The “Nant de Drance” pumped-storage power plant will use the downward gradient that already exists between two reservoirs to generate and store energy. At peak consumption times, the water stored in the Vieux Emosson reservoir will be fed to the turbines in the underground power house through two vertical shafts with a height of 425 m to generate power. From there, it will then be fed into the Emosson reservoir. The six latest generation pump turbines will be capable of feeding 900 MW of electricity into the grid or of pumping the equivalent amount of water up to the higher reservoir in a matter of minutes. The latter of these two operations will be performed when the demand for electricity is low. This will involve pumping water from the Lac d’Emosson reservoir back into the Vieux Emosson reservoir. In this way, the power plant can be used to store electricity. Its efficiency value of 80% is one of the highest ever achieved within the field of power generation. Harsh Environmental Conditions In 2016, Andritz Hydro commissioned Rittmeyer Austria with implementing a flow measurement solution in both pipelines running between the power house and the Emosson reservoir. The project scope included the following: a survey of the measuring point, determination of the ultrasonic sensor positions, coordination of the cabling, installation of the protective covers for the cabling (sensor cables), and connection of the sensors. Measuring 7.70 m in diameter, each pipe must be able to withstand a pressure of more than 30 bar and handle in excess of 170 m3 water per second. For this reason, the installation of the sensors and the pressure-resistant cables proved particularly challenging. Having to work underground, in poor light, and under humid conditions, and having to use scaffolding or cherry pickers meant that the environmental conditions did not make the task any easier. Sixteen inner sensor/measurement lines – some of which were very long – had to undergo pressure testing in advance. RISONIC Helps Meet the Stringent Requirements For the purpose of measuring the volume of water used, a measurement accuracy of less than 1% was stipulated. This requirement was imposed because of the Swiss water resource tax (“Wasserzins”), which power station operators in Switzerland are required to pay for using the water. To achieve this, the measuring point in each pipeline had to be surveyed precisely using theodolites. For this, Rittmeyer used its high-precision RISONIC modular ultrasonic flow measurement system. RISONIC was able to record 40 measurement points per pipe plus the positions of the sensors, angles, height, and distances. Using this data, it then generated the pipeline geometry and a special profile, which served as the basis for a simulation in order to check the required measurement accuracy prior to installation. Particularly within the context of harsh environments, RISONIC offers long-term stability, reproducible results, and requires no maintenance. To measure the flow, exclusive use was made of permanently installable sensors, with flow measurement possible in both directions. Final commissioning of the flow measurement solution is due to take place in 2019.