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Hydro Tidal And Wave Energy Engineering Essay

Section 1 - Introduction

The purpose of this assessment is to critically review wave power from its earliest development, to its modern day electrical generation potential. An investigative case study of one such wave energy conversion device will be presented, describing how it works at converting the wave motion into electrical energy and the opportunities and limitations for future development and possible commercial deployment.

Simply put, Wave energy is produced when electricity generators are placed on or near the surface of the ocean. Energy output is determined by wave height, wave speed, wavelength, and water density. To date there are just a few experimental wave generator plants in operation around the world but there are many more prototyped test tanks.

The Swedish energy company Vattenfall is one of the leaders in the renewable energy field and it has substantial research in progress with most of its attention being focused on wave energy. It suggests that after wind power, wave energy conversion will be the next renewable technology to be commercialised on a large scale.

The economic potential the various wave energy conversion technologies will depend on the costs of the specific conversion techniques employed the transmission costs and the production efficiencies. The most interesting places around Europe showing potential for Vattenfall, are on the Swedish and Norwegian west coasts, Denmark, the UK, Ireland and Portugal.

Section 2 – A brief history of Wave power

The oil crisis of the early 1970 was really the driver behind the renewed and focused research and development in renewables including wave energy conversion. The first known patent to utilise energy from ocean waves dates back to 1799 and was filed in Paris by Girard and son.[1] An early application of wave power was a device constructed around 1910 by Bochaux-Praceique to light and power his house near Bordeaux in France.

In the past decades Government sponsored programmes, particularly in Japan, Scandinavia, the UK and Ireland have lead the research. These programmes advanced the technology considerably and led to the installation of the first prototype devices. In the 1980s, as the oil price returned to more acceptable levels, renewables and wave-energy funding drastically reduced. More recently, following the issuing of climate change directives and global climate change agreements such as Kyoto and Copenhagen, there is again a growing interest worldwide for more alternatives to enter the renewable energy mix. Wave energy is seen by many to have the potential to contribute significantly to this energy mix. Normally industry-led development is required in order to achieve commercially viable technologies and this is now the case for wave energy conversion.

Early work on wave energy aimed at exploiting the maximum amount of resource from the energy source. The resulting elaborate schemes had associated large construction costs, prolonged construction times and significant technical challenges. Ultimately the high generating and large capital costs for the first prototypes made the technologies commercially unattractive. But lessons have been learned from these early experiences in wave energy. Most of the modern devices are relatively small in size and output (< 2 MW). This reduced size, decreases the prototype costs, which is especially critical in the current financial climate. The latest prototypes also benefit from two decades of experience in constructing and operating offshore oil and gas platforms, with the corresponding technological transfer. Nearly all of the devices studied for this assessment are being built by small to medium sized engineering companies, so there has been a focus on the economics of the technology from the start.

Section 3 – Potential energy generation and Current technological status

Section 3.1- Global Potential energy generation from Waves

Waves form a potentially large worldwide energy resource, estimated by the Word Energy Council at 2,000 GW, capable of contributing with more than 2,000 TWh/yr, which represents more than 10% of present global electricity demand, equivalent to the contribution by large scale hydro or nuclear power. There are several regions around the world with high incident wave power levels, which are particularly well suited to exploiting this renewable energy source. Because of the direction of the prevailing winds and the size of fetch across the Atlantic Ocean, the Irish west coast along with Scotland, and Cornwall have wave power levels that are amongst the highest in the world. Increased wave activity is found between the latitudes of ~30o and ~60o on both hemispheres. Again in its very nature the wave power is induced by the prevailing western winds blowing in these regions so the western coasts of the Americas, Europe and Australia, also rank high among regions suited to capturing the power of the sea for energy generation.

The power in a wave is proportional to the square of the amplitude and to the period of the motion. Long period (~7-10 s), large amplitude (~2 m) waves have energy fluxes commonly exceeding 40-50 kW per meter width of oncoming wave. See figure 1.

Figure 1, Sourced from Wave Energy paper. IMechE, 1991 and European Directory of Renewable Energy (Suppliers and Services) 1991[2] demonstrates the potential with figures quoted in kW per metre of wave front.

Figure 1 – Global Wave energy potential (greatest between 30o - 60o latitudes)

Early attempts to design and deploy cost-efficient devices met with limited success. However, the last five years have seen a resurgence of interest in wave energy throughout the world, with several companies currently developing and deploying new devices that represent a significant improvement over older concepts.

An important aspect to emphasise in relation to wave energy potential is the high energy density of sea waves, which the European Thematic Network on Wave Energy state is the highest among all the renewable energy sources. Clearly, it is now up to the wave energy industry to co-ordinate itself more effectively and to address the issues that undermine further investor support and full commercial realisation.

Section 3.2- Irish potential energy generation from Waves

Ireland is ideally located to benefit from the waves generated in the Atlantic Ocean. Globally, the Northern Temperate Zone has the best sites for capturing wave power. Ireland possesses one of the richest wave energy climates in the world and lies close to a large consumer market in N.W. Europe, and with the UK-Ireland interconnector gives it a unrestricted export potential.

Off the west coast of Ireland, the annual average wave height is 2.5m-3m, see figure 2a, however winter events are substantially greater. In December 2007, 14m waves were measured at the data buoys.

Figure 2b – Indicative Irish Wave Resource

Levels in Annual MWh/m/yr [3]

Figure 2a – Annual Average wave height [3]

But can wave generated power compete with other renewable sources? The assessment of the commercial prospects for wave energy is difficult, because estimates of the cost of power from wave energy devices represent a snapshot of the current status and costs of evolving designs at their current level of development. The electricity costs of a number of devices have been evaluated over the past 10 years using a peer-reviewed methodology [4]

The plot of Fig. 3 of the resulting costs against the year in which the design of a device was completed shows a significant reduction in generating costs up to 2000. This cost reduction is similar to improvements of generating costs for wind turbines, and in the past decade where devices have achieved their anticipated performance the costs with fossil fuel based production have continued the trend and have now become comparable.

Figure 3 - Historic electricity cost reduction for wave energy technologies

Section 3.2- Current technological status

In contrast to other renewables the number of concepts for wave energy conversion is very large; however, they can be classified within a few basic applications. Here, some representative and promising technologies will be presented and this will lead to the presentation of a more detailed case study. As many of the technologies, such as oscillating water columns, can be implemented onshore or offshore, the devices can be logically categorised according to the distance of the location of installation from the shore. Therefore on the shoreline, near shore < 50m water depth and offshore devices located in water depth of 100m or more.

These categories can then be sub divided into either

Fluid based energy conversion such as the Pelamis, which has articulated sections that stream from an anchor towards the shore and waves passing overhead produce hydraulic pressure in rams between sections. This pressure then drives hydraulic motors that spin generators.

Air based conversion technologies such as the Ocean Energy buoy, in which incoming waves pressurise air within a chamber or oscillating water column (OWC), the trapped air rushes up through the column and spins a two-way Wells turbine1 to produce electricity.

Section 3.2.1- Onshore devices

Fluid based conversion

An Israeli company SDE Energy Ltd, has developed a onshore based breakwater wave energy converter (WEC). Its method of producing electrical energy from waves consists of using sea wave motion to generate hydraulic pressure, which is then transformed into electricity. The system takes advantage of the wave’s speed, height, depth, rise and fall, and the flow beneath the approaching wave, thus producing energy. A full-scale model was tested successfully in Israel and produced 40ekW for almost one year. See figure 2

Figure 2 – SDE Energy – Onshore breakwater WEC

_________________

1 – The Wells turbine is spun to starting speed by external or stored electrical power, and spins the same direction due to uniform blades, regardless of air flow direction

Air based conversion

Probably one of the most well known WEC devices invented by Wavegen, the LIMPET, or Land Installed Marine Powered Energy Transformer, see figure 3a. It is a shoreline device which produces power based on the oscillating water column principle, see figure 3b. Back in 2000, Wavegen became the first company in the world to connect a commercial scale wave energy device to the grid, on the Scottish island of Islay.

Figure 3a – Wavegen’s LIMPET Figure 3b – The Oscillating Water Column principle

Fixed onshore systems have some significant advantages over floating systems, especially in the area of maintenance. However, the number of suitable sites available for fixed devices will of course be limited.

Section 3.2.2- Near-shore devices

Fluid based conversion

The Oyster, reportedly the world's largest working WEC device has been developed by a Scottish company Aquamarine Power, see figure 4a. The device doesn’t generate electricity directly but in essence is simply a large pump, which provides the power source for a conventional onshore hydroelectric power plant. The buoyant, hinged flap near-shore "oyster" is attached to the seabed at around ten metres and sways backwards and forwards in near shore waves. This motion drives hydraulic pistons producing either power by pumping high-pressure water to its onshore hydroelectric turbine (See figure 4b) or to a reverse osmosis filter to produce fresh water. BioPower Systems of Sydney Australia have a similar device called the BioWave. They call the application of the swaying motion of the waves, biomimicry, as it is based on the swaying motion of sea plants in the presence of ocean waves.

Figure 4a – The ‘Oyster’ pump WEC Figure 4b – High pressure water pumped to onshore turbine

Air based conversion

AWS Ocean Energy is another Scottish company which has developed the Archimedes Wave Swing or “AWS” wave energy conversion technology, see figure 5a. The Archimedes Wave Swing is a submerged air-filled cylinder with a 'floater' that rises and falls as the waves pass above. The movement relative to the fixed lower part is converted to electricity by a linear generator, see figure 5b.

Figure 5a – The AWS being submerged off Portugal in 2004 Figure 5b – the AWS principle

Section 3.2.3 - Offshore devices

Offshore applications probably present the largest array of both researched and developed devices for WEC. All technologies including OWC, overtopping, point absorbers and hydraulic pump have devices that are suitable for offshore deployment. See figure 6. A simple robust construction is essential for any device bound for operating offshore where the extreme forces of water, wind and also inevitable marine fouling could seriously affect any moving parts.

Figure 6: Usage of power take-off systems among various wave energy technologies [5]

Fluid based conversion

The Wave Dragon is an overtopping device that elevates ocean waves to a reservoir above sea level where water is let out via gravity through a number of turbines and in this way transformed into electricity. As the waves reach the reflectors they elevate and reflect towards the ramp increasing the amount of overtopping water thereby increasing the possible energy output, see Figure 7a. The Wave Dragon is a very simple construction and has only the turbines as the moving parts, see figure 7b. It is moored as stationary as possible in relatively deep water to take advantage of the ocean waves before they lose energy as they reach the coastal area.

Figure 7a – Wave Dragon with wave reflectors Figure 7b – Overtopping principle

Air based conversion

The Oceanlinx is a WEC in its third phase of design with the Mk III and developed by Oceanlinx Australia, see figure 8a. It again works on the OWC principle of a partially submerged structure with an underwater opening, which allows seawater to rise and fall within it thus creating bi-directional air-flow, see figure 8b, capable of driving a generator and it has a rated capacity of 2.5 MW.

Figure 8a – Oceanlinx - Mk III

Figure 8b – Offshore OWC principle

The offshore class of device exploits the more powerful wave regimes available in deep water. Many of the more recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays. But constructing devices that can survive storm damage without being so heavily over engineered and therefore prohibitively expensive to construct and maintain is a major challenge to developers. This point leads on to the case study.

Section 4 – Case Study - CETO Wave Energy Converter

To be a leading global wave power conversion device, the device must be a reliable, efficient, tuneable and a cost effective means of producing useful energy. The device chosen for presentation in this case study has these attributes. Carnegie Wave Energy Limited of Perth Western Australia, have just begun commercially demonstrating its CETO wave energy technology which is capable of producing zero-emission power and/or desalinated water. The CETO is effectively a fully submerged pump and generates power onshore rather than offshore through a Pelton turbine. See figure 9.

Figure 9 – CETO wave energy conversion principle

The device is in its third phase of development and CETO III units can be adjusted to operate in any wave environment or combination of wave environments. The standard design is based to operate at 50 % capacity in a combined 2 m swell. With the standard setup the system will not be able to operate efficiently in swells less than 1 m. However if a site has regular swells less than 1 m the system can be altered easily to allow production in swells as low as 0.5m, albeit with a reduction in efficiency.

Due to the modular setup and symmetry of the system CETO units are omni-directional and can operate equally effectively regardless of the direction of the waves, see figure 10a and 10b. With a multiple array system such as in figure 10c the direction of the wave will effect how much residual energy the wave has as it reaches each unit.

Figure 10b - CETO 2 deployment and operation, Fremantle, Western Australia, 2008

Figure 10c - CETO Wave Farms will be deployed in arrays of multiple units with many MWs of production capacity

Figure 10a -CETO 2 unit on

display at Fremantle Wave

Energy Research Facility

Section 4.1 – Advantages of CETO Wave Energy Converter

Some of the main advantages of CETO over other WEC devices are

It contains no oils, lubricants, or offshore electrical components. CETO is built from components with a known subsea life of over 30 years.

There is no need for undersea grids or high voltage transmission and costly marine qualified substation plants.

Wave energy harnessed by CETO can be used for permanent base load power or for fresh water desalination utilising standard reverse osmosis desalination technology.

The ratio of electrical generation to fresh water production can be quickly varied from 100% to 0% allowing for rapid variations in power demand.

CETO units are submerged and designed to operate in harmony with the waves rather than attempting to resist them. This means there survivability factor in adverse weather conditions is high.

CETO uses a great multiplicity of identical units each of which can be mass produced and containerised for shipping to anywhere in the world.

An important design feature of CETO, which should be a definite driver for its commercial deployment, is the concept of pumping water directly ashore under high pressure. In addition to the convenience of generating electricity onshore, the dual working principle of CETO also makes it a most efficient and cost effective way to desalinate freshwater from wave energy. By acting as an offshore pump, CETO delivers large volumes of high pressure seawater ashore ready for desalination via traditional reverse osmosis means but without the greenhouse gas emissions.

The world is running increasingly short of freshwater. It is estimated that nearly 2/3rds of the entire world will be water scarce by the year 2030. See figure 11. The yellow and red areas represent 62% of world population.

Figure 11 - Global Water Scarcity - 2030.

Comparing figure 11 with Figure 1, it is clear that there are areas where there is a scarcity of water and a promising wave energy resource, including the West coast of the Americas, Southern Africa and Western Australia.

Another environmental benefit of this technology is CO2 emissions reduction in the desalination process. The Australian Institute claim that a proposed $2 billion AUD, 500kL/day desalination plant operating on Australia's eastern seaboard would emit the equivalent of nearly 1,000,000 tonnes of CO2 per year [6],with the application of CETO these emission could be reduced to zero.

Section 4.2 – CETO efficiency and Power production

Under the standard design wave, Carnegie Energy has calculated that a single CETO unit will convert 20 % of the wave’s total energy into pumped high-pressure water. Depending on distance from shore, up to 5 % capacity may be lost transferring the high-pressure water to shore. A basic table for energy conversion is shown in figure 12:

Figure 12 – CETO Wave energy conversion

Based on the figures from table 12, and utilising the maximum anticipated losses given at 5% which will be dependent on each site location and number of units installed, the power generation for each wave height and period would be as seen in figure 13.

Figure 13 – CETO power generation

Finally looking at the desalination potential. The by product of the production of water by desalination is power generation through energy recovery, this would amount to 45 % of the values in figure 13. As a comparison with a standard energy cost for water desalination of 2 kWh/m3, the following total water quantities, could still be derived, see figure 14, if power was transferred to a desalination plant.

Figure 14 – CETO fresh water production

Section 5 – Conclusion

Huge swell sinks wave energy generator

http://www.abc.net.au/news/stories/2010/05/17/2901059.htm

The success of wave energy hinges on its perceived strategic benefit to the future of a particular society. Isn’t the time ripe for Ireland to prove its metal in the industrial world as a leader in technical innovation? Perhaps more importantly, as one of the highest per capita energy consumers in the world and with an enormous wave resource in our back yard, there is a moral duty on us to contribute to this field. Surely we are not content to sit on our laurels and simply hope that someone else will continue to solve impending problems for us? The next phase in the Irish strategy will be to go to full scale ocean prototype testing of one or more devices in the full rigour of the Atlantic. This is due to begin from 2008 onwards. That will require resources far beyond lip service. The ability to commit the very significant funding required for this phase and then to deliver a successful test, will be a true measure of Ireland’s ability to meet the wave energy challenge. One can only hope that the Irish decision making community and investors have the courage, and that researchers and engineers possesses the knowledge, to ensure that this island will stand to harvest the rewards of a potential energy revolution.

Withdrawal of wave energy

Near-shore effects on sedimentary processes, biological communities, competing uses for wave resources

Interactions with marine life and seabirds, Marine organism intake, fish aggregation, whale migration, hauling out of sea lions and seals, colonization by birds, marine growth on submerged surfaces, scouring of sea bottom by mooring catenaries

Atmospheric and oceanic emissionsWorking fluid spills & leaks, anti-fouling hull coating, underwater noise,atmospheric noise

Visual appearanceVisual intrusion on seascape, mandatory navigation hazard warnings,extent of required marking•

Conflicts with other uses of sea spaceMarine protected areas, commercial shipping & fishing, military

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