River Derwent Longbridge turbine coming on stream

 

The Longbridge Turbine building on the River Derwent in Derby

After many months watching the progress of the works to install a turbine across the Longbridge Weir in the centre of Derby, earlier this week I joined other members of the public to see and hear more.

Construction of the new turbine is complete and commissioning checks are in progress.  This will be followed very soon by electrical checks to allow the  installation to connect to the grid.  In operation it will generate up to 230kW of electricity depending on the flow of water down the river.  This is equivalent to the electricity used by just over 350 ‘average’ households.

The whole installation will have cost about £2 million to build.  Around three quarters of this was for the civil engineering – the new concrete channels reshaping of the edge of the river.  These guide the river water into the vertical axis turbine, and delivers it back to the river below the weir.  The shape is carefully designed to maximise the efficiency of the turbine.  These should last for maybe 200 years.

The turbine itself, housed in the building, is a kaplan design.  These were originally developed in the late 1800s, and are suitable where there is quite a small height difference (‘head’) but larger flow.  Here the height difference is around 2.5m.  This has a design life of around 70 years.  When the time comes to renew it, this should be a straightforward job as the building has been sized to allow a simple lift out of the old, lift in of the new – with even a specially strengthened platform available outside for the crane to stand on.  

Sticks and other rubbish in the river is prevented

The brash cleaner has three main parts: the grating (only the top edge is visible), the hydraulic cleaner arm (in foreground with scoop partly hidden by the wall) and ditch (with water running directly away in this view).

from reaching the turbine by a ‘brash cleaner’ in the intake.  This galvanised steel mesh reaching across the whole entry area is cleaned automatically by a mechanical arm to stop the entry getting blocked.  The collected material is scooped into a purpose-made ‘ditch’ and a small flow of river water is used to wash this back into the river downstream of the weir.

The fish pass has two flights with a resting pool half way. When the turbine is operating the water will exit between the fish pass and the shore.

 

 

 

 

 

A second flow of bypass water will make the weir more wildlife friendly by creating a ‘fish pass’.  This has been designed to allow migrating fish to reach the upstream side of the weir more easily, and was a requirement of the Environment Agency.  They may set up equipment to monitor how many fish make use of it.

When operating, the turbine will be need very little attention with most adjustments happening automatically.  For example the flow rate will be adjusted (by altering the angle of inlet vanes above the turbine) to ensure that there remains a minimum of 5cm of water falling over the weir.  If there are any problems, telemetry will alert relevant people to attend, and if needed the turbine will shut down automatically.

The connection to the electrical grid is fed through a new cable to a substation on the roof of the Council House.  The council will use this to power its offices, and any excess will be sold to the grid.  Generating it’s own electricity will reduce the uncertainty of the council’s electricity bills making it easier to budget.  At currrent electricity prices (including incentives for generating low carbon electricity) the project has a ‘payback’ of about 23 years – but as electricity prices are expected to rise this time period is likely to shorten.

The increasingly attractive return on investment for this sort of installation means that the Environment Agency is dealing with many more applications for turbines.  As this scheme was seeking approval, it was only one of four on their books – now they are considering applications for over 60 schemes.  This is good news for renewable electricity in theUK!

I have had a keen interest in this project, and not only because I like green engineering projects.  A turbine on the weir was suggested to me when I was first a cabinet member on the council in 2003-5 and I was keen the idea was explored further.  I was again the cabinet member dealing with planning when formal approvals for the scheme were put together in 2008.  The project has taken a lot longer to complete than was hoped, with the Environment Agency in particular finding it difficult to balance its roles to protect wildlife and support reductions in carbon emissions.

The Derwent Valley Mills Heritage Trust had arranged these drop-in tours as part of a week of activities.  This event was supported by Transition Belper.  Transition Belper have had funding to take forward an exciting project looking to install turbines across other weirs on the River Derwent.  They would hope to fund the installations by setting up a “community interest company”.  Residents all along the Derwent valley would be able to invest in this company, knowing that it should provide not only a financial return, but also make the area more self-sufficient in ‘green’ energy.

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