Maximizing the Efficiency of Offshore Wave Energy Converters: A Vital but Challenging Task

Although approximately 4 MW of wave energy have been installed worldwide, questions of how to maximize converter efficiency still exist (Bedard et al. 2010). Igic et al. (2011) explored this question by investigating how the overall performance of the Wave Dragon (WD) wave energy converter changed based on different control strategies and electrical system configurations. The authors modeled these dependencies using a computer simulation of one turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–>-generator connected to an AC/DC/AC converter and an infinite grid.  Results for torque, DC link voltage, power, speed, output voltage, and current were presented in relation to the height of the turbine head. Using a permanent magnet generator (PMG), Igic et al. found that the line to line voltage of their simulation was 690 V and the maximum current value was 50 A. In addition, their simulation was deemed appropriate for use in prospective studies of wave energy power take-off systems. Even though further research is still needed, the authors’ case study and simulation results should be considered in the design of future offshore wave energy converters. —Juliet Archer
Igic, P., Zhou, Z., Knapp, W., MacEnri, J., Sørensen, H., Friis-Madsen, E., 2011. Multi-megawatt offshore wave energy converters – electrical system configuration and generator control strategy. Renewable Power Generation, IET 5, 10–17.

Igic and colleagues examined various electrical system configurations and control strategies of the WD offshore wave energy converter in regards to overall system performance. First, Igic et al. presented a case study which thoroughly described the potential electrical systems, grid connections, and generators that could be used with the WD. Next, they described models and presented equations representing PMG and frequency converter control. In these models and equations, both generator and grid side control mechanisms were considered. Lastly, the team built a MATLAB simulation model by utilizing the “power system tool box” in order to investigate how the overall system performance was impacted by generator characteristics and control strategies. The authors made some minor assumptions while constructing their model. For example, the influence of converter harmonics and torque fluctuation was ignored. In addition, the AC/DC and DC/AC converters were both portrayed as voltage-controlled voltage sources (VCVS). Furthermore, the authors chose various parameters, such as stator resistance, inductance, and flux<!–[if supportFields]> XE “flux” <![endif]–><!–[if supportFields]><![endif]–> induced by magnet, for the PMG simulation. Finally, to focus solely on performance of the system control, Igic et al. assumed that the turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> was connected to an infinite utility grid.
The simulation results of this paper pertain specifically to the WD wave energy converter. The WD is a floating barrage that creates electrical energy from wave power<!–[if supportFields]> XE “wave power” <![endif]–><!–[if supportFields]><![endif]–>. It is composed of three parts which are analogous to a human’s mouth, arms and stomach, which work together in the process of deriving energy from food.  The main part of the WD is a large floating reservoir that faces incoming waves. This part is analogous to the mouth because it is where the waves enter the system via a curved ramp. As the waves overtop the ramp and enter the reservoir, potential energy is created by the difference in relative elevation. The arm-like reflectors assist in directing waves towards the reservoir and typically increase the rate of energy capture by 70%. Finally, the 16–20 low-head water turbines are like the stomach because they are used to convert the hydraulic head[1] within the reservoir into the end product, electricity. The multiple small turbines provide the many advantages, such as efficient flow rate regulation, shorter draft tubes, higher speeds and allowing the performance of maintenance activities while production continues. In order to maximize efficiency and thereby performance, a control scheme of three phases is applied to the production process. The first phase is the careful regulation of the platform’s floating level in order to maximize the amount of power flowing over the ramp, given ocean conditions. The second phase is controlling the water level inside the reservoir in order to minimize energy loss from losses of pressure head and due to overflowed water. The last phase controls turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–> and generator speed in order to maximize turbine efficiency based on the instantaneous turbine head[2].
Results from the single turbine<!–[if supportFields]> XE “turbine” <![endif]–><!–[if supportFields]><![endif]–>-generator-frequency converter unit are shown in relation to turbine head height. When water head (m) is relatively high, torque (Nm), turbine speed (rpm), and the amount of power (W) delivered to the grid are also relatively high. The DC link voltage (V) does not have a similar positive correlation with the turbine head. Output phase voltage (V) is constant throughout the simulation and shows no relationship to turbine head. The output phase current (A) shows a pattern similar but not analogous to the height of the turbine head. The authors also found that torque quickly decreases to zero as the cylinder gate closes. This ensures that the WD will use the greatest amount of water potential energy within the system. Igic et al. conclude by recommending that the relationship between power fluctuation and voltage near the grid connection point be examined by incorporating a grid model into their simulation.

[1] “The force exerted by a column of liquid expressed by the height of the liquid above the point at which the pressure is measured. Although head refers to a distance or height, it is used to express pressure, since the force of the liquid column is directly proportional to its height.” (Engineering Dictionary)
[2] “the level of difference between the reservoir level and the mean sea level”

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