Jens Peter Kofoed

Marine Renewable Energies: Perspectives and Implications for Marine Ecosystems

Countries with coastlines may have valuable renewable energy resources in the form of tides, currents, waves, and offshore wind. The potential to gather energy from the sea has recently gained interest in several nations [1–3], so Marine Renewable Energy Installations (hereinafter MREIs) will likely become very diffuse in the near future and determine a further transformation of our coastal seas.

Experimental Study of the WEPTOS Wave Energy Converter

This paper presents the power performance results of the experimental study of the WEPTOS wave energy converter (WEC). This novel device combines an established and efficient wave energy absorbing mechanism with an adjustable structure that can regulate the amount of incoming wave energy and reduce loads in extreme wave conditions. This A-shaped floating structure absorbs the energy in the waves through a multitude of rotors, the shape of which is based on the renowned Salter’s Duck. These rotors pivot around a common axle, one for each leg of the structure, to which the rotors transfer the absorbed wave energy and which is connected to a common power take off system (one for each leg). The study investigates the performance of the device in a large range of wave states and estimates the performance in terms of mechanical power available to the power take off system of the WEPTOS WEC for two locations of interest. These are a generic offshore location in the Danish part of the North Sea (Point 3) and the location of the Danish wave energy centre (DanWEC) in front of Hanstholm harbour.Copyright

Operation and maintenance strategies for wave energy converters

Inspection and maintenance costs are a significant contributor to the cost of energy for wave energy converters. There are different operation and maintenance strategies for wave energy converters. Maintenance can be performed after failure (corrective) or before a breakdown (preventive) occurs. Furthermore, a helicopter and boats can be used to transport equipment and personnel to the device, or the whole device can be towed to a harbour for operation and maintenance actions. This article describes, among others, a risk-based inspection and maintenance planning approach where the overall repair costs including costs due to lost electricity production are minimized. The risk-based approach is compared with an approach where only boats are used and another approach where the target is to minimize the downtime of the device. This article presents a dynamic approach for total operation and maintenance costs estimations for wave energy converter applications including real weather data and damage accumulation. Furthermore, uncertainties related with costs, structural damage accumulation, inspection accuracy and different maintenance strategies can be included. This article contains a case study where different maintenance strategies are applied for the Wavestar device, and the influences of the different parameters, for example, failure rate, inspection quality and inspection interval, are evaluated for the overall costs and the number of repairs needed during its lifetime.

A Marine Spatial Planning framework for the optimal siting of Marine Renewable Energy Installations: two Danish case studies.

ABSTRACT Azzellino A., Kofoed J.P., Lanfredi C., Margheritini L., Pedersen M., 2013. A Marine Spatial Planning framework for the optimal siting of Marine Renewable Energy Installations: two Danish case studies. In this analysis two Danish case studies are investigated using a spatial planning approach. The first case study concerns the area on the west coast of Denmark that has been elected as test site by the Danish Wave Energy Center (DanWEC), a foundation constituted by local authorities, Aalborg University supported by the national wave energy industry. The second case study attains the Danish portion of the western Baltic sea, where many offshore windfarms are already installed and many projects are in construction or in the planning stage. The environmental background for the two areas is considered through set of multiple indicators (e.g. sea bottom topography and characteristics, marine biodiversity, presence of vulnerable species). Environmental indicators are aggregated into environmental impact indexes that constitute the basis for evaluating the site suitability for Marine Renewable Energy Installations (MREIs). Concurrenlty, areas of potential conflicts between the interests of MREI developers and commercial, recreational users are identified. Multivariate analysis techniques allow to disentangle the different components of the environmental vulnerability of the two areas and suggest sound criteria for the optimal siting of these infrastructures. The two case studies, concerning respectively a regional and local scale, offer good examples about how spatial planning has the potential to guide the transition from the single sector management toward the integrated management of sea uses.

Full-Scale WECs

In this chapter, some of the concepts that have reached the full-scale stage are de-scribed in detail. Other examples could be given, but the analysis was limited to four main technologies: OWC (Oscillating Water Column), AWS (Archimedes Wave Swing), Pelamis and Wave Dragon presented in 7.1 to 7.4. Each illustrates one particular power conversion mechanism that was addressed in Chapter 6, namely air turbines, direct drive linear generators and hydraulics. A subsection re-garding overtopping theory is also presented, as it was not addressed in the previ-ous chapter and is linked with one of the technologies (Wave Dragon). Section 7.5 gives an account of the operational experience gathered by the several technology developers. Such detail provides valuable lessons to those interested in the field and to a wide engineering audience. To conclude, section 7.6 provides a brief up-date on test centres, pilot zones, a review on the most relevant EU funded projects and a case study related to one of the technologies.

Determining Mean Annual Energy Production

Abstract The calculation of the mean annual energy production (MAEP) is critical to the assessment of the levelized cost of energy for a wave energy converter or wave farm. Fundamentally, the MAEP is equal to the sum of the product of the power capture of a set of sea-states and their average annual occurrence. In general, it is necessary in the calculation of the MAEP to achieve a balance between computational demand and accuracy. A high accuracy can be obtained using a large number of sea-states with a high fidelity power capture model; however, this is likely to result in a high computational demand. Typically, the models most suitable for calculating the MAEP are time-domain models, spectral-domain models, and models obtained through system identification. The traditional method for representing the wave climate is using a scatter table, indexed by significant wave height and energy period; however, it has been found that this can lead to high errors in the MAEP due to the necessary assumptions regarding spectral shape. Alternative representations include an extensive time series of all the sea-states or an abridged set, where the set is chosen to cover the range of sea-states as completely as possible using techniques such as the k-means algorithm or the maximum distance algorithm. Once the wave climate is defined the power capture for each representative sea-state in the wave climate can be determined using a power matrix, indexed by significant wave height and energy period, modelling all the sea-states or modelling a representative set of sea-states defined using the radial basis functions method. The use of the power matrix is most popular, but also least accurate, whilst modelling all the sea-states is the most computationally demanding, but also the most accurate.

Coupled BEM/hp-FEM Modelling of Moored Floaters

A coupling between a dynamic mooring solver based on high-order finite element techniques (MooDy) and a radiation-diffraction based hydrodynamic solver (WEC-Sim) is presented. The high-order scheme gives fast convergence resulting in high-resolution simulations at a lower computational cost. The model is compared against a lumped mass mooring code (MoorDyn) that has an existing coupling to WEC-Sim. The two models are compared for a standard test case and the results are similar, giving confidence in the new WEC-Sim-MooDy coupling. Finally, the coupled model is validated using experimental data of a spread moored cylinder with good agreement.

The Wave Energy Sector

When entering the field of wave energy utilization it is relevant to ask—why is it important to start utilizing this resource? The reasons for this are shared with other renewable energy sources, such as hydro, wind, solar, biomass and other ocean energy forms such as tidal, currents, thermal and salinity driven systems.

Hydrodynamic Behavior of Overtopping Wave Energy Converters Built in Sea Defense Structures

Many sea defense structures need to be adapted to the rising sea water level and changing wave climate due to global warming. The accordingly required investments open perspectives for wave energy converters (WECs) — that are built as part of the sea defense structures — to become economically viable. In this paper the average overtopping discharges q of overtopping wave energy converters built in sea defense structures are studied. Physical model tests with this type of devices have been carried out in a wave flume leading to experimentally determined values for the average overtopping discharge q. These experimental data are compared with predicted average overtopping discharges using existing empirical formulae from literature — derived mainly for sea defense structures. Overtopping WECs have small relative crest freeboard heights and smooth slopes to maximize overtopping, which is contradictive to the basic role of sea defense structures. As a consequence, the experimentally achieved average overtopping discharges are situated in a range that is not well covered by the existing traditional prediction formulae. The presented results for linear-slope overtopping WECs fill the gap between those for smooth dikes and those for plain vertical walls. The overtopping behavior in that particular range is discussed in this paper.Copyright

WAVE IMPACTS ON CAISSON BREAKWATERS SITUATED IN MULTIDIRECTIONALLY BREAKING SEAS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.