Articles & Case Studies

Cadair Idris: A Flexible Approach to Penstock Specification

Posted: Monday 9th December 2013

The rocky mountainsides of Snowdonia have been a Mecca for walkers and the quarrying industry for generations and, amidst the tourists, farming has provided the bedrock of the Snowdonia National Park’s rural communities. Some areas of the terrain are too rugged even for hardy Welsh sheep, however, but the landscape continues to deliver new opportunities for jobs, commerce and essential commodities…..this time in the shape of Hydro Electric Power.

One of Snowdonia’s most popular peaks with hikers, Cadair Idris is located in Gwynedd, at the Southern edge of the National Park, near Dolgellau. The idea of a hydro-electric power station was first mooted by one of the farmers that occupies the steep hillside several years ago and the current scheme first began when hydro development company, Dragon Hydro, identified the farmland as a potential hydro site in 2008.

Explains David Roberts, Dragon Hydro’s consultant on the scheme: “The site we identified borders two farms but the land itself is not viable as farmland as it is too steep, boggy and covered in bracken and gorse for grazing livestock. The landowners we able to see the benefits of the scheme both in terms of generating clean energy and generating an income from the feed in tariff, but we also had to gain planning permission and environment agency approval and secure funding from a venture capital investor.”

Dragon hydro secured investment for the project from Albion Ventures LLP and construction of the £1.3 million scheme began in April 2013. The Cadair Idris hydro-electric power station will be completed in early September and fully operational by mid-September following testing and commissioning. Once operational, it will deliver a maximum electrical output of 315kWe to the grid, enough to power up to 300 households.

A Flexible Solution

One of the key challenges for the design and installation of the Cadair Idris Hydro-electric powerstation was the specification of the penstock. Not only does the location, gradient and rough terrain make the site a testing environment for specialist contractor, Jim Dorricott Construction, it also means that the chosen pipe system must be durable enough to provide a long-lasting and maintenance-free solution and flexible enough to bend and twist with the landscape.

David continues: “PE pipe was really the only viable solution – both technically and commercially. It wasn’t going to be feasible to import any backfill material up the mountainside for the installation of concrete or ductile iron pipe and only PE could offer the flexibility required for the terrain. Commercially, the ability to tailor the wall thickness of the pipe to the pressure requirement of the penstock at different points along the route meant that we could minimise the amount of raw material used, helping us to manage the cost of the penstock, which is one of the main costs of any hydro-electric installation.”

Having decided upon PE pipe as the best material for the hydro penstock, Dragon Hydro considered several PE pipe suppliers before opting for PE pipe specialist, GPS PE Pipe systems. The company has supplied several other hydro-electric projects, including the National Trust’s scheme at Mount Snowdon, and has in-house expertise that helps to deliver technical support for hydro customers.

David continues: “Obviously price was a factor but technical expertise in helping to ensure the penstock was as technically appropriate and cost effective as possible was also key. So too was GPS’s ability to respond to the specific needs of the project: the nature of the site means that there’s restricted space for manoeuvring materials and GPS agreed to supply pipe in 10m lengths to help address this. They were also able to supply us from stock with fortnightly deliveries which helped to overcome the project’s issues with limited storage facilities at the site.”

Tailored for Pressure

The Cadair Idris hydro-electric scheme has two intakes from two tributaries of the Afon Cadair river and the Jim Dorricott team constructed a weir to direct the flow and ensure that 50% of the water from each source remains in the river. From the intakes, the penstock follows a 400m route across open, boggy terrain before joining the route of an existing farm track for 1.2km and then passing through grazed fields for the final 800m stretch. GPS has provided black PE 100 pipe for the installation with a steadily decreasing SDR (Standard Dimension Ratio) as the water pressure increases during its descent.

Explains Ken Wilson from GPS: “Where the water enters the penstock at the intake the water pressure is low so 315mm diameter pipe was used in SDR 33, which gives a wall thickness of just 5mm. The pipe diameter then increases to 500mm at SDR 33, then the wall thickness increases to SDR 26, then SDR 21, SDR 17, SDR 13.5 and finally SDR 11 where the water enters the powerhouse at 15 bar pressure and a wall thickness of around 50mm is required for the 560mm diameter pipe.”

The penstock will deliver the water to the power house which is of brick-faced construction with a slate roof to help it blend into the landscape as a barn-like outbuilding. It will enter the turbine at 15 bar which will spin the generator to produce electricity that is delivered directly into the grid. The water will then be released back into the Afon Cadair, just as clean as when it entered the penstock at the intakes.

Environmental Legacy

Installation of the penstock began in April and, despite the challenges of the terrain, has been progressing quickly, with the first 400m stretch completed by the beginning of July. The Jim Dorricott team began by making a track for the route and stripping the soil back before digging a trench for installation. Lengths of pipe were welded together on site using butt fusion change pieces each time the SDR of the pipeline changes along the route. Once the pipe is installed, it is simply buried with specially treated top soil over the top to encourage fast re-growth of vegetation.

David adds: “With a project like this there is huge environmental responsibility and we have worked closely with the National Park and the Environment Agency and employed an ecologist on site to observe the installation team throughout the programme. Schemes like this one will help us further the use of renewable energy to protect the environment for future generations while blending into the landscape more-or-less from day one and the use of PE pipe for the penstock will help ensure reliable, maintenance-free operation for the next century.”

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