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Supplemental material, Appendices for City-wide building height determination using light detection and ranging data by Yue Wu, Luke S Blunden and AbuBakr S Bahaj in Environment and Planning B: Urban Analytics and City Science

International audience Experiments have been performed in the Shallow Water Wave Basin of DHI, in Denmark, on large arrays of up to 25 heaving point absorbers for a range of layout configurations and wave conditions. Float response and modification of the wave field are measured to provide data suitable for the evaluation of array interaction models and environmental scale models. Each wave energy converter unit has a diameter of 0.315 m and power absorption is due to friction of both a power take off system and bearings. Response is measured on all floats and surge force on five floats. Wave gauges are located within and around the array. Wave conditions studied include regular waves and both long- and short-crested irregular waves. A rectilinear arrangement of support structures is employed such that several float configurations can be studied. A summary is presented of the experimental arrangement with particular emphasis on the individual wave energy converters and wave conditions employed. Reasonable agreement is observed between measured response for single floats and power output and float response predicted using a linear time domain model. For an array of 25 floats, up to 16.3% reduction of significant wave height is observed down-wave and 10.8% increase observed up-wave for unidirectional irregular waves due to wave radiation by the heaving WECs. Spectra at different locations within and around the array show the wave field modifications.

The power production of a point absorber wave energy converter is optimized in irregular waves by means of a stochastic analysis of the wave process. The dynamic response is assumed to be a stationary random process with zero mean value. Based on the assumption that the control moment constrains the velocity of the oscillator to be in phase with the incoming wave moment, the damping coefficient is expressed by the autocovariance function of the stationary excitation process. The paper examines four different control strategies by means of numerical simulations in the time domain. The effects on the structural loads are analyzed and compared for the different control cases. The load distribution show that the reactive system has larger moment amplitudes than the passive controlled system. However, for mild and relatively short waves the difference of the loading distribution seems to be small while for larger and longer waves the amount of reactive power which is fed into the system is significant. The structural loads i.e. the mechanical control moment and the inertia due to the angular acceleration are therefore increased.

The paper discusses results of regular wave tests conducted at the University of Naples “Federico II” with the purpose of investigating the qualitative features and the magnitude of the wave pressures acting onto the front face of a Seawave Slot-Cone Generator (SSG). Various wave conditions have been considered, ranging from standing to plunging. Calculation formulae for engineering applications are proposed and discussed.

Recently has been suggested by some authors that resource diversity may be used to manage the variability of renewable power and lower the system integration costs of renewables. The key benefit, deriving from the diversification of the mix of renewable technologies, lies in the possibility of reducing the variability of the produced power. As MREIs develop, it is likely to result in further transformation of coastal seas, already affected by significant pressure from human activities. In this perspective, both the potential for combining different renewable technologies, and the impact associated to such development should be considered in the context of the existing pressures. The spatial conflicts of sea uses and the demand for sea space are in fact increasingly growing and quantitative MSP criteria may help to evaluate the sustainability of conflicting human activities in the perspective of the minimization of the overall environmental impacts. In this study the opportunity of co-locating offshore wind turbines and wave energy converters is analyzed through a MSP approach.

In order to deliver cost competitive solutions, Wave Energy Converters (WEC) must be optimized in several fields, e.g. transportation, installation, structure, machinery etc. Large expenses lie on the superstructure support, i.e. the offshore foundation. Geotechnical analysis and optimization of six possible foundation solutions for WEC are presented in this article. The study is performed for WEC superstructure supported by two or four foundations. In the four-column system horizontal wind and wave loads are transformed as a pair of vertical forces, acting in tension and compression. When the overturning moment is large, it might be desirable to increase the width distance, achieving smaller vertical forces. For this reason, several distances between the supporting columns are investigated. Additionally, a two-column system is analysed as it is expected to be more cost effective. In such a solution horizontal wind and wave loads are dominant.Gravity based, pile and bucket foundations are universally applied solutions for the offshore structures. The suitability of these types for a WEC is analysed and commented. The foundations are designed to satisfy ultimate and serviceability limit state requirements. For each of the foundation solutions, three geotechnical design steps are followed, employing analytical calculations, numerical 2D and 3D finite element programs.

The fully instrumented Lever Operated Pivoting Float LOPF wave energy buoy model has gone through the first stage of testing in regular waves in scale 1:25 of the North Sea wave conditions, in the 3D deep wave basin at the Hydraulic and Coastal Engineering Laboratory of Aalborg University. Some direct measurements from the model device are: voltage output, the torque on the generator, the arm bending moment produced by the mooring line and the absolute angle of the pivoting float. These allowed to follow the conversion of power in the power train from mechanic to electric power, the identification of areas for improvements and the preliminary result revealing that it is expected that a 15 m buoy in North Sea conditions will generate between 222 MWh/y and 723 MWh/y, depending on the degree of improvement achieved from the first model.

The paper presents the preliminary results of new physical tests carried out in the directional wave basin of Aalborg University (DK). The devices under exams are two floating 7 Degrees of Freedom Wave Activated Bodies moored with a spread system composed by 4 steel chains. The devices were subject to ordinary North Sea wave climate conditions and deployed in 1:60 scale. The main purpose of this paper is to analyse the performance of a Wave Energy Converter considering the interdependencies among energy production, loads on real moorings and device movements. The mooring effects on power production and on device movements are specifically investigated by varying the chain pre-tension level. Results suggest that the power production optimization is achieved with a slack mooring system providing a quasi-static response to the ordinary wave attacks

Wave energy devices are often located in intermediate or shallow water close to the free water surface where the wave exposure is extreme. The structures typically consist of large thin curved plates, spherical shells, or cylindrical structures which are sensitive to pressure loads. The lack of proper methods to calculate design pressure distributions has led to structural failures such as buckling in the shells in wave energy prototypes.As a step towards understanding the complex loading from high order non-linear waves, this paper presents a practical approach to estimate wave excitation forces accounting for both non-linearity and diffraction effects. The method is validated by laboratory experiments using a hemispherical point absorber with a 6-axis force transducer, but the technique is believed to be applicable for most types of submerged or semi-submerged floating devices.The applied method is based on calculation of a peak force coefficient defined as experimentally measured forces on the structure divided by forces estimated by the chosen theoretical method. Methods used include an integration of the undisturbed wave pressure over the surface of the structure, corresponding to the Froude-Krylov force, and a numerical solution to linear potential diffraction theory. Since the two methods have mutual limitations in describing higher order waves and diffraction, a combination of the two will be introduced.

Aalborg University together with Energinet.dk, the Danish Transmission System Operator, have released a freely available online spreadsheet to evaluate the Levelised Cost of Energy (LCOE) for wave energy projects. The open-access tool calculates the LCOE based on the power production of a Wave Energy Converter (WEC) at a particular location. Production data may derive from laboratory testing, numerical modelling or from sea trials.The tool has been developed as a transparent and simple model that evaluates WEC’s economic feasibility in a range of locations, while scaling WEC’s features to the selected site. The available locations are (i.e. three locations in West Denmark, two locations in the Danish North Sea, two locations off Portugal, off Ireland and off France, and at EMEC), Moreover, the tool provides good agreement between WECs’ scales and locations, and the twelve selected sites represent different wave conditions.The paper describes the parameters behind the calculations, provides COE numbers of WECs and other technologies, presents the COE calculation tool and its features, and introduces an ongoing project which continues the development of the tool.Significantly this is conceived as an open-access tool, available to device developers as well as academia. It can eventually help in the development of wave energy by contributing to open talks with investors, stakeholders, politicians and the general public.

The Danish wave energy sector has during 2011-12 carried out a project called ‘A new strategy for wave energy through industrial partnership’ described in the document ‘Wave Energy Technology, Strategy for Research, Development and Demonstration 2012’ [1]. The project was funded by the Danish Energy Agency’s EUDP programme. By 2030 at the latest, the development of wave energy technology will provide a cost-effective and sustainable electricity supply from offshore energy farms in Denmark. The strategy assumes that wave energy in combinations with wind farms will become more cost-effective than off-shore wind power, placed in deep sea far from shore and that the development will take place through an extended cooperation revolving around the technology development and supported by improved financial conditions for wave energy.The vision that the Danish wave energy industry will be able to sell competitive wave energy technologies both on the national and the international market and that utilization of wave energy will become a prerequisite for building offshore energy farms at greater water depths in the future, is the key reason for suggesting improved financial conditions for wave energy.This paper will present the development plan described by the partnership in order to realize their vision, including the collaborative focus areas for technological development, the timeframe and the required support structure identified by the Danish Partnership for Wave Energy.

The aim of this study is to validate and assess the capability of an advanced hydrodynamic time-domain model through the comparison of numerical and experimental results of a small-scale bottom-fixed pitch buoy wave energy converter. The series of tests have been conducted in the hydraulic laboratory of Aalborg University. The device is a prototype of a singleWavestar float, scaled at 1:20, considering one degree of freedom only. The tests included a various range of incoming waves, from small regular waves to storm conditions irregular waves.The numerical simulations have been performed at the WavEC using an in-house time-domain model. The code, based on the linear potential theory, solves the system of motion equations using state-space representations of the radiation terms and includes nonlinear hydrostatic forces and nonlinear Froude-Krylov excitation terms, which require the instantaneous detection of the floating body wetted surface. The idea behind this approach is first to increase the model accuracy (compared to the fully linear model method), but also to keep the computational time relatively low.Results show a good agreement between experimental and numerical data, which enhance significantly the confidence in using such sort of models to simulate the behavior of wave energy converters.

Lowering the overall cost of wave energy converters is a necessity for creating a feasible solution to renewable energy. The design of wave energy converters is in general based on traditional steel design methods. In the design of steel structures subjected to significant dynamical loading and a harsh environment issues such fatigue resistance and durability are of major concern. The welded joints in steel structures significantly reduce the fatigue resistance and give a low utilization ratio of the steel material. Furthermore is coating of all exposed steel surfaces a necessity to secure a sufficient durability of the entire structure. Consequently it will be of great concern to optimize the structural concepts of the wave energy converters and secondly find materials tailored to the harsh environment at sea.The paper shows that utilizing Ultra High Performance Fibre Reinforced Concrete as primary material in the design of wave energy converters is a feasible and promising solution, which reduce the overall cost of the structure significantly. This will be illustrated by means of a feasibility study carried out on the Wavestar project, where special attention is pointed at the arm and float structure . The arm structure is previously designed as a steel structure, whereas the float has been designed as a light weight glass fibre structure. The new UHPFRC design gives a very beneficial solution that will contribute to the reduction of the manufacturing and the maintenance cost of the Wavestar wave energy converter.

In this paper, an optical non-contact low budget method for tracking the position of multi-object is presented for marine/off-shore laboratory applications. Particular focus is given at the wave energy field and the analysis of floating wave energy converters dynamics. The measurement of the position and orientation is often a key point in this context, achieved nowadays with different standard technologies. Typically a base requirement for a measurement system is to be accurate and precise, without affecting the dynamical and statical behavior of the system itself. Video based systems satisfy these points, but they are typically prohibitively expensive. Through the article a low budget one-camera video based system is presented and compared with other two standard methods: direct physical measurement and inertial motion unit (IMU). The system was tested both, in a dry environment and in waves. Results show an average error below 4%, with increased high signal accuracy and precision in the low frequency components but also increased high frequency noise components. Further investigation will be addressed to the implementation of a hybrid system, where the corrupted high frequency information can be replaced by the IMU signal.

At the present pre-commercial phase of the wave energy sector, device developers are called to provide reliable estimates on power performance and production at possible deployment locations. The EU EquiMar project has proposed a novel approach, where the performance assessment is based mainly on experimental data deriving from sea trials rather than solely on numerical predictions. The study applies this methodology to evaluate the performance of Wave Dragon at two locations in the North Sea, based on the data acquired during the sea trials of a 1:4.5 scale prototype. Indications about power performance and production of the device at the target locations, as well as on the applicability of the methodology, are provided.

In the field of wave energy it is well known that control of point absorbers is essential in order to increase energy capture from waves. Correspondingly, advanced control is an integrated part of the Wavestar design. This paper presents the control method, referred to as the Wave Power Extraction Algorithm (WPEA), applied to the full-scale Wavestar Prototype for maximizing energy extraction. The WPEA is optimized based on simulations of the point absorbers in different sea states. Hence, a presentation of a hydrodynamic model of the Wavestar is included in the paper. A simplified Power Take-Off (PTO) is also added to the model, enabling the optimization of the WPEA to take into account the PTO constraints of PTO bandwidth and force limitations. The predicted results of the optimized WPEA are compared to real measurements from theWavestar Prototype, showing good compliance.

Aalborg University carried out wave tank testing a 1:20 scale model of Langlee, an oscillating wave-surge type of Wave Energy Converter (WEC). Langlee is designed to operate in deep water, with the hinged flaps attached to a, moored, semi-submerged reference frame. Langlee has a novel flap arrangement, with the flaps placed symmetrically opposing each other on a floating reference structure. This minimises the net force on the reference frame and increases the stability of the reference frame under optimal wave conditions. This paper presents the results and analysis from the wave tanks, which addressed the following: The Power Take Offs (PTOs) were simulated using a motor to resist the motion of the wings, according to the damping profile. Torque and velocity measurements were used to predict the wave- to mechanical-power conversion efficiency of the device. A number of wing types and configurations were tested to compare their power conversion efficiency. The power production for five selected irregular wave states was used to estimate the power matrix of the full scale the device, by making certain assumptions about the power conversion efficiencies dependency on the peak wave height and mean period of the waves. Ultrasonic sensors were used to obtain motion data in three degrees-of-freedom, from which the free oscillation and the response to irregular waves were determined. Load cells were used to measure the mooring forces and used to obtain a statistical description of them.The buoyancy of the flaps was found to have a large impact on efficiency, and surface piercing flaps were found to increase the efficiency over a fully submerged flap configuration. A peak efficiency of approximately 35 % was found with the mean wave length of the irregular wave state equal to approximately twice the length of the device. From the natural oscillation tests the natural periods in heave and pitch was found to be approximately 2.4 s (in lab. scale), which is an approximately 10% improvement over the previous iteration of Langlee.

The aim of the study is to retrieve the incident wave information that coincides with former Pico plant operation periods. The recent implementation of a directional pressure sensor for wave measurement as well as the recovery of the data gathered by a directional wave rider buoy allowed embarking on the validation of two different models by using both wave measurements: a model for wave propagation (SWAN) and an Artificial Neural Network (ANN). This paper, as a first step of a comprehensive study, leads to several recommendations to improve both methodologies in future works. The aim of the study is to retrieve the incident wave information that coincides with former Pico plant operation periods. The recent implementation of a directional pressure sensor for wave measurement as well as the recovery of the data gathered by a directional wave rider buoy allowed embarking on the validation of two different models by using both wave measurements: a model for wave propagation (SWAN) and an Artificial Neural Network (ANN). This paper, as a first step of a comprehensive study, leads to several recommendations to improve both methodologies in future works.

The purpose of this paper is to examine the hydrodynamics around floating wave energy converters (f-WECs). In particular, the paper considers the case of the f-WEC of the Wave Activated Body type, named DEXA. Based on 3D wave experiments in the Laboratory of the Aalborg University (DK), the modified wave field around a wave energy farm (composed by three 1:60 scale models) and around a single device (1:30 scale model) is investigated. Specific results include wave reflection, wave transmission and wave disturbance around the device. The results are examined considering scale effects, influence of wave length and wave steepness.

Wave Star has produced and installed a test and demonstration Wave Energy Converter (WEC) by Roshage pier near Hanstholm at the west coast of Denmark. The test unit is a prototype test section of a complete commercial WEC. After an initial period of finalizing the installation and testing, the WEC was launched for production in January 2010 and in May 2010 automatic unmanned operation was initiated. The number of operational hours per month has increased significantly during the following months, but experience with the initial period of continuous automatic unmanned operation has shown difficulties in ensuring the ability to be in daily operation 24/7/365. During the period from January 2010 to April 2011 the control software has been updated three times, and significant increases in the measured power levels have been confirmed. The measurement points have verified the expected level of harvested power, although further development of the control system is expected to increase the level further.

One of the most urgent environmental issues in 415 coastal systems worldwide is eutrophication and hypoxia. Cause of eutrophication is the rapid increase in intensive agricultural practices, industrial activities, and population growth which together have increased nitrogen and phosphorus flows in the environment. This is a major problem in the Baltic Sea. The present study introduces the Wave-Energized Baltic Aeration Pump (WEBAP). This is one floating overtopping device that uses the collected overtopping water to oxygenate the sea bottom in areas suffering from eutrophication. This is done by using the driving head of the overtopping water temporarily stored in the reservoir flowing out through a long vertical pipe leading close to the bottom of the sea. The necessary amounts of oxygen and corresponding overtopping volumes in order to have a positive impact in a damaged environment have been considered for the Baltic Sea case and laboratory tests in scale 1:25 to the North Sea have been conducted in the 3D deep wave basin at the Hydraulics and Coastal Engineering Laboratories of Aalborg University. The "prove the concept" and investigation of motions forces are here presented with focus on extreme wave conditions providing relevant insight on technical aspects.

At the present time there are no approved standards or recognised best practices being implemented for the performance appraisal and benchmarking of wave and tidal energy converters. As such, this develops considerable misunderstanding between device developers, testing centres, investors/ financiers etc when attempting to quantify the performance of a device since it makes it very difficult to reference and benchmark the performance of a marine energy converter. The EC Framework Programme VII EquiMar project has set out to develop a suite of Best Practices to be adopted when undertaking the performance evaluation of such systems in order to address this deficiency. This paper reports the development of a set of ‘Best Practices’ within the ECFPVII EquiMar project to be adopted for the performance quantification of wave and tidal energy converters as they evolve from an engineering concept to commercial scale deployment.

The paper reports on 2D small scale experiments conducted to investigate wave loadings acting on a pilot project of device for the conversion of wave energy into electricity. The conversion concept is based on the overtopping principle and the structure is worldwide known with the acronym SSG. The hydraulic model tests have been carried out at the LInC laboratory of the University of Naples Federico II using random waves. Results indicate wave overtopping is able to cause a sudden inversion of vertical force under wave crest, so that it is alternatively upward and downward directed over a short time interval. It is also shown that two calculation methods widely employed in the Japanese design practice of vertical face breakwaters, could be used to achieve safe estimates of the hydrodynamic loadings.

The wave energy sector is in need of showing positive experience from the real sea trial in order to prove its feasibility. For this purpose, an accurate knowledge of wave conditions at the selected location of installation is fundamental. A design challenge for wave energy devices is the large differences between the extreme wave conditions in which the device is designed to survive and the average wave conditions for which the device is to be optimised. Indeed, the ratio between extreme loads and operational loads has been identified to be a fundamental factor for the design and the cost analysis of the wave energy units. The present paper provides an estimate of everyday wave conditions at Hanstholm location, by mean of scatter diagrams for different locations within the harbour vicinity. Results on wave heights transformation from offshore to shore realized with numerical model for operational and extreme waves are then presented and compared in different strategic points for WECs installation in the proximity of Hanstholm harbour within the Danish Wave Energy Centre (DanWEC).

This paper analyses the performance of a new Wave Energy Converter (WEC) of the Oscillating Water Column type (OWC), named WavePiston. This near-shore floating device is composed of plates (i.e. energy collectors) sliding around a cylinder, that is placed perpendicular to the shore. Tests in the wave basin at Aalborg University allowed to investigate power production in the North Sea typical wave climate, with varying design parameters such as plate dimensions and their mutual distance. The power produced per meter by each collector is about the 5% of the available wave power. Experimental results and survivability considerations suggest that the WavePiston would be particularly suited for installations in milder seas. An example application is therefore presented in the Mediterranean Sea, off-shore the island of Sicily. In this case, each collector harvests the 10% of the available wave power. In order to allow the extension of the experimental dataset, an analytical model, based on mass-spring-damper system concept, is under development. The model is calibrated in order to represent the laboratory results for regular waves.

The paper addresses an important challenge ahead the integration of the electricity generated by wave energy conversion technologies into the electric grid. Particularly, it looks into the role of wave energy within the day-ahead electricity market. For that the predictability of the theoretical power outputs of three wave energy technologies in the Danish North Sea are examined. The simultaneous and co-located forecast and buoy-measured wave parameters at Hanstholm, Denmark, during a non-consecutive autumn and winter 3-month period form the basis of the investigation.The objective of the study is to provide an indication on the accuracy of the forecast of i) wave parameters, ii) the normalised theoretical power productions from each of the selected technologies (Pelamis, Wave Dragon and Wavestar), and iii) the normalised theoretical power production of a combination of the three devices, during a very energetic time period.Results show that for the 12 to 36 hours time horizon forecast, the accuracy in the predictions (in terms of scatter index) of the significant wave height, zero crossing period and wave power are 22%, 11% and 68%, respectively; and the accuracy in the predictions of the normalised theoretical power outputs of Pelamis, Wave Dragon and Wavestar are 44%, 52% and 62%, respectively. The best compromise between forecast accuracy and mean power production results when considering the combined production of the three devices.