Solar Trackers: Facing the Sun

Generally, modules are fixed at the optimum angle for their specific latitude. However, this is the angle optimized over the course of a year, and (depending on latitude) can vary by 30° as the sun appears lower or higher in the sky. Fixing PV modules at the optimum angle typically yields an improvement of around 15% compared with simply laying them flat. Trackers, on the other hand, adapt to both the daily passage of the sun and potentially the changing seasons too. And in many concentrating solar technologies (PV and thermal), tracking is an essential component.

Whatever the case, in any solar application the conversion efficiency is improved when the modules are continually adjusted to the optimum angle as the sun traverses the sky. As improved efficiency means improved yield, use of trackers can make quite a difference to the income from a large plant. This is why utility-scale solar installations are increasingly being mounted on tracking systems. (Left: A 4-MW tracking solar PV plant, Spain. Credit: Poulek Solar Co Ltd)

The development of feed-in tariffs and other similar support measures that reward PV producers per kilowatt hour delivered to the grid, has stimulated this growing interest in maximizing output from a given area, along with the relatively high prices for silicon-based PV modules that have been witnessed over recent years. With key economic drivers dictating that developers maximize system output, interest in tracking technology has soared.

Indeed, Paula Mints, principal PV analyst at Navigant Consulting, has reportedly forecast that between 2009 and 2012, tracking systems will be used in at least 85% of PV installations above 1 MW.

In Spain, for example, which has seen its solar market surge in response to government policy initiatives, tracker projects went from making up an insignificant part of the market in 2006 to perhaps 25%–30% of new projects in 2008, estimates Maria Lahuerta Antoune, international marketing manager for ADES – a tracker manufacturer based in Zaragoza, Spain. ADES reportedly saw a 40% increase in production in 2008 as the country installed a total PV capacity estimated at around 2.5 GW.

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, azimuthally moving from east to west over the course of a day. Double-axis trackers rotate both east to west and zenithally (vertically).

The numbers are latitude-dependent, but compared with modules fixed at the optimum angle, single-axis trackers typically increase electricity output by between 27%–32%. Meanwhile, dual-axis trackers typically add a further 6% on performance and see a 35%–40% improvement in output when compared to fixed panels. (Right: By tracking the sun throughout the day, tracker systems will boost daily energy production; this system is in Puertollano, Spain.)

Such figures are particularly impressive considering the lengths that the solar PV industry goes to in order to improve cell conversion performance by just a percentage point or less.

With the economics apparently clear, the choice of what type of tracker to use is largely a decision concerning the extra investment and the cost of maintenance over time, in relation to the increased energy (and financial) yield delivered by the system.

One of the first decisions is the choice between active or passive tracking systems. Active trackers, which use motors, gear trains or hydraulics to move the module, consume energy, while passive trackers use a low boiling point compressed fluid to move the system. Driven by solar heat creating gas pressure in the system, passive systems do not consume any energy, but they are also less precise than driven systems which rely on light-sensing technologies.

A further type of actuator responds to the motion of the earth relative to the sun through a simple mechanical geared system which rotates the module at an equal speed, but in the opposite direction. Theoretically such a system would complete one revolution per day, though in practice the module is likely to be reset to the start position at night.

Single-axis trackers

Single-axis tracking is one of the most straightforward ways to improve the potential performance and economics of a commercial solar installation. By using relatively simple equipment, considerably more performance can be expected. Tracking system manufacturer RayTracker, for example, says its systems have been proven to improve energy yield over fixed angle modules by up to 23% and over flat modules by 38%. (Left: Trackers adapt to both the daily passage of the sun and potentially the changing seasons, too. Credit: Concentrix Solar)

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 on-going maintenance costs. And, such systems are at greater risk of failure, having more moving parts than a single-axis tracker. 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.

The commercial opportunities represented by single-axis tracking technology are clearly not lost on the market.

For instance, RayTracker Inc. was recently formed in a move by Energy Innovations, Inc., which spun off the group to further expand, develop and capitalize on the growing demand for its RayTracker GC product line, which the company says was included in more than 2 MW of solar installations in 2008. ‘The RayTracker business unit has seen, and continues to see, tremendous demand from customers looking for a cost effective way to maximize the return on their solar PV installations,’ commented Bill Gross, founder of Energy Innovations, in announcing the decision.

The company says the machine’s precision, a shadow avoidance algorithm, no scheduled maintenance and low cost installation, are the key selling points for the design, adding that RayTracker, Inc. – an operating company of Idealab – was founded with the belief that ‘trackers provide a clear opportunity to increase the return on investment of every new commercial and utility solar installation.’

SunPower’s big deal

One of the world’s largest single-axis manufacturers is California-based SunPower Corp, which signed a number of significant deals during April 2009.

FPL Group, one of the largest renewable developer utilities in the United States, has inked a solar power supply agreement with SunPower – which has in turn said it will establish a Florida R&D centre with up to 50 employees if the state continues to pursue a ‘robust’ solar programme. SunPower will work with FPL and the state to identify a suitable location for such a research facility and the company adds that continued strong demand in Florida could also lead to manufacturing and distribution centres for solar panels and tracking systems being located there too.

The FPL supply agreement for SunPower’s solar modules and its tracker technology begins in 2010 and runs through to 2012 with deliveries to FPL Group subsidiaries Florida Power & Light Company (FPL) and NextEra Energy Resources.

FPL Group president and COO Jim Robo said: ‘For NextEra Energy Resources, this agreement will further advance our solar development efforts in key markets such as Colorado, California, Arizona, and New Jersey. In addition, assuming continued support by Florida’s legislative and regulatory leadership for the deployment of solar power in Florida, this agreement benefits the state through the creation of more clean energy jobs and will help to ensure that our utility customers in Florida will get the best pricing and technology available for solar projects.’

The latest move from FPL follows a deal with SunPower for the 25 MW DeSoto Next Generation Solar Energy Center. The plant, currently under construction, will be the largest PV facility in the United States when it is completed at the end of this year, using 90,000 modules across 180 acres (72 ha) of land and providing more than US$2 million in annual tax revenues by the end of 2010.

A similar development, the Space Coast Next Generation Solar Energy Center, will also use SunPower’s latest solar panel technology and the projects are two of three solar developments from FPL in the state that will produce some 110 MW combined.

‘FPL Group is the leader in providing renewable energy, having invested nearly $10 billion in growing that business. It’s already the world leader in deploying utility-scale solar and we believe that it will continue to be a driving force in the industry,’ noted SunPower CEO Thomas H. Werner.

Meanwhile, close to 1000 miles (1600 km) north in Chicago, Exelon and SunPower have revealed plans to develop America’s largest urban tracking solar power plant, a 10 MW installation on a South Side brownfield site. The $60 million power plant at a former industrial site is scheduled for completion by the end of this year, contingent upon Exelon receiving a federal loan guarantee for up to 80% of the project cost, under the recently passed federal stimulus legislation bill – the American Recovery and Reinvestment Act. Utility group Exelon plans to lease 39 acres (15.6 ha) of the West Pullman Industrial Redevelopment Area from the City of Chicago for the project and will own and operate the plant, which will feature 32,800 modules.

Late last year SunPower also announced that it is to build a 505 kW solar tracking installation for Australia’s Horizon Power, a government-owned company. The ground-mounted development on two sites in Marble Bar and Nullagine, in the east Pilbara region of Western Australia, will be the largest solar tracking system in Australia. Construction is expected to be complete by September 2009.

And, in perhaps the company’s best known development, North America’s largest solar photovoltaic system at Nellis Air Force Base 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, which was commissioned in December 2007.

This development is expected to generate more than 30 GWh annually and supply approximately 25% of the total power used at the base from its 140 acre (56 ha) site.

SunPower designed and built the 72,000 module plant using its proprietary single-axis 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. Again, using its T20 Tracker system and covering a total of approximately 70 ha, SunPower says it has now completed construction of more than 165 MW of Spanish power plants, out of a worldwide portfolio of over 400 MW.

New market interest in single-axis

It is the 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 too. For example, Solon AG, which already manufactured its dual axis Mover system, expanded its product range late last year with the launch of its single-axis model. It began offering this newly developed complete single-axis photovoltaic system for large-scale projects early in 2009. The standard unit consists of 12 module rows containing 32 large Solon modules each, and the system is tracked hydraulically along the horizontal axis. 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 output by up to 25%. The system is designed for wind speeds of up to 80 km/hour in operating position and up to 130 km/hour in storm position.

Meanwhile, Mecasolar has expanded its product range with a new single-axis tracker that will join the dual-axis solar systems currently designed and manufactured by the company at its production plants in Spain and Greece, as well as plants that are due to come online in Italy and the US in late 2009. The company says its ground mounted single-axis Azimuthal seasonal tracker increases production by 5% compared to similar traditional trackers, and by 28% compared to fixed structures. (Right: Rear aspect of a tracker mounted module. Credit: Sun Surfs)

As with other designs, a key advantage of this new product is that it allows manual adjustment from 25° to 35° of the tilt on the polar axis in response to seasonal changes. Mecasolar says it will present its new product as this goes to press (early May) and in 2010 expects to reach a production capacity of 200 MW a year for its mounting systems, including fixed structures and trackers, both single and dual-axes, in 2010.

The tracker allows a maximum of 12 kWp to be positioned on its 90 m² mounting surface and 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 consumption not exceeding 40 kWh/year.

Moving larger still, Germany’s a+f – a subsidiary of Gildemeister – recently presented its new ‘SkyCarrier’ for the first time. With a module surface of 247 m², 30.5 metres high and 8.1 metres across, the group notes that depending on the type of module used, more than 33 kWp can be installed on a single SkyCarrier unit.

This tracking system for open-space solar installations is designed for use between the equator and latitudes of 30°. In some places, the angle of the sun here can be up to 90°, the company says. The rotational axis of the tracking system ensures that the pitch angle of the module surface moves at an angle of +/-30° relative to the horizontal plane, and the modules are therefore always precisely aligned with respect to the sun. The system guarantees an additional yield of up to 25% compared with conventional fixed solutions available in the market, says a+f.

The drive system is provided by two electrically synchronized motors with a planetary gear and a steel chain, which the company says requires virtually no maintenance. The device joins its existing product, the single-axis SunCarrier tracking system, launched in 2006. With a module mounting surface of up to 287.5 m², depending on the type of module used, an output of up to 40 kWp can be installed on the device, which uses an average of about 3500 kWh per year.

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.

Double-axis trackers

Dual or double-axis trackers are also attracting considerable market interest, for example, since its market launch in 2005, more than 10,000 Solon Movers have been installed worldwide, the company says. Meanwhile, along with Mecasolar, Solon, Titan and so on, ADES is another of the larger manufacturers of such systems, with some 150 MW already in service.

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

Titan Tracker SA, for instance, has developed two new models of solar trackers with an adjustable maximum height, specifically designed in order to reduce the visual impact of solar installations.

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% more than fixed systems at 40° north latitude, together with reliability similar to fixed systems, and cost-effectiveness over the life cycle of the solar installation.

The company has also just signed a strategic alliance with Jiménez Belinchon, licensing the manufacture and marketing of its dual-axis trackers in Europe and Central America.

Titan, which specializes in the manufacture and marketing of dual-axis solar trackers, high concentration PV (HCPV) and thermal (CSP) tower and dish Stirling systems, 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 two-axis solar trackers and which was commissioned in August 2008. Located in Flix, the 12 MW installation is one of the largest double-axis tracking solar parks in Spain.

Meanwhile, earlier this year ADES announced a first project using its dual-axis technology in Greece, with machines installed in early March. The company says that height limitations of trackers on the Greek market have led to the sale of a new elliptical model, which allows a higher power density of 1 MW per 3.7 ha and is adjustable in height to the requirements of the Greek legislation. The project will feature three solar trackers each holding 33 kW of modules for a total power of 100 kWp for developers Mechanodomiki, based in Thessalonica.

The largest dual-axis tracking system currently operating in the US was commissioned in February this year by Solar Power Partners (SPP), an independent power producer which sells the output from its owned and operated facilities to consumers through long-term Power Purchase Agreements (PPAs), in this case 20 years.

With 89 units producing 1 MW at the West County Wastewater District (WCWD) in Mill Valley, Richmond, California, Premier Power was SPP’s EPC contractor on the 5 acre (2 ha) project. ET Solar trackers, measuring approximately 22 by 36 feet each (7 metres by 12 metres), are used at the site and the arrays contain 56 modules with a total peak power of about 11.24 kW each. The system produces approximately 1900 MWh annually, replacing some 35% of WCWD’s electricity usage.

Of the trackers installed on the site, 28 rise above the potential high-water line of a storm-water storage pond and all of the tracking towers are designed to withstand flooding and submersion while meeting all seismic restrictions and solving soil engineering challenges.

President of Premier Power, Dean R. Marks, observed: ‘Our engineering team discovered this large-scale tracker solution out of our European operations and felt that their design and capabilities provided the best option for meeting the site challenges and production needs of WCWD and SPP.’

Market development

Despite the prospects for growth of the tracking sector, inevitably, tracker developments are impacted by the wider economics of the PV sector, and this has clearly been hit hard by the on-going 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. On-going 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 – maximizing 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 be outweighed by forecast growth in the concentrating solar PV (CPV) and the concentrating solar power (CSP) sectors, where 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. For instance, Spanish engineering company SENER Ingenieria y Sistemas, S.A., is to supply 2650 dual-axis electromechanical drives for a CSP facility that is being built in Fuentes de Andalucia, Seville, Spain. The biggest contract awarded to date for two-axis drives for heliostats, the deal will see SENER supply the units for the Gemasolar central tower project belonging to Torresol Energy and due for completion in 2011.

SENER says it has specifically designed, developed and patented its high accuracy two-axis drive to cover the needs of Central Tower Receiver, dish-Stirling and CPV plants.

Titan, meanwhile, has just finished commissioning a CPV system using its dual-axis tracker units at Spain’s Institute of Concentration Photovoltaics Systems – the Instituto de Sistemas Fotovoltaicos de Concentración (ISFOC) in the Castilla-La Mancha Region. The company says the units installed at the site provide accuracy of better than 0.01°, and it is possibly the most accurate and reliable solar tracker around the world.

ISFOC is also the location for the installation of a new tracking system by Abengoa Solar which was customized for use with FLATCON concentrator modules from Concentrix Solar, which has now qualified tracking systems from two different manufacturers. In November 2008 Concentrix installed a system at the Abengoa test field in Seville. At the site in Puertollano some 38 Concentrix are installed, 27 of which use trackers from the company Pairan. Concentrix Solar’s installation for the ISFOC project in Puertollano is now complete with a total capacity of 300 kW. The company says that ISFOC confirmed a nominal power of 13.3 kW for each of the new tracking systems in March 2009.

Looking to the US market, José C. Martin, director of SENER Engineering and Systems Inc., the SENER subsidiary based in San Francisco, noted: ‘The US market has big opportunities for developing CSP plants and we are closing agreements with local companies for the construction of projects in the short term, the first of which could start in 2010.’ It is clear that Martin’s words apply just as well to other concentrating solar technologies and other regional markets.

Elsewhere, 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 it.

David Appleyard is associate editor of Renewable Energy World. e-mail: rew@pennwell.com.