The Manchester Bobber

Technical Description

The major components and the principle of operation are shown in Fig.1

The major components and the principle of operation are shown in Fig.1 where, for clarity, the components are not drawn to the same relative scale. A floating mass rises and falls under the action of waves in the water and this causes a pulley and its shaft to oscillate. A counterweight controls tension in the suspending medium over the pulley and the pulley shaft is connected to an output shaft through a freewheel clutch. As the float descends, the pulley speed attempts to exceed the output speed causing the clutch to engage and accelerating the entire shaft system. At maximum speed the clutch disengages, allowing the output shaft to continue its forward rotation whilst the pulley decelerates and reverses during ascent of the float. Whilst the clutch is disengaged, the output shaft continues to rotate due to the inertia of a flywheel but decelerates due to energy extraction (i.e. the power output). A gearbox is used to increase the output shaft speed hence reducing the size of flywheel and generator required to produce a given output power. The generator will be a conventional robust induction machine operating at varying speeds and interfaced to the grid through well developed solid-state power conditioning equipment.

The major innovative and attractive features of the system are:

• Power output can be constant over a wave cycle as a result of the energy storing properties of the simple, low-technology flywheel. No other cost effective and reliable alternative exists for smoothing the high power outputs that occur due to irregular wave loading.
• An array of floats will be disposed on a common platform. Each float will have an independent power take-off hence, whilst one drive is being serviced the remaining floats in the array will continue to supply electricity to the grid.
• Safe mode is achieved in extreme conditions (>10m significant wave height) via the ability to rapidly flood each float to achieve total submersion and allow the float to rest safely on the sea bed.
• Only the passive and inert float comes into contact with the water. All vulnerable mechanical and electrical components are housed in a protected environment above the crest of storm waves.
• All mechanical and electrical components are available from well- established suppliers and require minimal development.
• High reliability is expected due to the track-record of component developers and the above-sea location of the drivetrain. In particular, unlike many wave schemes, the design does not incorporate novel components and does not require immersion of critical components.
• Device will respond to waves from any direction with minimal adjustment. However, net output will change depending on wave direction and depending on the spread of the wave field.
• Maintenance and/or repair is greatly eased by the accessibility of components.

(a) Resonance can be used successfully to enhance the heaving motion. The float will have a natural resonant bobbing (heaving) frequency which will be designed, through choice of mass and shape, to suit the prevailing wave climate and so improve the energy capture characteristics. A patented design enables efficient adjustment of individual float response and allows operation to continue during relatively severe wave conditions.

(b) A computer simulation has been developed which gives very good agreement with observed behaviour when the system is delivering an output power. The simulation has been verified by tests on 1/100th and 1/10th scale prototypes indicating that the hydrodynamics and mechanical dynamics are understood and can be satisfactorily modelled.

(c) A method of guidance of the float has been devised that maintains stable vertical oscillation of the float without rubbing or rolling contacts. This is a low cost system which also reduces structural loading.

(d) Optimum design of float shape and system masses is being refined with funding in place.

(e) Power capability is proportional to approximately the cube of the float dimension but a limit is imposed by typical wave characteristics. A full scale platform will support an array of 25 floats and their power-take-off trains, each with a rating of 500kW so that the platform will act as an offshore power station with a total rating of over 12.5MW. Modelling predicts that the annual average from such a platform will be greater than about 5MW.