Playing with DC-AC ratio on PVwatts.

Thats about right. Your clipping losses go way up after 1.2. That's a system sized properly..

There a good paper somewhere on over sizing the invertors to achieve 1.6.
 
electrofelon said:
Something isn't right. As I increase the DC-AC ratio from one, production goes UP until about 1.2, then it starts going down 🤨
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Plausibly correct if inverter idle losses are being considered; but you will need to dig into the actual equations to confirm this.

The PVWatts technical reference is https://www.nrel.gov/docs/fy14osti/62641.pdf

The inverter efficiency model used is shown on page 15. The model isn't aware of specific products at specific power ratings; you enter your DC kW rating and your DC:AC ratio, and that defines the 100% rating of the inverter. Then the modeled DC output of the array is compared to that 100% rating, and the inverter efficiency scaling is applied.

The inverter efficiency model curve on page 15 is efficiency (%) vs per unit power. Re-graph it as efficiency (%) vs power (kW). Decreasing the DC:AC ratio has the effect of shifting this curve to the right, so that more of your actual production gets lost to the low efficiency region when the inverter is operating at low % power.

That is just me hand-waving an explanation. It would be nice to see intermediate results of the PVWatts model, so that you could see 'this much production was lost to clipping' 'that much production was lost to inverter efficiency', etc.
 
jaggedben said:
Nah, electrofelon is right. Production should approach a limit, not start going down.
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Huh? In PVWatts you input DC array size and DC/AC ratio. So by raising the DC/AC ratio, you are lowering the inverter AC rating. Of course production will go down.

If the model used an inverter efficiency that was fixed (independent of input DC power), then the behavior you'd get is constant production as DC/AC ratio increases to the point where clipping starts, and then decreasing production due to clipping.

As the model uses an inverter efficiency curve that is lower when the inverter's DC power is much lower than its rating, increasing DC/AC ratio will very slightly increase production up to the point that you clipping starts to be significant.

But if you look at the production estimates over the full range of DC/AC ratios from 0.4 to 2.0 that PVWatts allows, the difference between 0.4 and 1.2 due to the efficiency curve effect is much smaller than the difference between 1.2 and 2.0 due to clipping.

Cheers, Wayne
 
Sorry, I was thinking about it backwards, as in increasing the DC:AC ratio by actually increasing the DC size, rather than by changing the parameter in the software.

What you said makes sense.
 
I have done a lot of AC to DC ratio conversions mostly on industrial plant. For me it is simply fixed ratio.
 
Besoeker3 said:
I have done a lot of AC to DC ratio conversions mostly on industrial plant. For me it is simply fixed ratio.
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This is a different AC:DC ratio.

In solar power systems, the PV array has a nominal power rating and the inverter which converts DC to AC has a nominal power rating.

The ratio of array rating to inverter rating is the DC:AC ratio. When the DC:AC ratio is greater than 1, then the inverter is not capable of handling the maximum output of the PV array, causing 'clipping'. Because peak conditions are quite rate, some amount of clipping is not only tolerable in an optimal system, it is sometimes the case that over-all average production will be greater with a DC:AC ratio greater than 1.

-Jonathan
 
A DC/AC ratio of 1 means the STC power curve would peak at the AC rating of the inverter. It's rare that the STC power peak occurs in the real world so the inverter is under loaded.
A DC/AC ratio of 1.2 is generally considered the point where the real world DC power peak matches the inverter rating.
Now if you imagine a graph of the array power curve over a day and add to this the maximum inverter power, so a curve and a flat line. The inverter can process the DC power below this line, power above this line is clipped. Now increase the array power over 1.2 DC/AC, the curve peak goes up but the power available below the inverter max line increases too as the width of the curve widens where it meets the inverter rating. The inverter can process the extra power from the increase in the width of the power curve below the maximum inverter power line but the power above the line is clipped. Overall, this increases the amount of energy produced by the inverter as the increase in the width is greater than the loss at the peak. But only up to a point. As the array power is increased, and the DC/AC ratio increases, the power gained from the increase in width minus the power lost due to clipping approaches zero and nothing can be gained by increasing the DC/AC ratio past this point. This point is usually above 1.6 DC/AC ratio. I've seen DC/AC ratios as high as 2.0 to 2.5 in some systems.
 
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pv_n00b said:
As the array power is increased, and the DC/AC ratio increases, the power gained from the increase in width minus the power lost due to clipping approaches zero and nothing can be gained by increasing the DC/AC ratio past this point.
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"Approaches zero" yes. But "nothing" is not correct, let's say "little" or "nothing practical."

Anyway, your description is otherwise correct, but not applicable to the OP. The OP is discussing the situation where the DC size is fixed, and the DC/AC ratio is varying. So the size of the inverter is varying, not the size of the array.

Cheers, Wayne
 
Of course, a DC:AC ratio based on the STC numbers for the modules is virtually never what the inverter will see, because STC conditions are virtually never seen by the modules, and even if they are, it won't be for much of the day and any clipping at that time by a well designed "oversized" array will likely be more than made up for on the shoulders of the daylight hours DC power curve. To accurately predict what a system will actually produce takes a more elaborate (and expensive) program than PVWatts like Pvsyst. PVWatts is a useful tool for ballpark predictions but it is pretty crude. I believe that NREL says the results from PVWatts are + or - 10%.
 
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