I am unfamiliar with any palm trees in the free air on the UW campus. Nonetheless, the overall idea needs work, as two feet down is not nearly far enough to collect geothermal 'heat'. Misting will cause mold and fungal issues. The thickness of the glazing is insufficient R- value to retain what tiny amount of heat is collected on the third day of overcast at 600 ft and rain where the temp hasn't changed from 41 degrees F either. Surely a greenhouse film that diffuses light, and has anti-condensation properties and reflects IR is needed for those purposes and to create an air gap to insulate. Energy input will be needed, either to heat water in tubes in the floor or electric cables or heater of some sort. And you'll have to dig down much farther than 6" for your base for cinder blocks - you'll want to go the library to get a book to understand how to lay a masonry floor/patio. The bottom line is that you will be paying to heat the structure. Insulating the ground and replacing the glazing and adding an air gap will help. But you'll get scant help from the sun, and you don't want to pay the price to do geothermal correctly. Dan

What about using SALT, that might solve your freezing problems, from my experience and the description of the system you should be fine. If you want the system to last from 5-10 years, you are going to have other problems before the salt corrodes anything. The copper should be fine, however I don't know what the coils are made of so that's the only things i might worry about. Pool pumps are typically plastic and most of them are designed for both fresh and salt water. Anti-freeze is evil! It sounds like you want to help the environment, so stay away from it. Do it the way nature does, There is lots of liquid water in the artic circles- because its salty.

The best way to heat your plant beds with warm water is to use 3/8" PEX pipe connected to a Noritz gas/lp instantaneous tankless water heater,circulate the water with a Taco pump using a Goldline thermostat control.Install the thermostat sensor in the plant bed soil.Make sure that the bottom of the beds are insulated with styrofoam and the top of the beds have sawdust or small wood chips.You can build yourself a passive water heater out of a couple of 50 gallon water heater tanks.Remove the tank shells and insulation from electric water heaters and install inside a concrete coffin crypt all painted with chassis black.The crypts can be mounted in the ground facing true south with the crypts tilted on angle to your geographical degree of latitude for the best solar exposure through the year.These passive solar heaters can preheat the Noritz heater.It is also very energy savings to build the greenhouse below the ground level for added thermal mass and insulation. Here is a link that might be useful: INDEKSOLAR POOL HEATING

Mike, in my post I said I did not use my lights to 'grow' plants, I said I use them to provide 'short night' response in my photoperiodic plants to postpone their bloom times. For that, I don't need 2K foot candles, I need 20 foot candles. For the purposes of determining the quantity of light your plant is actually getting you can't just spit out the wattage or lumen rating output of a bulb as rated on its package. For instance, a hundred watt, unshielded incandescent bulb, ten foot away from an object will only provide about 1.5 foot candles as measured at that objects surface. That isn't much, is it? So, you can't just say a lamp has X lumens and have that mean much of anything. That's the potential output of the bulb. What you are interested in are the readings at leaf level, measured in lux or light density and that is going to vary greatly depending on how far away the light source is, what design the reflectors are, angles, intensity at various points of the cone of the beam. Yadayadayada. And if you think you are actually doing much trying to supplement the natural light by hanging a few shop lamps from an overhead beam in a greenhouse with a flourescent tube in them............well, it's not going to hurt anything, but it probably has negligible actual benefit to the plant. So, if you are really needing to provide lighting by which a plant will actually grow.........then your supplemental growing light has to be quite close to the plant, or the output has to be considerably higher than you think if mounted from overhead beams. The only way to really tell is to meter it out at various points in a greenhouse. That's why I suggested the poster find out what intensity of illumination leafy veggies really take. If she/he can make do with natural light, it would save a lot of money, and I suspect they'll grow just fine without supplementation. That's why in all but special circumstances where high light levels are absolutely necessary for producing certain plants in a winter setting, most growers do not go the expense of installing supplemental lighting. It's expensive to install and it's expensive to run, unless you are just interested in starting a few seedlings or a very limited collection of conservatory plants.