Marine current power
Marine current power is a form of marine energy obtained from harnessing of the kinetic energy of marine currents, such as the Gulf stream. Although not widely used at present, marine current power has an important potential for future electricity generation. Marine currents are more predictable than wind and solar power.
A 2006 report from United States Department of the Interior estimates that capturing just 1/1,000th of the available energy from the Gulf Stream, which has 21,000 times more energy than Niagara Falls in a flow of water that is 50 times the total flow of all the world’s freshwater rivers, would supply Florida with 35% of its electrical needs.
Strong ocean currents are generated from a combination of temperature, wind, salinity, bathymetry, and the rotation of the earth. The sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with no changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines. Other effects such as regional differences in temperature and salinity and the Coriolis effect due to the rotation of the earth are also major inﬂuences. The kinetic energy of marine currents can be converted in much the same way that a wind turbine extracts energy from the wind, using various types of open-ﬂow rotors. The potential of electric power generation from marine tidal currents is enormous. There are several factors that make electricity generation from marine currents very appealing when compared to other renewables:
- The high load factors resulting from the ﬂuid properties. The predictability of the resource, so that, unlike most of other renewables, the future availability of energy can be known and planned for.
- The potentially large resource that can be exploited with little environmental impact, thereby offering one of the least damaging methods for large-scale electricity generation.
- The feasibility of marine-current power installations to provide also base grid power, especially if two or more separate arrays with offset peak-ﬂow periods are interconnected.
The possible use of marine currents as an energy resource began to draw attention in the mid-1970s after the ﬁrst oil crisis. In 1974 several conceptual designs were presented at the MacArthur Workshop on Energy, and in 1976 the British General Electric Co. undertook a partially government-funded study which concluded that Marine Current Power deserved more detailed research. Soon after, the ITD-Group in UK implemented a research program involving a year performance testing of a 3-m hydroDarrieus rotor deployed at Juba on the Nile.
The 1980s saw a number of small research projects to evaluate Marine Current Power systems. The main countries where studies were carried out were the UK, Canada, and Japan. In 1992–1993 the Tidal Stream Energy Review identiﬁed speciﬁc sites in UK waters with suitable current speed to generate up to 58 TWh/year. It conﬁrmed a total Marine Current Power resource capable theoretically of meeting some 19% of the UK electricity demand.
In 1994–1995 the EU-JOULE CENEX project involved a resource-assessment compilation of a database of European locations in which over 100 sites ranging from 2 to 200 km2of sea-bed area were identiﬁed, many with power densities above 10 MW/km2. Both the UK Government and the EU have committed themselves to internationally negotiated agreements designed to combat global warming. In order to comply with such agreements, an increase in large-scale electricity generation from renewable resources will be required. Marine currents have the potential to supply a substantial share of future EU electricity needs. The study of 106 possible sites for tidal turbines in the EU showed a total potential for power generation of about 50 TWh/year. If this resource is to be successfully utilized, the technology required could form the basis of a major new industry to produce clean power for the 21st century.
Alternative technologies in marine-current-power applications
There are several types of open-ﬂow devices that can be used in marine-current-power applications; many of them are modern descendants of the old concept of the waterwheel or similar. However, the more technically sophisticated designs, derived from wind-power rotors, are the most likely to achieve enough cost-effectiveness and reliability to be practical in a massive marine-current-power future scenario. Even though there is no generally accepted term for these open-ﬂow hydro-turbines, some sources refer to them as water-current turbines. There are two main types of Water Current-Turbines that might be considered: axial-ﬂow horizontal-axis propellers (with both variable-pitch or ﬁxed-pitch), and cross-ﬂow vertical-axis Darrieus rotors. Both rotor types may be combined with any of the three main methods for supporting Water-Current Turbines: ﬂoating moored systems, sea-bed mounted systems, and intermediate systems. Sea-bed-mounted monopile structures constitute the ﬁrst-generation Marine Current Power systems. They have the advantage of using existing (and reliable) engineering know-how, but they are limited to relatively shallow waters (about 20 to 40 m depth).
Ocean currents are instrumental in determining the climate in many regions around the world. While little is known about the effects of removing ocean current energy, the impacts of removing current energy on the farfield environment may be a significant environmental concern. The typical turbine issues with blade strike, entanglement of marine organisms, and acoustic effects still exists; however, these may be magnified due to the presence of more diverse populations of marine organisms using ocean currents for migration purposes. Locations can be further offshore and therefore require longer power cables that could affect the marine environment with electromagnetic output.
- "Ocean currents". How Stuff Works. p. 4. Retrieved 2 November 2010.
- US Department of the Interior (May 2006). "Ocean Current Energy Potential on the U.S. Outer Continental Shelf" (pdf). Retrieved 2 November 2010.
- Ponta, F.L.; P.M. Jacovkis (April 2008). "Marine-current power generation by diffuser-augmented floating hydro-turbines". Renewable Energy 33 (4): 665–673. doi:10.1016/j.renene.2007.04.008. Retrieved 2011-04-12.
- Bahaj, A.S.; L.E. Myers (November 2003). "Fundamentals applicable to the utilisation of marine current turbines for energy production" (Article). Renewable Energy 28 (14): 2205–2211. doi:10.1016/S0960-1481(03)00103-4. Retrieved 2011-04-12.