kathy,

If you go the route of solar-electric, it will probably be MUCH more expensive than necessary and there are many inefficiencies in the system (insert electrical jargon here) where you'll wind up losing much of the sun's energy before it ever gets a chance to warm the greenhouse. It will be far cheaper and more efficient to first try converting the sun's radiant energy into heat and storing that heat in what we call a "thermal mass".

One approach to do this might be to blacken 1 gallon milk jugs or other containers, and place several filled with water inside your structure. The black surface absorbs sunlight and converts it into heat which will be absorbed by the water during periods of sunshine (Water is the "thermal mass"). The heat is released later as the temperature cools. The thermal mass helps prevent temperature spikes during the day since excess heat is absorbed when its hottest and is then released later.

Now, a couple of thoughts on the thermal mass. It is simply a place to store heat and people use many things: Water, stones, metal, etc. However, there are added benefits to using water for the thermal mass.

Benefit 1) Water has one of the highest abilities to absorb and release heat of any common, safe substance. It works MUCH better than stone or metal.

Benefit 2) Water also has a built-in safety feature as you approach freezing temperatures. It can give up enormous amounts of heat energy (scientists this the "latent heat of fusion) while remaining at the freezing point of 32 Fahrenheit. This can help maintain the temperature inside the greenhouse at freezing while the environment outside drops below. Citrus growers use this technique by spraying liquid water on their trees when the air temperature falls below freezing. The fruit never drops below 32F and is saved. Of course, once the liquid is all frozen in your greenhouse the temperature will start to drop so put enough jugs of water in there to make a difference.

In closing, I'd try out the thermal mass approach first. If that doesn't do what you want it to do, then a more expensive approach might be necessary. If you have any questions about what I've described, please write back and I'll do my level best to clarify.

mc

I don't know anything about active heating, but if you put a good layer of mulch around your plants, straw for instance, you will help retain heat in your soil and also you will create a small amount of heat. As the straw slowly decomposes it will give off heat.

Kathy, With your setup I think you could go into the fall with minimum heat. If you get things growing through October or so, you could be harvesting in december (carrots, spinach, kale.) without adding extra heat. It's pretty tough to grow into the winter as lettuce needs to be over about 50 degrees to grow much. Last February I built a 4'x8'x18" insulated box (2" polystyrene insulating foam (from Menards) and covered it with a layer of plastic. I put a couple of strings of Christmas lights inside and with the insulted cover it maintained about 50 degrees when it was 15 degrees outside. If you seal it up well, it does not take much heat to keep it above 50 degrees.

I bought a cheap 4x8 greenhouse ($79 at Menards) and draped it with plastic. A 1500 watt heater kept it a toasty 70 degrees at 15 degrees. Ther has to be sufficient airspace between the two layers of plastic for this to work well.

Another alternative would be to heat your house with composting manure. It's a bit tricky to get it heated up and find a way to plant in it. You might set potted plants into the compost. The compost can start as high as 150 degrees and will gradually decline for a month or so. On a warm day be sure to take the cover off or you might fry all your plants.(I did) If you used the potted plants idea, you might be able to change the compost periodically.

How about finding a way to take advantage of earth heat? About 6 feet unter your bed the temperature should be about 55 degrees. With some help and an old refrige, you might be able to rig up a simple heat pump; would require some power, though.

Winter growing in Illinois is only for horti-oddballs like me, any maybe you too.

mcStargazer is right on with the gallon milk jug idea. I used it in NH to keep chickens outside over the winter (lowest temperature was minus 10). In addition to the milk jugs I backed it up with straw around the outside of their house on the north, east and west sides for added insulation and thermal mass.
Using something again and again appeals to me and it especially appeals because it costs virtually nothing. I am going to try a small coldframe here in Piedmont, NC for tomatoes, lettuce, peppers and spinach. I have a much milder climate than you do and my coldframe is on the south-facing wall of my house up against a dark brick foundation. I will place a navy-blue-painted milk jug between each plant. The lettuce and spinach will be in the front further from the wall and the jugs. I don't expect to keep the tomatoes and peppers throughout the winter (unless we have a really mild one) but I want to see how low it can go before they are killed.