[RE-wrenches] Multiple angle conundrum

[Howie Michaelson]

My understanding is that if all the modules in a string have the same tilt
and azimuth, the MPPT "hit" is nominal with 2 strings of differing angles.
 Particularly, if they have the same azimuth, their voltages will be close
to identical almost regardless of pitch, and therefore the current from
the different strings will be virtually cumulative and therefore not a
problem.  However if there is differential shading on strings functionally
being combined, then you have a cumulative problem.  In that case, that is
where separate MPPT algorithms (via multiple MPPT inputs in a single
inverter, or separate string inverters, or micro inverters/optimizers)
become useful and appropriate.


Others please chime in to correct me if needed.

Howie
-- 
Howie Michaelson
NABCEP Certified Solar PV Installer™

Catamount Solar, LLC
Renewable Energy Systems Sales and Service
VT Solar Electric & Hot Water Incentive Partner
http://www.CatamountSolar.com
802-272-0004


On Wed, April 4, 2012 12:38 pm, Kirpal Khalsa wrote:
> Hilton....there are a couple ways to approach this.....There are some
> string inverters with multiple MPPT inputs....each different slope of roof
> would be installed into its own input.....the other way is to use micro
> inverters or other individual module optimizers....in which case you are
> completely free from mismatched orientation and slope constraints.....
> If you are looking to use a single MPPT channel string inverter you may
> consider splitting the arrays and having two single string inverters one
> for each tilt angle....That said the tilt angle difference feeding the
> same
> inverter does not have as big of a negative impact as if the sub arrays
> are
> facing different directions....
> good luck,
> Kirpal Khalsa
>
> On Wed, Apr 4, 2012 at 9:14 AM, Hilton Dier III
> <hiltondier at gmail.com>wrote:
>
>>  Wrenches,
>>
>> I am talking with a potential client about a roof mounted PV array. The
>> client has a reasonably large south facing roof, but it is broken up by
>> a
>> large (18' W x 16' H) shed dormer. The roof is 10:12 and the dormer is
>> 6:12.
>> I designed a layout that gave the dormer some room to avoid shading. The
>> client was disappointed by how much wattage I could get on his roof. I
>> pointed out that installing modules in the shade of the dormer was a bad
>> idea.
>>
>> He is also talking to another installer. The other installer proposed
>> installing part of the array on the 10:12 roof and part on the 6:12
>> dormer.
>> The client had already asked me about that and I gave him a rudimentary
>> explanation of maximum power point tracking and the inefficiency of such
>> an
>> arrangement. He asked them about that and their guy said "No big deal."
>> He
>> is still considering the two-angle scenario.
>>
>> So, how much efficiency will he lose by going with two angles? It seems
>> like the ultimate MPPT design error to me, but I don't have a number for
>> him to make my case. I suppose it all comes down to cents per delivered
>> kWh
>> in the end, but two different roof angles seems the the wrong way to go.
>> I'd like to be able to give him a definitive idea of what he would be
>> sacrificing with that scenario.
>>
>> Side note: The other installer is using the new Sunpower E20 modules.
>> They
>> seem to be available only from the manufacturer. Anybody have a source?
>> Has
>> anybody used them yet? Do they live up to the hype?
>>
>> Many thanks,
>>
>> Hilton
>>
>> --
>> Hilton Dier III
>> Renewable Energy Design
>> Partner, Solar Gain LLC
>> 453 East Hill Rd.
>> Middlesex, VT 05602
>>
>>
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