PV north facing in US [RE-wrenches]

Bill Brooks billb at endecon.com
Wed Jul 23 21:51:22 PDT 2003


Graham,

The thermal efficiency of a thermal collector goes to pot at low irradiance
depending if the ambient temperature is low so there is some difference to
the response of PV vs. Thermal at low irradiance. Off-angle irradiance would
have a greater effect on a thermal collector and I think that is born out in
the incident angle modifiers that are used in the performance models for
each.

To develop a valid performance model you have to take into account
irradiance, temperature, and incident angle modifier to get the answer. You
also must validate that information against a real world installation. There
are some very well validated models out there that do this stuff.

There are a lot more people that use various computer models without a full
understanding of how they work (e.g. PVWATTS) and end up with ridiculous
results. Garbage in is, in fact, garbage out.

Mounting collectors on the north side of a very low pitch roof would
admittedly be a last resort, but I can't say that I would never do it. I
have seen plenty of people install systems that perform below 75% of optimal
(like about half the systems in California if the truth were known). It does
not justify bad performance, but many installations have far worse
performance than expected for a variety of reasons. The pictures you posted
on your website are an excellent case in point.

The desire of that installer to get better performance by facing the panels
to the south had a much worse effect than laying them in the roof
orientation--but the installer clearly had no idea--that's the scary part
and why I will continue to teach basic PV courses to neophytes.

Bill.


-----Original Message-----
From: Graham Owen [mailto:graham at solarexpert.com]
Sent: Wednesday, July 23, 2003 5:41 PM
To: RE-wrenches at topica.com
Subject: RE: PV north facing in US [RE-wrenches]


Hi Doug, The street name was changed in Photoshop.

I would NEVER install a solar panel facing north in So Cal.  If you look
at the performance of solar water heaters on tract homes the water in
the north facing systems will be approximately the same as ambient, when
the south facing systems are HOT.  Maybe the photon absorption on PV
modules is completely different than thermal collectors, but as a solar
guy it seems wrong to install solar panels facing north.  Am I wrong?

Graham


-----Original Message-----
From: Doug Pratt [mailto:dmpratt at sbcglobal.net]
Sent: Wednesday, July 23, 2003 5:27 PM
To: RE-wrenches at topica.com
Subject: RE: PV north facing in US [RE-wrenches]


Is this wonderful example REALLY on "Total Eclipse Dr.", or did someone
Photoshop in that street sign?  Too cute... -Doug

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