[RE-wrenches] best way to connect 12v fans directly to panels with a higher voltage?

Fri Jan 20 14:01:57 PST 2012

Hi James,

I second Jay's SNAP-Fan recommendation. They are the highest quality DC fans that I've seen. Here's their website:

http://www.snap-fan.com/index.html

You might be able to use those Grainer fans on direct PV, but you will have to find out what the fan's motor specs are. Many "12VDC" motors can't handle voltages above about 15VDC, making them unsuitable for direct PV applications. If that is the case, you will have to add batteries (and a charge controller) to the system. You will also need to control the fans with a thermostat or timer; otherwise the fans will just run constantly until the battery is drained. This adds cost and complexity to what could otherwise be a very simple set-up.

The motors on the SNAP-Fans are rated up to 40VDC, so they can run PV-direct on 12V nominal (36-cell) or 24V nominal (72-cell) modules. At 24VDC nominal (operating at about 35VDC) they will draw almost three times the amperage (meaning 5-6 times the wattage), and develop almost twice the RPM as they do on a 12V nominal module (operating at about 17.5VDC). They will also move 2-3 times the CFM at 24V nominal as they do at 12V nominal.

You can also run the SNAP-Fans with 60-cell modules of course, and that will likely be the cheapest way to go. The fans will operate at around 30VDC on a typical 60-cell module. I'm not familiar with the ASW modules, but as long as they have 72 or fewer cells, you'll be fine with the SNAP-Fans

See the installation manual on the SNAP-Fan website for some great operating charts, and how to wire multiple fans in series-parallel arrangements with PV (you can wire two 12VDC fans in series and power them at 24V and the fans will run at their 12V speed). Unfortunately, the charts only show performance at 12V, 18V, and 24V, and not at 30 or 35V, but I'll bet the manufacturer has those figures too.

Of course, the faster you run the fans, the shorter the brush life will be, so take that into consideration.

One more point about running motors PV-direct: the larger the PV array is, the more hours per day the motor will run at full speed. A motor that draws 100W, powered by a 100W module, will only run at full speed at noon each day. However, a 200W module will develop 100W in half the amount of sunlight, so the fan will run at full speed for many more hours per day, and even on cloudy days. A 100W motor will draw only 100W even with a 1000W PV array, but will run at full speed in 10% sunlight (this is a little oversimplified, as a 100W motor would barely load down a 1kW PV array, so the motor will operate at a higher voltage, drawing more power).

As far as "Code compliant" hookup, that should be easy...or hard. I doubt that any of the DC fans are Listed, so that's a problem right there. Otherwise just use good wiring methods and you'll be fine. If you size the wire to 156% of the max PV amps, you don't even need overcurrent protection, although you should have a PV disconnect. I'd use a MidNite Big Baby box and 150VDC breakers.

Brian Teitelbaum

AEE Solar

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> Hi James

I suggest you use Snap Fans. These are DC fans. And super efficient. They come in a few sizes/CFM rates.

Check out Greenwired.net they carry them.

Jay

Peltz Power

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of James Rudolph
Sent: Friday, January 20, 2012 8:03 AM
To: RE-wrenches
Subject: [RE-wrenches] best way to connect 12v fans directly to panels with a higher voltage?

Dearest Wrenchies,

So here is the situation,  we have a customer with a passively heated kiln for drying lumber. Basically it is a greenhouse. There is no supplemental heat, or power at the site where it is. When it begins to be heated moisture will be driven from the lumber, which needs to be exhausted by fans. We need PV because there is no power there, and because we want to be completely solar. Rather than set up a charger controller and batteries, along with humidity controls for the fans, I thought it would be simplest to get some panels and wire them directly to the fans. That way, when the sun is shining and the kiln is being heated, the fans will be on to exhaust the moisture, and when the sun is not they are off. With that said the customer has alredy bought the 4,12v/18a  fans from grainger and has some ASW 240w 37V panels. I was wondering( since my work is always grid-tie applications) what would be the best code compliant way to hook up all this together.
Many thanks in advance for the help.

Sunny Regards,

James B Rudolph
NABCEP Certified PV Installer
Certified Californian Journeyman

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