1 Introduction
The known initiatives for harnessing ocean waves were started in the early of 18th century. Owing to the improved technological support, many different concepts to capture wave energy have been brought to daylight in recent times. But still the technical and conceptual convergence has not yet been obtained to say any one device as the best suited for the wave energy application. As a result, the proliferation of wave energy technologies occurs globally. There have been thousands of patents filed around the world on different methodologies to capture the ocean waves and its energy. The first patent was filed in 1799 [
1]. Since then more than thousand different methods have been filed for patent by 1980 around the world [
2]. The annual reports on ocean energy from the International Energy Agency are the sources of regularly updated information on the status of developments in wave energy research. Annex 9 of the 2008 report [AEA Energy & Environment and Sustainable Energy Ireland, 2006] showcased approximately 80 different wave energy concepts under development. Various authors have proposed the wave energy as a sustainable energy [
4,
5] and have also put forward different classifications based on the selection criteria.
An illustrative diagram proposed by Hagerman based on most well-known principles of wave energy conversion is still useful in assessing wave energy converters [
6]. His classifications are based on the mode of motion of energy absorption (pitch, heave and surge), fixed or flexible structure, type of force reaction (fixed structure, anchored support and inertial reference) and the type of working fluid (air, water and hydraulic oil). These classifications are valid today with an additional recent technology with direct energy conversion method.
Various wave energy conversion devices based on their working principle was reviewed [
2]. The devices considered were at prototype stage and under extensive development stage. The review covered almost all the devices tested so far. Various wave energy conversion concepts based on their location and operating principle were categorized [
7]. However, most of the on-shore wave activated body type devices were not considered. The significant developmental activity on on-shore devices and its commercialization in recent times can be seen in Refs. [
8−
10]. These works projected various mooring possibilities and their design constraints for different types of wave energy converters. The common classification addressed by these authors is dividing the wave energy converters into oscillating water column (OWC), over-topping devices (OTD) and wave activated bodies (WAB), based on the way they work.
Although numerous investigations have been conducted on each of these wave energy converters (WEC), wave activated body type WECs are noted as the most successful and diverse ones [
11−
14]. Any WAB type wave energy converter works in three stages to convert energy in the waves into electrical energy. Figure 1 shows the different stages of a typical wave energy converter where, the front end interface is the portion of device which directly interacts with the incident wave. These interfaces are mostly of one or more buoyant objects made to move with reference to fully or partially fixed references. Further, a power take off (PTO) system is coupled to convert displacement of front end interfaces into other usable forms of energy. Most of the PTO systems produce mechanical rotation but a few of them produce linear motion to energize linear generators. The generated electricity from electricity generating unit coupled with the PTO system is transmitted into the main land through underwater cables.
A simple modification in any one or more of these stages will constitute a new device and many such devices were invented and patented for commercial testing. The present paper focuses on the front end interface stage.
A detailed representation of recent development in various stages of a WAB type WEC is given in Fig. 2. It can be understood from the above flowchart that most of the developments are focused on the front-end conversion of a WAB type WEC.
2 Wave activated body (WAB)
WABs are devices that work with the relative motion of oscillating bodies resulting from wave motion. WAB type devices can be categorized as point absorbing devices (possess small dimensions relative to the incident wavelength), Attenuator-recline parallel to the predominant wave direction and oscillate relative to each other (Pelamis), Terminator-large devices aligned with wave crest (Principle axis perpendicular to the predominant wave direction) and sea floor mounted devices work with pressure differential. These devices involve one unwavering reference with which the wave absorbing body can oscillate and energy is harnessed by PTO systems through resisting or damping the motion of bodies.
3 Front end energy conversion
The front-end interfaces are primarily made of buoyant bodies and are kept afloat on the ocean surface. A few devices keep such buoyant bodies under water and use pressure difference created by waves above. The most of the recent WABs are classified based on the buoyancy of the front-end interface, shape and orientation of the front-end interfaces, modes of motion, frame of reference and principle of working. Each of these devices is discussed in the proceeding section.
3.1 Buoys
Any object with a specific weight lower than the surrounding water body is used as a buoy or floater in WAB type WECs in order to move along the wave motion. These buoys are usually hallowing cylinders or hallow objects made to move with reference to any reference point. These buoys can either be floating on the surface as shown in Fig. 3 or submerged under the water which are classified based on their displacement type during the interaction with the incoming waves [
15−
19].
Many engineers and scientist designed their WABs for increasing the overall energy conversion rate. Table 1 gives the summary of different design approach involved in buoys to increase the efficiency.
3.1.1 Floating buoy
Buoy as mentioned above may be of floating or submerged. The floating buoys are made to float on the water waves and displace in a particular mode of oscillation by harvesting the energy from the waves. These relative motions are further transferred to the transmission devices and then finally to the energy generator. Float may be of single or multiple [
25] to extract the wave energy. The proposed energy conversion model [
26] as shown in Fig. 4 is an example of the floating buoy which extracts the energy from waves by heave and transmit to the generator through suitable transmission systems.
3.1.2 Submerged buoy
These energy converters [
27] are totally submerged under the water and utilize the wave crest and trough to generate relative motions among their structures to produce electricity. As a submerged WEC, the device gets an advantage of having less visual impact on the people and being prevented from the slamming forces of the waves. For example, consider the submerged buoy (Fig. 5) which consists of an air-filled cylinder chamber and a movable upper cylinder both fixed to the sea bed through appropriate structure. The upper cylinder compress the air filled in the lower cylinder due to the wave motion, and the compressed air is used to drive the power generating unit.
3.1.3 Frames of references
Most of the WECs harness energy from ocean waves by inducing the relative displacement between rigid bodies through wave action. This relative displacement is then fed to a PTO system and converted into other usable forms of energy. There are various methods used in different devices to produce the relative displacement. A few devices anchored to sea bed and use it as their frame of reference to oscillate, a few use submerged horizontal plate as dampers and oscillate in heave, a few use many floating objects with different natural period and create relative displacement and a few use deep water spar anchored with sea floor as its frame of reference. Though there are varieties of designs, these entire devices harness ocean waves by generating relative displacement among bodies.
3.1.3.1 Sea floor mounted/anchored
Wave energy converters are positioned on the sea floor by placing its heavy weighted substance on the sea bed or by using an anchor. The sea floor mounted substance settles on the sea bed due to its gravity and prevents the floating buoys to displace along with the buoys. Figure 6 depicts the sea floor mounted reference frame.
3.1.3.2 Submerged plates
Submerged plates as shown in Fig. 7 also act as the reference frames to the floating buoys, which are actually not in contact with the sea floor. These plates are sufficient enough to offer damping against heave displacement. The wave energy converter with a floating buoy and reference frame was proposed [
30], where the float is suspended to the submerged plate using an elastomeric hose and further the submerged plate is moored to the sea floor. When the float heaves with reference to the submerged plates, the hose extends and decreases the internal volume. The water present inside the hose is discharged through a check valve to drive the turbine. When the float drops down, the hose attains its original volume and allows the water to enter through another valve.
3.1.4 Other floating bodies
Apart from the formal floating bodies, there are other several floats proposed to harvest the energy from ocean waves. The CPT WEC (Manta) shown in Fig. 8 is designed to convert the heave and surge motion of the float to a high torque rotation by a direct drive rotary (DDR) generator to provide simple and reliable energy conversion.
A prototype named as Charlotte (Fig. 9) and developed by Beatty [
32] consists of a central pillar surrounded by an arrangement of four cylinders referred to as the float. The PTO is placed within the central pillar. The prototype is designed in such a way that the geometries of the pillar and float are chosen so that a resonant condition is achieved at both ends of the wave frequency range. Tuning mechanism is achieved in this model to optimize energy extraction from the targeted waves.
A model (Fig. 10) consists of a float and a hallow cylinder attached to it was proposed [
33]. The hollow cylinder encloses two butterfly flaps at the mid portion and during the heave motion of the float, the flaps open and close which in turn fill the upper and lower portion of the cylinder alternatively due to the pressure difference in the water and obtain the resonance.
3.1.5 Spar buoy
A spar is a floating caisson (Fig.11) which is generally a hollow cylindrical structure similar to a very large buoy. The spar relies on a conventional mooring system to maintain its position. About 90% of the structure is underwater. The spar is designed and generally, is placed between the float and the submerged plate. The ultimate function of the spar in WEC is to stay relatively stationary when the float heaves due to energized waves. This plays an interconnection between the float and the reference frame. All the energy transmission system and power generating unit are positioned inside the spur thus, preventing it from ocean environment.
3.2 Non-buoyant bodies
Apart from buoys as front end energy converter, the non-buoyant type of bodies also plays a vital role in energy harvesting. The researchers convey that these types of devices are better than the float types as the formers are less prone to failure during extreme wave conditions and rough weather. Generally, the front end energy converters work effectively if the incident wave period is close to the natural period of the device. However, in practice, the natural period of any buoyant body is far less than the periods of most occurring waves. This constrain is successfully overcome by the non-buoyant bodies by obtaining a higher specific weight and hence, increased natural period. Figure 12 demonstrates a non-buoyant wave energy converter [
35] which consists of an arm pivoted at the center, a water filled non-buoyant container at one end of the arm and a counter mass at the other end. When the wave approaches the semi immersed container, the arm oscillates because of the unbalance between the two ends and this oscillation is further converted into a rotary motion and finally to the generator through appropriate transmission system.
3.3 Modes of motion
The buoys are classified into heave buoy, surge buoy and sway buoy, depending on their motion.
3.3.1 WECs in heave
These are the simplest forms of point absorbing devices with buoys moving only in heave and in general these are much smaller than the wave length [
36]. The buoys in these devices move up and down during each wave pass; the upward motion is caused by the buoyancy force exerted by waves on the buoy while the downward motion results from the acceleration due to gravity. During the upward motion, the energy is stored as potential energy and the remaining energy is captured by the PTO system. Whereas, during the downfall, the stored potential energy is released by the buoy and harnessed by the PTO for further energy conversion. The WaveStar [
37] device with multiple buoys (Fig.13) in which each of these buoys is linked with a platform through arms and the platform is secured to the sea floor using legs. A hydraulic system with accumulators is used as PTO, which converts and stores the heave displacement of individual buoys as hydraulic pressure.
3.3.2 WECs in Surge
Surge wave energy converters are most preferable for the near shore wave climate where the wave height ranges from 10 to 15 m. Surge WECs is installed at a shorter distance to land thus, the electricity can be delivered to the grid with less power losses. These devices generally consist of a bottom-hinged oscillator or flap as shown in Fig. 14, which completely penetrates the water column from above the surface to the seabed. When the wave strikes the flap, it swings with respect to the hinged reference. The hydraulic PTO system is connected to the flap pumps and the Pelton turbine is used to convert the hydraulic energy in pumped water into electric energy.
3.3.3 WECs in sway
The sway buoy extracts the energy from the waves and moves in side to side over the sea bed. These relative motions are utilized to compress the working fluid which may be of hydraulic or pneumatic to drive the turbine, which is incorporated with the generator to generate electricity. The sway wave energy converter (Fig. 15) developed by the BioPower Systems [
39] is an array of blades mounted on the sea floor with a pivot near the bottom. The energetic waves interact with the blades, as a result, the pivoting structure sways and this motion is converted into electricity by an onboard self-contained power conversion module.
3.4 Long cylinder/Tubes
In WEC, to transfer the wave motions to the PTO system, appropriate front end converters are required. Various techniques are proposed which have their own relative motions to convey the wave energy to the power generator via the PTO system. Apart from the general front end converters, flexible and rigid tubes are also in great demand.
3.4.1 Flexible tubes
The structure fundamentally consists of a water filled rubber tube which is placed in the sea. Both ends of the tube are sealed and it is anchored with its head to the waves. The rubber is squeezed or enlarged by waves causing pressure variations along its length, due to which, a running bulge wave is generated. The speed of the bulge wave depends on the dimension and material property of the tube which is designed in such a way that the bulge wave travels at the same speed as that of the external wave. At this resonant condition, the bulge wave grows as it moves through the tube and this permits energy to be extracted indirectly and it is transmitted to the PTO. Figure 16 illustrates the anaconda which is a flible tube WEC.
3.4.2 Rigid cylinder
The rigid cylinder WEC is an off-shore energy harvester consisting of a semi-submerged snake like structured device composed of articulated cylindrical section linked by hinged joints. The cylinders flex is relative to one another when the wave passes through and this relative motion is resisted by the hydraulic cylinders. Further, these hydraulic cylinders pump the pressurized oil through hydraulic motors via hydraulic accumulators. In turn, the hydraulic motor drives the power generating unit. Figure 17 shows the Pelamis [
41] which is an example of the rigid cylinder WEC.
3.5 Surfing
The front end energy converter may be also classified as surfers based on its typical surfing motion on the water surface when the wave passes through it. These devices are well suited for near shore installations. Generally, the surfing WECs (Fig.18) consist of a set of paddles attached to a common drive train and mechanically linked to the electric machine. These paddles move in the horizontal direction when the wave strikes and in turn, the electric machine generates power. Once the paddle reaches the downstream end of the system, the electric machine is switched to motor mode and lifts the paddle out of the path, thus lowering the next paddle for operation. It is also noticed that the efficiency of these devices is affected by various losses such as friction, inertia and drag. It is concluded that the efficiency is dependent on the wave angle and hence it is important to optimize the paddle geometry to overcome friction and inertial losses.
3.6 Gyro balancers
This technique implements the gyroscopic effect to harvest the wave energy [
43,
44]. Generally, the gyroscopic effect is obtained by placing a spinning flywheel inside the sealed floating device. When this flywheel is subjected to the incoming waves, a torque is generated which drives the power generation unit. Most of these devices achieve significant amount of durable and reliable operation in the ocean environment. Figure 19 is a typical example of a gyro balancer WEC.
3.7 Pendulums
Pendulum type of energy capturing devices exploit their horizontally moving components in the wave motion to produce the primary displacement as oscillations [
45−
48]. These are uncomplicated methods of wave power extraction which are potentially superior in terms of durability and maintenance. The relative motion obtained from the pendulum oscillations is utilized to compress the working fluid instead of direct electricity generation. Further, the compressed working medium is used to drive the power generating unit. The proposed pendulum type energy converter shown in Fig. 20 consists of a dual-stroke pendulum as front end interface and the pendulum oscillates when the wave impact is felt. The rods of the hydraulic cylinder move accordingly, thereby pressurizing the fluid and transmitting into the next stages for power generation.
4 Discussions
This review propounds all varieties of front end energy conversion techniques developed for point absorbing WECs. Different techniques are introduced and each one has its own advantage and besides these, there are certain features which play a vital role in the design of front end energy converter to function effectively. The inferences obtained during the review which will contribute to the designing of new and improved wave energy converters. To be efficient, any WEC should be working in closer to its natural period. The period of real sea waves are usually much longer than the natural period of small point absorbing devices, which makes most of the small WECs less efficient. Making the device larger or making the device heavier are two options to increase the natural period of device. Hence, any point absorber needs to be designed by keeping these two points in mind. It is also found through the findings of Heikkinen et al [
27] that any point absorber should harness waves in more than one mode of motion (Heave, Surge or Sway). Since most of the existing technologies focus only on heave, future WECs should incorporate more than one mode of motion. It is suggested the surge mode be preferred for a WEC if it is placed deep sea [
38]. The deep sea waves usually exert force on any floating object in heave, whereas the broken waves will exert horizontal forces on floating structures and induce huge structural stresses. To avoid the damage of device by rough and broken waves, any device should be placed well above the maximum wave reach and the front end energy converters should be connected using flexible connectors such as ropes and chains. For the above device setup, using a spar platform anchored with sea bed is best suited [
34]. The flexible or rigid cylindrical tube type front end energy converters provide promising results with improved energy conversion possibility [
40,
41]. These tube type front end energy converters align themselves with incoming waves and move in more than one mode automatically. It is also suggested that any device with multiple front end energy converter connected with single electrical PTO be much efficient and cost effective than individual devices [
37]. Besides the above conventional WEC’s, the pendulum type and gyro type WECs offer a new perspective of having no external moving parts exposed to waves and still harness ocean waves using its variable surface irregularities. Not exposing any moving parts outside, the waves provide the device a potential advantage of improved safety during rough weathers and waves. To be the best, any future WEC should focus on having no external moving objects but still capable of harnessing ocean waves. Apart from the device design, it is also found that the tuning of the PTO system of the WEC provides an additional advantage of keeping the device in phase with the incoming wave though the disparity between wave period and device natural period is far away. Hence, any future WEC should be compatible to various tuning schemes.
5 Conclusions
The emergence of new devices and method of wave energy conversion is happening by the way of improving any one or more of the energy conversion stages of a wave energy converter. Though many recent developments are found to be happening around the processing of electrical energy to improve electrical efficiency of the device, the real efficiency improvement requires effective front end interface. The front end energy converter is the only responsible stage to capture as maximum energy as possible from the incoming wave and the rest of the stages only process the captured energy. Unless the front end energy conversion happens with the maximum effectiveness throughout the wave spectrum, the device efficiency could not be improved. The present review briefs the various developments in the front end conversion and the significances of each in the present context. The following observations are made out of this review on various front end interfaces.
To be an efficient front end interface, the device should:
(1)Exploit energy in more than one mode of oscillation;
(2)The natural frequency of the device should match with the wave frequency;
(3)The device should cover minimum wave front yet exploit maximum energy;
(4)The device should harness energy from both the up and down motion of waves and it is found using a non-buoyant body and balancing of masses could be best suited for this purpose;
(5)The device should expose minimum moving parts outside and conceal entire energy conversion cycle inside an aerodynamically designed chamber;
(6)The frame of reference for the motion of a front end conversion should not be influenced by the external hydrodynamic forces to ensure device safety and long life of the device.
Any future development of front end interfaces should take into account the above points while making suitable modifications in the device.
Higher Education Press and Springer-Verlag Berlin Heidelberg
PDF
(1861KB)
