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In order to determine what wave energy conversion devices would be suited to installation on or around the UK shoreline, we must first understand the wave energy resource. This involves understanding wave formation and transport. It is also important to understand the changes which occur to waves as they enter shallow water and approach the shore.
In the following sections I will attempt to cover as much of these questions as is necessary. When describing a wave, I will follow the path the wave would take, from formation in deep sea, through deep water, and then onto shore through shallow water.
Also of importance are the effects of daily tides and extreme conditions such as storms, both of which will be discussed.
Before we can begin to model a wave we must first define the properties and characteristics of a wave. The physical properties of a wave are shown in .
Figure 5.1. - An Ideal wave with wave parameters labelled (drawn in Microsoft Visio).
The height (H) is the distance from the trough to the crest. The wavelength (L) is the distance from one crest to the next. The amplitude (a) is the height from the mean water level (X) to either trough or crest and the depth of the water is (d). The period of a wave (T) is the time taken between two consecutive waves from a fixed point, the inverse of T is wave frequency (f). When talking about waves in the open sea, there is also the direction of wave propagation to consider. This is defined as direction of wave crest motion drom due north. Finally, the speed with which a wave crest moves horizontally across the ocean surface is defined as wave celerity (c) and is defined in equation (x.x).
The wave number,, a measure often used in the quantitative analysis of waves. It is the reciprocal of the wavelength and is hence the number of wave cycles per metre. The wave energy resource is typically defined as the wave power that crosses into a circle of 1 m circumference, which is more accurately termed the omni-directional or gross wave power .
depicts an ideal situation, and this is not the case in the real life situation. In the real life situation there will be many waves of all in different phases and periods which combine together to produce what we know as Sea as shown in .
Figure 5.1. - A Picture showing the wave patterns in open sea (Personal Photograph).
In the ocean there are a number of different types of waves all with different sizes and periods. There are waves formed by the wind, waves formed by earthquakes, waves formed by the sun and the moon as well as waves formed by ships. A distribution of the total ocean wave energy is shown in .
Figure 5.1. - A schematic showing the energy spectrum of ocean variability
shows that the majority of energy in a wave is transferred to it by the wind. It is also wind waves that the majority of WEC focus on for extracting their energy as they are the most abundant and energy full waves [CITE]. It is for these reasons that I will concentrate on a discussion of wave formation via wind.
There are five main variables that effect wind waves as listed below. The greater each of the variables is, the more energy is transferred to the water .
The velocity of the wind.
The distance of the water (fetch) over which the wind has blown.
The duration of time the wind has blown over the surface of the water.
The width of the area affected by the fetch
And the water depth
The exact interactions between the wind and the water can be complex, and are not always well understood. At least three different mechanisms are involved in the development of wind waves [cite]
The wind exerts a tangential stress on the water surface of the sea, causing wave formation and growth.
Near the water surface, air flow may cause further wave development if the pressure fluctuations are in phase with existing waves.
When a wave is large enough, wind can exert a more direct force on the upwind side of the wave, leading to further growth of the wave.
Deep Water Waves
Shallow Water Waves
Historically it has always been seen to be more economically viable to install a WEC at an offshore / deep water location. This is because the gross wave energy is significantly less at nearshore compared to deep water. While this is still true, this is not the whole picture. A recent study by Folley and Wittaker takes into account the variability of the waves and the exploitable energy that they contain.
In their study they conclude that the majority of the reduction in gross wave energy from offshore to nearshore locations is due to refraction and not bottom friction or wave breaking. They also conclude that refraction does not change the directionally resolved wave energy propagating orthogonally to the seabed depth contours, and thus that the reduction in the net wave energy from nearshore to offshore is attributed entirely to bottom friction and wave breaking. This results in a loss of less than 10% of the net wave energy resource . This has been confirmed by [CITE] but [CITE] say otherwise.
Other Areas of Interest (Tides / Storms)
Wave Energy Conversion
(description, cost, potential energy << COE??)
There are many different types of wave energy converter (WEC) with a growing number of patents world-wide; in 2002 there were over 1000 patents for different WEC's in Japan, North America and Europe and as of 7/9/2010 there are 2000 and growing [cite]. There are nearly as many ideas for WEC devices as there are research groups, and it would therefore be a difficult and lengthy process to describe each one in this dissertation. Therefore, some of the larger projects with are close to, or at, commercial viability will discussed in this section. This large number of patents can be classified to within a few classes and sub classes. There are three main classes of WEC. Attenuators, Terminators and Point Absorber .
Attenuators are relatively long devices that are placed parallel to the general direction of wave travel (e.g. Pelamis).
Terminators are placed perpendicularly to the waves and aim to absorb a large proportion of the energy of the wave (e.g. WaveDragon).
Point absorbers are devices whose surface area is very small in comparison to the wavelength of the incoming waves (e.g. AquaBuOY).
Pelamis Worlds First commercial
Have we got any other type of attenuators?
Attenuators sit parallel to the predominant wave direction and effectively ride the incoming waves. An example of an attenuator WEC is the Pelamis, developed by Pelamis Wave Power (previously known as Ocean Power Delivery [cite]). The Pelamis device is a floating, articulated structure composed of cylindrical sections linked by hinged joints and anchored to the seabed. The wave-induced motion of the joints is resisted by four hydraulic rams at each joint that accommodate both horizontal and vertical motion. The rams pump high-pressure fluid through hydraulic motors which in turn drive electrical generators to produce electricity. Power from each device is fed down a single cable to the sea bed and then sent to shore. Several devices can be connected together and linked to shore through a single seabed cable .
The Pelamis WEC deserves a special mention as it was the world's first commercial scale machine to generate electricity into the grid from offshore wave energy and the first to be used in a commercial wave farm project . The first full scale prototype was successfully installed and generated electricity to the UK grid at the European Marine Energy Centre in Orkney, Scotland in August 2004 . The first wave farm consisting of three Pelamis machines and located off the coast of Portugal was officially opened in September 2008 [http://www.pelamiswave.com/news?archive=1HYPERLINK "http://www.pelamiswave.com/news?archive=1&mm=9&yy=2008"&HYPERLINK "http://www.pelamiswave.com/news?archive=1&mm=9&yy=2008"mm=9HYPERLINK "http://www.pelamiswave.com/news?archive=1&mm=9&yy=2008"&HYPERLINK "http://www.pelamiswave.com/news?archive=1&mm=9&yy=2008"yy=2008].
Need to say why we are talking about this? Do I want to do a little bit up top to say the method of investigation wave power resource? Need to do some talking about tariff ratesâ€¦ Whats a capacity factor? Do I need to do full economic analysis.. im sure I don't need toâ€¦ that would be too deep? Why have is COE my measurement of choice?
When discussing renewable energy projects, it is convenient to use the Cost of Electricity (COE) measurement for economic comparison [cite]. The cost of electricity measures the cost of generating electricity including initial capital, return on investment, as well as the costs of continuous operation, fuel, and maintenance [cite]. There have been numerous papers which discuss the COE of a Pelamis Wave Farm, the most recent of which attempts to create a "comparative standardised assessment of wave energy economic indicators" . In their paper they created an Excel model (NAVITAS) to estimate the annual energy output of Pelamis at a number of locations using wave height and period data to produce financial results dependant on various input parameters. Their model returned a COE of â‚¬0.05/kWh modelling over 100 WEC at 2004 cost of materials, and â‚¬0.15/kWh at 2008 prices for a wave farm at an Irish location.
A report filed by Previsic for EPRI conducted a design, performance and cost study of a single WEC installation and a large commercial-scale offshore wave power plant installed off the coast of San Francisco California. The device they selected to study was the Pelamis. Full COE analysis was only conducted on the large commercial project. The COE quoted was $0.11/kWh (approximately â‚¬0.09/kWh at 2010 prices) for an electricity tariff rate of $0.07/kWh . Although this number is substantially different from the report by Dalton et al. there are a number of variables which are different between the two studies. When the Previsic data was input into the NAVITAS model in the Dalton case study the NAVITAS model yields a COE of â‚¬0.07/kWh. This shows that the two papers agree. What are the differences between the two papers methods of calculation?
Another recent report conducted by D.Dunnett on a Pelamis installation in Canada examined a proposed 25GWh (27 Pelamis) wave power plant . The COE calculated ranged from â‚¬0.23-0.38/ kWh, depending on the location. The article mentions that the output was low in comparison to the EPRI report which used the same device design. It commented that the design was intended for North Atlantic waters, which have different (lower) sea states to those of Canada. When the data from the Dunnet and Wallace report was input into the NAVITAS model created in the Dalton paper, the NAVITAS model yielded a COE of â‚¬0.34/kWh. This is within the COE quoted in the Dunnet and Wallace paper, and therefore I believe that this shows merit for the NAVITAS model. Again what are the differences between the two calculations?
Oscillating water column (OWC)
OWCs are the most mature of the wave technologies, though they are still in their infancy compared to other renewable energy devices such as photovoltaic arrays and wind turbines. An OWC consists of a chamber with one end extending below the surface of the sea and a turbine and vent at the other end. The surface of the sea seals a column of air in the chamber and the moving water surface caused by incoming waves pressurises the air in the column. An incoming wave will push the air through the turbine and out through the vent, and a retreating wave will pull air back through the vent and turbine [INSERT FIGURE]. The turbine captures some of the energy transferred to the air by the waves and a generator attached to the turbine shaft produces electricity.
The majority of OWC installations are fixed onshore but there are a number of floating OWCs that are tethered to the seabed. Two of the leading onshore OWCs are the LIMPET on the Isle of Islay and the Pico device in the Portuguese Azores [cite]. One of the offshore devices close to commercialisation is the Oceanlinx Mark 3 Pre Commercial (or MK3PC). It was deployed from February to May 2010 in Port Kembla in Australia and was successfully rated to produce 2.5MW [cite].
The LIMPET installation was operational in 2000 and still operates today, with an installed capacity of 500 kW [INSERT FIGURE]. The LIMPET uses a counter-rotating Wells turbine to convert the air's kinetic energy into rotational motion. Wells turbines are frequently used in wave energy devices because the rotor (or rotors) moves in the same direction regardless of the direction of the airflow through the turbine. Counter-rotating turbines, where two rotors travel in opposite directions, are sometimes used to increase the operational range of the turbine, as stalling can be a problem at higher airflows , though more recently there has been a lot of work to prevent this stalling .
[INSERT TABLE OF LIMPET PERFORMANCE (maybe)]
The Pico project is a 400kW OWC built into a small natural gully near Porto Cachorro on the island of Pico in the Azores. Construction of the civil structural works began in 1996 and was completed in the summer of 1998. The plant was first commissioned at the end of 1999, however flood damage to the electrical and control equipment led to a further year delay. The plant comprises an insitu concrete collector, with the back wall stepped and the generation unit installed immediately behind the upper constriction of the collector wall. The plant is equipped with a horizontal-axis Wells turbine-generator set enclosed by a small turbine hall.
Oceanlinx (formerly Energetech) has designed a nearshore OWC device which includes parabolic walls to focus the energy of the incoming waves into the opening of the OWC. The structure is not built into the shoreline but rests on feet tethered to the sea floor, as shown in Figure 4.5 . The first of these OWCS has been built in Port Kembla, near Sydney, Australia, and was installed and tested in June 2005 .
In this type of device, waves are focused onto a ramp, such that the wave height increases enough to spill water into a floating reservoir. Electricity is then extracted by using a low-head turbine similar to those used in hydroelectric facilities. The Wave Dragon, an overtopping device developed in Denmark, will eventually have a rated power of between 4 and 10 MW. A 1:4.5 scale prototype is currently in the water off the coast of Denmark, and is also one of the few grid-connected wave energy devices .
Oscillating wave surge converter
(Surge and pitch)
Submerged pressure differential
Salter's Duck << Most Famous that started it all off
The most well-known Point Absorber is the Salter Duck. It was one of the original modern WEC device designs. Salter's device, also called a Nodding Duck but officially known as the Edinburgh Duck [cite], is a tear-shaped hydraulic device that converts the motion of the waves into rotational motion, causing an internal fluid to run unidirectionally through a specially designed pump to generate electricity . This device is illustrated in [INSERT FIGURE]. Though the Salter Duck project ran into financial difficulties and never made it to the commercialisation stage, many other point absorbers exist which are based on the concept of using hydraulics to convert wave motion into rotational motion and then to electricity.
Pros and Cons of Shoreline / Nearshore Technology
Avoiding the need for armoured subsea cables is one of the main advantages to building nearshore or shoreline WEC devices. These cables are costly, on the order of US $115,000 per km , and in the case of a farm of offshore devices, the distance between the shore and the farm may need to be covered multiple times depending on the number of WEC devices. Another advantage to shoreline devices is the accessibility for maintenance and repair. [Any Citations?]
The major disadvantage to shoreline and nearshore technologies is the substantial decrease in wave power which occurs as waves enter shallow waters and approach the shores. This is the main reason for the trend towards offshore WEC devices. Expand a bit more - cite the trend?
Social impact of having the wave power 'in the way'
Large scale WEC (Wave Farms)
Other Types of power generation related to wave power
Wind Wave Power