INTRODUCTION
I was always interested in solar power, but for twenty-eight years we lived in a swamp surrounded by sun-blocking trees. Here in Prince Edward County we are completely exposed to wind and sunshine. The sight of all that energy going unused while we paid for propane bothered me. With my usual, unwarranted optimism I set out to do something about it.
The smartest thing I did was go to www.builditsolar.com There's a lot of experience there. The dumbest thing that I did was to think big. With all that land and all that sunlight, surely a large array was what I wanted. With a large array I could help heat the house as well as produce hot water. All true, but there were problems.
The best location for a large array is the back of our property (30 metres behind the house). In that area there are no trees, buildings or other sun-blocking inconveniences to interfere with free energy. Thirty metres is not an impossible hot-water run, but bedrock is just a foot down (no apology for mixed measurement systems. It's the way my head works). To get the panel-to-house run below the frost line would require blasting. A big storage tank beside the panels with water drawn to a smaller tank in the house only when needed seemed reasonable. The long-run pipes could be empty and safe from freezing when not used.
The simplest system for cold climates is a drain-back system. When the pump turns off (no Sun) the water in the panel drains down into the storage tank; the panel is safe from freezing. To drain completely, the surface of the water in the tank must be lower than the bottom of the panels. The options were to raise the panels (not happening in this wind-whipped area) or lower the storage tank (also not happening. See "bedrock" above). What I needed was a way to have the panels at ground level with the large storage tank behind them at the same level.
I came up with what I thought was a simple solution. The supply and return lines between the storage tank and the panel would go to the top of the panel. As usual, the water in those lines would drain back into the tank when the water pump turned off. The change was that the water in the panel risers would drain to a reservoir at the bottom of the panel. That reservoir would have enough volume to hold all of the water in the panel risers plus an air space. That way the water would have room to expand when it froze. The panel tubes wouldn't rupture.
That was two years ago. I now have a test system in operation. It consists of 2 panels and a storage tank for preheating the cold water supply to the propane hot-water heater.
Good news: lots of hot water, and the first panel has been frozen, by accident, several times without damage.
Bad news: it's more complicated than I expected.
Embarrassing news: the test setup has the panels beside the house and the tank in the basement where (yes) it's well below the bottom of the panels. A simple drain-back system could have been used.
THE RESERVOIR-IN-PANEL SYSTEM
RIP - the Reservoir in Panel System
The left side of the diagram above shows a drain-back system. When the pump turns off, the water in the panel drains back into the storage tank. The panel is then safe from damage if the temperature drops below freezing.
The right side of the diagram shows the reservoir-in-panel system. When the pump turns off, the water in the supply line drains back to the tank as before. The return line also drains back, but in this case a large ABS plastic pipe is used to ensure that as water drains to the tank, air can travel in the reverse direction to allow the water in the risers to drop into the reservoir. The other difference is that the supply line goes to the top of the panel. The water then drops to the reservoir through a large, copper pipe. The large pipe ensures that, when the pump turns off, the water in that pipe travels one way (dropping down to the reservoir) while the air in the pipe travels up, allowing the supply line water to drain back to the tank and the water in the risers to drop to the reservoir.
Both of the diagrams are simplified. Slightly, in the case of the drain-back system. Considerably, in the case of the reservoir-in-panel. Before getting into agonizing detail, let's have a look at the real thing.
The right side of the diagram shows the reservoir-in-panel system. When the pump turns off, the water in the supply line drains back to the tank as before. The return line also drains back, but in this case a large ABS plastic pipe is used to ensure that as water drains to the tank, air can travel in the reverse direction to allow the water in the risers to drop into the reservoir. The other difference is that the supply line goes to the top of the panel. The water then drops to the reservoir through a large, copper pipe. The large pipe ensures that, when the pump turns off, the water in that pipe travels one way (dropping down to the reservoir) while the air in the pipe travels up, allowing the supply line water to drain back to the tank and the water in the risers to drop to the reservoir.
Both of the diagrams are simplified. Slightly, in the case of the drain-back system. Considerably, in the case of the reservoir-in-panel. Before getting into agonizing detail, let's have a look at the real thing.
THE FINISHED SYSTEM
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The tank (before adding the control system)
Control system (don't panic. It doesn't need to be this complicated)
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The panels in place
A clear day in September. The water pump doesn't turn on until the panel temperature reaches 40C above the ambient temperature. It turns off when the returning water drops below the temperature of the tank. The two steps at 17:00 and 21:00 show the result of removing 30 litres of water from a hot water faucet.
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PANEL CONSTRUCTION DETAILS
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From top left to bottom right:
-the tube support. 3/4" (19mm) plywood with grooves routed to accept the down tube and risers.
-shaping the first layer of aluminum for the risers (the vise thread showed signs of distress when finished).
-shaping the first layer of aluminum for the down tube. Ignore the colour change. I ran out of beige.
-the first aluminum layer stapled into place.
-perforating the reservoir tube to accept the risers.
-the perforations give a large surface area for soldering the risers. The joint is strong.
-soldering. Preforms were made by flattening and rolling solder.
-screwing the second layer of aluminum into place. The aluminum comes in 24" (61 cm) widths.
-the finished inner panel.
-insulating the box with polyisocyanurate insulation. 50mm on the back. 25mm on the sides.
-the outer polycarbonate window being installed into its frame. The inner window has no outer frame.
-the panel on its cradle. The cradle is on a trailer for transport.
-the tube support. 3/4" (19mm) plywood with grooves routed to accept the down tube and risers.
-shaping the first layer of aluminum for the risers (the vise thread showed signs of distress when finished).
-shaping the first layer of aluminum for the down tube. Ignore the colour change. I ran out of beige.
-the first aluminum layer stapled into place.
-perforating the reservoir tube to accept the risers.
-the perforations give a large surface area for soldering the risers. The joint is strong.
-soldering. Preforms were made by flattening and rolling solder.
-screwing the second layer of aluminum into place. The aluminum comes in 24" (61 cm) widths.
-the finished inner panel.
-insulating the box with polyisocyanurate insulation. 50mm on the back. 25mm on the sides.
-the outer polycarbonate window being installed into its frame. The inner window has no outer frame.
-the panel on its cradle. The cradle is on a trailer for transport.
Update 20161221
Both panels have been working now for months. That's the good news. The bad news is the same as it was for the last two years. During November and December we live under constant cloud. The fanciest solar system is useless under that condition. There are larger, industrial solar systems here in The County. Misery loves company.
Shown above: changing the panels from the summer to winder inclinations. Extender pipes are added to the supply and return lines to adjust for the different run lengths.
Both panels have been working now for months. That's the good news. The bad news is the same as it was for the last two years. During November and December we live under constant cloud. The fanciest solar system is useless under that condition. There are larger, industrial solar systems here in The County. Misery loves company.
Shown above: changing the panels from the summer to winder inclinations. Extender pipes are added to the supply and return lines to adjust for the different run lengths.
