Electricity Fact Sheet
TIDAL and WAVE ENERGY
Tidal Energy
Tidal energy is a form of hydropower that converts the energy from tides into electricity.
Tidal power is predictable and relies on the gravitational pull of the moon and the sun to create currents in the oceans. Generating electricity from the tides in oceans is not widely used and there is an emerging marine renewable energy industry. The potential has been recognised since Roman times and in the Middle Ages with evidence of early tide mills found in Europe and on the Atlantic coast of America.
The first tidal power station was built at La Rance in France. It was commissioned in 1966 with an installed capacity of 240MW. The first tidal power site in North America is the 18 MW Annapolis Royal Generating Station, Nova Scotia, which opened in 1984. The first in-stream tidal current generator in North America was positioned at Race Rocks on Southern Vancouver Island in 2006.
There are two main types of tidal energy – tidal stream systems and barrages.
- Tidal stream systems use the kinetic energy of the moving water to power turbines.
- Barrages use the potential energy in the difference between the height of the high and low tides.
Tidal Stream Systems
Tidal stream systems can be placed where there are strong, fast currents. Good locations can be found at the entrances to bays and rivers and between islands or other land masses where the currents become concentrated forces. There are a number of designs for tidal stream systems.
Following a successful testing period for its prototype Free Flow system of three-blade horizontal-axis turbines, which look similar to wind turbines, the New York company Verdant Power Inc. set a world record. Verdant Power’s Roosevelt Island Tidal Energy (RITE) Project operates in New York City’s East River. The project was completed in May 2007 and incorporates an underwater array of six three-blade horizontal-axis turbines that convert the kinetic energy of tides and currents into electricity. It is the world’s first array of grid-connected tidal turbines and has delivered around 50 MWh of electricity into the New York grid for city businesses.
Turbines such as the ones used in the RITE Free Flow system are deployed in tidal positions and are assembled specifically to allow the turbines to pivot with the changing tides. The pivotal design captures the energy for the majority of the day. Turbines in rivers are fixed and generate power on the continuous flow of the river, providing almost 24-hour electricity.
Barrage Systems
There are currently only three operating barrage systems in the world – on the Rance River in France, in the Bay of Fundy, Nova Scotia and the Barents Sea at Kislaya Guba in Russia. Barrage turbines generate electricity as the tide flows in and out of the bay, river or estuary. Barrage systems involve high civil infrastructure costs and there are potential environmental issues to address related to altering the ecosystem.
Wave Energy
Ocean waves are a tertiary form of solar energy. Due to the contours of the earth’s surface, there is unequal heating of the earth from the sun. This generates wind and wind blowing over water makes waves. Wave power refers to the energy of ocean waves and the capture of that energy to do useful work, including generating electricity, desalination processes and pumping water. When the wind blows over the surface of offshore water, such as oceans and seas, waves are created by the progressive transference of energy from the wind. Wave energy is stored, concentrated wind energy. The larger the wave, the more powerful it is. Wave power is determined by wave height, wave speed and water density. Almost 75% of the world’s surface is water. However, waves are a largely unexplored source of energy compared with the progress of solar and wind energy systems.
The Aguçadora Wave Farm is the world’s first multi-unit wave farm and the world’s first commercial wave energy project. It is located 5km off the Atlantic coastline of northern Portugal. When wave energy systems are placed five to ten kilometres from shore, it is possible to access the high energy levels found in deep swell waves.
The wave farm has three Pelamis 750 kW wave energy converters with a total installed capacity of 2.25 MW. The Pelamis Wave Energy Converter is a semi-submerged, articulated structure made up of cylindrical sections linked by hinged joints. The waves create movement of the joints. This movement is resisted by hydraulic rams pumping high-pressure oil through hydraulic motors. The motors drive electrical generators to produce electricity. Power from all the joints is fed through an ‘umbilical cable’ to a junction on the sea bed. Several devices can be connected and linked to shore through a single seabed cable. The flexible mooring technique keeps the Pelamis system in place while allowing it to swing head-on to the incoming waves.
Ocean Thermal Energy Conversion
The process of Ocean Thermal Energy Conversion (OTEC) uses the heat energy stored in the oceans to generate electricity. It is most efficient when there is a difference in temperatures from the warmer surface to the colder depth of the ocean bed of around 20°C. These conditions exist in tropical regions, mainly between the Tropic of Capricorn and the Tropic of Cancer. Types of OTEC include Closed-Cycle and Open-Cycle systems and Hybrid systems, which combine the two.. The United States became involved in OTEC research in 1974 with the establishment of a leading test facility for OTEC technology at the Natural Energy Laboratory of Hawaii Authority. There are currently more disadvantages than advantages with OTEC, including environmental issues, the relatively small ocean-based locations and the substantial capital investment, which would be required.
Spain, R, ‘A Possible Roman Tide Mill’ in the Kent Archaeological Society
Minchinton, W.E., ‘Early Tide Mills: Some Problems’ in Technology and Culture 20 (4) October 1979
Spain and Minchinton cited in Wikipedia ‘Tidal Power’.
L’Usine maremotrice de la Rance. Nova Scotia Power. Race Rocks Demonstration Project. Sources cited in Wikipedia ‘Tidal Power’.
www.verdantpower.com ‘The RITE Project’
www.verdantpower.com ‘Free Flow Systems’
Wikipedia ‘Tidal Power’.
Wavegen ‘Waves and Whistles’ online information.
Pelamis Wave Power ‘What is Wave Energy?’, www.pelamiswave.com
Wikipedia, ‘Wave Power’.
Wavegen, ‘Waves and Whistles’ online information.
Pelamis P-750 Wave Energy Converter www.pelamiswave.com
Pelamis P-750 Wave Energy Converter www.pelamiswave.com
US Department of Energy, Energy Efficiency and Renewable Energy, Ocean Thermal Energy Conversion at www.eere.energy.gov/consumer/renewable_energy/ocean |
