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Always on the sunny side

Lubricant-free polymer bearings for shade-free solar tracking systems

Photovoltaic tracking systems offer a series of benefits. By automatically following the position of the sun, they perfectly exploit the sun's energy and therefore have a particularly high level of efficiency. This supplier guarantees that the modules in its system remain completely out of the shade, moreover. So that these can work in wind and weather over decades without problems, the company relies on maintenance-free, durable plastic plain bearings. “Tracking systems pay for themselves very quickly despite higher purchasing costs,” explains graduate engineer Gerhard Dirscherl, Head of Development at RWenergy GmbH from the Bavarian town of Schwandorf.
“Compared to a fixed inclination, they achieve an additional yield of up to 30 percent. This results from a more efficient utilization of the morning and afternoon periods, whereby the tracker systems align themselves to the sun in an optimal manner. That naturally increases their level of efficiency in the long run. ” The various systems from the photovoltaic specialist are based on components and design principles that have proven themselves thousands of times over in the automotive industry. Basically it is a moving steel construction. All the components used here must be totally robust and wear-free. That particularly applies to the bearing technology. The solar plants use tribo-optimized polymer bearings from the bearings specialist igus® GmbH, Cologne. Besides being lubricant and maintenance-free, they are sturdy and ensure a long service life. “This is the only way that we can give our customers a 25-year operational guarantee,” says head of development Dirscherl.
Solar tracking system “s:track” solar tracking system from RWenergy GmbH in horizontal midday position

Optimally aligned

In 2006 the company came into the market with the single axis “s:wheel” tracking system. The ground-level system, which is hence not affected by wind, achieves up to a 30 percent higher energy yield than stationary systems. The azimutal tracking always the sun's energy to be perfectly utilized. The consistent alignment takes place at ten minute intervals. The system guarantees maximum yield due to this exact alignment with the actual course of the sun. At the same time, both the longitude and latitude of the place of installation is taken into account, as well as the shift between the actual solar time and the local time. “In recent years we have gained other footholds in the market besides this,” reports Gerhard Dirscherl. “Besides our tracker systems, we provide solutions both for rooftop systems and for ground installations.” In the tracking systems sector, the sturdy “s:track” model rounds off the product range at the bottom end. Compared to stationary systems, the yield here is up to 15 percent higher. The first text plant was put into operation recently in Mexico. More plants are in the pipeline.

Whereas the “s:wheel” single axis system rotates on a vertical axis, the “s:track” involves a horizontal tilting system. One axis runs in a north-south direction and makes the pivoting movements from when the sun rises in the east, through to the midday position, and continues westwards in the horizontal plane to the evening position in short motion intervals.
Robust lubricant and maintenance-free plastic plain bearings take care of the tilting movement on the tracking system. Robust lubricant and maintenance-free plastic plain bearings take care of the tilting movement on the tracking system.
Steel construction systems are not always aligned to the last millimeter. This generates high forces, which the plastic plain bearings must be able to withstand. Steel construction systems are not always aligned to the last millimeter. This generates high forces, which the plastic plain bearings must be able to withstand.

Yield curve must be right

“Other manufacturers also make the classic horizontal tilting unit,” explains Gerhard Dirscherl. “We guarantee total freedom from shade, however, as a unique selling point in our systems. Highly complex control software ensures that the modules stay out of the shade both in the early morning and late evening hours. The unit continually moves into the ideal position in increments of 2°, whereby the system is optimally aligned with the sun to give the best yield curve for the operator. ” The young company RWenergy GmbH, which belongs to the Josef Rädlinger group of companies in Cham, has settled on the premises of a former power plant. Here it operates its test plants on the roof of a decommissioned flue gas desulfurization plant at a height of almost 70 m. Although the photovoltaic market is undergoing change, the company is looking to the future with optimism. “From home owners to investor groups – with the right systems we can satisfy all the market requirements,” declares the head of development.
The “s:track” tracking system is 26 meters long and 3.2 meters high. Simple in design and profitable in use, the horizontal tilting unit proves its strengths above all in regions near to the equator. The number of cloud-free hours is good here over the year, meaning that the morning and evening hours can also be effectively used. However, there is often a lack of experience fitters in these regions to erect the plant. “The systems must be accordingly simple to assemble,” points out Gerhard Dirscherl.

Robust maintenance-free polymer plain bearings

The photovoltaic specialist uses lubricant and maintenance-free plastic plain bearings from igus® GmbH in all its solar tracking systems. In “s:wheel” they take on a centering function above all. And in the pivoting axis of the “s:track” horizontal tilting system, they prove their strengths in many ways. Pressed into a fully galvanized standard tube, combined with a steel shaft, they must work reliably despite the prevailing installation tolerances. At the same time they tilt daily by +/- 45°. Above all this comes down to insensitivity during assembly, smooth handling and a long service life.

The systems are not always aligned to the last millimeter. “You have to take tolerances into account in steel constructions,” says Dirscherl. High forces are generated in this case, which the polymer bearings must be able to withstand. Added to that are difficult environmental conditions: accordingly, dust, sand, rain and heat must not have any effects on the functional capability of the overall system. All components must hold their own outdoors over the long term. “Should, for example, the plastic bearing assembly happen to fall onto the dusty ground during assembly, that does not play any part in its functional capability,” says the head of development. “With a sensitive ball bearing assembly, on the other hand, we would experience serious problems by now at the latest. “

Endurance tests conducted in advance

“At the focus of our overall systems is the simplicity for the user,” explains development head, Dirscherl. “Our customers are spread all over the world. We cannot replace the bearings every few months. Furthermore, we give our customers the guarantee that they can operate their plants for at least 25 years. ” The igus® technical sales consultant, Bernhard Hofstetter, reports on a service life calculation that was carried out in advance in this respect. “We incorporated all the critical plant parameters, such as temperature, friction and stress. We were able to prove that the service life of the polymer bearings far exceeds even the service life of the plant. With these results we are in a position to show that the machine elements work out for the customer. “
The “iglidur® G” polymer bearings work in dry operations at temperatures of –40 °C to +130 °C and are insensitive to dirt and sharp edges. The “iglidur G” polymer bearings work in dry operations at temperatures of –40 °C to +130 °C and are insensitive to dirt and sharp edges.

Insensitive to dirt and sharp edges

“iglidur® G” was chosen as the polymer bearing material. These cost-effective all-round bearings cover a very wide range of applications. They work in dry operations at temperatures of –40 °C to +130 °C. At the same time they are insensitive to dust and dirt, suitable for simple rotational and pivoting movements. The robust bearings also cope with peak and edge loads without problems.

Altogether, 15 “iglidur® G” bushings are currently installed in the five steel struts for the north-south axis on the “s:track”. In total there is an available installation space of approx. 100 mm. Due to the high tolerances, three bushings are installed on each strut. “The third bushing basically has no function. It is only installed as a precaution,” says Gerhard Dirscherl. Hence there is the option of inserting a special bearing assembly with a length of 47 mm from left and right as a flange bushing. In this way one bearing can be saved, which further minimizes sources of faults and makes assembly even easier. Dirscherl appears to be very happy with the technical advice, moreover: “The collaboration is good, as can already be seen on the igus® proposal for the special bearing assembly. Although the numbers on new developments can often not be foreseen to start with, we always feel very well looked after. “

energy chain for photovoltaic plant

The “s:wheel” solar tracking system for medium and large solar energy plants also uses a multi-dimensionally moving “triflex® R” energy chain system from igus®. This securely guides the solar cables. The power is discharged at the module's rotating centre. The plastic energy chain is put under the highest stress at this point. Whereas the complete housing remains rigid, the chain takes a radial movement with every switching, so that it is increasingly stressed with each step. The high tensile strength is obtained by the so-called "trailer principle", which interconnects the individual elements similar to a towing hitch. The required mobility in all directions is ensured by this ball-shaped formation. In this way a trouble-free operating period can be guaranteed beyond the feed-in period.

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