Archimedes Screw

Small hydro delivers local benefits

E&T looks how the UK is using classic ideas to tap vast amounts of energy from its modestly sized waterways.

Whatever the detractors may state, renewable energy is here to stay. Despite the lack of a coherent binding agreement at last year's climate change conference, countries and regions are still pushing ahead with their own targets for carbon reduction and the adoption of renewable energy. The focus of Europe's nations is on meeting the EU 2020 targets.

As part of this target the UK Renewable Energy Strategy, published in July 2009, envisages that by 2020 electricity from renewable sources may contribute 117TWh,or 30 per cent of the total expected electricity demand. This would be a major contribution towards the binding requirement on the UK to generate 15 per cent of all energy consumed in 2020 from renewable sources.

One surprising source that has the potential to contribute significantly to that figure is small-scale hydropower.

'This is one of the most cost-effective means of producing clean renewable electricity, generally with a higher efficiency, reliability, and capacity factor than solar, wind, and ocean energy (wave and tidal energy) technologies,' David Williams, president of the British Hydropower Association (BHA) says.

Hydropower accounts for nearly 1.5GW of total electricity generation capacity in the UK, from reservoirs and run-of-river schemes – pumped storage is not included since it is a net user of electrical power. That amount, however, is dwarfed by the figures in a new report from the BHA, which claims that small hydro in England and Wales alone could contribute up to 248MW of clean energy, over 15 times the current total.

How hydropower works

Hydropower can be captured wherever a flow of water falls from a higher level to a lower level. This may occur where a stream runs down a hillside, or a river passes over a waterfall or manmade weir, or where a reservoir discharges water back into the main river.

The vertical fall of the water, known as the head, is essential for hydropower generation; fast-flowing water on its own does not contain sufficient energy for useful power production except on a very large scale, such as offshore marine currents.

A standard small hydro scheme would take water from a river by diverting it through an intake at a weir. In medium or high-head installations water may first be carried horizontally to a settling tank or 'forebay' by a small canal or 'leat'.

In the forebay, the water is slowed sufficiently for suspended particles to settle out. The forebay is usually protected by a trash rack – metal bars that filter out water-borne debris. A pressure pipe, or 'penstock', conveys the water from the forebay to the turbine, which is enclosed in the powerhouse together with the generator and control equipment. After leaving the turbine, the water discharges down a 'tailrace' canal back into the river.

In practice, sites that are suitable for small-scale hydro schemes vary greatly. They include mountainous locations where there are fast-flowing mountain streams and lowland areas with wide rivers. In some cases, development would involve the refurbishment of historic water power sites. In others it would require entirely new constructions.

The Archimedes screw

Small hydro has typically employed either impulse turbines, such as Pelton, Turgo and Crossflow, or reactive turbines, such as the Kaplan or Francis, but there is a new penchant for using the Archimedean screw.

This throwback to grass-roots physics has traditionally been used like a pump, to convey water or other materials upwards. In this configuration a prime mover is required to drive the screw to pump water or convey other materials. The approach used here, however, sees the screw running in reverse, allowing water at a higher level to flow to a lower level, which serves to produce power, which can then be used to drive an electric generator to produce electricity.

'We started out looking for a solution to a very small hydro site with a very low head that was difficult to make work viably with a conventional turbine system,' explains David Mann, managing director of Mann Power Consulting, who brought the Archimedes screw to the UK five years ago. 'This was not only because the head was very low, but also because there were a lot of protected fish in the river that had to be kept out of the conventional turbine.

'We looked for solutions for these issues and finally came up with the Archimedes screw because that allows fish to go through the machine unharmed,' says Mann.

Essentially, the screw is a spiral scroll mounted about a central tube. It is set at an angle of usually 20° or 22° and the water comes in at the top. As the water fills the chamber formed by this spiral it acts to push on the spiral, which turns the screws as the water moves through it. The power comes from the rotation of the shaft and screw, which are connected to a gearbox to bring the screw up to the right speed to power a generator.

'The screws are not all the same size, but are custom built to suit each site,' explains Mann. 'The length of the screw will depend on the amount of head available. Because the machine is taking water right from the top to the bottom, the higher the head the longer the machine needs to be. Then the diameter of the machine determines the flow of water through the machine, as at any given site there may be more or less water. For each particular site you need to look at its characteristics and build a machine suitable for those.'

Tackling conservation concerns

It is a fact of life that setting any mechanical device to extract energy from rivers or waterways is sure to draw the critical eye of conservationists. However, extensive research indicates that the Archimedes screw sits comfortably with most parties.

'We carried out lots of tests when we first brought the machine over here to demonstrate to a very sceptical regulatory board and fishing community that these machines were fish friendly,' Mann says.

'We actually commissioned a biologist to run some tests where he deliberately introduced fish into the top of the machine and caught them at the end. At the end he would inspect them and photograph them to prove beyond doubt that there wasn't any damage caused as they passed through. We did that for over a thousand actual fish of different sizes and species.

'We filmed the passage of the fish with an underwater camera as it moved from the top to the bottom of the screw and in actual fact they are swimming around in these chambers as if they were in the river.'

Comparative efficiency

There have been questions raised about the efficiency of the Archimedes screw when compared to devices such as the Kaplan turbine, and Mann understands the concerns.

'If you are looking at pure technical efficiency of the turbine itself then ours is not as good, but if you look at the efficiency of the overall capture of electricity from a weir site then it is very comparable,' he says.

'When you take into account the need to move the water through these screens into the system you actually end up losing some of the head as it moves through the screening system and the pressure pipe that move into the Kaplan system. So you end up operating with a lower head for any particular given site, and there is effectively a loss of efficiency.'

There is certainly huge potential in the UK for small hydro schemes. It is estimated that there are around 20,000 old mill sites – more than enough to keep small hydro companies busy far into the future.

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