Dual trackers, two opinions


Solon has announced that it is discontinuing its dual-axis trackers, because module prices have fallen so low. Deger disagrees. Why?

Schwedhelm: If other manufacturers of tracking systems want to leave the market, that’s their decision. The first big difference is that a lot of manufacturers sell astronomically controlled systems. We offer sensor-control systems, which allow us to increase power yield far more than comparable astronomic systems. We conducted studies in solar farms and collected evidence from case studies to demonstrate that our systems provide some 40 to 45 percent more power than a fixed array. On some days, the difference is even greater. That’s certainly one reason why providers of astronomic systems are leaving the market.

Do these figures hold true only for Germany, or also internationally?

Schwedhelm: The yield increase depends, of course, on the given location. We studied our own solar farms in Baden-Württemberg and found yield increases of around 45 percent. But compared to a system on a stand in Spain, where I have a lot more hours of sunlight than in Germany, the difference is not as significant. You can certainly compare one region to another. Northern Europe, Southern Europe, the U.S. and Canada. California is comparable to southern European countries such as Spain, Italy, and Greece.

So what are the specific differences between the North and the South?

Steinhöfel: In Northern Europe, we basically always have 40 percent; the German average is 38.5 to 39 percent. The last yield data we received from customers were far above 45 percent.

Are the data public?

Steinhöfel: We send them out on request and collect the data in a reference book that will probably be published by the summer. It contains references from countries where we have major installations, such as Spain, Italy, Greece and Germany.

Why have you not already published the book?

Steinhöfel: We have installations all over the world. We have to go to the far corners of Canada, the U.S. and India. One decisive factor is how the solar farm is managed. If you simply set up the trackers and let them run, you will probably reach 35 to 40 percent. But if your project is important to you and you take care of your systems, you will probably service them well. Then you will have much different figures.

What mistakes can be made in operation?

Schwedhelm: Often, mistakes are already made during installation. In some cases, installation staff ignore explicit instructions. We have seen cases where sensors were improperly installed although the manual clearly describes the process. Naturally, systems then do not then run optimally. We offer an installation check sheet that operators can fill out and return to us. After that we can say whether everything was properly implemented.

Experts say that such fine-tuning hardly makes a difference with flat panels.

Schwedhelm: Different orientations produce different yields. If you deviate by several degrees, the power yield of each individual module drops. With each tracker covering 40 to 50 modules and with 100 to 200 such systems in one solar farm, losses add up quickly.

What are the problems in operating such systems?

Schwedhelm: An installation with hundreds of kilowatts or more than a megawatt costs millions of euros. No one can expect to be able to just set these things up somewhere and forget about them. You have to make sure that all of the trackers are working optimally and tweak the system if necessary. You have to make sure that all of the modules are truly at a 90 degree angle facing the sun. These nuances determine whether I get 40 or 43 percent more yield.

If module prices are not decisive, then what is?

Schwedhelm: Module prices are definitely decisive. They play a major role even though they have plummeted. Modules and inverters make up a large part of the total investment in a solar farm.

Which costs determine a system’s economic feasibility?

Schwedhelm: Property prices certainly make a difference in the installation’s rate of return. How cheap is the land? Can I buy it, or do I have to lease it? And then there are financing costs, installation costs, the cost of modules, inverters and stands.

How do you calculate feasibility?

Schwedhelm: We start with an overall system that produces a gigawatt-hour of energy. You can then calculate what the financial returns will be depending on the specific country and rate paid for solar power. On the other side of the balance sheet, you then have investment costs for property, modules, inverters, installation, and maintenance. And once you've done the math, you quickly see that tracking systems pay for themselves much faster. In most cases, there is an investor who wants to have a certain return on the capital invested. That return largely depends on the size of my investment and the yield I can expect.

How long does it take for a system to pay for itself?

Schwedhelm: In around 95 percent of the countries we work in, a specific feed-in tariff is paid. Here, the benefits of tracking systems really make themselves felt. If I can sell 40 to 45 percent more power to the grid, my income is that much greater. But a tracking system does not add anywhere close to 40 percent more to the cost. As a result, the whole array pays for itself much faster.
Steinhöfel: We are roughly 20 to 25 percent faster than a fixed system as a result of the significant additional power yield.

And all of this is due to optimal orientation to the brightest spot in the sky?

Steinhöfel: That’s right.

How does the mechanism work exactly?

Schwedhelm: Our control system is mainly based on the sensor, the Deger Connector. The dual-axis system has two sensors: one for azimuth tracking from east to west; another for elevation control to track the height of the sun on the horizon. The sensors measure the brightest point and output a value for the tracker system to focus on. The motors react and start tracking until the sensor gets the same signals from the two cells. When that equilibrium is reached, the system is focused on the brightest point. It doesn’t matter if the sky is cloudy or cloudless – the system always focuses on the brightest point.

If a cloud passes in front of the sun, is that still the brightest point in the sky?

Steinhöfel: Yes, but if the cloud is big enough and dense enough, it refracts the light. We then have diffuse light. The Deger system then moves into horizontal position because the brightest point now comes from the whole cloud, not from a specific point. In contrast, an astronomic system will still follow the sun even though it is no longer the brightest point. Astronomically controlled systems track the sun even when the sky is relatively evenly cloudy and no tracking is needed. Our systems therefore consume less energy for tracking than astronomic systems do. A lot of our competitors don’t believe us, but we know it’s true.

When clouds hide the sun, the energy can drop down to 100 watts per square meter. Is that still worthwhile?

Schwedhelm: Every watt is worth it.
Steinhöfel: Power yield analyses show that the system pays for itself the most in the winter. We have far more than 50 percent greater yield then than we would have from fixed arrays. The difference can even be so great that you have 200 kilowatt-hours on your invoice instead of zero.

How fast does the tracker react?

Schwedhelm: Normally, adjustments take place every three or four minutes. But if the change would only produce an extra watt per square meter, our systems won’t move. The threshold value is around 100 watts of incident energy per square meter.

And how much energy does the tracker itself consume?

Schwedhelm: A single system consumes around nine kilowatt-hours per year. Our largest system, the 7000 NT with an installed capacity of 10 kilowatts, generates around 1,000 to 1,200 kilowatt-hours per year for each kilowatt at our latitudes. So you see that the tracker only consumes around a tenth of a percent of the energy the system produces.

What are the costs compared to an astronomic tracking system?

Schwedhelm: If you only consider the control system, then there is hardly any difference. The overall costs are probably also comparable. The sensor technology can be compared to the angle sensors in astronomic systems. The drives are also more or less the same. But we do not publicize the percentage that tracking systems cost as part of the system.

From 2006 to 2008, the Spanish market gave quite an upswing to dual-axis tracking. In 2009, the market collapsed and left a vacuum behind.

Schwedhelm: The combination of insolation and compensation provided ideal conditions there.
Steinhöfel: Module prices were also twice as high back then. Photovoltaics therefore really only paid for itself if you used dual-axis tracking. That’s why we now are having this discussion about whether tracking systems are still worthwhile. They certainly are. If you can produce forty percent more power, it does not matter whether a module costs a euro or three euros.

Solon is abandoning dual-axis trackers altogether. Will other companies follow suit?

Steinhöfel: They're quite a big firm that originally only manufactured modules. They merely included dual-axis trackers to take advantage of that market segment while it was especially good. Things are different with us. We specialize in tracking systems and have more than 170 competitors worldwide. But that number is shrinking quickly.
Schwedhelm: We have a lot of locally operating competitors, especially in Spain due to the boom in tracking systems. If they did not manage to expand to other countries in 2009, they have lost their market completely. It's logical then for them to disappear from this market segment altogether.

What do you think are the main future markets?

Schwedhelm: In 2009, we sold a bundle in Germany, Spain, in Greece. But now other markets are opening up. North America is interesting and Eastern Europe too, with Romania, Bulgaria and the Czech Republic. There is no gold rush, as was the case in Spain, but demand is growing in these countries. And looking further, the Emirates, India and Asia offer great potential. Companies that have not positioned themselves internationally in this selection process will not survive.

Control systems for dual axis tracking

Two different types of control systems are commonly in use for dual axis tracking: astronomical and sensor-driven systems. The first technique uses a tracking algorithm that calculates the sun’s position based on astronomical data, automatically following the position of the sun in the sky during the course of the day. Sensor-driven systems use sensors that detect the brightest spot in the sky and position the modules towards it. While some manufacturers claim that one system allows for an extra gain of energy compared to the other, researchers usually express a placid view.
Matthew Muller, engineer at the National Renewable Energy Laboratory (NREL) says: “I don’t think one system is more favorable than the other when considering non-concentrator PV modules. A tracking system with flat panels does not require high accuracy to gain additional energy compared to fixed installations, he says. “Tracking within five degrees is perfectly adequate to achieve these benefits. Sensor-based and astronomically tracked systems should both achieve these goals fairly easily.”
During days with a clear sky both tracking systems behave the same way: they follow the course of the sun. The difference is their activity during cloudy days. “The system that is using astronomical calculations will always be pointing at the sun, independent of cloud cover,” says Muller. “When the sun comes out, the modules will achieve full production without delay.” When the sun is not the brightest spot in the sky, sensor-based systems are able to focus on a point with higher irradiance, although this makes little difference to energy production as energy availability drops significantly with cloud cover.
If sensor based controllers are not appropriately designed they suffer from a problem called “hunting”: they search through the sky without finding a bright spot, using extra energy to turn motors on. A design that follows the sun based on calculations could produce slightly more energy on a partly cloudy day, but the irradiance can quickly drop as clouds obstruct the sun. “I speculate that there is not much energy to be gained from one system to the other,” says Muller. To address the energy that trackers are using, he says: “Working on an IEC technical specification for trackers, we suggest that manufacturers report the average daily energy consumption of their tracker. I wouldn't be critically worried about the type of controller used in flat plate trackings," says Matthew Muller. “I would be concerned about how cost effective the entire system is in comparison to a fixed installation.”
Britta Danger


This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com.