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How a floating solar farm powers water treatment

Floating solar installation

United Utilities is using innovative technology, updated designs and modular thinking for its floating solar panel array in the North West.

When the Langthwaite reservoir opened in 1935, it had a simple purpose: the 900M.l of water it held would supply households in Lancashire’s surrounding towns.

Fast-forward more than 80 years and the reservoir is now being put to a second use, as civil engineers face the new challenge of using existing infrastructure in more efficient, greener ways. A floating array of solar panels is being constructed on the reservoir. Energy from the panels is expected to save hundreds of tonnes of carbon per year.  

Client United Utilities (UU) is in the final stages of constructing the solar farm with contractor Engie delivering the scheme which covers 5,200m2 – the size of 22 tennis courts. 

The array’s 3,250 panels are divided up into interchangeable “figure of eight” style pontoon frames, each measuring 4m by 3.8m. The array is capable of producing 895,000kWh of electricity and the energy generated is already powering the UU water treatment works on the same site. 

Excess electricity is delivered to the National Grid. 

Floating solar

Floating solar

The Langthwaite array is not UU’s first venture into floating solar panel arrays. But this latest project has allowed UU to draw on lessons learned from the  Godley reservoir array in Greater Manchester, which was constructed in 2016. The Langthwaite project has come in at £1M, considerably cheaper than Godley’s £3.5M cost. 

“This is a really innovative design,” says UU head of renewable energy Richard Waggitt. 

“Taking lessons learnt from the array we constructed at Godley, we have adapted and come up with a completely different design using a different system that provides greater resilience that avoids a lot of the issues we have had with the Godley array.” 

Floating solar panel arrays have several advantages over their terrestrial cousins – exploiting unused space, the water keeps the panels cool and the panels protect the water from the sunlight which causes algae growth. 

But they do require more maintenance and there are obvious challenges posed when mixing electrics and water.

This is a really innovative design

At Langthwaite rows of panels have been assembled on the side of the reservoir, floated out via a launch platform and then connected to concrete anchors with pre-tensioned moorings.

Setting the angle of the solar panels posed one of the first challenges, said Engie head of energy Barry Tayburn. 

Engineers had to find the balance between the optimal angle for generation and the “sail” effect created by the wind and the stress that exerts on the moorings. 

“There is a 10° angle on the panels. Normally they would be mounted at 22° for the UK,” Tayburn says.

Floating solar installation

Floating solar – the panels are installed at an angle of 10°

“This had to be balanced against the weight of the anchors and the extra anchorage needed, because a higher angle means more of a sail effect putting stress on the structure. We have found that the change in angle doesn’t make a noticeable difference in performance for this area.” 

The pontoons supporting the panels lift them and the associated electronics a clear 300mm above the water surface, an important lesson learned from the Godley project.

The increased distance between the panels and the water is important in reducing the development of damp on the panels by allowing air flow between the structure and the water surface to clear mist that would otherwise condense on sensitive components. 

The redesigned pontoons also support much more weight, allowing engineers to freely walk across the array to conduct inspections and maintenance, another updated design element resulting from the Godley experience. 

Below the surface, 36 vast 1.25t concrete blocks anchor the array to the reservoir bed. 

Pretensioned cabling allows a constant mooring that means the array can rise and fall up to 12m vertically as the level of water in the reservoir changes. 

In contrast, the Godley array is held in place with anchors which are secured to the side of the reservoir by horizontal cables. 

While these are easier to install, the cables at Godley restrict movement around the perimeter of the pontoons. 

The concrete used at Langthwaite is not marine concrete but it is of dense non-porous specification that protects the steel inside the blocks from contact with the water.

To ensure the anchors were placed correctly and safely, UU used of two drones, one in the air and one underwater. 

The anchors were moved by engineers using an A-frame, while the drones fed back GPS data to ensure each anchor was placed correctly.

The submersible drone can also be used to inspect the moorings and cables, meaning divers never need to enter the reservoir – a hazardous environment for diving created by the mooring cables. 

Local company, Northern Pontoons built the pontoons, which were assembled on site by Bolton-based Engie. 

Each pontoon unit was then bolted to its neighbour and floated out on the water to be joined to the main structure. 

The modular nature of the system, which uses off the shelf parts, means UU can replicate the design on other suitable sites and expand the Langthwaite or other arrays with ease if requirements change. 

We could build these four or five times bigger by simply producing more units and bolting them together

“There is nothing bespoke about the design. If the water treatment works here grew and needed more power, then it is no problem to just expand the array,” Tayburn added. 

“This was built in a modular format, we could build these four or five times bigger by simply producing more units and bolting them together,” Waggitt said. 

“One of the things we are looking at for other sites is storage, using the panels to charge batteries throughout the day, and then using the stored energy during high tariff periods, but what you need then is more panels to be able to charge batteries and provide power at the same time during the day.” 

The Langthwaite array is the latest project in UU’s 24MW, two-year pipeline of renewable energy projects.

The aim of these floating solar farms and other renewable projects including the potential for further solar energy developments as well as onshore wind power is to reduce the utility’s energy bills by 10% – at 800GWh it uses as much energy every year as the whole of Manchester.

Floating solar

Floating solar

Readers' comments (2)

  • Interesting and nicely detailed report, especially concerning the design development from the previous project . Algae growth can pose huge problems for stored river water. It's possible that we might see a lot more of these installations giving double benefits.

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  • "The array is capable of producing 895,000kWh of electricity" i.e 895 MWh of energy?"
    Is this figure correct? Over what period of time?

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