CEC vs. European weighted large PV inverter efficiency 1

[ters]

I’m tasked to compare two 1MW PV inverters. One of the items which seems to be rather difficult to compare is the efficiency. One unit is specified with CEC (California) weighted efficiency, while for the other one European weighted efficiency is provided.

I know that inverter efficiency, aside from the type and quality of electronic components used, also depends on the DC voltage, inverter output power and ambient temperature as well, but I don't know how it is actually measured according to various standards.

So after doing some research, I determined that CEC efficiency is calculated as an average value of DC/AC conversion efficiency at six pre-defined outputs between 5 % and 100 % nominal power, while calculation of European-weighted efficiency is more fuzzy and seems to be somehow determined from several efficiencies at different operating points that are weighted according to the frequency with which they occur at a specific location somewhere in Europe.

Web sources which compare the above two methods/standards of calculating PV inverter efficiency and provide some sort of conversion seem to be non-existent, while some inverter manufacturers occasionally do give efficiencies according to both standards, where more often than not “European" efficiency is higher than the CEC one, for example CEC-weighted is efficiency of 97.5% is sometimes given as 98.1% European-weighted. However, examples where this is oposite also exists, for instance one partular make and model of the inverter has CEC efficiency of 97.00% while Euro-eta is 96.60%.

Could anyone put some more light on this, or my only option is the ask both manufactures to provide efficiency rating according to both standards, if they are willing to do so?

 

[IRstuff]

That would be my route, as well as having them show the actual detailed calculations, preferrably supplied as an Excel spreadsheet so that you can compare apples to apples, as much as possible.

TTFN

FAQ731-376

 

[jfpe]

ters,
CEC efficiency is a weighted average of measurements taken at different power levels and input voltages. I beleive Euro efficiency is a weighted avaerage at a single input voltage. The weigting factors are slightly different as well. I beleive CEC efficiency gives more weight to the higher power levels.

CEC test data is available at

http://www.gosolarcalifornia.org/equipment/inverter_tests/summaries/

-John

 

[Sparweb]

I was recently reminded that the efficiency of one component of a system can fade in comparison to its place in the entire system. When comparing two inverters with a 1% difference in efficiency, if the more efficient one has more harmonic distortion that the other, the heating, life reduction and/or reduced efficiency of the end use appliances could negate the benefits of the inverter.

Another useful comparison is the relative down-time if the unit fails. The one available domestically arrives for replacement 3 days sooner than the one coming from overseas. Or maybe it's the other way around because the latter company has a better track record for service. A few extra days down-time would cost you more than the efficiency savings all year.


Steven Fahey, CET

 

[ccfowler]

SparWeb's comment makes an excellent point.

It is rare when a simplistic comparison or analysis is either wise or adequate. Almost always, it is important to evaluate how a particular component will function within the greater system in which it will be incorporated. An item that may work superbly in one situation may well be a disaster in another system.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[ters]

Thank you all for replies and very useful comments. However, the purpose of this comparison was of different nature - not an actual project application but rather someone wanted a simplified comparison for the sake of importing, certifying and selling a Chinese inverter vs. another already existing inverter on the N. American market.

For this particular exercise there is rather total lack of information such as average down time for either unit or even more detailed technical specs such as max harmonics distortion. It was more comparison of declared input and output values, ratings, physical dimensions, etc. Prices weere not compared either.

On a side note, has anyone dealt with optimization of number and size of inverters for large scale solar projects. For example, if a 10MW solar park is located on a land parcel with regular dimensions, is it more economical to use 20 500kW inverters each covering 5% of the area, or 10 1MW units each covering 10% or 5 2MW units (provided that they exist at all) each covering 20%.

It is obvious that that 5 large units with cost much less than 20 small ones. However, in case of 2MW units, each would be serving an area of 5-7 acres (2.5-3 hectares), where DC cable will be extremely long and unless heavily oversized (up to 3 times nominal current capacity) losses will be way to high. So factoring in tens of kilometers of very thick cable and increased losses on DC side, any savings achieved buying big inverters seems to melt. With smaller inverters serving much smaller area, DC cables are becoming much shorter (and as such don’t have to be extremely thick). On the other side, HV collector cable becomes longer, but the lenght is still very reasonble and losses on the HV side are much lower anyway.

 

[BrunoPuntzJones]

Okay, I'm not an electrical engineer or anything but it's not obvious to me that 5 large units would cost less than 20 small ones. It may be true, but it's not obviously true.

If 2MW is the high end for inverters, aren't they likely to be sold at a premium to the run of the mill sizes?

 

[Sparweb]

Ters,

This is a topic that scientists at Sandia like to publish about. Although I can't name a specific report or anything off the top of my head, I would be very surprise not to find studies that address your concern. I'd start searching there first. Be warned that Sandia is part of a network of research labs and is involved deeply with NREL, too, so searches will take you in many directions, many of which will lead in useful directions.

CCFowler,

That signature line of yours reminds me of the "One Hoss Shay". I wonder if you've heard of it. If not, you'd probably get a kick out of it: look up Oliver Wendell Holmes.
Good and timeless advice in that piece.

Steven Fahey, CET

 

[ters]

Steven, thank you for the info, will try searching there but I doubt that anyone have arrived to a very firm conclusion as what may be better. Especially given that 2MW inverters are still almost non-existent.

As I indicated, depending on many factors, a 2MW inverter would be serving an area of about 10 acres (4 hectares) or even more. Where thousands of long LV wires (DC side) would be served by one single HV cable on the AC side. Why is this good? Seems to be just opposite, rather suboptimal – a 500kW unit would serve 4 times smaller area, so DC wires are much shorter and so are DC losses. While, yes, that single HV cable will be longer, but who cares, 3km vs. 1 km of the HV cable is still in the range of probably 0.1% of the total project cost for 10MW. And loses are much easier to optimize on the HV side.

BrunoPuntz, it was not electrical engineers deciding that big inverters are more cost effective, probaly more investors, project managers and other similar disciplines .

Yes, I may be wrong - since 2MW units hardly exists, they really may cost more than 2x1MW. At this time, it may as well be like buying a hard disk of 2TB, still uncommon, vs 1TB...

But generally speaking it is the nature of commonly avaialbe electrical equipment that the price doesn’t increase proportionally with the rating. A 2MVA transformer doesn't cost twice 1MVA. It cost perhaps 50% more. Similarly, 2MW generator, cost perhaps 1.5 times 1MW, etc. The some sort of scale should apply to inverters and it does when we are talking about already widely available products, such as 250kW vs. 500kW.

You should also factor-in the installation cost – it cost much more to install 10 units than 5. You need 10 foundations, instead five, ten step up transformers, etc, so I do believe that 10 units cost noticebly more but considering all factors, I seriously doubt that very large units are a preferred choice.

Or if you want a very simplified comparison, building 10 smaller houses is usually more expensive that 5 of double size with similar features.

 

[Sparweb]

The two solar panels on my barn don't really inform me enough to help with the rest of your problem

It sounds like you have enough information already to set up a spreadsheet and run it several times with different configurations and costs until you find the optimal.

To me the calculation sounds easy, but I'm not in the thick of the matter. On the other hand, you may be drowning yourself with too much information.


Steven Fahey, CET