[RE-wrenches] Inverter with two strings of different orientation

[Mark Frye]

Peter,

What are you modelling for annualized soiling loss?  What is the basis for
that value, margin of error? With industry standards for soiling in the
range 5 to 7%, are you not seeing the forrest for the trees, looking so
closely at effects of multiple orientations of separate strings.

Go with the Fronius White paper: one inverter, multiple strings, all modules
in any given string with the same orientation. If you must, throw a number
at the orientation loss, but how meaningful will it be in the overall
picture?
 
Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City,  CA 95959
(530) 401-8024
www.berkeleysolar.com 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Peter
Parrish
Sent: Monday, December 29, 2008 10:07 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Inverter with two strings of different
orientation

Thanks Bob, these are really great reference articles. The equation for Vmp
had four terms, three of which are temperature dependent. So I think we put
that issue to bed. The reason that this issue is so important for us is
this: if Vmp is temperature dependent, then there must be some loss when
combining two strings together with different tilts/azimuths. How much, I
don't know. Specifically, I have a client for whom we are designing a system
with two strings with different tilts/azimuths (otherwise identical)
18deg/East and 15deg/South. The questions is, "Go with one inverter and wire
the strings in parallel, or go with two inverters one for each string."
Since the cost for two inverters is considerably more than one inverter of
twice the capacity, I would like to go with the single inverter, if the
losses are in the 1-2% range.

I am also very skeptical about MMP tracking algorithms. I documented the
"saw-toothing" on the Fronius IG-4000 input (July 2006), and we still have
an outstanding malfunction with a Xantrex XW6048 with an output power
oscillation.

Right now I will be telling our clients that "using a single inverter --
with two or more subarrays of widely differing orientations (see above) --
is not recommended, because there may be significant losses (significantly
more than 1-2%)". 

- Peter

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 CA Lic. 854779,
NABCEP Cert. 031806-26 peter.parrish at calsolareng.com