Issue 10

Tidal Stream Energy - The Role of Efficiency

A synopsis of a 2011 Open Day talk given by Dr Richard Willden

The talk started with a brief summary of the potential for tidal power in the UK: EU Commission figures from the 1990s estimate that it could provide 5% (18 TWh/yr) of the UK's requirement, although Dr Willden believes it could be closer to 10% - the EU figures are quite old. Tidal power is therefore of considerable interest to the UK in particular.

Although they might at first sight appear similar, tidal power is not quite like wind power: the environment is harsher, with heavier loadings, and of course tidal power is predictable - the time, volume and direction of flow are known in advance - which enables different strategies to be employed.

There is a balance to be found between power coefficient (ratio of power obtained to power available in the stream) and efficiency when extracting energy from tidal flow: developers want maximum power coefficient, while regulators want maximally efficient use of the water resource. Predictably, these two goals cannot be met simultaneously!

The speaker's speciality is device modelling, using numerical modelling and simulation tools: as well as discussing the performance of various different device types, he also laid to rest some basic misconceptions, chief among which is that no device can out-perform simple momentum theory (which sounds obvious, but apparently some people claim otherwise).

During a wide-ranging overview of the field, four different types of turbine were discussed in particular:

1. Simple axial-flow rotors (the tidal equivalent of conventional wind turbines): these come quite close to "disc" theory (86% at peak), but perform more badly on efficiency.
2. Ducted turbines have converge/diverge ducting to increase flow speed, which gives more power (power is proportional to the cube of flow speed), but - despite some extravagant manufacturer claims that this gives you something for nothing - water spills around the outside of the ducting, so not as much water is processed as might be expected.
3. Open-centre turbines use a jet to reduce pressure behind the turbine and suck more flow through, but the overall effect is poor because the hole in the middle of the rotor reduces its area.
4. Cross-flow turbines (i.e. where the turbine axis is perpendicular to the stream: see Ross McAdam's article in SOUE News issue 7) are able to block the entire channel better than circular axial-flow devices, but generate vortices which reduce efficiency, and also suffer badly from viscous effects, blade lift and drag - the blades only spend part of their cycle time "working".

Common problems of tidal stream turbines were also discussed:

• Fatigue poses a problem for tidal turbines, especially cross-flow, due to torque variations around the cycle (for cross-flow, when the blades are at the back of the rotor; for axial-flow, when the blade passes the tower holding the turbine in place).
• Flow stream through a turbine needs to expand downstream, which causes particular problems in confined flows.
• Extracting energy from a stream reduces its free surface height (so it is potential, not kinetic, energy that is extracted); this has environmental impact downstream, a problem that turbines share with tidal barrage schemes.

Comparing the various types, Dr Willden observed that axial-flow rotors are very hard to beat (while engaging in some good-natured banter with the Head of Department, who is currently involved in an attempt to develop a commercial cross-flow turbine!)

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