ASK THE EXPERTS: PV-Powered Fans

Intermediate

I’d like to use DC fans powered by photovoltaic modules to circulate air in my greenhouse. I favor the high-volume axial fans, like those used to cool desktop computers. I realize that these are too small, but I’m trying to understand the basics. If I know the motor’s volt and amp specs, how do I determine what solar-electric module to buy? Take me to school, please.

Norman Anderson • via e-mail

PV-powered fans have been used successfully to exhaust greenhouses and sunrooms since the 1980s. They are quite appropriate because they vary in speed with the intensity of the sun, moving the most air when there is the most need to remove heat. Solar attic fans are commercially available and also may be applicable to this situation. Some manufacturers, like Solar National Air Propulsion (www.snap-fan.com), make solar fans that are suitable for wall installation

Electrically, a solar fan consists of nothing more than a PV module, a fan with a DC motor, sometimes an on/off switch (usually a thermostat), and some wire. There is no need for a current booster, other electronics, or batteries to help it start, because there is no load at startup except slight friction.

Greenhouse designers choose fans based on their air-moving capacity, rated in cubic feet per minute (cfm) relative to the loads, and the space’s size and the solar gain. To make an informed decision, you should estimate your cfm requirements—you may be able to find guidelines from a greenhouse supplier. Of course, you can use more than one fan or larger sizes to add capacity. Try one perhaps, and add more if you need to.

For low-cost DIY project, you can try a salvaged or surplus fan or blower. A fan is usually best (blowers are optimum where more pressure is required). I suggest you only use a conventional brush-type motor, rather than a high-tech brushless motor (like a computer fan). Brushless types use electronic inverter circuits that may not handle the wide voltage range of a nonbattery PV source. These may not work in diffuse sun, and may fail in bright sun when the voltage climbs to 17 V or more. The old brush-type motors are more forgiving. If the motor has a long cylindrical shape, it is usually brush-type. Short the two wires together and spin it by hand. If shorting makes it harder to turn, it’s brush-type. Although motor brushes eventually wear out, I’ve seen them last as long as 10 years.

It’s simple to select a PV module for a fan. Determine the current that the fan will require. You may need to measure it yourself, using a 12 V battery and a multimeter capable of 10 A. A 12 V PV module will apply voltage of at least 16 V. That causes the fan to draw about 20% more current than it draws at 12 V. To start and run the fan early and late in the day, the PV module needs to be rated for yet another 20%. If you get a module rated for at least 50% greater current than the 12 V draw of the fan, it will work well. Consult a low-voltage wire size chart to determine the minimum wire size required.

You will probably want a thermostat in the system (unless there is already one built-in to a ready-made solar fan). It will prevent the fan from coming on when the sun is bright, but the greenhouse is cold. I have had good results with a “line-voltage thermostat,” designed to switch 120 VAC at several amps. It has no trouble handling a few amps of DC current at low voltage. It can be mounted on the wall or elsewhere in the greenhouse. These thermostats are typically available from electrical or heating and air-conditioning suppliers. Ask for Grainger 2E158 or similar.

For a small greenhouse or sunspace of 300 square feet (floor area), your material cost may be about $500—or much less if you are resourceful.

Windy Dankoff, founder (retired), Dankoff Solar Products (now Conergy, Inc.) • Sante Fe, New Mexico

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