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First anniversary for underground power station Gletsch-Oberwald in the Upper Valais region

The newly constructed Gletsch-Oberwald power station in the Upper Valais region of Switzerland has been producing clean energy since early 2018, making efficient use of the energy potential of the upper Rhône river.

Financing for the logistically complex CHF 67 million project was provided by Sion-based energy supplier FMV SA. From the turbine house and 2.2 km pressure tunnel to the integrated desilter, most of the facility is installed underground. The plant’s three-year implementation phase included extensive rock blasting work and the use of a tunnel boring machine with a 3.9 m diameter to excavate around 90,000 cubic metres of rock. In view of the impressive performance of the finished plant, however, it is safe to say that the effort was well worth it. With full water resources available, the two identical Pelton turbines provided by ANDRITZ Hydro achieve a total of more than 14 MW. In normal years, this high-tech facility supplies the power for around 9,000 Upper Valais households. To comply with the environmental regulations for power stations, amelioration measures had to be implemen-i ted in an extensive riparian area.

The fact that there was considerable hydropower potential along the upstream stretch of the Rhône near the Valaisian municipality of Obergoms, between the districts of Gletsch and Oberwald, had been known for many decades. In a current article published in the Swiss trade journal “Wasser Energie Luft” (“Water, Energy, Air”) Raoul Albrecht, Member of the Board and Head of Production at FMV SA in Sion, notes that various concepts for exploiting the hydropower potential in the project area in the Upper Valais region had been under consideration since the 1970s. Among others, these included the construction of a 100-million cubic metre reservoir in Gletsch, as well as various pumped-storage concepts. However, due to the growing public sensitivity with respect to environmental and landscape protection in the 1980s, and in view of changing economic conditions, the plans for the construction of a large hydropower facility were subsequently put on ice. However, the introduction of the “Compensatory Feed-in Remuneration” (CFR) in 2009 as a means of subsidising energy from renewable resources, marked a turnabout in the economic situation for Swiss energy providers. Energy provider FMV SA had had the foresight to prepare a feasibility study for a regional power station project already in 2007, and one year after the introduction of the CFR was able to submit a license application for the construction of a hydropower station in the area under consideration.

Ecologic package of measures
An operating license for a period of 80 years was granted to FMV by the Valais cantonal authorities in 2013. This was possible only because a settlement could be agreed with the regional environmental agencies, which had initially objected to granting the requested license. Intense negotiations between all affected parties led to a package of several measures, the most crucial of which called for comprehensive ecological amelioration measures in the riparian “Sand” area near Oberwald. Additionally, the plans included to flood protection measures for the community, and various touristic aspects. For example, additional tributary streams of the Rhône in the riparian area were to be opened, a footbridge was to be installed, and hiking paths were to be prepared. Once the license was granted, the planning approval application could be submitted in late 2013, which also marked the start of the tendering process. Actual construction work commenced in spring 2015.

Logistic challenge
Commenting the on-site inspection by zek Hydro of the latest-generation underground turbine house, Raoul Albrecht names the organisational and logistic aspects of the project as the greatest challenge during the construction phase: “As usual with large-scale projects in the Alpine regions, we were faced with a lot of logistic challenges. We had worked out a basic concept that would allow construction work to continue all year round. However, work at the forebay in Gletsch was possible only during the summer and autumn months, as access via the Furka pass road was closed for around seven months due to the avalanche risk there. That made the completion of the Oberwald tunnel all the more urgent as the only was to enable year-round access to the construction site at the forebay.” Moreover, despite the limited spatial conditions inside the mountain and at the gallery access points, the engineers had to prepare the space required for the transport of material and equipment, along with a suitable location for preparing the concrete. As far as coordination planning was concerned, it was necessary to organise the work into multiple shifts with around 40 staff to work them. Detailed planning for the individual sub-installations was provided by four contracted engineering offices.

Tunnel boring machine cuts headrace tunnel
Internationally successful underground construction specialist Strabag AG won the contract for the entire drilling, rock blasting and concrete placement work. April 2015 marked the simultaneous kick-off to the blasting work for the two access galleries in Gletsch and Oberwald. In Oberwald, the first 300 metre stretch of the access gallery leading up to the turbine house was excavated exclusively by blasting. The gallery was also used for getting the 120 m long tunnel boring machine in pace so the construction of the headrace tunnel could go ahead. Measuring 3.9 m in diameter, the boring head began its work in October, cutting its way through the rocky crag of the Grimsel massif. Excavation work for the underground turbine house and the tailrace tunnel, through which the processed motive water is returned to the Rhône river, was also begun in autumn 2015. About half of the 90,000 cubic metres of excavated material that was blasted out of the mountain was directly recycled and used as concrete and gravel material for construction. Thanks to the favourable geological conditions and the perfect collaboration among the various firms involved in the project, the tunnel breakthrough to Gletsch was achieved by April 2016, after just six months. With the gallery complete, the water intake in Gletsch was now accessible all year round for the entire rest of the construction period.

GRP material reduces installation time
In the area in front of the transit to the underground turbine house, the final section of the penstock (approximately 60 m) was implemented in the form of a solid steel construction. In the bottom third of the gallery, this followed by an approximately 700 m run of GRP pipework. As the pipe elements had already been lined with concrete at the factory, they could be installed using the rails that had been installed for the tunnel boring machine. Once the pipe elements – each measuring 6 m and weighing 15 tonnes – were installed, they were backfilled with concrete along the entire run. This method, says Albrecht, was much faster than using steel armour plating. Thanks to the excellent condition of the rock, the subsequent 1.3 km penstock run leading to the desilter at the forebay could be completed using concrete lining without the need for any additional reinforcement. In total, the headrace tunnel with its steady 13-degree gradient measures 2,150 m in length and includes an almost 90-degree bend.

Forebay and desilter
The construction of the forebay in the form of a bed intake, along with interior work on the desilter cavern, were completed during the summer and autumn months of 2015 to 2017. A temporary diversion of the Rhône with its flow capacity of 20 m³/s provided a dry working area with added protection in case of high water levels. As Albrecht points out, completing the interior construction for the entirely rock-blasted desilter cavern came with its own set of challenges. In an extremely confined space and in very short time, strong concrete walls 10 m high were put up while the hydraulic steelwork was put into place. In its finished state, the desilter consists basically of two high-volume reservoirs, each measuring 45 m in length, where fine-grained debris from the glacier and the sediments from the Rhône waters can slowly settle. Thanks to the well-coordinated collaboration of the executing contractors, the work on the forebay and close-by desilter cavern were completed on schedule in autumn 2017.

Cutting-edge underground technology
Starting with the installation of the two turbine casings, work on the technical equipment for the cavern in Oberwald commenced in spring 2017. The delivery included the electromechanical equipment and the automation and control system – an all-in-one package provided by internationally active all-around hydropower specialist ANDRITZ Hydro. With the electrical engineering work complete, the facility was taken into trial operation in November 2017. Two 6-nozzzle high-efficiency Pelton turbines are used to ensure maximum utilisation of the Rhône’s energy potential both at full and reduced flow rates. The two vertically aligned turbines have a total design flow rate of 5.7 m³/s as well as a gross head of 288 m to put to optimum use. When the full water volume is available, each of the machines with hydraulic nozzle control can achieve a bottleneck capacity of 7,500 kW. Each of the 1,150 mm turbine rotors was milled from a stainless steel monobloc and rotates at 600 rpm. The directly coupled vertical synchronous generators are operated at precisely the same rotational speed. Water cooling in the generators’ casing ensures optimum operating conditions during heat-intensive full-load operation. A previously installed indoor crane proved extremely useful in installing the generators, each weighing 35 tonnes and generating a rated apparent power of 8,100 kVA. The power generated this way is fed into the medium-voltage switchgear and then stepped-up to 16 kV by two transformers inside the cavern. At this voltage the electrical energy is transported via underground cables to the “Pfarrwerk” sub-station and on to the nearby (65 kV) Ulrichen sub-station, from where it is finally fed into the next-higher grid level. ANDRITZ Hydro’s control and automation system also provides fully automated regulation of the energy production process. The system is operated via control cabinets with the help of large touch screen panels.  

FMV trusts in hydropower potential
The Gletsch-Oberwald power station commenced regular operation already more than a year ago, in January 2018. In late August 2018, the facility was formally inaugurated in an official ceremony. “Despite tight energy markets, FMV decided in 2015 to implement the construction of the new Gletsch-Oberwald hydropower plant on their own account and as the sole building owner. This is in line with the cantonal and corporate strategy of putting the hydropower potential of the Valaisian municipalities to efficient use,” commented FMV’s Managing Director, Paul Michellod in an official press release. Project manager Albrecht is highly satisfied with the energy production of the FMV’s new hydopower plant after its first year in operation, and he commends all firms that helped to implement the project. With its annual standard capacity of 41 GWh, the power station satisfies the annual energy requirements of 9,000 average households.

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