Civil engineering works proceed at Allt na Fearna hydro scheme, Scotland

Civil engineering works for the Allt na Fearna hydropower scheme are well under way.

Tree felling (above) was first thing to be undertaken, in order to create a corridor for excavating a trench for the long penstock pipe. The bulk of the trees were commercial crop which would have been harvested in the near future. Others were large unhealthy trees which had grown too big to harvest. The disturbed ground between the penstock and the burn will be left to re-wild with some planting of native species which will improve ecological diversity.

Preparing the site for the penstock (see images above) involved excavation of a 780 m long trench approximately 0.5 m wide and 1 m deep, running parallel to the Burn. This involved the use of small excavation plant on moderately steep terrain. The penstock pipe has a diameter of 355 mm, with a short section of 400 mm in the lower section. The penstock is made of HDPE and was laid in 12 m long sections, then fusion welded together. The trench was then backfilled ready for replanting. There was a requirement for a pipe bridge to enable the penstock to cross above the burn.

At the head of the penstock run is the water intake structure (see above) which will be fitted with a Coanda screen. Modern Turgo turbine systems almost exclusively use Coanda intake screens. These screens are built into the main channel so that the water flows over them. The screen has hundreds of tiny slots of around 1 to 2 mm wide that the water gets drawn through by the ‘coanda effect’. Coanda screens have the great advantage of being passively self-cleaning, because as debris builds up on the upper surface of the screen it is washed downstream by the water flowing over the screen. Due to the fine bar-spacing they also exclude all but the tiniest debris which ensures minimum wear on the turbine.

The penstock will terminate at the turbine house (above), currently under construction, which will house the 100 kW Turgo turbine.

The civil engineering works should be completed by the end of October. This will be followed by the mechanical and electrical engineering works which includes the turbine installation, electrical grid connection works and final commissioning. It is anticipated that the scheme will be completed in November this year.