Tidal Impoundment And Tidal Streams Engineering Essay

Published:

With the huge dimensions of its waters (an approximated area of 361 million km2 and a volume of 1.370 million km3), oceans are one of the most powerful sources of energy known to date (Miguélez, 2009). Although the potential of marine power is huge, it still remains on the list of the less exploited renewable energies, fact that could change in the coming decades.

As part of the Solar System, The Earth and therefore the oceanic waters are constantly receiving the energy of the Sun not only as heat by means of radiation, but also by the effects of the gravitational attraction that both the Sun and the Moon exert on them.

The harness of renewable and clean energy from the sea can be achieved in the following ways:

Tidal Power: Tidal Impoundment and Tidal Streams

Wave Power

Ocean Thermal Energy

Osmotic Power

Ocean Thermal Energy Conversion (OTEC) technology is probably the most advanced one among the above mentioned and vast amounts of energy can be achieved by using it. However, as these systems to be efficient a minimum temperature difference of about 20°C is needed between surface and deep waters, only tropical waters at specific latitudes are suitable. Europe is totally out of those areas and does not have the required external inputs to get profit from this technology.

Lady using a tablet
Lady using a tablet

Professional

Essay Writers

Lady Using Tablet

Get your grade
or your money back

using our Essay Writing Service!

Essay Writing Service

In respect to Osmotic Power, i.e., the technology taking advantage of the salinity gradient between river and sea waters is at the very beginning of the investigation phase and it is limited to very few studies, laboratory models, prototypes and experimental installations.

In consequence, so as to set a priority in relation to our needs, only the energies related to the motion of sea and oceanic waters are going to be developed on the following pages: tidal power, including both impoundment and streams, and the power of waves.

DESCRIPTION

TECHNOLOGY

TIDAL POWER

Tides are cyclic variations in the level of sea and oceans created by the gravitational effect of the Moon and the Sun, combined to the rotation of the Earth. Even though the attraction of the first one is more significant due to mass and distance factors (the sun is way larger than the moon, but at the same time it is way farther from the Earth than the moon), both account on the final scale of the created tides.

The interaction between Solar and Lunar tides results in the creation of spring tides in those moments when the Sun and the Moon are aligned (new moon and full moon), and neap tides (first and third quarters) when their position creates an angle of 90°. That makes tides something as predictable as the lunar calendar, having certain energy peaks along the year.

Certain patterns based on a wide range of temporary cycles are strictly followed by tides:

A daily 12-hour cycle due to the rotation of Planet Earth within Moon's gravitational field: high and low tides

A monthly 14-day cycle: spring and neap tides

An annual 6-month cycle: remarkable spring tides during March and September (equinoxes)

Other more complex gravitational interactions that take place each 19 and 1600 years, for example.

Apart from the mentioned phenomenon of vertical variations, the periodical changes in the sea level originated by the displacement of water bodies create streams. Thus, there are two different ways to take advantage of tidal power:

Using potential energy by means of impoundment of water

Using kinetic energy generated by the horizontal streams

TIDAL IMPOUNDMENT

Tidal impoundment is achieved by constructing barrages in strategic locations such as estuaries harnessing the potential energy generated by the rise and fall of sea levels to produce clean electricity at the end. As tides are based on complex astronomical processes they are totally predictable, just the opposite to the wind phenomenon, which is based on random atmospheric processes.

This basic concept was historically used for grain grinding in tide mills with similar applications to those of the river mills. But it was not until the XX Century when tides were seriously studied as a potential source for the electrical industry.

Lady using a tablet
Lady using a tablet

Comprehensive

Writing Services

Lady Using Tablet

Plagiarism-free
Always on Time

Marked to Standard

Order Now

The concept is similar to that described in the hydropower chapter: potential energy is transformed in kinetic energy and is used to run turbines so as to create electricity. The creation of a plant is needed which will be similar to dams with low head-difference requirements. The main difference is that an inflow and outflow is naturally created by the effect of tides. Therefore, the technology related to tidal impoundment involves large scale systems due to the fact that they require constructing quite large plants to generate electricity, instead of installing offshore or near shore devices.

Usually there are two different kinds of lock gates: the main lock gate and the turbine lock gate. Combining their opening and closing the flow is controlled and adapted to the needs. 3 different ways are possible depending on how the flows are utilized to run the turbines:

Using the outflow (Ebb):

This is the most common operating way of tidal plants. The production of electricity lasts up to 40% of each tidal cycle. Simply explained, the same process is followed for each daily tide-cycle:

Both the main and turbine lock gates are opened as the sea level rises.

Once high tide is reached lock gates are closed to maintain the level in the interior of the dam.

When the water level is low enough outside the turbine lock gates are opened to start generating electricity as the water falls down.

Using the inflow (Flood):

The output is lower and the environmental impact higher. Basic process:

Both the main and turbine lock gates are closed as the sea level rises.

Once high tide is reached the water is accumulated in the exterior of the dam.

When the height difference between the interior and the exterior is enough the turbine lock gates are opened and the water flows to the interior, generating electricity.

Using both the outflow and inflow → Bidirectional System (Ebb + Flood):

This system combines the previous two systems, generating electricity during the whole cycle. Even though the bidirectional system is possible, it is only used in extraordinary occasions, such as spring tides, because the performance is not considered to be significantly improved. In addition, it entails a higher cost due to the need of using bidirectional turbines, which are more expensive than the conventional ones.

Depending on the features of the mentioned constructions there are also 3 different methods or technologies for harnessing tidal power by impoundment:

TIDAL BARRAGES

This system involves the construction of a dam across an estuary, i.e., totally obstructing it from one bank to the other. This is the most common and utilized way for tidal impoundment known to date.

TIDAL BUNDED LAGOONS

This system consists of a barrage constructed against one of the banks of an estuary. Unlike the conventional tidal barrages, the bunded lagoons do not completely obstruct the estuary.

TIDAL OFFSHORE LAGOONS

As the previous system, the modern concept of offshore lagoons avoids obstructing an estuary, but this time by means of impoundment walls within an offshore area. Therefore, they are independent of the shoreline because none of the banks of the estuary is in contact with the walls.

C:\Users\ALBERTO\Desktop\MASTER\4. SUBJECTS\GROUP PROJECT\GROUP PROJECT I\MARINE ENERGY\INFORMATION\4.PICTURES\TIDAL POWER\tidal barrage.jpgC:\Users\ALBERTO\Desktop\MASTER\4. SUBJECTS\GROUP PROJECT\GROUP PROJECT I\MARINE ENERGY\INFORMATION\4.PICTURES\TIDAL POWER\bunded tidal lagoon.jpgC:\Users\ALBERTO\Desktop\MASTER\4. SUBJECTS\GROUP PROJECT\GROUP PROJECT I\MARINE ENERGY\INFORMATION\4.PICTURES\TIDAL POWER\offshore tidal lagoon.jpg

Figure E.3.6: From left to the right: Tidal barrage, tidal bunded lagoon

& tidal offshore lagoon. (Aqua-RET Consortium, 2012)

Even though the concepts of tidal lagoons were revealed few years ago, there is no real application of these systems to date. Double impoundments systems, i.e, tidal lagoons including more than one reservoir, could help enhance the productivity of electricity adjusting it to the demand, providing energy during those moments when the system is out of the tidal cycle.

TIDAL STREAMS

Tidal streams are part of the phenomenon of tides and their embodied kinetic energy harness is the topic to be treated on this point. They consist in horizontal flows that can vary depending on the layout of the seabed and other geographical features. They are more significant in narrow passages like channels and straits due to the higher speed these flows can reach, phenomenon also known as "funneling effect". The concept of tidal streams conversion presents many similarities with the harnessing of the kinetic energy of wind.

Lady using a tablet
Lady using a tablet

This Essay is

a Student's Work

Lady Using Tablet

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Examples of our work

Concepts of tidal impoundment were present far long before, whilst it took longer for the concepts of tidal streams to arise. This emergence is related to the development of wind energy during the last decades, which helped realizing that similar principles taken from it could be introduced in this new technology. A serious approach with the aim of producing electricity to these systems was not made until the late XX Century, in the 1990s.

While large scale systems are used for the harnessing of tidal impoundment, smaller devices are generally required to take profit of tidal streams. These devices can be set up either individually or in group, creating tidal farms or tidal fences. This last option seems to be the most appealing one, because this is when significant energy amounts could be generated. The concept of tidal fences has also been aim of study, where the creation of a barrage including stream turbines all along its length is needed, but it has not been deployed.

Depending on the concept used by these devices, clean energy can be generated by using 3 different technologies (Aqua-RET Consortium, 2012):

TIDAL CURRENT TURBINES

Marine current turbines can be either horizontal or vertical axis turbines. It is essential for horizontal ones to take into account the direction of the streams, while this factor is not decisive for vertical axis turbines because the flow is mainly horizontal. Horizontal axis turbines are the most commonly used, though. The most common tidal turbine types are normal bladed turbines (Marine Current Turbines Ltd., 2012) and open-centre turbines (OpenHydro Group Ltd., 2012). They are very similar to wind turbines, especially to those installed offshore in spite of the existing shape-based and operational differences:

Smaller in size (around half the diameter): Ø = 15-30 m

Less current speeds are required 2-5 m/s (five times less)

Density of water is much greater 1025 kg/m3 (about 800 times more dense)

A very significant datum is that marine turbines can produce as much energy as wind turbines with a third of their size.

http://aquaret.com/images/stories/aquaret/stills/horizontal%20axis%20turbine.jpg http://aquaret.com/images/stories/aquaret/stills/vertical%20axis%20turbine.jpg

Figure E.3.7: Horizontal Axis Turbines Figure E.3.8: Vertical Axis Turbines

(Aqua-RET Consortium, 2012) (Aqua-RET Consortium, 2012)

RECIPROCATING HYDROFOILS

The operating principle of these devices is the creation of an oscillating motion which linked to a complex hydraulic system is able to produce electricity. These devices usually have one or more hydrofoils attached to an oscillating arm, which is supported by a structure lying on the seabed. Therefore, the hydrofoils take care of facing the flow with their big area, whilst the articulated arm creates the oscillation.

VENTURI EFFECT DEVICES

This last harnessing principle among tidal streams consists of an artificial duct built around a turbine so as to create a space with a considerably greater flow-speed. Apart from that they are able to redirect the flow direction, up to a 40° offset, having always a perpendicular facing towards turbines for a better efficiency. They are totally suitable for deep waters as they use to simply rest on the seabed. Furthermore, for devices with a relatively big diameter, shallow waters would not be as suitable as deep waters.

http://aquaret.com/images/stories/aquaret/stills/reciprocating%20hydrofoil.jpg http://aquaret.com/images/stories/aquaret/stills/venturi%20effect%20device.jpg

Figure E.3.9: Reciprocating hydrofoil Figure E.3.10: Venturi effect device

(Aqua-RET Consortium, 2012) (Aqua-RET Consortium, 2012)

These systems can be secured to the seabed in different ways depending on its state and the depth at which they are situated. They are very similar to those for wind offshore systems:

Monopile systems: they consist of one pile covering all the water depth, from the seabed to the surface. They are not suitable for very deep waters.

Gravity systems: these systems are deployed on straight on the seabed and are more suitable for deeper waters.

Anchors: they are also suitable for higher depths.

WAVE POWER

Waves are created by the effect of wind moving on the oceans. Compared to the tides and streams phenomena, waves offer more diverse options to harness their embodied kinetic energy and these is reflected in the higher amount of different devices that have been proposed during the past and recent years. Though, they are not as predictable as them, because they do not follow precise cycles as tides.

Depending on the location of these systems, onshore, near shore and offshore devices and installations can be distinguished. The direction of these devices in respect to the wave direction is important: parallel or perpendicular to the wave front, in order to take profit of the motion or the terminating effect (by impact, when they break down) of waves, respectively. The power generated by these devices is generally transmitted ashore via underwater power cables. The nearer to the shore the easier the system is, and the less expensive it results.

Another classification criterion is their relative position to the sea: they can be either floating, supported or submerged systems, or a combination of them.

There are many different principles (Aqua-RET Consortium, 2012) of wave power obtaining and the following are worth mentioned:

OSCILLATING WATER COLUMN (OWC)

They mainly involve large scale structures built up in the shoreline with an entrance below the water surface. A water column is created so that when waves break down (terminating effect) the water enters the hollow structure and compresses the air contained above it, creating an upward air-flow. With the aid of bidirectional air turbines both the upward and downward air-flow is harnessed to produce electricity. There are three plants to be mentioned in Europe: PICO (Portugal), Mutriku (Spain) and LIMPET (Scottland). The one in Mutriku is the most recent one, consisting of 16 Wells 18,5 kW turbines installed along a breakwater, with an installed capacity of 300 kW (Ente Vasco de la Energía, 2011).

ATTENUATORS

These devices take advantage of the up-and-down motion of waves to create clean electricity. This is achieved by means of several floating cylinders tied to each other and positioned perpendicular to the wave front. These cylinders are flexible in both vertical and horizontal direction. The most significant one is the "Pelamis" (Pelamis Wave Power Ltd., 2012) wave converter device.

POINT ABSORBERS

Point absorbers are relatively small devices which can be installed independently to the direction of the incident wave. The most common solutions are based on buoys. They are suitable for the idea of wave power farms. The "PowerBuoy" (OPT, 2012) point absorber is the one of the most developed one.

OSCILLATING WAVE SURGE CONVERTERS (OWSC)

They consist of an oscillating arm in the form of an inverted pendulum with a considerable area, which placed on the seabed take profit of the movement of the sea-surface. This oscillation movement is converted into clean electricity. The "Oyster" (Aquamarine Power, 2012) near shore device is a good example.

OVERTOPPING DEVICES

The concept of overtopping devices is similar to that of hydro power plants, but with an offshore small-scale application. A reservoir is created in order to get water accumulation by the action of waves going over it. This accumulation of water is equal to potential energy that is harnessed using low-head turbines located in the lower part of the device. A good example is the prototype "Wave Dragon" (Wave Dragon, 2005), which is being tested since many years in Denmark, west of Jutland.

Image

Figure E.3.11: Overtopping devices basic concept.

(Wave Dragon, 2005)

ARCHIMEDES EFFECT DEVICES

Typically installed on the seabed they harness the pressure differential created by the motion of the waves above them. The level rises and falls as waves pass by, moving the top part of these devices up and down. This oscillating motion produces electricity by means of a lineal generator. The "Archimedes Wave Swing" (AWS Ocean Energy Ltd., 2012) is the best example.

http://aquaret.com/images/stories/aquaret/stills/attenuator.jpg http://aquaret.com/images/stories/aquaret/stills/point%20absorber.jpg http://aquaret.com/images/stories/aquaret/stills/oscillatingwavesurgeconverter.jpg

Figure E.3.12: Attenuators Figure E.3.13: Point Absorbers Figure E.3.14: OWSC

http://aquaret.com/images/stories/aquaret/stills/oscillatingwatercolumn.jpg http://aquaret.com/images/stories/aquaret/stills/overtopping%20device.jpghttp://aquaret.com/images/stories/aquaret/stills/submergedpressuredifferential.jpg

Figure E.3.15: OWC Figure E.3.16: Overtopping Figure E.3.17: Archimedes Devices Effect Devices

Source of figures: (Aqua-RET Consortium, 2012)

There is a wide range of devices for harnessing tidal streams and waves. The most of them are prototypes, but there are also manufactured solutions being the ones shown in the Table 1 some of the most worth mentioned.

TIDAL STREAM DEVICES

TYPE

DESCRIPTION

PICTURE

Seagen

Marine Current Turbines Ltd.

Type: Horizontal Axis Turbine

Place: Strangford Narrows

(Northern Ireland)

Capacity: 1,2 MW

Dimensions: Total height 40 m, 16 m diameter turbines.

(Connected to the grid since 2008

with a production of 6,000 MWh/year

It is able to supply 1000 average dwellings)

http://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Seaflow_raised_16_jun_03.jpg/250px-Seaflow_raised_16_jun_03.jpg

OpenHydro

OpenHydro Group Ltd.

Type: Open-centre turbine

Capacity: 2 MW

Dimensions: 22 m high with a weight of 850 T.

(This is the model of the 4 turbines to be installed off the coast of Paimpol-Bréhat in Brittany, France) 

openhydro tidal turbine

Clean Current

Clean Current Power Systems

Type: Venturi effect device

Capacity: 65-500 kW

Dimensions: 3,5-10 m diameter

(There are 4 different models provided by Clean Current Power Systems and they range within the mentioned capacities and dimensions)

http://gigaom2.files.wordpress.com/2009/05/clean_current_turbine.jpg

WAVE POWER DEVICES

Pelamis

Pelamis Wave Power Ltd.

Type: Attenuator

Capacity: 750 kW

Dimensions: Up to 180 m long and 4 m diameter

(A three machine farm was installed in Portugal [2008] with a capacity of 2.25 MW)

http://www.greenlivinganswers.com/blog/wp-content/uploads/2008/03/21/wave-farms-hydropower-and-the-pelamis-wave-energy-converter/Pelamis-Wave-Energy-Converter.jpg

PowerBuoy

OPT, Ocean Power Technologies

Type: Point Absorber

Capacity: 150 KW (PB150)

Dimensions: 45 m high (9 m above the surface) and a weight of 150 T.

(The company is developing a new 500 kW model, PB500)

http://www.robotikka.com/wp-content/uploads/2011/09/hawaii_photo1.jpg

Wave Dragon

Wave Dragon

Type: Overtopping Device

Place: Nissum Bredning Fjord (Denmark)

Capacity: 4-7 MW

Dimensions: 300x170x17 m

(It has been built for a 36 kW/m wave climate)

http://upload.wikimedia.org/wikipedia/commons/c/ce/WaveDragon.JPG

Figure E.3.18: - Tidal Streams & Wave Power devices

Source of figures:

(Marine Current Turbines Ltd., 2012)

(OpenHydro Group Ltd., 2012)

(Clean Current Power Systems Inc., 2012)

(Pelamis Wave Power Ltd., 2012)

(OPT, 2012)

(Wave Dragon, 2005)

CURRENT STATUS

Marine energy is at the bottom of the list of renewable energies, both in terms of energy production and installed capacity. Compared to other renewable energies the global installed capacity is irrelevant: 527 MW to 2011. Nevertheless, the total installed capacity has been doubled in 2011and this could happen year after year during the coming decade (REN21, 2012).

In regard to the countries with the highest potential for an implementation of these technologies, the United Kingdom has one of the largest marine energy resources in the world, as it is estimated in over 10 GW. This number represents around 50% of the possible tidal energy capacity in Europe. Globally speaking the tidal stream energy capacity is considered that could be around 120 GW (Marine Current Turbines Ltd., 2012). These are theoretical values that could be reached in a relatively distant future, since in view of the figures of currently installed capacity it is obvious that we are far from achieving them.

TIDAL IMPOUNDMENT

There are important differences between the development statuses of the previously mentioned technologies for marine energy harnessing. Tidal impoundment technology is the most developed one among the 3 of them. It is been long since it was introduced in the market: 1960s.

Among the different systems mentioned within tidal impoundment only tidal barrages have been erected so far. No tidal lagoons have been built yet, although serious proposals do already exist for the Severn Estuary Project in the United Kingdom, in order to avoid the construction of a barrage, which would have a greater environmental impact.

Even in a much smaller number than conventional dams, there are some existing tidal plants in the globe with different capacities, being Annapolis in Canada (1984 - Capacity: 20 MW), La Rance in France (1966 - Capacity: 240 MW) and Sihwa Lake in South Korea (2011 - Capacity: 254 MW) the best examples. Another one is under construction in South Korea, which is expected to have a generating capacity of over 1300 MW. Many others are at proposal stage, among which the one in the Severn Estuary (Wales, United Kingdom) is one of the most significant.

Even so, the most developed European country in terms of tidal impoundment energy harnessing is France, basically thanks to the Tidal Power Plant of La Rance located in the estuary between Dinard and Sain-Malo. According to data from the International Energy Agency (IEA) 497 GWh were provided to the French electricity grid during the year 2009. It is stated that since it was built it contributes with an average of 544 GWh/year, discounting the 64,5 GWh/year required for pumping. This is an idea of what these systems can provide, being La Rance the only real example existing in Europe.

TIDAL STREAMS & WAVE POWER

A total of 6,8 MW of tidal stream and wave power are installed within the UK (REN21, 2012), the European country with the highest potential. That is at the same time the total installed capacity of these technologies globally speaking.

Even though they have suffered a slow implementation process taking long periods of time, tidal stream and wave power systems are just emerging technologies. Indeed, they are currently experiencing an important turn because it is now when they are starting to be deployed. A good indicator of this fact is that more and more projects to be realized by 2015 are coming to light worldwide.

Within these technologies vertical axis turbines, venture effect devices, wave attenuators and point absorbers are some of the devices (shown previously in Table 1) that are starting to be introduced in the market by companies such as "Marine Current Turbines Ltd.", "OpenHydro Group Ltd.", "Hammerfest Strom UK Ltd." (in collaboration with Andritz Hydro and Iberdrola), "Clean Current Power Systems" or "Pelamis Wave Power Ltd.".

Many areas in different European countries are proposed as interesting possible spots for the harnessing of tidal stream and wave power: United Kingdom, France, Portugal, Ireland, Spain,…A significant project to be mentioned is the Tidal Power array of Paimpol-Bréhat being installed off the French coast. It will consist of a farm with an installed capacity of 8 MW using several OpenHydro tidal turbines (HydroWorld, 2012).

FACTORS

EXTERNAL INPUTS

These are some of the external inputs needed for the application of these systems:

TIDAL IMPOUNDMENT

The most important one is an area with a high tidal range. A tidal range of at least 5 m is needed to make these systems viable.

An estuary is needed to create a tidal barrage.

Shallow waters, especially for the tidal lagoons. As these systems use low head-difference turbines, no excessively deep waters are required (less than 10 m).

Figure E.3.19: Tidal Range in the Severn Estuary (United Kingdom): up to 12 m. (Hammons, 1993)

TIDAL STREAMS

Current speed: The real minimum velocity to make these systems viable is 1 m/s, but the desired range of speeds is comprised between 2-5 m/s. Current or flow speed is related to the tidal range of an area, the speed tends to be higher with higher tidal ranges. The higher flow speed the more efficient the devices within the aforementioned range.

Direction of the streams, especially for vertical axis turbines. For horizontal axis turbines it is not an important requirement as they are placed horizontally, in the same direction of the flow. Venturi Effect devices solve this problem by redirecting the flow.

A narrow passage in which the tidal stream speed is probable to be higher. In relation with the bathymetry of the seabed.

Water depth. The depth of the seabed is important because some systems need a minimum water depth to be installed, while others have maximum depth restrictions.

WAVE ENERGY

Wave power of the area (KW/m). Values ranging from 20 to 70 kW/m are considered good for implementing these systems.

Direction of the waves

Height of the waves

ENVIRONMENTAL FACTORS

TIDAL IMPOUNDMENT

The environmental interactions during the whole life of these systems can be either negative or positive.

NEGATIVE POINTS

Obstruction of the estuary (*).

Barrages are erected all the way across it.

Impact to the sea bed.

During surveying, site preparation and construction phases.

Marine wildlife impact.

In relation with the previous impact, during the surveying, site preparation, construction, operation and maintenance of these systems, marine plants and animals can be affected. (**)

Changes in the water flow of the surroundings and alteration of tidal patterns, especially during operation. Loss of intertidal zones as a consequence.

Noise pollution. Especially during construction and operating phases.

Not only would the animals be affected, but the human beings too.

Visual impact.

Manmade new constructions are introduced within natural ecosystems.

Pollution of water.

Debris generated in the construction phase.

Risk of accidental events. Chemical or oil/fuel spills could occur.

POSITIVE

POINTS

Clean electricity generation.

After the completion of the installation and while operating, it helps reduce hazardous emissions to the environment.

(*) Lagoons are less responsible for this impact. They are actually solutions proposed to reduce the environmental impact of barrages.

(**) These systems can provide protection to some sub-marine species such as shells that find a proper settlement on the foundations. So, this fact could in a way be considered as a positive interaction too.

TIDAL STREAMS & WAVE DEVICES

The environmental interactions mainly depend on the amount of devices installed in a given area: the more devices the higher the environmental impact. The way they are installed also has an effect, in particular due to the density of devices placed within an area.

As we have seen different systems, the way they are fixed to the seabed is another relevant factor in regards to environmental impact. The monopile system is the less respectful one with the environment.

NEGATIVE POINTS

Impact to the sea bed.

Especially during site preparation and construction phases. The surveying phase does not require much work.

Marine wildlife impact.

In relation with the previous impact, during the site preparation, transportation, construction, operation and maintenance of these systems, marine plants and animals can be affected. (*)

Noise pollution. Especially during construction and operating phases. Not very significant.

Reduction in tidal current and wave action.

POSITIVE

POINTS

Clean electricity generation.

After the completion of the installation and while operating, it helps reduce hazardous emissions to the environment.

No visual impact. They are hidden below the ocean surface. This visual impact varies depending on whether they are located offshore or near-shore, but in general, due to their small scale, it is considered as a positive point

(*) These systems can provide protection to some sub-marine species such as shells that find a proper settlement on the foundations. So, this fact could in a way be considered as a positive interaction too.

The environmental negative interactions of tidal stream and wave devices are way less important than the ones produced by tidal impoundment systems. This is the main reason why these systems are likely to be developed in a not too distant future.

ECONOMICAL FACTORS

The fact that tidal impoundment involves large scale projects makes them have a high construction cost, and therefore the initial investment is important. Obviously everything depends on the characteristics of the project, such as width and depth of the estuary where it is to be implemented or the total potential installed (e.g., amount of turbines and their performance). Since few projects have been developed to date, they are the only source of information as far as the cost is concerned.

The construction cost for the Tidal Power Plant of La Rance in France was 95 million € in 1967, quite a high sum for those times, because it currently would mean about 580 million € (The Green Age, 2012). The payback period for this plant has been 20 years. The construction cost of the most recent plant in South Korea, Sihwa Lake, was around 280 million € (PEMSEA, 2012). As previously mentioned, there are no real executed projects involving tidal lagoon schemes and therefore no relative data can be provided about their cost. When it comes to large scale projects, decommissioning cost has to be considered, because one day the life of the plant may come to an end with it consequent dismantling.

In order to compare the costs of the 3 different technologies we are going to look at an indicator of a study from 2008 (International Energy Agency [IEA], 2008), in which the investment and the production of electricity costs are calculated, comparing the present and the expected implementation. As among marine energy technologies tidal impoundment is the most developed one, the cost concerning power generation is not as high as for the others. Although, it does not have a relevant development potential and predictions show that cost will not vary much in the coming decades. The investment cost for tidal impoundment harnessing was 2000-4000 USD/kW in the year 2005, whereas the cost of production of electricity was 0,06-0,10 USD/kWh. By the year 2030 the cost should be lower: 0,05-0,08 USD/kWh and 1700-3500 USD/kW.

The investment cost for tidal stream power was 7000-10000 USD/kW in the year 2005, whereas the cost of production of electricity was 0,15-0,20 USD/kWh. Due to the development potential of this energy technology, both the production and investment costs are predicted to fall by 2030: 0,08-0,10 USD/kWh and 5000-8000 USD/kW.

The cost of the investment is still very high for wave source energy, 6000-15000 USD/kW in the year 2005. In terms of electricity production costs, it also has the highest among the 3 sources of energy we are analyzing: 0,20-0,30 USD/kWh (2005). As it is considered that this technologies will experience an important development by 2030, both the production and investment costs should be much lower: 0,045-0,09 USD/kWh and 2500-5000 USD/kW.

Another point concerning costs is that near-shore locations involve a far lower investment per MW of generating capacity. Even though the offshore resources are higher, the conditions are more extreme and the access to them is significantly more difficult.

SOCIAL FACTORS

The social acceptance of marine energy is quite different for some of these technologies in respect to the others. This is due to different factors:

Tidal impoundment technology involves such a long constructing period that decisions have to follow a long acceptance period that may take years or even decades. The main reason why people very often refuse using them is the environmental impact they suppose into the natural areas.

Another fact to be taken into account when considering applying these technologies is the possible affectation to other trades such as fisheries and shipping industries. Totally closing an estuary, apart from having an important environmental impact, would affect those industries operating through them, becoming as a result in important enemies of tidal power plants.

The creation of new jobs in the area where these kinds of projects are constructed is important, due to their large scale. As a figure, it is estimated that the Severn Estuary project would create around "20,000 jobs in construction and another 30,000 in activity around the barrage". This is an example of the positive impact these large scale projects could have in the society.

Due to the huge amount of power hidden in the oceans and the implementation opportunity that it entails, marine energy development is one of the investigation priorities of the energy sector since some decades. The tendency is to try to replace large scale systems with high environmental impact, and the future is focused in tidal streams and wave potential. A good indicator is the fact that the European Commission has invested more than 55 million € on ocean energy so far from the 1980s, especially during the last decade (European Commission, 2012).

In order to accelerate the development of marine energy, a clear policy framework is needed to address the requirements of a commercial Ocean Energy Industry. As a starting point, the European Ocean Energy Association has recently drawn a Roadmap for Marine Energy. They want it to experience a similar grow to that the wind energy has experimented during the last decades. For that some targets (Fig. E.3.20) have been set to be fulfilled by 2020 and 2050.

Figure E.3.20: Estimated benefits of developing a world leading European Ocean Energy Industry

(European Ocean Energy Association, 2010)

CONCLUSION

SWOT ANALYSIS

All the technologies are included in the same SWOT analysis so as at the end we have a clear idea in mind of which of them has more opportunities in a future.

- Social acceptance: shipping industries, certain population (TI)

- Big investments are required

- High construction cost (TI)

- Long constructing periods (TI)

- Not so easy integration to the grid (TS & W offshore devices)

- Creation of jobs

- Diverse ways to harness energy (TS & W)

- Immediate commissioning opportunity (TI)

- Similarities with Wind Energy (TS)

- Possibility to create farms (TS & W devices)

- European Market

- Huge available marine energy resource

- Clean energy generation

- Tides are totally predictable (TI & TS)

- Well developed technology (TI)

- Small scale systems (TS & W)

- Reduced environmental impact

(TS & W devices)

- Reduced visual impact (TS & W devices)

- Performance peaks with tides

- The harness is not constant, low/high tides

- Large scale systems (TI)

- Considerable environmental impact (TI)

- Considerable visual impact (TI)

- Undeveloped systems (TS & W)

- Lack of policies

C:\Users\hp\Desktop\SWOT\SWOT.png

TI = Tidal Impoundment; TS = Tidal Streams; W = Waves

Therefore, we can predict that the future trend to let marine energy grow will be the deployment of tidal stream and wave energy devices.

SUITABILITY FOR BUILDINGS

None of these systems seem to be suitable at all for their direct integration in buildings.

SUITABILITY FOR SMALL VILLAGES

Near-shore devices could provide clean energy for a reduced amount of houses. The rest of the solutions would not be justified and therefore would not be suitable for their application in small villages. The "SeaGen" tidal turbine prototype is a good example because it is estimated that it is able to supply 1000 average dwellings. Offshore devices would be justified if they are built in farms to send great amounts of energy to the main grid, but not for a small scale application.

LIST OF FIGURES WATER ENERGY

Figure E.3.1: Small hydro site layout.

Figure E.3.2: Table with different plant configurations. Most popular configurations.

Figure E.3.3: The three main types of water turbine: (A) the Pelton turbine (or wheel); (B) the Francis turbine; (C) the Kaplan turbine.

Figure E.3.4: Table with ranges and best options for each case

Figure E.3.5: Table with emissions generated per TWh.

Figure E.3.6: From left to the right: Tidal barrage, tidal bunded lagoon & tidal offshore lagoon.

Figure E.3.7: Horizontal Axis Turbines

Figure E.3.8: Vertical Axis Turbines

Figure E.3.9: Reciprocating hydrofoil

Figure E.3.10: Venturi effect device

Figure E.3.11: Overtopping devices basic concept.

Figure E.3.12: Attenuators

Figure E.3.13: Point Absorbers

Figure E.3.14: OWSC (Oscillating Water

Figure E.3.15: OWC (Oscillating Water Column)

Figure E.3.16: Overtopping Devices

Figure E.3.17: Archimedes Effect Devices

Figure E.3.18: - Tidal Streams & Wave Power devices

Figure E.3.19: Tidal Range in the Severn Estuary (United Kingdom): up to 12 m

Figure E.3.20: Estimated benefits of developing a world leading European Ocean Energy Industry