Whereas you just plunk a photovoltaic solar panel on the roof and run some wires, with solar hot water, you have a number of decision points and then you need a roofer, plumber, electrician and controls installer. What makes it worse is that there is not one flavor of solar hot water. You have to consider water quality and climate as well as your lifestyle.
Water quality: Water with high dissolved solids or other contaminants can cause the impurities to plate out on the solar panel thus reducing its efficiency and eventually plugging it up. The solution is to use a secondary heat transfer fluid such as propylene glycol. One major decision point.
Climate: In cold climates, water pumped through the panels can freeze, so you either have to use propylene glycol or drain the panels. Where do you drain the panels? Not in your neighbor's yard!
Lifestyle: A simple solar panel often seen in less developed countries is a panel with a lump at the top. The tank is put on the roof. The problem is that this adds weight to the roof, so the size of the tank is limited. If your lifestyle involves long showers, this is not for you. In addition, in cold climates, the tank can freeze.
Where did that leave our contractor friend who wanted to build an ultra-efficient solar system for his large home? In heaven.
Peter bought two top-of-the-line glazed 4x8 foot solar panels and mounted them on his roof. He then ran insulated copper tube off the roof to an electric hot water heater fitted with a tube heat exchanger designed for solar hot water.
He then bought a high-efficiency 50W pump to handle the propylene glycol heat transfer fluid. (Propylene glycol is far less toxic than automotive anti-freeze, ethylene glycol, so leaks won't kill you!)
He installed temperature sensors at the top of the panel, the bottom of the tank and the top of the tank. For good measure, he put in a paddle wheel mass flow meter and a glycol pressure transducer.
Now what did he do? Peter talked with his friend who brought a Soft-I/O module and proceeded to hook everything up. Compared to the commercial solar hot water controllers that actually cost more than Soft-I/O, this new approach to solar hot water control ran rings around the older "delta-T" systems.
While it's true that solar hot water system are driven by the difference in temperature between the top of the solar panel on the roof and the bottom of the hot water tank, there is much more to do. One thing is to measure efficiency. Peter did that by multiplying mass flow (measured by the flow meter) by the difference in temperature going into and coming out of the heat exchanger. Once he had that, he converted KWhours to dollars and proudly displayed on his browser the "money pumped from the sky".
Sensors used for the Solar Hot Water System
· Top of panel temperature
· Bottom of main tank temperature
· Top of main tank temperature
· Bottom of kitchen tank temperature
· Glycol pressure
· Glycol mass flow rate
Actuators used for the Solar Hot Water System
· Main glycol pump drive
· Kitchen pump drive
All sensors and actuators were connected to pins on Soft-I/O.
Now, we have to confess, this application was done on a lark just to see how well Soft-I/O could tackle a fairly simple application and do it well. Peter put in his Soft-I/O system about one year ago. He has turned off his electric heat. Soft-I/O does an admirable job of wringing heat out of the sky.
A few months into the process, Peter realized that he could pump water from his solar tank into his kitchen water heater located at the other side of his house. He ran two Pex tubes, insulating them one from the other, and put in another small 50W pump. He added a couple more logic statements, and Soft-I/O took charge of pumping hot water from his primary solar tank to his kitchen electric water heater. After a short time, he turned off the electricity to his kitchen hot water heater.
This simple system may convince folks that solar hot water is worth tackling. Our advice is simple and self-serving: do solar right. Do it with Soft-I/O!