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A New Pumping-Turbine Power Station Adapted from Information by Herrenknecht AG and Nant de Drance SA. The Nant de Drance pumping-turbine plant in Switzerland will use the elevation difference between two existing retention lakes to produce and store electrical energy, depending on whether there is high demand or a surplus on the network. When demand for electricity is high, the water stored in the Vieux-Emosson lake will flow down two vertical wells to a depth of nearly 1400 feet into the underground power station. The water will go through a turbine to produce electricity and then be dumped into Lake Emosson. Conversely, when demand for electricity is lower, water from the Emosson lake will be pumped to the lake of Vieux-Emosson. Construction began on the plant in 2008, and commissioning should be carried out in stages beginning this year. To bore the access tunnel, which measures over three miles in length, a Herrenknecht Gripper tunnel boring machine (TBM) was used. Following successful previous projects, the hard rock TBM was specially modified for this project, and it had to deal with a gradient of up to 12% and curve radii of 1640 feet. Systematic predrilling operations explored disturbance zones. In areas with very high water pressures, concrete injections were used to stabilize the rock where the tunnel would be bored. Two vertical pressure shafts, the penstocks, were bored to connect the Vieux Emosson reservoir with the power house cavern inside theAlps. The Herrenknecht Raise Boring Rig (RBR) was used to drill these vertical shafts downward, and in the next working step, the pilot borehole was reamed to a diam- eter of eight feet. Subsequently, the borehole was enlarged to almost 30 feet using a Shaft Drilling Jumbo. The pumping station-turbine will be equipped with six Francis-type turbine pumps of 150 megawatts (MW) each. They will deliver a pumping and turbine power of 900 MW and will produce about 2.5 billion kilowatt-hours per year with a frequency of 50 hertz. Since the installation is reversible, it will function as a turbine when electricity is produced and as a pump when electricity is consumed. The plant will be able to switch from pumping at full power to operating as a full power turbine in less than ten minutes. One of the major assets of the Nant de Drance project is this very high flexibility, which allows for a quick response to the electricity demand or need for storage. Pumping-turbine plants do not generate additional electricity because the current used for the pumping cycles is greater than the current produced during the turbine. However, thanks to the use of innovative tech- nology, the Nant de Drance pump-turbine plant will offer a yield of more than 80%, which is currently one of the highest yields available for electricity storage. Pump-turbine, courtesy of Nant de Drance. Nant de Drance pumping-turbine plant illustration, courtesy of Herrenknecht AG. ! " # ! 26 FEBRUARY 2018 WorldWide Drilling Resource ®