[RE-wrenches] high insolation value and inverter undersizing

Mon Jan 28 17:35:08 PST 2013

Hi Marco

Here is our experience with that knowing that we are at 6000 to 8000 foot
elevations. 

We have seen significant differences in module delta temperature to ambient
for ground mounts vs roof mounts and that is where we become concerned about
oversizing the module or array.  For a ground mount on a nice breezy day
(especially in open areas or ridgetops) we have seen delta T’s at or below
10 degrees C. (SMA’s string sizing allowed you to select mounting type so
that delta T could be accounted for in sizing the inverter). Then add to
that high irradiance and a low coefficient of power module and you have the
perfect conditions for serious clipping.  I would say that it is
particularly true for trackers – maximum irradiance at 9:00AM until 3:00PM
with lower ambient temperatures during the morning.  

We also see very low delta T’s with white roofs where albedo is high and
roof temperatures are low because of the reflected light.  In this case, the
decision on module size would depend on the tilt angle of the array relative
to the season’s high irradiance levels and ambient temperatures.

Thanks

Randy

Randy Sadewic

Positive Energy

Office: 505 424-1112

Cell:    505 570-0137

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Chris Mason
Sent: Monday, January 28, 2013 5:16 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] high insolation value and inverter undersizing

The initial issue was concerned with waste power, but that presumes all
power is priced evenly. Take a familiar situation, you specify and sell the
system based on a number of 240W modules. Let's presume time passes and when
you go to buy out the job, you find there is a deal on 260W modules which
makes them as cheap as the 240W would have been. Why would you not just buy
the larger output if all costs are the same. If they clip the inverter a
little, oh well.

On Mon, Jan 28, 2013 at 7:35 PM, Nick Soleil <nsoleil at enphaseenergy.com>
wrote:

Hi John,

It's important to clarify that the "clipped" power does not turn into heat
within the microinverter. The microinverter only draws enough current to
achieve it's max output rating, and the remaining current is "left in the
module", so to speak. So, the heat generated at peak times is the same,
whether using a 260W module or a 280W module. 

Many string inverters actually work similarly. What's different for them,
however, is their overall thermal situation. The primary thermal driver for
a string inverter is the amount of heat generated by their electronics, and
the ability of their "fans & fins" to expel that heat.

For example, a 5kW string inverter generates ~200W of heat at peak, and it
has fans and fins sized accordingly.  But, the effectiveness of the fans and
fins depends on the ambient temperature (and on elevation/air mass), and if
your inverter was operating at max power during hot weather (which would
require significant oversizing due to the modules' temperature derates), it
could exceed the capability of the fans and fins. This would then require
the inverter to ramp-down output.  (Actually, some string inverters can
respond improperly--depending on whether their fan is continuous,
load-controlled or thermally-controlled--and actually fry the electronics.)

But, the thermal environment of a microinverter is very different. 

The main thermal driver in our situation is actually the external
environment, not the electronics. Regardless of sizing ratio, an M215
generates less than 10 watts of heat at peak, whereas the heat radiated from
the module at peak times will be substantially greater than that.
Consequently, we've already over-engineered the microinverter for extreme
heat management, and the thermal effects of sizing ratio are simply a drop
in the bucket compared to the other factors.

For that reason, we don't put limits in our warranty with regard to sizing
ratio (which contrasts with at least some string inverter warranties), and
we would not expect to see any microinverter ramp-down behaviors driven by
sizing ratio.

On Mon, Jan 28, 2013 at 11:06 AM, John Berdner <John.Berdner at solaredge.com>
wrote:

Jason:

Be careful of drawing false comparisons.  There is no free lunch.

Beyond the energy yield issues there are some reliability issues.

You should also consider that running an inverter flat out all the time will
shorten it’s life (due to the Arrhenius effect).
http://en.wikipedia.org/wiki/Arrhenius_equation

Think of the inverter temperature in the same way you would the power output
curve (the two are closely related).  Moving the whole power curve up “more
on the shoulders” also moves the whole temperature curve up as well.

The average lifetime (all this stuff is only probabilities) was projected
with certain assumptions about how many hours at what temperature.  As
module sizes increase a given inverter will spend more time at a higher
power level, i.e. runs hotter longer, which non-linearly reduces the life of
the product.

Just my $0.02
 With the high labor cost of replacement it might be better to
undersize the module relative to the power electronics or vice versa.   This
may or may not increase the installed system cost depending on the products
you choose. Running everything flat out all the time is probably not the
greatest idea.  

Best Regards,

John Berdner

General Manager, North America

SolarEdge Technologies, Inc.

3347 Gateway Boulevard, Fremont CA 94538 USA  (*Please note of our new
address.)
T: 510.498.3200, X 747 <tel:510.498.3200%2C%20X%20747> 

M: 530.277.4894 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Monday, January 28, 2013 5:30 AM

To: RE-wrenches
Subject: Re: [RE-wrenches] high insolation value and inverter undersizing

I found the latest module "rightsizing" paper by Enphase pretty interesting.
It does stand to reason that there are a lot of hours in the day where the
inverter is producing less than rated power, so increasing the module size
makes sense. Obviously there is an optimum point in every system where
clipping exceeds gains, but that's virtually impossible to nail down due to
so many factors being involved. I think the point would be that PV mods are
coming down in price so much that "rightsizing" is basically equivalent to
"upsizing," within reason, of course. Spending an extra 25% on PV might
increase the installed system cost by 5-10%.

Jason Szumlanski

Fafco Solar

On Sun, Jan 27, 2013 at 6:00 PM, Marco Mangelsdorf <marco at pvthawaii.com>
wrote:

I have had a concern for a long time that as PV modules have gone up in
output that there is inevitably going to be more and more clipping taking
place as they’re paired with comparatively lower output inverters.  In
effect, matching, say. a 260-watt module with a micro inverter with a max
output of 225 watts reduces that 260-watt mod to 225 watts.  It’s not a
matter of if clipping will take place, but how much and how much actual
harvestable energy will be lost.

Out here in the tropics, even in January with the lower sun angle, we see
irradiance levels well above 1,000 watts/square meter range.  Take a look at
what I copied today from a weather station that we installed at nearby
installation.

Given the clipping issue inherent in using undersized inverters, it seems to
me that moving to larger output micros makes a whole lotta sense.  I’ve got
one of the first installs using the Power-One 250-watt and 300-watt micros
and am so far quite impressed.  Why use a 225-watt rated micro with a
225-watt or higher output mod when larger micros are now readily available?

What are the views out there regarding this clipping issue?  Is it as
minimal as our Enphase friends suggest in their latest white paper or more
of an issue as I conclude?

marco

Environmentals from Weather Station at a Hilo, Hawaii installation

Sunday, January 27th 2013, 12:40:00 pm

Ambient Temp. 25.30 °C

Cell Temp. 41.70 °C

Insolation 127.81 kWh/m²

Irradiance 1,175.00 W/m²

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