I was thinking of a plastic 750 gallon septic tank ($500-600) buried below the green house filled with water .With the garden hoses drop in circulating the system water and easily removed for cleaning purposes. I know I would not achieve a constant 55F but it would definitely be lower than the surface temp.I also like the idea of the partially buried FT.Along with good insulation around beds and plumbing FT.
Come to think about it I wonder how those septic tanks would work for fish tanks ????
The way that I worked it out, was for the winter condition because that is where one needs to heat up at night. To get rid of heat in summer, is easy.
Working on a plain water storage tank, what I said was at day when it is easy to heat up the greenhouse air to 25⁰C, one simply has an air to water coil with a pump circulating to the storage tank, and a fan to circulate the greenhouse air through it for exchange. So what you do is keeping the greenhouse on 25⁰C and heating up the volume of water from the night.
Right. Now lets say we manage to exchange whatever heat we can during the day and have the volume of water at 22⁰C at the end of the day. Night comes and once again, sets say we are generous and willing to drop the greenhouse air temperature to 10⁰C minimum and to achieve that, swing the volume of water down to 13⁰C.
Lets for argument sake say we do it for 2 scenarios. Take a Greenhosue with 100mē total surface area and for a single wall construction, we assume a u-value of 5.8 W/mē ⁰C. For a double layer we will work on 3.5 W/mē ⁰C.
From real weather data for my town, I would need 78 kWh of energy to get through a cold night. By swinging a volume of water through 9⁰C to manage that, I would need 7500 litres. That means in any case that the volume of water can be used for nothing else than thermal barrier. You cannot put fish through that swing twice a day and the growth beds would not like it either even if you do manage a way to store it there.
For a double layer greenhouse, one would need only 47 kWh for that same night to stay on 10⁰C.
Going geothermal, you would need an even larger volume of soil at least 1.8m down to manage that, so prepare to dig.
I have included the PDF of the excell sheet that I use to calculate these things.
That is why I for now, will concentrate to rather make a better greenhouse construction with decent thermal properties.
Am buying some rural land in southern US. Got a quote of about $1,200 from a contractor who will drill a hole about 60 meters deep (200 feet) big enough for a U-tube to circulate fluid (anti-freeze or water). They pour in an insulating grout to hold the U-tube in place and to isolate one line from the other. Then, it is ready to reject or supply heat. They claim that this U-tube system will be equivalent to 3.5 kW (1 ton). I'll have to spend other money to link it to an end-use. That seems reasonable for 3.5 kW/1 ton of heating/cooling for the price of running a pump.
Hello Ms, Ga here, and am working on something similar. I also Know that WIS in TX is also working on a heat sink. I find some interesting facts just reading this post, and I am sure you will find even more info if you search this site. There is a search function top right of this site that should help you find that very information. I have just started a system here in my basement, as my learning curve, with the intention of going outdoors in the spring. To that end, I have been digging down about 3 feet to bury my IBC totes on top of buried pipes routed to an old dried up well. Don't know how well it will work, as it is still just an idea. But like I said, WIS, myself and others are working on this very idea, and I for one, welcome your input, your trials, and yes, your errors, so that we may all find success in this venture. Keep us posted, and PLEASE, provide Pics.
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