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The test model seen underwater

 

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Summary

Phase I & II of the development and evaluation trials for the WavePlane wave energy converter (WEC) have now been completed. A total of 500 individual experimental runs were conducted, of which 200 were with the idealised device during Phase I and 300 on the design model for Phase II.

During each stage of the programme different device geometry's were investigated to qualify (Phase I) then quantify (Phase II) their influence on the hydrodynamic characteristics of the machine. A schedule of experimental & design variables is given below. Not all the component options could be accommodated in the two models for either time constraint or practical considerations. Some aspects will require independent testing on special jigs at large scale whilst the influence of others could be accurately inferred from cross analysis of the existing data.

For this stage of the development project the main instrumentation was restricted to the basic device performance criteria of total water volume capture and the induced whirl component to the fluid flow. Visual observations were recorded at all stages & video monitoring of special dye trace test conducted both above and below the still waters level (SWL).

Photographs of the two models are attached, display illustrations show the device

designs together with a lower. operational, in situ shot, Fig 1 & 2.

Methodology

The development and performance evaluation schedule was conducted to the specifications directed by the European Commission's Wave Energy Converter Experimental Protocol, Phases I & II. Basically the requirement is to prove the operational theory and isolate the main hydrodynamic factors in monochromatic waves of appropriate period and height. Accurate power takeoff characteristic modelling is not necessary at this time since it is one of the control variables itself.

Fundamentally the WavePlane device is a novel system that can extract the potential and kinetic energy contained in ocean gravity waves by physically arresting the water wave and then behaving as a dynamic hydro converter. Open ocean WEC's usually operate on the principal of conversion of the bound energy into device motion. The DW-E approach allows the device to be mechanically uncomplicated and conventional, requiring only a standard water turbine for the primary power take-off. However, the hydrodynamics of the machine required investigating to develop the design codes that would enable the inventor to optimise the geometry and component arrangements. These requirements often vary relative to the local wave conditions or the seasonal sea state changes.

Schedule

Phase I ; to investigate the basic control criteria a stylised model was constructed from clear acrylic material. This manufacturing choice enabled through viewing to facilitated both internal and external operating conditions monitoring.

The main aspects of the investigation were combinations of the following variables;

bulletinlet vane configuration
bulletinlet vane number
bulletoutlet duct diameter
bulletoutlet duct intake slot width
bulletdevice immersion depth
bulletartificial beach profile
bulletwave approach direction / device angle
bulletwave period and height (monochromatic)

Performance was assessed for each layout by measuring the wave water volume capture

factor and monitoring the induct flow whirl, by use of a free spinning tachometer.

Extensive dye trace trial were video recorded to observe the water particle path through the machine. Flow marker streamers were also liberally attached to the various component surfaces, both externally and internally. Model scale was 1 : 10.

Phase II; following analysis of the Phase I data a design proposal was developed and a second model constructed. For this stage of testing the device configuration was fixed so a more realistic unit could be manufactured. Due to the more complex geometry required to accommodate the device to wave crest angle of 45o the material choice became a mixture of clear acrylic and opaque aluminium. The model represented one section of one side of the envisaged prototype device at a scale of 1 : 18.

The model also included an extraction tower into which the power take-off duct exhausted. This unit can be seen in Fig.2. to starboard of the inlet section.

The physical parameters to be measure were ;

bullet

water flow axial velocity through the duct

bullet

flow whirl component (calibrated tachometer)

bullet

tower internal pressure (operator & wave induced)

Performance variable for Phase II test were;

bullet

artificial approach beach

bullet

incident wave guide walls

bullet

extraction tower

bullet

inlet vane arrangement

bullet

wave period & height (monochromatic; 8.5 > T > 4.5, 4.5 > H > 0.5))

bullet

real sea spectra

As with the Phase I idealised trials dye trace experiments were conducted to observe the water flow patters at the inlet, exhaust & inside the device.

Results

For both Phases of the device testing the model was fixed. This was regarded as an acceptable approach since a floating, moored unit would have sufficient ballast to respond only to the long period (> 25 seconds) waves and tidal changes.

Assessment of the experimental results is still underway at both HMRC and DW-E. Performance evaluation is based on the ability of the WavePlane to capture the water volume available in the incident wave & the combination of axial velocity and whirl induced in flow through the device. Analysis of these parameters will develop an understanding of the detailed operational characteristics of the device, rather than the less informative evaluation by power input and output comparisons.

Typical axial velocity time series are shown in the oscilloscope traces in Fig 3. The upper graph depicts a high flow rate (T= 5seconds, H= 3.5metres) and the lower diagram a slower but steadier flow (T= 8.5seconds, H= 1.5metres). On each plot the top line indicates the tower internal pressure and bottom line the water velocity at the centre of the outlet pipe.

The mean flow velocity, after averaging the oscillations, was used to calculate the volume flow for each wave period temporal and spatial combination tested. A corresponding available water volume was estimated from the deep water wave measurement such that the capture efficiency for all conditions could be made. The results of the trials are presented in Fig 4. In this example the abscissa is determined as the available water volume (=f{T & H}).

Also presented as Fig 5 is the whirl component of the through flow of water in the device, measured by a tachometer close to the duct exhaust into the tower. Again a typical case is shown with the whirl units in revolutions per minute (RPM) along the graph ordinate axis. In this example the wave characteristic on the abscissa is height, so the periods appears as the family of curves.

Conclusion

Analysis of the test data is still underway with regard to isolating the individual component hydrodynamic & behavioural effects. The ideal combination of layout for the best design that produces the optimal flow pattern to match power take-off turbine characteristics is also being investigated. Since, as with all WEC's, performance relates to the incident wave characteristics considerable variation require assessment and it is expected further testing of a slightly modified design will be required. However, early evaluation of the results do indicate the following aspects.

bulletthe device performs as predicted by the theory
bulletvolumetric capture efficiency was consistently over 30%
bulletvariation of the device geometry can be used to produce the required flow characteristics of axial or rotary motion
bulletlayout options did not influence performance to any significant degree indicating either the device is insensitive to modification or has not yet been optimally configured
bulletmax capture efficiency was 60% @ T= 4.5secs & H= 2.75m
bulletmax whirl was 110 rpm @ T= 5.5secs & H= 4m
bulletmax free kinetic energy flux was 70 kW @ ~T= 5.5secs & H= 4m
bulletmax power efficiency was 36% @ T= 5.5secs & H= 2m
bulletthe Phase II, practical design may be too effective at transmitting the wave since the stern inlet slots did not function as header tanks to smooth out the fluid flow
bulletthe Phase II artificial approach beach requires further investigation
bulletthe max water flow kinetic energy flux consisted of 50% linear and 50% rotary motion
bulletall performance characteristics must be interpreted considering operation in high energy sea states when the device will be fully awash or even submerged

Click for picture Idealised WavePlane model

Click for picture Practical WavePlane model

Click for picture Axial flow velocity profile

Click for picture Capture efficiency Vīs water volume

Click for picture Whirl Vīs wave height