A twin-screw hydroelectric generator on the River Wharfe at Otley

Hydroelectric generators tap the backwaters for energy

Hydroelectric power is moving away from giant dams to generators that work on a more human scale.

For a country that receives so much rain, hydroelectric energy seems to be curiously underexploited in the UK. Just 1.5 per cent of the electricity supply is derived from water. For traditional hydropower, though, Britain has the wrong kind of water. Norway’s large array of fjords provides the country with most of its electricity needs. The gentler, smaller rivers and streams that characterise the UK’s landscape provide a poor fit for hydro. Typical hydro turbines require speeds of at least five knots to make them financially viable: most rivers flow at less than two knots. And damming rivers in the UK is an unpopular choice.

Big hydro projects using dams and tidal barriers can wreak havoc with ecosystems, causing extensive flooding, soil erosion and deforestation. Does this leave hydro up the creek? Several projects have found ways to extract the latent energy from rivers, with some based on very old ideas.

David Wilson Homes has installed a pair of hydroelectric generators in a weir on the River Wharfe next to a new 300-home estate in Otley, West Yorkshire. The 10m-long generators use the classic screw design developed by Greek inventor Archimedes - turning as the water descends through them. The builder says the screw generator will provide up to 1300MWh of energy per year.

Other techniques are based on principles discovered more recently. The Venturi-Enhanced Turbine Technology (VETT) developed by UK-based VerdErg is designed to amplify the small drops in level at fast-flowing points in a river. It does this by exploiting the principles of the Venturi effect - that a fluid forced through a constriction speeds up, causing a drop in pressure. Attached to sections of water where there is an existing low ‘head’ - or drop - such as weirs or locks, the VETT sends 80 per cent of the available water through a Venturi pump. The low-pressure environment inside the constriction is then connected with water in the remaining 20 per cent high-pressure regime to drive a turbine. The combination amplifies the effect of the existing head by two-and-a-half to three times.

VerdErg operations director Lars Boerner says the increased pressure differential created by the Venturi pump allows for more cost-effective turbines: “You can use a much smaller turbine because it sees only 20 per cent of the water. But it sees a higher pressure, which means it spins faster. It doesn’t require a gear box and it needs less civil infrastructure, so the cost comes down.”

As only 20 per cent of the water goes through a turbine, the installation of the VerdEng system should have far less effect on wildlife than the building of a dam.

VerdErg is currently aiming at the small-scale end of the generation market. A typical scheme could provide energy for tens or hundreds of households, depending on the water source available. The company has had interest from private landowners, small businesses, community energy schemes and trusts. With permits needed for each scheme, the planning process could be drawn-out but, on the upside, sources of low-head hydro are numerous. The Environment Agency has identified over 20,000 sites across England and Wales - and they are very accessible to communities and power grid locations. “It’s a very important source, a very accessible source and a very predictable source of energy,” Boerner claims.

With the vast majority of the UK’s waterways being slow-moving and flat, new technologies are seeking to exploit even this previously untapped source. One is from US firm Vortex Hydro Energy, which has developed a system to capture the energy from vortices formed in flowing water - the same energy used by fish to swim upstream.

So-called vortex-induced vibrations (VIVs) have been known of for at least 500 years. Leonardo Da Vinci observed them in the form of Aeolian tones created by wind passing across the strings of an instrument. They are responsible for destructive phenomena such as the violent shaking of bridges in heavy winds. In water they are caused when a current flows around so-called bluff bodies such as cylinders or spheres. VIVs occur when vortices form on the downstream side of the object. These vortices shed on alternate sides to create an oscillating motion. Fish exploit this phenomenon to propel them through water. 

This phenomenon interested Michael Bernitsas, a professor of marine engineering at the University of Michigan. He was intrigued that such an obvious source of energy had never been harnessed before. So in 2005 he founded Vortex Hydro Energy, a commercial venture for the technology he patented, the vortex-induced vibration for aquatic clean energy (Vivace). The Vivace consists of a three-dimensional array of horizontal cylinders that use VIVs to oscillate in a current, generating electricity by moving magnets up and down metal coils attached to their ends. The device consists of four 4m-long cylinders producing 4kW of power - enough to supply all the energy needs for two to four typical homes.

Overall, the river-scale Vivace is about the size of a room and can generate 4kW from a current of 3.2 knots. In the lab, however, Bernitsas has managed to generate energy from speeds as low as 0.8 knots. Because Vivace sits on the river bed, there are no ugly structures popping up above the surface. There is even evidence from a joint study by Harvard University and the Massachusetts Institute of Technology (MIT) that fish seem to like the device. “Fish go behind cylinders and enjoy the ride in their wake,” says Bernitsas. “They ride between vortices just as when they trail another fish in a school.”

Another device seeking to obtain energy from slow flows in flat stretches of water is a turbine system from a UK start-up, Lunagen, which claims to produce usable amounts of energy from flows as low as 1 knot. Lunagen uses a vertical-axis Savonius turbine. The design operates at high torque and so generates more energy from slower speeds. The Lunagen team has optimised the turbine to make it up to 30 per cent more efficient for river-based generation.

Lunagen’s design deploys the turbines across the entire stream, creating a blockage effect so that all of the water has to pass through the device. This, according to Lunagen founder Will Penfold, gives the turbines a tenfold increase in efficiency. A simple 2kW array consisting of two turbines could provide the energy needs of a typical UK household and units can be daisy-chained together to scale the system up. “We can gather useful levels of energy at a good levelised cost of electricity and we open up a vast resource which has previously been uneconomic,” Penfold claims.

Lunagen is looking for investors. Although the technology is basically operational, the team has yet to conduct fish trials, which will take place later this year. Despite using turbines to block the river, Penfold says the design of the turbines plus the ability to space them at different intervals means they can allow all sizes of wildlife, even otters, to pass through unharmed. He isn’t so confident, however, of securing the backing of important stakeholders such as river preservation societies and fishing groups, who could see blocking rivers with arrays of turbines as a step too far.

The most promising market for Lunagen therefore might come from countries like India and Australia, where huge networks of irrigation channels offer a vast resource of slow-moving hydro energy. “We’ve spoken with people who run medical centres slap bang next to irrigation channels and they’re running off diesel generators,” says Penfold. “If we can take 80 per cent of the load off that diesel generator, that’s a massive benefit.”

If streams and irrigation channels aren’t slow enough, how about water that isn’t moving at all? As far back as 1852 Lord Kelvin wrote about the possibility of obtaining heat energy by cooling a fluid.  It was the same paper in which he suggested refrigeration, so it is perhaps fitting that an industrial refrigeration company is now at the cutting edge of manufacturing Kelvin’s so-called ‘heat engines’. Star Renewable Energy, an offshoot of Scottish company Star Refrigeration, makes water-source heat pumps using what is essentially the reverse of the refrigeration process to produce heat energy from natural sources of water.

Star Renewable Energy’s first large-scale project was to provide the district heating for an entire city. In 2009 Norwegian city Drammen began looking for a source of heating that was efficient and environmentally friendly. It settled on water-source heat pumps and Star Renewable Energy instantly stood out as it was the only company not using hydrofluorocarbons, which are greenhouse gases,  as the coolant. Instead Star Renewable proposed using ammonia, a chemical that was not only greenhouse-neutral but also proved to be 20-25 per cent more efficient than expected.

Star Renewable Energy won the contract and now provides the city’s 65,000 inhabitants with 85 per cent of their annual heating, saving 80 per cent of the district heating bill. It does this by taking the water from the nearby fjord, at 8°C, and using it to heat pressurised ammonia to 2°C, at which point it evaporates. The ammonia gas is compressed further until it reaches 110°C and used to heat the water in the city’s heating system to 90°C.

“We are taking heat out of something relatively cold,” says Star Renewable Energy’s director, Dave Pearson, “but the concept is, so long as you can make it cooler, you are taking heat away that can be used.”

Later this year Star Renewable Energy will start a similar scheme to provide district heating for the Shetland Isles. Other parts of the UK, from Inverness and Dundee to London and Bristol, have shown an interest. The potential for water-source heat pumps is clear - the Thames alone, according to Pearson, could provide the heating for half a million homes - and the UK government has produced a water source heat map indicating potential sites for such pumps. It has set a target of 4.5 million heat pumps across the country, which could help to reduce household bills by 20 per cent.

The biggest benefit, says Pearson, comes from combining water-source heat pumps with electricity from renewable sources. “There are increased cases of wind farms not running at full capacity because there’s not enough demand for electricity at the right times,” says Pearson. “When the wind blows is when you need the most heat so rather than paying wind farms not to run, we should be grabbing the electricity from them and using it for heat pumps.”

With so much untapped energy in Britain’s river systems, it perhaps raises the question of why it isn’t already being used. For Pearson it is that water-source heat pumps, like hydro, suffer from a reputation problem. “There’s a mistrust,” he says, “which means when people have heard of them, they don’t think they work.” 

Similarly, VerdErg’s Boerner believes that existing low-head hydro techniques are too small-scale to provide the kind of quick energy fix the government seeks. “Energy production for 20, 30 or 40 households is a significant number,” says Boerner. “But for government level it’s not going to have the big impact of an offshore wind farm that can probably cover 1,000 houses.”

However, as Boerner is at pains to point out, there is no magical quick fix to the world’s energy challenges, which means small-scale hydro could play an important part in a jigsaw of future solutions. “It’s vital in the long term to develop these small projects,” says Boerner. “With hydro it does all add up and that does help contribute to securing your energy supply as well as reducing emissions.” *

Three-way power

Vortex Hydro Energy’s Vivace harnesses the power of slow-flowing water by combining three natural phenomena: vortex shedding, galloping and flow-induced motion.

For vortex shedding, cylinders placed in a current shed vortices on their downstream side in an alternating fashion. When these vortices shed they push the cylinder away, causing it to oscillate from side to side in vortex-induced vibrations (VIVs). Fish use the same phenomenon by curving then straightening their bodies, shedding vortices on alternate sides to propel them through water.

Galloping is a geometric instability phenomenon occurring at higher speeds, seen in the bouncing motion of suspension bridges in high winds. It causes the cylinder to vibrate at even greater amplitudes. Vivace is designed so that, as flow speed increases and before VIV ends, galloping starts, allowing the device to operate effectively over a wide range of speeds.

The final effect, flow-induced motion, is seen as fish swim in schools. They organise themselves in specific three-dimensional arrangements to maximise the effect of vortex shedding. This provides a boost from the fish ahead, in the same way as a racing driver uses the slipstream of the car in front to reduce drag. The cylinders in the Vivace are positioned to take advantage of the same flow-induced motion.

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