• Energy Research

Quantitative reliability, availability, and maintainability (RAM) assessments are of fundamental importance at the early design stages, as well as planning and operation of marine renewable energy systems. This paper presents an RAM framework adaptable to different offshore renewable technologies, conceived to provide support in the choice of the device components and subsequent planning of the O&M strategies. A case study, characterizing a pilot farm of oscillating water column (OWC) wave energy converters (WECs), is illustrated together with the method used to obtain reliable estimate of its key performance indicators (KPIs). Based on a fixed feed-in-tariff for the project, economic figures are estimated, showing a direct relationship with the availability of the farm and the cost of maintenance interventions. Consequently, the probability distributions of the most relevant output variables are presented, and the mutual correlations between them investigated using principal components analysis (PCA) with the aim of discovering the relationships influencing the performance of the offshore farm. In this way, the contributions of the individual factors on the profitability of the project are quantified, and generic guidelines to support the decision-making process are derived. It is shown how this type of analysis provides important insights not only to ocean energy farm operators after the deployment of the devices, but also to device developers at the early design stage of wave energy concepts.

The paper concerns the phase control by latching of a floating oscillating-water-column (OWC) wave energy converter of spar-buoy type in irregular random waves. The device is equipped with a two-position fast-acting valve in series with the turbine. The instantaneous rotational speed of the turbine is controlled through the power electronics according to a power law relating the electromagnetic torque on the generator rotor to the rotational speed, an algorithm whose adequacy had been numerically tested in earlier papers. Two alternative strategies (1 and 2) for the latching/unlatching timings are investigated. Strategy 1 is based on the knowledge of the zero-crossings of the excitation force on the floater-tube set. This is difficult to implement in practice, since the excitation force can neither be measured directly nor predicted. Strategy 2 uses as input easily measurable physical variables: air pressure in the chamber and turbine rotational speed. Both strategies are investigated by numerical simulation based on a time-domain analysis of a spar-buoy OWC equipped with a self-rectifying radial-flow air turbine of biradial type. Air compressibility in the chamber plays an important role and was modelled as isentropic in a fully non-linear way. Numerical results show that significant gains up to about 28% are achievable through strategy 1, as compared with no phase control. Strategy 2, while being much easier to implement in practice, was found to yield more modest gains (up to about 15%).

AbstractThe quest for conquering the ocean and understanding its behaviour has been a challenge with increasing needs for innovation and technology investments in many areas of strategic value for the promotion, growth and competitiveness of the marine economy worldwide. Current oceanographic buoy systems are limited to low power levels and intermittency of data acquisition and transmission, among other aspects that need to be overcome to comply with new and more demanding applications. The development of marine activities requires more powerful and reliable data-acquisition systems to guarantee their future sustainability. This work presents a new systematic methodology for optimum design of wave energy converters. The methodology was applied to design two self-powered sensor buoys for long term monitoring based on the oscillating-water-column principle. The optimisation focussed on buoy hydrodynamic shape, sizing and selection of the turbine and the generator, as well as the control law of the generator electromagnetic torque. The performance was assessed through the use of the power matrix and a set of performance indicators. These performance indicators were defined to allow a simple comparison between different wave energy concepts. The results confirm the applicability of the designed buoys for a next generation of oceanographic monitoring systems.

Abstract The quest for exploiting the ocean resources and understanding its behaviour has been a challenge with increasing needs for innovation and technology. Model testing is an essential step in offshore renewable energy technology development. It involves challenges that require experience and guidance. Costly mistakes might arise with the subsequent waste of time and resources. This paper presents the model design and testing processes as part of wave energy projects and the results of experimental testing of two types of oscillating-water-column (OWC) wave energy converters (WEC). The model design aims at the creation of a reduced-scale model to simulate the physical phenomena found in full-scale devices. It is a process that requires several skills and an adequate compromise among all variables. This design involves several approaches as different physical phenomena do not follow the same similarity conditions, requiring adjustments in scale, materials, and other relevant properties. Besides, the model testing process comprises the necessary planning and actions to execute the tests and post-processing of data. This process is addressed here through model design and testing of two WECs: the coaxial-duct and the spar-buoy OWCs. The configurations have been designed and studied for large-scale energy production and small-scale power in oceanographic applications. Although the devices are both OWCs, the designs exhibit significant differences. The development process of the models and results are presented for the two OWC devices. Free-decay tests, hydrodynamic performance and mooring tension results are presented and discussed. These may serve as guidelines and numerical modelling validation.

Abstract The paper presents a detailed analysis of the dynamics and control of air turbines and electrical generators in oscillating-water-columns (OWCs). The aim is to explain the performance of an OWC device based on the physical behaviour of each system component. The Mutriku wave power plant was chosen as the test case. The power plant is a breakwater located in the Bay of Biscay, in Basque Country, Spain. The contributions of the work are: i) development of a hydrodynamic model of the power plant in the frequency domain; ii) implementation of a non-linear time-domain wave-to-wire model; iii) real-valued implementation of the Prony method for the computation of the wave-radiation force; iv) a detailed generator model based on experimental data to assess the influence of the generator efficiency on the power take-off performance; v) a critical performance comparison of the Wells and biradial turbines; vi) a sensitivity analysis of the control parameters of the turbine/generator set; and vii) an explanation of the comparative performance of both turbines based on statistical data. The turbine performance curves were taken from developers’ published experimental data. The results were obtained with a simplified model of the wave climate off the Mutriku test site comprising 14 sea states.

Abstract This work focuses on the investigation of bent-duct buoy oscillating-water-column (OWC) wave energy converters (WEC), which have been generally studied in their backward configuration (BBDB). This work studies both backward and frontward configurations through systematic phases starting from a reference model of a BBDB OWC WEC. Firstly, an appropriate numerical model is defined to assess the performance of BBDBs equipped with linear turbines. Secondly, having variations from the reference model, a parametric sensitivity analysis is carried out to study the influence of the main geometric variables, when designing a bent-duct buoy in both backward and frontward configurations. Results show a better performance of frontward configurations in most cases. Afterwards, an optimisation study is performed for a specific site. Results confirmed that the frontward configuration for the optimised geometry outperformed the backward configuration, and the selected geometry also performs better than the original reference at the site selected. Finally, a techno-economic assessment is carried out for the reference BBDB and for the optimised bent-duct buoy. Despite the substantial increase in annual mean power output, results confirm that the wave commercial projects are not economically feasible under current assumptions. This requires more efforts from developers and from regulators/policy makers to develop mechanisms to promote wave energy projects.