Other proposals

4.2.1 BioPower Systems

Few details and no published data are available on this device, but the tidal energy conversion system, bioSTREAM™ (Figure 4.21 (left)) is apparently ‘based on the highly efficient propulsion of Thunniform-mode swimming species, such as shark, tuna, and mackerel’ (BioPower Systems, 2008). The motions, mechanisms, and cau­dal fin hydrofoil shapes of such species are said ‘to be up to 90 % efficient at converting body energy into propulsive force’. The bioSTREAM™ mimics the shape and motion characteristics of these species but is a fixed device in a moving stream. In this configuration, the propulsion mechanism is reversed and the energy in the passing flow is used to drive the device motion against the resisting torque of the O-DRIVE™ electrical generator. Owing to the single point of rotation, this device can align with the flow in any direction, and can assume a streamlined configura­tion to avoid excess loading in extreme conditions. Finally, bioSTREAM™’s website claims that systems are being developed for 500 kW, 1000 kW, and 2000 kW capac­ities to match conditions in various locations. This may prove difficult because, if we use the potential hydraulic power equation, then the device would have to be a massive 65 m high with a 20 m sweep to achieve the last named output in a stream of 2 m s-1 at 40 % efficiency. There are few if any sites of such depth with such currents. Another problem stems from the physics of the device. Shapes (such as fishtails) that may be very efficient at converting oscillating motions into propulsive forces because of the added mass generated in the wake and against which the fin reacts are necessarily inefficient when operating in the opposite sense because the added mass is simply a drain on the system.