How much does a batch solar heater cost? That’s a question I can answer, because I built one for use in our cheese facility.
I started with a tank from a doscarded 30-gallon propane hot water heater. Freecycle is great for stuff like this.
I stripped off the exterior metal and the insulation, wire brushed the residue, and spray painted the tank flat black.
The cost of the tank so far: $0.
Then I built a frame for the tank.
Since the most important element of the frame is the glazing, I designed the frame around a discarded window. Last year we upgraded our windows, and I kept the old ones anticipating that I would build a greenhouse– something I haven’t gotten to yet. So I chose a 48″ x 59″ double-paned window. That was a little large for what I wanted, but I calculated (remember the Pythagorean Theorem?) that if I set the window at a 45-degree angle, the interior width of the frame would be 33″, perfect for the tank.
I built the frame out of 10 2×3 and 2 sheets of OSB. Cost: about $22. You can see from the photo that I’m no framer, so if I can build this, anyone can. The most important thing was to leave a flat edge on four sides for the window to rest on, so I double-checked (and adjusted) this opening along the way.
The plan was to leave the large side open and position it against the south wall of the building. The other three walls would be painted to seal the OSB, insulated, and covered with OSB on the inside as well. I would put a layer of clean gravel in the bottom as a heat sink, then position the tank on top of the gravel, just under the glass of the window. Then I would use pex pipe and fittings to connect the tank to the existing hot water inlet. The cost of the fittings was about $20, bringing the total cost of the unit to $42.
Our plans changed when we decided to buy more equipment, and we decided to go with a much larger, active solar heating system. That should give us enough free hot water to run the pasteurizer.
We never installed the solar preheater, so I can’t provide actual cost savings. But as a wild guess, I expected it to reduce our propane usage for water heating by 30%. That would have saved us about $50 per year. Not bad for a $42 unit!
Though batch preheating ended up not being right for our situation, this kind of unit represents a simple, cheap way to harness the power of the sun, reduce our use of fossil fuels, and save money too.
A batch heater may be right for you if you get a good percentage of sunny days, and if your hot water heater is located on a south-facing wall. Especially in cold climates, it doesn’t make much sense to run your preheated water through yard of pipe in chilly weather.
