Response and conversion efficiency of a wave energy device consisting of double cylindrical floats with two degrees of freedom

Double cylindrical floats have been proposed as a device for extracting energy from waves by the carrier oscillating. The study on the oscillating property of the device can help to design the device and improve the conversion efficiency of the device. This paper simplifies the physics model of the two-body system as two degrees of freedom forced vibration system which has viscous damper. Based on the linear wave theory, wave excitation forces, added masses and damping coefficients can be derived by using the method of matched eigenfuntion expansions. The expressions of conversion efficiency and response of the device can be deduced from the equations of the carriers which have two degrees of freedom. The optimal principle is put forward on this basis. Numerical calculations indicate that damping optimal curve has a wave peak that is independent of the spring, and the value of the peak and the corresponding frequency are only related to the working conditions. Optimal curve presents two peaks when the spring exists, the corresponding frequency width between them becomes narrow while the elastic coefficient increases The damping is small at low frequency, but the response is relative high. The damping is relative high at the corresponding optimal peak frequency, but the response is relative small and nearly smaller than the amplitude of the incident wave.