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A DYNAMIC MODEL OF A BUOYANCY SYSTEM IN A WAVE
ENERGY POWER PLANT
Tom S. Pedersen and Kirsten M. Nielsen
Department of Automation and Control
Aalborg University, Fr. Bajersvej 7, Aalborg, Denmark
Keywords: Dynamic model, wave energy, simulation, buoyancy control, verification, renewable energy.
Abstract: A nonlinear dynamic model of the buoyancy system in a wave energy power plant is presented. The plant
(“Wave Dragon”) is a floating device using the potential energy in overtopping waves to produce power. A
water reservoir is placed on top of the WD, and hydro turbines lead the water to the sea producing electrical
power. Through air chambers it is possible to control the level, the trim and the heel of the WD. It is
important to control the level (and trim, heel) of the WD in order to maximize the power production in
proportion to the wave height, here the amount of overtopping water and the amount of potential energy is
conflicting. Five separate air chambers, all open to the sea, makes the device float. The pressures in the air
chambers may be individually controlled by an air fan through an array of valves. In order to make a model-
based control system, this paper presents a model describing the dynamics from the air inlet to the level,
trim and heel. The model is derived from first principles and is characterized by physical parameters.
Results from validation of the model against plant data are presented.
1 INTRODUCTION
Renewable energy is an important issue due to the
global warming problem and utilisation of wave
power is one of the energy resources to be exploited.
The wave power system “Wave Dragon”, on
which this paper focuses, was invented by Erik Friis
Madsen, Löwenmark and tested at Aalborg
University and University of Cork. An EU based
European consortium has been involved in the
construction and implementation of a 1:4 scaled test
site - 57x27 m wide and with a weight of 237
tonnes- which is placed in Nissum Bredning in
Denmark. Large numbers of tests have been carried
out during a two years operating period. One goal
for energy production improvement is a better
control of the Wave Dragon buoyancy.
Wave Dragon (WD) is an offshore wave energy
converter of the overtopping type, a description is
found in (Kofoed, 2006) and (W.D.Aps, 2006). The
main structure consists of a ramp where the waves
are overtopping and led to a reservoir (basin). Two
reflectors are focusing the waves towards the ramp
as seen on figure 1. WD is fastened to an anchor
making it possible to turn the ramp towards the
dominant wave direction.
Figure 1: Main components of the Wave Dragon (Kofoed,
2006).
The WD use the potential energy of the waves,
meaning that for a given wave type there exist an
optimal level of the reservoir. As shown on figure 2
the reservoir water is led through a turbine.
The WD floats on open air chambers used to
adjust the floating level. Control of the floating level
is a part of optimizing the overtopping and a
dynamic model for a model-based control system is
the topic of this paper. It should be noted that the
wave conditions are measured online and may be
used as reference to the level control system.
First the wave dragon buoyancy system is
presented. A dynamic model of the air supply system
controlling the pressure in the air chambers is set up.
13
S. Pedersen T. and M. Nielsen K. (2008).
A DYNAMIC MODEL OF A BUOYANCY SYSTEM IN A WAVE ENERGY POWER PLANT.
In Proceedings of the Fifth International Conference on Informatics in Control, Automation and Robotics - SPSMC, pages 13-17
DOI: 10.5220/0001479200130017
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