ASK THE EXPERTS: Solar Hot Water Pump Module

Beginner
March and Hartel DC circulation pumps, suitable for use with a PV-powered SWH system (pictured: a thin-film PV module).

I have a non-functional PV module that powers the pump of a 17-year-old still-functioning solar water heating system (manufactured by a company that is no longer in business). I’d like to replace the module, but how do I determine its wattage so I can buy a suitable replacement? The ink on the module’s label has faded, though the name looks like Solarex, which is also no longer in business. The module has a black plastic base and 13 5/8-inch square frame.

David W. Potter • Orange Grove, Texas

The module is probably 10 watts, which, back in the day, is in the range of power you could get out of a square foot of PV. You can count the number of cells to determine the voltage, but it’s likely a 36-cell module, which is for 12 V nominal applications. Any full-service retailer should be able to supply you with a new module.

But are you sure it’s bad, and that it’s not something else? After that much time, loose connections are likely, possibly anywhere between the module and where it hooks up (direct to pump or controller).

Michael Welch • Home Power senior editor

I concur with Michael’s advice, but my primary suspect would be the pump in the 17-year-old system you describe. If it is a pump motor with brushes, that is what I would check first, but wiring and connections could also be the problem. The PV module would be last on my list to check.

A 12 VDC pump can easily be checked with a 12 V battery. Connect it after ensuring the polarity is correct—red wire to positive; black to negative. Many pumps will just run backward with reverse polarity, but a few high-efficiency models made for solar water heating can be damaged if the wires are reversed. If the pump runs when connected to the battery, that eliminates the pump as the problem.

Most small SWH pumps are not economically repairable, with the exception of brush replacement on those types. Be cautious of replacing a failed pump with a product from the marine or RV industries. In a batteryless system, in which the module is connected directly to the pump, most of these pumps will have a limited life. Marine RV pumps are designed to be powered by an electrical system with a battery. Connected directly, PV voltages can often exceed the pump’s maximum voltage rating, causing damage.

Any replacement pump should be accompanied with a module recommendation. March, Milton Roy (Hartell), and El Sid have been making small DC pumps for the solar industry for decades. El Sid is a reliable pump also, but double the recommended PV module size if you’ll be pumping an antifreeze-based system. The bottom line is to buy a pump from a manufacturer that makes pumps for PV-direct systems.

Module testing is straightforward. You need a multimeter and a sunny day for the best results. Connect the two PV module’s wires to a multimeter that’s set for DC voltage to measure VOC (voltage, open-circuit). An old 12-volt crystalline module may read as little as 18 volts. An older thin-film module could read as little as 13 to 14 V.

In rare cases, a module can have a good VOC measurement but still not produce any power under a load. A much better test is to connect the module to a known, operational load (a small motor or light, tested with a battery) under good sunlight conditions.

Chuck Marken • Home Power solar thermal editor

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