OCEAN ENERGY

Generating technologies for deriving electrical power from the ocean include tidal power, wave power, ocean thermal energy conversion, ocean currents, ocean winds and salinity gradients. Of these, the three most well-developed technologies are tidal power, wave power and ocean thermal energy conversion.

 

The oceans have a tremendous amount of energy and are close to many if not most concentrated populations. Some believe that ocean power will provide a substantial amount of new renewable energy around the world. Difficulties arising from marine life attaching to energy systems in the seas require these to be easily cleanable.

In a context of increased fossil fuel scarcity and with environmental questions being taken more into consideration, ocean energies are certain of a future in the European and worldwide energy scene. These energies have to be considered in the plural form because the sector covers the energy exploitation of all energy flows specifically supplied by the seas and oceans: waves, tidal currents, ocean currents, osmotic pressure (the differential in salinity of marine currents that can create a flow and which, in turn, can be used to produce electricity) and thermal gradients.

At present, most efforts in both R&D and experimental implementation are concentrated on tidal currents and wave power. There is an amazing diversity in converters capable of transforming these flows into electricity, and more than thirty can be counted. The most effective solutions should emerge as the sector matures.

The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.

Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.

Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems. 

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds.

As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.

A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.

90% of today’s worldwide ocean energy production is represented by a single site: the La Rance Tidal Power Plant (240 MW) that was commissioned in 1966. This type of installation has remained unique in the world and has only been reproduced at much smaller capacities in Canada (20 MW), China (5 MW) and Russia (0.4 MW). This type of project was abandoned for many years because of very high initial investment costs as well as the strong local impact that results from it. However, the present economic situation has encouraged South Korea to build a 260 MW dam closing off Sihwa Lake, which is set to be commissioned in 2009. Lighter new techniques, like hydro turbines, are being developed today to harness ocean currents. The leader in this field, the British company, Marine Current Turbine (MCT), should install 1.2MW in Northern Ireland following its 300 kW pilot project in Bristol Bay.

Among the different converters capable of exploiting wave power, the most advanced is unquestionably the Pelamis Wave Energy Converter, a kind of “undulating sea serpent” developed by Ocean Power Delivery. This technology is the object of a commercial contract for installation of a farm in Portugal. At present, three machines, with a total capacity of 2.25 MW, are in installation phase, and should be joined by 27 others in the years to come. Another 5 MW project is being studied for England this time.

It should be noted that this sector is attracting more and more interest. In this way, the Coordination Action for Ocean Energy Project (CA-OE), which groups together 41 members, was created at the end of 2005, with the support of the European Commission, the European Ocean Energy Association (EUOEA). In terms of the large industrial groups, Voigth Siemens Hydro has integrated the English company, Wavegen, which has been exploiting a rival to the Pelamis converter since 2000, while the English subsidiary of EDF has taken a 25% share in MTC’s SeaGen Project. For its part, the Total oil company has invested 10% of a pilot project located in Spain, using the PowerBuoy, manufactured by the American company OPT.

Ocean energies must face up to two challenges. First of all, progress has to be made in finalising and perfecting technologies and pilot projects have to be validated: the density of water, which is 800 times greater than that of air, makes the installation and maintenance of sites that are constantly subjected to extreme forces difficult. And, above all, costs must be brought under control. The French Ministry of Industry estimates investments between 1,000 and 3,000 €/kW. Production costs should reach a price range included between 36 and 59 €/MWh by 2015.

A process called Ocean Thermal Energy Conversion (OTEC) uses the heat energy stored in the Earth's oceans to generate electricity.  OTEC works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water is about 20°C (36°F). These conditions exist in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer. To bring the cold water to the surface, OTEC plants require an expensive, large diameter intake pipe, which is submerged a mile or more into the ocean's depths. Some energy experts believe that if it could become cost-competitive with conventional power technologies, OTEC could produce billions of watts of electrical power.

Technologies

The types of OTEC systems include the following:

• Closed-Cycle

These systems use fluid with a low-boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Cold deep-seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system.

In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini OTEC experiment, which achieved the first successful at-sea production of net electrical power from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers and televisions.

In 1999, the Natural Energy Laboratory tested a 250-kW pilot OTEC closed-cycle plant, the largest such plant ever put into operation.

• Open-Cycle

These systems use the tropical oceans' warm surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.

In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Energy conversion efficiencies as high as 97% were achieved. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment.

• Hybrid

These systems combine the features of both the closed-cycle and open-cycle systems. In a hybrid system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.