Vianden Pumped Storage Plant

The Vianden Pumped Storage Plant is located just north of Vianden in Diekirch District, Luxembourg. The power plant uses the pumped-storage hydroelectric method to generate electricity and serves as a peaking power plant. Its lower reservoir is located on the Our River, bordering Germany, and the upper is elevated above on the nearby Saint Nicholas Mountain. Construction on the plant began in 1959 and the first pump-generators were commissioned in 1962. A tenth pump-generator was installed in 1976 bringing the plant's installed generating capacity to 1096 MW. The plant generates an average of 1650 GWh annually but of course consumes even more. Generally the efficiency of this energy storage method is around 70–80%. The plant is owned by Société électrique de l'Our and RWE. Construction on an eleventh pump-generator began in 2010 and it is expected to be commissioned in 2013, which will bring the plant's installed capacity to 1296 MW.

Background
Planning for the project began in 1925 but the idea failed due to a lack of funding and political pressure. On 10 July 1958, a treaty was signed between Luxembourg and the German state of Rhineland-Palatinate, which also shares the Our River, allowing construction. Construction began in August 1959. The first four pump-generators were commissioned in the winter of 1962/63 and five through nine in 1964. The official inauguration of the plant was held on 17 April 1964. Construction on a tenth pump-generator began in 1970 and was operational in late 1976. It required the expansion of the upper reservoir, known as Upper Reservoir II.

Second expansion
Planning for a 200 MW eleventh unit began in 2006 and the designs were completed in 2009 by Lahmeyer International. Construction on the unit began in January 2010. To accommodate the new unit, the capacities of both the upper and lower reservoirs are being increased by 500000 m3. To accomplish this, the upper reservoir wall will be raised 100 cm and the lower Vianden Dam raised 50 cm. Additionally, a new cavern and set of tunnels just east of the main power cavern will be constructed to house the turbine-generator and penstock. Excavation of these tunnels and caverns was completed on 31 May 2011. On 15 June 2010, Upper Reservoir I was drained to install a new intake structure, which was completed in November 2010. Construction on the new suction/discharge pipe began on 16 May 2011. A cofferdam was constructed around the site in the lower reservoir. The entire expansion is expected to be complete in late 2013.

Design and operation
The power plant consists of two reservoirs (upper and lower), two power stations and appurtenant structures such as tunnels, intakes and transformers. The upper reservoir for the plant is separated into two sections, I & II. It is formed by a continuous dam and both sections are divided by a dam with floodgates. The total storage capacity of the upper reservoir (both I & II) is 7230000 m3. The active (or usable) capacity of the upper reservoir is 6840000 m3. Upper Reservoir I itself has as active capacity of 3010000 m3 and Upper Reservoir II: 3830000 m3. To supply water to the generators and to serve as a discharge for the pumps, both Upper Reservoirs I & II have combined intakes/outlets. Upper Reservoir I primarily supplies the main powerhouse (900 MW) with water and Upper Reservoir II supplies a secondary powerhouse (196 MW) with water although both reservoirs are at the same altitude and can balance one another. Water from Upper Reservoir I is sent to the main power house, which contains nine Francis pump-turbine-generators, via a system of tunnels and penstocks. When generating power, each generator has an installed capacity of 100 MW and when pumping the capacity is 70 MW. Each of the nine pump-generators can discharge 39.5 m3/s of water when generating electricity and can pump 21 m3/s back to the upper reservoir when in pumping mode. Upper Reservoir II's intake/outlet sends water down to the secondary powerhouse which contains one Francis turbine-pump-generator. When generating electricity, the pump-generator has an installed capacity of 196 MW and when pumping the capacity is 220 MW. This pump-generator can discharge 77 m3/s of water and can pump 74 m3/s. After electricity is generated, both power stations discharge water into the lower reservoir which is created on the Our River by the 30 m tall and 130 m long gravity dam, Vianden Dam. The lower reservoir has a gross storage capacity of 10080000 m3 of which 6840000 m3 of water is active (or usable for pumping to the upper reservoir). The lower reservoir is at an elevation of 227.5 m while the upper reservoir is situated at 510.4 m. The difference in elevation between the reservoirs affords a maximum hydraulic head of 291.3 m and minimum of 266.5 m.

Water is pumped from the lower reservoir to the upper during periods of low demand, usually during the night for a period of eight hours. When energy demand is high, water from the upper reservoir is used to generate power and meet peak energy demand. The process repeats daily or as needed. During pumping and generating about 5500000 m3 is oscillated between both reservoirs. When the 200 MW expansion is completed, the size of both the upper and lower reservoirs will be expanded and the installed generating capacity of the power plant will increase from 1,096 MW to 1,296 MW.