PV's on tower [RE-wrenches]

[Kelly Keilwitz]

Wrenches,

I'm designing an interesting off-grid system where the PV array will be
mounted on the
same free-standing lattice tower (160-ft) used for the wind turbine. The 2
to 3 KW
array will start at 50 feet due to tree shading and go up. Mounting the
array flush to the tower face (~3 deg from vert) allows the array to be the
most compact, minimizing the
overturning moment on the tower due to wind loading on the array. It also
simplifies the array mounting. However, summer PV production is considerably
reduced due to the low angle of incidence of the sun on the array.
Unfortunately the summer is the worst time for wind power, so we
need all the PV we can get.

The site latitude is 48 deg. Optimum angles are considered to be 33 deg for
summer, 48 deg for fall and 63 deg for winter. The differences in actual
panel orientation from optimum, if mounted flush with the tower, are 54 deg
for summer, 39 deg for fall, and 26 deg for winter. For these deviation
angles efficiencies of 65% for summer (May-Aug), 80% for Fall (Aug-Nov), and
95% for Winter (Nov-Feb) were used (these periods were chosen because of
changes in wind production during the periods). The efficiencies used were
interpolated from a figure in a dated SEI PV notebook.

Using the efficiencies above, we will not have enough PV production
during the summer without using a bigger array, tilting the modules or using
a generator to supplement the production. Tilting the modules to 63 deg from
horiz changes the angle deviation to 30 deg from optimum. For that deviation
an efficiency of 90% of full production was used, which provides sufficient
summer production. However, in order to prevent shading from successive
modules as they run up the tower (mounted in landscape orientation) the
vertical seperation between modules must be 26" to 29", depending on the
module. This will increase the height of the array more than 36
feet, which would mean that the top of the array would be around 135 feet on
the tower, instead of 99 feet. Obviously this would present a tremendous
increase in
overturning moment for the tower and is, in my view, impractical.

The efficiencies of the modules at an angle to the insolation is critical in
this design. The modules in consideration include Shell SQ160, Sharp 185,
and Sanyo 180HIT. Any feedback or suggestions concerning the PV efficiencies
at an angle, or this system design will be appreciated (other than
suggesting to cut the trees down). I'd especially like any feedback on the
Sanyo modules (durability, increased production at low angles of incidence,
mounting, etc) as I've never seen one.

Thanks,
-Kelly

Kelly Keilwitz, P.E.
Whidbey Sun & Wind
Renewable Energy Systems
986 Wanamaker Rd, Coupeville, WA 98239
ph & fax: 360-678-7131
sunwind at whidbeysunwind.com
www.whidbeysunwind.com
Renewable Energy Now!

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