Nick,
You said "the microinverter only draws enough current..." Could you
explain how this works? I primarily work with battery based systems
and the charge controllers that perform current limiting do so by
shunting the excess current as heat. How are you leaving current "in
the module"?
Thank you,
Larry Crutcher
Starlight Solar Power Systems
On Jan 28, 2013, at 4:35 PM, Nick Soleil 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
<[email protected] <mailto:[email protected]>> 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
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*From:*[email protected]
<mailto:[email protected]>
[mailto:[email protected]
<mailto:[email protected]>] *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
<[email protected] <mailto:[email protected]>> 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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