Meeting Swiss power peaks

Kraftwerke Linth-Limmern AG, a major Swiss high pressure storage power plant, is expanding its existing facilities under the Linthal 2015 project to be able to ensure security of electricity supply in the future. The core of the project is the Limmern pumped storage power plant with a pump and turbine capacity of 1000MW. Construction on the project commenced at the end of September 2009.

The construction sites are high up in the mountains, with the highest site at an altitude of 2500m asl. They will be served by two 25 tonne construction cableways. The logistics and scale of the project, the weather and natural dangers all pose major challenges to those involved. The largest component of the construction work encompasses the underground galleries, the tunnels and the dam wall and is the responsibility of the ARGE KW Limmern consortium, which also operates the logistics chain.

The construction of the Limmern pumped storage power plant is expected to take almost seven years in total. The commissioning of the first generating unit is planned for 2015 and the last for 2016.

Kraftwerke Linth-Limmern

Kraftwerke Linth-Limmern AG (KLL) is one of the most important Swiss high pressure storage power plant complexes. The company is a partnership between the canton of Glarus and Axpo. Together with its partners, Axpo delivers electricity to around three million people in Switzerland.

KLL draws on the inflow from a catchment area that is 140km² in extent. At present, KLL has a turbine capacity of 460MW and a pump capacity of 170MW and produces approximately 460M kWh of energy per year, which is enough to supply the electricity requirements of a town of around 60,000 residents.

There are four power plant stages stretching from Lake Mutt at around 2500m asl to Linthal at 670m asl. The Limmern dam is the largest reservoir in the KLL complex with a volume of 92Mm³. KLL is a storage power plant mainly producing peak-load energy.

Peak load energy

In Switzerland, base load energy is primarily produced by nuclear power plants and run-of-river power plants. The need for peak load energy is continually growing throughout the European grid network. In addition to the yearly rise in consumption, another important reason for this growth in demand is the fact that many of the coastal countries in the European Union have been extensively expanding electricity production from wind energy. This results in a growing proportion of energy production that depends on wind conditions and which cannot be planned. Another issue is the liberalisation of the electricity market. In a free market, electricity can be purchased anywhere by consumers so the electricity grid has to be increasingly regulated in order to ensure security of supply.

Peak load energy from storage power plants plays a key role in security of supply. It ensures sufficient output and flexible electricity production, and can be stored in the form of water in reservoirs at competitive market prices. Additional pumped storage plants will make it possible to meet the growing demand for peak-load energy.

The Axpo study Outlook For Electricity In 2020 shows that Switzerland will require additional power plant capacity between 2013 and 2018. Axpo has therefore planned various measures to ensure that we continue to enjoy security of electricity supply going forward. The Linthal 2015 project is one of these measures.

Linthal 2015

The Linthal 2015 project will result in the expansion of the existing KLL power plants. The Limmern pumped storage power plant is the core of the project. The Limmern plant is an underground facility that will have a pump and turbine capacity of 1000MW. It will pump water from the existing Limmern dam to Lake Mutt and then use this water to drive turbines that will produce electricity; as part of the project, the level of Lake Mutt will be raised. Depending on operating conditions and management of the Limmern dam, the Limmern plant will have a drop of between 560m and 710m.

Lake Mutt is a natural reservoir; the lake’s surface level will be raised by up to 30m through the construction of a 1050m long concrete gravity wall. During turbine operations, water from Lake Mutt will be collected from the inlet/outlet structure and channelled via the 570m long headwater pressure tunnel, which has an inner diameter of 8m, in the direction of the pressure shafts. A service chamber with two butterfly control valves is located at the top of the pressure shafts. The two reinforced pressure shafts are 1050m in length with an inner diameter of 4m; they channel the water down a drop of 680m to the machinery gallery.

Just before the machine gallery, each of the pressure shafts splits into two headwater connecting tunnels that then lead to the four turbines. There is a ball valve on the headwater side of each turbine to control the operational, emergency and maintenance settings. The water surge pressure created in the water system when the shutoff device is activated is dampened by the surge chamber. The water is channelled from the turbines to the Limmern dam via the tailwater tunnels.

The four tailwater tunnels are reduced to two tunnels with a length of 420m and 370m and an inner diameter of 6m. The water is channelled into the Limmern dam via the two inlet/outlet structures.

The underground gallery centre is made up of a machinery gallery and a transformer gallery and the two are connected by various tunnels. The machinery gallery is 160m in length, 30m in breadth and 50m high, while the transformer gallery is 140m long, 20m wide and 25m high. A new access tunnel called ZS 1 will be built from Tierfehd to the machinery gallery and will be 4000m in length with a bored diameter of 8m. The access tunnel will be fitted with a funicular railway to transport heavy loads, and will facilitate the delivery of the generator-transformer units to the site; each unit weighs 225 tonnes.

The four pumped storage units will be installed in the machinery gallery; each unit is made up of a single stage pump turbine with an installed pump and turbine output of 250MW and a variable rotary asynchronous motor-generator with the necessary support and ancillary equipment. The designed throughput volume for the pump action is approximately 35m³/sec and approximately 50m³/sec for the turbine action. The pump turbine units are reversible Francis turbines. Each motor-generator is coupled via its transformer to the 380kV substation. Two 380kV cable systems lead from the substation to the new Tierfehd transformer substation via the ZS1 access tunnel.

A 17km long 380kV overhead line is being constructed from the Tierfehd substation to connect the Limmern pumped storage plant to the Swiss high voltage grid.

Environmental measures

A comprehensive two-stage environmental impact survey was undertaken as part of the project work for the Limmern plant. The construction work has been planned in such a manner so as to minimise the negative environmental impact that is unavoidable in a project of this nature and size. Project planning paid special attention to the protection of pristine biomes and contiguous areas.

The planning work was undertaken in ongoing consultation with the Linthal 2015 stakeholder working group, which included representatives of various authorities and stakeholder groups; this was a key aspect in the process of finding successful consensus-based solutions. The most important environmental measures involve:

• New regulations for residual flow charges and hydro peaking in the headwaters.

• Various offset measures in terms of nature conservation and landscape protection laws.

• Minimising the surface area required and the transport routes to the construction site.

• Ongoing monitoring of the environmental components.

• Creation of an environmental liaison unit.

KLL followed a two-stage procedure to apply for planning permission to build the Limmern plant. The first part of the process was the application for a concession to use the water and the second was the construction permit to build the pumped storage plant.

In mid-2006, KLL submitted the application for the water concession, and the application for the construction permit was submitted at the end of October 2008. The concession was awarded to KLL at the end of October 2007 and the construction permit was awarded in July 2009. By engaging with the authorities and the various stakeholder groups well in advance, and by collaborating closely with them, no objections were lodged during the application processes or against the issuing of the permits.

Implementation

In mid-September 2009, the boards of directors of Axpo and KLL approved the construction project and authorised a construction budget of CHF 2.1B (US$1.9B). The main construction work commenced immediately after this decision. The following sub-groups have been formed for the construction of the Limmern plant: Sub-group A for facilities construction; Sub-group B for hydraulic steel construction; Sub-group C for electrical engineering; Sub-group D for utilities services; Sub-group E for railways.

Sub-group A has been further subdivided into sub-group A1, which will be responsible for the construction of access tunnel 1, and sub-group A2, which will be responsible for the construction of all other tunnels, the underground galleries and the Lake Mutt dam wall, as well as the logistic chain.

Access and logistics

The construction sites are high up in the mountains and will be served by two temporary 25 tonne construction cableways with a maximum load of 40 tonnes and 30 tonnes respectively. The two cableways are connected via a 3km long horizontal road tunnel. Until the funicular railway in access tunnel 1 has been completed, the only access to the construction sites will be via the cableways or by helicopter.

The logistics represent an enormous challenge for the companies. Bulk materials such as cement and steel will be transported by rail to Linthal and then by truck for 10km to the construction site logistics centre at Tierfehd. Together with other material and equipment, the bulk materials will then be transported from the logistics centre using a container system via the first section of cableway to an altitude of 1800m asl. From here, the materials are transported by trucks along the horizontal tunnel to the second cableway and then up to Lake Mutt at an altitude of 2500m.

A team of more than 70 logistics specialists will be responsible for the logistical work on a round-the-clock basis, seven days a week. The logistical planning kicked off as soon as the major construction site equipment was ordered. Major pieces of equipment such as the tunnel boring machines, concrete mixing plants and gravel plants will have to be packed in suitable containers for transport.

Weather and natural hazards

The weather conditions on the mountain construction sites also present a major challenge. Construction at an altitude of 2500m can only take place for five or six months in the year due to snow. Even in summer, regular construction work can be interrupted by high altitude snow, fog and thunderstorms. The underground construction sites will operate throughout the year. Potential avalanches, snow and storms will interrupt or restrict work by making the logistical sites inaccessible. KLL will operate an avalanche service to reduce the problem of avalanches.

Technical construction challenges

The construction work of sub-group A2, which will be undertaken by the ARGE KW Limmern consortium (including the companies Marti Gruppe and Toneatti), covers the construction of the transformer and machinery galleries, the complex water tunnel system (including the two pressure shafts with a length of 1100m) and the dam wall.

Key performance metrics for this work include approximately 80,000m³ of dam wall excavation, 250,000m³ of dam wall concrete, 630,000m³ of excavation for galleries and tunnels, 200,000m³ of concrete and liquid concrete for underground work, 25,000t of reinforced steel and 130,000t of cement

Despite complex mountainous conditions, the site is considered to have good stable medium-hard stone. The underground construction areas are mainly located in Malm limestone bedrock (Quintnerkalk). During the formation of the Alps, this has been repeatedly broken, folded and superimposed in layers. The mountain is characterised by two rupture zones, shear zones and a well-developed Karst ground water system.

Special technical installations

Concrete plants and crusher units for the underground construction work will be installed and operated in their own underground galleries specially created for this purpose. The excavated material will be removed using a complex conveyor system and then processed in a traditional gravel plant. Discarded material will have to be stored in difficult topographical conditions. The processed aggregate will be used for concrete for the central gallery and for finishing the tunnels in the form of sprayed concrete, wall cladding, steel reinforcement backfill, etc.

In addition, the bulk of the processed aggregate will be transported via the second cableway to the dam wall construction site to be used for making concrete. A twin-shaft mixing plant with an output of 250m3/hr will be used for the production of concrete for the dam wall. The concrete will be delivered using two remote-controlled 180 tonne caterpillar cranes and concrete hoppers with a capacity of 6m3.

Construction of the Limmern pumped storage plant is expected to take almost seven years in total. According to plan, the first generating unit will be commissioned in 2015 and the other three in 2016.

Martin Gmür, Assistant to the head of Axpo Hydroenergy,5401 Baden, Switzerland. Email: martin.gmuer@axpo.ch