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Wave power offers a huge potential renewable source for energy generation because more than 70% of the world's surface is covered by water. That is why engineers and scientists have been studying for many years to find useful ways to harvest the power generated from waves.
At the moment, because of the high oil prices, the world petroleum crisis, the imminent catastrophic effect of global warming and greenhouse effect, there is an urgent need for mankind to tackle immediately the problem of climate changes and global warming and start to make serious and effective efforts to find alternative and reliable energy sources.
Ocean and sea waves can play a very important role on achieving it, and they can also help solving a big part of the problem. It is well know that wave power is not involving large amount of CO2 emissions.
Sea and ocean waves potentially and ideally can travel for long distances without losing energy; also the power density of waves is much higher than any other renewable source (for example wind or solar power).
The potential worldwide energy that can be generated from waves has been estimated to be of about 2 TW.
Sea And Ocean Waves: Physical Concepts
Ocean waves can be seen as a secondary form of solar energy because wind is derived from solar energy. In fact, sun heats the earth differently depending on the latitude and longitude. This difference in temperature causes differences in air pressure which results in wind generation.
In more details, sea waves are generated by wind blowing over the sea surface. This is because air flowing over the sea, exerts a stress on the water surface that results in the formation and growth of waves as it is depicted below in figure 1.0.
It is clear that the wave's size and energy carried by the wave depend upon three factors:
· Wind speed
- Period of time during which wind blows over the sea surface
- The distance over which the wind is blowing over sea surface, called Fetch.
Even if real waves are not actually sinusoids, they can be seen as a sum of sinusoids. It can make easier the analysis of ocean and sea waves as it will be seen later.
Ideal waves, called Monochromatic waves are characterized by their Wavelength (λ), Height (H), Period (T) and the Direction.
The power (P) of monochromatic waves can be seen as:
- ρ is the sea water's density
- g is the acceleration due to the gravity
- H is the wave Height
- T is the wave period
Stochastic Nature Of Waves
Ocean and sea waves have a random and a not predictable nature. This means that their characteristic waves cannot be predicted very well in advance and that, in real sea wave condition, waver have all different heights, periods and directions compared with each other.
They are also said to be irregular or polychromatic waves. Example of annual variation of Polychromatic waves Wave-height during a period of one year. As it can be seen from figure 1.2 represents a random result and therefore not predictable in advance.
This is way, when dealing with real waves, two new terms are introduced: one is Significant Wave-height (Hs) and the other is Zero-up-crossing Period (Te).
The first is defined as 4 times the root mean square of wave height. The second is defined as the average time between upwards movements of the surface through the mean level.
Therefore, for a real wave the average power can be seen as:
Waves can be classified depending on how much distance is from sea surface and sea bed:
Waves in shallow waters. They are close to shoreline and are characterized by having a small amount of potential energy. Shallow waters are considered when the Height (H) is:
H is the distance between the sea surface and the seabed.
Waves in intermediate waters. They are located in between shallow and deep waters.
Waves in deep waters. They are far away from shoreline and have a huge amount of potential energy. The direction of waves in deep water is usually also the direction of the wind.
A schematic representation of different water locations is depicted.
However, there are some places in the world, where the shoreline is actually followed by a cliff which drops immediately in deep water and different notations have to be made.
From what said above, it is quite obvious, that waves travelling from deep water, lost a great energy potential, when reaching shallow water. This is mainly due to the frictional coupling between the water particles and the sea bed that are closer in shallow water having thus a stronger frictional coupling.
For example, a travelling wave located in deep water carrying power density of 50 kW/m, when reaching shallow waters might contains only 20 kW/m of power density . There is therefore about a 60% of the original potential which was lost on getting closer to the shore which must be carefully considered by engineers.
Waves can also be classified by their wavelength into:
Swell waves. They are waves that can travel for long distances from the point where they are first generated. Usually they have a small amount of energy loss during the travelling. These waves are generated by storms that blow on a sea surface usually bigger than 100 km.
Sea waves. They are waves that have a shorter wavelength compared to swell waves. These are generated by winds acting on a sea surface of usually 100km.
Capillary waves. Also called ripples, they are waves with a very small wavelength and so are affected by surface tension which causes the wave to have a circular crest. Surface tensions means that pressure of a liquid will be actually different between the edge area and the outside are of the liquid itself. The maximum wavelength of a capillary wave is of about 1.73 cm.
Wave Energy: History
Although human being has been attracted to the idea of extracting energy from ocean and sea waves for many centuries it was only in the early 1970s that practically implementations begin to exist. The actual research on wave energy in the UK begin with the Central Policy Review Staff Report , which actually identified the government responsibilities in order to make a good use of alternatives energies for generating electricity.
The UK Energy crisis of 1973 was one of the reasons of an increase interest in renewable energy and especially of wave energy because of the great potential as a source of electricity for the UK National Grid.
During the period from 1974 to 1983 the UK Department of Energy (DEn) funded programs of research for wave energy conversion and extraction. It was included in the program called Wave Energy Programme (WEP). One of the main aims of the WEP was to estimate the cost of energy production from waves for the National Grid.
Many devices were contrived and then experimentally tested but they never succeeded because there was not a very big commercial support for them to develop. In fact insufficient effort and money were made available in order to permit these technologies to evolve into a good and stable way of generating electricity for public and domestic use.
However, this situation, was fortunately, not like that all around the world, in fact some other countries, like Japan and Norway, put a great effect and increase their research and development programmes.
After this black period for renewable energies, in the 1990s, the EU starts to provide funding to a few numbers of projects and launched the European Wave Energy Technology and Support Unit Chief Scientists' Group (ETSU) which is a “group” that basically supports and provides information about Renewable Energy.
A few years later the ETSU estimated that the technical potential of the UK offshore wave energy resource consists in an average of 61-87 TW per year.
In 1999 the Scottish government presented the Scottish Renewable Obligation (SRO) which requires that public electricity suppliers generate some of their electricity from renewable energy sources.
In the meantime in England and Wales a sort of SRO was presented. It was called the Non Fossil Fuel Obligation (NFFO) which objective was to exclude fossil fuels, coal, natural gas and petroleum from energy generation purposes.
In November 2001, the Prime Minister of UK announced that the Government will spend a further £100 million pounds for renewable energy. The Cabinet Office Performance and Innovation Unit (PIU) suggested allocating £5 million of the above for demonstration and testing of wave and tidal technologies.
Wave Energy: Current Status
As time goes by there has been a very significant change in the way devices are designed and also performances and construction techniques have increased their quality. Some of the problems that wave energy encountered during many years of tests have been overtaken and now the all process of renewable energies for domestic and public use is quite mature.
However, at this very moment, wave energy is competitive almost only in remote places where there is a great potential of waves and where traditional energy sources are expensive or not available.
One useful way of comparing the energy extracted from waves and the energy extracted for example from coal fired is by comparing their cost of electricity generation. Therefore it is important to remember that the cost of generation of electricity from waves is now remarkable decreased.
In 2001, Thorpe said that the actual annual potential of wave resource is of about 30 TW at a cost of 4p per kW. The coal-fired cost of generating electricity is at the moment of about 3p per kW.
This is one of the reasons of why wave energy could not yet become a reality. Practically it is becoming every day more competitive but at the moment it is still far away from its best use.
U.K. is situated in a very well position for wave energy potential extraction. This is because the U.K. is surrounded by stormy waters and because it lies at the end of the Atlantic Ocean. The Atlantic Ocean can be seen as a very long Fetch over which the wind blows most of the times directed toward the U.K.
Wave Energy: Different Uses
Wave energy can be used for many purposes. The main ones are listed and described briefly below.
- Seawater desalination
- Refrigeration plants
- Mineral recovery from sea water
- Hydrogen generation
- Pumping of clean sea water
- Heating of sea water
- Propulsion of vessels
- Coastal protection
- Marine culture (i.e. cultivation of marine life for food purposes)
However the main use for human being at the moment is to convert ocean and sea waves into electricity which is also the main purpose of this project.
Wave Energy: Perspectives
Waves' power varies between locations around the world as shown below in figure 1.4. As it can be seen, the U.K. has an annual average wave power of 70 kW per metre. It is also known that the areas where the wind blows regularly over the whole year are the areas where the wave energy is greater. One of the areas with the major potential is the Atlantic Ocean as mentioned before.
Since sea waves can travel for very long distances with only a small amount of power loss, at any location we can find waves arriving from different places around the world with different characteristics and different directions. This is actually why is practically impossible to predict in advance the real wave pattern which is presented in an irregular and random manner.
As said previously, it is known that the potential worldwide energy can been estimated to be of about 2 TW.
However, wave energy cannot be exploited all around the planet at the same way. This is because ideally some factors are required, and some of them are that the wave climate needs to be suitable (i.e. high wave energy with frequent waves during all day) and that deep water should be near shore.
In fact, there are some countries like The United Kingdom, the United States of America, Japan, Sweden, The Netherlands, Denmark and Australia that have a greater potential of wave energy compared to other countries.
In 1992 it was estimated that the total annual average of power extracted from waves in UK ranges from 260 TW to 700 TW. This data can be compared to the actual annual electrical energy used in U.K. This was of about 360 TW in 2005. This confirms that, by exploiting the full resource of wave energy available in U.K., a big percentage of the demand for electricity can be satisfied.
The PIU also suggested that, by 2020, 20% of the energy used in U.K., would be derived from renewable energy sources.
But it is also known that political attitude is not very helpful and favourable with the wave energy exploitation development. However, as said before in the Introduction, the urgent need to address immediately the problem of global climate change, force them to consider seriously a valid alternative for global energy production.
A very optimistic view for the future of wave energy is given by the fact that the prediction costs of generating electricity from waves are much lower than the actual costs are. This decrease in production costs will exist because of the improvement in performance of the devices and because of the increase in the scale production. I.e. many devices will be operating at the same location. Also, as we will discuss later, Engineering Control can play an important role in achieving this decrease on the costs of production.
Wave Energy: Drivers And Barriers
But if the potential is so great why it is not exploited as it could be done?
This is because there are some barriers that are affecting the natural development of exploiting wave energy and renewable energies in general. Drivers and barriers for a flourishing development of wave energy are listed and analyzed briefly below.
- Wave energy is naturally replenished. It means that there is always availability of it in abundance around the globe and that waves are generated by nature on a day basis.
- Climate changes and global warming emphasize the urgently need for mankind to reduce the CO2 emission into the atmosphere.
- High oil prices and world petroleum crisis highlight the need to find out a valid and reliable alternative source of energy production.
- The practical energy that can be extracted at the moment is much smaller than the potential one because of operational and economic constraints. In fact, at the moment, wave energy technologies have a very high cost of production and maintenance.
- Not every country has optimal conditions to develop wave energy extraction; some countries have good resources but not during the whole year.
- Industries of oil and fossil fuel, which are now very powerful, can influence governments and obstruct the natural development of renewable energies.
- Renewable industries often need government sponsorships to help them to be competitive in the actual market. Otherwise, they cannot be competitive.
- Consumers are very lazy to shift from an already well know system to new technologies, therefore it is fundamental to provide them the new energies in the same manner they have been provided before.
Wave Energy Converters (Wecs): An Introduction
Wave energy converters (WECs) are devices that convert the kinetic energy and potential energy of ocean and sea waves into a more useful way of energy, mainly electricity.
During the last three decades many different devices have been proposed and tested.
The main concept exploited by WECs, is using induced motion (of bodies or masses) to actually drive a Power Take-Off System (PTO System). Some examples of PTO systems are electrical generators, air turbines and hydraulic rams. Some examples of WECs that make use of a PTO system are The Pelamis and The Searev (discussed later).
The WECs are usually mechanical oscillators which, hence, perform better if waves' frequency is close to their natural frequency.
Wave Energy Converters: Factors To Choose A Wec
It is known that waves are composed of orbiting particles of water. Near the sea surface these orbits have the same dimension as the wave-height but as we go deeper toward the sea-bed these orbits becomes smaller in an exponentially way.
Knowing this, it is important to consider that, in order to extract the maximum energy from a wave, ideally, the device should capture all these orbiting parts. However, in the real world, this is not possible achievable. Therefore, the designers need to decide how deep the device should actually be built.
Doing so, is good to know that about 95% of the wave energy is actually in the area between the sea surface and a depth (H) equal to a quarter of the wavelength. Therefore, in mathematically terms, for a given sea surface at a height (h), it is known that the 95% of the wave energy is in the area (A) given by:
h < A < H = (m)
Another important factor on choosing the WEC location is on trying to avoid the so called Breaking waves. These waves usually exist in shallow waters or close to it. These are the ones that are so high that they no longer cannot support their top and it collapse. These waves can be very dangerous for the structure of the WECs.
The size of the WEC structure is also a factor that needs to be considered in order to maximise performances and efficiency. Usually the size of the device needs to be appropriate to the amount of water that this will deal with.
Wecs Classification By Location
There are many ways of classifying WECs: the first is to classify them by their position:
These are usually in shallow waters and fixed to the seabed.
Are for obvious reasons more difficult to construct and to maintain and also are more expensive. An advantage of off-shore devices is that they can harvest a greater amount of energy than on-shore devices do. This is because they are in located in deep water which have a greater potential energy available than shallow water (where on-shore devices are situated) as discussed before.
One efficient way of generating electricity from wave energy is to construct an array of devices off-shore. They are able to capture a great amount of energy that can then be converted in electricity and transmitted to shore by means of underwater cables.
Wecs Classification By Geometry
The other, more useful, way of classifying WECs is by their Geometry. By doing so, four types of devices are identified: Point absorbers, Attenuator, Terminator and Overtopping devices.
Point Absorbers Devices
It is well known that when an object is submerged in water or any other fluid, a force acts on the body moving it upwards. This is force is called Buoyant force and is equal to the weight of the liquid that has been displaced by the submerged object.
And is following these principles that Point Absorbers work. In fact, Point absorber devices are characterized by a floating structure (usually a buoy) that moves with movement of incident waves. They usually have a very small physical size comparing it with respect to the incident waves.
The relative motion between buoy and the rest of the structure is then used to generate electricity by using hydraulic or electromechanical energy converters. In figure 2.1 a representation of a Point Absorber device is depicted.
Theoretically these devices are able to generate more power than other WECs because Point absorbers are dealing with waves in deep water which have a great potential energy.
These devices include Edinburgh Duck, Clam and Pelamis from U.K. Wave dragon and Backward Bent Duct Buoy (BBDB) are also very important examples of Point absorbers devices. Some of them will be discussed later.
Archimede Wave Swing (Aws)
The Archimede Wave Swing is a Point Absorber WEC device which first was built and tested in 2004 in Portugal.
This device consists in a submerged cylindrical buoy linked to the sea bed. The Archimede Wave Swing has only one moving part, the floater, which is an air-filled device connected to a fixed cylinder. A schematic representation of an AWS.
Within this cylinder there is a generator that converts the up-and-down motion of the floater into energy. This energy is said to be made from the reciprocal movement of the floater and the incident wave.
In fact, when the waves approach the device, there is an increase in pressure at the top of the floater, which pushes the mechanism inside the cylinder downward.
Lately, when the waves pass over the floater, forces it upwards. This is the reciprocal motion generated by the floater we mentioned before which converts, using a hydraulic system, this motion into electricity.
Advantages And Disadvantages
- Having only one moving part (as said before) the AWC device is more reliable compared with the other devices and it needs, therefore, less efforts and costs for its maintenance.
- Power density of AWC is about ten times greater than any other device for renewable energy.
- AWC is actually a device that does not produce any noise.
- It has no high-speed rotational equipment. Therefore it does not incur in pollution problems.
- Point absorbers, in general, are potentially more economical compared with other WECs. This is principally because they have smaller dimensions, sizes and weights.
Others Point Absorber Devices
The Edinburgh Duck
The first Edinburgh Duck prototype was proposed in the early 1970s by Professor Salter at Edinburgh University.
However the first official model was first tested in 1982. This first model consisted in a floating boom, the segment of which fluctuates as the waves reach the device. This fluctuating motion was then used to drive motors/pumps and spin generators. A schematic representation of an Edinburgh Duck.
The 1982 Duck was able to generate an average power of 164 kW. However another model of Duck was tested and was called 1991 Duck, this latest device was able to generate an average power of 498 kW .
Salter's ducks have been developed over two decades after this first prototype was proposed.
The Wraspa Device
The Wraspa WEC was first proposed by the Lancaster University and it is meant to deal with water between 20 and 25 m depth.
When waves reach the Wraspa WEC they force the collector body, which is carried on an arm, to rotate, about a fixed horizontal axis located below sea level.
An example of the Wraspa device is depicted.
In Wraspa, therefore, the body oscillates at about the same frequency of the sea waves. Thus, it can be said, that a very high power from a relatively small and cheap device can be generated by using a Wraspa WEC. This very good result is demonstrated.
Another advantage of the Wraspa is that in extreme wind situations, the arm is moved to a position that minimise fatigue, so as to ensure that the device does not get damaged under such conditions. Because of this, the Wraspa WEC can represent a very valid alternative for those areas where the wave energy has good potential resources but, where extreme wind situations are often likely to occur.
The Sea Clam
This Sea Clam device was presented in 1986 at Coventry University and was firstly proposed to be used in remote communities and Islands.
The structure was a floating toroidal dodecagon of approximately 60 m circumference and of 8 m high. The Sea Clam was designed to extract energy from waves by using the displacement of air.
The device consists in twelve air chambers placed around the toroidal dodecagon. Each air charmer was composed by flexible membranes in the outer part of it. As waves reach the structure, the membranes move in and out the structure, forcing the air to move from one chamber to another.
This air flow drives Wells turbines place between each air chamber and, lately, this energy created by Wells turbines, is converted into electricity by means of some generators.
An example of Sea Clam tested in Loch Ness in depicted below in picture 2.5. This structure was contrived to be moored a few kilometres off-shore.
A latest prototype of Sea Clam called the 1991 Clam was able of generating 598 kW of average power.
Attenuator devices, also called Surface following devices, are characterized by being oriented parallel to the direction of the waves.
The movement of the waves along the length of the device cause the device to flex where the segment are connected. This movement, in the connection from section to section, is used to generate electricity by means of converters. An example of Attenuator device.
The “Sea Snake” Pelamis
The Pelamis wave energy converter is an attenuator device, able of generating 750 kW of electricity, which was first installed at the European Marine Energy Centre in Orkney in Scotland.
This device is a submerged structure which consists of four tubular steel floats linked one another by hinged joints. The force exerted by the incident waves in moving each segment of this structure is resisted and captured by hydraulic rams which pump a biodegradable fluid through hydraulic motors via accumulators. Lately, these hydraulic motors drive electrical generators which produce electricity.
Power from every single segment of the Pelamis is fed and conjoint down into a single junction on the seabed via an underwater sea cable. The Pelamis device is depicted.
The Pelamis contains in total three power conversion modules which contains an electro-hydraulic power generation system. Each of these is capable of generating 250 kW of electricity.
The latest version of the Pelamis device is of about 180 m long and 4 m of diameter.
The Pelamis wave power, the company that develops Pelamis devices, states that this WEC is able to produce an average between 25% and 40% of the full rated output per year.
On their web site, they also said, that each Pelamis, is able to generate energy from waves in order to provide electricity for 500 homes.
The inventor of Pelamis, Richard Yemm, declared that he expected the future versions of Pelamis, to be able to produce easily a kW of electricity with less than 3p and hopefully to be able to produce a kW of electricity with even less than 2p.
Advantages And Disadvantages
- At the moment Pelamis offers a kW of electricity at the lowest price in the market.
- The Pelamis within its structure has only smaller parts of metals and other materials that can be dangerous for the surrounding environment.
- An increase of the structure's mass, due to the marine growth, can be neglected as it does not affect the overall performances of the device.
- The hydraulic rams use a biodegradable fluid that is not, therefore, harmful for the surrounding marine environment.
- Pelamis produces a low amount of CO2 compared with the electricity output that generates.
- As it can be easily expect, the power generated from Pelamis is dependent upon the location and the condition of the place where the device is actually mounted and built. Therefore, its utilization cannot be expected to be florid all around the globe at the same way.
Other Attenuator Devices
The Searev Wec Device
The Searev WEC is a 25 m of length and 13 m of width. The Searev is a floating device with an embarked moving mass within its structure as depicted below in figure 2.8.
Floating device and moving mass are forced to move as waves reach the structure. This relative motion is later used to drive a generator.
One of the latest version of the Searev WEC has the advantage that the moving mass within the structure, that in normal sea state operates like a pendulum, in extreme cases (i.e. when wind and waves are much stronger than normally) is actually free to complete a full rotation. Therefore this device is able to take advantage under extreme wind situations. Therefore, these, usually severe and dangerous conditions, can become favourable conditions. In fact, during extreme events, the device is able to produce more energy than under normal sea state conditions. This is a very rare feature for a WEC.
Terminators devices are characterized to be parallel with respect to the incident wave and they actually capture and intercept the power of the waves.
Terminator devices are very easy to maintain which is an advantage with respect to the Point absorbers discussed before.
These WECs usually operates only in shallow waters and therefore the water that they exploit has not a very high amount of potential energy on itself.
From Water To Air
Many Terminator devices transform water into air and from air then generate useful energy (usually electricity).
This is achieved by means of air turbines; the most used are Wells turbines after its inventor Alan Wells in the late 1980s.
These turbines are driven by the up-and-down movement of the water into the air chamber.
Water represents a big source of energy because it is able to rotate an air turbine at 1200 rpm.
Oscillating Water Column (Owc)
Oscillating water column devices are the most common form of WECs devices and have been tested and used in several countries with quite success for many years. This success is mainly due to its robustness and simplicity of use.
In OWC there is a chamber filled with air that is pressed up when waves reach the device. This air is therefore passing from the chamber through an air turbine connected to a generator which generates electricity. It is depicted below in figure 2.9 The air turbines used are usually Wells turbines (introduced before).
Advantages And Disadvantages
- OWC devices have been tested already for many years and they demonstrated already to be a reliable device for wave energy harvesting.
- Cables and generators are on-shore, therefore maintenance costs are lower.
- Costs of generating electricity can become even lower by improving the structure and by using natural hollows instead of artificial ones.
Other Terminator Devices
The Shoreline Gully Owc
The Shoreline Gully device was first presented in 1985 at the Queen's University of Belfast. It was a 10 m deep device and 10 m width capable of generating an average power of 250 kW.
The intent of the designers was to develop a cheap device which could be built in island and remote location. In fact the project was built in a natural gully in the Scottish Isles. By using a natural gully it was possible to save some money.
The operational principles of the Shoreline Gully OWC were the same as for a generic Oscillating Water Column discussed before.
The Shorereline Gully OWC was, later, included in a 150 kW scheme project which included a Wells turbine.
The Osprey Owc
The Osprey is an OWC designed by Wavegen and its first prototype, called Osprey1, was presented in 1995 in Scotland.
The structure of the Osprey1 is depicted below in figure 2.91 and it incorporates a collector of 20 m wide which was located between two ballast tanks. The main function of these two tanks was to concentrate the waves into the collector.
The latest version of Osprey OWC is capable of generating an annual power output of about 4955 Mw/h.
Overtopping devices are characterized by having a reservoir filled with waves. This reservoir is located above the level of the surrounding sea as it is shown below in picture 2.92.
At some point the water is free to go through an opening and to come back to the ocean due to the gravity force. The energy of this travelling water is used to drive a hydro Turbine.
Wave Dragon Device
The first prototype of Wave Dragon WEC was developed and tested in Denmark. The Wave Dragon is usually situated off-shore and consists in a large floating reservoir above the level of the sea. Waves reaching the structure are concentrated by a pair of reflector arms and then are pulled into the reservoir. Water is let out through an opening and on coming back to the sea drives a hydro turbine. Thus, the device generates electricity by means of generators.
The latest version of Wave Dragon device is a huge structure of 48 kW per metre rated at 11 MW which has a reservoir that contains approximately 14,000 of water. Its weight is of 54,000 tons, it is 390 m wide and its length is 220 m. The total height of the structure is of about 19 m and it drives from 16 to 24 turbines.
Although it is a massive structure it is able to produce an annual power of 35 GW/h.
Advantages And Disadvantages
- There are only few moving parts (i.e. the turbines); therefore the structure is reliable and cheap to be maintained. Usually off-shore devices have to deal with extreme winds situations and water with great potential, therefore the moving parts are the most likely to get damaged.
- The Wave Dragon is simple and it uses the wave energy directly without the need of converting it into mechanical motion or anything else.
- Wave Dragon can govern situations with intense wind and it is environmental friendly.
- Since it is the largest wave energy convertor in the world it has a low Mass-Power ratio. In fact a proposed structure rated at 4MW of 300 m length has a mass of about 30,000 tons.
A Brief History Of Wecs
The idea of extracting energy from waves is an ancient idea of humankind. In fact the first device discovered was in 200 BC. It was a Persian water wheel (figure 2.93) placed in a stream of water (usually a river) and the wheel turned as the water flowed against it. It was used for lifting the water from the water stream to a higher location.
A Brief History Of Wecs
On the April 1976, in London, Dr Walter Marshall, announced that the British Government was to spend about £1 million pounds on investigating the ways of harvesting energy from sea waves . It was then, actually, the beginning of the wave energy modern exploitation.
In 1977 the Edinburgh Wave Tank was designed, which was a tank that scaled an ocean area of about 3 km wide and 1 km of length. With the help of this tank it was possible to recreate real ocean conditions and in fact it was first used for testing the effectiveness of the so called Ducks.
In 1985 a 350 kW prototype called Tapchan started its operation in the Norwegian islands. As it can be seen below on figure 2.94 the Tapchan device consists on a Tapered channel, a Reservoir and a Turbine house.
A Brief History Of Wecs
The channel walls on the prototype were of 10 m high and 170 m long.
The mechanism was simple: water entering into the collector passes through the tapered channel which converges the water into the reservoir. Because the waves are forces into a tapered channel, their height is amplified until the crests spill over the walls into the reservoir at a level of about 3 meters above the sea level. The energy of these waves is, hence, converted into electricity when the water passes through a turbine back into the sea. Tapchan was able to produce 350 kW of electricity that is, then, sent to the Norwegian national grid.
Since the Tapchan has only a few moving part it is simple, easy and cheap to maintain. Also, because of its shape, it permits to actually smooth the electrical output. In other words, since Tapchan gathers waves into its reservoir, the turbine output is only relative to the difference in water levels between the sea and the reservoir. This is a very peculiar characteristic for a WEC that makes it reliable.
A challenge for the designers of Tapchan is to be able to actually predict the future. In fact, by doing so, it is possible to output a greater amount energy during the period before the arrival of large waves and so when they arrive the reservoirs is almost empty and there is more room to collect the large waves. This project can be also good in order to save money, because, by predicting the future, designers can have devices with smaller reservoirs.
In 1989, a project fully funded by Den, in Scotland, at the Islay, a 75 kW prototype of a particular Oscillating Water Column (OWC) was installed. The challenge of this prototype was to develop a cheap device which could be built on small islands and remote locations. It consisted on a wedge-shaped chamber, open at the bottom, into which sea water enters. This project was development by the Queen's University of Belfast (QUB) which, in collaboration with the Wavegen, developed the Designer Gully OWC. It was a better version of the original prototype including some differences in the construction and the shape of the OWC. The Designer gully was excavated behind a rock wall which was, at the end of the installation process, removed. Here the water column, which moves up-and-down with the waves' movement, acts as a piston drawing air into-and-out the chamber. The air flow drives a Wells turbine which is connected to an electric generator which generates electricity. The latest version of this project started in 1989 was developed in 2000 and was known as Limpet. The Limpet drives two Wells turbine connected to a generator of 250 kW each giving the total output of 500 kW.
Another more recent device developed and tested in the U.K. was the Bristol Cylinder.
The Bristol Cylinder device, depicted below in figure 2.95 was contrived in England by the mathematician David Evans of the University of Bristol. The Bristol Cylinder consisted on a large cylinder submerged in water but allowed to move freely in response to the incident waves. This cylinder is connected to the seabed by two anchors that actually provide the power take-off mechanism.
A Brief History Of Wecs
Wave Energy Converters: Environmental Considerations
Although there are not yet well identified and serious problems caused by wave energy extraction, the major environmental issues and considerations are listed and described below:
- The physical condition of the coastline: sand beaches are the most affected by the wave action in the coastline. Detritus and other material from the WECs can be, very easily, transported on shore.
- WECs usually contain lubricating oils but, however, the dispersion of them in the surrounding environment, can be easily avoided with little care and effort.
- Visual landscape: on-shore WECs can be seen from coastline and, in some rare cases, off-shore WECs can also be seen from the coastline.
- WECs usually produce a very low noise. The higher noise can be in fact, represented by the waves that crash on the structure.
- It will be extremely important to remove the unused structures when they will not be any longer operating.
- Ecological balance: Fish, birds and plants lives can be affected by the presence of WECs. For example, there are some fishes that spend most part of their lives close to shore, and it would be impossible or difficult in presence of an on-shore WEC. The WEC affect also the natural flowing of the currents some fishes (such as the Scottish Salmon) may be lost when returning to the spawning grounds. It may affect the Scottish economy as well since its commerce is very important for the country.
- Navigation and fishing: there is a real risk for boat to hit the WECs devices and mechanisms. Because WECs, sometimes, are submerged or semi-submerged and so not easily visible. There is also a risk for the very small boat, because of the incident waves reflected from the WEC; they can be very high and powerful sometimes and can so knock over the small boat.
These risks can be avoided by implementing more sophisticated systems on fishing boats but they may be very expensive.
As it can be soon realized, wave energy conversion is a very good alternative resource but by using it we can incur in very severe problems.
It is therefore very important, for WEC's designers, to consider all this aspects above and to try to avoid them or at least limit them.
In fact, a derangement of the sea-habitat, by the name of the renewable energies, will not be tolerated.
Wave Energy Converters: Issues
There are many factors and situations that may damage the structure or the moving parts of the WECs. They are briefly listed and describe below:
- Extreme winds situations must be considered. Extreme wind situations happen when wind blows at a very high speed and therefore making waves to have a tremendous amount of power. This can be a very good situation for a WEC able to support and deal with it because of the great potential amount of energy carried by the waves. However, the problem exists when the WEC is not able to work under extreme situations and is there when it can be seriously dangerous and destructive for the all structure.
- Seawater has a very high percentage of salt in itself and it is well known that salt is very corrosive and can damage the WEC, especially where the structure is exposed alternatively to air-and-water.
- Since WECs are subject always to fluctuating forces they have to be able to sustain fatigue. However, this is not usually easily achieved, especially on a long-term basis.
- Marine plants and animal growing or settling on the structure of the WEC can increase its mass by an amount of about 50 kg/ per year. That is why, on a long-term basis, the WEC must be able to sustain, as years pass, more fatigue.
- Transmission costs also need to be carefully considered. These refer to the costs required in order to connect the WEC to the nearest part of the grid. In some cases, effort for strengthening the already existing grid are required, which means that extra costs are to be undertaken if the WEC needs to be connected to the main grid.
- Finally, it was observed and demonstrated by Falnes and Budal in  that a good wave absorber is also a good wave-maker. Therefore, for WECs, it is fundamental to displace a great amount of water (possible in phase with respect to the incident wave).
Wave Energy Converters: Other Uses
There are other uses for which WECs can be useful. One of this is the desalination of sea water by means of a WEC.
From the Collins English Dictionary desalination means “the process of removing salt from sea water so that it can be used for drinking or for watering crops”.
Several methods exist for desalination of sea water but in this example desalination is achieved by using a membrane process.
The desalination full system discussed here includes a buoy (Point Absorber WEC) and a Reverse Osmosis (RO).
Reverse Osmosis is a specific type of membrane process in which sea water (salted) passing through a semi-permeable membrane lost his salt concentration. Salt is actually being stopped by the membrane.
Besides, the WEC used in the full system here is a Point Absorber.
This WEC is connected to a hydraulic system which transforms the buoy's oscillations into a seawater flow at high pressure. High pressure, passing through the semi-impermeable membrane, lost is salt concentration.
There are a few advantages of using a WEC into this system, and these are listed below:
- Since WECs generates a variable source of energy it is helpful to stabilize the pressure.
- By having a WEC, and therefore a constant pressure, the device can be cheaper in terms of energy consumption.
Control Of Wave Energy Converters
Introduction To Control In Wave Energy Converters
As said before, the WECs are mechanical oscillator devices that perform better if the waves' frequency is close to their natural frequency.
But, since “real waves” are irregular and each sea wave has different frequency from one to another, the efficiency of WECs can be seriously affected when dealing with monochromatic waves. One effective way to solve or at least to attenuate this problem is by means of active control which can be included on the WECs and, by doing it their frequency can be adjusted relative to the frequency of the incident wave.
Definition Of Control In Wave Energy Conversion
The term control in wave energy literature often refers to a system optimisation rather than a control. This is because, usually, in control engineering, a set-point, which indicates point of maximum output, is given. Whereas, in the context of wave energy, usually, a predefined set-point is not given since the value of the maximum power that we can extract from waves, is not known. In fact, it is only know, that, it is required to extract as much energy as possible from the incident waves.
Control strategies are therefore methods to increase WECs power output by increasing their efficiencies and performances.
Wave Energy Converters Control
Many devices for wave energy extraction and conversion, discussed before, are actually oscillating systems with a frequency-dependant response that highlight the importance of the resonance frequency.
Starting from first principle, the resonance frequency is the tendency of a system to oscillate at larger amplitude only at certain and particular frequencies. These are known as resonant frequencies. At these frequencies larger amplitude can easily be expected and achieved.
The resonant frequency, as said before, in the wave energy context, occurs when the period of the WEC is coincident with the natural period of the wave.
Therefore, knowing this, at resonance frequencies output with larger amplitude are expected while smaller amplitudes are expected at non-resonance frequency.
The output is usually even less powerful when resonance bandwidth is narrow. This knowledge leads to the conclusion that attenuator and terminators devices have a larger bandwidth compared with point absorber devices.
On the other hand, is well known that the smaller the structure of the WEC is, the higher the Mass-Power ratio will be. This is therefore a good advantage for point absorbers and other small devices.
Knowing that Point Absorber devices would therefore operate most of the time at non-resonance frequency, it is very important to have an optimum control strategy. This will be necessary in order to achieve the greatest possible output from waves when using Point absorber devices. Vice versa, for wider structures such as attenuators and terminators, control strategies have been demonstrated to be less influent on the overall output.
The optimum power output can be obtained by controlling the oscillation frequency of the WEC in order to achieve an optimum interaction between the WEC and the incident wave.
Since most methods of control deal with real and irregular waves, their require prediction of the future. It is, therefore, required that
- Waves' velocity is predicted, for those WECs that work by opposing velocity such as point absorber.
- Waves' flow is predicted for those WECs that work by opposing flow.
It is also known that larger WECs, like Attenuator and Overtopping devices, depend more on prediction of the future than smaller WECs such as Point Absorber devices. This is because they have a narrow bandwidth and therefore they have a larger memory duration.
It has also been demonstrated in  that information collected locally is not always enough to achieve optimal control. Sometimes, in fact, it is necessary to have also information collected remotely to increase the result of the control strategy.
There are two considerations to be made when data is required to be collected both locally and remotely:
- Locally information should be collected relevantly close to the WEC in order to avoid corruption of the data caused by irregular waves.
- Remote information should be collected far away enough so that the time of waves to reach the WEC is bigger than the time for the control system to process the data.
Sensors can be used to help in collecting the remote information. These Sensors can be either fixed or movable sensors depending on the available time for waves to reach the WEC.
The aim of sensors in wave energy extraction context is to convert collected data, (wave-speed, temperature, position, pressure and so on) into a more useful form of data: such as an analogue or a digital signal for control purposes.
An example of movable sensor is the LIDAR (Light Detection And Ranging) sensor. This sensor is a kind of active sensor, which transmits signals to a body and record the time it takes to go, hit the body, and return back to the receiver.
Whereas fixed sensors, also called passive sensors, are the ones that collect the data without actually manipulate the surrounding environment with active investigation.
Control: Historical Review
The utilisation of control, to optimise wave energy conversion and maximize WECs output, was first proposed in the 1970s by Budal and by Salter.
The intent of their studies was to achieve optimum phase angle and optimum amplitude of oscillation by using a Power-take-off (PTO) device. Some examples of PTO are hydraulic rams and air turbines.
The theory said that by controlling the phase angle also the reactive power is controlled. This control of the reactive power can lead to achieve the maximum possible value of active power. This strategy was lately called continuous phase control.
A few years later Budal proposed that phase angle optimisation could be achieved by latching the wave absorber during specific time intervals of the cycle of oscillation. This method was lately called discrete phase control.
It was during this period that was realized the importance for discrete phase control of predicting the future. In fact, in order to achieve maximum output from a WEC was very important to be able to predict in advance the characteristics of a future incident wave. After a few years time was proved that also for continuous phase control prediction of the future was necessary.
In the 1980s some studies and experiments were done in order to achieve maximum optimum from OWC and from oscillating bodies by using control engineering.
Lately there have been many attempts to demonstrate that prediction of the future was not compulsory to get an optimal control strategy, but, however, they never obtained satisfactorily results.
The Benefits And The Importance Of Control
WECs working under optimally controlled strategies can operate for a longer period of time at a full capacity. Therefore, an optimal control strategy can increase significantly WECs lifespan. This increase in lifespan can lead to an improvement on the economic prospects for wave energy extraction.
By having, thus, WECs more efficient, more power would be available with less effort and with less monetary costs.
Control: Strategies And Methods
Control methods can be divided into two main categories:
Control methods where the former can operates at any time. This method is based on measurement of the waves.
Reactive control is a continuous method strategy and refers to the control by reversing the energy flow direction during part of the wave cycle. In this way it should be possible to increase the total absorbed energy from the WEC. This reactive phase control can be achieved by means of a Power take-off device as demonstrated in . However, this method of control would not be recommended when the reactive power peaks are big and losses cannot be neglected.
Control methods where the former can only acts on the system just one or few times during each oscillating cycle. This method is based on WEC's oscillation and is reliable when the natural frequency of a buoy (for example) is shorter than the incident wave period. In this way, an optimum approximation of the phase is achieved by latching the buoy at an instant when its velocity is actually zero.
The idea of latching control started in the early 80s, when Budal and Falnes proved that for WECs, dealing with monochromatic waves, in order to maximise their efficiency, was necessary to keep the velocity of the attenuator or floater body in phase with the natural velocity of the waves.
In fact, latching control actually locks the motion of the body (buoy or floater) when its velocity is vanished, and then releases it when the maximum value of velocity is expected to be coincident with the maximum value of the wave excitation force (or in the most favourable situation possible). Latching control is depicted below in figure 3.1
However, there is still the need, for ocean engineers, to find out what is actually the most favourable situation mentioned above and also to find out for how long period of time the system needs to be locked.
One way of applying, in practical uses, latching control is by means of brakes.
As time went by, it was showed that latching control can improve considerably the performances and efficiency of a WEC but only if it is assume that the actuators are locked instantaneously. However, this is not possible in real situations, in fact, since latching forces can be extremely large, the braking does not apply instantaneously and there will be a period of time before it actually happens. Therefore, the real system cannot be any longer considered without taking into account these time constants. In it has been proved that these time constants can affect considerably the latching control efficiency. In figure 3.2 the mean power absorber, in normal sea state, by the Searev WEC device is depicted. As it can be noted, different results are obtained by using different time constants. In more details, as the time constant increase the mean power absorbed by the WEC decreases and vice versa.
Reactive Vs Latching Control
Although both methods are really valid in, using a point absorber device was demonstrated that reactive phase control can increase considerably the efficiency of the WEC under irregular waves' condition. The same increase in power output was not found by using latching control. This was probably because the latching control was hardly achieved in a body with such a small mass (due to low inertia of body). In other words, the idea was to immobilize the body during part of the wave cycle, but it was not possible in a body with such a small inertia.
Control: Classification By Power Flow
There is another way of classify control strategies, and it is by power flow.
It is the method of controlling the power flowing from wave generation to the actual delivery of it. Power flow control can be subdivided into other 3 main categories: Geometry Control, PTO Force Control and Power Regulation Control.
Geometry control has been used recently in to define a type of control that actually changes the geometry of the WECs.
The geometry of WECs can be modified using the following techniques:
- By changing the spring coefficient or mass. For example by pumping water in and out a tank to change the buoyancy of the device which therefore leads to a change in its mass. This particular method is usually referred as Slow Tuning and it can be helpful to better match the WEC with the wave climate. More energy can be captured by doing so.
- By changing the degrees of freedom of the WEC's motion with respect to the water surface. It has been demonstrated that a change in degrees of freedom leads to different results.
- By matching the WEC's direction with respect to the wave's direction.
Force Control (Pto Force Control)
PTO force control is the control by which the amount of power returned back to the sea after WEC utilisation is regulated. It was, in fact, showed that by changing this amount of energy different results can be observed.
Power Regulation Control
By using this method of control it is possible to change the amount of electricity generated and delivered to the grid.
However, in this project we will deal more with the previous classification (i.e. continuous and discrete control strategies) because it is actually the most used and the one that leads to better results in terms of WEC's efficiency and performance.
Control: Operation Issues
- Extreme wind situations need to be considered in the overall control strategy. Usually, extreme events represent a constraint for controlling strategies. In fact, many systems can be destroyed or damage when the sea is in such conditions.
- Even if both methods of control, reactive and latching, are valid, engineers must consider, very carefully, whether to use one of them rather than the other. It is fundamental then, to carry out some tests and simulations before the actual implementation in the real wave conditions. These help to evaluate the best possible method for control for that particular harvesting device.