Page 60: of Offshore Engineer Magazine (Sep/Oct 2015)

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EPIC

Making waves pressure drop across 95% of the rotor

Assumptions around tidal arrays need to be rethought, diameter, decreasing turbulence and according to Professor Stephen Salter. Emma Gordon reports. improving the performance of successive rows of blades, he adds.

he renewables industry needs the accepted assumptions for restricted “What I want to do is have the wake to contest the assumption that ? ow channel turbine design for areas much cleaner than what came in, and a

T turbines operating in arrays in such as the Pentland Firth. pressure drop across rotors that is even,” tidal streams should behave like under- He says the equation for hydrau- Salter says. “You do this by having a water wind turbines, a leading expert in lic machines in a closed duct, which vertical-axis rotor with blades at varying the ? eld says. calculates power based on the head and pitch, with two contra-rotating rotors.

Stephen Salter, emeritus professor of ? ow rate, should be used when design- These pitch angles deliver nearly con- engineering design at the University of ing turbines in large, close-packed tidal stant pressure.”

Edinburgh, is often credited with being stream arrays. And, to pinpoint the size Delegates also heard the vertical-axis the founding father of the UK’s wave and of the resource, Salter adds that the head, design allows for better blade mount- tidal industry, having pioneered the devel- which would increase as more turbines ing, with rotors supported at both ends: opment of wave energy in the 1970s. were installed, needs to be accurately reducing pressure differences across the

Speaking at the All Energy Conference measured. blades. and Exhibition in Glasgow, UK, early Salter says it is important to ? nd out Salter adds that supporting the beam

May, Salter says if tidal energy is to real- how much energy is currently being (or blade) at both ends reduces the ize the full potential from restricted ? ow wasted; adding that, if this dissipation bending moment by a factor of four, channels like Scotland’s Pentland Firth, is high, installing more turbines will compared with a cantilever blade seen different assumptions need to be made, not substantially reduce ? ow capacity. on traditional three-blade horizontal axis which would imply different turbine The of? cial estimate, which uses a bed turbines, which are only supported at designs should be assessed. friction coef? cient appropriate for ? ghter one end. Salter’s design also reduces the

Salter has been working on an alterna- aircraft and does not take certain fac- vortexes in the water created by tradi- tive design to the traditional three-blade tors, such as channel obstructions, into tional blades, which lead to energy loss horizontal axis turbine design, a vertical- account, is conservative, Salter says. and turbulence. axis, variable pitch rotor design, com- The effectiveness of this approach has Salter also says contra-rotating rotors, prising multiple vertical, pitch-variable been con? rmed in tests carried out by using pitch control blades, mean the blades supported on horizontal rings, McAdam at Oxford, Salter says. McAdam turbines could also be installed close which would occupy a large cross section measured double the Betz limit with a together, canceling out each other’s cross- of a restricted ? ow channel. “sweepage” fraction of 0.59. force. In addition, due to the number of

As part of an array of 200m diameter, Salter’s vertical-axis, variable pitch blades on an individual machine, having 50m deep versions of these machines, rotor design increases the resistance for some blades fail would not impact per- in a restricted ? ow channel such as the the water to ? ow through its openings. formance, adding redundancy.

Pentland Firth, Salter says each pair Because the water has to exit through a To ensure suf? cient stability in pitch of devices could generate a staggering smaller ? ow area, it will go faster, giving and roll, the (horizontally orientated) 400MW. performance well above the Betz limit: diameter of the rotor needs to be around

The ? gure is based on research by Ross the maximum coef? cient of power for three times the water depth. Many areas

McAdam at the University of Oxford, wind turbines, Salter says. of the Pentland Firth have depths of which has reinforced Salter’s questioning Further, the design achieves an even around 70m.

September 2015 | OE oedigital.com 62 062-OE0915_EPIC3_Salter.indd 62 8/19/15 10:48 PM