This is an interesting solar water heater design, and includes some ideas you may be able to use.
It is a thermosyphon solar water heater. In this kind of design, the tank is located above the collector, and natural convection circulates the water. When the water in the collector is heated by the sun, it becomes less dense and rises up out of the collector and into the tank. This draws cool water from the tank to the low side of the collector, and a steady circulation develops.
When the collector cools to below the tank temperature, circulation stops because the cool water in the collector is stable where it is. No pumps or controls of any kind are needed.
The collector uses closely spaced PVC pipe risers. Each riser is encased in a fluorescent tube that has had its ends removed and the inside cleaned out.
A layer is glazing is used over the whole collector housing, so its essentially a double glazed collector.
Water systems in this part of the world typically use gravity feed from a tank located above the water using appliances, so the storage tank is not pressurized. A simple float valve on the tank intake pipe to the tank keeps the tank full of water. A gravity feed outlet pipe takes water to the shower when the shower valve is opened.
So, look how simple this all turns out to be:
- A simple, non-pressurized storage tank.
- A simple collector made from pipe.
- No pump
- No controller
- No heat exchanger
- No electrical power
You could probably make the whole thing for a couple hundred bucks.
A lot of what makes the system simple is the gravity feed water system that eliminates the need for a pressurized tank and the need for a heat exchanger to transfer heat to the pressurized tank. The fact that no freeze protection is needed also makes things simple.
So, maybe simple, gravity feed plumbing has its advantages.
There are things about the design that I think are right on the edge (if not over the edge):
- I think that the PVC pipe may prove to be a bad choice. PVC is not good for much more than 130F even when not pressurized, and I think that it may prove to be a bad choice over time. For an inexpensive plastic choice, PEX might work out better (if available).
But, Jake has done two things that improve the odds for the PVC.
He enclosed the manifolds (the T fittings) in foam insulation, so that they will see lower temperatures. This is a technique that might be used on other collectors to protect the manifold fittings from the full collector temperature. It would be best to insulate the side of the manifold toward the collector, with no or less insulation on the side of the manifold that faces out (I think).
He reduced the chance that the collector will be stagnated (no water flow) by choosing the thermosyphon design, and by making sure that the tank has an automatic mechanism to keep water in it.
Without a pump and controller, the chance that some kind of failure will result in stagnation of the collector is much less.
But, I still feel that collectors should be designed to withstand stagnation, because (I think) that at some point stagnation will happen no matter how careful you are to try to avoid it.
- I'm a bit skeptical that the fluorescent tubes are a big plus. It seems to me that aluminum fins on the tubes would be no more work, and would work better at getting the heat transferred into the PVC pipes. It would be nice to see a side by side comparison.
When I look at commercial solar water heating systems that cost upwards of $8000 in the US, I can't help but think that there are some good lessons to be learned from simple but effective systems like this. Quite a nice job!
Gary November 15, 2009