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CPV Trackers: A Crucial Aspect of Project Success?

Tracking is crucial to concentrating photovoltaics (CPV), but can it enable the technology to earn a larger portion of the solar power sector? Key players in the trackers field tell us where the sector is focused, and how it is coping with the challenge of low-cost PV.

As anyone involved in the solar industry knows, cost reduction is crucial to survival in a challenging global market. Although concentrating photovoltaic (CPV) technology currently supplies just 0.1 per cent of this market - with just 100 MW installed worldwide, compared with PV's 70 GW - its manufacturers believe they can ultimately offer a lower-cost alternative to conventional panels.

CPV uses mirrors or curved lenses to focus (concentrate) sunlight up to hundreds of times onto a smaller number of solar cells than traditional PV. (The amount of concentration is measured in "suns".) Because the materials used are relatively low-cost when compared with silicon wafers, CPV advocates claim their technology is more efficient than PV and ideal for high direct normal irradiation (DNI) areas. However, in order to achieve higher energy yields, CPV panels must remain trained on the sun at a precise angle throughout the day as a result of the narrow effective incidence angle. This is where trackers come in.

Players in the CPV tracking space include Solaria, Mecasolar, newcomer Nexteer, Opel Solar, Soitec, SolFocus, SunPower, Titan Tracker and, until recently, Amonix and DEGERenergie.

Accuracy

Trackers are vital to CPV — and they need to be extremely precise in how they follow the sun. Hansjörg Lerchenmüller, senior vice president of Soitec's solar energy business unit customer group, explains: "With a CPV system the alignment of the module towards the sun has to be very accurate, in the range of 0.5°. This accuracy is achieved with precise dual axis tracking." Stavros Mastorakis, technical director at Mecasolar, comments: "For a low-concentrating CPV module such precision was requested to be less than 1.5°, while in a typical PV [module] a precision of 3° would not affect the results in energy yield." SolFocus considers that for its mirror-based systems the acceptance angle can go up to 1.0°.

How hard is it to achieve such accuracy? When automotive manufacturer Nexteer moved into trackers, it discovered the scale of the challenge through site visits, says David Westphal, executive director of the company's SunSteer CPV tracker program. "We visited fields and every tracker had the panels pointed in different directions. They had tried multiple times to fix the problem but it didn't work."

Tracking accuracy can be affected by temperature changes, wind, low DNI, condensation, soiled sensors and even more subtle events such as ground movement and gradual degradation of mechanical or electrical systems. Environmental conditions definitely affect system performance, says Lerchenmüller: "We found that, for example, low temperatures influence the system. Consequently different environmental conditions require different system layouts."

Nancy Hartsoch, senior vice president of marketing and sales at SolFocus and the chairperson of the CPV Consortium, says, "When we designed and tested our trackers we felt it was critical to have them operating in different temperatures and climates around the globe to understand performance in the unique conditions of various geographies. For example, hydraulic trackers can run into difficulties when temperatures drop well below freezing, so we decided to go with electric motors."

Westphal argues that, ultimately, "Output remains a function of the device that you're moving. Even different coatings on PV will give them more or less tolerance to exact perpendicularity to the sun. CPV wants to be tracked even more accurately. What type of mirror, how good is the polish, are they using Fresnel lenses, ... determine the angle they need to hold."

Single or dual axis?

Dual axis trackers can follow the sun both vertically and horizontally, while single axis trackers are generally north-south aligned. The consensus seems to be that dual axis tracking is required for high concentration PV (HCPV) — of 100 suns or more — while for low concentration (2-100 suns) single axis can be used.

"With the current feed-in tariff (FiT) applied, two axis tracking for low-concentrating CPV modules is not paying off its investment," says Mecasolar's Mastorakis. "On the other hand, if we are talking about high-concentrating CPV modules then a two-axis system is a must in order to obtain the higher yields."

For Westphal, the choice also depends on "where you are in relation to the equatorial plane. We find we get the best gains using dual-axis and being below 1°, perpendicular to the sun, at 1/10 of a degree. Everybody's particular CPV panel has different aspect characteristics as to tolerance to being out of alignment to the sun. How accurately you need to track to get efficiency out of a particular device is based on that technology."

Hartsoch sees dual axis trackers as vital for HCPV, while silicon PV or flat-plate panels can be fixed or can employ single or dual axis tracking: "It becomes an economic and terrain-based decision, trading off CAPEX costs against generation. The higher the value of the energy, the more likely that a tracked approach will be better due to the higher energy generation," she says. 

"For HCPV, tracking on two axes is mandatory, and that is why suppliers of dual axis trackers have to be diligent in the quality of their equipment and push the bar on reliability testing, failure analysis, and all aspects of assuring the trackers will be operating in the field for 25 years. The critical importance of trackers to CPV systems is why SolFocus has designed its own tracking technology, to ensure it is perfectly married to the CPV panels it deploys. There is no shared accountability for energy production with this approach." 

Lerchenmüller considers that dual-axis tracking will no longer be cost competitive for silicon modules. "Module costs went down so much, going to tracking almost immediately means that you have to use the highest module efficiencies available with CPV technology. So, effectively, for fixed installations and for CPV installations, in terms of energy yield per m2 of module, tracking for a CPV system can be much cheaper per kW because even if it's slightly more expensive per m2, the cost per kW or kWh is much smaller because module efficiency is almost exactly twice as much as a good silicon module.

"If you go for silicon and thin film, your choice will be either fixed or maybe single-axis tracking depending on location and specific technology. It's not worth having dual-axis tracking for silicon. Of course, due to the fact that [it is] twice as efficient as a good silicon module, dual axis tracking makes a lot of sense [for CPV] and provides maximum distribution of energy."

Cost: the Holy Grail

CPV allows manufacturers to produce fewer solar cells, which can offer a cost advantage over PV. However, this saving plus the efficiency boost given by trackers can be offset by the extra cost of components, a tracking system and maintenance. (Trackers make up about 20 per cent of CPV system costs.) The challenge for CPV is that developers might choose instead to simply install more PV panels given their current low price and lack of extra maintenance costs.

As Westphal puts it, "As PV panels drop towards the $1/W range, you have to be able to look at [a customer] and say 'I can give you a large enough performance increase over a fixed array' so they'll understand why they shouldn't just install more fixed panels."

Lerchenmüller says, "Some people quote or believe that going to a more precise tracker leads to an increase in cost, which is not true. Look at the forces and constraints of a tracker. A tracker consists of a drive, a precision mechanical device - which is also used in other industrial products where things are turning - and the structure. It's about positioning accuracy: the drive is responsible for play or backlash — whatever accuracy you need, you cannot afford any significant backlash or play in design. Wind which blows onto the tracker would immediately start to move and swing it, and would create a lot of stress, putting stress on the drive. So you have to go to minimum backlash anyway, then the difference between high tracking accuracy is down to software and algorithm. What is important for cost is efficiency of the module."

Hartsoch says, "When you look at a CPV system, the tracker is only part of the picture. With inexpensive PV panels you can get 20 per cent more energy with a single-axis tracker. The question is: will CPV give you a lower cost of energy in high-sun areas? Today, the answer is yes, it can."

Cost reduction strategies

Price and quality of materials, ease and speed of installation, energy use and even the size of a tracker can reduce costs.

One of Soitec's cost-lowering strategies is, says Lerchenmüller, "Larger modules for fast assembly. You cannot place large modules by hand, which is why we use small cranes for our new system. The previous system incorporated 168 smaller modules, and it used to be that two people were mounting module by module on the tracker, and for each of the modules they adjusted three screws.

"Now we have one person using a small crane to mount 12 big modules per system. He or she places the module on the tracker, then gets down of the crane and manually fixes the module with screws. The modules are still small enough to be easily handled, both in the factory but also — very importantly — in transport. Because they fit into containers we can easily transport them to the site."

Mastorakis also cites transport as an issue, saying that Mecasolar's new product "is modular and scalable and can be assembled on site, which allows for easy transport and installation, and reduces costs."

Soitec studied the ideal size for CPV trackers and, says Lerchenmüller, found that "if you just look at the tracker costs themselves, you find a minimum of cost reduction between 60 and 80 or 90 m2 of CPV module area. If you make small trackers then the motor cost etc. is very expensive because you can apply only a small CPV module area. If you go to large CPV module areas, you have to strengthen the steel etc., so the tracker cost goes up again. But we also considered the costs for the foundation and trenches for the cables and so on. For the CPV system plus installation cost plus foundation cost plus trench cost, the minimum CPV module area needed shifts to 90-110 m2, which is why we decided to go for a 105 m2 CPV module area."

Lerchenmüller claims that "the most sophisticated or accurate way of [tracking is] measuring the output of the system at each single point in time and using this measurement data to find the tracking algorithm, which helps to get the highest power out of the system. It took us some time to develop that algorithm but once it was developed it cut costs."

Westphal cites a different approach. "When we first started talking to people, very few had considered that although their equipment was working hard on the front end to make energy, they put hydraulic or large motor systems under it. They weren't concerned about consuming parasitic power loss to move these things." Nexteer's background in fuel efficiency led the company to build high efficiency into the system, he says.

"Based on our high-efficiency Electronic Power Steering portfolio, the electric automative-based technology that we are bringing to the solar market cuts parasitic losses through the use high-efficiency mechanical and electrical components and advanced software algorithms to reduce the power required to operate the system."

Reliability

Trackers are generally viewed as the weak link in CPV system reliability. Westphal says, "When we benchmarked current solar trackers we found reliability was a problem as well as efficiency. Some [customers] we talked to had field failure rates in the 70th percentile. Only 50 per cent of original installations were still working to their satisfaction after five to six years."

Hartsoch says, "Today, as a whole, trackers are more durable than four years ago. Still, lots get thrown out because they have not gone through testing requirements. We build our own tracking systems because if the trackers aren't as good as the panels, the panels aren't good. In the past trackers have developed a bad name from being thrown in the field before reliability and durability testing. We test to failure in desert, higher humidity, and temperature-contrast climates. For the last four years we've been doing accelerated life testing. We have tracking systems running in three places around the globe, running 24/7 at an accelerated rate to simulate a 25-year life.

"One other part of testing is breaking our test beds into segments.With some trackers we maintain the gears and motors based on the manufacturer's recommendation; with some we maintain them more often, and some we don't maintain. We find that the amount of maintenance required is extremely low; with our current systems scheduled maintenance of the motors and gears occurs in the sixth year."

Third-party engineering firms evaluate stress points in the tracker structure, looking at stability and wind load standards. Hartsoch says there are currently no international standards; instead criteria are based on local regulations. The International Electrotechnical Commission (IEC) is working on draft standards for trackers, but Hartsoch says a final version is at least a year away. The IEC has, however, developed stress-test procedures for CPV systems that evaluate the impact of extreme temperature variations, wind, snow and ice loads.

Looking to shine

EnergyTrend predicts "sweeping growth" for the CPV industry in high-DNI areas over the next three years, and there are around 700 MW of CPV projects currently in the planning or construction stages. However, cheap PV panels are giving CPV a run for its money in its traditional markets. Amonix, one of the "big three" CPV firms last year, announced in July 2012 that it was closing its U.S. manufacturing plant because of low demand due to "challenging" PV panel prices, and DEGERenergie discontinued production of its trackers in March of the same year, saying CPV was "unprofitable in the foreseeable future".

But CPV is also moving into new markets. Italy has a CPV feed-in tariff, making it an attractive arena, and some in the sector have a project pipeline in markets in markets such as Mexico (SolFocus) and South Africa (Soitec). Many believe that, despite its current challenges, CPV's long-term future looks bright.

Mastorakis says, "The sun is there, it makes our days brighter, therefore it is a matter of using what we have available and with no cost. [And] as long as the technology of CPV exists, the tracking technology that supports it will follow."

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