The OWEL Wave Energy Converter as a Platform for Combined Wave & Wind Power Generation
Authors: Professor John Kemp, Anthony Derrick, Jamie O’Nians, and Dr. Drona Upadhyay

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ABSTRACT

This paper aims to provide a concise description of the development stages to date for the Offshore Wave Energy Ltd wave energy converter. It will outline the principles upon which the device works and the details of the scaled test models that have been undertaken, including those from the current phase.

The paper will also examine the predicted scaling effects on pressure generation and discuss the application and deployment of the WEC offshore winds turbines and other applications. The next steps for the OWEL device will also be presented along with preliminary cost estimates of the demonstrator and prototype devices.

Introduction

The immediate future for renewable energy appears to comprise mainly of the exploitation of onshore wind. This cannot provide all the power needed to meet established UK and European targets so that offshore wind farms will become of increasing importance in the medium term.

As the percentage of power produced by wind increases, continuity of supply becomes a matter of increasing importance for both commercial and political reasons. To provide the required continuity of supply, major offshore wind generator sites will have to be widely spaced geographically, and it is suggested that some will need to be sited on floating platforms in deep water along the European “Atlantic Arc”. This is a region of highly energetic ocean waves, the number of platforms required could be halved and continuity of supply enhanced, by combining wind and wave generation on the same platforms.

OWEL wave energy converters will be integrated into large, concrete, floating platforms, eminently suited for hosting wind generators. An update on the development of the OWEL device will be presented. The suitability of an OWEL platform for combined wind/wave power generation will be discussed, and the possible advantages of using wave only in the Atlantic Arc will also be considered.

Outline of the device principles

The OWEL Wave Energy Converter (WEC) is designed for deployment in offshore sites to take advantage of energetic wave conditions before the energy of the waves is attenuated by friction from the seabed. The design is sized to capture the energy from long lengths of wave crest, and optimised to absorb a large proportion of the energy from small waves, but a small proportion of the energy from storm waves. The device will be simple and robust in construction, with no mechanical moving parts in contact with the water, and thus the large structures will have low cost and minimal maintenance.

The WEC unit is designed in the general form of a horizontal duct floating such that the freeboard is approximately equal to the amplitude of the ambient waves. A unit has a length related to the longest design wave, and this feature, together with a small waterplane area, ensures that a platform formed from multiple units will remain steady in a seaway.

The duct in each unit is open at one end, and the mooring system of the platform ensures that this end is presented to the incoming waves. The duct has angled side-plates and an angled bottom-plate or ramp. The air in the trough behind an incoming wave is trapped as the following wave seals against the top of the duct. This air is then compressed by the angled side-plates and delivered to a compression manifold. An air take-off, through a non-return valve, feeds the compressed air to a reservoir, and a baffle system disperses the remnant energy in spent waves.

The baffle system also creates a reactionary force on the compressed air in the manifold. The air collected in the reservoir may then be used to drive a compressed air turbine and thence generate electricity. A conceptual design of the WEC is shown in Figure 1.

Figure 1: Schematic of the OWEL WEC

A full-scale WEC would be formed of a number of units consolidated into a large, concrete platform, with a wave capture length of 200 metres. In West Coast wave conditions, this would have a rated output of 12 Megawatts and an average output of 4 Megawatts. In less energetic wave conditions, the output would be correspondingly reduced. However, since the platforms are large and stable, weighing of the order of 30,000 tonnes, this could be compensated by siting wind generators on a platform.

The OWEL WEC has the huge advantage that its primary output is compressed air. A further option currently under consideration, is using the compressed air in conjunction with a gas-turbine, to provide a continuously available source of power irrespective of weather conditions. Approximately two-thirds of the power generated by a gas-turbine is absorbed by the compressor. When wave energy is available, WEC compressed air could be delivered to the appropriate compression stages of a gas-turbine, thus saving up to 60% of the fuel consumption for a given power output.

In calm conditions, the gas-turbine could provide its own air compression so there need be no reduction in power output. Overall, it is estimated that a WEC/gas-turbine system could save of the order of 20% of the fossil fuel consumption as compared with a gas turbine alone. Little new technology would be required since the principle of feeding externally generated compressed air to gas turbines is well established. Electricity is already produced in Germany and the USA by gas-turbines supplied with compressed air stored in disused mines.

Steps taken to date for device development

All of the competing wave energy devices proposed or under development appeared to have shortcomings of one sort or another and thus at the Wave Energy Converter (WEC) concept stage the perceived shortcomings were identified and proposals for their resolution formulated and subsequently Offshore Wave Energy Limited (OWEL) was set up. Steps taken so far during the development of the OWEL WEC are outlined here.

1:100 Scale Model Tests

A DTI SMART grant was obtained to carry out a feasibility study to investigate and develop a new approach in the form of a patented wave energy converter (WEC) which traps and compresses the air in successive wave troughs. The feasibility study comprised of a mathematical modelling study, a tank-testing programme for a 1:100 scale model and an assessment of the commercial potential of the WEC design. The mathematical modelling and the tank-testing components were conducted for OWEL by a team from the QinetiQ.

The model for the 1:100 tank testing was supplied by OWEL and was a redesigned version of an earlier model which had been the subject of preliminary tank testing in October 1999. The model was approximately 2 metres in length, and for the test programme it was secured near the centre of the towing tank. A picture of the test model is shown in Figure 2.

Figure 2: 1:100 Scale Model

This tank-testing programme carried out in 2001-2002, in its own right, provided results that were encouraging for the future potential of the OWEL WEC concept. However, the main purpose of the programme was to support the mathematical study and to input results that could be used for calibrating the mathematical model. Direct comparisons between the mathematical predictions and the tank-testing results were made and good agreement achieved for an idealized model of this type, which cannot take into account all of the fluid dynamic loss mechanisms.

In addition to the tank tests, mathematical modelling of the 1:100 scale model was carried out by QinetiQ. The mathematical model of the performance of the WEC was used in conjunction with hydrodynamic model experiments to build on the knowledge of the mode of operation of the device. Based on this knowledge, the performance of a full-scale system was been predicted. The benefit of this study was to reduce the risks associated with the further development of the concept.

The study concluded that, subject to the need for an improved knowledge of the flow processes involved, the prospects of a full-scale Wave Energy Converter were promising.

1:10 Scale Model Tests

Offshore Wave Energy have recently tank-tested a tenth-scale, 15 metre long, models of their Wave Energy Converter (in a number of different configurations) at the New and Renewable Energy Centre (NaREC) facility in Blyth, Northumberland. The testing programme was supported by the Carbon Trust RD&D programme as part of an ongoing project. A series of model scale tests on the prototype 1:10 scale Wave Energy Converter (WEC) model was carried out. The purpose of the tests was to obtain an increased physical understanding of the underlying principles of operation of the scaled up model and to gather information which would be used to validate a CFD numerical model.

Three different models, of the WEC (each weighing approximately 4 tonnes) with differing ramp and side-wall angles were designed and manufactured. Of the three models, two had fixed geometries whilst the third had an adjustable floor which could be moved to achieve three different ramp angles. Five different configurations of the WEC were tested over a repeated range of wave spectra.

The various configurations tested within the NaREC wave dock were mounted within a framework which itself was located upon two standard 40 feet transportation containers. This design approach resulted in statically fixed, non buoyant models which were fully constrained against potential pitch, sway, heave, yaw, roll and surge motions resulting from wave excitation forces. A picture of the 1:10 model is shown in Figure 3.

Figure 3: 1:10 scale model being prepared for tests

Over 100 tests runs were completed during the entire testing programme. Pressure, temperature and wave data were recorded for each run at a high sample rate and fluid mechanical principles were used to derive secondary parameters. For certain wave spectra and configuration combinations, excellent wave to compressed air power conversion efficiencies of the order of 50% were recorded.

Mathematical Modelling

Mathematical modelling has been an integral part of the development of the WEC. The feasibility study stage (1:100 scale) had a component of mathematical modelling, which was used to extrapolate to the performance of a full-scale Wave Energy Converter. In addition to pressure, the power output, efficiency and flow rate of the device were predicted.

More numerical modelling of the 1:100 scale WEC using Computational Fluid Dynamics (CFD) software ANSYS CFX was carried out during the development phase. The purpose of recreating the original 1:100 test model in CFD was to validate the methodology to be adopted for the CFD analysis of the 1:10 scale model.

Extensive 2D and 3D CFD simulations with 1:100 models were carried out and the results from the mathematical modelling were compared with the available experimental data. Multiphase flow methodology combined with a transient solution of the flow problem was employed. A close correlation was achieved between the experimental results obtained from 1:100 scale tank testing and the CFD results, thus validating the methodology used in the CFD.

A three-dimensional CAD model of the new WEC was created and was then used to generate the geometrical model for mathematical modelling. The CAD model was meshed with appropriate densities at various locations of the WEC. For example, at the upper one third of the WEC, finer mesh was used to capture the movement of the wave and the lower two third was coarsely meshed to keep the size of the model within the limits of the computing power available.

Further CFD modelling was carried out for the 1:10 scale WEC model using the validated methodology, which consisted of, among others, appropriate initial and boundary conditions applicable to the model. A list of boundary conditions employed for the model is shown in Table 1.

Table 1: Boundary Conditions for WEC

The boundary conditions and their locations are graphically represented in Figure 4.

Figure 4: Boundary Conditions for WEC

CFD was used as a design tool to assist the project team to optimise the design, before and after the tank testing. Several simulations were carried out for different geometric configurations of the WEC for each set of wave parameters such as wave length, period and amplitude. Qualitative and quantitative results were obtained from each simulation and the results were analysed to assess the performance of the WEC and to assist the design team to modify the design.

One such qualitative result obtained is the the fluid volume fraction within the WEC which helps to trace the wave profile along the length of the duct of the WEC and is shown in Figure 5. In the figure, red is water and blue is air and other colours show various proportions of water and air mixture. The simulation results were used in conjunction with other design criteria to arrive at the final 1:10 scale WEC model design that has been built and tested.

Figure 5: Fluid Volume Fraction within the WEC

Predicted scale performance

The 1:100 numerical model study by QinetiQ also carried out an analysis to predict pressure rise and efficiencies at other scales of the WEC. The experimental model was used as the baseline for this study, since the size of a full-scale unit had not yet been finalised. For the 1:10 scale, CFD results were used to arrive at the predicted pressure rise for this scale of the WEC. A comparison of the predicted pressure rise calculated by QinetiQ study and CFD study carried out recently is shown in Figure 6.

Figure 6: Scaling effects on pressure

It can be seen from the figure that both CFD and QinetiQ study give near linear trend in rise in pressure as the scale goes from 1 to 100. Here a scale factor of 1.0 corresponds to the experiment test model of 1:100, and a scale factor of 100 corresponds to a unit that is 180m in overall length with the results spanning tank tests, intermediate scale testing and full-scale units.

Future STEPS

Testing of a marine based fully operable grid connected device

Following completion of the the 1:10 scale model at NaREC, OWEL will now progress to a three-quarter scale sea-going demonstrator. This demonstrator, comprising of a single OWEL unit, will be rated at 750kW and will undergo sea trials at EMEC or a similar grid connected test facility. The demonstrator phase, with a projected cost of £3.5 million, is expected to have a two-year duration. During this phase, detailed design and optimization will be undertaken with further refinement to modelling the device in irregular sea states.

It is scheduled that a prototype single array rated at 12MW will follow on from the demonstrator as an OWEL ‘first farm’. The aim is to have the farm deployed in early 2009 making use of the proposed Wave Hub facility off the north Cornish coast.

Figure 7: Costs breakdown for a 12MW ‘first farm’

The capex costs are estimated to be in the order of £1.5 to £2 million per MW with a cost per MWh of £55 to £65. It is expected that these costs will reduce as the number of MW arrays deployed increases and that these costs will become even more competitive. It is estimated that the costs will be at least in line with those predicted for comparable, large off shore wind installations.

synergy with offshore wind

Whilst the primary focus of OWEL is to develop a robust, low maintenance and low cost per MW wave energy converter the nature of the OWEL device lends itself to being exploited in other ways.

The current depth limitations (of less than 30m) for the deployment of offshore wind turbines is a considerable restriction. A single OWEL array comprising of 6 units forms a large, stable platform in the order of 100x200m. With such a platform it is conceivable that wind turbines could be integrated onto the platform raising the array rating significantly and reducing the £/MW of the power transmission cabling and deployment significantly.

There are of course a number of matters that will need to be considered in detail such as the structural integrity and buoyancy of the WEC and of course the economic modelling.

A full-scale WEC would be formed of a number of units, as shown in artist’s impression in Figure 8.

Figure 8: Artist's impression of full scale array of WEC