Derat Factor for big commericals!

Re: Derat Factor for big commericals!

You don't really know the "proper" deratings over time until you get a chance to ACCURATELY measure a big installation...

Just in PV watts, they have a whole bunch of derating factors that add up to their 77% derating.

[B]Derate Factors for AC Power Rating at STC
[/B]Component Derate Factors         PVWatts Default         Range

PV module nameplate DC rating         0.95         0.80–1.05                 
Inverter and transformer         0.92         0.88–0.98                             
Mismatch         0.98         0.97–0.995              
Diodes and connections         0.995         0.99–0.997             
DC wiring         0.98         0.97–0.99              
AC wiring         0.99         0.98–0.993              
Soiling         0.95         0.30–0.995              
System availability         0.98         0.00–0.995             
Shading         1.00         0.00–1.00              
Sun-tracking         1.00         0.95–1.00              
Age         1.00         0.70–1.00              
Overall DC-to-AC derate factor         0.77         0.09999–0.96001

With a large customer--Do you have to have a performance guarantee? And how would somebody "prove" one way or another that you are meeting the specifications or not?

You can have a 10-20% year over year change in yearly average solar irradiation.

A 5-10% error in production output could be the difference between profit and loss...

The little I have seen here--A site may not even choose to instrument each solar array string--But instead base power production measurements on a 100-500+kW solar block... There could be a dozen dead panels in a block before generic measurements could catch a 2% variance.

But instrumenting each string (an option supported by some/many/all? GT systems vendors) is expensive and, apparently, not always chosen.

-Bill

Re: Derat Factor for big commericals!

My opinion is probably pretty much next to worthless...:cool::roll:

Not many people on our forum here have experience with large commercial systems.

Finding a consulting engineer / firm with a good CV (lots of successful installations) for your first project or two may be worth the money. You learn and your customers have systems that perform to specifications.

I would also assume that many of the GT Inverter companies offer courses and design support/modeling to their distributors/installers. Pick a vendor and work closely with them on first few installs.

Try to interview/take a tour at a couple of local large PV sites and and get some understanding of what made them happy/unhappy.

I am sorry I do not have any better answers... But I am not sure there are any "simple" answers either.

-Bill