March 01

Dear Solar Guy,

I am considering installing a solar-powered attic fan. I would
like a lot of flow, so I am considering using more PV panels.
Would doubling the number of panels double the air flow, and
if so, how should they be connected?

- Fannie Atticus

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Dear Joe,

The first thing that I would like to point out is that the
combination of continuous soffit vents and ridge vents is
a naturally solar-powered attic cooling system, and is quite
effective, especially when combined with a radiant barrier.
In terms of cost-effectiveness and reliability, it is hard to
beat.

If you really want a PV-powered attic fan, here are some things
to think about.

First, the air moving capacity of a fan generally doesn't
double when the input power doubles. So, hooking up two
solar panels to a fan will not yield twice the air flow.

Secondly, fans vary widely in their efficiency. Compare
fans and choose one that delivers a lot of flow (CFM) for
the number of watts it consumes. This will reduce the
cost of the PV panel that drives it.
 
Thirdly, the PV panel has a complex electrical characteristic,
and this interacts with the motor's complex electrical
characteristic. Difficult analysis is required to predict
how well the combination behaves at anything other than
at both devices' rated operating conditions. And, overall
efficiency will be the best at that rated operating point. 

So, I suggest the following methodology:

First, find the most eficient fan you can. Then, choose a PV
panel rated at the same voltage and current as the fan. This
will match the panel to the fan, so that the PV operates at
its most efficient point, and is not oversized (and unduly
expensive).

If you want more capacity, either use a larger fan and
size the PV array to it, or simply duplicate your original
design.

- The Solar Guy's friend