39.3-F is the temperature at which water is most dense. This is the temperature of the water in the deep zones of Sepp's ponds and lakes. There are some very interesting things going on with the physics of water at this temperature, this is one of the huge benefits of the deep zone. Those who are interested in this kind of stuff
should look into Viktor Schauberger and water and cell membranes as dipoles.
I like this question, this is something that Chad and I were actually discussing in January. SHCS sounds like a great concept, I can see how water could provide some amazing benefits by the heat it releases as water vapor turns to liquid water underground. This releases 540 calories per gram in this phase change from vapor to liquid. I haven't seen any of the systems for myself, or felt the heat coming from them, but I am not a big fan because of all the electricity and technology involved. I'm sure you can have a windmill or battery bank power the fans at night, but this all seems overly complicated when I know you can get the same benefits with less inputs and technology. I look forward to seeing Jerome's place, hopefully sometime soon. It's certainly something I would like to experiment with in the future.
The greenhouses that we build use natural systems. No ventilation fans, no electricity, only working with nature. Ventilation is passive, and we can nearly triple the growing season in Bozeman with just passive solar and passive geothermal. Winter is an important part of the natural cycle, and you can still harvest plenty of foods during it with a 4-season harvest approach. Even if it is only for a brief period it helps keep the rhythms and cycles of the plants and animals natural. The tree frogs go dormant and the
trees get their rest to fruit well the next year. In Bozeman there's not really enough light in December to grow the tropical plants that require a year round 60-F. As I'm sure you can guess I'm not a fan of artificial lighting; when humans try to replicate the sun we have taken it way too far.
If you did want to keep your soil at a constant 60-F (the only use I can see is a cutting bed) you would just need a
rocket mass heater. We will be building one of these into the prototype greenhouse during a
workshop on March 16th, this spring before Sepp arrives. This will give us a bed with heated soil, providing excellent conditions for rooting cuttings. This is how I am going to propagate my favorite figs, among many many other things
Sepp uses the water to capture the
energy from the sun during the day. When the temperatures drop you can see the heat rising off of the lake. Pulling this warm humid air through the greenhouse and capturing that phase change energy without any electricity is a really interesting concept. All you would need is a psychos or solar chimney that heats up during the day to draw the air. I don't think this would provide any warmth in the dead of winter but it could provide some big benefits during the spring and fall. We might try something like this at Chad's when we build his greenhouse.