Hydropower plant at the Eisack River

Autor: David Tscholl , 01.06.2011

It has been a long family tradition for the sawmill Plunger & Sohn to operate a small hydropower plant on the Eisack River in the South Tyrolean village of Franzensfeste.

The plant is small and reliable, but the site has an even greater energy efficiency potential.

Changes have been made with the help of local hydropower experts. Within the past two years Troyer AG from Sterzing, South Tyrol, and experts in steel construction for hydraulic engineering, Gufler Metall, have created a state-of-theart low-pressure power plant. Now a higher electricity output meets ecological improvements on this sensitive power plant site. The two new Kaplan turbines have been in operation without intermission since end of September 2010.

When the South Tyrolean provincial government placed the Eisack River under protection as part of a new water management plan in 2009, this put an end to various large-scale hydropower projects. But at the same time smaller projects got a better chance of being realized, as plants already possessing riparian rights and projects with a medial effective output of 3 MW were exempt from this regulation. The output of the traditional plant of Plunger & Sohn, a re-nowned sawmill in Franzensfeste on the Eisack, was nowhere close to this limit. After all it had been built for the self-supply of the nearby sawmill. The operators knew that it was necessary to make improvements regarding ecology as well as the plant's energy efficiency. They had already come up with the idea of reconstructing the plant a couple of years earlier, but plans had regularly failed each time talks of large-scale hydropower projects had arisen in Franzensfeste. However, through the water exploitation plan all projects for large-scale power plants were put on hold, and Plunger & Sohn now had the option to renew their plant and expand it moderately within the riparian rights specifications.

Troyer AG was commissioned with the construction project in 2009. Under the direction of Troyer AG a project for a diversion power plant was developed, where improvements regarding energy efficiency met ecological improvements. “The main condition was to reconstruct the plant with the focus on its ecological features. In early summer, when high water levels occur more often, the Eisack River has 28 m3/s on average. The plant had been constructed for an output of 18 m3/s. On basis of these data we could assume that the plant had surplus water from May until August. In addition to this, relatively high standards for residual flow had been set. The solution was to leave 5 l/s in the course of the river for each km2 of the drainage basin. Since the drainage basin of the Eisack spreads across an area of 650 km2, the compensation water amount is 3.25 m3/s. During the period from April until the end of October this amount is increased by an additional 1.3 l/s3 - a whole lot of residual flow“, says Markus Zössmayer, project manager of Troyer AG.

The heart of the power plant, in the first level below ground, consists of two identical Kaplan spiral turbines with an outer diameter of 6,4 m. They have been manufactured by Troyer AG and are the biggest Kaplan turbines ever built by the traditional manufacturer from Sterzing. Two years before, Troyer AG was selected best bidder in the public tenders. The two 5-blade Kaplan spiral turbines are constructed for a design output of 1.4 MW each at a head of 17.7 m and a usable design flow of 9 m3/s each. The power plant will generate about 13.8 million kWh of clean electricity for South Tyrol each year. For the Franzensfeste operators the renewed power plant represents a new era of hydropower. Today the generated electricity is not only used for self-supply, but is also delivered into the network of the Brixen Municipal Utilities Company.

A challenge for the plans of Troyer AG were the conditions of tight space on site. In spite of the shortage of space a complex water abstraction system was created. It consists of a weir plant with two radial gates including hydraulically operated fish belly flap gates and the fish passage, an about 100 m long sedimentation basin with three chambers and the water abstraction chamber, all of which are attached to the penstock. The latter is structured by a rectangular armored concrete pressure channel, whose walls were cast on site and attached to each other by means of special connecting elements.

It is no secret that South Tyrol can rely on local technological expertise regarding small-scalehydropower. Some companies have emerged in the field of steel construction for hydraulic engineering and offer state-of-the-art solutions for hydropower plants. They have all accumulated an impressionable reference list. One of the top companies in this field is Gufler Metall from Moos in Passeier. Within the last couple of years there has hardly been a large-scale hydropower plant in South Tyrol that was constructed without the help of this company. It is not surprising that Gufler Metall has provided the entire hydraulic steel construction for the new flagship power plant Plunger on the Eisack. Gotthard Gufler founded the company in 1991. It has evolved into an essential player in the field of steel construction for hydraulic engineering for South Tyrol's power plants ever since. In 2008 the business was turned into a corporation. With 12 employees, Gufler Metall focuses on metal construction, steel construction for hydraulic engineering and casting of steel pressure pipes.

In 2009 the company was commissioned to provide the entire hydraulic steel construction for the small-scale power plant on the Eisack. This included the 3.45 m long fine rack, two flap gates with a clear width of 11.75 m and a height of 1.8 m each, three sluices as pipe burst safety devices with a clear width of 3 m each and a height of 2.7 m. In addition to this, three scouring sluices, a scouring sluice for coarser sediments, three shut-off flaps, shut-off sluices for the power plant and two pipe burst safety lugs DN2400 had to be provided. Furthermore all casting work of the water catchment as well as on the power plant was done by Gufler Metall. The DIN EN805 pressure test for the concrete duct has also been successfully completed by the team from the Passeier Valley.

Another task performed by the company was constructing a pipe bridge for a drinking waterpipeline. A steel pipe type DN150 had to be installed on a framework with a length of 48 m across the Eisack. Even in this field Gufler Metall has gained a lot of experience and know-how. According to Alfred Gufler, director of Gufler Metall, the biggest challenge during construction work was to build in the big flap gates, which were not easy to handle due to their clear width of 11.75 m. Both flap gates are operated by two hydraulic cylinders and function as an effective flood discharge. Even the construction of the pipe burst safety sluices and the shut-off flaps on the sedimentation basin had some hidden challenges. The time schedule for construction was very tight and the existing space between the Eisack River and the nearby main road was extremely narrow. However, with the Franzensfeste project Gufler Metall could again live up to its reputation as a reliable and competent expert in steel construction for hydraulic engineering and could add another flagship power plant to its reference list with the Plunger power plant.

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Flap Gates



The two flap gates at the water catchment of the new Plunger power plant are just under 12 m wide. (photo: Gufler)

Pipe Bridge

pipe bridge2


Gufler Metall built a 48 m long pipe bridge for a drinking water pipeline by laying a DN 150 steel pipe across the Eisack River. (photo: Gufler)