Introduction
One of the most difficult design problems faced by the wave energy device designers is the disparity between wave conditions during normal operating conditions and those during rough weather conditions. In most of the cases, designers provide mechanical, structural or other forms of damping to prevent the point absorbers from having large heave motions. Providing damping requires heavy structures and systems to withstand the extreme stresses developed by waves during stormy conditions. To be economically viable, wave devices must generate useful output in the wide range of operating conditions. Making the device more robust enables the device to efficiently harness waves during normal wave climate. The proposed novel method addresses these issues in different perspective and provides high electrical energy conversion and safety.
Point absorbers in heave
Amongst the existing devices reported in the literature, heaving point absorbers are common; the proposed device is developed on a similar principle. Critical review of the existing mechanisms is carried out to compare the advantages of the newly proposed device. Most of the heaving point absorbers are designed for offshore applications; they are either submerged in water or floating on the surface [
1]. As the point absorbers are small in dimension in comparison to the average wave length, they are restricted to heave motion only. A rotary, direct-drive wave energy device proposes a ball screw arrangement for converting heave motion of the float into rotation [
2]; float stores energy during its upward stroke and supplies to the internal system during down stroke. A full scale prototype of linear generator, mounted on sea floor is connected to a floating buoy for harnessing wave energy [
3]. Spring stores energy during upward stroke of the piston as the wave crest passes. An improved device employed deep-draught spar and annular saucer shaped buoy that is restricted to heave motion with respect to the spar; spring is loaded to restore the forces for down stroke [
4]. A mechanical type of float and counter weight system of wave energy uses a surface floating buoy that is kept hanging by a cable; counter weight is attached to the other end of the cable. The cable is running through a pulley, enabling it to rotate when the float heaves. Cables slag during upward stroke of the buoy and hence the generator gets disconnected through a ratchet mechanism [
5]. Archimedes wave swing (AWS) [
6] is a completely submerged linear generator type wave energy converter built at the Portuguese coast. The device comprises of a floor-mounted cylindrical chamber enclosed by a floater which moves relative to the cylinder when encountered by waves. The air inside the cylinder acts as a spring and provides restoring force to the floater. Linear generators installed inside the cylinder produce electrical energy from the heave movement of the floater. A similar device was also employed on heaving body wave energy conversion and its performance is analyzed by adding a supplementary mass [
7]. A buoyant floating buoy connected by metal cable with a linear generator mounted on sea floor is disclosed [
8], a spring is used to pull the float down during the wave trough. In 2006 a latching control is proposed [
9] to limit the heave response of buoy during extreme wave climate. In 2009 the concept of changing natural period of the device by adjusting float draft with upper surface immersion was presented [
10].
All these devices consists of a single floating buoy or two bodies combined together working with the similar principle except in shape of the buoy and the design of power take off system. Incident wave crest pushes the floating buoy up due to the buoyancy force and drops because of gravity during the approach of wave trough. Wave energy is stored during the upward motion of float as potential energy or spring tension and released to the power take off during the down stroke. Further, these float motion is transferred to mechanical, hydraulic, pneumatic or direct power takeoff system to produce electrical energy. This action makes these devices to harness waves only during upward motion and release the stored energy during down stroke. To maximize power capture, designers have designed the devices with response amplitude ratios (ratio of float displacement to wave amplitude) greater than unity, so higher float motions will occur during operational conditions. For economical reasons, it is desirable to ensure that the float response does not exceed the allowable limits during extreme conditions; since, the effect of extreme load is considered as failure mode. To prevent the device from having extreme responses, structural, mechanical, hydrodynamic and other forms of damping are proposed by designers. The extreme nature of waves makes any form of damping a failure and leads to the total device failure.
The proposed device addresses these problems from a different perspective and eliminates the issue of excessive heave response during extreme wave climates without any form of damping and with increased conversion rate than any other point absorbers. The devise also provides a few additional advantages like simplified installation and device shutdown during device maintenance.
Non-floating body for heave response
The proposed wave energy converter is a near shore device to be kept on a rigid platform. The device consists of a cylindrical non-floating object, an oscillating arm, a unidirectional gearbox to convert alternative rotations into continuous unidirectional rotation, a step-up gearbox and an electricity generating unit.
Figure 1 shows an experimental setup designed, fabricated and tested in a small scale wave maker. The model is designed to conduct the experiment at 2 m wave flume at Indian Institute of Technology Madras. A 0.7 m high, 0.3 m diameter water filled steel container is used as the non-floating body and kept hanging from one end of the 2 m long steel arm. A 6 mm metal rope is used to hang the container. Solid weights are used as counter mass instead of ballasting tank for the experimental purpose. The arm is supported by a rotatable shaft and the assembly is mounted on a base plate. The said shaft is coupled with a unidirectional gearbox which converts alternative rotational input into continuous rotation. Further, an electrical generator is coupled with the unidirectional gearbox through a step-up gearbox.
Oscillating arm assembly
An oscillating arm consists of a straight frame pivoted at its middle, a non-floating body kept hanging at one end via metal rope and a counter mass assembly at another end. A water filled steel container is used as the non-floating object in the present case and metal plates are used as counter mass.
Unidirectional gearbox
There are many different methods to convert both positive and negative directional rotation into continuous unidirectional rotation. The proposed unidirectional gearbox is also one such unique technology which provides energy in the form of continuous unidirectional rotation from alternatively rotating arm shaft. The unidirectional gearbox has an input gear and output gear. The output gear provides continuous rotation for the further conversion.
Step-up gearbox
Because the frequency of ocean waves is very low, the speed of rotation from the unidirectional gearbox will be much lower than what is required by any conventional generator. Hence another gearbox is coupled with the unidirectional gearbox to increase the speed of the rotation.
Electrical generator
A conventional rotary type electrical generator is coupled with the output shaft of the step-up gearbox to convert mechanical rotation into electrical energy.
Working of heaving non-floating body type wave energy converter
Figure 2 presents the schematic drawing of various components of oscillating arm.
Initial condition
The entire setup is to be mounted on a platform such that the hanging water container is completely immersed in water. When the counter mass is loaded, the container starts to surface, as an action of balancing the additional force exerted by the counter mass. The cable length is adjusted such that the arm is horizontal after the exposed height of the container reaches the required level. Once all the initial arrangements are made, the arm will be in equilibrium due to the balancing of effective mass of the semi immersed container (m) and counter mass (M).
Working
When incidental wave passes the semi immersed container, the effective mass (m) of the container gets reduced due to the increase in surrounding water level and the arm becomes unbalanced between its two ends. The counter mass (M) pulls the container up as an action of balancing. This action makes the arm to oscillate in one direction and when the wave trough approaches the container, the effective weight of the container increases on account of the decrease in water level around it. This, in turn, makes the container side heavy and pulls the counter mass side up. This alternative balancing of forces makes the arm to continuously oscillate with respect to the point O.
This oscillation of arm makes the input gear of unidirectional gearbox to rotate alternatively and the unidirectional gearbox produces unidirectional rotation at its output shaft. This low speed high torque unidirectional energy is converted into high speed rotation by the step-up gearbox and converted into electrical energy by the rotary electrical generator.
Uniqueness
The following are the features which make the proposed design very unique and highly efficient. Oscillation of the heaving body is caused by the variation in effective mass of the container during wave action rather than by the pushing action of the wave crest. The maximum oscillation can be limited by changing the water in the ballasting tank. The heaving mass is connected via cable with the device and hence extreme forces will not be transferred to the internal components of the device. The device harnesses both the up and the down motions of the waves to produce useful energy but all conventional systems harnesses in one stroke and uses the stored energy during the next stroke.
Experimental results
An experimental setup is designed and fabricated to test in the 2 m deep wave flume at Indian Institute of Technology Madras. Preliminary studies were conducted to find the heave response of water filled container with respect to wide range of regular waves. It was observed from the initial study that the device performs well between 2 to 3 s period waves and the wave maker can generate wave amplitudes up to 30 cm. The model was kept 10 m from the wave pedal and the container was immersed 0.8 m from each wall. A water filled steel container of 0.4 m in diameter and 0.7 m in height was used for the experimental purpose. An 8 pole permanent magnet DC motor was used as the electricity generating unit and electrical bulbs were used as load.
The experiments were conducted by varying wave heights between 10 cm and 30 cm, time period between 2 and3 s and with electrical loadings of 30 and 40 W. The generated electrical energy was recorded using a power analyzer and device performance was monitored. A characteristic study was made and the variation in device performance was plotted. Figure 3 illustrates the impact of incident wave amplitude in generated power. It is found from the experiment that the maximum power generated is linearly proportional to the wave amplitude in all wave periods. Figure 4 depicts the impact of wave period in output shaft speed. The curve indicates that the output speed of the device is significantly reduced outside the band of 2.3–2.7 s. It is also observed that the device speed is under control even for higher wave amplitudes outside these bands of wave period. Figure 5 demonstrates the performance of the device under 60 W loading. It is seen from the curve that the device performance increases to 30 watts loading for higher wave amplitudes and between 2.3–2.7 s wave periods.
Discussion
The purpose of the study was to find out the possibility of using a non-floating object to harness ocean waves in heave displacement. The experiments indicate that the non-floating objects provide greater energy conversion in linear waves. The device shows significantly higher efficiency in the range of 60% between 2.3–2.7 s wave periods and higher wave amplitudes. With no additional modifications or damping attachments, the energy captured significantly reduces when the wave period goes beyond the band of 2.3–2.7 s. These characteristics demonstrate that the extreme variation in the wave climate makes the device to capture very little energy and keeps the device safe. It is also observed clearly from Fig. 4 that the speed of output shaft significantly reduces even for higher wave amplitudes beyond the wave period band of 2.3 and 2.7 s. The increase in electrical loading makes the device to have a higher energy conversion rate between 2.3–2.7 s wave period and reduces the device performance beyond this range of wave periods. It was also observed that the energy capture of the device depends on the counter mass which is varied with ease. In real sea application, the counter mass tank can be de-ballasted and the container can be immersed in water to protect the device from extreme circumstances. It is also possible to drain the container and bring the entire container assembly out of the water.
Conclusions
Offshore engineers are concerned with protecting the wave energy converters from extreme conditions. They are more particular on limiting the heave response of the floating body during extreme conditions. It was clearly understood that providing mechanical stoppers or limits will not be practically possible as the structural load will be very high in extreme conditions. The engineers also want the device to perform with maximum efficiency in defined range of wave environments. In this paper, a novel method of using a non-floating object to harness waves in heave motion with maximum efficiency and safety was proposed. The experimental results showed that the energy capture of the device is significantly higher per unit wave front compared to other existing point absorber technologies. It was also proved that the device automatically damped during extreme wave climates with no additional damping techniques. The unique working principle of the device makes all these advantages possible.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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