In order to reduce the impact of fossil fuels on our climate, the contribution of renewable energy to energy production must be enhanced. Within the European Union, targets have been set for member countries to produce a percentage of their energy from renewable sources [1
]; for example, the UK must produce 15% of its energy share from renewable sources by 2020. In order to reach the targets set by the EU, alternative energy generation methods must be explored.
Marine renewable energy is a relatively underutilised area of energy extraction, with avenues in offshore wind, tidal stream, tidal range and wave energy available. Worldwide, wave energy potential is estimated to be 17 TW h/year [2
] with the largest concentrations at mid-latitudes, 30° to 60°, which Europe is in a prime position to exploit.
For the potential of wave energy to be fully realised and commercially viable, several fundamental steps must be completed. Firstly, the resource must be assessed at each proposed site, as it can present significant spatial and temporal variation in a local area [3
]. The uncertainty of the resource as well as the potential for weather windows allowing access to the device for operation and maintenance tasks should also be considered [14
]. Secondly, the impact on the local marine environment must be assessed in terms of the effect on the coastline [16
] and the immediate marine ecosystem [19
], amongst others. The above impacts are not necessarily negative, as a wave farm extracting energy from an incoming wave field can protect vulnerable coastlines [20
] or other renewable energy installations [23
]. Thirdly, a WEC must be chosen to suit the conditions in which energy extraction is occurring, both to minimise negative impacts and to efficiently capture energy in a commercially viable manner. Finally, the device must be able to survive at the location it is deployed as the local wave climate will impart wave loading forces to the device, which it must resist [25
This article focuses on the WaveCat WEC and continues from previous proof of concept work [26
] with further physical modelling. The objectives of the experiments were three-fold. First, device motions are examined in preparation for validation of a numerical model and overtopping rates analysed to characterise power performance for the tested waves. Second, baseline data were acquired with which to begin the design optimisation process both physically and numerically. Third, the control systems designed for this model were shown to work in test conditions.
The WaveCat, Figure 1
, is an offshore, floating WEC which operates on the principle of oblique overtopping, where waves impinge the device at an angle, compared to other devices which overtop front-on [30
The WaveCat consists of two symmetrical hulls joined at the stern via a hinge, allowing the relative angle between the hulls to be varied depending on the sea state. In addition, the freeboard of the device decreases along the inner edge towards the stern allowing incoming waves to continue overtopping despite the reduction in height caused by the overtopping itself. Furthermore, the draft and trim of the device can be altered through the use of ballast tanks to adapt to sea states and tune the freeboard to spread overtopping volumes throughout the device. Volumes of overtopping water are collected in onboard tanks contained within the hulls and released through low-head turbines to generate electricity. The overall length of the planned prototype is 90 m and is intended to operate in water depths of between 50 m and 100 m. Typically these water depths are found further offshore, where the low profile of the device will limit visual impacts compared to large offshore structures such as wind turbines.
The device is moored via a single point to the seabed, using a Catenary Anchor Leg Mooring (CALM). This allows the device to orient itself along the direction of wave propagation passively, reducing the need for complex systems devoted to maintaining device direction. The survivability of the device is closely linked to the wedge angle. By reducing the angle to 0°, effectively closing the wedge, the device acts as a single hull body.
3. Results and Discussion
The Qualisys motion capture system recorded motion data for all tests performed, with and without overtopping. Figure 7
shows an example of the main movements of interest to the numerical model: heave and pitch. Heave is the translational movement in the vertical axis and pitch is the rotational motion around the axis parallel to the wave fronts. The device experiences the largest rotational and translational movements when the largest waves impinge on the model.
shows the levels recorded in the starboard overtopping tanks during an example test of Tpm
= 2.19 s and Hsm
= 0.117 m. The rearmost tank fills up first during the test, and the second tank is filled both by larger waves and when the first tank fills and cascades forward. The third tank then fills after the second tank has reached maximum capacity. Ideally the device would overtop at a rate proportional to the volume of the tank and the flow rate capacity, resulting in a constant level within the tank. This presents potential for design optimisation, to be addressed in future experiments.
In the example of the wave spectra from WG1 and WG5, shown in Figure 9
, corresponding to a test with Hsm
= 0.1 m and Tpm
= 1.64 s, the effect of the device interacting with the wave field is apparent. The device attenuates waves of a higher frequency more than those of a lower frequency, showing that there is absorption of energy bands corresponding to higher frequencies within the sea spectra.
The potential power generation can be estimated based on the overtopping flowrate and the water head in the tanks:
is the density of water (ρ
= 1000 kg/m3
is acceleration due to gravity (g
= 9.81 m/s2
is the head of the water in the tanks, Q
is the instantaneous volumetric flowrate and η
is the efficiency of the energy conversion system. Efficiency was assumed to be 75% from the combination of the efficiencies of the individual systems: 85% for the turbine, 95% for the drive and 93% for the generator. The potential power generated for tests with overtopping is presented in Table 2
. The capture width, Cw
, of the device is defined as:
is the power as defined in Equation (1) and Pw
is the power available per metre of wave front, detailed in Table 2
. The WaveCat device, when at a wedge angle of 60°, will collect 3 m of wave crest at model scale and 90 m at prototype scale.
The device generated more potential power at lower values of Tp
for each wave height, and increased as wave height increased, as seen in Figure 10
. Wave steepness is defined by the ratio of Hs
to wavelength, L
, and Figure 11
shows the relationship between Cw
and wave steepness. The potential power generated by the device increases as wave steepness increases. Considering only the tests that experienced significant overtopping, shown with red and black markers, the results show good correlation with R2
= 0.88. The maximum power generated was for Tp
of 9 s and Hs
of 3.0 m, at prototype scale, and would have generated 71 kW at prototype scale.
There is a large difference in overtopping rates between the fore and aft tanks, Figure 8
, with the aft tanks collecting significantly more water. This is true for both hulls due to the models symmetry and the lack of angle spread on the incident waves. It is also observed that lower wave periods generate higher quantities of power, consistent with the spectra attenuation shown in Figure 9
The efficiency of the device, for the device configuration tested, depends on the characteristics of the incoming wave field. With a lower Tp
the device generates more power for similar Hs
values. To compare devices which may have significantly different dimensions, a capture width ratio (CWR) was developed, in which the Cw
of a device is divided by its significant dimension. For the WaveCat the significant dimension is the width of the wave front captured by the wedge. The typical CWR of an overtopping device is approximately 0.17, implying a device will generate power equivalent to 17% of the power contained in the total wave front acting on the device [34
]. This version of the WaveCat device has a CWR of between 2.5% and 0.5% during its most productive states. Whilst this is lower than other overtopping devices it is crucial to note that the device has yet to undergo optimisation tests to increase the power generated for the same incoming wave power, whereas other overtopping devices are prototype testing in real sea conditions. The main factor in reduced CWR is the imbalance in overtopping rates along the device, where the rear tanks were becoming swamped and the fore tanks would experience reduced overtopping until the previous tank was full and cascaded forward. In addition, this study only takes into account a limited set of tests with one constant freeboard (draft) and wedge angle. This configuration has not been optimised for the sea states, which will be performed as part of future experimental and numerical campaigns. Both the freeboard/draft and the wedge angle should be varied according to the sea state to maximise overtopping rates, and this will be addressed in future optimisation. Upcoming tests are likely to produce higher CWR as they approach ideal conditions for the device and the model begins the optimisation process in future design iterations.
To potentially generate more power at lower Hsm, a number of aspects of the device configuration can be altered, and the effect of doing so will require future optimisation. Avenues to consider include widening the wedge angle for smaller Hsm values, and narrowing the angle at larger Hsm values; setting the device to have a freeboard lower at the rear of the device and focusing overtopping into the rearmost tanks; and adjusting the ballast of the device so the entire model sits lower in the water whilst increasing the level at which the valve opens to retain head.
In addition, due to the unequal filling of tanks, several solutions are proposed to explore in future iterations of the model. The first is to increase the size of the rear tanks, reducing the fore tank size, so levels remain equal during overtopping. To perform this efficiently, however, a numerical model would be preferable to test several different tank sizes in several configurations. The second solution is to use larger pipes on the rear tanks to allow the overtopped water to drain faster, again the specific pipe gauge would require optimisation. The third solution is to link the tanks so that water can be transferred between, allowing a uniform distribution of overtopping even with unequal overtopping rates along the hull. The tank walls, in this case, would act as slosh buffers and the model would effectively be one tank with several outlets.
As indicated, several key features of the device must be optimised individually and as a whole to ensure the optimum overtopping rates for any sea state, following similar optimisation pathways to those of other WECs, e.g., Wave Dragon [31
] or SSG [32
]. Features that require optimisation include, but are not limited to: the freeboard/draft, trim, size of tanks relative to both the device and one another, geometry of the freeboard, wedge angle, and overall shape of the hulls of the device.
A numerical model using the presented data, along with the 6DOF motion data, is currently under development with the aim of providing optimisation routines for power generation. Once the design is optimised through the numerical model, a more exhaustive campaign of tests will be carried out to fully determine the power generation properties of the device.
An experimental campaign of the WaveCat WEC at a 1:30 scale was conducted in the Ocean Basin at the University of Plymouth. The campaign had three objectives: to obtain 6DOF data for the calibration of a numerical model; to form a basis for future optimisation of power conversion rates by measuring the power generation for this specific configuration of tanks and freeboard; and to test the concept of the on-board control system.
The device experienced overtopping for waves above Hsm of 0.083 m, and more overtopping at lower Tpm. The CWR of the device was between 0.5% and 2.5% for Hsm of 0.1 m, showing the need for device optimisation.
Three main conclusions were drawn from these experimental tests. First, for a given Hsm the power generation depends heavily on Tpm. The wave period that led to the greatest potential power was Tpm = 1.64 s; both Hsm of 0.083 m and 0.1 m saw the potential power generated drop at higher Tpm values. The Cw of the device depends on the wave steepness, with the greatest Cw value corresponding with the greatest wave steepness tested and showing positive correlation. Second, the model tanks must be optimised to accept greater quantities of water in the rear tanks compared to the fore tanks, with exact values determined from a numerical modelling campaign based on the experimental results presented in this paper. Third, and most critically, the optimisation of the model is crucial to the device’s ability to generate power and a greater CWR, and steps towards this goal have been outlined.