The Kárahnjúkar Power Plant
The installed capacity of the Kárahnjúkar Hydropower Plant (HPP) is 690 MW, and its power-generating capacity is 4,600 GWh/year. This single installation harnesses the potential of the Rivers Jökulsá á Dal and Jökulsá í Fljótsdal and tributaries in the Hraun area to the east.
The main storage reservoir is Hálslón, on the River Jökulsá á Dal, alongside the Kárahnjúkar peaks. There is also a small intake reservoir, called Ufsarlón, in the River Jökulsá í Fljótsdal north of Eyjabakki, and a storage reservoir in the River Kelduá to the east, known as Kelduárlón. The water passes through a headrace tunnel to the Fljótsdalur powerhouse at Valthjófsstadarfjall and then through further tailrace tunnels and canals to the River Jökulsá í Fljótsdal. Kárahnjúkar HPP has substantial underground structures with tunnels totalling over 70 km in length.
This storage reservoir is contained by three dams. The largest is the Kárahnjúkar Dam, 200 m high, at the inner end of the canyon Hafrahvammagljúfur. It is a concrete-faced rockfill dam (CFRD), among the highest of its type in the world. There is an outlet at the base of the Kárahnjúkar Dam and a concrete spillway on the west bank. Secondary dams are located on both sides of the Kárahnjúkar Dam; Desjarár Dam to the east and Saudárdalur Dam to the west are made of gravel and rock with a watertight earthen core. Hálslón, reaching from Kárahnjúkar to Brúarkökull, is 25 km long, with an area of 57 km2.
A 40-km-long headrace tunnel with a diameter of over 7 m runs from Hálslón. It leads west under Vesturöræfum and then north under Fljótsdalsheidi. A 13 km tunnel from Ufsarlón connects with it from the south. The headrace tunnel was mostly excavated using three full-face tunnel boring machines. On reaching Valthjófsstadarfjall, the tunnel splits into two 420-m-high, steel-lined vertical penstocks.
Generation takes place in the Fljótsdalur Powerhouse. It houses six 115 MW generating units with ancillary equipment. The turbines are of the Francis type. The station tunnel is connected to a special transformer hall housing the main transformers. Transmission lines then carry the 245 kV electric current away from the mountainside to a Landsnet switching station. The station service centre stands in front of the entrance tunnel, at the head of the Fljótsdalur valley