[RE-wrenches] high insolation value and inverter undersizing

Tue Jan 29 06:48:11 PST 2013

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 <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

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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