The OWEL Wave Energy Converter – Demonstrator Phase
Authors: Professor John Kemp (OWEL) & Jamie O’Nians (IT Power)
 | SYNOPSIS |
This paper presents an outline of progress made by Offshore Wave Energy Ltd (OWEL) in developing a wave energy converter (WEC). It builds on a paper presented at the 2002 MAREC Conference, which described a feasibility study conducted for OWEL by the QinetiQ organisation. Research and development since 2002 is now discussed, including the tank-testing of a large-scale model at the NaREC wave facility in Blyth. The paper concludes a look-ahead to the planned demonstrator phase which will comprise the construction and proving of a three-quarter scale WEC at sea and, beyond that, to the deployment of the first commercial platforms.
| INTRODUCTION |
Offshore Wave Energy Ltd (OWEL) was formed in 2001 to investigate and develop a wave energy device which traps and compresses the air in successive wave troughs. The compressed air is accumulated in a reservoir for use in driving a turbine. The devices are designed to be installed on floating platforms, moored offshore in sea areas where suitable energetic wave spectra are experienced.
A feasibility study, supported by a DTI SMART award, commenced in June 2001. The Feasibility Study involved Mathematical Modelling and extensive Tank Testing of a scale model, both these studies being carried out for OWEL by the QinetiQ Organisation, Haslar.
The feasibility study was completed on time and within budget,. The results were positive, and were presented in a paper at the MAREC Conference in Newcastle in 2002. The Feasibility Study has now been followed in 2004/2005 by an initial development phase, with support from a Carbon Trust grant. This has involved CFD computer simulation, and the design, construction and testing of a 15 metre, 1/10th scale model in the New and Renewable Energy Centre (NaREC) wave facility at Blyth in Northumberland. This development phase is due to be completed in December 2005. It is currently on schedule to do so, and within the Carbon Trust budget
Following completion of the tank-testing programme, NaREC has become a shareholding partner in OWEL, thus joining the existing shareholders, Professor Kemp, IT Power Ltd, Sycamore Innovation Engineering Ltd, and Business Link Wessex.
This paper will present some results of the initial development phase, and will then go on to discuss the planned demonstrator phase and, beyond that, the commercial exploitation of OWEL technology.
| AUTHORS' BIOGRAPHIES |
John Kemp spent 10 years at sea before becoming an academic. He was professor at London (Guildhall) University and a special professor at the University of Nottingham. Following retirement, he became a director of Offshore Wave Energy Ltd. He is the author of many papers on maritime safety and navigation.
Jamie O’Nians has experience of offshore wave renewable energy devices includes design work engineering and project management of the OWEL WEC. He has also worked on the Seaflow marine current turbine and other marine energy devices. He has a strong background R&D especially in water power having designed hydropower plants.
| PRINCIPLE OF OPERATION |
Design Criteria
Following a review of existing and proposed systems for harnessing wave energy, the OWEL wave energy converter (WEC) was designed with a number of criteria in mind.
These included:
1 | Bearing in mind the rapid decrease in wave energy in shallow water, the WEC should be sited offshore, preferably in water deeper than 40 metres. | 2 | A floating unit should be used to facilitate offshore deployment and to ensure that performance is independent of tidal range. | 3 | A robust structure with low maintenance is required for operation in energetic wave spectra. There should be no moving parts in contact with the water and the structure should be simple and inexpensive to construct. | 4 | The horizontal component of wave motion should be used since wavelengths are many times greater than wave heights. Also, the horizontal motion is uni-directional whereas the vertical motion is oscillatory. | 5 | The low-grade wave energy should be amplified to produce higher air pressure for supply to a turbine. | 6 | Floating units should be designed to absorb only a small proportion of the energy in storm waves. |
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Implementation
With the above criteria in mind, a WEC unit was designed in the general form of a horizontal duct floating such that the freeboard would be approximately equal to the amplitude of the ambient waves. A unit has a length related to the wave regime in which it is intended to operate 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 one-way valve, feeds the compressed air to a reservoir. Adjacent to the manifold is a baffle system which disperses the remnant energy in spent waves so that they do not reflect back along the duct to interfere with following waves. The air collected in the reservoir may be used to drive a compressed air turbine and thence generate electricity. A schematic of the system is shown in Figure 1.
Figure 1: Schematic of the OWEL WEC principle. |
A full-scale WEC would be formed of a number of units consolidated into a large, concrete platform, with a total wave capture length of 200 metres. In UK 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 32,000 tonnes, this could be compensated by integrating a wind generator on each platform. At this stage, no detailed study has been made of the technical factors involved in hosting wind turbines. Research and development is concentrated on proving the viability of stand-alone wave power.
| RESEARCH AND DEVELOPMENT |
Feasibilty Study
As noted in section 1, a previously reported feasibility study was conducted in 2001/02. The results of a tank-testing programme, using a 1.8 metre physical model, were used to calibrate a mathematical model. The mathematical model was then used to predict full-scale performance, as illustrated in Figure 2.
3.2 Development Phase
Figure 2: Power output by scale factor. |
The scale factor in figure 2 is the linear scale factor, so that a scale factor of 100 refers to a WEC 180 metres in length.
The curve in figure 2 was generated from mathematical modeling conducted by the QinetiQ organization, but its general form can be explained as follows. Generated air pressures are expected to be directly proportional to linear scale, while the volume of air per second entering a WEC is expected to increase as somewhat more than the square of the scale. Thus the overall power developed is expected to vary as somewhat more than the cube of the scale. An alternative approach is to consider the power in the trapped waves. This increases as the square of the wave height, directly as the wave period and directly as the length of wave crest captured. Since longer waves have increased periods, the power in waves trapped by a WEC increases as rather more than the cube of the linear scale.
Further details of the feasibility study are contained in the proceedings of the 2002 MAREC conference.
Development phase
Following successful completion of the feasibility study, a grant was provided by the Carbon Trust for a two-year development phase. This included the tank-testing of a 15 metre physical model in the wave facility operated by the New and Renewable Energy Centre (NaREC) at Blyth. Again, the results of the tank-testing were used to calibrate a mathematical model. The mathematical modelling is being developed by IT Power engineers in association with the University of Bristol.
Tank-testing set-up
Five different configurations of the WEC model were each tested in nine different wave spectra, as shown in the matrix of Table I.
Table I: Wave spectra matrix
Wave Height | Wave Period | 2.1 secs | 3.0 secs | 3.8 secs | 0.2 Metres | Test 1 | Test 2 | Test 3 | 0.3 Metres | Test 4 | Test 5 | Test 6 | 0.4 Metres | Test 7 | Test 8 | Test 9 |
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The configurations differed in terms of the angle of the side-plates, the angle of the ramp, the design of the baffle system, and the fitting of an air-retaining “pelmet” across the top of the WEC intake. The device freeboard was set to 0.15m.
The wave periods of 2.1s, 3.0s and 3.8s were chosen so that the corresponding wave lengths of 7m, 14m, and 21m had a relationship of approximately 0.5, 1.0 and 1.5 to the effective length of the WEC duct respectively. This allowed a study of the effect of wave/duct length ratios on generated air pressure and power.
Tank-testing results – Air pressure
Measurements of WEC output were primarily in terms of air pressures (P1) developed in the compression section of the WEC duct. The mean peak P1 pressures developed are presented in figure 4.
Figure 3: Configuration comparisons by developed air pressure. |
The main feature to note in Figure 3 is that the mean P1 peak pressures do not vary greatly amongst the five denoted configurations.
Comparison between appropriate pairs of configurations showed that there was no significant difference in P1 pressures due to changes in side-plate angle, to changes in ramp angle, or to fitting the air-retaining pelmet.
These findings confirm similar findings from the feasibility study. They are important because they show that fixed angles can be chosen for the side-plates and ramp with little penalty in performance. Also, that an air-retaining pelmet need not be fitted. The findings accord with the OWEL policy of having no moving parts in contact with the water.
The only pair of configurations giving a significantly different result in terms of P1 pressure were configurations 3.2(i) and 3.2(iii). These were similar except for the baffle design. The sensitivity of developed air pressure to the baffle design is being investigated further by CFD modeling, and it is expected that an optimal baffle design will be identified.
Tank-testing results – Power
The physical model outputs were also analysed in terms of the power developed under differing wave conditions. Some results are shown in Figure 4.
Figure 4: Power output by wave characteristics. |
As one would expect, there was little power produced from waves of 0.2 metre height since the WEC freeboard was such that waves of this height did not seal against the top of the duct. (In a practical system, this would be addressed by decreasing the WEC freeboard to equal the wave amplitude so that a seal would be obtained). For higher waves, there was clearly a pronounced increase in power as the wave period increased from 3.0 seconds to 3.8 seconds.
Prior the tank-testing, it had been expected that the optimum power output would be achieved when the duct length (11 metres) was of the same order as the wave length (the case for 3.0 second waves). However, the results show that the power produced is greatly improved when the duct length is around half the wave length (the case for 3.8 second waves)
This relationship between duct length and wave length has since been confirmed by CFD modelling. It is an important finding for the design of a full-scale WEC in relation to the wave climate in which it is to be deployed. One implication is that, for a particular wave regime, the duct length of a WEC can be made shorter than was originally assumed, with a consequent saving in construction costs.
For the best combination of wave height and wave period, as they interacted with the duct length and freeboard of the WEC model, powers of the order of 1 kilowatt were produced.
Mathematical Modelling
The mathematical modelling, using computational fluid dynamics (CFD) techniques was initially aimed at predicting the results from the 1.8 metre tank-test model, as used in the feasibility study. Using multiphase flow methodology combined with transient solution of the flow parameters, a close correlation was achieved with the actual tank-test results, thus validating the CFD model.
The CFD model was then used as a design tool for the 15 metre physical model which was tested at NaREC. As noted above, five configurations of the physical model were tested. The results in terms of air pressures developed and power generated were from these tests were compared with the CFD model, the results again correlated with those recorded in the tank-testing.
Transparent Model
In order to verify the action at the air/water face within a WEC, a small model was constructed of Perspex, and tested in a transparent tank of water. A wave-making paddle was also fitted. Movie sequences enabled the action to be studied. Figure 5 shows a pocket of air within the WEC duct, with the exciting wave sealing against the roof of the duct at about the half-way point along its length. Outside the duct the profile of the undisturbed wave can be seen passing from left to right. The crest of this wave can be seen breaking as it begins to spill over the roof of the duct.
Figure 5: A side view of a Perspex WEC model, wave movement is from left to right. |
In this simple model, the air is allowed to exit through a small diameter, vertical pipe, and there is no air reservoir to accumulate the compressed air. However, the venting air does rotate a vertical axis turbine.
| DEMONSTRATOR PHASE |
The Carbon Trust supported development programme was completed in December 2005. The next phase will comprise the design, construction, deployment and proving of a three-quarter-scale, seagoing demonstrator. This will consist of a single OWEL unit, and it is hoped to deploy it at the EMEC facility in Orkney.
The demonstrator will be approximately 55 metres in length by 20 metres in width (tapering to 10 metres). The depth will be 12 metres. Constructed most likely in concrete, the demonstrator would weigh of the order of 2,000 tonnes; a steel equivalent would be in the order of 400 tonnes.
Initially, a considerable computer simulation effort will be conducted, using the CFD model calibrated during the development phase, as noted above. This will refine the design of the WEC demonstrator and lead to the production of detailed drawings. At the same time, studies on the mooring system, the turbo-machinery, the electrical system and the ballasting system will build on work already conducted during the feasibility and development phases.
Figure 3: 1:10 scale model (15 metre) being prepared for tests at Blyth.
A seagoing demonstrator would be around 55 metres in length. |
The demonstrator will have a rated power of 750 kilowatts. Over the planned two years covering design and deployment, the expenditure is estimated to be of the order of £4 million. The risk at this stage is limited because of previous experience with a large-scale physical model in the NaREC test tank (see Figure 6). A successful demonstration will limit the risk for the next stage, which will be the construction and deployment of a full-scale commercial WEC platform.
| COMMERCIAL DEPLOYMENT |
As noted in section 4, the first commercial WEC platform can be designed, constructed and deployed with considerable confidence, since it will be dependent on satisfactory results being obtained during the demonstrator phase. Problems which arise during the sea-going deployment of the demonstrator can be addressed, and analysis of the demonstrator performance will be expected to lead to further improvements in design. Over a two-year period, for the design, construction and deployment of a commercial platform, costs are expected to be of the order of £18 million. This figure would include capital costs, development costs and operational costs for installing a platform on a site such as the proposed Wave Hub.
A commercial WEC will comprise some 6 units connected rigidly together, side by side (see Figure 7), to form a large concrete platform. The platform will have a rated output of 12 megawatts and will weigh in the order of 32,000 tonnes. It will be moored so that its wave capture length of 200 metres faces towards the incoming waves.
Figure 7: Artist's impression of full scale WEC platform. |
Costs for subsequent commercial WEC platforms are expected to decrease as manufacturing and operating techniques become more efficient. However, for the first few platforms, the cost of constructing and installing platforms is expected to be around £1.6 million per megawatt of rated power. It is estimated that the cost of producing electricity will be of the order of 5p per kilowatt-hour over a twenty-year operating period.
Economic studies confirm that the OWEL concept is financially sound, and that it will provide excellent long-term returns to investors. The Government policy that 20% of the UK’s energy should come from renewable sources by 2020, and that there should be a 60% reduction in carbon-dioxide emissions by 2050, ensures that there will be a stable market for WEC platforms for at least 40 years.
| CONCLUSIONS |
As a small company, working on a modest budget, OWEL has made remarkable progress in developing a wave energy converter which has good energy conversion efficiency, excellent survival characteristics and very low operational costs. Since the WEC is a floating system, it is easily and cheaply deployed, and equally easily decommissioned. Predicted costs for producing electricity from the first commercial platforms are less than half the central estimates of 22p/25p per kilowatt hour suggested for first generation wave energy converters in the Future Marine Energy report by the Carbon Trust.
Tests of a large-scale physical model, and complementary CFD studies, provide a solid base from which to proceed to the construction and proving of a sea-going WEC demonstrator, and thence to the commissioning of full-scale commercial platforms.
