Abstract
Several manufacturers have developed devices with which to harness tidal/current power in areas where the depth does not exceed 40 m. These are the so-called first generation Tidal Energy Converters (TEC), and they are usually fixed to the seabed by gravity. When carrying out maintenance tasks on these devices it is, therefore, necessary to remove the nacelles from their bases and raise them to the surface of the sea. They must subsequently be placed back on their bases. These tasks require special high performance ships, signifying high maintenance costs. The automation of emersion and immersion maneuvers will undoubtedly lead to lower costs, given that ships with less demanding requirements will be required for the aforementioned maintenance tasks. This research presents a simple two degrees of freedom dynamic model that can be used to control a first generation TEC that has been conceived of to harness energy from marine currents. The control of the system is carried out by means of a water ballast system located inside the nacelle of the main power unit and is used as an actuator to produce buoying vertical forces. A nonlinear control law based on a decoupling term for the closed loop depth and/or orientation control is also proposed in order to ensure adequate behavior when the TEC performs emersion and immersion maneuvers with only hydrostatic buoyancy forces. The control scheme is composed of an inner loop consisting of a linear and decoupled input/output relationship and the vector of friction and compressibility terms and an outer loop that operates with the tracking error vector in order to ensure the asymptotically exponential stability of the TEC posture. Finally, the effectiveness of the dynamic model and the controller approach is demonstrated by means of numerical simulations when the TEC is carrying out an emersion maneuver for the development of general maintenance tasks and an emersion maneuver for blade-cleaning maintenance tasks.
Highlights
Tidal energy is a renewable source that may both help attain the EU climate change targets and provide additional value in a future energy market with regard to other renewable energy sources thanks to its high predictability [1,2]
Emersion/immersion maneuvers are being sought in order to improve their economic competitiveness
After an appropriate review and carrying out the appropriate maintenance tasks, the power unit must be returned to the same place on the base
Summary
Tidal energy is a renewable source that may both help attain the EU climate change targets and provide additional value in a future energy market with regard to other renewable energy sources thanks to its high predictability [1,2]. One of the first steps that should be taken to accomplish these maneuvers automatically is that of implementing a closed loop depth and/or orientation control in order to: (i) extract the main power generation unit from its normal depth of operation (on the base placed on the seabed) to the sea surface and ; (ii) return it from the sea surface to the base These automatic maneuvers can be performed by controlling the inner ballast water inside the device and with the help of small guide wires, as is illustrated in Figure 4 [25]. The dynamic model for an approximately cylindrical body is composed of only two lumped masses handled solely by hydrostatic forces, which are conceived of as volume-increasing devices It is, necessary to design the nonlinear control law on the basis of an uncoupling term and nonlinear term compensation for the closed loop depth and/or orientation control in order to ensure adequate behavior when the TEC performs emersion and immersion maneuvers with only passive buoyancy forces.
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