Pneumatic Wave-Power Conversion By A Phase-Controlled Buoy

The design of a pneumatic wave-power buoy is described. The hull of the buoy is spherically shaped with an opening to the sea in its lower end. To obtain a fixed reference, a strut is passing through the buoy to a universal joint on the sea bed. The heave motion of the buoy may be phase controlled by clamping the hull to the strut during one or two intervals of each wave period. The water column enclosed by the buoy acts as a piston forcing air through a turbine . The air flow is rectified by non-return valves. The buoy can produce electrical energy when it is fixed, when it is freely floating, or when it is phase controlled. Even in the most severe wave conditions the power production is maintained . Model experiments with the freely floating buoy in rough seas have demonstrated that the vertical anchoring forces in extreme waves are smaller than the largest anchoring forces which may occur when the buoy is phase controlled in waves of modest height. From these experiments, locations with water depths of 80 m were found to require prohibitively expensive mooring struts. A water depth of 40 m was then chosen . A time-domain mathematical model of the pneumatic phase controlled buoy is presented which uses a time-series of the wave elevation as input to calculate the motion of the buoy and its power production . A computer program for numerical simulation of the power buoy is prepared. The hydrodynamical performance of the buoy is described by an approximat i on to applied- pressure parameters computed from known values of the radiation impedance matrix and of the excitation forces of two concentric sections of a rigid, heaving hemisphere. It has been shown that the phase angles of the excitation forces of the two sections are equal and , - ·1 I I identical to the phase angle of the excitation force of the hemisphere. IV The impulse response functions used when computing the excitation quantities of the buoy for a given time-series of wave elevation, are found to be non-causal. In order to compute the excitation force and the excitation volume flux with a reasonable degree of accuracy, information is req uired on the future surface elevation for a time, which is one seventh, respectively one twelvth of the time for which information on the past is required. From numerical simulations the turbine shaft power is computed for one single 10 m diameter buoy operated in regular waves of 2 m height and period 5-14 s. The results clearly show that for the pneumatic buoy, phase control is essential in order to absorb any significant amount of power from waves of modest heights. When the buoy is freely floating, the turbine shaft power is 2 kW or lower for all wave periods. In the same waves the fixed buoy delivers 4-19 kW on the turbine shaft. If the buoy is phase controlled with latching in both upper and lower position, the turbine shaft power exceeds 110 kW for wave periods in the range 6-12 s. A peak production of about 150 kW is obtained for wave periods of 7-10 s. For longer periods the power production falls off due to the limited energy storing capacity of the buoy . If the buoy is phase controlled with latching in the lower position only, more than 55 kW is produced for wave periods of 6-12 s. A peak production of 98 kW is obtained for wave periods of 7-8 s. The power production of a buoy of 12 m diameter was computed whose linear dimensions are obtained by scaling those · of the 10 m buoy. The turbine used in the 12 m buoy was, however, identical to that of the 10 m buoy. Compared to the 10 m buoy, the generated power was increased for l onge r wave periods due to increased energy storing capacity. The heave amplitude of the 12 m buoy was, howe v er, for all periods smaller than the amplitude of the 10 m buoy. This fact indicates that the turbine used in the 10 m buoy do not provide optimum damping of the 12 m buoy. I ' In irregular waves, with the energy transport equal to that of a regular wave of 2 m height and of 10 s period, the power production of the buoy is reduced by 32-45 % compared to the power production in the corresponding regular wave. The lower figure pertain to a JONSWAP sea and the higher to a Pierson-Moscowitz sea. The reduction is partly due to the transient motion of the buoy in irregular waves, and partly due to the simple strategy of unlatching used when phasecontrolling the buoy in the simulator. The smoothing effect of the air chambers on the air V flow through the turbine is substantial. The air flow through the non-return valves which commuriicate with the outer atmosphere is much smaller than the flow through the valves communicating with the pump chamber. Hence, the area of the former valves may be reduced compared to the area of the latter. With no heat transfer included in the model, the temperature of the high pressure chamber always exceeds the temperature of the low pressure chamber. Hence, there will be a continuous heat loss between these chambers unles s the walls between them is thermally insulated