Experiments by Invited Researchers


Scaling Effects in Wave Loading and Performance of a Breakwater-Integrated Oscillating Water Column Wave Energy Converter

Project acronym: HyIV-FZK-02
Name of Group Leader: Dr. Thomas Bruce
User-Project Title: Scaling Effects in Wave Loading and Performance of a Breakwater-Integrated Oscillating Water Column Wave Energy Converter
Facility: Large Wave Channel (GWK)
Data Management Report: Report

User-Project Objectives

The aim of the project is to explore the scaling issues involved in the physical modeling of wave energy converters (WECs) based upon the Oscillating Water Column (OWC). In pursuit of this overall aim, the project’s specific objectives were: 1. To deliver detailed, quantitative advice on the scaling of key hydraulic loads on the shoreline-mounted, simple OWC WEC. Specifically: wave loadings on the front face of the caisson, in-chamber loads on the rear wall, on the ceiling, through possible runup and slam. 2. To deliver detailed, quantitative advice on the scaling of the air spring and its relation to power take off and performance. 3. To deliver generic, quantitative advice on the scaling of overall plant performance, rooted in a physically rational framework. To meet these objectives, the following were the key elements of the physical model test design: 1. Waves: both regular and irregular waves will be used. Focussed wave groups may be used to create impact loadings. 2. Loadings will be inferred via multiple pressure transducer measurements, on front face; on rear wall of chamber (facing in); on chamber ceiling (facing down/in). 3. Power take-off will be via an orifice in chamber roof (as per most physical model studies). Flow rate in orifice was inferred from three measurements; by: (i) orifice meter; (ii) propeller meter; and (iii) indirectly from movement of water surface in chamber. In addition, matching CFD and small-scale physical model studies are being performed at HR Wallingford and University of Edinburgh respectively.

Short description of the work carried out

During the access period, testing took place on 18 days, during which 139 tests were performed with short sequences of regular waves (ranging from 80s to 140s), and a further 43 with irregular waves, with “1000-wave” runs ranging from 3000s to 6000s. There was also a solitary solitary wave test! 4 different power take-off settings were explored (via 4 orifice diameters), plus the situation of a closed chamber. The influence of water level and curtain wall submergence depth were also explored. The tests were staffed by a rota of 6 personnel , from Professor to Senior Undergraduate Student, from 4 institutes, in the UK and Italy. Data was gathered from 8 external wave gauges; 13 pressure transducers located on the outside of the structure and within the chamber, and 5 further water elevation gauges set out within the chamber. Additional information was gathered from 2 video cameras; an external camera viewing the front of the OWC, and a second one located within the chamber viewing the excursions of the in-chamber water. In all, 6.2 Gb of data was collected, checked and archived, together with a further 188 Gb of video from the camera located within the chamber and 179 Gb from the conventional, external-view video camera. Throughout this access period, there was not a single significant difficulty encountered! There were only minor issues with air leakage and with propeller meter function. This is testament to the outstanding planning, construction, commissioning and attention to detail throughout of the GWK team.

Highlights of important research results

While, at the time of writing, detailed analysis of the data remains in progress, some potentially important observations include: 1. Under larger, irregular sea conditions, the behaviour of the water column in the OWC chamber is very far from the idealised “piston-type” movement. An agitated water surface is generally observed, with instances of extreme wave run-up on the rear wall resulting in an overturning jet returning to strike the inside of the front (curtain) wall. It is anticipated that full analysis will show that under such conditions, the performance of the device as an energy converter will be significantly compromised. 2. The chamber pressures are strongly affected by the occurrence of “venting” at the front opening, when the local water surface falls to the extent that the chamber opening is momentarily exposed. 3. The pressure distribution on the rear (in-chamber) wall are quite far from what would be predicted by calculating the “Goda pressures” that would have prevailed had the chamber opening been a solid wall, and “stretching” these over the wetted area of the rear wall, as postulated by Patterson et al. (2009). [Patterson, C., Dunsire, R. & Hillier, S., 2009, “Development of Wave Energy Breakwater at Siadar, Isle of Lewis”, Proc. Coasts, Maritime Structures and Breakwaters, ICE / Thomas Telford]