Micro-hydroelectricity takes off in the UK

Engineerblogger
April 19, 2012

A 450KW project in the lake district's Duddon Valley.

Power stream: London-based Ellergreen is on a mission to operate more micro-hydro schemes in the UK.

Mark Cropper is in his Portobello Road office in London Poring over remote live flow data for one of the 20 or so micro-hydroelectric schemes he oversees around the country.

The motto of his company, Ellergreen Hydro, is: ‘We’re happy when it rains.’ Given that we have had one of the driest winters on record, you might expect some gloominess, but he’s surprisingly upbeat.

‘We’ve got two schemes now that have not stopped since 1 September [2011]. Probably in about a week they might go down because there’s not enough water, but they’ve been running continuously for that period.’

Ellergreen is one of a handful of companies building and consulting on micro-hydro schemes (defined as sub-1MW) in an industry that has been steadily maturing off the back of the April 2010 Feed-in Tariff scheme guaranteeing a set price for generated capacity.

‘There is at least 1GW of hydro that could be built out right now. Provided that the regulatory environment doesn’t change and provided the feed-in tariff remains where it is, this will get built,’ Cropper said.

The majority of micro-hydro schemes employ ‘run-of-the-river’ (ROR) designs using water that is available at any instant in the watercourse with little or no storage behind a dam for later use. The river flow is diverted through turbines that spin generators before returning water back to the river downstream. They are often defined as low-head and high-head, although there is clearly a spectrum: low-head has a total water drop somewhere in the region of 2-5m and high-head can use anything up to hundreds of metres out of the drop of a watercourse.

While working on the same basic energy principles, low-head and high-head are very different beasts suited to distinct site locations, requiring different equipment and bringing certain advantages and disadvantages. Low-head installations tend to use existing redundant mill infrastructure at river points and feature open bypass channels coupled with large generators such as Archimedes screws or Kaplan turbines. By contrast, high-head schemes are located around streams that fall very steeply down the side of a valley, for example. They may require a small intake weir connected to a penstock pipe that is buried under the ground, travels down the hillside and feeds pressurised water through a small turbine, typically of the Pelton wheel or similar design.

Ellergreen has involvement with both head types, but favours the latter - a general trend within the industry. ‘With high-head, because you’re channelling a huge volume of water, not only does the civil [engineering] - like the channel you create - have to be very big, but you’ve got to put this water through a machine, whether an Archimedes screw or a more conventional turbine, and that’s got to be very big too,’ Cropper said.

Indeed, high-head schemes create a greater water pressure, giving a higher flow rate and a faster rotating turbine, which, in turn, means lower torque. The cost of the drivetrain is closely related to how much torque it has to transmit, so high-head costs less.

The environmental impact is also lower with high-head since the pipeline is buried, coming out at a shed or small barn at the bottom housing the generating equipment. Nevertheless, there is a strict regulatory framework ensuring minimal impact to local wildlife. Screens are needed to make sure fish cannot enter the channel or pipe used for the generating head, and sufficient, viable flow must remain in the native part of the stream.

In addition, when dealing with remote locations in Scotland, the Lake District or north Wales, there can be challenges around the export of electricity. ‘There are some places where you just simply cannot get the power away at economic cost; if you’re having to upgrade an electricity line all the way along the valley, that’s meaningful. But we have been really pioneering on the grid side,’ Cropper said.

Some high-head hydro schemes cannot connect directly to the National Grid but must make do with the local distribution networks in single phase, which puts a limit on generating capacity. ‘With some of those, we’ve had great success persuading them that we can put far more kilowatts into a single-phase network than anybody thought was possible. Now we’re looking at split-phase generators. That way, you can double up what you can put into single phase, so we’re now looking at 92kW in single phase,’ Cropper said.

The largest of Ellergreen’s completed schemes is a 450kW, £1.5m site at Logan Gill in the Lake District, while the smallest is a 15kW, £90,000 scheme. Construction has recently begun for a client on a 920kW site.

‘We’ve changed the business model and become a full service provider, so we will deliver hydro schemes for other people. We’ve built up a knowledge and experience of doing it ourselves, which is invaluable, and you learn far more by building, owning and operating your own hydro schemes - the subtle things can make a huge difference,’ Cropper said.

The first stage of this service is feasibility assessment, including topological surveying, axial flow gauging and computer modelling, to build up a profile of the site and the amount of energy it will generate (see box). This will be submitted to the relevant environmental agencies and electricity providers and thus plays a very important part in the success of a scheme. The technology solution is then selected on a case-by-case basis .

‘What’s brilliant about hydro is that of all the renewable technologies it’s the only one that is, and can be, totally UK plc - the supply chain, the whole project from start to finish, can be UK.’

Because much of the core generating equipment used is established and robust, Ellergreen has been using old generators dating as far back as the 1930s, which still achieve efficiencies in the mid-to-high 80 per cents. This keeps capital costs to a minimum, which is important given that many clients are farmers or landowners looking to diversify their income stream. ‘A lot of that kit is sitting around in dilapidated sheds, and I go and hunt it down and we refurbish it and automate it,’ said Cropper.Nevertheless the hydro industry hasn’t completely stood still. ‘What has changed enormously is the process controls, so now these schemes are fully automated and can be remotely accessed,’ he added.

‘You have a sensor at the top where your water comes in to your channel on a low-head or pipe on a high-head, and it can measure the volume of water available, and if it isn’t enough to cover your compensation flow you’ll be off, but then it will open up the values on the turbine from 10 per cent flow to 100 per cent depending on what’s available.’

There are still some considerable challenges ahead if the industry is to fulfil Cropper’s vision of more than 80GW of installed generating capacity. The fisheries lobby is currently engaged in an emotive anti-hydro campaign featuring dead fish, which Cropper says are not in connection with any UK scheme. Meanwhile, there are proposals afoot in England and Wales to halve the amount of water that hydro schemes are allowed to take. ‘That would literally stop us dead in our tracks, and we’re going to fight that,’ Cropper said.

Learningcurves

Flow duration graphs provide information on energy generation and ecological effects

Flow duration curves tell a prospective developer how much energy a watercourse will generate, its ecological impact and whether it will overwhelm equipment during a flood.

Data is gathered over a set period, be it months or decades. Flow rate in m3/sec is plotted in descending order with the highest flow rates on the left and progressively lower flow rates to the right. The X axis is percentage time exceedence; the Y axis is flow rate.

So a given percentage on the curve shows the flow rate equalled or exceeded for that percentage of time.

In the top graph of Cunsey Beck, Windermere, the flow rate is around 0.6m3/sec or more for 40 per cent of the year.

The average of all the flows can be calculated as Qmean; meanwhile, the flow rate exceeded for 95 per cent of the year (Q95) is often set by environmental regulators as the minimum that must flow before diversion for hydro.

The ratio of Q95:Qmean is a good indicator of how ‘flashy’ a watercourse is - that is how fast it rises and falls in response to rain. In the example curves, Cunsey Beck has a large catchment containing a lake (Esthwaite Water) and drains slowly while Black Beck (bottom graph) is down a hillside in Longsleddale and drains more quickly.

Source: The Engineer

Additional Information:

• To read more about Ellergreen 
• To read about the Case Studies