Generating energy from the tide, or other marine currents, has the potential to be a significant part of the energy mix, particularly in Scotland. Conventional tidal turbines are attached to towers sunk into the seabed, much like wind turbines. But sinking steel towers into the seabed is expensive and only viable in shallow waters, while much of the available tidal energy is in deep water channels such as the stretch between Ireland and the Mull of Kintyre.
In response to these drawbacks, a team from Strathclyde University’s Energy Systems Research Unit (ESRU) has designed a novel marine current turbine that does not need a solid support structure.
The new design is called a contra-rotating turbine, because it has two sets of rotor blades, one in front of the other, turning in opposite directions on a single axis. The contra-rotating design was first tested for wind turbines in the 1980s, but was never fully developed. However, in the much denser fluid environment of water, the advantages of the design are more important.
Contra-rotating blades minimise to almost nothing the twisting forces acting on the support structure as the rotors go round. “Having two rotors means the twisting forces cancel each other out,” explains Andrew Grant, a member of the ESRU team. “This means we don’t need a support tower at all. We think we can use a flexible mooring that is very cheap and very simple, and can be used in deep water.”
Having two rotors on one turbine also reduces the vortex, or “wake”, which forms behind a single rotating turbine. “For marine turbines, this wake persists for long distances,” says Grant, “which is a problem if you want to put several rows of turbines in a channel. We are pretty sure that the wake behind a contra-rotating turbine is different in nature and disperses more quickly, which could be a major advantage.”
The team at ESRU built their first model contra-rotating turbine three years ago, designed using computer simulation to calculate the most efficient shape and angles for the blades. They have since built a prototype, with 2.5 m diameter rotor blades, and a second smaller prototype with a generator and mooring attached. Their design has three blades on the upstream rotor, and four blades on the downstream rotor.
Both prototypes have gone through initial testing in the sea off south west Scotland, and performed as predicted in simulations. Now the team is looking for funding to test how the contra-rotating turbine performs when it is deployed longer term. “We are developing the second generation of tidal energy machines here,” says Grant, “anticipating problems that will come up perhaps ten years from now.” The ESRU has patented the design and is considering setting up a spin-out company to develop it commercially.
The scientists presented their design at the World Renewable Energy Congress in Glasgow, in July.