PV Trackers Seek Their Place in the Sun

Generally, modules are fixed at the optimum angle for their specific latitude. However, this is averaged over the course of a year, and (depending on latitude) the optimal angle can in fact vary by 30° as the sun appears lower or higher in the sky.

The development of feed-in tariffs and other similar support measures that reward PV producers per kilowatt hour delivered to the grid has stimulated growing interest in maximising output from a given area.

As with other technologies, increasing the complexity inevitably introduces additional possibilities for malfunction and failure. However, while tracker technology may be perceived as inherently more risky than extremely simple fixed angle systems, growing market exposure is boosting developers' confidence in the reliability of tracking technology.

Tracker Types

There are two basic types of tracker system. Single-axis trackers simply rotate about one axis, moving along an azimuth from east to west over the course of a day. Double-axis trackers move not just along a fixed azimuth but can alter the angle of elevation.

The numbers are latitude-dependent, but manufacturers' figures suggest that compared with fixed modules, single-axis trackers typically increase electricity output by between 27-32 percent, while dual-axis trackers typically see a 35-45 percent improvement in output. Such figures are impressive considering the lengths that the PV industry goes to in order to improve cell performance by just a percentage point or less.

Single-Axis Trackers

Single-axis tracking is one of the most straightforward ways to improve the potential performance of a commercial solar installation.

OPEL Solar International Inc of Toronto makes high-concentration photovoltaic (HCPV) panels and both roof- and ground-based solar tracker systems. The company's TF-800 series horizontal single axis tracker is a utility-scale tracking system that will increase the yield of photovoltaic panels by up to 25 percent, the company says. The TF-800 incorporates an advanced programming feature that reverses the motion of the tracker to eliminate inter-row shadowing, allowing for larger systems to be installed in a smaller areas. A complete 10-kW system can be assembled in less than four hours without the need of special machinery, tools or welding in the field, OPEL adds.

Manufacturers claim tracking technology can increase production by up to 45 percent in some regions, compared with modules at a fixed angle (Source: SunPower)

In April, the company announced that it is supplying TF-850 single-axis trackers to Greenlight Power Company, a solar energy development firm, for the launch of a 125-kW project to be installed at a solar farm in a business park in Kent County, Maryland. This is the first phase of a 1.4 MW solar development at the site.

In this case, the trackers should increase the project's energy production by about 24% over what would be expected from the same solar panels using a fixed installation, the company says. This first phase of the Greenlight project is expected to produce about 182 MWh of energy annually. OPEL says that its tracker product line can be used with any type of solar panel technology, providing greater layout flexibility for integrators and engineering firms choosing a tracker.

Manufacturers of the simpler, single-axis devices have claimed that the additional net energy yield delivered by a dual-axis system over a single-axis system is frequently lost as a result of additional installation, permitting and ongoing maintenance costs. And, such systems are inevitably at greater risk of failure, having more moving parts than a single-axis system. Furthermore, single-axis trackers tend to have a lower profile, sometimes half the height of dual-axis trackers, and are therefore more likely to receive planning permits where the visual impact of an installation is important.

SunPower’s Big Deal

One of the world's largest single-axis manufacturers is California-based SunPower Corp. The company has more than 400 MW installed on four continents, though not all of this capacity is mounted on tracking units. Nonetheless, its T20 tracker produces up to 30% more energy than fixed-tilt ground systems, the company says.

In late July, SunPower announced the completion of Australia's largest solar farm to use a tracking system. The 1-MW Uterne solar power plant in Alice Springs incorporates over 3000 SunPower solar panels installed on the company's T20 tracker system.

Florida Power and Light (FPL) teamed up with SunPower to build the DeSoto Next Generation Solar Energy Center, completed in 2010 — which, at 25 MW, opened as one of the largest solar PV plants in the US. Its annual estimated generation is about 42 TWh. The plant consists of more than 90,500 solar panels.

A similar development, the Space Coast Next Generation Solar Energy Center, was commissioned in 2010 and also uses SunPower's solar panel technology. The projects are two of three solar developments from FPL in the state that will produce some 110 MW combined. Officials from NASA and FPL commissioned the 10-MW Space Coast project on the basis of a public-private partnership. It is located on NASA property at the Kennedy Space Center and comprises some 35,000 modules.

Single-axis manufacturers have claimed that the additional net energy yield delivered by dual-axis systems is often lost due to increased costs (Source: Sedona Energy Labs)

Meanwhile, close to 1000 miles (1600 km) north in Chicago, Exelon and SunPower developed the largest urban tracking solar power plant in the U.S., a $60 million 10 MW installation which opened in 2010. And, in perhaps the company's best known solar energy development — and another of North America's largest photovoltaic systems — at Nellis Air Force Base (AFB) in Nevada, SunPower was part of a joint project of the US Air Force, MMA Renewable Ventures LLC, and Nevada Power Company to develop the 14 MW system at the site. Over the next 30 years this solar system will help Nellis reduce carbon emissions by 24,000 tonnes, and will save the Air Force some $1 million per year in energy costs, the developers say.

At Nellis AFB SunPower designed and built the 72,000 module plant, again using its T20 Tracker system, while MMA financed and is operating the facility, selling electricity to the base at a guaranteed fixed rate for the next 20 years.

On the other side of the Atlantic, in Spain, the 18 MW Olivenza solar power plant in Badajoz was completed by SunPower just prior to the Nellis project commissioning using its T20 Tracker system and covering a total of approximately 70 ha.

Market Interest in Single-Axis

It is perhaps the high profile of such headline projects that continues to excite interest in single-axis trackers, not just among developers and consumers, but also from manufacturers. For example, Solon AG, which already manufactures its dual axis Mover tracking system, has now expanded its product range to include its Tauri system, a complete PV module-based single-axis tracker. The standard unit consists of 12 module rows, each containing 32 large Solon modules, and the system is tracked along the horizontal axis using hydraulics. In addition, automatic backtracking corrects the position of the units as needed to prevent the modules from shading one another. Depending on latitude, the company says its device can increase the electrical output by up to 25 percent. The system is also robust, its developers say, adding that it is designed for wind speeds of up to 80 km/hour in its operating position and up to 130 km/hour in storm position.

Meanwhile, Mecasolar has also expanded its product range with a single-axis tracker. The company says its ground-mounted single-axis azimuthal seasonal tracker increases production by five percent compared to similar traditional trackers, and by 28 percent when compared with fixed solar structures.

In its most recent figures Mecasolar reported that it had supplied 311 MW of trackers worldwide in the first half of 2011, amounting to 24,000 units. For the second half of 2011 Mecasolar has an order book of 22 MW. The company also claims to lead the tracker system market in Italy, Spain, Greece, Canada and the U.S., and has furthermore expanded into new markets such as Egypt, Israel, Colombia, Belgium, Algeria, China, Sweden, Cyprus, Bulgaria, Britain and Australia, among others. Additionally, plans are under way to enter markets in Mexico, Brazil and India, where the company plans to open an office in 2012.

As concerns over reliability and O&M costs subside, the use of tracking technology in the solar sector is expected to become far more prevalent (Source: Sedona Energy Labs)

As with other designs, a key advantage of the azimuthal tracker is that it allows manual adjustment from 25°-35° on the polar axis in response to seasonal changes. This tracker system allows a maximum of 12 kWp to be positioned on its 90 m² mounting surface and, like the Solon system, can withstand winds exceeding 130 km/hour. Each tracker carries its own automatic tracking device with astronomical programming, as well as a three-phase electrical tracker, with parasitic consumption not exceeding 40 kWh/year, the company claims.

Late last year a+f GmbH concluded a €12 million order that will see its SunCarrier devices installed in Italy for the first time. In the Apulia region in southern Italy, solar parks will be constructed with a total output of about 2 MW. The SunCarrier is a single-axis tracking system that achieves an additional yield of up to 35% in comparison to conventional market solutions, says the company.

With a module surface area of up to 287.5 m², the SunCarrier — presuming typical modules are fitted — provides an average output of around 38 kWp. If high-efficiency modules are used, output can be increased up to 53 kWp per tracking system.

Zomeworks of New Mexico has also produced a number of novel single-axis designs. In the company's Cable Tracker system, PV modules are mounted on two suspended cables. The system is oriented on a north-south axis and tracks east and west. There are two models to choose from depending on location and power requirements, using either active or passive tracking.

The passive tracking system uses the shifting weight of a liquid refrigerant and a proprietary shadow plate to keep the array of modules pointed at the sun. Meanwhile the company's active tracking system uses a controller and one or more actuators for targeting. The active tracking is more accurate — but more expensive — than passive tracking, says the company, and requires some electronics maintenance. The active system also requires a power source for the controller and various actuators.

Rather than going large, some single-axis tracker manufacturers are focusing in on a slightly different market. For instance, Germany's DEGERenergie, which manufactures large tracking systems, has developed the single-axis TOPtraker 8.5 system with an elevation angle of 20o-30o. This system is designed for a module area surface of 8.5 m² and can be operated with all available solar modules. Depending on the module type, the system's rated power is between 500 and 1300 Wp. The TOPtraker 8.5 system was designed for the requirements of the US market, being suitable for small and medium-sized solar plants and individual self-supply. It is optimised for building integration.

"We noticed that the importance of self-supply is rapidly increasing worldwide. With our extensive tracking portfolio, we are perfectly suited to support and promote this development," said Michael Heck, vice president for marketing and sales at DEGERenergie.

Double-Axis Trackers

Dual or double-axis trackers are also attracting considerable market interest. For example, since the company's market launch in 2005, more than 10,000 of its Movers have been installed worldwide, Solon says.

And, as manufacturers of double-axis machines are launching products into the single-axis sector, the previously single-axis manufacturers are also pursuing new product developments.

Titan Tracker has released a new a generation of dual-axis trackers which it claims can reduce costs by about 30 percent and increase yields by up to 45 percent thanks to improvements such as the centralisation of the elevation driving and reduction of personnel hours in the manufacturing process.

These models are aimed at solar parks located in areas with environmental restrictions, such as visual impact, and are rated for a wind speed of 125 km/hour at any position. With a maximum surface area of 124 m² for modules, Titan claims energy production of up to 45 percent more than fixed systems at 40° north latitude.

Titan saw more than 15 MW of its devices installed in Spain in the first year of launch. Several major projects followed, including the biggest solar park in Catalonia under the terms of an agreement with Flix Solar, from Finances Personals Group, to supply 128 dual-axis solar trackers. Located in Flix, the 12-MW installation is one of the largest double-axis tracking solar parks in Spain.

Titan's R&D department is also currently developing tracker prototypes for CSP systems, both tower and dish-Stirling.

ADES, a research and design engineering company with more than 150 MW of solar trackers installed since 2004 and activity in Spain, Italy, Greece and Canada, features a dual axis solar tracker design with efficient land use (1 MW on 3.5 ha), a patented 'traction clamp', an elliptical shape and lowered height to decrease shadowing. In this autonomous design a battery system provides the voltage and current necessary for all tracker movements, protecting against electrical failure due to grid proximity.

Market Development

Despite the prospects for growth of the tracking sector, tracker developments are inevitably impacted by the wider economics of the PV sector, and this has clearly been hit hard by the ongoing financial crisis. The higher capital costs of such installations, together with a residual perception of lower reliability and therefore greater project risk, are potentially making tracker developments more difficult to finance in the current risk-averse economic climate. Ongoing operations and maintenance costs also are higher.

There are a number of other influences at work too. As the costs of silicon-based modules fall in response to expanded production, a key driver for the installation of trackers — maximising their output to improve return on investment — becomes less critical. If the site is large enough, it may be as cost-effective to simply cover a larger area with fixed modules. Furthermore, the growing strength of the thin-film segment may also potentially erode the tracker market. The logic is that with a lower overall efficiency, thin-film modules require more tracker units for a given level of output, increasing their relative cost in a project's development.

However, these potentially negative factors may still be outweighed by the fact that in the concentrating solar PV (CPV) and the concentrating solar power (CSP) sectors, precise alignment is critical. Although still a relatively small market, the impact of CPV and CSP installations on the tracking sector is clearly becoming influential.

The production gains available from the installation of tracking technology simply cannot be ignored by developers and, as concerns over reliability and operating and maintenance costs subside, the use of tracking technology in the solar sector is anticipated to become far more prevalent. After all, the resources are there and it seems a shame not to use them.