Howie is absolutely right. Strings with different orientations on the same MPPT are not a problem. Modules with different orientations on the same string is a big problem. Search the archives here and you'll find a good discussion on this. The short version is that voltage is a function of temp, which won't change much based on orientation and the MPPT will favor the strings with better orientation at any given time because they're making more power.<br>
<br>We have a system with 6 strings on each of two opposite facing roofs. One Sunny Boy has four strings from one side, one has four from the other and a third has two from each side. The one with two from each side produces the exact average of the other two. The theory works!<br clear="all">
<br>Dana<br><br>Dana Brandt<br>Ecotech Energy Systems, LLC<br><a href="http://www.ecotechenergy.com">www.ecotechenergy.com</a><br><a href="mailto:dana@ecotechenergy.com">dana@ecotechenergy.com</a><br>360.510.0433<br>
<br><br><div class="gmail_quote">On Wed, Apr 4, 2012 at 11:39 AM, Howie Michaelson <span dir="ltr"><<a href="mailto:howie@suncatchervt.com">howie@suncatchervt.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
My understanding is that if all the modules in a string have the same tilt<br>
and azimuth, the MPPT "hit" is nominal with 2 strings of differing angles.<br>
Particularly, if they have the same azimuth, their voltages will be close<br>
to identical almost regardless of pitch, and therefore the current from<br>
the different strings will be virtually cumulative and therefore not a<br>
problem. However if there is differential shading on strings functionally<br>
being combined, then you have a cumulative problem. In that case, that is<br>
where separate MPPT algorithms (via multiple MPPT inputs in a single<br>
inverter, or separate string inverters, or micro inverters/optimizers)<br>
become useful and appropriate.<br>
<br>
<br>
Others please chime in to correct me if needed.<br>
<span class="HOEnZb"><font color="#888888"><br>
Howie<br>
--<br>
Howie Michaelson<br>
NABCEP Certified Solar PV Installer<br>
<br>
Catamount Solar, LLC<br>
Renewable Energy Systems Sales and Service<br>
VT Solar Electric & Hot Water Incentive Partner<br>
<a href="http://www.CatamountSolar.com" target="_blank">http://www.CatamountSolar.com</a><br>
<a href="tel:802-272-0004" value="+18022720004">802-272-0004</a><br>
</font></span><div class="HOEnZb"><div class="h5"><br>
<br>
On Wed, April 4, 2012 12:38 pm, Kirpal Khalsa wrote:<br>
> Hilton....there are a couple ways to approach this.....There are some<br>
> string inverters with multiple MPPT inputs....each different slope of roof<br>
> would be installed into its own input.....the other way is to use micro<br>
> inverters or other individual module optimizers....in which case you are<br>
> completely free from mismatched orientation and slope constraints.....<br>
> If you are looking to use a single MPPT channel string inverter you may<br>
> consider splitting the arrays and having two single string inverters one<br>
> for each tilt angle....That said the tilt angle difference feeding the<br>
> same<br>
> inverter does not have as big of a negative impact as if the sub arrays<br>
> are<br>
> facing different directions....<br>
> good luck,<br>
> Kirpal Khalsa<br>
><br>
> On Wed, Apr 4, 2012 at 9:14 AM, Hilton Dier III<br>
> <<a href="mailto:hiltondier@gmail.com">hiltondier@gmail.com</a>>wrote:<br>
><br>
>> Wrenches,<br>
>><br>
>> I am talking with a potential client about a roof mounted PV array. The<br>
>> client has a reasonably large south facing roof, but it is broken up by<br>
>> a<br>
>> large (18' W x 16' H) shed dormer. The roof is 10:12 and the dormer is<br>
>> 6:12.<br>
>> I designed a layout that gave the dormer some room to avoid shading. The<br>
>> client was disappointed by how much wattage I could get on his roof. I<br>
>> pointed out that installing modules in the shade of the dormer was a bad<br>
>> idea.<br>
>><br>
>> He is also talking to another installer. The other installer proposed<br>
>> installing part of the array on the 10:12 roof and part on the 6:12<br>
>> dormer.<br>
>> The client had already asked me about that and I gave him a rudimentary<br>
>> explanation of maximum power point tracking and the inefficiency of such<br>
>> an<br>
>> arrangement. He asked them about that and their guy said "No big deal."<br>
>> He<br>
>> is still considering the two-angle scenario.<br>
>><br>
>> So, how much efficiency will he lose by going with two angles? It seems<br>
>> like the ultimate MPPT design error to me, but I don't have a number for<br>
>> him to make my case. I suppose it all comes down to cents per delivered<br>
>> kWh<br>
>> in the end, but two different roof angles seems the the wrong way to go.<br>
>> I'd like to be able to give him a definitive idea of what he would be<br>
>> sacrificing with that scenario.<br>
>><br>
>> Side note: The other installer is using the new Sunpower E20 modules.<br>
>> They<br>
>> seem to be available only from the manufacturer. Anybody have a source?<br>
>> Has<br>
>> anybody used them yet? Do they live up to the hype?<br>
>><br>
>> Many thanks,<br>
>><br>
>> Hilton<br>
>><br>
>> --<br>
>> Hilton Dier III<br>
>> Renewable Energy Design<br>
>> Partner, Solar Gain LLC<br>
>> 453 East Hill Rd.<br>
>> Middlesex, VT 05602<br>
>><br>
>><br>
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