Sealing a Concrete Thermal Water Storage Tank
I am building a new off-grid house in the mountains of Colorado this coming spring. Climate is about 4 though it rarely gets below -5F. The electrical solar system is already in place (16KW) Canadian Solar 430 watt panels. The current plan is to insulate with R40 double stud walls and R60 roof. Hydronic heated pex tube floor with 4-6 inches of rigid foam under the slab. It is in a good sun area, but the house will not be passive designed. I don’t want to burn propane. My current plan is to use 17 30-tube evacuated solar collectors or a second electric array, to heat a 5-10,000 gallon insulated, buried, thermal storage tank. I want to keep the water at a high of around 180 degrees. All surplus power from the existing electric array will be relayed to a heating element in the thermal storage as well (if it isn’t already up to temp). The current problem is to build the thermal storage tank AND be cost effective. I can’t fit a $30K+ fiberglass thermal tank into the budget. Because of thermal cycling many have said concrete tanks will crack over time. I can’t find any EPDM etc. liners that are rated for 200F+ degree that don’t also say intermittent and that the manufacturers won’t guarantee for more than 5 years at 180 degree temps. If I do a concrete tank and then put 1/8 to 1/4 inch of RTV high temp silicone (troweled onto the entire inside) would that hold up and/or if any cracks do get big enough to separate the silicone can I drain the tank and fill the cracks with more of the same. The engineering numbers seem to work with everything else, but I can’t find a cost effective thermal storage tank solution as of yet. Yes, I have looked at sand and a few other solutions, but have determined a water filled tank buried just outside the house will be the most effective solution whether it is heated by a second electric solar array or evacuated tube panels. I am open to other solutions, one guy even told me if I heat the floor during the day I should be good, but he didn’t have any numbers to prove it. As a backup heat system I am either going to run an Arctic air-to-water heat pump or at worst a high efficiency on demand propane boiler. Any ideas or help with thermal storage will be greatly appreciated. Well any ideas on any of it are welcome!
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