Design and Control of the PowerTake-Off System for a Wave Energy Converter with Multiple Absorbers

Research output: Book/ReportPh.D. thesisResearch

Abstract

Most active Wave Energy Converter (WEC) concepts are based on harvesting energy from ocean waves by placing buoyant bodies in the sea. As the bodies are forced to oscillate by the waves, power is produced by converting the oscillations into electricity, which is performed by the Power Take-Off (PTO).

Despite 40 years of research activities within wave energy, the PTO is still a hindrance. No matured designs exist and no installed prototypes have demonstrated average electrical production above 250kW. Looking beyond 1MW, limited research exists, making the lack of advances in PTO research a contributing factor to wave energy remaining in a pre-commercialisation phase - a phase where electrical power production has been demonstrated, but needs to find a road to larger power scales and effective production.

The purpose of the research documented in this dissertation is to find a PTO capable of meeting current and future needs of WECs, i.e. meeting requirements of efficiency, controllability and durability while having the important feature of scalability. The case study for the research is the Wavestar 600kW 20 float multiple absorber, but the research has general applicability to WECs based on converting bi-directional motions into electricity.

All PTO technologies are kept open and based on a thorough analysis and evaluation of state-of-the-art, the PTO technologies with greatest potential are identified. The state-of-the-art covers both PTOs for wave energy, but also general advances in high powertransmissions, which have applicability to wave energy.

One of the difficulties in PTO design is performing the trade-off between contradicting PTO characteristics, e.g. controllability, efficiency and peak power capacity. A PTO system for wave energy is a classic example of a mechatronic design problem, where all aspects of the design couples. To this end a framework is presented where all classic wave power extraction methods (reactive control, linear damping, latching control, declutching, etc.) are analysed according to a generic PTO formulation and optimised using numerical simulations in irregular waves. This enables a comparison of the performance of the wave power extraction methods according to PTO requirements. The framework also allows comparing performance of fundamentally different PTOs.

The idea of reactive control for increasing power absorption dates back to the 1970’s, and today its feasibility for real PTO systems still causes dispute. In this dissertation an analytical result is provided, proving that reactive control is highly beneficial at even “low” PTO efficiencies.

The formulated reactive control is tested in a wave tank with 1:20 scale absorbers, validating the expected performance. The wave tank tests also verify the derived wave and absorber models, which are based on linear wave theory. This increases the confidence in the heavy use of models through-out the work.

A new high performing control method is developed for wave power extraction characterised in that the Oscillation Control is Implemented Resistively (OCIR). The OCIR control implements a causal non-linear control, which achieve similar manipulation of the absorber’s behaviour as reactive control, but through non-linear damping techniques. The control is shown to be superior to other resistive control techniques.

The research leads to three potential PTO systems, where one is a magnetic gear based PTO. The gear is based on implementing the function of a screw and nut magnetically by placing permanent magnets in a helical pattern. A PTO layout with the magnetic lead screw is found and analysed using simulations. The feasibility leads to having a group of master students designing a working prototype at a scale of 17kN with a half meter stroke. The magnetic lead screw is able to directly convert a linear motion of 0.5m/s to a rotational motion above 1000rpm, driving a conventional generator.

Two other hydraulic PTO solutions are also found highly feasible. One of them is based on discrete control of a hydraulic cylinder, and is assessed to be the most promising solution. It is therefore analysed in depth. The solution is named a Discrete Displacement Cylinder (DDC). The developed DDC allows discrete force control of a multi-chambered cylinder driven by the absorber, while efficiently transferring the generated power directly into a battery of high pressure accumulators. The concept allows DDCs of multiple absorbers to supply the same accumulator battery, where a hydraulic motor may use the stored energy to drive a generator at near constant load.

A complete PTO with the DDC is designed and simulated for a 20 absorber Wavestar 600kW WEC. The simulation comprises a 20 absorber hydrodynamic model, all PTO component models, and all main system control. The plus 600 state simulation model proves the expected PTO performance.

A working full scale 420kN prototype of the DDC for one absorber is designed and tested. The DDC consists of a multi-chambered cylinder with 2m stroke and a prototype valve manifold. The manifold is implemented using high performance proportional valves instead of on/off valves. This allows emulating an arbitrary on/off valve. The prototype DDC has a peak power capacity of 210kW. To test the DDC prototype, commissioning of a full-scale test-bench was necessary. The test-bench uses a second hydraulic cylinder to emulate the movement of an absorber in waves. A control solution based on state-space control is developed, which tracks a real-time implemented simulation model of the absorber. This enables the test-bench to emulate the absorber dynamics while suppressing its natural modes. The test-bench is shown to emulate the
absorber dynamics, including being able to respond correctly to PTO loads.

Systematic tests are performed on the prototype DDC, validating the calculated requirements of valves etc. Anticipated problems with line dynamics are experienced, where the impact pressure during shifting gives a 30% extra pressure peak. The measured responses could be simulated exactly, and by using the models, an improved shifting technique is developed solving the problem.

Finally, an initial test of the prototype DDC in irregular waves is successfully performed on the test-bench, verifying the applied models and approaches.
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Details

Most active Wave Energy Converter (WEC) concepts are based on harvesting energy from ocean waves by placing buoyant bodies in the sea. As the bodies are forced to oscillate by the waves, power is produced by converting the oscillations into electricity, which is performed by the Power Take-Off (PTO).

Despite 40 years of research activities within wave energy, the PTO is still a hindrance. No matured designs exist and no installed prototypes have demonstrated average electrical production above 250kW. Looking beyond 1MW, limited research exists, making the lack of advances in PTO research a contributing factor to wave energy remaining in a pre-commercialisation phase - a phase where electrical power production has been demonstrated, but needs to find a road to larger power scales and effective production.

The purpose of the research documented in this dissertation is to find a PTO capable of meeting current and future needs of WECs, i.e. meeting requirements of efficiency, controllability and durability while having the important feature of scalability. The case study for the research is the Wavestar 600kW 20 float multiple absorber, but the research has general applicability to WECs based on converting bi-directional motions into electricity.

All PTO technologies are kept open and based on a thorough analysis and evaluation of state-of-the-art, the PTO technologies with greatest potential are identified. The state-of-the-art covers both PTOs for wave energy, but also general advances in high powertransmissions, which have applicability to wave energy.

One of the difficulties in PTO design is performing the trade-off between contradicting PTO characteristics, e.g. controllability, efficiency and peak power capacity. A PTO system for wave energy is a classic example of a mechatronic design problem, where all aspects of the design couples. To this end a framework is presented where all classic wave power extraction methods (reactive control, linear damping, latching control, declutching, etc.) are analysed according to a generic PTO formulation and optimised using numerical simulations in irregular waves. This enables a comparison of the performance of the wave power extraction methods according to PTO requirements. The framework also allows comparing performance of fundamentally different PTOs.

The idea of reactive control for increasing power absorption dates back to the 1970’s, and today its feasibility for real PTO systems still causes dispute. In this dissertation an analytical result is provided, proving that reactive control is highly beneficial at even “low” PTO efficiencies.

The formulated reactive control is tested in a wave tank with 1:20 scale absorbers, validating the expected performance. The wave tank tests also verify the derived wave and absorber models, which are based on linear wave theory. This increases the confidence in the heavy use of models through-out the work.

A new high performing control method is developed for wave power extraction characterised in that the Oscillation Control is Implemented Resistively (OCIR). The OCIR control implements a causal non-linear control, which achieve similar manipulation of the absorber’s behaviour as reactive control, but through non-linear damping techniques. The control is shown to be superior to other resistive control techniques.

The research leads to three potential PTO systems, where one is a magnetic gear based PTO. The gear is based on implementing the function of a screw and nut magnetically by placing permanent magnets in a helical pattern. A PTO layout with the magnetic lead screw is found and analysed using simulations. The feasibility leads to having a group of master students designing a working prototype at a scale of 17kN with a half meter stroke. The magnetic lead screw is able to directly convert a linear motion of 0.5m/s to a rotational motion above 1000rpm, driving a conventional generator.

Two other hydraulic PTO solutions are also found highly feasible. One of them is based on discrete control of a hydraulic cylinder, and is assessed to be the most promising solution. It is therefore analysed in depth. The solution is named a Discrete Displacement Cylinder (DDC). The developed DDC allows discrete force control of a multi-chambered cylinder driven by the absorber, while efficiently transferring the generated power directly into a battery of high pressure accumulators. The concept allows DDCs of multiple absorbers to supply the same accumulator battery, where a hydraulic motor may use the stored energy to drive a generator at near constant load.

A complete PTO with the DDC is designed and simulated for a 20 absorber Wavestar 600kW WEC. The simulation comprises a 20 absorber hydrodynamic model, all PTO component models, and all main system control. The plus 600 state simulation model proves the expected PTO performance.

A working full scale 420kN prototype of the DDC for one absorber is designed and tested. The DDC consists of a multi-chambered cylinder with 2m stroke and a prototype valve manifold. The manifold is implemented using high performance proportional valves instead of on/off valves. This allows emulating an arbitrary on/off valve. The prototype DDC has a peak power capacity of 210kW. To test the DDC prototype, commissioning of a full-scale test-bench was necessary. The test-bench uses a second hydraulic cylinder to emulate the movement of an absorber in waves. A control solution based on state-space control is developed, which tracks a real-time implemented simulation model of the absorber. This enables the test-bench to emulate the absorber dynamics while suppressing its natural modes. The test-bench is shown to emulate the
absorber dynamics, including being able to respond correctly to PTO loads.

Systematic tests are performed on the prototype DDC, validating the calculated requirements of valves etc. Anticipated problems with line dynamics are experienced, where the impact pressure during shifting gives a 30% extra pressure peak. The measured responses could be simulated exactly, and by using the models, an improved shifting technique is developed solving the problem.

Finally, an initial test of the prototype DDC in irregular waves is successfully performed on the test-bench, verifying the applied models and approaches.
Original languageEnglish
PublisherDepartment of Energy Technology, Aalborg University
Number of pages266
ISBN (Print)978-87-92846-34-1
StatePublished - 2013
Publication categoryResearch

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