If you’re interested in passive solar greenhouses, I believe that you must look at the Chinese model. A few years ago I joined a tour of greenhouse operators visiting passive solar greenhouses in China and I’d like to tell you a bit about them.
The basic principle is simple. Build something with a lot of mass (brick in this case, but sometimes cement, and sometimes the walls are filled with sand or another material to add extra heat-holding thermal mass). Add a plastic cover to create an area that holds in some of the heat. Have an insulated material that covers the plastic at night (usually straw mats, sometimes another material). Plant things. That’s it. You have a greenhouse.
1. It’s super simple to build, and not too capital intensive. (This is great because it takes a relatively small investment to get to the minimal viable product.)
2. You don’t have to pay for heat. My half-acre greenhouse is heated with waste wood and a lot of the costs are labour so hard to calculate, but it would cost about $60,000 to heat if I used natural gas–heating is a big cost in a cold climate. This area of China (within an hour of Beijing) gets fairly cold, about like Michigan or Southern Ontario. Heating there would be a bit less than where I am, but still costly. Those costs would be greatly reduced with a system like this.
1. It is difficult to cool this enough to use it effectively in the summer.
2. It is more stretched out than a normal greenhouse, so I think labour costs would increase (more walking, less compact).
3. You couldn’t grow high crops (not only because the roof is too low in many areas, but also because you have to balance heating with growing. Any space blocked by plants won’t get as much benefit from the sun (it either helps the plant grow or it heats the space). Which means lower crops.
4. Crops that give off a lot of moisture would create problems, both because that would make heating more difficult and because you don’t generally want higher humidity in the greenhouse.
5. The light in the east and west is blocked by the brickwork.
6. It is difficult to automate the insulated cover.
7. It is more difficult to regulate the humidity.
I list these downsides, but I have to admit that almost every crop I saw was beautiful. And if you can find inexpensive labour, and a market for short plants, these would be viable even in a fairly cold climate.
Overall, I was impressed with the ability of the people running these greenhouses. Even in their non-ideal internal climates, the crops grown were beautiful.
One of the greenhouse owners asked me about my greenhouse, and how its operated. I talked with him a little about our computer system. He laughed and said, “computers are expensive! You should get a peasant.” Every greenhouse we saw had a little header house entry on either end of the greenhouse. A peasant lived in each one of those houses, with a cot, coal stove, and a trunk. I understand that this peasant would be in charge of their one greenhouse. I tried to imagine what living and working in that space would be like, and perhaps its a bit better than some of the factory jobs in the cities, but I cannot really see it being a happy situation for anyone with real choice.
Nevertheless, the peasants in these houses are highly skilled growers and the system seems to work there.
Many of the greenhouses we saw had a back up heating system in addition to the passive heat. Coal was the primary system here, and often it was burned without a chimney, the smoke used as additional disease control (I think because it has a lot of sulfur–which is a natural fungicide).
The designs are simple, elegant and they simply work. With the low cost of labour in China, I imagine that they are also profitable. When I studied greenhouse management, there was a lot of talk about the possibility of commercial-scale passive solar greenhouses, but there are a lot of things about it that can be criticized. Ultimately, it is the type of direction that we may need to go.