SUMMARY

Location:                                   Poundsgate, Devon

Rating:                                      90kW

Turbine:                                    Crossflow turbine

Average available head:             16m

Design flow:                              800 litres/sec

Year of commissioning:               1995

OVERVIEW

Old Walls hydro scheme is on a farm on the West Webburn, a tributary of the River Dart.  A previous hydro scheme had operated since the 1930s but the equipment was in need of replacement.  This offered the opportunity to redesign the site to maximise the output. 

It took the owner three years to achieve the planning and environmental consents to proceed with the project.  However the scheme is now seen as a good example of what can be done to mitigate the minor environmental damage caused in the construction and operation of a micro-hydro site.

The whole 90kWscheme cost around £80,000 in 1995.  The scheme has had a NFFO contract providing an income of £21,000 per annum.  The expected design life of the equipment is at least 30 years.

In June 2004, Old Walls Hydro received an Ashden Award for this best-practice example of a modern micro-hydro scheme in the UK.

DESIGN ASPECTS

·          The site is permitted to abstract 800 litres/sec.  A notch in the weir ensures an appropriate residual flow is maintained in the river. 

·          A new 500 m canal was dug from the intake to the forebay.  The velocity in the canal is only 0.4 m/sec which allows grit to settle out and provides a gentle flow for salmon smolts to develop.  The canal passed through a wood and five mature trees had to be felled.  As compensation, 800 hardwood saplings were planted on the adjacent land.  The new trees and natural growth along the canal have increased biodiversity.

·          The penstock inlet is protected by a stainless steel screen consisting of a continuous belt of 10 mm mesh mounted on nylon rollers.  As the rollers rotate, a clean section of mesh rolls up from the bottom and the blocked mesh deposits the accumulated debris into a composting heap.

·          A 700 mm buried penstock pipe carries the water to the powerhouse by the river bank; a pipe length of 115 m was required to cover the vertical drop of 16 m.  The powerhouse is set back 6 m from the river bank and 2 m above water level to prevent flooding. 

·          The turbine is a crossflow machine supplied by Cink of the Czech Republic.  The turbine produces 90kW in full flow conditions.  The control system and screen were supplied by G P Electronics of Bovey Tracey.

·          The water-to-wire efficiency of the scheme has been measured at 71.7%.  This excellent result is due to a number of factors:

The large leat ensures a slow flow velocity; the water falls just 100 mm over the 500 m canal length. 

The design of wire-mesh screen produces negligible friction losses, and the screen cleaning mechanism keeps it clear of leaves at all times. 

The penstock is over-sized, minimising the losses due to pipe friction.

The turbine has a patented design of control valve which minimises the turbulence as the flow enters the runner.

·          The only ongoing maintenance is to check the weir and leat for large items of debris and occasional inspection of the screen cleaner and occasional removal of the leaf pile.

Nature rapidly recovered after site construction.  The banks of the canal show a good growth of new tree and shrub species, and the canal itself contains young trout.  The cross-flow turbine also aerates the water, a benefit which will be carried downstream for some distance.