I read online about a guy in Lake City, MN, using thermal banking to build his greenhouse for growing vegetables year round at this website: http://cookingupastory.com/sustainable-energy-thermal-banking-greenhouse-design

My plan was to put mine on the south side of my one car garage but to make it even more efficient, I am planning on having the floor of the greenhouse be 3 feet below grade, and digging another 3 feet down to lay corrugated plastic piping and top that with gravel. I would like to install a large duct fan and pipe that into an octopus of many different corrugated pipes.

Does anyone have experience with a mostly off the grid thermal banking greenhouse in zone 5? I would ideally like to grow a fan trained fig as well as some hardy citrus in the ground using the thermal banking fan system as well as some solar panels and batteries to store excess juice for running the duct fan or for running an electric heater or water heater to heat large black barrels in the greenhouse.
My dimensions will be 18 feet long, 10 feet wide, 12 feet tall lean-to top, with 6 foot height in the front or south side. I am willing to use space heaters on subzero nights, but do not want to significantly add to electric bill because of my obsessive hobby.
Anybody try this?
Dave[/url]

I have a simplied version of what you are talking about.

None of the pipes underground stuff, but basically a 90% passive greenhouse.

Mine is South facing, the floor is about 8 inches below the grade outside the greenhouse, and the floor is also covered with dark, flat rocks. Those rocks are my "thermal bank" , though I have had black painted water jugs also in the past.

I do have AC power run to the greenhouse. This is connected to a greenhouse thermostat set at around 30-32 degrees (F) that turns on a couple of 40 watt bulbs hanging directly over two Owari Satsuma mandarin trees.
Additionally I have some pepper plants, various potted plants,etc in there.
For me in zone 7 the lights only come on a handful of nights each winter, so its practically free to run.

I don't know about using solar power / batteries to run electric heaters because you would have to have a pretty hefty solar system to run a 1500 watt or more heater for any length of time. (Maybe I misunderstood you though, you mentioned your electric bill?)

The black barrels of water is a good idea, like big sun batteries.

Generally having black or dark colored floors in a greenhouse is not considered a good situation. By far the most important item in any greenhouse is light. Nothing is more important than light. Light is the force that growth is most depended upon. If a grower desires their plants to obtain optimum growth throughout the year he would be better served if the floor remained a light reflective color, and also the north wall. As an example, growers that use water storage in barrels as passive solar, have the side of the barrel facing the sun black, and the portion of the barrel facing away from the sun (normally north) painted white. The reason is, of course, to absorb heat from the sun while at the same time reflecting all possible light to the plants. That is unless the barrels are up against against the north wall.- Millet

zone 7 would be my ideal location for gardening and everything else, but I am in zone 5 in Iowa. I guess it has some pluses like not usually baking for 4 months, we just steam for 3 months due to the heat and humidity.

I am ordering a Owari Satsuma as it's the hardiest of mandarins, but would mine be ok if I let the temperature drop down to the 20s at night in January? When it gets older, I was going to take it out of the pot and put it in the soil in the greenhouse. I know the fruit ripens in November through early December and usually it's not horribly cold up here yet.
That's encouraging that your greenhouse stays that mild even when lows get into the single digits or teens I am guessing.

The heating system wouldn't be much more than simple water heaters and the solar panels would also power the duct fan that would blow warmer geothermally heated (or thermally banked) air back up out of the greenhouse ground into the air in the greenhouse.

Any idea how cold it was inside your green house with just the light bulbs for heat? Tom

That is awesome about the Owari mandarin...guess it's Japanese heritage (I think) has helped its hardiness for us. I will take your advice and wait til the tree trunk is thickening up to mature size before putting it in the ground. My only concern with a geothermally or thermal bank design is that it won't be cold enough to chill my fan trained peach and fan trained fig. Guess you can't have it all unless you have two growing zones in your greenhouse.

You have any luck with growing avocados in the greenhouse? I have one in a pot but it looks horrible in the winter...like death, some die back every winter. It's a "Day" avocado from Logees in Conneticut

I believe I remember reading that the original owari was the only tree that survived an deep freeze in Louisiana. My miho has survived in ground down to 15 degrees without damage. I would hate to go much lower that that without covering it with a large sheet. If you get container owari, protect them if it's going into the low to mid 20's.

Best of luck!

David

hmmm, I am JUST NOW seeing the comments regarding my earlier post.
To answer Tom, I don't think the INSIDE of my greenhouse got lower than the mid 20's F with teens outside.

In any case I've completely rebuilt and expanded my greenhouse this year and I suspect I may need more than just a couple of 40 watt bulbs.
But then I'm adding a lot more thermal mass to keep temperature more consistent.

If you do an internet search with the words 'earth tubes greenhouse' you will find information about keeping your greenhouse from freezing in the winter. It also keeps condensation down.

Since citrus could manage well with lower temperatures, it could be a fairly simple set up.

other words to check out are 'passive annual heat storage' and 'annual heat storage'

Wikipedia has this:
http://en.wikipedia.org/wiki/Seasonal_thermal_energy_storage

I would love to be able to do something like that, but it looks expensive and complicated to install. That is some interesting reading though.

I'm leaning towards cheaper and simpler thermal storage at the moment.
For instance, concrete pads insulated from the Earth, rain water cached in barrels,etc.