[RE-wrenches] P1 micro performance

Sat Mar 23 18:19:10 PDT 2013

Friends,

I am a little puzzled by this topic.

What has happened to the good engineering practice of sizing electronic
equipment so that it is not driven to the maximum.

My understanding is that the MTBF increases significantly the harder you
drive the unit.

This seems to be a case of overdriving the units for short term gain.  

Carl Emerson

Free Power Co. 

Auckland N.Z.

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Exeltech
Sent: 24 March 2013 5:50 a.m.
To: RE-wrenches
Subject: Re: [RE-wrenches] P1 micro performance

Marco .. and Wrenches ..

I'm going to take a run at this -- just once.

First ...

Without proper and *accurate* data acquisition equipment, there's no way
to know what the actual available PV wattage is compared to the inverter's
output wattage when its output is being *LIMITED*.  This limiting action
occurs in an inverter when there's more available power at the input than
the inverter can produce at its output.  Subsequently, you don't know how
much potential energy wasn't harvested.

Now then ...

Let's say the inverter is producing 216 watts, and the PV  *could* produce
227.3 watts at max power point under those specific conditions if every
PV-generated milliwatt were used.

Next, and presuming the inverter is 95% efficient, that's a "limited loss"
of
ONE WATT.  227.3 x 95% = 215.94 watts (OK, so I fudged 0.06 watt).
Under what conditions (and since you're in Hawaii, I'll use 70F) would this
occur, and with what size PV?

I went to my magic spreadsheet and grabbed the first 270-watt-rated PV I
could find.  NESL DJ-270P,.

YOU think it's producing 270 watts.  It's not.  At 77F (25C), and under the
conditions of 100% irradiance, perfectly orthogonal to the sun at mid-day,
light wind, that particular PV will produce 227 watts +/- its tolerance.
(Let's say the tolerance is dead on.)

Now, consider the benefit of increased energy output due to more rapid
output wattage rise experienced during early morning, and the higher
output later in the afternoon, AS WELL AS the increased power output
realized during periods of less than 100% irradiance one derives from
using larger PV compared to PV you might consider "perfectly matched"
to the inverter.

ALL of this adds up to more kilowatt-hours produced annually than had the
inverter been connected to your "perfect" PV that doesn't produce enough
wattage to have the inverter begin limiting its output.  The "shoulders" of
the
output wattage curve are steeper than with lower-wattage PV.

Granted you *could* connect the inverter to [say] a 450 watt PV module,
and >that< would truly be a waste of the PV wattage.  There *is* a broad
"sweet spot" for AC Module inverters and microinverters alike, and it's
actually on the higher side of the PV's rated output wattage versus the
inverter's wattage rating. So, can you over-do it?  Sure.  But there IS an
overall kilowatt-hours-produced benefit for *modest* over-sizing the PV.

What I *can't* answer is the long-term effect this may have on the overall
life of the inverter.  THAT depends on various intricate design
considerations
that went into creating the inverter in the first place.

This isn't an Enphase issue, nor are they trying to mislead you on this
topic.
It's an industry-wide issue.  Wrenches face it every time you designed a
string
system, especially those being installed in regions with wide temperature
swings.

Trying to explain in depth the "how and why" slightly larger PV is of
benefit
to a customer is like trying to explain photovoltaic equipment to the
general
public.

As a competitor to Enphase .. I'm not coming to their defense.  However,
what Nick said (below) IS fact.

Whether you elect to believe this or not is up to you.

Regards to all,

Dan Lepinski, Senior Engineer
Exeltech / Exeltech Solar Products

With 41 years experience as a design engineer in solar energy.

--- On Sat, 3/23/13, Marco Mangelsdorf <marco at pvthawaii.com> wrote:

From: Marco Mangelsdorf <marco at pvthawaii.com>
Subject: [RE-wrenches] P1 micro performance
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Saturday, March 23, 2013, 4:04 AM

Yes, I know that that screen shot was only a moment in time.  Here it's only
March and clipping is already taking place.  Imagine what kind of clipping
is going to take place at higher irradiance levels later in the year.  Yes,
the monitoring program cannot as of now quantify what kind of harvesting
losses would take place over time compared to an identical array using
Enphase micros.  But the principle remains unchallengeable: not allowing for
maximum kWh harvesting is plain and simple NOT the best design strategy.

"Some clipping is good"?  You've got to be joking.  Not being able to
harvest usable solar energy is good?  What kind of optimal design philosophy
is that?

As module outputs have been going up, Enphase has a vested interest in
continuing to move product with little regard for the harvestable energy
being essentially lost.  Using larger micros that reduce or eliminate that
clipping is prima facie a good thing if one cares about maximizing kWh
harvest.  As more micro products come on the product with higher outputs
than the venerable and solid M215, Enphase risks being left behind and
losing market share.  I for one find that "white paper" overly self-serving.

marco

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Nick Soleil
Sent: Thursday, March 21, 2013 8:04 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] P1 micro performance

Good points David, we are in peak season for inverters to be limiting power.

Marco, your diagram shows that the modules are only overproducing the
Enphase M215's output of 225 watts for a single 15 minute period during the
day.  That would only equate to about 3 watt-hours of lost power on a day
when the modules are producing more than 1000 watt-hours.  That tends to be
in agreement with our study, which can be viewed at;
http://enphase.com/wp-uploads/enphase.com/2011/12/Enphase_White_Paper_Module
_Rightsizing.pdf. 

Averaged across the entire year, this loss of power would total less than
0.1%, and would be less than 0.2% for a 265 watt module.  Keep in mind that
with degradation accounted for, you will see even less limiting in future
years.  I'd encourage Wrenches to look closely at the attached document.  It
is based upon real system production data from Enlighten compared against
actual irradiance data.

Most analysts would agree that some "clipping" is good.  You will have a
better return on your investment when your DC to AC ratio is greater than
one.  In this case, bigger is better.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20130324/42647259/attachment-0003.html>