where needed.
Other ideas and thoughts:
Ground-source geothermal. Pumping 1 gallon per minute from a well can deliver 10,000 BTU/hr (at the yearly mean temperature ~45 deg.F). This alone isn't warm enough but could reduce the heating requirements significantly, or reduce the insulation and heating requirements needed for passive solar thermal heating.
So if we combine all of our best ideas it might make a good design. Maybe take UV-stabilized greenhouse film, make it into bubble-wrap. Take 2 layers of this with a channel in between that is a few inches thick. Blow soap bubbles in between. Maybe use a bit of heat from the 45 deg.F well-water to blow the air bubbles. Make the greenhouse with a strip of this say 1-2 feet wide. The total amount of plastic would be about the same as a cheap tunnel, just quadrupled up for better insulation, and relying on solar concentration to bounce the light through the smaller window. Use reflectors, whether polished aluminum sheet metal flashing, or aluminized mylar or mirrors to concentrate the light by say 3 fold. Put this on the south side of a building with a Trombe wall. Add some grow
lights for a bit of extra light and warmth.... Something like this might work in a colder climate without breaking the bank.