Lubrication-free polymer plain bearings for photovoltaic tracking systems

"The basic idea of our company was to realise solar tracking with smart technology in an affordable and effective way," explains Thomas Vogel, Managing Director and company spokesman of TecnoSun Solar Systems AG in Neumarkt/Oberpfalz. Founded in April 2010, the company employs around 12 people and manufactures photovoltaic (PV) systems.

The company's location is favourable: the conditions in a city that focuses on renewable energies offer good framework conditions for the young company. As the decentralisation of the electricity markets remains an important topic for the future, it is primarily local authorities and municipalities, but also private investors, who are focusing on renewable energy.

While the basic system originally consisted of a two-axis guideway, it is now supplemented by a single-axis guideway. In addition, there is an aerodynamic system for flat roofs. This is a permanently mounted solution that can be installed on flat roofs without damaging the roof cladding. "And at Intersolar, we are presenting a system for noise barriers that is already attracting a lot of interest in advance," says Thomas Vogel. "It will be in series production by the middle of the year at the latest."

Problem

TecnoSun Solar Systems AG was looking for plain bearings for vertical tracking systems. The requirements for the bearings were quite high: they had to be low-friction and characterised by high performance. In addition, the bearings had to be weather-resistant for decades, as the systems are exposed to heat, cold, rain and snow. Maintenance and servicing work must be quick and uncomplicated.

Solution

With iglidur ® plain bearings, the photovoltaic expert has found a cost-effective solution that fulfils its requirements. Thanks to the incorporated solid lubricants, the plain bearings have low friction and make the bearing points maintenance-free. This makes additional lubricants superfluous. The plain bearings are insensitive to extreme weather conditions and even dirt is no obstacle to their functionality.

Tracking systems for PV systems (trackers) are systems that are independent of the orientation of the base area. They follow the course of the sun during the day and thus create an optimum angle between the sun and the solar module, optimising the utilisation of the modules.

The additional yield from the use of a tracking system has been proven to be up to 40 %. A significantly higher output and a consistent power yield is achieved throughout the day. "Until now, tracker systems have often been criticised for their disadvantages," says Thomas Vogel. "They are generally considered to be too expensive and disfigure the landscape due to their size, as they are usually several metres high. We took a different approach and wanted to keep the installation height low, which also greatly reduced costs compared to other systems."

The advantages of this provider's tracking systems are therefore quickly summarised: they include the low installation height of around 1.5 m and the associated material efficiency. As a result, the PV systems can be integrated well into the landscape and offer a small area exposed to the wind. Then there is the drive concept. Up to 120 trackers are operated with just one motor. This corresponds to a module area of around 400 m2. This has significantly reduced the system costs.

Normally, a tracked system is anchored to the ground with concrete elements that have to be laboriously removed at some point. "Our anchoring consists of ground screws, on which the relatively simple mechanical system, a pure metal construction, is built," says Thomas Vogel. The friction in the system is completely negligible. This is ensured by lubrication-free polymer plain bearings from igus® GmbH, Cologne. There is almost no application in which the lubrication-free bearing technology does not prove its versatility.

Three unlubricated bearings are used between the drum and the traction ring, the base plate. "At this point, we were looking for a high-quality, high-performance solution that, on the one hand, had to run completely dry and, on the other, must not be susceptible to damage due to the ambient conditions," says Thomas Vogel, looking back. System service lives of several decades are part of the requirement profile. And if the worst comes to the worst, it must be possible to replace the bearings quickly and easily during maintenance and servicing work.

The all-round material iglidur® P fulfils the requirement profile. Due to its thermal stability and low total moisture absorption, this bearing is one of the most dimensionally stable in changing ambient conditions. It is generally recommended for rotary movements with medium to high loads. It is also cost-effective, which in turn has a noticeable effect on the economic side.

"In the run-up to the system development, we carried out a series of extensive practical tests with a wide variety of bearings in order to be able to make reliable statements about the behaviour under load. Various suppliers presented their products," says Thomas Vogel. "The test results always spoke clearly in favour of the lubrication-free polymer plain bearings. Added to this is the good price-performance ratio and the comprehensive support. Our sales representative, Bernhard Hofstetter, is in constant dialogue with us and keeps us informed about new and further developments so that we are always up to date with the latest trade show technology."

The requirements profile for the plain bearings is extremely high. They must be weather-resistant for decades, withstand dirt and moisture and be easy to install. Weight due to snow loads also plays a role. They are also subjected to heavy loads in some areas, as relatively strong rotational movements act on the basic mechanical structure. In addition, tensile loads of several 100 kg can occur at the corner points where the cable pulls.

Before deciding which material is best suited for this requirement profile, the polymer supplier also carried out various load calculations and recommended the material iglidur® P according to the exclusion procedure.

Before deciding which material is best suited for this requirement profile, the polymer supplier also carried out various load calculations and recommended the material iglidur® P after the exclusion procedure.

In contrast to applications in mechanical engineering, the non-pressed-in bearings rotate around fully galvanised surfaces. Due to the necessary installation tolerances, there is therefore a residual risk that the bearing can seize both externally and internally. Here, too, preliminary tests have shown that the polymer plain bearing has no problems with this. The bearings run in the tracking systems from day one without any unnecessary downtime.

"Our standard material iglidur® G, which has proven itself in countless applications worldwide, was also up for discussion," reports Bernhard Hofstetter, the technical sales consultant responsible at the igus® Bavaria North-East office. "We initially carried out the first samples. But due to the lower total moisture absorption and thermal expansion, the decision was finally made in favour of the iglidur® P material."

The iglidur® Q2 material also proves its wear resistance in a two-axis system located in the Swiss mountains at an altitude of 1,800 metres. Dimensionally stable under high loads, it is predestined for highly stressed swivelling applications under extreme conditions.

"At this altitude, the temperatures fluctuate extremely throughout the year, not to mention the snowfall in winter," explains Thomas Vogel. "To make our tracking system even more robust and easy to maintain, we use the wear-resistant bearing. Under these difficult climatic conditions, the load is naturally the heaviest. And our initial experiences are very good. The polymer bearing also fulfils our basic requirements for economical solutions without compromising on quality!"