A modern version of the classic overshot waterwheel is at the centre of an imaginative restoration project in the Outer Hebrides.
Back in the early nineteenth century, the subsistence economy of western Scottish islands like Lewis was based on a plethora of small vertical axis or 'Norse' waterwheels. These powered the grindstones that ground the barley and oatmeal which formed the staple diet of the locals - wheat was unsuited to the infertile, acid soils and cool, damp climate of the Hebridean islands, and potatoes had not yet arrived. Change was in the air, however. Britain's Industrial Revolution was taking off, agricultural production on the mainland had taken great strides, and local landowners were keen to modernise.
In those days Lewis was owned by the Mackenzies, who built three modern horizontal axis watermills on the island in the years up to 1816. The 250 local Norse mills were eventually closed or destroyed in the pursuit of more efficient production (and greater profits). One of the new watermills was actually in the grounds of Seaforth Lodge itself, the home of the Mackenzies. The site was known as Willowglen, the water came from the River Glen. What was initially dubbed the Stornoway Mill later became known as Latta's Mill after the death of John Latta in an accident there in 1834 — or possibly because the Latta family operated the mill for more than 20 years.
In 1844 the Mackenzies sold Lewis and Seaforth Lodge to James Matheson, one of the founders of the world famous Jardine Matheson trading company of Hong Kong. The wealth that Matheson amassed from the China trade funded major improvement works throughout Lewis, including nearly 250 km of roads and a peat distillery which produced paraffin for lighting, sheep dip, marine anti-fouling, roofing pitch and candlewax. He also demolished Seaforth Lodge and a nearby whisky distillery, then built Lews Castle on the site and even laid out extensive grounds surrounding it.
After more than 70 years of operation, Latta's Mill burnt down in February 1890 and was never rebuilt. By the end of the twentieth century, all that remained to mark its passing was the overgrown mill lade and mill-pond, and the stonework that once housed the overshot wheel itself. The Category 'A' listed Lews Castle had also fallen on harder times: since it was gifted to the local community in 1923 by the last owner, Lord Leverhulme, it has served variously as a wartime naval hospital and the first home for Lews Castle College. Major structural defects were discovered at the end of the 1980s, and by the turn of the century it was unoccupied.
This decline mirrored a similar decline in Stornoway itself. In 2000, however, a major regeneration effort by several agencies began. In the Burgh itself the Stornoway Amenity Trust undertook a number of projects that began to revitalise the built environment, and transform the ambience of public spaces. The Stornoway Trust built a new visitor centre on the site of the old whisky distillery in the Castle grounds, which had suffered from years of neglect. Both trusts then combined their resources in 2003. The joint objective was to restore the Castle grounds, and the bold decision was taken to install a modern water-wheel in the sad remains of Latta's Mill. This would be no cosmetic exercise; the wheel would generate electricity - which would light the Castle grounds.
Design and project management of the project was undertaken by local civil engineer Tony Robson on behalf of the Amenity Trust. Advice and assistance on the detail design of the wheel itself came from the Pedley Waterwheel Trust. The first overshot Pedley waterwheel was installed at Pedley Wood in Cheshire in 1991, and was intended to demonstrate that with modern technology the low speed traditional waterwheel could be used effectively to generate low cost mains voltage and frequency AC power. After six years the original 2.4 m diameter wooden wheel was replaced by a modern steel version running on self-aligning roller bearings and coupled directly to a readily available modern integral geared induction motor running 'in reverse' . The gearing increased the wheel's 12 revolutions per minute to a much more useful 1,600. Between 1997 and 2005 a number of Pedley waterwheels with outputs of up to 5 kW were installed in low head installations in Sri Lanka.
At the Willowglen site the available head was 8.5 m, maximum flow was 150 l/s. Wheel size was limited to a diameter of 4.5 m and a width of 650 mm by the configuration of the existing stonework. It was calculated that such a wheel could generate up to 4 kW, enough for the intended end use. To supply water to the wheel some 340 m of the lade had to be cleaned out and relined and a v-notch weir constructed in the river, which is used by salmon and seatrout so a mesh fish screen is also needed. Gradient in the lade was 1:200. The mill pool was dug out and put back into service and a 50 X 25 mm mesh trashrack installed between it and the wheel. (Overshot wheels cope better with trash than most hydropower alternatives.) A short tailrace takes the water 20 m back into the river.
Most of the time, the wheel will generate surplus power. This surplus power is fed to the local grid. (Control of the supply is down to an induction generator controller supplied by Nottingham-based Sustainable Control Systems.) As the water begins to flow, this ensures
Adapting a readily available geared induction motor is the key to the wheel's efficiency (Reproduced with permission from Tony Robson Consultant)
The visitor centre and waterwheel during construction (Reproduced with permission from Tony Robson consultant)
that the power from the wheel matches the grid in terms of frequency, voltage and phase before the 'soft connection' to the grid is made. Control is via a motorised sluice immediately before the wheel, which allows small variations in the wheel's speed to be made to maintain synchronisation. Should the local grid suddenly go down, there would be the risk of the wheel accelerating to dangerous speeds as the load comes off and before the sluices can reduce the flow enough. In such a case 'ballast heaters' absorb the power that would have gone to the grid and convert it to heat which is dissipated to atmosphere.
Fabrication of the wheel from 6 mm galvanised sheet steel was undertaken by a local blacksmith. Construction began in early 2005 and the visitor centre and the waterwheel were officially opened in October 2005. There are plans afoot to refurbish Lews Castle itself as a mixed-use development comprising a new home for the Museum nan Eilean and a high quality hotel with function suites.
According to the Pedley Waterwheel Trust, a modern waterwheel like the one in Stornoway should extract more than 80% of the energy in the water. Gearbox efficiency is 97%, generator efficiency 90%. 'Water to wire' efficiency, therefore, is approximately 65%, which few turbines could match at such low heads and flow rates.
Client: Stornoway Trust in partnership with Stornoway Amenity Trust Designer: Tony Robson Consultant assisted by Pedley Wheel Trust and SCS Contractor: Bardon Hebrides Millwheel fabricator: I. M. Murray Engineering
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