Studying the Performance of Solar PV Systems

A couple of weeks ago, I noted the importance of examining parameters other than module costs when gauging the economic competitiveness of solar PV energy. I noted how multiple factors influence the levelized cost of energy produced by solar PV systems, and thus its relative cost position on the grid. Nothing new here.

However, besides standard test conditions (STC) conversion efficiency, or nameplate conversion efficiency, public data on parameters other than cost per watt-peak is not always easy to come by. That's why I found reading "Potential of photovoltaic systems in countries with high solar irradiation", a paper about to be published in the journal Renewable and Sustainable Energy Reviews, particularly interesting.

The Study

In the authors' own words, the paper reports the results of the following study (funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)):

Thirteen grid-connected PV systems of nominal power 1 kWp each have been installed in Nicosia, Cyprus and Stuttgart, Germany [...] providing the opportunity for direct comparisons under the different climatic conditions of the two countries.

More specifically, the installed PV technologies [...] consist of twelve fixed plate mounted systems, a two-axis tracking system and a flatcon concentrator system. The systems range from monocrystalline, multi-crystalline silicon to amorphous silicon, CdTe, CIGS, HIT-cell and other solar cell technologies from a range of manufacturers such as Atersa, BP Solar, Mitsubishi, Sanyo, Solon, SunPower, etc.

The PV modules are mounted on mounting racks at the optimal inclination to provide maximum annual yield for each respective location.

This study thus examines the performance of the main commercially-available solar PV cell technologies under the same real-world conditions, rather than in the lab. The annual solar irradiation measured on-site at the ideal inclination was 1997 kWh/m2 in Cyprus and 1460 kWh/m2 in Germany. This equates to roughly 5.5 kWh/m 2/day and 4.0 kWh/m 2/day , respectively. The NREL Photovoltaic Solar Resource map provides a rough guide to equivalent US locations, while Solar4Power's global maps do the same for the rest of the globe.

The systems were initially deployed in June 2006 and the data reported is for the first year of operation, so until June 2007.

The systems under study are as follows:

Manufacturer (Ticker)	Technology	System Power (Wp)	Size (m2)	Nameplate Module Efficiency (%)
Atersa (uses Q-Cells cells, QCLSF.PK)	Mono-crystalline silicon (tracker)	1020	7.90	12.9
Atersa (uses Q-Cells cells, QCLSF.PK)	Mono-crystalline silicon	1020	7.90	12.9
BP Solar (BP)	Mono-crystalline silicon (Saturn-cell)	1110	7.52	14.8
Sanyo (SANYY.PK)	Mono-crystalline silicon (HIT-cell)	1025	6.26	16.4
Suntechnics (Uses Sunpower cells, SPWRA)	Mono-crystalline silicon (back contact-cell)	1000	6.22	16.1
Schott Solar (Private)	Multi-crystalline silicon (MAIN-cell)	1020	7.87	13.0
Schott Solar (Private)	Multi-crystalline EFG silicon	1000	8.58	11.7
SolarWorld (SRWRF.PK)	Multi-crystalline silicon	990	7.82	12.7
Solon AG (SGFRF.PK)	Multi-crystalline silicon	1540	11.50	13.4
Mitsubishi (MIELY.PK)	Amorphous silicon (single cell)	1000	15.74	6.4
Schott Solar (Private)	Amorphous silicon (tandem cell)	960	18.00	5.4
First Solar (FSLR)	Cadmium Telluride	1080	12.96	8.3
Wurth (Private)	Copper–Indium–Gallium– Diselenide	900	8.75	10.3

The study uses energy yield - kWh produced divided by nameplate kWp - to directly compare the performance of each system. Theoretically, this should normalize out conversion efficiency differences between the various systems and, because other key factors such as inclination are kept equal, the performances of the systems should be roughly equal.

The figure below displays the annual energy yield for the Cyprus location. Ignoring the tracker -equipped system, we note some non-trivial differences in AC energy yields between the various systems, with the Suntechnics (SunPower (SPWRA)), Wurth, Sanyo (SANYY.PK) and First Solar (FSLR) systems performing best, and the BP Solar and Schott a-Si systems performing worst.

The figure below depicts the energy yield by season for the Cyprus location. As can be noted, the thin-film technologies (a-Si CIGS and CdTe) tend to have higher energy yields in the summer months than most crystalline technologies, but perform in roughly similar fashions or even slightly worse in winter months.

The seemingly wider variations between summer and winter months for thin-film systems are not actually due to the properties of thin-film materials, but rather to the properties of crystalline materials. The table below displays deviation from the average AC energy yield across all systems, as well as the MPP power temperature coefficient. The latter metric shows the drop in system power per one kelvin increase in temperature.

As can be noted, overall, the crystalline technologies tend to experience much greater performance declines under warmer conditions than do their thin-film brethren. The authors note that the technologies with the lowest MPP power temperature coefficients showed the highest average energy yields during the summer period.

The phenomenon discussed above is perhaps best captured by the graph below, which displays seasonal module efficiency for the Cyprus systems. Once again, by-and-large, thin-film technologies tend to experience much lower drops in efficiency with higher temperatures than do crystalline technologies, with the First Solar CdTe system showing the most stability.

The authors note that the systems installed in Cyprus showed a lower average measured performance ratio than those installed in Germany because of higher temperatures.



Conclusion

A couple of fairly obvious insights emerge from this article.

First, at least for the time being, crystalline technologies retain an edge over thin-film for applications where available space is an issue. Lower efficiencies in thin-film are forcing much larger system sizes, as depicted in the first table above. The urban roof-top market thus remains crystalline technologies' domain.

However, and far more interestingly in my opinion, thin-film technologies' relative performance stability in warm weathers, as demonstrated by lower MPP power temperature coefficients, makes them superior alternatives for areas where temperatures between seasons range from very hot to hot, and where module temperatures are likely to be fairly high year-round. In Cyprus, according to data in the study, average monthly temperatures stood near or below 15 degrees Celsius (~60 degrees

Fahrenheit) during six months out of the whole year. Several potentially large markets will show much higher temperatures throughout the year.

Incidentally, such regions could become, because of their solar irradiation regimes, very attractive solar PV markets. Areas such as India, North Africa, the Middle East and Australia all come to mind (the scale shows kWh/m 2/day).

India recently announced it would be targeting 20 GW installed by 2020, and it was reported that it would institute a production-based incentive , which generally takes the form of a production tax credit or a feed-in tariff. In regions of Southern India with very hot summers and hot winters, thin-film technologies would probably offer the best alternative for ground-mounted installations, which will likely spring up in fields across the region if the incentive is generous enough.

DISCLOSURE: None

Comments

[Davewmart:]

Thanks for a fine article, which I have bookmarked.
An additional criteria outside of the artificial world of subsidies which hopefully solar is starting to emerge from is how well the profile of daily, and more importantly, annual solar incidence matches demand.
So for instance in Arizona which uses a lot of power for cooling, the daily variation is not too bad, and a few hours of storage can be envisaged as maybe being relatively economic for the evening and early night when temperatures are still high but sunlight is weak or not available.
You can't do that for annual variation - storing the energy is out of the question, and you would have to massively overbuild to meet winter demand.
So for somewhere like Cyprus with relatively modest winter demand due to fairly high temperatures and it's reasonably low latitude which gives you more winter sunshine than further north.
Germany is a truly lousy place for solar power, as not only is it a long way north but has high cloud cover in the winter.
Summer cooling needs there are relatively modest, but winter heating needs are large.
It all means that if solar was any significant proportion of the grid, then it would simply make the economics and efficiency worse.
In practise it would build in natural gas burn at high rates for the winter, so that the natural gas could not be used in the most efficient manner, as it is now becoming practical to install
fuel cells in the home utilising waste heat to provide hot water and electricity to run the home.
I would conclude therefore that solar power in this sort of area is unlikely to provide any net benefit at all and no savings in the use of fossil fuels for it's considerable cost.

Another factor in costs is ease of installation and maintenance.
In this respect it makes a lot of sense to build 2-10MW units on the ground - big enough for cheap maintenance, assisted by not having to get up on a roof, and reduced installation costs, but small enough that power does not need transmitting by long-distance lines or stepping up for that purpose then stepping down at it's destination.
This means that low-latitude regions with high summer cooling needs and plenty of spare land are the most favourable locations, and Arizona in this respect is better than Cyprus.
Counterbalancing this is that power costs a lot more in Cyprus, so solar can be competitive whilst costing more.

Aug 12 07:33 AM

[Jigar Shah:]

The person that has done the most work on how well solar production tracks air conditioning peaks is Dr. Richard Perez from SUNY Albany. See here: www.asrc.cestm.albany..../

On the article broadly, the Most important factor very quickly will be profit per area. When the land is free, First Solar might be able to get away with a $0.20/Wdc discount to medium efficiency crystalline. But if you are maximizing profit per fixed land area, my guess is that this delta will rise to $0.50/Wdc and be almost $0.80/Wdc for amorphous silicon 6.5% efficiency AMAT panels.


On Aug 12 07:33 AM Davewmart wrote:

> Thanks for a fine article, which I have bookmarked.
> An additional criteria outside of the artificial world of subsidies
> which hopefully solar is starting to emerge from is how well the
> profile of daily, and more importantly, annual solar incidence matches
> demand.
> So for instance in Arizona which uses a lot of power for cooling,
> the daily variation is not too bad, and a few hours of storage can
> be envisaged as maybe being relatively economic for the evening and
> early night when temperatures are still high but sunlight is weak
> or not available.
> You can't do that for annual variation - storing the energy is out
> of the question, and you would have to massively overbuild to meet
> winter demand.
> So for somewhere like Cyprus with relatively modest winter demand
> due to fairly high temperatures and it's reasonably low latitude
> which gives you more winter sunshine than further north.
> Germany is a truly lousy place for solar power, as not only is it
> a long way north but has high cloud cover in the winter.
> Summer cooling needs there are relatively modest, but winter heating
> needs are large.
> It all means that if solar was any significant proportion of the
> grid, then it would simply make the economics and efficiency worse.
>
> In practise it would build in natural gas burn at high rates for
> the winter, so that the natural gas could not be used in the most
> efficient manner, as it is now becoming practical to install
> fuel cells in the home utilising waste heat to provide hot water
> and electricity to run the home.
> I would conclude therefore that solar power in this sort of area
> is unlikely to provide any net benefit at all and no savings in the
> use of fossil fuels for it's considerable cost.
>
> Another factor in costs is ease of installation and maintenance.
>
> In this respect it makes a lot of sense to build 2-10MW units on
> the ground - big enough for cheap maintenance, assisted by not having
> to get up on a roof, and reduced installation costs, but small enough
> that power does not need transmitting by long-distance lines or stepping
> up for that purpose then stepping down at it's destination.
> This means that low-latitude regions with high summer cooling needs
> and plenty of spare land are the most favourable locations, and Arizona
> in this respect is better than Cyprus.
> Counterbalancing this is that power costs a lot more in Cyprus, so
> solar can be competitive whilst costing more.

Aug 12 07:44 AM

[Davewmart:]

Jigar, thanks for the info, however your link did not work and googling showed a load of different papers by the chap you refer to.
Have you got a name for the paper or a working link?

Aug 12 10:34 AM

[Road Runner:]

The graph shows that Suntechnics (Uses Sunpower cells, SPWRA) has a module yield of only 16.1%. I'm baffled by why this is so low since Sunpower cells are now at 22.5% efficient and have been above 20% for years. Maybe they bought these cells years before the study was concluded.

Aug 12 10:47 AM

[Chad Brown:]

There's a lot here, but investors will need a lot more help to convert this to the investment implications....

Aug 12 11:18 AM