FIELD

This invention relates to a wave energy converter.

BACKGROUND

Natural waves in seas and oceans arise from transfer of energy from wind to water. The greater the frequency and height of the waves, the more energy there is present in the waters. There are existing methodologies for harvesting energy from waves using linear motion in the transverse or longitudinal directions of waves. Such methods commonly use turbines, vanes, mills or pistons to drive the rotors of generators. However, most of the existing technologies developed have a rated operating range that does not address capturing energy in low wave height conditions.

Existing mechanisms are typically submerged or semi-submerged in offshore of nearshore locations, often requiring a certain depth and wave height to be effective. Such devices require drivetrain components to interact with water volume under normal operating conditions, therefore being costly in their materials to avoid, minimize or slow down corrosion. In addition, operating and maintenance costs of offshore and near-shore installations are extremely high. The existing technologies are classified according to the following methodologies:

1. Point Absorber Buoy

2. Surface Attenuator

3. Oscillating Water Column

4. Overtopping Device

5. Oscillating Wave Surge Converter

Table 1 below lists some existing wave energy conversion devices, in which the onshore Islay LIMPET device requires civic works for a concrete infrastructure similar to that of a small barrage.

l Device Proponent Mechanism Country of Notes

Origin

PowerBuoy Ocean Power Point US Transverse wave motions

Technologies Absorber drives generator via rack

Buoy and pinion. Requires

depth of 60m, 8km offshore and submerged transmission cables.

Pelamis Pelamis Wave Surface UK Wave motion pumps high Attenuator Power Attenuator pressure oil to drive

generator via hydraulic cylinders and motors. Semi-submerged off shore design.

Islay Islay LIMPET Oscillating UK Shoreline onshore device LIMPET Water uses oscillating water level

Column to drive air through turbine housed in a concrete infrastructure.

Complicated civil works.

Wave Wave Dragon Overtopping Denmark Offshore reservoir traps Dragon ApS Device and collects water from deflect waves to drive hydroelectric turbines using gravity.

Oyster Aquamarine Oscillating UK Hinged mechanical flap Wave Power Wave attached to seabed drives Energy Surge high pressure water to Converter Converter onshore turbine via

hydraulic pistons.

Table 1

There is therefore a need for a plug-and-play unsubmerged wave energy converter that can effectively convert wave energy to electricity across both high and low wave height conditions to overcome the disadvantages of existing devices.

SUMMARY

The presently disclosed wave energy converter translates linear to rotary motion and uses vertical linear motion of floating structures caused by transverse motion of waves appropriately to drive typical generators (e.g. rotational, linear).

The wave energy converter is a modular onshore device that is completely above the water surface, therefore not requiring drivetrain components to interact with water volume under normal operating conditions.

Power derivation of the wave energy converter in low wave height and frequency conditions (Hs<0.5m, Tp=2-10s) makes it suitable for small scale power generation.

The wave energy converter can be primarily provided for power generation, or as an added functionality for structures such as jetty pontoons or floating platforms where the primary function of such structures is for movement/storage of persons/objects, and stability of said structure is required.

The wave energy converter is intended for power generation for jetties, harbours, marine ports, oil rigs, offshore installations, shore-based loads, berthed/anchored ships. This is applicable as long as there is a relatively static structure that enables the floating platform and the roller to engage and rotate due to wave motion.

The wave energy converter has a combined functionality (structural support, stability and power generation) for both passive structures (e.g. jetties, pontoons) and dynamic bodies (e.g. ships).

Advantages of the wave energy converter include:

1. Power generation being possible from both wind induced waves and ship-induced waves in both seawater and freshwater conditions.

2. Fully unsubmerged design, resulting in:

a. significantly reduced costs in operating and maintenance costs

b. reduced vulnerability to adverse sea conditions like biofouling

c. elimination of interaction with seawater, leading to reduced corrosion of device components, thereby extending operating lifespan of the wave energy converter d. elimination of need for support in sea to land power transmission

e. minimal adverse impact on environment due to reduced interaction with marine life such as fish, corals and so on

3. Being able to be incorporated in remote islands for powering of micro-grids

4. Allowing for variations in design-to-scale according to conditions in area of installation (onshore, near shore, or offshore installations can be accommodated) 5. Allowing for plug-and-play wave energy conversion, giving better flexibility and ease of installation. For example, multiple units of the wave energy converter may be installed on one pontoon.

According to a first aspect, there is provided a wave energy converter comprising: a roller connected to a floating structure on water having surface waves, the roller configured to roll against a guide to which the floating structure is coupled, wherein relative motion between the guide and the floating structure caused by the surface waves results in rotational motion of the roller; and a generator configured to convert the rotational motion of the roller into electricity.

The wave energy converter may further comprise a surface engager configured to ensure continual contact of the roller against the guide

The surface engager may comprise a compression spring in connection between the roller and the floating structure.

The wave energy converter may further comprise a linear guide having linear shaft guides supporting linearly moveable shafts, a first end of the linearly moveable shafts attached to the roller and a second end of the linearly moveable shafts attached to the compression spring, the linear shaft guides being immoveable relative to the floating structure.

The wave energy converter may further comprise a mounting configured to be attached to the floating structure, wherein the roller, the compression spring and the linear guide are provided on the mounting.

Alternatively, the surface engager may comprise a clamping arrangement comprising the roller and a secondary roller, the roller and the secondary roller rolleably clamping the guide therebetween, the clamping arrangement coupling the floating structure to the guide.

The wave energy converter may further comprise a gearbox provided between the roller and the generator to step up rotations from the roller to the generator. The wave energy converter may further comprise an energy storage device to store the electricity.

All parts of the wave energy converter may be out of the water. The floating structure may be a pontoon and the guide may be a stationary pile. The guide may be a vertical beam attached to a side of a ship. Alternatively, the guide may be an anchor chain of a ship. Alternatively, the guide may be a leg of an oil platform. BRIEF DESCRIPTION OF FIGURES

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments of the present invention, the description being with reference to the accompanying illustrative drawings.

Fig. 1 is a plan view of a first embodiment of the wave energy converter attached to a pontoon.

Fig. 2 is a side view of the wave energy converter of Fig. 1.

Fig. 3 is a schematic illustration of operation of a roller and gearbox of the wave energy converter.

Fig. 4 is a photograph of a generator of an experimental prototype of the wave energy converter.

Fig. 5 shows curves of voltage, torque and power against rotation speed of the generator of Fig. 4.

Fig. 6 is a photograph of a gearbox of the experimental prototype of the wave energy

converter.

Fig. 7 is a photograph of a laboratory drop test set-up of the experimental prototype of the wave energy converter.

Fig. 8 is a photograph of an existing roller of a pontoon acting on a stationary steel pile as guide posts for the pontoon.

Fig. 9 is a schematic illustration of installation of the experimental prototype of the wave energy converter on the pontoon of Fig. 8. Fig. 10 is a photograph of the experimental prototype of the wave energy converter installed on the pontoon of Fig. 8.

Fig. 1 1 is a photograph of an ultrasonic displacement sensor installed on an existing roller of the pontoon of Fig. 8.

Fig. 12 is a graph of wave conditions at the pontoon of Fig. 8 during a field test of the experimental prototype of the wave energy converter.

Fig. 13 is graph of power output of the experimental prototype of the wave energy

converter during the field test.

Fig. 14 shows schematic illustrations of alternative embodiments of the wave energy

converter.

Fig. 15 is a schematic illustration of a further embodiment of the wave energy converter used with a ship.

DETAILED DESCRIPTION

Exemplary embodiments of the wave energy converter 10 will be described below with reference to Figs. 1 to 15. The same reference numerals are used throughout the figures to denote the same or similar parts among the various embodiments.

In general, the wave energy converter 10 comprises a roller 30 and a generator 60. The wave energy converter 10 preferably also comprises a mounting 20 configured to be immoveably attached to a floating structure 100, such as a pontoon 100, that is floating on water where surface waves are present. The surface waves cause sinusoidal motion of the floating structure 00. The pontoon 100 is generally constrained from substantial horizontal translational motion by stationary guide pylons or piles 200 to which the pontoon 100 is coupled, while being free to move vertically relative to the piles 200 according to the surface waves in the water on which it is floating. The piles 200 are typically made of steel.

The roller 30 is configured to roll against a guide 200, such as the vertical surface 202 of the stationary pile 200, to convert sinusoidal motion of the pontoon 100 caused by the surface waves into rotational motion. The roller 30 is preferably made of rubber, and is configured to roll against one of the piles 200 at the pontoon 100. A surface engager 40 is preferably provided to ensure continual contact of the roller 30 with the steel pile guides 200 at all times to maintain necessary friction between the pontoon pile 200 and the roller 30. The surface engager 40 is connected between the roller 30 and the floating structure 100. In one embodiment, the surface engager 40 may comprise a compression spring 40. The spring 40 is provided to overcome frictional losses between the steel pile 200 and the rubber roller 30. The high performance compression spring 40 is loaded behind the roller 30 to ensure contact of the roller 30 with the pile 200 all the time. The compression spring 40 is attached to the mounting 20 and biases the roller 30 away from the mounting 20. In use, the mounting 20 is attached to the floating structure 100 with the spring 40 under some compression when the roller 30 is in contact with a vertical surface of the pile 200. In this way, the roller 30 is kept in constant contact with the pile 200 so that when the floating structure 100 moves up and down on the water as a result of the surface waves, the roller 30 attached to the floating structure 100 rolls against the pile 200 and moves vertically relative to the pile 200 corresponding to vertical movement of the pontoon 200.

In a first embodiment of the wave energy converter 10 as shown in Figs. 1 and 2, the mounting 20 comprises a mounting table 20 to which the different components are attached. This may be by means of bolts in order to be easily detachable for maintenance and future design changes. A linear guide 50 is further provided to attach the roller 30 to the mounting table 20 under bias of the spring 40. Linear shaft guides 52 of the linear guide 50 are immoveably attached to the mounting table 20 and support linearly moveable shafts 54. The roller 30 is attached to a first end 54-1 of the shafts 54 while a first end 40- 1 of the compression spring 40 is attached to a second end 54-2 of the shafts 54. A second end 40-2 is attached to the mounting table 20. The shafts 54 extend the roller 30 away from the mounting table 20 to contact the steel pylon 200 with the spring 40 under some compression when the mounting table 20 is attached to the pontoon 100.

The generator 60 is configured to receive rotations from the roller 30 to generate electricity. A gearbox 70 may be provided to increase the rotational speed of the roller 30 to the rated speed of the generator 60 if necessary. The gearbox 70 is preferably a right- angled gearbox 70 installed between the rubber roller 30 and the generator 60 to increase rotational speed input to the rotor of the generator 60. The rated rotational speed required by off-the-shelf generators 60 is generally higher than the rotational speed of the roller 30 at the pontoon 100 such that the gearbox 70 is required to step up the rotational speed, as shown in Fig. 3. In one embodiment, the gearbox has a gear ratio of 10.

The power of the generator 60 is controlled by installing a charge controller after the generator 60 for maximized power production. During a testing phase of the wave energy converter 10, instead of the charge controller, a constant resistor bank was installed to track power conversion.

Experimental Prototype

An experimental prototype of the wave energy converter 10 was built and tested.

Analytical calculations used for designing the experimental prototype of the wave energy converter 10 are given below:

Airy Waves Theory (linear wave)

( = ζ _α cos (-ωί + kx)

( ζ : free surface elevation; ζ _α : wave amplitude; ω: frequency in radians/sec; k: wave number )

At x = 0, ζ _α = A cos (-cot) cos (kx) - sin f-iot) sin (kx) = A cos wt 2π

where, ω = 2nf = —

Velocity, U =— =—ζ _α ω sin ωΐ, ά ² ζ

Acceleration, U = -τ- =—ζ _α ω ² cos tt

dt ^{2 a}

At the peak velocity, sin ωί— 1

Thus,

d _ _ , 2π

Έ ^{~ ~ζαί ~ ~ζα} T

ά ² ζ

(T = wave period in second)

Morrison Equation:

Total force (N) on body of projected area, (A=LD), displaced volume. No radiation forces considered. For Pontoon No. 1 at TMFT, C _D = 2.1 ^{1 1} C ₀ value extracted from http://www.engineeringtootbox.com/drag-coeffictent-d_627.htm l

C _m = (1 + C _a ) = 2.36 (Wendel, 1950)

Designed wave period, T = 4 sec

Designed wave amplitude, ζ _α = 0.15 m

Pontoon Dimension = 16m (L) x 3.5m (B) x 0.8m (D) = 3.3 kM

WEC Roller and Gearbox Specifications Gear ratio = 10

(Ιζ

U = ~77 = r. ω

at

r = radius of WEC roller (m) ω = angular velocity (r ad /sec)

Designed WEC roller diameter = 150mm

Thus, designed angular velocity, ω = ζ _α ~ ~— 3.14 rad/ sec = 30 rpni

In the experimental prototype 10, the generator 60 used was a permanent magnet generator supplied by Ginlong Technologies, China, model no. GL-PMG-500A, as shown in Fig. 4. Specifications of the motor are given in Table 1 below.

Model number GL-PMG-500A

Rated Rotatoin 450

Speed(RPM)

Rated Output Power(W) 500

Outer frame material High standard Aluminium alloy with TF T6 heat treatment

(TF/T6 full heat treatment for increasing the performance of aluminium alloy as follows. Heat 4-12 hours at 525- 545 degrees Celsius, quench with hot water, and

precipitation heat treatment for 8-12 hours at 155-175 degrees Celsius.) Outer frame finish Aluminium surface is anodised then power painted for anti-corrosion protection

Shaft material High standard Stainless Steel

Shaft bearing High standard NSK 6207DDUC3(Front) NSK C(Rear)

Weight(Kgs) 14.4

2

Rotor inertia (Kg.m ): 0.006

Fasteners (nuts and bolts) High standard Stainless Steel

Lamination stack High specification cold-rolled Steel

Windings temperature rating 180 degrees Celsius

Magnet material NdFeB (Neodymium Iron Boron)

Magnets temperature rating Rated at 150 degree Celsius

Generator configuration 3 Phase star connected AC output

Safety Capable of withstand short term shorting of the windings for braking effect at rated rotation speed. Class 1 electrical safety rated for prevention of electrical shocks.

Starting torque(NM) <0.5

Phase resistance(ohms) 0.4

Recified DC Current at 20

Rated Output(A)

Requied Torque at Rated 14.8

Power(NM)

Table 2

Voltage, torque and power curves of the motor 60 against rotation speed are shown in Fig. 5.

Rotation speed of the roller 30 was expected to be at about 30 rpm. To match the rotation speed of the roller 30 with the input rated of the generator 60, a gearbox 70 was required. The following shows the calculations performed to obtain the correct gearbox 70 rating. 'WEC refers to the wave energy converter 10.

Generator and Gearbox Matching

At designed condition, WEC roller rpm _design = 30 rpm After gearbox, rpm _inpu t = 300 rpm

According to GL-500-A generator specs, at 300 rpm, Torque _input = 11 N.m Power output of WEC, at 300 rpm = 250 W Therefore, required torque at the WEC roller before gearbox, Torque _{r a} ^ _d - 110 N.m Frictional force required at the WEC roller = ^{Toi quermfutr d} = 1, 66 N

WEC roller radius

Notably, the frictional force required at the WAVE ENERGY CONVERTER roller was less than available heave forces on the pontoon which were calculated to be 3.3 kN.

The gearbox 70 selected for the experimental prototype of the wave energy converter was an ABR-Series High Precision Planetary Gearbox by Apex Dynamics, USA, as shown in Fig. 6.

Specifications of the gearbox 70 used in the experimental prototype are given in Table 3 below.

Table 3

To determine the compression spring 40 suitable for the experimental prototype, frictional forces and normal forces required at the roller 30 against the steel pile 200 were computed with the following equations:

F _$ = β Ftf, (where ¾ = Frictional Force; F _N = Normal Force; μ = Coefficient of friction} **The frictional coefficient of rubber on steel data was not available for the rubber roller used in the experimental prototype. An approximate value was derived from a drop test that will be described below.

The normal spring force F _N required for roller rotation of the wave energy converter 10 was calculated as follows: F _N = 1 ,466 / 0.7 = 2,094 N

Thus, a spring 40 having loading capacity of 5, 185.8 N at 00% stroke was selected for prototype testing of the experimental prototype and a desirable stroke length was reconfigured for final site installation.

Although the above calculations were done with existing theories and formulae, there were assumptions made in the calculation steps. Therefore, it should be taken into consideration that the power output of the experimental prototype of the wave energy converter 10 presented in this application may not be accurate because some of the hydrodynamic forces (6-DOF) and wave radiation effects were not considered. Power output may be re-calculated (preferably with the help of hydrodynamic software) with 6- DOF motion options with different wave spectrum models to validate the concept.

Laboratory Drop Test

Wave motion for the wave energy converter 10 was replicated as a single stroke drop test using the experimental prototype to characterize its performance in a lab.

The drop test set-up is shown in Fig. 7 and procedures of the drop test were as follows:

1 ) The spring 40 was loaded at desired stiffness to keep the roller 30 and a wall simulating the guide pile 200 at optimal position in contact.

2) A chain hoist was used to lift up the experimental prototype 10 to a desired height.

3) The experimental prototype 0 was released from the set height. This was achieved by cutting a rope carrying the experimental prototype 10 (the rope being attached to the chain hoist) to let the roller 30 drop freely, while rolling against the wall 200, to simulate one downward stroke motion.

4) The generator 60 was preloaded with resistor bank for the desired charging current. 5) Displacement of the drop test, RPM of the roller 30 and power output from the generator 60 were logged, and given in table 4 below:

Table 4

It should be noted that the drop test setup did not have control over the dropping velocity and its acceleration was governed entirely by gravity. The objective of the drop test was to confirm the smooth operation of the gearbox 70 and roller assembly 30 and the frictional control by spring stiffness of the spring 50.

Field Test

The experimental prototype wave energy converter 10 was tested on a pontoon 100 (Fig. 8) at a ferry terminal in Singapore. A frame 22 of the mounting table 20 was welded to a bracket 120 of an existing roller 130 of the pontoon 100, as indicated in Fig. 9. Fig. 10 shows the wave energy converter 10 installed on the pontoon 100.

Up and down motion of the pontoon 100 was captured using ultrasonic displacement sensor (Maxbotix MB7360) 300 mounted onto the pontoon 100, as shown in Fig. 1 1. Simultaneously, wave conditions from deployed wave sensors (not shown) were recorded for comparison against pontoon movement. The pontoon 100 and the wave sensors were distant from each other because of different installation spots (despite being in the close vicinity). Consequently, the wave height detected by the wave sensors and the pontoon displacement detected by the displacement sensor 300 were noticed to be slightly out of phase.

Power output of the generator 60 was tracked and logged using DEWETRON data acquisition system and a personal computer using an analogue channel measuring the voltage across sense resistor (0.15 ohms). The generator 60 was loaded with a series of resistive load of 8 ohms in total, which was connected to the generator 60 via a rectifier. Before activating the roller device or wave energy converter 10, two threaded rods were installed together with the spring 40 of the roller device or wave energy converter 10 to control the spring stiffness before engaging it. Once the desired stiffness was set, the threaded rods were released by unscrewing the nuts and fixing the spring holder position onto the base-plate of the table or the mounting table 20.

The spring 40 was loaded at 2,000 N to produce required friction between the pontoon pylon or pile 200 and the rubber roller 30.

The field tests were carried out on 10 December 2014. Wave condition of the site during the experiment is shown in Fig. 12 while Fig. 13 shows power output of the wave energy converter 10 during each upward and downward stroke of wave motion.

The wave energy converter 10 produced power output of ~ 333Watts (Peak) at 265mm displacement of the pontoon. Concurrently, the wave conditions measured by wave sensors at the site was Hs =0.257m and Tp=3.2s.

During upward and downward motion of the pontoon 100, it can be seen in the graph of Fig. 13 that the roller 30 starts to accelerate and decelerate during the respective motion. However, it was noted that the roller motion was not uniform in both upward and downward strokes. It was found to be due to lateral movement of pontoon 100 which interferes with up and down motion of the roller 30. This caused knocking or shock pattern in the roller motion and prevented the roller 30 from rolling smoothly. Further design modification can be made to rectify such interruption.

It was noted that the wave energy converter 10 was operating in an approximate range at the estimated designed loading and the power output of the wave energy converter 10 was within the range of the expected power matrix. The estimated power output matrix of the wave energy converter 10 is shown in Table 4 below at different wave heights and periods. WEC Power Wave Period, T (sec)

Output (Watts)-' 2 3 4 5 6 7 8 9 10

W _h H _{i (} ⁾ H _t ae _g vem, 0,1 106 46 26 16 11 8 6 4 3

0.2 429 190 106 67 6 34 26 20 16

0.3 - 429 241 154 106 78 59 46 37

0.4 - 429 274 190 139 106 84 67

0.5 - - 672 429 298 218 167 131 106

0.6 - - - 619 429 315 241 190 154

Table 4

² WEC power output was directly derived from the power vs rpm curve of the generator 60 used in the experimental prototype. Battery charging and electrical loads were not considered in the power output estimation. Pontoon motion was assumed to be at the same frequency as wave period. Maximum generator power output of generator was set at 770W.

Besides the configuration of the wave energy converter 10 as described above for the experimental prototype, other configurations and embodiments may be devised, as illustrated in Fig. 14.

In another embodiment, the wave energy converter 10 may be used with ships, as shown in Fig. 15. In such an embodiment, a guide 200 having a vertical surface 202 is secured to the ship 400. The guide 200 may be in the form of a vertical beam attached to the side of the ship 400. The roller 30 is connected to a floating structure 100 such as a buoy or platform that floats alongside the ship 400. The roller 30 is positioned to roll against the vertical surface 202 of the guide 200. The difference in buoyancy between the ship 400 and the floating structure 100 will result in rotary motion in the roller 30 when the ship 400 and the floating structure 100 encounter a wave. This rotary motion is converted to electricity in a generator 60 of the wave energy converter 10, in a similar manner as in the pontoon version of the wave energy converter 10 as described above with reference to Figs. 1 to 14. Depending on the weight of the wave energy converter 10, symmetrical deployment of wave energy converters 10 on both sides of the ship 400 may be required.

To ensure continual contact of the roller 30 with the guide 200, a surface engager 40 comprising a clamping arrangement 40 is provided, as shown in Fig. 15. The clamping arrangement 40 comprises the roller 30 and a secondary roller 32 clamping the vertical guide 200 therebetween. Via the clamping arrangement 40, the floating structure 100 is coupled to the ship 400 to allow movement of the floating structure 100 relative to the ship 400 only along one axis. Alternatively, the surface engager 40 may comprise a compression spring 40 connected between the roller 30 and the floating structure 100.

An alternative design of the wave energy converter 10 for use with ships would be to use a deployed anchor chain in tension of the ship 400 as the guide 200 against which the roller 30 rolls, the wave energy converter 10 being mounted on a float 100 alongside the ship 400. Similar to the embodiment described with reference to Fig. 15, a secondary roller 32 may be provided such that the roller 30 and the secondary roller 32 clamp onto the anchor chain 200 serving as the guide 200. The clamping can be achieved by means of passing the chain 200 between the roller 30 and the secondary roller 32 that exert compressive forces on the chain links with the use of springs or pistons. The roller 30 and secondary roller 32 can take the form of gears to allow greater engagement between the wave energy converter 10 and the anchor chain 200 and thus reduce slippage. Rotation from the roller 30 is input to the generator for conversion to electricity.

Variants of the wave energy converter that can be mounted on a miniature pontoon or float that is coupled to a leg of an oil platform can also be developed. Another possibility of a variant is one which is designed to be a stand-alone electricity generation installation in deeper waters for higher power output. Pile-like structures engaged by the rollers of wave energy converters mounted on floating pontoons on all sides can serve as a wave power generation installation similar to off-shore wind turbine or tidal turbine installation. A further possible embodiment of the wave energy converter is to incorporate a roller fender of a pontoon as the roller of the wave energy converter. A roller fender is basically a rubberised roller that is mounted on floating pontoons to cushion any impact of the pontoon against a jetty or piles due to the waves.

Another variant for nearshore deployment is to apply the wave energy converter at existing marine structures such as breakwater dikes or barrages. A roller 30 connected to a generator 60 on an independent float/buoyant body 100 can function as a wave energy converter 10 when the roller 30 engages a guide 200 such as a surface of the existing marine structure 200 in a secured manner.

The wave energy converter 10 may additionally comprise an energy storage device (not shown) to store the electricity produced by the generator 60 from the rotational motion from the roller 30. Advantageously, for all embodiments, all parts of the wave energy converter 10 are above the water level, thereby minimizing corrosion, and reducing the need for maintenance and change of parts.

Whilst there has been described in the foregoing description exemplary embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations and combinations in details of design, construction and/or operation may be made without departing from the present invention. For example, the different embodiments of the surface engager (e.g. spring, clamping arrangement) may be provided in different combinations for the different guides (e.g. stationary pile, vertical beam of ship, anchor chain) with which the wave energy converter is used. In place of the compression spring or clamping arrangement, yet other configurations of the surface engager may be provided to ensure continual contact of the roller with guide. Such means may include hydraulic or pneumatic pistons. While embodiments have been described with reference to a ship, the wave energy converter may be deployed with other vessels. Although relative motion between the floating structure and the guide has been described in some examples as being vertical, the relative motion need not be strictly vertical so long as there is relative motion between the guide and the floating structure arising from surface waves in the water on which the floating structure floats that results in the roller rolling against the guide to give rise to rotational motion of the roller for conversion into electricity by the generator.