[RE-wrenches] P1 micro performance

Mon Mar 25 10:54:03 PDT 2013

Hi Carl, Marco, and Wrenches,

As a longtime installer, I understand your point of view.  Historically, I
designed my string and microinverter systems with the same consideration
for maintaining conservative DC to AC ratios.  That being said, the costs
of modules have decreased significantly and the dynamics have changed.
Solar companies should be focused on selling systems that offers a great
rate of return.  Maximizing the customer's investment is most important.
Isn't that what your customer wants?

What Enphase is encouraging is the development of cost effective PV systems
that will generate a healthy return.  Considering that the modules are only
20% of the total system costs today, it is smart to give up 0.2% or more of
the module production to lower the overall system costs by 5-10%.  The NEC
requires that the AC panel boards, conductors, and circuit breakers are
sized to the inverter continuous output current rating.  We should be
maximizing this infrastructure.

The data available indicates that when a system is installed with a 1.2 to
1.25 DC to AC ratio, it will rarely operate at peak output.  Installing a
PV system with a 1.25 DC to AC ratio is not driving the equipment to the
maximum.  In the case of the Enphase M215s; they are designed to operate
continuously at peak output, so reaching that level a few hours in the
first years is not problematic.  To be clear, this limiting will occur the
most during the spring months, because you have a combination of both cool
weather and high irradiance levels.

As Dan mentioned, this applies to string inverters as well as
microinverters.  One of the most common system designs of the early US
grid-tied market was installing 18- 165 watt modules on an SWR-2500.  I
designed hundreds of projects like that.  That was a ~1.2 multiplier, and
was at a time when the modules cost $5 per watt; not $1 per watt.  Why be
more conservative now?

On Sat, Mar 23, 2013 at 6:19 PM, Carl Emerson <Carl at solarking.net.nz> wrote:

>  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.****
>
> ** **
>
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-- 

Cordially,

*Nick Soleil*

*Field Applications Engineer
*

*Enphase Energy*

Mobile: (707) 321-2937

**

*Enphase Commercial Solar.*
*Limitless.*<http://www.enphase.com/commercial?utm_source=email&utm_medium=sig&utm_campaign=Comm2012>

*
*<http://www.enphase.com/commercial?utm_source=email&utm_medium=sig&utm_campaign=Comm2012>

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E: nsoleil at enphaseenergy.com

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