The replacement for Cotlan AG’s traditional small hydroelectric station, a former textile factory in the Swiss canton of Glarus, was able to take shape thanks to a period of intensive construction work in recent months.

The construction of the now entirely underground headrace in particular made this power plant project a real challenge. The powerful Kaplan-SZ turbine, which was provided by the Austrian hydroelectric power specialists Kössler, has been in operation in the new plant on the Linth river since last autumn. Designed to produce an output of 2.5 MW, in an average year the machine ensures around 12 GWh of clean energy from regional resources. As a result, this new low-pressure facility is the most efficient power plant on the Linth. The ceremonial opening will take place in June this year.

You wouldn’t know it from looking at the romantic, sleepy Canton of Glarus, but it is nevertheless the most heavily industrialised canton in Switzerland – the very same canton in which the textile industry once burst onto the scene. The first textile factories that took the calculated decision to settle here did so principally because of the many streams and rivers in the area. These waterways allowed the machinery to be operated efficiently. By 1870, there were already around 22 textile printing plants and 24 spinning mills in Glarus. Around a third of the canton’s residents earned their living in these factories.
One of those old traditional factories was established in Rüti in 1847. Like so many others at that time, it was also reliant on water power from the Linth river. The hydropower station in Rüti formed the economic backbone of the textile company based there for many decades, initially providing mechanical drive and later producing hydroelectric power. In 1936, construction began on the Cotlan power ­station in order to supply the cotton mill with electricity. That same mill was taken over by Cotlan Textilfabriken AG in 1979, which continued to operate the business. The factory just about made it into the new millennium, although it was closed down shortly ­afterwards in 2002. The power plant that had undergone so many renovations throughout the decades was still running, but had long since reached the end of its technical lifetime. It was no longer economically feasible to operate and was even causing disruptions in the local distribution net. It became clear that renovation or re­placement had become inevitable.

Headrace drawn in from upstream
An important factor in the operator’s considerations was the fact that the power plant was built before 1918, and therefore that no new permits would be necessary for an expansion within the perimeter used at the time. With this in mind, it was decided that the new version of the power plant should make better use of the hydrological potential. To this end, a previously unused 400 m long watercourse segment to the upstream power station was incorporated into the plan. However, a new permit would have to be obtained for this stretch. “A key point in the new planning strategy is to move turbine-driven works water from the upstream facility through an underground pressure channel to a completely new under­ground pressure pipeline. This will allow us to use the already clean water from upstream a second time”, explained the project leader for Cotlan Wasserkraft AG, Friedrich Winkler. This approach opens up a number of further advantages: since the water has already been cleaned of sediments, the old weir systems with rakes, rake cleaning machines and sand traps become obsolete, which is an important cost-saving factor. It moreover removes an obstacle that previously prevented fish from migrating along the river. The transition from a headrace channel to a pressure channel also allows for a more efficient energy yield. What’s more, the recultivated area can now be used once more for farming purposes. Finally, these measures will allow for the dismantling of the existing version, the diversion channel and, lastly, even the old power house to go ahead.

Elaborate underground pipe jacking
In terms of construction, the production of the 1,150 m long underground penstock ­poses the greatest and most complicated challenge for the building contractors and the contracted companies. One part was constructed in an opencast mine, but a 640 m long section was built using an elaborate technique known as pipe jacking. The subsoil turned out to be particularly difficult as it contained a high proportion of loose rock. This meant a large wear load for the drill head of the pipe jacking machine. “An additional challenge presented itself in the form of the high permeability of the subsoil. Due to the minimal blanket under­neath the Linth, which the tunnel ­crossed below, and because of the high groundwater levels, the support pressure had to be handled very precisely. As part of the pipe jacking, a curve radius of 500 m was  ­d­rawn, the gradient of which measures ­between 0.5% and 6%” Friedrich Winkler ­explained. In total, the tunnel driving works required around 6 weeks, during which the drill head, which was roughly 3.8 m in dia­meter, cut through the subsoil. The construction work for the project started back in 2014 after the operators had acquired the necessary permission. The breakthrough at the end of the tunnel was celebrated in September 2016 – a vital milestone for the power plant project.
The underground drilling process required very exact planning across the board. In the end, the path of the drill passed very close by the old vaulted tailraces of two other power plants that are still operational. However ­the­re was no viable alternative to the pipe ­jacking method. By using this method, there were very few problems in designing the ­construction of the pressure pipeline. The 44 ton pipe components were each compressed with a weight of up to 1,500 tons and then joined together.

Technology from Lower Austria
While the drilling machine was working under­ground last summer, the machine ­assembly process in the facility was already on the home straight. The company contracted to produce the machinery, the Austrian hydro power specialists Kössler, had delivered the turbine in May 2016 and then started on the assembly process. The operators had decided to acquire a Kaplan-SZ turbine with a ­h­orizontal axis from the specialists at Kössler, who enjoy an excellent reputation far beyond the DACH countries. “During the bidding phase, we were able to win over our client’s decision makers with a technically sound ­design for a modern, 6-blade Kaplan turbine. We were awarded the contract in July 2015. As well as supplying the turbine, assembling it and making it operational, we also provided the generator, the hydraulic turbine control system, the cooling water system as well as the entire electrical apparatus, which was developed in collaboration with Schubert Elektro­anlagen” explained the project leader at Kössler, Georg Neuber.

Efficient 6-blade turbine with customised technical solutions
The particular version of solid cavitation blade wheel with 6 blades met the requirements of the operators perfectly. Using this equipment allowed the installation height to be refined and for the project to do without deep foundations. This resulted in reductions in the building costs while retaining maximum efficiency. Turbines produced by the specialists at Kössler usually have a reputation of being at the forefront of hydropower engineering. Customised solutions could therefore be ­developed, such as the incorporation of the blade wheel adjustment cylinder in the impeller hub, which is an unusual feature in Kaplan machines in small hydropower facilities. More­over, a 3-bearing arrangement was chosen for the project, consisting of 2 generator bearings and 1 turbine bearing. The turbine guide bearing acts as a slide bearing with clean water lubrication. Georg Neuber: “Due to this 3-bearing arrangement, fitting and aligning the shaft assembly of the turbine and generator posed a difficult challenge for the technicians. The procedure called for considerable precision and, of course, plenty of experience. In the end, it worked very well indeed.”
The control system specialists at Schubert Elektroanlagen were given very specific requirements in one particular detail. “Due to lack of space, the headwater reservoir had to be kept very small. This requirement proved to be very demanding in terms of the control system. Fortunately, the technicians at Schubert were able to overcome this challenge with an excellent solution” reported the project ­leader at Kössler.

The most efficient power plant on the Linth river
The dual-control Kaplan turbine is connected firmly to the generator and powers it with a nominal speed of 375 rpm. The Kössler turbine is designed for a net head of 18.74 m and a design flow of 15 m3/s. Under full load, the machine therefore achieves a nominal output of 2,537 kW. In comparison with the output of the old system, which could produced 320 kW, the new Cotlan power plant has almost eight times the amount of installed capacity. As a result, the facility is now the most efficient and powerful small hydropower plant on the Linth. “In an average year, we would expect an annual production of around 12 GWh. That’s enough to supply around 2,600 homes in Glarus with clean, locally-produced electricity” said Friedrich Winkler.
The operators invested around 22.5 million Swiss francs in the new power plant, which opens a new chapter in the region’s long history of using hydroelectric power from the ­Linth river. The facility became operational in January 2017, and successfully came through its first few months of trials. With the ceremonial opening of the new power plant at the end of June 2017, it will finally also re­ceive its official inauguration.