Heating the greenhouse

I have gotten a few questions about whether there is enough mass in my compost pit to heat the greenhouse, and frankly I don’t know, but probably not (see below). Although there is a lot of physics involved and there are easy calculations, there are a lot of variables that lead to uncertainty and imprecision, too, such as the following:

• Outside air temperature
• Composition of compost
• Type of manure
• Mixture (C:N)
• Type of carbon - e.g., straw, sawdust (generated from my woodworking shop)
• Frequency of turning the compost pile
• Frequency of C or N addition
• Size of greenhouse
• Insulation of greenhouse
• Thermal mass in the greenhouse (water, concrete, etc…)
• Other sources of heat

I have been researching this quite a bit, and although there are some great resources out there, I think the bottom line is I am going to have to try it and see how well it works.
Two great sources are the Physics of Composting by Cornell (it seems they have the answers to everything ) and “The Composting Greenhouse at New Alchemy Institute: A report on two years of operation and Monitoring March 1984 - January 1986”.

I based my calculations largely on the results from the above report. In essence, they used an active system to draw heat from a 25-cubic yard compost pile to heat a 576-sq. ft. hoop house. Their compost pile was composed of horse manure and sawdust (though it is not the hottest compost mixture, it is probably what I would use, plus our releatively insignificant contribution of kitchen scraps). This system gained them 23-35°F over the outside ambient winter temperature on cold, clear nights. The temperature at the highest point in their greenhouse dropped to 28°F (which I think was the lowest it got), but it did not cause problems for their lettuce (good news to me).

There is a huge range of information out there on how much compost can heat (see variables above). I estimate I will typically need between 4,100 and 8,000 BTU’s to heat the greenhouse to above 32°F in the winter (here is a great tool to begin determining your heating requirements). The range I am using in heating requirements is based on heating from low temperature between 0°F and +17 °F. Even though it does get to –30°F here, the average low temp is 17°F according to (http://www.weather.com/weather/climatology/monthly/59801?), and I used 0°F as a realistic good middle ground. I look at this range as designing for 90-95% for the time, rather than the 5% event. These BTU calculations have taken the glazing material, glazing surface area, insulation and greenhouse design into consideration.

Here are a few bits of information from various sources:

• A compost bin will produce about 1,660 BTU's/ cubic yard/ hour or 1,530 BTU/ lb
• Or heat 6-24 square feet of greenhouse/ ton of compost (interestingly, both these ratios seems validate my ratio below, too)

Another way to calculate how much compost I would need to heat the greenhouse is using the example greenhouse (from the Composting Greenhouse at New Alchemy Institute, above) because their results seemed to work for my needs. Using the ratio of of their greenhouse square footage to the quantity of compost they used was 576:25 so using that ratio, I would need about 2.6 yards of compost. My planned compost furnace is about 1 cubic yard, so my compost furnace would, in theory, supply 1/3 the heat to keep my greenhouse going in the winter.

In addition to the compost heat source I will use rain barrels as a heat sink, but perhaps more effectively, as a source of water for the plants in the greenhouse. Given the space restrictions, I will probably only use 100 gallons, or roughly 1/5 of what I would need to heat the greenhouse with passive alone (though I'd like to find out more about Glauber's salts or related). In addition to water, I will have a concrete floor, but again, the concrete will only account for a very small fraction of passive solar heating needs. The roof on the south side of the greenhouse is angled to intercept the low winter sun angle effectively (as opposed to standard, conventional kit greenhouses), so hopefully that will help. To better insulate the greenhouse, I will seasonally install a solar pool cover. I will have a ceiling fan in the greenhouse to help move the air around and finally I will rely on a thermostatically controlled oil-filled electric radiator-style heater (about 5,200 BTU’s). The heater is large enough to heat the greenhouse alone, since I roughly need between 4-8,000 BTU's (depending on outside temperature)with no other assistance.

So, knowing that one method alone will not effectively (or in the case of the electric heater, sustainably) heat the greenhouse in the winter, hopefully a combination of techniques will yield satisfactory results. Again, the main thing here is that I have no desire to grow orchids or tropical plants. In the winter I am only hoping to grow hardly vegetables- and I think I can do it with little use of the electric heater.