Research Papers: Ocean Renewable Energy

Latching Control of an Oscillating Water Column Spar-Buoy Wave Energy Converter in Regular Waves

[+] Author and Article Information
A. F. O. Falcão

e-mail: antonio.falcao@ist.utl.pt

L. M. C. Gato

Instituto Superior Técnico,
Technical University of Lisbon,
Av. Rovisco Pais,
1049-001 Lisbon, Portugal

1Corresponding author.

Contributed by the Ocean Offshore and Arctic Engineering Division of ASME for publication in the Journal of Offshore Mechanics and Arctic Engineering. Manuscript received March 20, 2012; final manuscript received August 23, 2012; published online February 25, 2013. Assoc. Editor: Daniel T. Valentine.

J. Offshore Mech. Arct. Eng 135(2), 021902 (Feb 25, 2013) (8 pages) Paper No: OMAE-12-1023; doi: 10.1115/1.4007595 History: Received March 20, 2012; Revised August 23, 2012

The present paper concerns an oscillating water column (OWC) spar-buoy, possibly the simplest concept for a floating OWC wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends and fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control through the latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular, such compressibility may remove the constraint of the latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modeling is performed in the time domain for a given device geometry and includes the numerical optimization of the air turbine rotational speed, chamber volume, and latching parameters. Results are obtained for regular waves.

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Fig. 1

Cross-section view of the axisymmetic floating OWC

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Fig. 6

Time series, without (a) with (b) latching control, of the mass flow rate of air m·, diffraction force on the buoy Fd1, buoy velocity x·1, and dimensionless relative chamber pressure p/patm, for regular wave period T=8 s and height H=2 m

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Fig. 5

Optimal air chamber height (a) and optimal turbine rotational speed (b), as function of the wave frequency, for a spar-buoy equipped with a biradial turbine without latching and a biradial turbine with latching for wave height H=2 m

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Fig. 4

Comparison of the dimensionless capture width versus wave frequency for a spar-buoy equipped with a biradial turbine with and without latching control, for wave height H=2 m. Also plotted is the theoretical maximum hydrodynamic capture width, λ/2πd, and frequency domain results for an optimized perfectly efficient linear turbine.

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Fig. 3

Efficiency η and dimensionless mass flow rate Φ as a function of the dimensionless pressure head Ψ of the biradial turbine, obtained from CFD simulations

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Fig. 2

Example of the convolution integral limits in the case of a 4th-order polynomial approximation of Fc(τ). The function Fc(τ) is computed at five points τ0,…,τ4 and divided in two parts: Fc+(τm) and Fc-(τm).




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