Evopod is a unique tidal energy device being developed by a UK-based company Oceanflow Energy Ltd for generating electricity from tidal streams and ocean currents. It can operate in exposed deep water sites where severe wind and waves also make up the environment.
Floating tethered turbines
Advantages
The flow speed in tidal streams and ocean currents tends to be fastest near the surface and falls off in speed as one descend in the water column. As the power that can be extracted from the free flowing water is proportional to the velocity cubed, then a 10% increase in flow speed equates to a 33% increase in power per unit swept area of the turbine. The flow is generally more consistent in the top 1/3 of the water column as it is well away from disturbances created by the seabed topography. The drag force on turbines of the same power output is proportionally less for a turbine in faster flow (positioned in the upper part of the water column) than a turbine in slower flow (positioned in the lower part of the water column). A floating device does not require a flat seabed as the pile anchors require relatively little space and there is no structure on the seabed. Turbines supported by floating platforms are more readily accessible for maintenance than those on the seabed. Maintaining watertight seals is less problematic for devices positioned higher in the water column as they are not subject to such extreme static pressures. Floating devices that are fitted with navigation lights and markings are more readily identifiable under international navigation regulations than unmarked submerged turbines.
Disadvantages
Floating devices are subject to ocean wave action which can induce motions that will impact on the performance of the turbines they support; semi-submerged devices such as Evopod are designed to be a stable platform in waves so that they can operate for longer and extract more energy from the wave particle velocity. Waves large enough to have an adverse effect on Evopod would affect turbines in all parts of the water column. Ocean waves create an orbital movement of the water particles which will add or subtract from the steady ocean current or tidal stream velocity as the wave passes the turbine. Without proper blade pitch or power take-off control systems this could lead to the blade stalling and loss of power output. With proper control systems it is possible to extract this kinetic energy from the waves, much as a wind turbine does in response to wind gusts. The wave particle velocity for short wave reduces with water depth and is therefore less of an issue for deeply submerged turbines. As the wavelength gets longer than it becomes a shallow water wave where there is little change in velocity in the water column. The vertical component of the mooring load induced by the drag of the turbine can pull a floating platform under the water unless it is adequately compensated for by change in buoyancy forces, e.g. the submergence of the struts on Evopod, or hydrodynamic lift forces (lifting foils). Tests have shown residual buoyancy in Evopods is sufficient to withstand these forces with the extra bonus of improve system stability. Floating devices have to be robust enough to withstand impact from flotsam and in Northern latitudes may need to be designed to cope with ice floes. This is however true for all turbines as flotsam may be fully submerged and therefore impact any seabed turbine.
Design features
Hull design and rotating midwater buoy
The device differentiates itself from other tidal turbines in that the turbine is mounted on a floating, semi-submerged body that is tethered to the seabed. The power generation equipment is similar to that of a wind turbine and is housed in the cylindrical shaped watertight lower hull, which is deeply submerged below the water line and supported by small waterplane area surface piercing struts.
One variant of this patented hull concept has three vertical struts that pierce the water surface, much like a multi-hull SWATH design. The two transversely separated aft struts provide the stability that is needed to resist the torque reacted by the single turbine/generator unit. The configuration of the struts also ensure that the device weathervanes about its midwater mooring buoy such that it always points into the direction of the current.