Glacier Water Drives Hydropower Plant

Autor: David Tscholl , 23.05.2013

Energie Schnals Konsortial- GmbH invested around 8 million euros into the construction of the hydropower plant, which uses two structurally identical machine units to generate around 13 gigawatts of electricity.

The facilities have been fully up and running since June 2012. The highest situated aerial cableway in South Tyrol leads from Kurzras at the head of the valley up to Mount Hochjochfern in the Ötztal Valley, around 3,200 m above sea level. The region owes its worldwide fame not so much to its touristic attractiveness but rather to the discovery of the “mummy of the Hauslabjoch”, better known as “Ötzi”. No doubt the cableway up the glacier, which was built here in the first half of the 1970s, has contributed greatly to the touristic development of the Schnalsertal region. To supply the cableway with local electricity, a hydropower plant was erected around a decade after the cableway had commenced operation. The plant uses the water of the Schnalserbach stream, which flows down a 400 m head towards the two twin-jet Pelton turbines, enabling them to generate a combined output of around 3.2 megawatts. This plant is still an important contributing supplier of electricity to the Schnalstaler Gletscherbahnen.  

OPPORTUNITIES OPEN UP AS SEWAGE WORKS CLOSE DOWN
The idea of using another gradient step down to the Vernagtsee for generating hydroelectricity was already being considered at the time. “There used to be a sewage works in the tail water area of the Kurzras power station, which used the Schnalserbach as a receiving water course. This made it impossible to get the necessary building permission for a second power plant,” explains the town’s former mayor, Hubert Variola. “The opportunity came when the sewage plant was closed down and the sewage water from Kurzras could be redirected to the central sewage works in Schnals,” says Variola, who currently serves as chairman of the Supervisory Board of Energie Schnals Konsortial GmbH. Subject to the local regulations for environmentally compatible, nature and landscape friendly building construction, the project was given the green light by South Tyrolean authorities in 2008. Experts in limnology (i.e., the study of fresh water bodies) and landscape preservation were consulted to help in working out the plans. Initially, the water rights were linked to the permission of the Gletscherbahnen, which is due to run out in 2015. This was unacceptable to the operators. They immediately appealed the decision, which was revised in their favour when they pleaded their case. In the end, they were grated a permission for 30 years. Regarding plant operations, the communal government finally negotiated a solid partnership with SEL AG, which now holds a 40% stake in Energie Schnals Konsortial GmbH. Like many other hydropower plants throughout South Tyrol, this one is now also operated by Hydros AG.

TIGHT SCHEDULE FOR CONSTRUCTION
When construction was began in the summer of 2010, it was clear that the project would involve massive building and engineeringwork. “The Schnalserbach intake lies at around 2,150 m above sea level. This gave us only a very narrow time window to complete our work during the warm season. In view of this situation, construction work was contracted out to several firms so that they could work the various construction sites in parallel. As a result, the teams were working up to ten machines at the same time,” says Dr. Ing. Peter Pohl of the Pohl engineering team from Kastelbell.

MOTIVE WATER FROM TWO INTAKES
Like the upstream hydropower plant in Kurzras, the new facilities of Schnals Energie Konsortial GmbH utilise the water of the Schnalserbach stream. The water is captured directly in the downstream area of the plant and guided to a reservoir by way of a non-pressurised DN1000 steel penstock. The 2,700 m3 reservoir was built underground, with only a slight elevation in the hilly Kurzras landscape hinting at its existence. “Having it above ground wasn’t an option,” as the planning engineer explains, “because of our landscape preservation laws”. From the reservoir, the motive water is guided through a 2,500 m long DN700 drain pipe to the pipe intersection, where a Y-pipe section combines this stream with the water from the Lagaunbach. Thelatter flows to the intersection through a 1,150 m DN500 penstock. The combined motive water stream is then fed to the power house at the shores of the picturesque Vernagtsee by way of a 1,400 m DN800 penstock. This way, the power station is supplied with motive water from two intakes.

DUCTILE CAST IRON PIPES USED
A total of 5 km of penstock piping was installed for the new hydropower plant. The products used for this purpose are ductile cast iron pipe systems by Duktus. There are specific reasons for that. “To us it was important that the pipes had to be easy to install – more or less independent of the weather situation – to help us keep to our schedule. That was just one of the reasons why we went for the Duktus cast iron pipes,” explains Pohl. Another reason was the comparatively small trench size. Says Pohl, “We also had to dig the channel, which gave us quite a bit of excavation work to do. With that in mind, it was essential to keep the earth-moving work within reasonable limits and avoid disturbing the landscape unnecessarily.” Depending on the inclination of the terrain, the engineers used classic pipe sockets or longitudinally stable Class K9 – K12 BLS® socket joints.

EFFECTIVE CORROSION PROTECTION
“As for everyone in hydropower plant construction, our focus in this project was on ensuring a long lifespan of the materials, especially where the long penstocks are concerned. It’s a well-known fact that penstocks made from ductile cast iron usually keep for many decades. But in this case we had something else to take into consideration: the motive water has a low hardness level and a particularly low pH value, which means a relatively high acidity level. So, to protect the piping against corrosion we had to apply a durable inside protective layer. This is why we decided to go with the Duktus pipe systems with an added coating of alumina refractory concrete,” says Pohl. Large sections of the penstock pipework was installed underground beneath roads and paths. This sounds much easier than it turned out to be. Among other things, the engineers had to install three pipe bridges across various rifts and ditches. The longest of these measured a full 28 m in length. Still, the contracted firms all managed to keep to their schedules. “All the firms did an excellent job. They were able to complete the majority of the work between mid-June and late October. This made it possible for us to put one of the two machine units into partial operation already in summer 2011,” Variola recalls.

ELECTRICITY FROM TWO MACHINE UNITS
Where machine equipment was concerned, the operators favoured a solution with two machine units to be able to adapt to the seasonal fluctuations in available water resources. In particular, they selected two identical twin-jet Pelton turbines by Troyer AG, each with a design flow rate of 750 l/s at a net head of around 225 m. The two horizontally aligned machines generate a rated output of 1.7 megawatts each. Both turbines are coupled to a 750 rpm synchronous generator. The rated apparent power of the generators (made by WKV) is stated as 2,000 kVA. With the acquisition of this electrical machine equipment, the operators have made a solid investment in ultimate hydropower quality. Both turbines and generators stand for high output efficiency, durability and a long useful life. It was already apparent during the first season of operation that the machines do live up to the operators’ expectations. A particular characteristic of the WKV generators is the fact that they not only look powerful but are indeed extraordinarily powerful. A lot of material went into the construction of the housing and end shields, providing a solid basis for the overall durability and smooth operation of the machine. In particular, the combination of high-quality stator sheets and a sophisticated ventilation system ensures the high level of efficiency in this machine. WKV offers no “off the shelf ” generators. Every one of their machines is fine-tuned precisely to the specific application environment and the individual requirements for its intended purpose. The generators are therefore just as “tailor made” as the turbines.

DEPENDENT ON THE UPSTREAM PLANT
While the new power station completed its first year in partial operation, it was waiting for the water from the Lagaunbach to awaken its yet dormant potential. After all, work on the supply intake at the Lagaunbach was not scheduled to complete before June 2012. But once the work was finished, the time had come: the new power station with its two machine units was finally able to switch into high gear and full operation. “The motive water from the Lagaunbach stream is very important for this power plant. It’s not just about the additional volume of water. The main point is that it makes the facilities independent from the production regime of the Kurzras plant. In accordance with the requirements of its operator, the cableway now receives its electricity from the upstream power plant. Thanks to a water reservoir, the plant can generate both supply electricity and peak electricity, which means it is operated in hydro-peaking mode,” as Pohl explains. “Also, our reservoir allows us to generate peak electricity at the downstream facilities. But since the electricity that’s generated here is sold on the open market, the production curves of the two plants are not synchronised very well. That doesn’t always make things easy for the operator, Hydros GmbH.”

TURBINE BUCKETS PROTECTED AGAINST GLACIAL ABRASION
By utilising the supply intake from the Lagaunbach in combination with the small daily storage reservoir downstream of the Kurzras plant, it was possible to minimise the hydro-peaking effect of the Schnalserbach – something quite undesirable, not least for ecological reasons. “The reservoir serves an important purpose. For one thing, it supports our demand-based production and allows us to plan efficiently in times of low water levels. On the other hand, it is essential for protecting our turbines. We know from experience with the upstream plant what kind of damage glacial abrasion can do to the Pelton buckets. It tends to wear out most conventional rotors within a single season. But we have our reservoir: this allows the fine-grained glacial sediment to settle, which prevents it from getting into the penstock,” explains Pohl. A first check of the rotor after its first year of operation confirmed the reservoir’s bucket protecting qualities. The verdict: no visible damage from glacial sediment.

PEOPLE FOR HYDROPOWER
For both the SEL AG and the community of Schnals the new power plant at the Vernagtsee has turned out to be a true success story. After all, it is by no means typical or expected for a local population to welcome hydropower projects like this one with an emphatic “yes” – especially at a place where only 60 years ago an entire community was evacuated to make room for the artificial lake. Back then a largescale power plant was erected against the will of the people – a course of action that caused much suffering and has left scars on the souls of many locals that are still felt today. However, the new hydropower facilities were built with the consent of the people. Today they are actually proud of their new small-scale power station. Generating around 12 million kWh of clean electricity in a normal year, it will not only help to support and secure the energy supply to the Vinschgau region, but even provide a nice income for the local community as well.

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turbines

 

The core of the facilities consists of two structurally identical machine units, which deliver around 12 million kWh of clean electricity in a normal year. (photo credits: Troyer AG)

 

The penstocks are constructed from ductile cast iron pipes with an inside protecting layer of alumina refractory concrete. (photo credits: Duktus)