[RE-wrenches] 14 PV-Powered Fans- best method?

[Mick Abraham]

Did Nextek Power take over production of the DC ceiling fans from Foster
Hankins of Rocky Creek Hydro? One might assume, because those are the same
product with similar product names.

Addressing Dave's question: If each fan is fed through a breaker, the energy
through that breaker would be determined by how the load behaves on your
supplied DC voltage. If one of the fans were to drop out so it's no longer
loading the solar array, the array would tend to operate at a higher voltage
than before...and yes, the other fans would then spin faster and draw more
amperage. I can see how Dave's cascading breaker interrupt scenario could
develop...but this should be easy to avoid.

Suggestions:

(a) Make sure that your open circuit PV voltage is not too high for even a
solitary fan to run OK. This is because the solar module may operate close
to OCV if it is big relative to the load.

(b) Hook up that solitary fan to the module and measure the amp draw before
you choose the value for the circuit breakers. Choose the breaker rating a
bit higher than measured current, of course, so you don't get nuisance
trip-outs.

Final note: Since a solar array is current limited by its very nature, the
circuit breakers may be acting more like on-off switches instead of
overcurrent devices. As you do the math you could find that a single fan
motor with its feed wires could handle the short circuit PV amps forever
without heating up.

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675


On Mon, Sep 14, 2009 at 8:11 AM, Dave Click <daveclick at fsec.ucf.edu> wrote:

> Hi all,
>
> I'm working on a project for a state park, where they're looking to add
> some PV-powered fans to a outdoor bus stop. The customer is hoping to have
> the fans spin more quickly as irradiance increases, and they want a master
> cutoff switch. I'm initially planning to use 2 10W modules (series to 24V)
> powering each of 14 Nextek (http://www.nextekpower.com/) DC-powered
> Gossamer fans, since that seemed to be the easiest way.
>
> However, I'd like something that looks better than 30 of those small
> modules (best 10W module I found that can do 24V is series-connected Sunwize
> modules with semicircle cells) up there on the roof. I was thinking about
> having a 24V module or two up there that backfeeds the output of a 14-pole
> source circuit combiner, with each pole a 1A breaker feeding a fan. Then I
> got concerned that if a breaker or two ever tripped (hopefully each fan
> presents an equal load to the PV, but still), the remaining breakers would
> trip as it tried to feed 200W through fewer and fewer loads.
>
> Any recommended method for implementing this, and for installing the cutoff
> switch in the first scenario? For the second scenario, I'd probably just use
> an unfused safety switch on the PV output.
>
> Thanks,
> Dave
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