Site: Cotehele Mill, Cornwall
Turbine: Crossflow turbine
Head: 7 metres
Flow: 135 l/s
Screen: WRE automatic screen
Fish: Piped fish bywash and integrated flood spillway
Role: Design – Construction – Screening – Mechanical & Electrical Engineering – Commissioning
Cotehele Mill is owned and managed by the National Trust, and is both a working flour mill and open to the public. Located in a steep sided valley, the site already possessed some of the infrastructure required for the National Trust hydro scheme, including the weir, leat and of course a working waterwheel. The flour mill operations only run for two days a week, thus opening the possibility of putting in a modern turbine upstream of the mill. It was important, however, given the space and limited availability of water, that the existing waterwheel and flour mill operations could continue, and coexist with the new turbine.
How was this achieved? Water destined for the new Crossflow turbine is first collected from the leat in a newly constructed forebay tank – where as, water destined for the waterwheel, which has first priority, just carries on down the leat. If the waterwheel is opened up, it starves the Crossflow turbine of water and causes it to shut down. Conversely, if the waterwheel is closed, the water level rises in the forebay tank, then the Crossflow turbine automatically switches on again. This system ensures that the water resource is always pit to good use – when water isn’t being used to turn the waterwheel to grind flour, it is used by the Crossflow turbine to generate electricity.
What about fish? The river has been identified as a migratory river. Therefore, the leat may have fish in it which need to be able to re-enter the river. Provision was made for fish to bypass the screen and re-enter the river via a fish bywash that runs adjacent to the powerhouse.
Western Renewable Energy (now owned by Renewables First) were brought in to design, install and commission the system. The construction process was planned and executed in a manner that enabled both the working mill and visitor centre operations to continue unaffected.
The images (above and below left)) shows excavation works commencing. The valley is compact and steep sided, which meant access was difficult. A special bridge had to be constructed (above right) to enable access for equipment (below right).
The powerhouse, shown under construction (below left), is located upstream of the existing mill. The design is deliberately small, compact and discreet, with a turf roof. For ease of installation, the Crossflow turbine was installed immediately prior to the walls and roof of the powerhouse being assembled (below right).
All visible construction work had to be finished in a manner consistent with the historic mill site, for example, stone facing in the forebay (below left) and timber cladding and a turf roof on the powerhouse (below right).
The pipe shown running down the hillside to the right of the powerhouse (above right) is the fish bywash. This was specified and installed to regulations. The final images are taken from within the powerhouse, showing the Crossflow turbine during and after installation.
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