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Revised 4/23/08

Why solar power and not AC to run your circulation pump?
To quote Chuck Marken's Article "Pick the Right Pump: Choosing a Circulator for Solar Hot Water Systems" in Home Power magazine (issue #121):

"One way to approach the DC and AC pump choice is to examine relative system efficiencies. The efficiency of some heating systems is rated by the relationship of the amount of energy output to the energy input. If you have a system that produces a certain amount of heat with half the equivalent electrical input, the “coefficient of performance” (COP) is 2. Produce four times as much hot water as the amount of energy input from electricity and the COP is 4. We can use this same methodology in evaluating the efficiency of SHW pumps. Using a utility-powered AC pump for your solar water heating system will give you a COP between 12 and 25, and this is an excellent value compared to electric water heaters, which have a COP of 1.
But the COP will never be as good as a DC PV-powered SHW system. DC hot water circulation pumps can have a higher COP than AC pumps because there is no traditional energy input if a PV module powers the system. If you use a solar-electric module to power the pump, your COP is infinite—you’re not adding any input energy. The sun provides it all, and you get something for nothing after the initial investment. PV-powered systems are also immune to utility outages. This is a big plus with antifreeze systems, since the collectors can overheat on sunny days if the pump stops operating due to a power failure. An overheated collector can actuate the pressure-relief valve, which will make it necessary to recharge the system with antifreeze solution. In some cases, the overheating can be so severe that the antifreeze solution will be compromised to the point of needing replacement.