Sea wave energy is being increasingly regarded in many countries as a major and promising resource. The paper deals with the development of wave energy utilization since the 1970s. Several topics are addressed: the characterization of the wave energy resource; theoretical background, with especial relevance to hydrodynamics of wave energy absorption and control; how a large range of devices kept being proposed and studied, and how such devices can be organized into classes; the conception, design, model-testing, construction and deployment into real sea of prototypes; and the development of specific equipment (air and water turbines, high-pressure hydraulics, linear electrical generators) and mooring systems.

Wave energy utilization: A review of the technologies

When a molecule absorbs a photon of appropriate energy, a chain of photophysical events ensues, such as internal conversion or vibrational relaxation (loss of energy in the absence of light emission), fluorescence, intersystem crossing (from singlet state to a triplet state) and phosphorescence, as shown in the Jablonski diagram for organic molecules (Fig. 1). Each of the processes occurs with a certain probability, characterized by decay rate constants (k). It can be shown that the average length of time τ for the set of molecules to decay from one state to another is reciprocally proportional to the rate of decay: τ = 1/k. This average length of time is called the mean lifetime, or simply lifetime. It can also be shown that the lifetime of a photophysical process is the time required by a population of N electronically excited molecules to be reduced by a factor of e. Correspondingly, the fluorescence lifetime is the time required by a population of excited fluorophores to decrease exponentially to N/e via the loss of energy through fluorescence and other non-radiative processes. The lifetime of photophycal processes vary significantly from tens of femotoseconds for internal conversion1,2 to nanoseconds for fluorescence and microseconds or seconds for phosphorescence.1






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Figure 1


Jablonski diagram and a timescale of photophysical processes for organic molecules.

/pdf/fluorescence-lifetime-measurements-and-biological-imaging-xfalzh4w7e.pdf

Fluorescence Lifetime Measurements and Biological Imaging

The technology of hydrodynamic modeling and marine craft motion control systems has progressed greatly in recent years. This timely survey includes the latest tools for analysis and design of advanced guidance, navigation and control systems and presents new material on underwater vehicles and surface vessels. Each section presents numerous case studies and applications, providing a practical understanding of how model-based motion control systems are designed.

/pdf/handbook-of-marine-craft-hydrodynamics-and-motion-control-2qu7epb0pt.pdf

Handbook of Marine Craft Hydrodynamics and Motion Control: Fossen/Handbook of Marine Craft Hydrodynamics and Motion Control

Ocean waves are a huge, largely untapped energy resource, and the potential for extracting energy from waves is considerable. Research in this area is driven by the need to meet renewable energy targets, but is relatively immature compared to other renewable energy technologies. This review introduces the general status of wave energy and evaluates the device types that represent current wave energy converter (WEC) technology, particularly focusing on work being undertaken within the United Kingdom. The possible power take-off systems are identified, followed by a consideration of some of the control strategies to enhance the efficiency of point absorber-type WECs. There is a lack of convergence on the best method of extracting energy from the waves and, although previous innovation has generally focused on the concept and design of the primary interface, questions arise concerning how best to optimize the powertrain. This article concludes with some suggestions of future developments.

https://journals.sagepub.com/doi/pdf/10.1243/09576509JPE782

A review of wave energy converter technology

A review of wave-energy extraction

Ocean Waves and Oscillating Systems

1. Introduction 2. Mathematical description of oscillations 3. Interaction between oscillations and waves 4. Gravity waves on water 5. Wave-body interactions 6. Wave-energy absorption by oscillating bodies 7. Wave interactions with oscillating water columns Bibliography Index.

https://assets.cambridge.org/97805217/82111/copyright/9780521782111_copyright.pdf

Ocean waves and oscillating systems : linear interactions including wave-energy extraction

VARIOUS large scale systems have been proposed for absorbing and utilising the energy carried by ocean waves1–4. In principle, such a system consists of a damped oscillator which interacts with the wave. The net result is that energy is transferred from the wave to a load. Here we call a system a ‘point absorber’ when its horizontal extent is much smaller than one wavelength, and a ‘linear absorber’ when the system is made as a straight construction, at least a few wavelengths long1.

A resonant point absorber of ocean-wave power

Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply. Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings. This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used. The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments. A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum. Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller. An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

Johannes Falnes

Papers

A review of wave-energy extraction

Ocean Waves and Oscillating Systems

Ocean waves and oscillating systems : linear interactions including wave-energy extraction

A resonant point absorber of ocean-wave power

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter