Go Solar for Free Hot Water
(Page 4 of 6)
February/March 2007
By Bob Ramlow
If the circulating pump is DC-powered, it’s wired directly to a small photovoltaic (PV) panel mounted outside. Whenever the sun shines on the PV panel, electricity flows to the circulating pump, and it starts running. When the sun stops shining, the pump turns off. DC-powered systems are becoming the most popular kind of closed-loop antifreeze system on the market. The PV supplies free power to the pump; the pump runs whenever the sun is out, preventing stagnation and overheating; and it runs at variable speeds depending on the amount of sunlight, so it automatically matches the collector’s heat output. These pumps have limited power, but it’s sufficient to move the fluid through a piping system that’s always full. The absence of a pump controller, the component most prone to failure, makes PV-powered systems quite reliable.
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Drainback systems are popular in moderate and hot climates. They’re similar to closed-loop antifreeze systems, but the big difference is that they include an additional tank, the drainback tank. When the system is off, the fluid is stored in the drainback tank and the pipes and collectors are empty. The fluid can be pure water or a weak solution of antifreeze in water. These fluids don’t resist freezing as well as those used in closed-loop antifreeze systems, which makes drainback systems vulnerable to freezing in extreme or extended cold.
Drainback systems usually use a controller and an AC pump. The pump is a special high-head pump because it has to lift the fluid to fill the piping and collectors every time the system turns on. This requires more power than in antifreeze systems, where the pump just circulates fluid through piping that’s always full. Otherwise, drainback systems operate similarly to an antifreeze system: When the sun warms the collectors, the high-head pump comes on and circulates the fluid into and throughout the system, and a heat exchanger transfers the heat from the fluid into the storage tank. There is no expansion tank and fewer valves and gauges than in an antifreeze system, but the piping must be carefully installed to facilitate proper drainage.
Additional Considerations
A few other factors will influence your choice of a solar water heater. If you find a system that is substantially cheaper than others, there’s probably a reason, such as lower quality components. There is no substitute for quality — don’t take shortcuts with an investment like this.
A solar water-heating system often can be expanded to include space heating as well. The collector array and other components will need to be larger. There are numerous options for storing and distributing heat in combination systems. Discuss your ideas with a solar professional (see Solar Sources, below).
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