Tidal energy

1. IntroductionThe available sea energy is enormous. If only a 10% were used to generate electricity, it would be a significant amount of energy regarding to the total world energy. In fact, the possible sea energy sources and consequently the conversion systems are divided in three main groups:

Tidal Energy: Making use of the potential energy of the different sea levels created by the tidal effect or by using directly the energy of the tidal streams

Wave Energy: It is mainly caused by the wind effect on the sea.

Ocean Thermal Energy: This energy is harnessed by using a thermodynamic cycle between the different temperatures of the ocean deep and surface water. During the last decades, several devices and equipments have been developed to convert the sea energy. But only a few ones have become real-scale prototypes to be tested on the sea. Nevertheless, an exception is La Rance (France) tidal power plant, which has been generating energy over the last 40 years. Apart from the tidal energy, the other kinds of sea energies are also being tested obtaining different results, some of them closed to the theoretical calculations and other ones quite differents.

That is why the real condition trials are essential to evaluate their possibilities, neither tank testing nor computer simulation can replace them. Besides their technical problems, economical ones should be taken into account. Nowadays, the cost of the produced sea energy can not compete with other renewable energies or with the traditional ones. Furthermore, it is usually difficult to obtain the project funding, due to the destruction risk associated to weatherconditions. In this paper, the most important technologies associated to the sea energy conversion are analyzed, specifically the following ones:

Tidal energy (conventional power plants and those that harness the energy of tidal streams)

Wave Energy (shoreline and offshore technologies)

Ocean Thermal Energy Conversion systems

The Department of Energys (DOE) Federal Energy Management Program (FEMP) facilitates the Federal Governments implementation of sound, cost-effective energy management and investment practices to enhance the nations energy security and environmental stewardship. FEMP works to reduce the cost and environmental impact of the Federal Government by advancing energy efficiency and promoting the use of distributed and renewable energy at Federal sites. The overall goal is to lower Federal fossil-fuel energy consumption to reduce environmental and national security risks. This goal is supported by renewable energy and energy efficiency mandates as set forth in the Energy Policy Act of 2005 (EPAct 2005), Energy Independence and Security Act of 2007 (EISA 2007), and other Federal regulations. Oceans cover 70 percent of the earths surface and represent an enormous amount of energy in the form of wave, tidal, marine current, and thermal resources. Though ocean energy is still in a developmental stage, researchers are seeking ways to capture that energy and convert it to electricity. EPAct 2005 provides the Department of the Interior (DOI) leasing authority to support production, transportation, and transmission of energy off the Outer Continental Shelf. This leasing authority provides jurisdiction over projects that produce alternative energy and can make use of existing oil and gas platforms in Federal waters. Marine technology was once considered too expensive to be a viable source of alternative clean energy, especially compared to already developed products such as wind and solar. However, with the increased price of oil and the issues of global warming and national security, U.S. coastal sites are looking to add ocean energy to their renewable energy portfolios. This paper gives an overview of ocean energy technologies, focusing on four different types: wave, tidal, marine current, and ocean thermal energy conversion (OTEC). It outlines the operating principles, the status, and the efficiency and cost of generating energy associated with each technology.

2. Background

Ocean and marine energy refers to various forms of renewable electric energy harnessed from the ocean. There are two primary types of ocean energy: mechanical and thermal. The rotation of the earth and the moons gravitational pull create mechanical forces. The rotation of the earth creates wind on the ocean surface that forms waves, while the gravitational pull of the moon creates coastal tides and currents. Thermal energy is derived from the sun, which heats the surface of the ocean while the depths remain colder. This temperature difference allows energy to be captured and converted to electric power. People have been fascinated with capturing ocean energy since the late 18th century. Monsieur Girard received the first recorded patent for wave energy conversion in 1799. The patented device consisted of a ship attached to shore with waves driving pumps and other machinery. Only occasional attempts to harness the oceans energy were made between 1800 and the late 1960s. However, in 1966, the largest tidal power station in the world was built in St. Malo, France [1]. This ocean tidal power station still operates today, producing 240 megawatt-hours (MWh) of power each year. Widespread attempts to harness ocean energy wax and wane with the price of oil. In 1973, an oil shortage crisis occurred when the Organization of Petroleum Exporting Countries (OPEC) placed an embargo on shipments of oil to the U.S. and Europe. During that time, an engineer from Scotland, Stephen Salter, took the first steps to develop an ocean-wave generator known as Salters Duck. The duck moves up and down with wave motion. A turbine converts this movement to electrical energy. This single wave-conversion device produced six megawatts (MW) of electricity at a cost of nearly $1 per kilowatt-hour (kWh). The initial cost of wave power was considered too high, and the Salter Duck never made it to production [2]. With fossil fuel prices increasing and expected to stay high in the future, the search for alternative energy resources is once again on the forefront. In the past few years, a growing interest emerged in ocean energy, and progress is being made to bring ocean energy technologies from development stages to the commercial market.

3. HistoryTidal energy is one of the oldest forms of energy used by humans. Indeed, tide mills,in use on the Spanish, French and British coasts, date back to 787 A.D. but it is likelythat there were predecessors lost in the anonymity of prehistory. Tide mills consistedof a storage pond, filled by the incoming (flood) tide through a sluice and emptiedduring the outgoing (ebb) tide through a water wheel. The tides turned waterwheels,producing mechanical power to mill grain and power was available for about two tothree hours, usually twice a day.The power requirements of the industrialized world dwarf the output of the early tidalbarrages and it was not until the 1960s that the first commercial-scale modern-eratidal power plant was built, near St. Malo, France. The hydro mechanical devices suchas the paddlewheel and the overshot waterwheel have given way to highly-efficientbulb-type hydroelectric turbine/generator sets. The tidal barrage at St. Malo usestwenty-four 10-megawatt low-head bulb-type turbine generator sets. Installed in 1965,the barrage has been functioning without missing a tide for more than 37 years.After that, more and more commercial-scale tidal barrage was put in service in France,Canada, Switzerland, UK, China and so forth.

4. Technology Types Four types of ocean energy conversion exist: wave energy, tidal energy, marine current energy, and ocean thermal energy conversion.

4.1 Wave Energy Wave energy is generated by the movement of a device either floating on the surface of the ocean or moored to the ocean floor. Many different techniques for converting wave energy to electric power have been studied. Wave conversion devices that float on the surface have joints hinged together that bend with the waves. This kinetic energy pumps fluid through turbines and creates electric power. Stationary wave energy conversion devices use pressure fluctuations produced in long tubes from the waves swelling up and down. This bobbing motion drives a turbine when critical pressure is reached. Other stationary platforms capture water from waves on their platforms. This water is allowed to runoff through narrow pipes that flow through a typical hydraulic turbine. Wave energy is proving to be the most commercially advanced of the ocean energy technologies with a number of companies competing for the lead.

4.2 Tidal Energy The tidal cycle occurs every 12 hours due to the gravitational force of the moon. The difference in water height from low tide and high tide is potential energy. Similar to traditional hydropower generated from dams, tidal water can be captured in a barrage across an estuary during high tide and forced through a hydro-turbine during low tide. To capture sufficient power from the tidal energy potential, the height of high tide must be at least five meters (16 feet) greater than low tide. There are only approximately 20 locations on earth with tides this high. The Bay of Fundy between Maine and Nova Scotia features the highest tides in the world, reaching 17 meters (56 feet). This area has the potential to produce 10,000 MW [3].

4.3 Current Energy Marine current is ocean water moving in one direction. In the U.S., it is found primarily off the coast of Florida. This ocean current is known as the Gulf Stream. Tides also create currents that flow in two directions. Kinetic energy can be captured from the Gulf Stream and other tidal currents with submerged turbines that are very similar in appearance to miniature wind turbines. As with wind turbines, the constant movement of the marine current moves the rotor blades to generate electric power.

4.4 Ocean Thermal Energy Conversion Ocean thermal energy conversion, or OTEC, uses ocean temperature differences from the surface to depths lower than 1,000 meters, to extract energy. A temperature difference of only 20C (36F) can yield usable energy. Research focuses on two types of OTEC technologies to extract thermal energy and convert it to electric power: closed cycle and open cycle. In the closed cycle method, a working fluid, such as ammonia, is pumped through a heat exchanger and vaporized. This vaporized steam runs a turbine. The cold water found at the depths of the ocean condenses the vapor back to a fluid where it returns to the heat exchanger. In the open cycle system, the warm surface water is pressurized in a vacuum chamber and converted to steam to run the turbine. The steam is then condensed using cold ocean water from lower depths [3].

5. Resources Ocean energy has great potential as a renewable energy resource for the U.S. Tidal current and wave potential alone are estimated to produce approximately 400 terawatt-hours per year (TWh/yr) or 10 percent of the national energy demand [4]. The Electric Power Research Institute (EPRI) performed an energy resource study in 2005 and 2006 that examined seven locations in North America. The estimated wave resource was found to be approximately 2,100 TWh/yr . EPRI also studied tidal energy in North America [4] and found the power potential to be 6.6 TWh/yr .

Figure 1: Preliminary Estimate of U.S. Coastal Wave Energy Resources (60-meter Depth and Greater than 10 kW/m) Courtesy of the Electric Power Research Institute

6. Tidal Energy

The tidal energy is the energy stored by the ocean due to the tides produced by the combination of different effects: The gravitational effect of the Sun and the Moon The Earths rotation Other factors, such as different ocean depths at different places, odd shapes of the continents, Earth tilt, etc. Depending on the location, it is also possible to find a once-a-day tide (one high-tide and one low-tide a day), twice-a-day tide (two high-tides and two low-tides a day), and so on. Therefore, tides can be considered as the longest sea waves, with high periods (12-24 hours) and wavelengths comparable to the length of the Earths equator circumference. Considering the different ways of using this type of energy, the technologies are classified in the following way:A.Traditional Tidal Power Plants Many centuries ago, the tidal energy was firstly used storing water in the high tide for a later use of its potential energy to move till wheels. Although the firstequipments on the 20th century also used this system, the potential energy has a different application. The water stored during the high tide by a river estuary dam is released to move a bulb turbine, generating electricity (i.e. La Rance Power Plant in France, . These systems can be considered as the first generation ones.

6.1 Definition of Tidal Power Tidal power is the power inherent in tides at sea or oceans, that is the power of motion of water actuated by tides. Tides are defined as the increase and decrease in water levels due to the motion of water from one place to the other. This motion of water is actuated by large amounts of energy due to the movement of the Sun, Moon and Earth relative to each other and also to their rotational movement. Thus there is a renewable source of energy in the tidal motion of water at seas and oceans. This source of energy could be used to generate other types of energy that could be useful in industrial applications. The generation of electricity using tidal power is basically the transformation of tidal power found in tidal motion of water in seas and oceans into electrical energy.This is done using a very basic idea involving the use of a barrage or small dam built at the entrance of a bay where tides are known to reach very high levels of variation. This barrage will trap tidal water behind it creating a difference in water level, which will in turn create potential energy. This potential energy will then be used in creating kinetic energy as doors in the barrage are opened and the water rush from the high level to the lower level. This kinetic energy will be converted into rotational kinetic energy that will rotate turbines giving electrical energy. Fig. 1 shows the process in very simple terms.

Fig 2. A diagram showing transformation of tidal energy to electric energy.

6.2 Physical Concepts of the Tidal Phenomena Tidal movements in seas are due to the increase of water levels at certain areas in the globe and the decrease of water levels at other areas. This is basically due to two factors: 1- The gravitational forces between the Sun, Moon and Earth. 2- The rotation of the moon and earth. As there are gravitational forces between the Moon and the Earth, seas or oceans water is pulled away from earth toward the moon at the area where the moon and the earth are in front of each other. At the opposite side of the earth the water is beingpushed away from the earth due to centrifugal forces. there are two areas where the water levels are high and other areas where the water level is low. Thus, the tidal motion of water is created. This is called the lunar tide. The same concepts that apply for the moon apply for the sun, yet, the sun has a smaller effect on the water levels but when that can only contribute or lessen the effect of the moons gravitational power. This is described by "spring tides" where the lunar tide and solar tide are aligned and contribute to each other and by "neap tides" where the lunar and solar tides are at right angles of each other and lessen each other.

Fig. 3. The spring and neap tides.

6.3 How Tidal Power Generation Systems Work In very simple terms a barrage is built at the entrance of a gulf and the water levels vary on both sides of the small dam. Passages are made inside the dam and water flows through these passages and turbines rotate due to this flow of water under head of water. Thus, electricity is created using the turbines. A general diagram of the system is shown in . What follows will be a description of a general tidal power station with its components. Also, many systems of power generation will be described.

Fig.4. General scheme of the tidal power station

The components of a tidal power station are: 6.3.1 A barrage: a barrage is a small wall built at the entrance of a gulf in order to trap water behind it. It will either trap it by keeping it from going into the gulf when water levels at the sea are high or it will keep water from going into the sea when water level at the sea is low. 6.3.2 Turbines: they are the components responsible for converting potential energy into kinetic energy. They are located in the passageways that the water flows through when gates of barrage are opened. There are many types of turbines used in tidal power stations: Bulb turbines: these are difficult to maintain as water flows around them and the generator is in water. Rim turbines: as shown in these are better maintained than the bulb turbines but are hard to regulate as generator is fixed in barrage. Tabular turbines: as shown in figure these turbines are fixed to long shafts and thus solve both problems that bulb and rim turbines have as they are easier to maintain and control.

Fig.5. A Bulb turbine.

Fig.6. Tabul ar turbines.

Fig.7. Rim turbines.6.3.3 Sluices: sluice gates are the ones responsible for the flow of water through the barrage they could be seen in Fig . 6.3.4 Embankments: they are caissons made out of concrete to prevent water from flowing at certain parts of the dam and to help maintenance work and electrical wiring to be connected or used to move equipment or cars over it. These embankments are shown in Fig .

Fig 8. Embankments

7. DIFFERENT METHODS OF OBTAINING POWER FROM TIDAL POWER STATIONS: 7.1- Ebb method: 1st- Water starts to ebb or go toward the sea. 2nd- The gates are left closed keeping the water trapped in basin to increase its level. 3rd- Then water is released out toward the sea rotating turbines creating electrical energy.

7.2- Flood method: 1st-Water is let into the basin when it is empty. 2nd-As the turbines are rotated the electrical energy is created.

7.3- Ebb plus pumping method: 1st-The turbines are operated as pumps pumping the water into the basin at the flood period. 2nd-The water level in the basin is increased creating greater head. 3rd-At the ebb phase the water is let out of the basin creating energy for longer time than usual due to the increased head.

7.4- Two way power generation: 1st-Starting with the basin full the gates are opened letting water flow out generating energy. 2nd- The turbines are reversed as the flow will be reversed. 3rd- The gates are closed when the flood period or cycle starts.

4th- Water starts to build up behind the dam. 5th- When a sufficient head is achieved the gates are opened to start flood generation cycle as the water flows into the basin.

7.5- Two basin generation method: 1st-Two basins are built one called a high-level basin and the other is the low-level basin. 2nd- The turbines are placed in the wall dividing the two basins. 3rd- The high level basin is filled at high tide or flood period. 4th- Then the low-level basin is filled through the turbines from the high level basin. 5th- The low level basin is emptied at low tide ebb period.

8. Simulation ModelThe tidal cycle and the performance characteristics of low-head hydroelectric generating equipment are known. By combining these two known sets of parameters, we can simulate the output from a tidal power plant and model a variety of configurations with accuracy and detail. Following is a description of the simulation model used by the author and created by John Haapala of Harza Engineering (now Montgomery Watson Harza Engineering).The model determines tide levels at 6-minute intervals for any day, month, or year for the period of years between 1949 and 2025. The tide levels are predicted from a series of harmonic equations as provided in a FORTRAN program developed by NOAA, in the case of US sites or by the British Admiralty, in the case of British sites. Predicted tide levels may vary from actual tide levels due to weather conditions such as wind atmospheric pressure. A comparison of tide levels predicted by the model and actual recorded tide levels showed agreement well within acceptable limits for a feasibility study. The constantly changing tidal regime would cause generation to vary by several percent from year to year. Other things being equal, it was found that during a high generation year (1998) project output would be about 8% greater than in a low generation year (2005). This cycle is known as the nodal cycle.A hypothetical equipment package is selected and its operating parameters are input and an impoundment size is selected, which defines the amount and timing of water available and the sequence of heads available. Together, these parameters determine the turbine flow and efficiency as well as the generator efficiency. Because the turbines have minimal heads, there are periods of sluicing that are essential to the operation in order to maximize or minimize the water levels in the pool in preparation for the next generation cycle. Some of the sluicing flows through the turbines and sluice gates provide addition sluicing capacity.Sluicing is determined from the following equation: Q = C*A*(2g*H) 1/2Where:Q is the sluice flow in cfsC is the sluice gate coefficient or turbine sluicing coefficientA is the sluice gate area (ft2) or turbine area for sluicing (ft2)H is the head on the sluice (feet)g is 32.2 ft/sec2The net energy is determined by using the following formula:E = 0.08464*Q*H*Et*Eg*Etr*Lt*Lsu*Lo*TWhere:E is the energy generation in kilowatt-hoursQ is the turbine flow in cfsH is the net head on the turbine in feetEt is the turbine efficiencyEg is the generator efficiencyEtr is the transformer efficiencyLt is the transmission line loss factorLsu is the station use loss factorLo is the forced and unforced outage loss factorT is the time increment (0.1 hour in the model)Hydraulic losses unavoidably occur near the intake and outlet of a turbine. These head losses are included based on the following equation: HL = k*Q2Where:HL is the head loss in feetk is the head loss coefficientQ is the turbine flow in cfsTransmission line losses are included to the point of interconnection where the sale of electricity is metered. Some electricity use normally occurs at the project site, which is accounted for in the station use factor. The forced outage loss factor should account for average outages of all types over the economic life of the project. Outage losses would include factors such as transmission line forced outages, and down time for equipment repair, both scheduled and unscheduled.

9. Advantages of Tidal Power Generation There are many advantages of generating power from the tide; some of them are listed below: Tidal power is a renewable and sustainable energy resource. It reduces dependence upon fossil fuels. It produces no liquid or solid pollution. It has little visual impact. Construction of large-scale offshore devices results in new areas of sheltered water, attractive for fish, sea birds, seals and seaweed. Tidal power exists on a worldwide scale from deep ocean waters. It offers short time scale between investing in the modular construction and benefiting from the revenue Tidally driven coastal currents provide an energy density four times greater than air, meaning that a 15-m diameter turbine will generate as much energy as a 60m-diameter windmill. Tidal currents are both predictable and reliable, a feature which gives them an advantage over both wind and solar systems. Power outputs can be accurately calculated far in advance, allowing for easy integration with existing electricity grids.

The tidal turbine offers significant environmental advantages over wind and solar systems; the majority of the assembly is hidden below the waterline, and all cabling is along the seabed. Seawater is 832 times as dense as air; therefore the kinetic energy available from a 5-knot ocean current is equivalent to a wind velocity of 270 km/h.

10. Disadvantages and Constraints to Tidal Power Generation Unfortunately, there are also disadvantages and limitations to generating tidal power. Some of these are: At the present time both tide and wave energy are suffering from orientation problems, in the sense that neither method is strictly economical (except in few locations throughout the world) on a large scale in comparison with conventional power sources. Tidal power systems do not generate electricity at a steady rate and thus not necessarily at times of peak demand, so unless a way can be found of storing energy efficiently - and any storage devices currently available incur a considerable loss - they would not help in reducing the overall need for fossil power stations, but only allow them to run at a lower rating for a certain amount of the time. Tidal fences could present some difficulty to migrating fish.

11. Examples of Tidal Power Stations Worldwide The most major tidal power station in operation today is a 240-megawatt at the mouth of the La Rance river estuary on the northern coast of France (a large coal or nuclear power plant generates about 1,000 MW of electricity). The La Rance generating station has been in operation since 1966 and has been a very reliable source of electricity for France. La Rance was supposed to be one of many tidal power plants in France, until their nuclear program was greatly expanded in the late 1960's. Elsewhere there is a 20 MW experimental facility at Annapolis Royal in Nova Scotia, and a 0.4 MW tidal power plant near Murmansk in Russia. This compares to 28 GW and 7-10 GW thought possible in the Bay of Fundy and the Severn Estuary (Great Britain) respectively. It has been estimated that this could supply as much as 10% of the country's electricity needs.Tidal power has been proposed in the Kimberley region of Western Australia since the 1960s, when a study of the Derby region identified a tidal resource of over 3000 MW. In recent years a proposal to construct a 50 MW tidal plant in the Derby region has been developed Derby Hydro Power. Studies have been undertaken to examine the potential of several other tidal power sites worldwide. Similarly, several sites in the Bay of Fundy, Cook Inlet in Alaska, and the White Sea in Russia have been found to have the potential to generate large amounts of electricity. Conclusion Tidal power has the potential to generate significant amounts of electricity at certain sites around the world. Although our entire electricity needs could never be met by tidal power alone, it can be a valuable source of renewable energy to an electrical system. The negative environmental impacts of tidal barrages are probably much smaller than those of other sources of electricity, but are not well understood at this time. The technology required for tidal power is well developed, and the main barrier to increased use of the tides is that of construction costs. The future costs of other sources of electricity, and concern over their environmental impacts, will ultimately determine whether humankind extensively harnesses the gravitational power of the moon.

Fig. 9. Bulb Turbine (La Rance Tidal Power Plant) [1]

Reversible turbines are usually installed in this kind of power plants, making use of the streams of both tides (high and low). It is also important to consider the importance of the difference between high and low-tide sea levels. The higher the difference is, the higher the energy produced will be. Nowadays, five meters is considered the minimum difference needed to produce commercial electricity. The theoretical energy available in a tide cycle dependson the reservoir area and on the square of the maximum tide amplitude, which fixes the reservoir maximum height. Considering the previously mentioned level difference required to produce energy, there is only a few places in the world with enough tide amplitude. Table 1 shows anexample of some of them.

Table 1. Suitable places to install tidal power plants [1]

An important disadvantage of this kind of energy is the high cost of the installed power (1500 /kW). Apart from that, the long construction time (5-15 years) and a low load factor (22-35%) due to the variable tide amplitude. The environmental impact produced by the changes ofhydrodynamic regime has also to be considered. Another negative factor is the long distance between the power plants and the load demand, requiring long transmission lines. The main advantages are the low operation and maintenance cost (< 0.5%) and its high availability (> 95%), with a high number of generator-turbine groups. Some examples of this kind of installations are: Kislaya Power Plant (Kislogubskaya), located on the White Sea in Russia. It uses a 0.4 MW bulb turbine, and started working as a pilot plant in 1968. It is the second in the world of its kind. Power plant located on the east coast of Canada, at the estuary of the Annapolis River on the Bay of Fundy. By an 18 MW Straflo turbine, it started working in 1984 [1]. All the mentioned negatives points influence many countries not to invest in this kind of power plants. However, R&D related to the tidal streams based power plants explained below have higher promotion due to its advantages and future perspectives.B. Tidal Streams Power Plants Todays tendency towards tidal energy production is the use of equipments to convert the tidal streams energy, also called second generation systems. These systems do not need large dams to collect the water in the reservoir. Although in some places it is not possible to make use of the existing tides, the tidal streams are usable to produce energy. To this purpose, different kinds of turbines have been developed for a submerged installation in suitable locations. The world available tidal stream energy is estimated at 5 TW (Isaacs and Seymur 1973), approximately the world energy demand. Nevertheless, in spite of the uncertainty about the existing resources, only a little part of that energy could be used because of the streamslocations. They can usually be found near the oceans periphery or through straits between islands or other geological shapes. Besides, from the economical point of view, only 1 m/s or above streams are usable for energy generation purposes. The most important positive factors to take into consideration are the high predictability of the sea streams and the high load factor of these installations (20-60%).The available power in tidal streams can approximately be obtained from the following expression:

Where: Ptma = Total mean annualised power (W). Vrmc = Cubed root of the mean of the cubes speeds (m/s). = Current water density (kg/m3) A = Cross-sectional area (m2)Since extracting all the available stream power could be environmentally damaging, it is necessary to use a factor apparently no damaging consequences. It is called SIF (Significant Impact Factor) and Black and Veatch (a British consultant company) considers it should be 20%

Four are the proposed tidal stream energy conversion systems considered nowadays:1) Horizontal-axis turbines They look similar to wind turbines, and can be installed either seabed-mounted or hanging from floating platforms. The extractable energy depends on the rotordiameter and current speed

Fig. 10. Extractable power with a 30% average performance(CEC, 1996) [3]

They may have pitch-controlled blades depending on the stream flow. 10-15 m diameter and 200-700 kW rated power turbines, are probably the most economic overall solution for first generation devices [3]. Some installations that use this kind of technology are: Seaflow, with a rated power of 300 kW and 11 m rotor diameter, installed in Lynmouth, Devon (UK) in 2003 [21]. Hammerfest Strom A.S., Norway, with 300 kW and 20 m rotor diameter, installed in 2003 [10].

2) Vertical-axis turbinesIn these turbines, the stream flow is perpendicular to the rotational axis and they are also similar to their wind homologues (Darrieus). However, in Darrieus turbines the rectilinear blade design produced efficiency but instability at the same time, with high tendency to rupture due to vibrations. After some laboratory tests, it was concluded that using helicoid blades (Gorlov turbine) the vibrations problem is solved and therefore the rupture risk problem. Another important advantage of Gorlov turbine is the amount of energy extracted from the tidal stream (35%), compared with rectilinear blade Darrieus turbine (23%) and with the conventional horizontal axis-turbine (20%). Besides, the Gorlov turbine always turns in the samedirection, regardless what the stream direction is, with higher versatility in energy conversion [4]. An example of this technology is the Enermar Project, which uses a 3-bladed Kobold turbine with a rated power of 130 kW (23% of global efficiency). It is located in the strait of Messina, Italy [10].

3) Oscillating hydroplane deviceIt is based on a seabed mounted device, called Stingray. main characteristic of this technology is its large wing-like hydroplane, which can be pitch-controlled, oscillating up and down and compressing oil of an hydraulic power converter. Several prototypes have been tested in several stages with 150 kW and 180 Ton [5]

4) Venturi effect tidal deviceIt is based on the Rochester Venturi device, developed by Hydro Venturi and installed in the north of England. This system consists of an open venturi tube, which uses the venturi effect to accelerate the water flow. While the water is accelerated, a pressure reduction is generated in the most constricted point, under atmospheric pressure. Subsequently, the pressure gradient force (from high to low pressure) is used to move a conventional turbineinside the tube which is connected with the constricted point [6].

C. AdvantagesThe most important advantages and disadvantages of tidal energy conversion compared with the other kinds of sea energies are the ones listed below. Some of them are common to both conventional tidal and tidal stream power plants, but other ones only refer to one technology.

1) Advantages Highly precisely predictable local energy resources, superior to other renewable energies, such as solar or wave energy. Conventional tidal power plants The bulb turbine technology used in conventional tidal power plants is far developed with reliability and good performance, obtaining low operation and maintenance costs. The high availability of conventional power plants is a positive factor to take into account. Extensive experience due to the power plant installed in La Rance (France), which has been working during decades. Possible transportation improvement due to its application to the development of traffic or rail bridges across estuaries.

Tidal stream power plants High sea streams energy density. Compared with the most known wind turbines, seawater is 832 times denser than the air. So the power available in a sea current of 14.81 km/h speed is equivalent to a 390 km/h wind one. Vertical axis turbines used in tidal streams conversion can be linked to each other in a grid to make better use of the streams energy in deepest areas.

2) Disadvantages There are only a few places where this energy can be commercially used and many times they are quite far away from the energy demand, so the transmission infrastructure could have too high costs. Conventional tidal power plants The conventional tidal power plants are far expensive, with very long construction periods, apart from their reduced load factor. Nowadays, the environmental impact of conventional tidal power plants is rather high to be assumed.

Tidal stream power plants There is an uncertainty about the existing world resources related to tidal streams. The turbines used for tidal stream energy conversion must be installed offshore and underwater. Consequently, high operation and maintenance costs can not be avoided. During its normal operation, the horizontal-axis turbines withstand high working stresses (higher under bad weather conditions). Besides, there is high probability for cavitation to appear at the blade tips, due to the pressure reduction with its increase in water relative speed. The offshore turbines increase the ship crash risk when they are installed near the maritime routes. Damaging effects, for both migratory sea species and seabed, due to existing underwater cables and anchorages. Todays offshore turbines cost is far higher than the other renewable energies. There have been not enough tests to get to know the resistance of these technologies in a sea environment. In case of an extended use of this energy conversion, it must be a maximum limit for the extracted energy. Otherwise, the environmental impact could be toohigh. Possible sea spills (lubricant oils, etc.)12. Wave energyThe ocean waves are mainly produced by the effect of the wind (due to the sun energy) blowing over the surface of the oceans. Considering a solar irradiance of 375 kW/m2 over the world surface, only 1 kW/m2 is transmitted to produce waves, which work like an energy storage. Then, these waves move quite easily from one place to another through the oceans, with a speed proportional to their wavelength.There is no perfect regularity in waves. Their amplitude, energy and direction vary randomly through the year. While sometimes they can change from an absolute calm (1%) to 1 MW/km, in other places they can reach 10 MW/km for a short period of time (minutes). Instantaneous variations are also possible. The power in a real wave depends on several factors (wave direction, sea depth, etc.), but it is mainly proportional to the square of the amplitude and to theperiod of its motion. A long period wave (7-10 s) with high amplitude (2 m) could have more than 40-50 kW/m. As in most renewable energies, the wave energy is not equally distributed over the world. Between the 30 and 60 latitudes of both hemispheres is located the highestenergy density, due to the characteristic west winds of these areas. The world wave power distribution is shown in figure .

Fig. 11. World wave power distribution (kW/m width of oncoming wave) [8]

The world wave energy resources have been commercially estimated in more than 2000 TWh/year (10% of world electric demand) with an expected investment of 735000 M [9]. Comparing with the wind energy forecast (40000 MW in 2010), 2000 MW of sea energy power is expected to be installed in 2020, the most of it using wave energy [10]. The most relevant characteristics of different generation technologies are analyzed below, also mentioning their advantages and disadvantages

A. First Generation DevicesThe first generation devices group is mainly composed of the shoreline technology (fixed devices). Inside this group the importance of the OWC (Oscillating Water Column) should be emphasize. This on-shore installed equipment has a turbine moved by the air flow generatedby an oscillating water-column, which harness the wave energy. Nowadays, the most used turbine for this purpose is the Wells turbine, because of its unidirectional torque, regardless of which the air flow direction is. This kind of turbines maximum performance is around 70-80% (see figure ).

Fig. 12. Oscillating Water Column device [10]

A good example of this technology is the LIMPET (Land Installed Marine Powered Energy Transformer). It is a 500 kW rated power plant, developed and operated by Wavegen company, in the coast of Islay, Scotland. It comprises a single counter-rotating Wells turbine in which each plane of blades drives a generator rated at 250 kW. Using this kind of devices, theoretically it would be possible to obtain up to 50% energy conversion performances [11]. Apart from the coastline OWC, breakwater OWC has been developed with no further environmental impact.Another example of this technology is the shoreline OWC pilot plant on the island of Pico, Azores, using a Wells turbine and with a rated power of 400 kW [10].

B. Second generation devicesThese are the floating, semi-submerged or submerged devices, commonly the offshore ones which may have a fixed connection to the seabed or another structure. They harness the oscillating wave motion to generate electricity. Their location is usually on the continental shelf up to 150 m depth waters, no further than 12 nautical miles from the coast. The importance of this distance limit is related with the increased cost of the device to the electric grid connection point.Nowadays, from the power conversion train point of view, several technologies have been developed. Some of them are mentioned below: Oscillating movement Fluid compression Hydraulic turbine drive Rotary electricalgenerator Oscillating movement Linear electric generator Oscillating movement Sea water pumping to coast Hydraulic turbine drive Rotary electrical generator (Seadog pumping system)There are countless patents and designs for this kind of wave energy conversion. However, just a few ones have reached the real prototype stage development, being tested in real conditions. As an example, the technology developed by the Ocean Power Technologies (OPT) company, composed of multiple buoys (PowerBuoy) and underwater transmission cable [12].Other examples are: Pelamis: It involves 4 large floating cylinders connected together. As the waves pass, a pendular motion is generated, which impulses a power conversion hydraulic system. Several prototypes of 750 rated power have been tested all over the world. Wave Dragon: This design has a parabolic wave concentrator, which increases the water volume in a higher reservoir, to be released through Kaplan turbines later. It is expected to have a real-size prototype of 7 MW rated power installed in the west coast of Scotland in 2007. Energetech: OWC system (500 kW 2 MW), installed in Port Kembla, Australia in 2005 [20].

C. Advantages and disadvantages1) Advantages Shoreline Devices Shoreline devices have far less difficulties in power grid connection than the offshore ones Maintenance and repair costs are lower than those for offshore devices, due to the unlikely storm damages. Less environmental impact than conventional tidal power plants. The visual impact may be reduced when small Wells turbine modules are installed in specific places such as breakwaters, harbour walls, etc. The shoreline prototypes installed a few years ago have already provided enough data to improve efficiency and reliability and to reduce the total cost. High availability. Offshore devices There are more places suitable for wave energy conversion than those for tidal stream energy. The available power offshore is higher than in shoreline installation. Easier calculation of real energy performance in offshore devices.

2) Disadvantages Shoreline Devices The coastal orography, when choosing the installation point, is essential to reach a pneumaticperformance closed to the theoretical one. There are less suitable places for shoreline technology than for offshore one. Although they have less environmental and visual impact than other sea energy conversion systems, this impact is quite high. Construction works usually modify part of the coastal orography, damaging possible locations of marine species. Available wave energy in shoreline installations is less than in offshore locations.

3) Offshore devices Higher maintenance and repair costs, considering the use of ships, due to both in-situ works and device transport. Collision risk with ships and marine species. The environmental and visual impact will mainly depend on the specific technology used, but it could be higher than in first generation devices. Higher power losses and cost than in shoreline devices, because of the longer connection cables used in offshore technologies. The structural strength in case of sea storms is a great handicap not assessed enough so far. There are only few data available obtained from realscale prototype trials. Due to the variability of wave energy, these devices need some storing system to increase the stability of thegenerated electrical energy (reducing power oscillations) and to improve the power quality of its grid connection. The power plant may be forced to auto-shutdown if power quality does not fulfil the electrical grid constraints [18].

13. The keys of tidal power technologies13.1 Barrage or damA barrage or dam is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. The basic components of a barrage are turbines, sluice gates and, usually, slip locks, all linked to the shore with embankments. When the tides produce an adequate difference in the level of the water on opposite sides of the dam, the sluice gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.

Figure 13 Tidal fenceTidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters. Some of these currents run at 58 knots (5.69 miles per hour) and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind). Tidal fences are composed of individual, vertical axis turbines which are mounted within the fence structure, known as a caisson, and they can be thought of as giant turn styles which completely block a channel, forcing all of the water through them as shown in figure .

Figure 14 model of a tidal fenceUnlike barrage tidal power stations, tidal fences can also be used in unconfined basins,such as in the channel between the mainland and a nearby off shore island, or betweentwo islands. As a result, tidal fences have much less impact on the environment, asthey do not require flooding of the basin and are significantly cheaper to install. Tidalfences also have the advantage of being able to generate electricity once the initialmodules are installed, rather than after complete installation as in the case of barragetechnologies.Tidal fences are not free of environmental as a caisson structure is still required,which can disrupt the movement of large marine animals and shipping.A 2.2GWp tidal fence using the Davis turbine, is being planned for the San BernadinoStrait in the Philippines. The project, estimated to cost $US 2.8 Billion and take 6years to complete.

14. Ocean Thermal Energy Conversion (OTEC)The ocean thermal energy can be harnessed by means of a thermodynamic cycle, which uses the temperature gradient between the cold deep waters and the warm surface waters. It is estimated that, the amount of solar energy absorbed annually by the oceans is equivalent to 4000 times the world energy demand in the same period [13].The main problem with this renewable energy is the necessity of a temperature gradient higher or equal to 20C, between the hot and cold reservoir. The higher the temperature difference, the better efficiency it will have. This requirement is only fulfilled in tropical and equatorial zones during the whole year. The OTEC systems efficiency is not quite high because of the little temperature difference used in the thermodynamic cycles. Although the ideal energy conversion using 26 C and 4 C warm and cold seawaters is 8%, due to several losses final 3-4% efficiency is get [13].Apart from thermal efficiency difficulties, long piping systems will be necessary to pump the cold water from 1000 m depth or more. Besides generating energy, OTEC systems can also beused for cold water production, for mariculture, hydrogenproduction and to desalinize seawater. That is why, and due to their special conditions, in some islands this would be a valid alternative to conventional energy power plants. Furthermore, its only environmental impact consists in its small cooling effect in seawater. Other collateral effects (minimum carbon dioxide out-gassing, etc), can be usually reduced with a proper plant design [13].There are two Ocean Thermal Energy Conversion systems:

A. Open cyclesThe thermal working fluid used is the sea water. The warm surface water at around 27 C, is evaporated at low pressure slightly below the saturation pressure. After expanding in the turbine to drive the generator, the steam turns into liquid again by the exposure to a cold deep water (5 C) condenser (figure 6). Finally, it is returned to the sea.

Fig. 15. Open cycle [13]

An example of this kind of OTEC is the Natural Energy Laboratory of Hawaii (HNEI), working between 1993 and 1998 with a turbine-generator of a designed output of 210 kW.

B. Closed cyclesThese type of cycles use ammonia, propane, freon, etc. as thermal working fluid (low-boiling point fluids). The liquid fluid is pumped through the evaporator, where it is vaporized by the warm surface water at 27 C. After that, it is expanded in the turbine (driving the generator) andthen the cold deep water at 5 C condenses it back to a liquid, closing the cycle .

Examples of closed-cycles OTEC are: Mini-OTEC: Small plant mounted in 1979 on a barge off Hawaii, producing 50 kW of gross power (net output of 18 kW). 100 kW gross power, land-based plant operating in the island nation of Nauru, in 1981.

C. Hybrid cyclesThese cycles produce both electricity, with a closed cycle system, and fresh water, by the desalinization with an open cycle system.

Fig.16. Closed cycle [13]

D. Advantages and disadvantagesThe most important advantages and disadvantages of this sea energy conversion technology are stated below.

1) Advantages Damaging environmental effects are minimized if the cold water is discharged to the ocean at enough depth. Desalinize seawater, produced cold water for mariculture and hydrogen by electrolysis, are other OTEC advantages. Highly available energy resources.

2) Disadvantages The small land based OTEC plants need kilometers of piping to move a high volume of cold water from deep ocean. Its cost could be up to the 75 % of thetotal power plant costs. Therefore, researches show that power plants with a rated power lower than 50 MW can not compete with other energy sources. As an example, a 50 MW power plant would require a 3 km long pipe with a 8 m diameter to pump 150 m3/s of cold water [14]. The suitable locations to harness this kind of energy are reduced to equatorial and tropical zones. Higher cost than other energy sources (hydroelectric, wave energy and diesel) in islands. Low thermal efficiency due to the low temperature difference between the cold and hot reservoir (around 22 C). Although floating OTEC plants could apparently be solution, maintenance and repair costs would also be high. Floating plants and piping of land based plants must withstand high stresses during storms.

15. Comparative analysisFinally, a comparative summary of the different sea energy resources is showed in table 2, considering their advantages and disadvantages in each case. The mentioned figures must be considered as average values. The compared characteristics are the following ones:

A. Power Unit CostNowadays, the unit cost of sea energy technologies is stillhigher than other kind of renewable energies. However, R&D trends show the installation of different pilot plants, both offshore and shoreline (also sea streams devices). Therefore, in the medium-long term, costs are expectedto decrease and considering their good features, they could finally be competitive with other energy resources. Above all, OTEC systems have the highest power unit cost (as an example, see Table 2 for a 40 MW power plant).

B. Total efficiency ()Wave energy and tidal streams devices have the highest efficiencies of all sea energy sources. The efficiencies of the power conversion train are expected to be improved by the use of the data obtained in the installed pilot plants. As it can be seen table 2 below, the OTEC systems efficiencies is quite low, although it could be balanced by its vast available energy quantity and the possibility of producing other goods, such as: fresh water, nutrient rich cold water for mariculture and hydrogen by electrolysis.

C. Global resourcesIn contrast with its efficiency, OTEC systems have an enormous quantity of energy resources, far higher than tidal stream, tidal power plants and wave energy technologies.

D. Energy densityTidal stream energy has the highest energy density, higher than the wave energy and the OTEC. Despite its highly available thermal energy, the OTEC energy density is quite low, so it is necessary a high cold water volume per generated MW.

E. PredictabilityThe OTEC systems hardly depend on seasonal nature to generate its rated power, so its predictability factor is higher than the others. On the contrary, the wave energy is very unpredictable in the medium-long term and with high seasonal dependence (heavier swells in winter than in summer, in warm climates). Streams are mainly produced by tides, so they are more predictable in the medium-long term and with less seasonal dependence than the wave energy

F. AvailabilityThe availability represents the probability of a sea energy based system to produce energy in a certain moment, and is directly influenced by the maintenance and repair periods. It must be consider that tidal stream and OTEC conversion technologies are still under development (pilot plants), so the real operation data is not available yet. The data showed in table are based on some realscale prototype tested so far, they are not average values of each type of energy source.

G. Operation and Maintenance costThe operation and maintenance cost of conventional tidal power plants is the lowest of all sea energy conversion devices. Mainly, it is due to the working experience of La Rance (France) tidal power plant, connected to the grid during decades. While Cost for OTEC is estimated in1.5% of capital cost [13], wave and tidal stream energies costs mentioned are only applicable for some of the technologies tested up to now.

Table 2. Comparative table of different sea energy resources

H. Environmentaland Visual ImpactAs it has been previously mentioned, the most environmentally damaging technology is theconventional tidal power plant. The other ones have mainly a visual impact, over commercial maritime routes or coastal ecosystem, but not as higher as the tidal power plants.

I. Future developmentConventional tidal power plants are not probable to be extensively installed, due to its important

disadvantages:high cost, construction period of several years, few proper locations and the already mentionedenvironmental impact. Because of this, energy development policies promote other renewable energies (such as solar or wind energy) and in the medium-long term wave and tidal stream energies are expected to be equally promoted as well. On the other hand, the OTEC systems will only be possible in those specific areas (mainly islands) when the temperature gradient is 20 C,at least. Not only for power generation, but also for different productions.Although 5800 MW of marine renewable power is expected to be installed between 2004 and 2008, only a small percentage of it will be composed by wave and tidal power technologies, due to their less development compared with offshore wind energy. This energy is rapidly growing nowadays, but the wave and tidal energy could have the same trend from around 2010 [6].

16. Social attitude to tidal power energyThe social attitude is closely connected with the environmental impacts, the financial factors and the application efficiency. The environmental concern is especially related to the impacts on fish and plant migration, some studies that have been undertaken to date to identify the environmental impacts of a tidal power scheme have determined that each specific site is different and the impacts depend greatly upon local geography, Local tideschanged only slightly due to the La Rance barrage, and the environmental impact has been negligible, but this may not be the case for all other sites, we still need information to prove that; when refer to silt and mud deposits, the barrage could have a compensating impact on the level of silt and sediment suspended in the water. The waters in the Severn Estuary currently carry in suspension much silt churned up by the tides, making the water impenetrable to sunlight. With the barrage in place and the tidal ebbs and flows reduced, some of this silt would drop out, making the water clearer.Regarding to the financial factor, currently, The long construction period for the largerschemes and low load factors would result in high unit costs of energy, which makes tidal projects have relatively high capital cost in relation to the usable output, compared with most other types of power plant, consequently with long capital payback times and low rates of return on the capital invested. Predicted unit costs of generation are therefore unlikely to change and currently remain uncompetitive with conventional fossil-fuel alternatives. However, Some non-energy benefits would stem from the development of tidal energy, which would yield a relatively minor monetary value in proportion to the total scheme cost. These benefits are difficult to quantify accurately and may not necessarily accrue to the barrage developer. Employment opportunities would be substantial at the height of construction, with the creation of permanent long-term employment from associated regional economic . Public opinion focuses more and more on these non-energy benefits and that would be an important force for the development of the tidal power energy.Tidal power generators use familiar and reliable low-head hydroelectric generating equipment, conventional marine construction techniques, and standard power transmission methods. The key points in determining the application efficiency of a tidal power plant are the size (length and height) of the barrage required, and the difference in height between high and low tides. Although the technology is not developing in a very fast rate, we have gained a lot of experience. According to the high capital cost for a tidal energy project, the electricity cost is very sensitive to the discount rate. Therefore, how to prove the tidal power energy is not a waste of money is what we need to do emergently.The role that public opinion plays should not be forgotten at any time; a very important difference between the countries where some renewable but costly energy has become widespread is largely depending on the public support. A sustainable Future can only be established if the reasons behind decisions are public knowledge and the possible dangers are researched well.

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