A quiet revolution29 November 2010
UK renewable energy legislation has driven innovative new applications of hydropower technology. Samantha Wood from civil engineering contractor JN Bentley reports
Hydropower currently provides just 40% of the UK’s renewable energy production, but changes in government policy have provided opportunities to develop hydropower projects which previously would have been unviable. The UK, which already has over 1500MW of installed hydropower capacity, generating over 4000GWh/yr, is now much better placed to make use of the water resource available.
Recent surveys have shown that there is potential in the UK for a further 2GW of capacity, with significant further development possible, particularly for new micro scale hydropower projects. Many of these micro generation projects will make use of existing infrastructures such as old mill sites and weirs, or water utility assets such as reservoirs and water transfer schemes, as well as opportunities to enhance existing hydro schemes.
In April 2010, the feed-in tariff (FIT) scheme was introduced, incentivising small scale (less than 5MW) low carbon electricity generation and making low head schemes a commercially viable option.
JN Bentley, a well established civil engineering contractor, has been quietly building its reputation in the hydropower sector for a number of years. Through the company’s collaborative working agreement with Archimedean screw supplier Spaans Babcock, it is now using its expertise to install screw powered hydro generators on a range of low head sites. Many more schemes are in the planning and outline development stages.
Closest to its heart is JN Bentley’s own hydro development site at Linton Falls in the beautiful Yorkshire Dales National Park, only a few miles from the head office in Skipton.
The Linton Falls site was originally built as a hydropower station and operated from the early 1900s to 1946 when the National Grid finally brought power to the Dales. Such is the importance of the site, as an exceptionally rare example of early industrialisation in the Yorkshire Dales, that what remains of the now derelict powerhouse is listed as a Scheduled Monument by English Heritage.
JN Bentley has received consent from English Heritage to sympathetically restore the 100-year-old crumbling turbine house to its former use and secured planning permission and Environment Agency licensing to build and operate a 100kW twin Archimedean screw hydro station.
By 2011, Linton Falls will be generating sustainable energy for the local communities once again, this time via the National Grid. This project is an example of a growing trend in the UK, with the Yorkshire Dales National Park Authority alone considering up to 50 further sites for small hydro schemes.
Large scale run-of-river
The skills developed during the design and development of Linton Falls have been taken forward by JN Bentley’s team of hydro specialists to help clients realise similar aspirations. Many of these are also weir based projects, but also include Kaplan turbine technology in addition to Archimedes screws.
JN Bentley is currently working with the Small Hydro Company, in partnership with British Waterways, to develop run-of-river hydro schemes on around 25 sites across the organisation’s 3540km waterway network. The wider project team also includes design consultants Mott MacDonald, fisheries experts Turnpenny Horsfield Associates and JN Bentley’s supply chain.
Early contractor involvement (ECI) during site appraisal and scheme development stages is ensuring that ‘buildability’ and commercial viability do not become issues, by quickly discounting unsuitable sites and minimising the cost of design development.
The 25 proposed hydro schemes, ranging in electrical capacity from 100kW to 1.5MW, will generate 210,000MWh of renewable energy a year, enough for about 40,000 homes. Gunthorpe and Sawley are two key sites currently under development which combined will contribute 33% of government targets for hydropower generation in the East Midlands region.
The Sawley Weir site, on the River Trent near Nottingham, will be the first of the two sites to be completed and will use two Kaplan turbines at the weir to generate up to 1MW of power. Sawley Weir, under normal flow conditions, is around 1.8m high, but the high flow rate of the River Trent is best used by increasing the head and using twin 2.5m diameter Kaplan turbines.
While increasing the head makes the site more commercially viable it also brings a number of challenges. The biggest issue was public perception of an increased flood risk.
In proposing an increase in weir height by 800mm, there was an immediate concern that the water level upstream of the installation would also increase by 800mm and therefore increase the risk of flooding. The challenge was in reassuring those concerned that Kaplan turbines of this size actually have the effect of drawing down the water level upstream, in this case by approximately 600mm.
Such a small net increase in water level is well within the natural variation of water level on the river, but to prove this required extensive use of a computational model of the River Trent, obtained from the Environment Agency, combined with site surveys and traditional calculation.
Once the team had overcome this, the next concern was the potential for the turbines to have to shut down due to mechanical/electrical issues or the weir flooding out and the head disappearing during floods.
Unchecked, this would exacerbate upstream water levels as the draw down effect would be lost. But to prevent this situation, the hydro team incorporated twin hydraulic flood gates in the design which will open and release trapped water from upstream of the installation. When open, the gates will allow the same volume of water through as the increased weir height had prevented, making the installation flood neutral and maintaining the river’s natural flood response.
JN Bentley’s ECI during design development, together with the flood assessment expertise of consultant Mott MacDonald, both overcame public concerns and maximised the power generation potential of the river. As a result, the scheme achieved planning consent within two years of scheme conception, an exceptionally short timescale for a UK hydro scheme of this size.
The scheme at Sawley has also presented the opportunity for additional environmental enhancement. The Small Hydro Company has made a commitment to install fish passes on all their hydro schemes, regardless of location or type of installation; in this case, supporting the objectives of the Environment Agency and the Trent River Trust to re-establish a sustainable salmon population in the River Trent by 2020.
JN Bentley has also secured contracts with RWE npower renewables. The £4M (US$6M) civils contract for the Black Rock run-of-river scheme in the northeast of Scotland involves a 3.5MW twin Francis turbine installation on the River Glass at Black Rock Gorge near Inverness. It also includes construction of a powerhouse and low profile concrete intake weir. The project utilises a 70m fall in river level between the new intake weir and powerhouse, with approximately 3.5km of buried 1.5m diameter pipeline installed to connect the two. The scheme is due for completion in autumn 2011 and will supply power to the National Grid.
The 47-week project will produce an average of more than 10GW hours each year – enough to supply around 2000 households with their annual electricity needs.
Value-add hydro applications
The same hydropower technology is also proving to be a significant value add-on for some major projects where power generation is not the primary purpose, making an important contribution to operational efficiencies.
In February 2007, in the first installation in the UK to use untreated sewage for hydropower generation, JN Bentley installed two new Archimedean screw hydro-turbine generators at Yorkshire Water’s Esholt wastewater treatment works as part of an overall site upgrade worth £19M (US$29M).
The two generators supplied by Spaans Babcock, each weighing more than 20 tonnes, were installed in series between the inlet works and the new primary settlement tanks. Up to 3240L/sec of wastewater now flows sequentially through the two screws over a total fall of around 10m, passing through an 1800mm diameter pipe to the hydro turbine station. Over 180kW is generated, reducing the imported power demand of the treatment works.
Another flagship UK project is the £4.6M (US$7M) redevelopment of the Tees barrage white water course in Stockton on Tees in northeast England, funded by regional development agency One North East, Stockton Borough Council, Sport England and British Waterways. The project is currently the only installation in the world where Archimedean screws are used as both pumps and generators. A number of improvements are underway to turn the course into a world-class water sports facility and it has already been confirmed as one of the official training camps ahead of the London 2012 Olympic and Paralympic Games.
Improvements will include reconfiguring the existing main course (a 250m long, 7m wide ‘U’ shaped loop, with a 2.5m drop and a flow of 10m3/sec) and building a second shorter, steeper course for high-speed paddling.
Before redevelopment, the course could only operate two or three hours either side of low tide in the River Tees. To extend the operating time, and improve course flow control, JN Bentley is installing four 12m long by 3.1m diameter Archimedean Screws, each weighing more than 30 tonnes, in a newly built pumping station.
The screws will pump water from the bottom pool to the top pool, creating guaranteed conditions for canoeists and rafters regardless of tide. When not pumping water around the course, the screws will turn into generators using excess river water to generate electricity for the National Grid, making Tees the UK’s first fully sustainable white water course.
When operating as a screw pump, the power absorbed at the duty point (3500 l/sec @ 4.94 m) will be 178.5 kW per screw pump. The screws will each be capable of lifting 3.5m3/sec at a maximum head of 3.68m, producing a maximum flow rate of 14m3/sec for the course. The screws are controlled by variable speed drives via a SCADA controls system in the main control tower, and can be run in either manual or automatic mode.
In manual mode the pumps will run at a fixed speed of 1500rpm from an input of 20mA. When the pump is running up to speed, the system will send signals to the hydraulic power pack to open the relevant Penstock. Automatic mode will control the number of pumps needed from those available and vary their speeds to keep the bottom pool at its desired level of -1m. If more than one pump is available, the system will only vary the speed of one pump to trim the level and run the others at their most economical speed. It will also select available pumps according to their accumulated hours to balance the running time as far as possible.
Generating mode will only be available when pumping mode is set to ‘off’. When operating in reverse, generating electricity, each screw pump will produce approximately 131kW. To maximise the opportunities for exporting electricity to the National Grid, the system will automatically alert the barrage operator when the conditions are appropriate i.e. no pumps in ‘pumping mode’, upstream level above 2.35m AOD and lower pool level below -1m.
The project, being led by British Waterways, is due for completion in winter 2010.
As populations grow and carbon reduction targets are set, the increasing need for sustainable energy is one of the few certainties the civil engineering and construction industry can expect over the coming years. Designers, contractors and clients alike, who are positioning themselves to address tomorrow’s needs today, will be well placed to be leaders when this still youthful market reaches maturity. The revolution may be quiet, but it’s coming.
Samantha Wood, Communications Officer, JN Bentley.
The author would like to thank the following at JN Bentley for their help and technical expertise in producing this article: Austin Flather, Rhianna Rose, Kate Crawford, Oliver Wilson, Graham Jessop and Paula Wyldbore.
Permission kindly granted by: British Waterways, RWE npower renewable, Small Hydro Company, Spaans Babcock, Turnpenny Horsfield Associates and Yorkshire Water.
Linton Falls, Yorkshire
*1 Homes powered based on typical 3 bed room house annual use of 4200kWhr/yr