The location and arrangement of the scheme is shown in the scheme layout plan below.

Inverlael Hydro Scheme takes water from a catchment area of approximately 16km2, including water flowing off Beinn Dearg and surrounding summits. Water is abstracted at two intake weirs on the River Lael and Allt Mor which feeds into 3.2km of buried pipelines to a powerhouse containing two turbines and generators as well as a transformer and associated equipment. The turbine discharges combine through a short buried tailrace pipe back to the River Lael before its discharge into the north side of Loch Broom. There is no reservoir storage so the turbines only use the water available in the rivers at the time.

The station is capable of generating 2.5 megawatts (MW) of renewable electricity, with a predicted output in a year of average rainfall of around 7200 MWhr, enough to supply approximately 1500 homes.

The scheme was consented under Section 36 of the Electricity Act 1989 by the Scottish Executive in July 2007, took 13 months to construct at a cost of just under £5M, and became operational in May 2009.

Environmental Issues

The Environmental Statement to support the planning application was prepared by ash design + assessment of
Glasgow. A number of environmental issues were addressed and incorporated into the design of the project before the scheme was consented. The site was surveyed for fish, birds and mammals as well as archaeology.River environment - in order to maintain the ecology of the river downstream of the intake, a “hands off” or
“compensation” flow is always released to the river below each intake before any water can be taken for generating
electricity. Depending on the natural flow in the river this flow varies above a minimum flow of 60 and 150 litres per
second, for the Allt Mor and Lael rivers respectively, in order to simulate the natural variation in river flows.

Fish - a fish survey found evidence of migratory salmon up to the discharge pool of the scheme so the tailrace screen has closely spaced bars to prevent the passage of fish up into the turbines and a large area to slow the flow and discourage fish from approaching. Both intakes also have appropriate measures to accommodate the passage of native brown trout.

Birds & mammals - no issues were raised in relation to birds, though as a precaution care was taken to avoid tree
felling in the breeding season. A bat survey showed the presence of Pippistrelle bats associated with the native
woodland so felling of potential bat roost trees was avoided, although no active roost had been found in the construction area. Otters use both the Allt Mor and River Lael, although again no active holts were found in the construction area.

Archaeology - the remains of a number of croft buildings are found in Glen Squiab and Inverlael. The design of the
hydro-electric scheme was modified to avoid areas of archeological importance.

Trees - the pipeline wayleave (corridor) involved the felling of around 6 hectare (ha) of immature conifers (mainly sitka spruce). On completion of the project 6000 native trees (mainly native broadleaves) have been planted as part of a habitat restoration programme.

Water supplies - the intakes were constructed above the two Ullapool water supply intakes (now replaced by new
boreholes in the valley bottom) which required close liaison with Scottish Water to ensure there was no compromise to the quality of Ullapool’s drinking water supply.

Civil Works

The civil works, the main components of which are the intake weirs, the buried pipelines and the powerhouse, were
designed by Mott Macdonald Ltd (Glasgow) and constructed by Global Construction Ltd (formerly Les Taylor
Construction Northern Ltd) (Muir of Ord).

Lael Intake

The Lael intake weir is a reinforced concrete structure incorporating a self cleaning COANDA screen mounted in the
face of the weir - this type of curved profile wedge-wire screen excludes material greater than 1mm in size, and requires only infrequent cleaning, thereby reducing  operating costs. The intake was constructed without a rock foundation and relies on deep foundations and long concrete aprons to prevent the water passing round the weir. A temporary diversion was used to allow the construction of the intake.

Allt Mor Intake

The Allt Mor intake weir is located in a narrow gorge where the lack of space for a COANDA screen necessitated the use of a conventional 10mm spaced overshot bar screen. Minimum excavation was required because the rocky gorge provided a perfect foundation. During construction the river was diverted through pipes in the base of the structure.

Pipeline

The pipeline is a total of 3.2km long and consists of two 800mm buried pipelines leading from the intakes combining to a common 1100mm pipe close to the river confluence which then splits again just before the powerhouse to feed the two turbines. It is constructed from glass reinforced plastic (GRP), a strong, lightweight and cost effective pipe material which required careful handling by the contractor. The “head” - the difference between the elevation of the discharge and the water at the intake - is around 120m.

Powerhouse

The powerhouse contains the turbines, generators, transformers, electrical substation and associated equipment. This is largely underground and required a 9m deep excavation into the rock to build a reinforced concrete substructure, which provides excellent noise attenuation. There is just a small above ground building containing the grid connection substation with an external transformer compound.

Power Generation

Generating equipment

The hydro generation equipment was designed, manufactured and installed by Gilkes. There are two horizontally shafted Francis type turbines directly coupled to two 415 volt, three phase synchronous electrical generators. The turbine is controlled by a et of 20 guide vanes mounted in the spiral casing which direct the water onto the runner, causing it to rotate at 1000rpm to drive the generator. After passing through the turbines the water is discharged down through the draft tube into the tailrace pipe and back into the river through a special fish friendly screen.

Control

When in operation the turbines are controlled automatically via the control panels in the powerhouse using level sensing equipment at the intakes. The turbines have a gross head of 120m and use between 0.4 and 2.9 cubic meters per second of water between them, supplied roughly equally from each intake, and can generate between 300 and 2,500kW of power, depending on the water available in the river.

Grid Connection

A station transformer converts the output from the generators from 415 to 33,000 volts which is then delivered to Scottish Hydro Electric Power Distribution’s local electrical distribution 33kV network via a buried cable and short overhead line. As this is near to the end of a spur feeding Ullapool, the output from the plant is tightly controlled to prevent it from interfering with the stability of local supplies.