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The new wave energy converter WaveCat: Concept and laboratory tests
Marine Structures 29 (2012) 58–70
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Marine Structures journal homepage: www.elsevier.com/locate/ marstruc
The new wave energy converter WaveCat: Concept and laboratory tests H. Fernandez a, G. Iglesias b, *, R. Carballo a, A. Castro a, J.A. Fraguela c, F. Taveira-Pinto d, M. Sanchez a a
Univ. of Santiago de Compostela, Hydraulic Eng., EPS, Campus Univ. s/n, 27002 Lugo, Spain University of Plymouth, School of Marine Science and Engineering, Marine Building, Drakes Circus, Plymouth PL4 8AA, United Kingdom c Univ. of A Coruña, Naval Architecture and Ocean Eng., EPS, Mendizábal s/n, 15403 Ferrol, Spain d Univ. of Porto, Inst. Hydraulics & Water Resources, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2011 Received in revised form 17 July 2012 Accepted 1 October 2012
For wave energy to become a fully-fledged renewable, efficient and reliable Wave Energy Converters (WECs) must be developed. The objectives of this article are to present WaveCat, a recently patented WEC, and its proof of concept by means of an experimental campaign in a large wave tank. WaveCat is a floating WEC whose principle of operation is oblique overtopping; designed for offshore deployment (in 50–100 m of water), it has two significant advantages: minimum (if at all) impact on the shoreline, and access to a greater resource than nearshore or shoreline WECs. It consists of two hulls, like a catamaran (hence its name); unlike a catamaran, however, these hulls are not parallel but converging. Using a single-point mooring to a CALM buoy, the bows of WaveCat are held to sea, so incident waves propagate into the space between the hulls. Eventually, wave crests overtop the inner hull sides, and overtopping water is collected in reservoirs at a level higher than the (outer) sea level. As the water is drained back to sea, it drives turbine-generator groups. The freeboard and draught, as well as the angle between the hulls, can be varied depending on the sea state. After preliminary tests with a fixed model of WaveCat in a wave flume, which constituted the first step in the development of the WaveCat patent, in this work a floating model was tested in a large wave tank. In addition to serving as a proof of concept of the WaveCat model, this experimental
Keywords: Wave energy Wave energy converter Overtopping Physical modelling Renewable energy
* Corresponding author. E-mail address: [email protected] (G. Iglesias). 0951-8339/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marstruc.2012.10.002
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campaign allowed to gather data that will be used to calibrate and validate a numerical model with which to optimise the design. In addition, it was found in the tests that the overtopping rates (and, therefore, the power performance) greatly depended on the angle between hulls, so that the possibility of varying this angle (as contemplated in the patent) should indeed be incorporated into the prototype. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The intense consumption of fossil fuels has well-known environmental repercussions, so it is necessary to enhance the contribution of renewables to the total energy production as rapidly as possible. In this line, many regions and countries in the world have specified new renewable energy targets, e.g. 20% of renewable energy by 2020 in the European Union, or 30% by 2050 in Denmark [1]. In order to reach these targets, the contribution of the renewable energies currently under exploitation, such as wind or PV, must be increased, and the new renewable energies that are not yet being exploited commercially (or only barely so) should be developed. Among the latter, wave energy is one of the most promising [2,3]. Ocean waves are generated by the wind blowing over the ocean surface, so wave energy may be seen as a concentrated form of wind energy [4]. With respect to other renewables, wave energy presents a number of advantages: high resource predictability, high power density, relatively high utilisation factor, and low environmental and visual impact relative to another renewable energies. With regard to tidal stream energy, there exist globally many more potential sites for a wave farm than for a tidal energy plant because strong tidal currents occur only in a relatively small number of areas (e.g [5].). For these reasons, wave energy is widely regarded as one of the renewable energy sources with the greatest potential for development over the next few years – for instance, the European Science Foundation estimates that “by 2050 Europe could source up to 50% of its electricity needs from Marine Renewable Energy” [6] – and intensive work is being devoted to the assessment of the wave energy resource in many regions worldwide [7–20]. However, for wave energy exploitation to become a fully-fledged renewable, action along two lines is necessary [21]. On the one hand, the distribution of the wave energy resource must be assessed in detail; and, on the other hand, Wave Energy Converters (WECs) must be further developed in order to get a high level of efficiency and reliability – both aspects being fundamental for the economic viability of wave energy. This article deals with WaveCat, a recently patented WEC. Its main objectives are two: to explain the WaveCat concept, and to present the physical model tests carried out as a proof of concept. WaveCat is an offshore floating WEC whose principle of operation is oblique wave overtopping (Fig. 1). It consists of two hulls, like a catamaran (hence its name). Unlike a catamaran, however, the hulls are not parallel but converging, forming a wedge in the plan view; they are joined at the stern by a hinge, which allows the angle between them to be varied depending on the sea state. The freeboard and draught can also be varied according to the sea state. Moreover, the freeboard varies along with the length of each hull, decreasing towards the stern so that overtopping continues as the wave propagates between the hulls (with a constant freeboard, the decrease in the height of the wave crest due to the initial overtopping could interrupt the overtopping before the wave reaches the stern reservoirs). The overall length of the prototype is 90 m. WaveCat is intended to operate offshore, in water depths between 50 and 100 m, where the wave energy resource is larger than in nearshore or onshore locations. Another advantage of the offshore deployment is the low environmental impact relative to nearshore or onshore WECs, and in particular, the low visual impact of WaveCat because of its floating nature and the absence of superstructures. The mooring system is a crucial part for the operation of an offshore WEC; it must be reliable and guarantee the WEC’s survivability under storm conditions [22]. WaveCat is moored to a catenary-buoy using a CALM configuration (Catenary Anchor Leg Mooring). This single-point mooring system ensures
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Fig. 1. Schematic of the WaveCat concept.
that WaveCat swings when the wind and wave direction changes, so that incident waves always propagate into the wedge between the hulls (Fig. 2). As regards survivability, the WaveCat concept presents the advantage of the variable wedge angle. When a storm approaches, the wedge is closed (the wedge angle is reduced to 0 ), so the device is similar to a monohull. As explained, overtopping occurs progressively as waves advance between the hulls, in a process that can be termed as oblique overtopping, for the direction of wave propagation forms a relatively small angle with the overtopped structure (the inner hull sides). This process is radically different from the frontal overtopping on which other designs of overtopping WECs are based, which occurs when the wave encounters a ramp placed perpendicular to its direction of propagation [23–26]. The main difference concerns the interaction between the wave and the WEC: with oblique overtopping, the process is distributed over some length (part of the hull length) along which the wave continues to propagate, while with frontal overtopping the process is concentrated at a point (the ramp). This difference signifies that the results of laboratory tests carried out with other overtopping WECs (based on frontal overtopping) could not be applied to WaveCat; therefore, specific physical model tests had to be carried out. Preliminary tests had been conducted with a fixed model of WaveCat [27] in the wave
Fig. 2. Plan view of the CALM (Catenary Anchor Leg Mooring) mooring system.
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flume of the University of Santiago de Compostela. The new tests presented and analysed in this work constitute a clear improvement with respect to the preliminary experimental campaign in that they were carried out on a floating model with a Catenary Anchor Leg Mooring (CALM). The WaveCat technology presents a number of advantages relative to other WECs. The hinge allowing to vary the angle between the hulls is normally stationary; indeed, the angle is only modified to adapt the configuration to changes in the sea state, but not to individual waves. Thus, the only parts of WaveCat that are continuously moving are the ultra-low-head turbines and the generators coupled to them – well-proven, reliable equipment; this may naturally be expected to result in greater reliability and lower maintenance costs, in particular compared to other WECs in which complex articulations move with the passage of each wave. Moreover, oblique overtopping has a number of advantages. To begin with, the structural loads imposed upon the device are far smaller than in the case of frontal overtopping. In addition, the effect of the motions of the device on the overtopping rates (and, hence, the power performance) may be expected to be far smaller. Indeed, in the case of frontal overtopping, pitch and, in particular, heave may substantially reduce the overtopping rates under certain wave conditions, if the device moves in phase with the waves. This reduction, which is made possible by the fact that overtopping occurs at only one point in the direction of wave propagation (the point where the ramp is located), is avoided in the case of WaveCat by substituting frontal overtopping by oblique overtopping, and thereby distributing the overtopping along a section of the hull length. Thus, the main effect of pitch and heave is a mere displacement of the point along the hulls at which overtopping starts, with only minor repercussions on the actual overtopping volumes. This article is structured as follows. The physical model, experimental setup and test campaign are described in Section 2. The results are presented and discussed in Section 3, which includes also a discussion on the advantages of the WaveCat concept. Finally, conclusions are drawn in Section 4.
2. Material and methods 2.1. Physical model, wave tank and motion capture system A 3D model of marine board was constructed at a 1:30 scale (Fig. 3). The main dimensions of each hull, in model scale, are: length overall, 3 m; beam, 0.4 m; hull height, 0.4 m; freeboard, 0.04 m; draught, 0.18 m (Fig. 4). The displacement of the whole model, also in model scale, is 360 kg. The inner hull sides have variable freeboard, decreasing towards the stern (Fig. 4) – the freeboard is 0.01 m smaller at the aft reservoir relative to the fore reservoir. The 1:30 scale was chosen as a compromise between the advantages of a small scale ratio (which reduces scale effects) and the wave generation capabilities and dimensions of the wave tank of the Faculty of Engineering of the University of Porto (Portugal), where the tests were conducted. The wave tank dimensions are 28 12 1.25 m; at its centre is a pit with a depth of 1.5 m relative to the tank floor and dimensions of 4.5 2 m in plan view
Fig. 3. The physical model in the wave tank with its four reservoirs for collecting overtopping water.
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Fig. 4. Plan and lateral view of the model (dimensions in mm and model scale).
(Fig. 5). This pit was used for the anchoring system – a catenary anchor leg mooring (CALM) system anchored at the pit bottom (Fig. 6). The characteristics of the mooring lines are shown in Table 1. Wave generation was conducted with a multi-element piston-type wave maker composed of two modules, each with eight 74 cm wide paddles. Moreover, a motion capture system was implemented in order to register the main motions of the device (heave, pitch and roll); this system is based on the detection of reflective marks (5 in total) placed on the model by means of three infrared cameras (strategically located around the tank); once the marks are detected (Fig. 7) the software creates a rigid body. The system monitors the marks in order to measure the translations and rotations of the model. For illustration, the time series of heave, pitch and roll during part of a test (AD07_I3) are presented in Fig. 8. 2.2. Experimental setup and test campaign Experimental tests were carried out with a constant water depth of 0.9 m (2.4 m at the tank pit). The physical model was placed at a distance of approx. 13.6 m from the wave maker (Fig. 5). The experimental setup consisted of three groups of wave gauges (six gauges in total) aligned with the centreline of the wave tank. The first group was composed of one gauge, WG1, close to the wave maker (2.7 m); the second group consisted of three wave gauges, WG2, WG3 and WG4, whose measurements would
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Fig. 5. Wave tank layout and experimental setup, showing the WaveCat model and the location of the wave gauges outside the model (WG1 to WG6). [Dimensions in m].
be used for performing the incident-reflected wave analysis. Finally, the third group consisted of one gauge to monitor the waves within the wedge (WG5) and another to measure their transmission past the model (WG6); for illustration, the recordings of these gauges in part of a test are shown in Fig. 9. Each reservoir in the model was equipped with a water level control system formed by a bilge pump (working at a constant flow rate), a resistance wave gauge and an electronic level control system (Fig. 10). The pumping system began to work when the water level was at its maximum (0.07 m from the bottom of the reservoir) and stopped when it reached its minimum (0.02 m from the bottom of the reservoir). The performance of this water level control system can be seen in Fig. 11, in which the variation of the water level in the two reservoirs of a hull throughout a test is shown alongside the displacements of the free surface recorded by the wave gauge between both hulls (WG5). The overtopping events and intervals of operation of the pumps (the quasivertical lines), are clearly visible. 25 tests were performed with significant wave heights in the range 0.067–0.100 m in model values (2.0–3.0 m in prototype values) and peak periods of 1.83 and 2.20 s in model values (10 and 12 s in
Fig. 6. Longitudinal section of the wave tank showing the model and the CALM system. [Dimensions in m].
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Table 1 Characteristics of the mooring lines employed in the anchoring system (dimensions in model scale). Wire
l (m)
D (mm)
Wu (N/m)
Bl (tn)
1 2 3
3 3 3.85
4 4 4
0.79 0.79 0.79
1.35 1.35 1.35
prototype values); these values were selected based on the wave climate of Galicia, NW Spain [8,28,29], where the WaveCat prototype will first be deployed. The JONSWAP spectrum was used to generate the waves, with a peaked ness parameter g ¼ 3.3. As regards the WaveCat configuration, four values of the angle between hulls (a ¼ 30 , 45 , 60 , and 90 ) and three values of the freeboard (Fm ¼ 0.04 m, 0.09 m, and 0.10 m, in model values, or Fp ¼ 1.20 m, 2.70 m, and 3.00 m, in prototype values) were tested. (Given that the freeboard varies along the length of the inner hull sides, decreasing towards the stern, the value used as a reference to describe a test is that at the aft end of the aft reservoirs, or the minimum freeboard). The tests with a freeboard Fp of 1.20 m were aimed at investigating the WaveCat in operation (i.e., in a configuration prone to overtopping), whereas those with the larger freeboards, 2.70 m and 3.00 m, were aimed at studying its seakeeping response while not in operation (without overtopping); indeed, no overtopping occurred in the latter. Of the tests carried out with the operational freeboard of 1.20 m, significant overtopping occurred only with the significant wave heights of 2.50 and 3.00 m waves; it is these tests simulating the WaveCat in operation, and its power performance, that are the object of the present article. The parameters of these tests and the overtopping and power data are shown in Table 2 and discussed below. 3. Results and discussion Before analysing the overtopping data, it is worth commenting on the interaction of the WaveCat model with the waves. The group of three wave gauges in front of the model (WG2, WG3 and WG4) was used, as explained, to separate the incident and reflected waves by means of the Baquerizo method [30]. On these grounds, the reflection coefficient was computed as follows:
Kr ¼
Hsr ; Hsi
(1)
Fig. 7. Motion capture system. The reflective white marks placed on the stern of the model (left) are detected by the motion capture system (right).
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Displacement [m]
Heave 0.1 0.05 0 −0.05 −0.1 800
850
900
950
1000 Time[s] Pitch
1050
1100
1150
1200
850
900
950
1000 Time[s] Roll
1050
1100
1150
1200
850
900
950
1000 Time[s]
1050
1100
1150
1200
Angle[º]
5
0
−5 800
Angle[º]
1 0 −1 800
Fig. 8. Heave, pitch and roll during part of a test (AD07_I3). (Dimensions in model scale).
where Hsr and Hsi are, respectively, the significant wave heights of the reflected and incident waves. It was found that the reflection coefficient did not present significant variations (Table 3); it is in the range 0.42–0.47 in all the tests, with the highest value corresponding to the tests with the largest angle between hulls (a ¼ 90 ), as might be expected. As regards wave transmission, the transmission coefficient is defined by:
Kt ¼
Hst Hsi
(2)
where Hst is the significant wave height at WG6 (the wave gauge in the lee of the model). The transmission coefficient did not vary significantly, Kt z 0.8, with the exception of one test (AE04_I3) in which it was lower (Table 3). Overtopping rates and energy production data for the overtopping tests described above (Section 2.2) are presented in Table 2 and Fig. 12. The power, P, was obtained from:
P ¼ rg qa Fa þ qf Ff ;
(3)
where r is the density of seawater (r ¼ 1025 kg m3), g is the acceleration of gravity (g ¼ 9.81 ms2), qa and qf are, respectively, the average overtopping rates of the aft and fore reservoirs; and Fa and Ff are the freeboards at the centre of the aft and fore reservoirs, respectively (Fa ¼ 1.275 m, Ff ¼ 1.425 m). Finally, the power per metre of wave crest was computed from:
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Wg5
Free surface position[m]
0.1 0.05 0 -0.05 -0.1 200
250
300
350
450
500
550
600
450
500
550
600
Wg6
0.1 Free surface position[m]
400 Time[s]
0.05 0 -0.05 -0.1 200
250
300
350
400 Time[s]
Fig. 9. Water level time series recorded by WG5 and WG6 during part of test AD07_I5.
Fig. 10. Pump and control system in one of the water reservoirs of the model.
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Table 2 Test parameters (a, angle between hulls; Hs, significant wave height; Tp, peak wave period) and overtopping and power data in prototype values (qa and qf, average overtopping rates at the aft (R2 þ R3) and fore (R1 þ R4) reservoirs, respectively; qt, total overtopping rate; P, power). (Prototype dimensions). Test case
a ( )
Hs (m)
Tp (s)
qa (m3 s1)
qf (m3 s1)
qt (m3 s1)
P (kW)
AE07_I3 AE07_I5 AD07_I3 AD07_I5 AA07_I3 AA07_I5 AB07_I3 AB07_I5
30 30 45 45 60 60 90 90
2.50 3.00 2.50 3.00 2.50 3.00 2.50 3.00
11.0 12.0 11.0 12.0 11.0 12.0 11.0 12.0
0.976 1.360 1.637 2.000 0.798 2.232 0.275 0.572
0.343 0.622 0.254 0.838 0.105 1.010 0.152 0.413
1.319 1.982 1.891 2.838 0.903 3.242 0.427 0.985
17.42 26.35 24.63 37.64 11.74 43.08 5.71 13.25
Table 3 Reflection (Kr) and transmission (Kt) coefficients. [The wave conditions for each test are given in Table 2]. Test case
a ( )
Kr ()
Kt ()
AA04_I3 AA04_I5 AD04_I3 AD04_I5 AE04_I3 AE04_I5 AB04_I3 AB04_I5
30 30 45 45 60 60 90 90
0.425 0.438 0.441 0.428 0.421 0.431 0.470 0.470
0.806 0.777 0.777 0.756 0.507 0.760 0.749 0.772
Pw ¼
P ; W
(4)
where w is the length of wave crest captured by the device, i.e. the beam between its bows (which depends on the angle between hulls?). Overall, the values of power output are indicative of a limited efficiency, as is usually the case of a nascent technology. As explained from the outset, these tests were primarily aimed at providing a proof of concept of the WaveCat patent. As a continuation of this line of research, a numerical model, calibrated and validated on the basis of the results of these laboratory tests, will be used to optimise the WaveCat design. Given the preliminary nature of the design tested, there is clearly room for improvement. Best results were obtained with the intermediate values of the angle between hulls, a ¼ 45 and a ¼ 60 . For wave condition I5 (Hs ¼ 3.0 m, Tp ¼ 12.0 s) the optimum value of the angle between hulls is 60 , whereas for wave condition I3 (Hs ¼ 2.5 m, Tp ¼ 11.0 s) the optimum is 45 . The largest angle (90 ) led to substantially lower performances in both cases. As might be expected given the symmetry of the WaveCat design, the difference in overtopping rates between the starboard and port reservoirs was well below the level of statistical significance, hence they were not taken into account for the computation of the power performance. However, significant differences occurred between the aft and fore reservoirs. The aft reservoirs consistently experienced larger overtopping that their fore counterparts, as may be seen by comparing the data for R3 (aft) and R4 (fore) in Table 2. This has, naturally, implications for the turbine-generator groups to be installed – those connected to the aft reservoirs should have a greater power rating. Alternatively, the volume (and length along the hulls) of the aft reservoirs could be increased at the expense of the fore reservoirs, so that similar overtopping rates occurred – in which case the same turbine-generator groups should be installed at all four reservoirs. By which amount the volumes of the aft and fore reservoirs should be modified is a question for the numerical model of WaveCat, currently under development; once calibrated and validated with the results of the present tests, it will be used to optimise the design of WaveCat.
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Water level [m]
Reservoir 3 0.08 0.06 0.04 0.02 0
500
1000
1500
1000
1500
1000
1500
Water level [m]
Time[s] Reservoir 4 0.08 0.06 0.04 0.02
Free surface position[m]
0
0
500 Time[s] Wg5
0.1 0.05 0 −0.05 −0.1
0
500 Time[s]
Fig. 11. Water level in reservoirs #3 (aft, starboard) and #4 (fore, starboard) and free surface between the hulls recorded by wave gauge WG5 during test AD07_I5.
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0
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Fig. 12. Power performance (P) vs. angle between hulls (a) for sea states I3 and I5.
4. Conclusions An experimental campaign of a 1:30 model of WaveCat was conducted in a large wave tank. Its objective was not to encompass the full variability of wave conditions and WaveCat configurations; this would have made little sense at this early stage of development of the patent because the design available is preliminary, and must yet be optimised for performance. For this reason, the objectives of the laboratory tests were: (i) to provide a proof of concept of WaveCat as a viable technology to convert wave energy into electricity using the novel approach of oblique overtopping; (ii) to obtain a preliminary assessment of its power performance – preliminary because the number of tests carried out was limited and, most importantly, the design tested was itself preliminary – and of the dependence (or otherwise) of the overtopping rates on the angle between the hulls and the wave conditions; and (iii) to gather experimental data with which to calibrate and validate a numerical model. These objectives accomplished, the next step in the development of the patent is the implementation, calibration, and validation of a 3D numerical model, which will be used to optimise the design of WaveCat. The optimised design will then be subjected to exhaustive laboratory tests and, subsequently, to sea trials. Four main conclusions can be extracted from the results of the physical model tests. First, the WaveCat patent is a valid concept to extract energy from waves. This proof of concept is the logical first step in the development of a new WEC. Second, for any particular value of the angle between hulls, the larger the wave height and period, the greater the overtopping rate; however, this does not hold true if any value of the angle between hulls is considered – for the sea state I3 (Hs ¼ 2.5 m and Tp ¼ 11.0 s) the overtopping rates with angles of 30 and 45 were greater than that for a sea state of smaller wave height, I5 (Hs ¼ 3.0 m, Tp ¼ 12.0 s) with an angle of 90 ; indeed, it was found that the overtopping rate presents a high sensitivity to the angle between hulls. Third, the optimum value of the angle between hulls – which may be defined as the value that maximizes the overtopping rate, and therefore, the power output for a given sea state – was found to depend on the sea state; for the wave conditions tested, the optimum values were 60 and 45 , the larger and smaller angle being best suited for the larger and smaller waves, respectively. Furthermore, in view of the latter conclusions, a fourth conclusion can be drawn, namely the importance of incorporating into the prototype the possibility of varying the angle between the hulls, as contemplated in the patent. Acknowledgements This research was supported by the Government of Galicia (Xunta de Galicia) through its Energy and Mining Resources programme (Contract No. PGIDIT07REM001CT)
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Marine Structures journal homepage: www.elsevier.com/locate/ marstruc
The new wave energy converter WaveCat: Concept and laboratory tests H. Fernandez a, G. Iglesias b, *, R. Carballo a, A. Castro a, J.A. Fraguela c, F. Taveira-Pinto d, M. Sanchez a a
Univ. of Santiago de Compostela, Hydraulic Eng., EPS, Campus Univ. s/n, 27002 Lugo, Spain University of Plymouth, School of Marine Science and Engineering, Marine Building, Drakes Circus, Plymouth PL4 8AA, United Kingdom c Univ. of A Coruña, Naval Architecture and Ocean Eng., EPS, Mendizábal s/n, 15403 Ferrol, Spain d Univ. of Porto, Inst. Hydraulics & Water Resources, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2011 Received in revised form 17 July 2012 Accepted 1 October 2012
For wave energy to become a fully-fledged renewable, efficient and reliable Wave Energy Converters (WECs) must be developed. The objectives of this article are to present WaveCat, a recently patented WEC, and its proof of concept by means of an experimental campaign in a large wave tank. WaveCat is a floating WEC whose principle of operation is oblique overtopping; designed for offshore deployment (in 50–100 m of water), it has two significant advantages: minimum (if at all) impact on the shoreline, and access to a greater resource than nearshore or shoreline WECs. It consists of two hulls, like a catamaran (hence its name); unlike a catamaran, however, these hulls are not parallel but converging. Using a single-point mooring to a CALM buoy, the bows of WaveCat are held to sea, so incident waves propagate into the space between the hulls. Eventually, wave crests overtop the inner hull sides, and overtopping water is collected in reservoirs at a level higher than the (outer) sea level. As the water is drained back to sea, it drives turbine-generator groups. The freeboard and draught, as well as the angle between the hulls, can be varied depending on the sea state. After preliminary tests with a fixed model of WaveCat in a wave flume, which constituted the first step in the development of the WaveCat patent, in this work a floating model was tested in a large wave tank. In addition to serving as a proof of concept of the WaveCat model, this experimental
Keywords: Wave energy Wave energy converter Overtopping Physical modelling Renewable energy
* Corresponding author. E-mail address: [email protected] (G. Iglesias). 0951-8339/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marstruc.2012.10.002
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campaign allowed to gather data that will be used to calibrate and validate a numerical model with which to optimise the design. In addition, it was found in the tests that the overtopping rates (and, therefore, the power performance) greatly depended on the angle between hulls, so that the possibility of varying this angle (as contemplated in the patent) should indeed be incorporated into the prototype. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The intense consumption of fossil fuels has well-known environmental repercussions, so it is necessary to enhance the contribution of renewables to the total energy production as rapidly as possible. In this line, many regions and countries in the world have specified new renewable energy targets, e.g. 20% of renewable energy by 2020 in the European Union, or 30% by 2050 in Denmark [1]. In order to reach these targets, the contribution of the renewable energies currently under exploitation, such as wind or PV, must be increased, and the new renewable energies that are not yet being exploited commercially (or only barely so) should be developed. Among the latter, wave energy is one of the most promising [2,3]. Ocean waves are generated by the wind blowing over the ocean surface, so wave energy may be seen as a concentrated form of wind energy [4]. With respect to other renewables, wave energy presents a number of advantages: high resource predictability, high power density, relatively high utilisation factor, and low environmental and visual impact relative to another renewable energies. With regard to tidal stream energy, there exist globally many more potential sites for a wave farm than for a tidal energy plant because strong tidal currents occur only in a relatively small number of areas (e.g [5].). For these reasons, wave energy is widely regarded as one of the renewable energy sources with the greatest potential for development over the next few years – for instance, the European Science Foundation estimates that “by 2050 Europe could source up to 50% of its electricity needs from Marine Renewable Energy” [6] – and intensive work is being devoted to the assessment of the wave energy resource in many regions worldwide [7–20]. However, for wave energy exploitation to become a fully-fledged renewable, action along two lines is necessary [21]. On the one hand, the distribution of the wave energy resource must be assessed in detail; and, on the other hand, Wave Energy Converters (WECs) must be further developed in order to get a high level of efficiency and reliability – both aspects being fundamental for the economic viability of wave energy. This article deals with WaveCat, a recently patented WEC. Its main objectives are two: to explain the WaveCat concept, and to present the physical model tests carried out as a proof of concept. WaveCat is an offshore floating WEC whose principle of operation is oblique wave overtopping (Fig. 1). It consists of two hulls, like a catamaran (hence its name). Unlike a catamaran, however, the hulls are not parallel but converging, forming a wedge in the plan view; they are joined at the stern by a hinge, which allows the angle between them to be varied depending on the sea state. The freeboard and draught can also be varied according to the sea state. Moreover, the freeboard varies along with the length of each hull, decreasing towards the stern so that overtopping continues as the wave propagates between the hulls (with a constant freeboard, the decrease in the height of the wave crest due to the initial overtopping could interrupt the overtopping before the wave reaches the stern reservoirs). The overall length of the prototype is 90 m. WaveCat is intended to operate offshore, in water depths between 50 and 100 m, where the wave energy resource is larger than in nearshore or onshore locations. Another advantage of the offshore deployment is the low environmental impact relative to nearshore or onshore WECs, and in particular, the low visual impact of WaveCat because of its floating nature and the absence of superstructures. The mooring system is a crucial part for the operation of an offshore WEC; it must be reliable and guarantee the WEC’s survivability under storm conditions [22]. WaveCat is moored to a catenary-buoy using a CALM configuration (Catenary Anchor Leg Mooring). This single-point mooring system ensures
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Fig. 1. Schematic of the WaveCat concept.
that WaveCat swings when the wind and wave direction changes, so that incident waves always propagate into the wedge between the hulls (Fig. 2). As regards survivability, the WaveCat concept presents the advantage of the variable wedge angle. When a storm approaches, the wedge is closed (the wedge angle is reduced to 0 ), so the device is similar to a monohull. As explained, overtopping occurs progressively as waves advance between the hulls, in a process that can be termed as oblique overtopping, for the direction of wave propagation forms a relatively small angle with the overtopped structure (the inner hull sides). This process is radically different from the frontal overtopping on which other designs of overtopping WECs are based, which occurs when the wave encounters a ramp placed perpendicular to its direction of propagation [23–26]. The main difference concerns the interaction between the wave and the WEC: with oblique overtopping, the process is distributed over some length (part of the hull length) along which the wave continues to propagate, while with frontal overtopping the process is concentrated at a point (the ramp). This difference signifies that the results of laboratory tests carried out with other overtopping WECs (based on frontal overtopping) could not be applied to WaveCat; therefore, specific physical model tests had to be carried out. Preliminary tests had been conducted with a fixed model of WaveCat [27] in the wave
Fig. 2. Plan view of the CALM (Catenary Anchor Leg Mooring) mooring system.
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flume of the University of Santiago de Compostela. The new tests presented and analysed in this work constitute a clear improvement with respect to the preliminary experimental campaign in that they were carried out on a floating model with a Catenary Anchor Leg Mooring (CALM). The WaveCat technology presents a number of advantages relative to other WECs. The hinge allowing to vary the angle between the hulls is normally stationary; indeed, the angle is only modified to adapt the configuration to changes in the sea state, but not to individual waves. Thus, the only parts of WaveCat that are continuously moving are the ultra-low-head turbines and the generators coupled to them – well-proven, reliable equipment; this may naturally be expected to result in greater reliability and lower maintenance costs, in particular compared to other WECs in which complex articulations move with the passage of each wave. Moreover, oblique overtopping has a number of advantages. To begin with, the structural loads imposed upon the device are far smaller than in the case of frontal overtopping. In addition, the effect of the motions of the device on the overtopping rates (and, hence, the power performance) may be expected to be far smaller. Indeed, in the case of frontal overtopping, pitch and, in particular, heave may substantially reduce the overtopping rates under certain wave conditions, if the device moves in phase with the waves. This reduction, which is made possible by the fact that overtopping occurs at only one point in the direction of wave propagation (the point where the ramp is located), is avoided in the case of WaveCat by substituting frontal overtopping by oblique overtopping, and thereby distributing the overtopping along a section of the hull length. Thus, the main effect of pitch and heave is a mere displacement of the point along the hulls at which overtopping starts, with only minor repercussions on the actual overtopping volumes. This article is structured as follows. The physical model, experimental setup and test campaign are described in Section 2. The results are presented and discussed in Section 3, which includes also a discussion on the advantages of the WaveCat concept. Finally, conclusions are drawn in Section 4.
2. Material and methods 2.1. Physical model, wave tank and motion capture system A 3D model of marine board was constructed at a 1:30 scale (Fig. 3). The main dimensions of each hull, in model scale, are: length overall, 3 m; beam, 0.4 m; hull height, 0.4 m; freeboard, 0.04 m; draught, 0.18 m (Fig. 4). The displacement of the whole model, also in model scale, is 360 kg. The inner hull sides have variable freeboard, decreasing towards the stern (Fig. 4) – the freeboard is 0.01 m smaller at the aft reservoir relative to the fore reservoir. The 1:30 scale was chosen as a compromise between the advantages of a small scale ratio (which reduces scale effects) and the wave generation capabilities and dimensions of the wave tank of the Faculty of Engineering of the University of Porto (Portugal), where the tests were conducted. The wave tank dimensions are 28 12 1.25 m; at its centre is a pit with a depth of 1.5 m relative to the tank floor and dimensions of 4.5 2 m in plan view
Fig. 3. The physical model in the wave tank with its four reservoirs for collecting overtopping water.
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Fig. 4. Plan and lateral view of the model (dimensions in mm and model scale).
(Fig. 5). This pit was used for the anchoring system – a catenary anchor leg mooring (CALM) system anchored at the pit bottom (Fig. 6). The characteristics of the mooring lines are shown in Table 1. Wave generation was conducted with a multi-element piston-type wave maker composed of two modules, each with eight 74 cm wide paddles. Moreover, a motion capture system was implemented in order to register the main motions of the device (heave, pitch and roll); this system is based on the detection of reflective marks (5 in total) placed on the model by means of three infrared cameras (strategically located around the tank); once the marks are detected (Fig. 7) the software creates a rigid body. The system monitors the marks in order to measure the translations and rotations of the model. For illustration, the time series of heave, pitch and roll during part of a test (AD07_I3) are presented in Fig. 8. 2.2. Experimental setup and test campaign Experimental tests were carried out with a constant water depth of 0.9 m (2.4 m at the tank pit). The physical model was placed at a distance of approx. 13.6 m from the wave maker (Fig. 5). The experimental setup consisted of three groups of wave gauges (six gauges in total) aligned with the centreline of the wave tank. The first group was composed of one gauge, WG1, close to the wave maker (2.7 m); the second group consisted of three wave gauges, WG2, WG3 and WG4, whose measurements would
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Fig. 5. Wave tank layout and experimental setup, showing the WaveCat model and the location of the wave gauges outside the model (WG1 to WG6). [Dimensions in m].
be used for performing the incident-reflected wave analysis. Finally, the third group consisted of one gauge to monitor the waves within the wedge (WG5) and another to measure their transmission past the model (WG6); for illustration, the recordings of these gauges in part of a test are shown in Fig. 9. Each reservoir in the model was equipped with a water level control system formed by a bilge pump (working at a constant flow rate), a resistance wave gauge and an electronic level control system (Fig. 10). The pumping system began to work when the water level was at its maximum (0.07 m from the bottom of the reservoir) and stopped when it reached its minimum (0.02 m from the bottom of the reservoir). The performance of this water level control system can be seen in Fig. 11, in which the variation of the water level in the two reservoirs of a hull throughout a test is shown alongside the displacements of the free surface recorded by the wave gauge between both hulls (WG5). The overtopping events and intervals of operation of the pumps (the quasivertical lines), are clearly visible. 25 tests were performed with significant wave heights in the range 0.067–0.100 m in model values (2.0–3.0 m in prototype values) and peak periods of 1.83 and 2.20 s in model values (10 and 12 s in
Fig. 6. Longitudinal section of the wave tank showing the model and the CALM system. [Dimensions in m].
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Table 1 Characteristics of the mooring lines employed in the anchoring system (dimensions in model scale). Wire
l (m)
D (mm)
Wu (N/m)
Bl (tn)
1 2 3
3 3 3.85
4 4 4
0.79 0.79 0.79
1.35 1.35 1.35
prototype values); these values were selected based on the wave climate of Galicia, NW Spain [8,28,29], where the WaveCat prototype will first be deployed. The JONSWAP spectrum was used to generate the waves, with a peaked ness parameter g ¼ 3.3. As regards the WaveCat configuration, four values of the angle between hulls (a ¼ 30 , 45 , 60 , and 90 ) and three values of the freeboard (Fm ¼ 0.04 m, 0.09 m, and 0.10 m, in model values, or Fp ¼ 1.20 m, 2.70 m, and 3.00 m, in prototype values) were tested. (Given that the freeboard varies along the length of the inner hull sides, decreasing towards the stern, the value used as a reference to describe a test is that at the aft end of the aft reservoirs, or the minimum freeboard). The tests with a freeboard Fp of 1.20 m were aimed at investigating the WaveCat in operation (i.e., in a configuration prone to overtopping), whereas those with the larger freeboards, 2.70 m and 3.00 m, were aimed at studying its seakeeping response while not in operation (without overtopping); indeed, no overtopping occurred in the latter. Of the tests carried out with the operational freeboard of 1.20 m, significant overtopping occurred only with the significant wave heights of 2.50 and 3.00 m waves; it is these tests simulating the WaveCat in operation, and its power performance, that are the object of the present article. The parameters of these tests and the overtopping and power data are shown in Table 2 and discussed below. 3. Results and discussion Before analysing the overtopping data, it is worth commenting on the interaction of the WaveCat model with the waves. The group of three wave gauges in front of the model (WG2, WG3 and WG4) was used, as explained, to separate the incident and reflected waves by means of the Baquerizo method [30]. On these grounds, the reflection coefficient was computed as follows:
Kr ¼
Hsr ; Hsi
(1)
Fig. 7. Motion capture system. The reflective white marks placed on the stern of the model (left) are detected by the motion capture system (right).
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Displacement [m]
Heave 0.1 0.05 0 −0.05 −0.1 800
850
900
950
1000 Time[s] Pitch
1050
1100
1150
1200
850
900
950
1000 Time[s] Roll
1050
1100
1150
1200
850
900
950
1000 Time[s]
1050
1100
1150
1200
Angle[º]
5
0
−5 800
Angle[º]
1 0 −1 800
Fig. 8. Heave, pitch and roll during part of a test (AD07_I3). (Dimensions in model scale).
where Hsr and Hsi are, respectively, the significant wave heights of the reflected and incident waves. It was found that the reflection coefficient did not present significant variations (Table 3); it is in the range 0.42–0.47 in all the tests, with the highest value corresponding to the tests with the largest angle between hulls (a ¼ 90 ), as might be expected. As regards wave transmission, the transmission coefficient is defined by:
Kt ¼
Hst Hsi
(2)
where Hst is the significant wave height at WG6 (the wave gauge in the lee of the model). The transmission coefficient did not vary significantly, Kt z 0.8, with the exception of one test (AE04_I3) in which it was lower (Table 3). Overtopping rates and energy production data for the overtopping tests described above (Section 2.2) are presented in Table 2 and Fig. 12. The power, P, was obtained from:
P ¼ rg qa Fa þ qf Ff ;
(3)
where r is the density of seawater (r ¼ 1025 kg m3), g is the acceleration of gravity (g ¼ 9.81 ms2), qa and qf are, respectively, the average overtopping rates of the aft and fore reservoirs; and Fa and Ff are the freeboards at the centre of the aft and fore reservoirs, respectively (Fa ¼ 1.275 m, Ff ¼ 1.425 m). Finally, the power per metre of wave crest was computed from:
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Wg5
Free surface position[m]
0.1 0.05 0 -0.05 -0.1 200
250
300
350
450
500
550
600
450
500
550
600
Wg6
0.1 Free surface position[m]
400 Time[s]
0.05 0 -0.05 -0.1 200
250
300
350
400 Time[s]
Fig. 9. Water level time series recorded by WG5 and WG6 during part of test AD07_I5.
Fig. 10. Pump and control system in one of the water reservoirs of the model.
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Table 2 Test parameters (a, angle between hulls; Hs, significant wave height; Tp, peak wave period) and overtopping and power data in prototype values (qa and qf, average overtopping rates at the aft (R2 þ R3) and fore (R1 þ R4) reservoirs, respectively; qt, total overtopping rate; P, power). (Prototype dimensions). Test case
a ( )
Hs (m)
Tp (s)
qa (m3 s1)
qf (m3 s1)
qt (m3 s1)
P (kW)
AE07_I3 AE07_I5 AD07_I3 AD07_I5 AA07_I3 AA07_I5 AB07_I3 AB07_I5
30 30 45 45 60 60 90 90
2.50 3.00 2.50 3.00 2.50 3.00 2.50 3.00
11.0 12.0 11.0 12.0 11.0 12.0 11.0 12.0
0.976 1.360 1.637 2.000 0.798 2.232 0.275 0.572
0.343 0.622 0.254 0.838 0.105 1.010 0.152 0.413
1.319 1.982 1.891 2.838 0.903 3.242 0.427 0.985
17.42 26.35 24.63 37.64 11.74 43.08 5.71 13.25
Table 3 Reflection (Kr) and transmission (Kt) coefficients. [The wave conditions for each test are given in Table 2]. Test case
a ( )
Kr ()
Kt ()
AA04_I3 AA04_I5 AD04_I3 AD04_I5 AE04_I3 AE04_I5 AB04_I3 AB04_I5
30 30 45 45 60 60 90 90
0.425 0.438 0.441 0.428 0.421 0.431 0.470 0.470
0.806 0.777 0.777 0.756 0.507 0.760 0.749 0.772
Pw ¼
P ; W
(4)
where w is the length of wave crest captured by the device, i.e. the beam between its bows (which depends on the angle between hulls?). Overall, the values of power output are indicative of a limited efficiency, as is usually the case of a nascent technology. As explained from the outset, these tests were primarily aimed at providing a proof of concept of the WaveCat patent. As a continuation of this line of research, a numerical model, calibrated and validated on the basis of the results of these laboratory tests, will be used to optimise the WaveCat design. Given the preliminary nature of the design tested, there is clearly room for improvement. Best results were obtained with the intermediate values of the angle between hulls, a ¼ 45 and a ¼ 60 . For wave condition I5 (Hs ¼ 3.0 m, Tp ¼ 12.0 s) the optimum value of the angle between hulls is 60 , whereas for wave condition I3 (Hs ¼ 2.5 m, Tp ¼ 11.0 s) the optimum is 45 . The largest angle (90 ) led to substantially lower performances in both cases. As might be expected given the symmetry of the WaveCat design, the difference in overtopping rates between the starboard and port reservoirs was well below the level of statistical significance, hence they were not taken into account for the computation of the power performance. However, significant differences occurred between the aft and fore reservoirs. The aft reservoirs consistently experienced larger overtopping that their fore counterparts, as may be seen by comparing the data for R3 (aft) and R4 (fore) in Table 2. This has, naturally, implications for the turbine-generator groups to be installed – those connected to the aft reservoirs should have a greater power rating. Alternatively, the volume (and length along the hulls) of the aft reservoirs could be increased at the expense of the fore reservoirs, so that similar overtopping rates occurred – in which case the same turbine-generator groups should be installed at all four reservoirs. By which amount the volumes of the aft and fore reservoirs should be modified is a question for the numerical model of WaveCat, currently under development; once calibrated and validated with the results of the present tests, it will be used to optimise the design of WaveCat.
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Water level [m]
Reservoir 3 0.08 0.06 0.04 0.02 0
500
1000
1500
1000
1500
1000
1500
Water level [m]
Time[s] Reservoir 4 0.08 0.06 0.04 0.02
Free surface position[m]
0
0
500 Time[s] Wg5
0.1 0.05 0 −0.05 −0.1
0
500 Time[s]
Fig. 11. Water level in reservoirs #3 (aft, starboard) and #4 (fore, starboard) and free surface between the hulls recorded by wave gauge WG5 during test AD07_I5.
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0
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Fig. 12. Power performance (P) vs. angle between hulls (a) for sea states I3 and I5.
4. Conclusions An experimental campaign of a 1:30 model of WaveCat was conducted in a large wave tank. Its objective was not to encompass the full variability of wave conditions and WaveCat configurations; this would have made little sense at this early stage of development of the patent because the design available is preliminary, and must yet be optimised for performance. For this reason, the objectives of the laboratory tests were: (i) to provide a proof of concept of WaveCat as a viable technology to convert wave energy into electricity using the novel approach of oblique overtopping; (ii) to obtain a preliminary assessment of its power performance – preliminary because the number of tests carried out was limited and, most importantly, the design tested was itself preliminary – and of the dependence (or otherwise) of the overtopping rates on the angle between the hulls and the wave conditions; and (iii) to gather experimental data with which to calibrate and validate a numerical model. These objectives accomplished, the next step in the development of the patent is the implementation, calibration, and validation of a 3D numerical model, which will be used to optimise the design of WaveCat. The optimised design will then be subjected to exhaustive laboratory tests and, subsequently, to sea trials. Four main conclusions can be extracted from the results of the physical model tests. First, the WaveCat patent is a valid concept to extract energy from waves. This proof of concept is the logical first step in the development of a new WEC. Second, for any particular value of the angle between hulls, the larger the wave height and period, the greater the overtopping rate; however, this does not hold true if any value of the angle between hulls is considered – for the sea state I3 (Hs ¼ 2.5 m and Tp ¼ 11.0 s) the overtopping rates with angles of 30 and 45 were greater than that for a sea state of smaller wave height, I5 (Hs ¼ 3.0 m, Tp ¼ 12.0 s) with an angle of 90 ; indeed, it was found that the overtopping rate presents a high sensitivity to the angle between hulls. Third, the optimum value of the angle between hulls – which may be defined as the value that maximizes the overtopping rate, and therefore, the power output for a given sea state – was found to depend on the sea state; for the wave conditions tested, the optimum values were 60 and 45 , the larger and smaller angle being best suited for the larger and smaller waves, respectively. Furthermore, in view of the latter conclusions, a fourth conclusion can be drawn, namely the importance of incorporating into the prototype the possibility of varying the angle between the hulls, as contemplated in the patent. Acknowledgements This research was supported by the Government of Galicia (Xunta de Galicia) through its Energy and Mining Resources programme (Contract No. PGIDIT07REM001CT)
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