Marco,
I still challenge those who believe that “some clipping is good” to make their
case.
Well, the way you've phrased it, you've made my job pretty easy-- though
it seems to me that several folks have already done this for you.
Some power limiting at some point over the 25-year life of a system is
definitely a good thing. A 25-year life corresponds to about 110,000
hours of daylight. Let's say that you can expect one total hour of those
110,000 in which a Montana system sees 1600 W/m2 (high slope, snow
reflection, edge-of-cloud) while simultaneously experiencing a record
low temperature and 50 mph winds. Would you put a 9kW inverter on your
5kW array to avoid any power limiting ever? Of course not-- you're
substantially increasing system cost for a $0.15 gain.
It comes down to figuring out probabilities and doing some complicated
and annoying math to figure out the best size for an inverter. Or more
realistically, attempting to get high-sample-rate weather data to
simulate array performance at your target location. When you oversize an
inverter, your system will be operating at a lower efficiency, on
average. It increases the cost to the customer not just from the base
cost of the larger unit, but also the larger output conductor and
raceway sizes, the output disconnect, the interconnection
breakers/fuses... and maybe even upsized panelboards that didn't
actually need to be upgraded had you correctly sized the inverters. So
you have to figure out whether the additional production is worth the
increased cost of installation.
Oversizing an inverter may extend its life but I don't know that we'll
ever have those numbers from manufacturers to better quantify that
impact. More current causes more heat, which is bad, but increasing an
inverter size to the next higher power rating available doesn't
guarantee that the larger inverter will be more reliable.
So that's a general response. As for your Power-One 250 vs Enphase 224
conundrum, if all else is equal but the power rating, then I'd probably
join you in choosing the Power-One. But as you know, there are other
factors to take into account-- [perceived] reliability, BOS cost (e.g.
more Enphase units fitting on a 20A breaker may save you a circuit), DAS
usefulness, resistance to corrosion, and the fact that Enphase operates
at a higher conversion efficiency in the lower half of its operating
range (where it spends most of its operating time). I am not saying that
Enphase is better than Power-One in anything but low-range efficiency--
I don't know one way or the other.
Efficiency Curves:
http://gosolarcalifornia.com/equipment/inverter_tests/summaries/Enphase%20M215%20IG-240V.pdf
http://gosolarcalifornia.com/equipment/inverter_tests/summaries/Power-One%20MICRO-0.25-I-OUTD-US-240.pdf
After all this discussion, it's pretty funny that the peak conversion
efficiency of the 250W Power-One micro occurs at... a 250W output. So
even though the marketing guys tell you you should only plug in a 265W
module, the engineers are clearly asking you to turn it up to 11.
"Do you, Mr./Mrs./Ms. Homeowner, want a PV system that produces as much
solar kWhs for your investment as possible?" isn't the right question to
ask. Try, "Do you, Mr./Mrs./Ms. Homeowner, want a PV system that
produces the best value for your investment?" Then show your super
impressive calculations, based on your years of experience, that your
recommended inverter is the best fit for them. Allowing for maximum kWh
harvesting, within reason, is the best design strategy.
Dave
5.376kWdc on a 5.000kWac, and loving it
On 2013/3/23 19:31, Marco Mangelsdorf wrote:
From Dan at Exeltech:
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.
I still challenge those who believe that “some clipping is good” to make
their case. And as far as the general buying public, I’m find that
people do in fact understand when you ask them the following questions:
Do you, Mr./Mrs./Ms. Homeowner, want a PV system that produces as much
solar kWhs for your investment as possible? Usual response:
absolutely. If I give you the choice of PV system using a 250-watt
module paired with a COMPARABLY priced 250-watt micro inverter OR that
same 250-watt module with a max output ~ 224-watt micro inverter that
will never under any circumstances allow that 250-watt module to put out
its max rated power output, which option do you think they’ll choose?
They get that. It doesn’t take someone with an engineering degree or
10-40 years in the field to get that simple premise.
For what it’s worth, being here in the tropics in the Hawaiian islands
we don’t get those bright and sunny and cold late fall/winter/early
spring days that will allow for an array to put out its STC-rated
power. But seeing regular times during the day—any time of year—where
the irradiance is more than 1,000 watts/sq. meter is not at all common.
marco
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
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Change email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Change email address & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
