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

[Peter Parrish]

Mark,

What do I tell my clients about soiling? 96% if they clean them once a year.
If they clean them on the 4th of July, they'll do a percentage point or two
better. 

The Forest and the Trees: That's just the point. A bunch of trees makes a
forest! A PV system can suffer from a score of small losses, each of which
may be only a few %. Individually they may be small, but cumulatively they
can add up to 10% or more. 

How do we know? Take a look at the DC-AC derate terms in PV Watts 2. By the
way I am a firm believer in PVWatts 2; I used to think the tool was overly
conservative, now I think it is spot on. 

I have read (and re-read) the Fronius White Paper. I find it unconvincing.
Remember these are the same guys who in 2006 couldn't get MPP tracking
correctly.

For my more astute clients, 5% is significant.  For our Company, we try to
reduce every loss contribution to its practical minimum. For example we are
conservative on (1) specifying wire gauges for minimum IR losses, (2)
strongly encouraging our clients to trim their trees/bushes and keep them
trimmed, (3) encouraging our clients to remember to clean their PV modules
on schedule, (4) using the inverters with the highest efficiency ratings,
(5) using PV modules with the tight tolerances. These five issues can easily
add up to 10%. Heck, shading can be 10% or more on its own if you don't pay
attention to it (e.g. because the utility company that disburses rebates
doesn't require a shading analysis).

- 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 


-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark Frye
Sent: Monday, December 29, 2008 10:39 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Inverter with two strings of different
orientation

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 

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