Experimental Validation of a Nonlinear MPC Strategy for a Wave Energy Converter Prototype

One of the major limitations to the development of advanced wave energy converters (WECs) control strategies are the associated computational costs. For instance, model predictive control (MPC) strategies have the potential to obtain almost optimal performance, provided that the imperfect power conversion in the power take-off (PTO) system is correctly taken into account in the optimization criterion and that the incoming wave force can be estimated and forecast. However, demanding computational requirements as well as the unresolved issue of wave force estimation have so far prevented real-time implementation and validation of such MPC strategies. In this paper, we present the successful experimental results obtained on a scaled-down prototype of the well-known Wavestar machine. Performance comparisons are provided for nonlinear MPC versus a reference PI controller.

Weather Window Analysis in Connection With Operation and Maintenance of Ocean Renewable Energy Devices

There is an increasing need for utilization of ocean renewable energy (ORE) around Japanese coast because Japan is surrounded by ocean. Because technologies for harnessing ORE have not been mature enough, Japanese government selects some demonstration sites for ORE devices and some demonstration projects are going. As these projects are progressed, the operation and maintenance (O&M) activities will increase and become essential factors for the success of demonstration projects. Hence, weather window analysis is required to quantify the levels of access for ORE devices in the demonstration projects, and commercial projects in the future. In this paper, two new parameters are proposed in order to evaluate accessibility to ORE devices. One is the operational probability, and the other is the forecasted waiting time. The operational probability assesses weather duration with considering variability of wave condition. The forecasted waiting time is an expectation value of waiting time before O&M planners get next chance to arrange the O&M activities. In order to check the effectivity of the proposed 2 parameters, accessibility is evaluated for significant wave height in terms of the 2 proposed parameters, these are • Operational probability • Forecasted waiting time and 3 conventional parameters, these are • Excess probability • Persistence probability • Waiting time between windows The accessibility is evaluated at two locations along the Japanese coast. This study reveals that large differences are caused between persistence probability and operational probability when operational wave height limit occurs intermittently and required window length is long. The forecasted waiting time has the same variation tendency as the waiting time between windows.

Fundamental Study on Elastic Behavior of Large-Scale Floating Coal Stockyard

This paper proposes a large-scale floating coal stockyard (LFCS) and discusses its elastic behavior. Indonesia has recently become the main country supplying coal in the Asia-Pacific region. However, there is concern that export to Japan will decrease as coal demand increases. Therefore, the trend of coal transport in Indonesia is a very important matter in ensuring the continued stable import of coal to Japan. It is difficult for bulk carriers to traverse the shallow terrain of the seabed of the Markham River in East Kalimantan to reach coastal areas. Additionally, an LFCS can be operated as a relay base for barges, and large coal carriers have been proposed for use in offshore areas. The LFCS is capable of loading, storing, and offloading coal. Installing an LFCS offshore Kalimantan is expected to improve coal transport productivity in the region. Under such circumstances, the design plan proposed in this paper can simultaneously perform independent loading and unloading without interference. The partial mass distribution and local rigidity of the LFCS varies depending on the coal loading conditions. In addition, because the structure has a planar shape, the response of the LFCS showed elastic behavior. Design example of such a huge floating structure with the great difference of the displacement is unparalleled, it is very important to clarify a design fundamental subject. The objectives of this study are to provide a preliminary LFCS design and investigate the impact of varying the mass distribution and local rigidity on not only the distribution of the distortion and internal stress but also on the dynamic hydroelastic motion of the LFCS when it is impacted by waves. Therefore, the wave response of the LFCS was analyzed under different loading conditions.

Starting System for Darrieus Water Turbine of Tidal Stream Electricity Generation

Darrieus type vertical axis water turbine in a cylindrical shape which consists of some straight blades is simple, efficient and easy to install a generator upward. However, it has difficulty in starting revolution. As a method to cope with such a problem, a starting revolution assist mechanism was fabricated and set on a prototype of the turbine. Assist experiment was carried out. It resulted helping well the starting revolution. The improved prototype of tidal stream turbine can generate 1.4 W under a water flow of 1 m/s where impossible to self-start. Besides that, Darrieus water turbine’s generating torque property was investigated by the famous original experimental data of lift coefficient Cl and drag coefficient Cd for straight blades of NACA63 3-018 cross section. It was found that setting two or four blades in a turbine would help to improve the difficulty of starting revolution.

Effects of Disturbance of Current Field on Power Characteristics of a Floating Type Pitch-Controlled VAMT in a Real Sea

While a type of marine turbine for tidal current generation can be chosen from several types, a vertical axis marine turbine (VAMT) should be better in Japan because sea areas around Japanese islands where current velocity is sufficient are limited. This study conducted a sea test of a VAMT of a floating type installed with six straight pitch-controllable blades. The cycloidal mechanism was adapted for the pitch control. The purpose of the study is to understand effects of ocean waves and motion of a floating body on turbine performance and behaviours of the VAMT in unideal current conditions. Besides, the data taken should be effective to consider that effects in order to design VAMTs. The setup with the setting angle of −30 degrees suggested highest performance from the sea tests, then 15% in maximum turbine power and maximum output was 40W. Ocean waves strongly affected on the turbine performance because fluid velocity changes due to ocean waves and it was unable to neglect the variation of the velocity in spite of small. The characteristics of the turbine sensitively varied because of ocean waves. The results suggested that during accelerating and decelerating incoming fluid speed, characteristics of the turbine were different in each case.

Experimental Study on a 3-Dimensional Hydro-Elastic Deformation of “Underwater Platform” With Multi-Towers in Waves

Underwater platform was proposed in OMAE 2015 for the purpose of enhancing productivity of various types of renewable energy converter on the sea and its feasibility study was carried out through 2 types of tank experiment [1]. The underwater platform which is a very large frame shape structure connects several floaters under the sea to share power cables and mooring lines and to keep relative distances between the floaters. In the experiment, 1/200 scale elastic model with three spar buoys was used. The buoys imitated spar type floating offshore wind turbines (FOWTs). From the experiment, it was shown that the platform with large draft can reduce its response in waves. In this paper, we report new result and knowledge obtained by additional model experiments use the 1/200 model. In the experiment, we changed the arrangement and draft of the model and measured hydro-elastic deformation of the underwater platform in waves. From the last experiment, relationship between draft settings and response was shown. In the experiment, relationship between wave angles and response was surveyed. From the experiment, we have confirmed followings: 1. Rigid-body motion is remarkable in beam waves, 2. Elastic response is remarkable in head waves, and 3. Remarkable torsional motion is occurred in 45 degrees’ waves. The more important thing, however, is that the experimental result indicated that the platform of large draft decreases its motion in the all the wave angles.

Analysis of Wave Energy Sources in the North Atlantic Waters in View of Design Challenges

This paper describes a wave energy analysis of North Atlantic waters and provides an overview of the available resources. The analysis was conducted using a scatter diagram data combined with wave statistics and empirical parameters given by wave height and periods. Such an overview is instrumental for modelling of wave energy sources, design of wave energy converter (WEC) devices and determination of locations of the devices. Previous survey of wave energy resources widely focused on determination of the reliability on installations of WECs. Though the renewable energy source that can be utilized from the waves is huge, the innovative work in design and development of WECs is insignificant and the available technologies still require further optimization. Furthermore, the wave potential of North Atlantic waters is not sufficiently studied and documented. Closer review of the literature also shows that wave energy conversion technology, compared with other conversion machines of renewable energy sources such as wind energy and solar energy, seems still immature and most of the research and development efforts in this direction are limited in scope. The design of energy converters is also highly dictated by the wave energy resource intensity distribution, which varies from North to South hemisphere. The immaturity of the technology can be attributed to several factors. Since there are a number of uncertainties on the accuracy of wave data, the design, location and installation of WECs face a number of challenges in terms of their service life, structural performance and topological configuration. As a result, collection and assessment of wave characteristics and the wave state conditions data serve as key inputs for development of robust, reliable, operable and affordable wave energy converters. The fact that a number of variables are involved in wave distribution characteristics and the extraction of wave power, treating these variables in the design process imposes immense challenges for the design optimization and hence the optimum energy conversion. The conversion machines are expected to extract as high wave energy as possible while their structural performance is ensured. The study reported in this paper is to analyse wave data over several years of return periods with a detailed validation for wave statistics and wave power. The analysis is intended to contribute in better understanding of the wave characteristics with influencing parameters that can serve as design optimization parameters. A method is proposed to conduct a survey and analysis of the available wave energy resources and the potential at cited locations. The paper concludes that wave energy data accuracy is the baseline for project scoping, coastal and offshore design, and environmental impact assessments.

A French Application Case of Tidal Turbine Certification

Tidal turbines are emerging technologies offering great potential for the harnessing of a renewable and predictable oceanic resource. However, exploitation at sea comes with significant design, installation, grid connection, and maintenance operations challenges. Consequently, guidelines and standards are required to ensure safety, quality, performance and accelerate tidal turbines development and commercialisation. Standardisation is also a necessity to support and improve safety and confidence of a wide range of Marine Renewable Energy (MRE) stakeholders such as designers, project operators, investors, insurers or final users. There are undergoing developments on guidelines, standards and certification systems within the International Electrotechnical Commission (IEC) Technical Committee TC 114 “Marine energy - Wave, tidal and other water current converters” and the IEC Renewable Energy “Marine Energy - Operational Management Committee” (IECRE ME – OMC). However, as the tidal energy concepts are only at the demonstration stage, only few guidelines and no dedicated certification scheme has been published so far within this organization, which guarantee an international, independent, non-governmental and consensus-based elaboration process. The aim of this paper is to present a proposal of certification methodology, developed by Bureau Veritas for the design assessment of current and tidal turbines, and its application to a French case study. This certification procedure was developed within the French research project Sabella D10 funded by ADEME and is published in the Bureau Veritas guideline NI603 “Current & Tidal Turbines”. The suggested certification procedure addresses prototype, component, type and project certification. Main objective, scope, intermediary steps to be completed and resulting certificates will be detailed for each certification scheme, as well as their interactions. This methodology will be illustrated by the case study on the Sabella D10 prototype, a French tidal turbine installed in 2015 in the Fromveur Passage, off Ushant Island. Sabella D10 is a 1 MW tidal turbine fully submerged laid on the seabed with a horizontal axis and 6 blades. It is the first French tidal turbine producing electricity and connected to the electrical network. The Sabella D10 case study will focus on prototype certification and computations performed for support structure and blades. The paper will describe the load cases that have been considered, the review procedure for the support structure and the blades design assessment, including description of a streamlined method for basic design and a detailed method for final design. In conclusion, the next steps will be introduced to continue the certification developments of tidal and current turbines.

Numerical Simulation of Dynamics of a Spar Type Floating Wind Turbine and Comparison With Laboratory Measurements

Wind turbines are popular for harnessing wind energy. Floating offshore wind turbines (FOWT) installed in relatively deep water may have advantages over their on-land or shallow-water cousins because winds over deep water are usually steadier and stronger. As the size of wind turbines becomes larger and larger for reducing the cost per kilowatt, it could bring installation and operation risks in the deep water due to the lack of track records. Thus, together with laboratory tests, numerical simulations of dynamics of FOWT are desirable to reduce the probability of failure. In this study, COUPLE-FAST was initially employed for the numerical simulations of the OC3-HYWIND, a spar type platform equipped with the 5-MW baseline wind turbine proposed by National Renewable Energy Laboratory (NREL). The model tests were conducted at the Deepwater Offshore Basin in Shanghai Jiao Tong University (SJTU) with a 1:50 Froude scaling [1]. In comparison of the simulation using COUPLE-FAST with the corresponding measurements, it was found that the predicted motions were in general significantly smaller than the related measurements. The main reason is that the wind loads predicted by FAST were well below the related measurements. Large discrepancies are expected because the prototype and laboratory wind loads do not follow Froude number similarity although the wind speed was increased (or decreased) in the tests such that the mean surge wind force matched that predicted by FAST at the nominal wind speed (Froude similarity) in the cases of a land wind turbine [1]. Therefore, an alternative numerical simulation was made by directly inputting the measured wind loads to COUPLE instead of the ones predicted by FAST. The related simulated results are much improved and in satisfactory agreement with the measurements.

Economic Seafloor Massive Sulfide Mining by Japan’s Model

Because of the higher Au, Ag, and Cu contents, seafloor massive sulfides (SMS) have received much attention as future commercial mining targets by private companies and nations. One of them, Solwara 1 Project in Papua New Guinea (PNG), is scheduled to start the commercial mining operation from 2018. Because the mining site is inter-island area and almost no cost is necessary for the waste disposal in PNG, the economy of the mining is expected very well. In contrast with this, because all the SMS distribution sites in Japan locate outer ocean areas and the waste disposal cost on land in Japan is very expensive, the economy of SMS mining in Japan is quite negative. In order to overcome the problems, a self-standing riser with flexible link to the sea surface platform and a primary ore separation on the seafloor prior to the ore lift-up are proposed. The improved SMS mining concept named Japan’s model is examined.