[Greenbuilding] Greenbuilding Digest, Vol 11, Issue 20

[Keith Winston]

From: RT <Archilogic at yahoo.ca>


> I would venture if one were to think about Keith's statement, (horizontal
> pipe oriented with long axis oriented East-West) his question would answer
> itself.
>
>
As mentioned in my response to Reuben's post the other day, the panels
> are attached to spars (4 panels per spar, not two as I had mentioned
> earlier in my response to Reuben's message) mounted perpendicular to the
> long axis of the pipe.
>
>
Alas, I read this list in gmail and digest. On the day of this post there
were so many posts that some got cut off and, as far as I can tell, thrown
away by gmail (actually, I think it's the Greenbuilding listserve). So I
couldn't see the original posting, and was going by the abbreviated
description others' proffered. And I also mis-spoke, a single-axis tracker
is typically a N-S axis (see the exception below). However, your description
(below) of E-W tracking is incorrect: it doesn't sweep across, like N-S
tracking, but rather rises and falls back to  the same position, depending
on time of year. That's what happens, if you think about it, on the second
axis of a 2-axis tracker. And in fact, a sweep would keep a 2-axis tracker
from working. The point in both cases is tracking, after all.

When I look at the solar radiation redbook data for MD (
http://rredc.nrel.gov/solar/pubs/redbook/), I see yearly totals for fixed
and single (with lattitude tilt), and two-axis tracking of 4.6, 5.9, and 6.0
kWh/m^2/d. That is an increase of 28% for single axis and 30% for two-axis.
Modules don't do as well with oblique light, which makes me wonder that
there isn't a bigger difference between the 1 & 2 axis tracking. But I'm
still surprised at the 44% you quote. An interesting experiment would be to
select a few sunny days in a row, and alternately track and fix the system
in the middle of it's range on alternate days. That  would give fully
comparable numbers: same system, same location, same tilt, same time of
year, with enough repetitions to confirm results. Now that's science.
Comparing different systems, especially (if I  understood you) one utilizes
microinverters and the other a central inverter system (and perhaps
different PV modules?), might be hard to control for error.

If the long pipe were in fact oriented East-West and if the panels were
> laid flat (ie no seasonal tilt), in the morning the panels would be aimed
> at the Southern horizon. Then as the pipe rotates, after a ~90 degree
> rotation, the panels would be aimed straight up, perpendicular to the
> Earth and then after a further 90 degree rotation of the pipe, aimed at
> the Northern horizon... which of course, makes no sense.
>
> WRT Keith's comment about Kanata being at "those far north latitudes" ...
> my and my neighbour's (rural) Kanata location is at ~ 48.38 degrees North
> Latitude, practically within (but not quite)the Banana Belt of Canada,
> hardly the "Far North".  "North of 60" is probably what most MooseHuggers
> would consider "Far North".
>

Yes, Canadians might have different definitions of North from 'Murricans,
even though much of Canada is South of significant fractions of the US. But
in any case, I didn't say Canada was at "far north latitudes": I simply said
that some of the rules shift as you get to far north latitudes: I didn't
even mention Canada there.

In case it interests anyone: tracking trough solar thermal collectors, which
have a parabolic mirrored surface reflecting up to a solar thermal collector
(a pipe of liquid typically), when aligned on an E-W axis, can be tracked
seasonally instead of daily, with the trough essentially functioning as a
N-S-axis tracker. Some loss in efficiency but gain in simplicity. I've seen
a similar design for tracking concentrating PV.

Keith
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