Offshore Wind Energy
What is offshore wind energy?
Wind energy is an indirect form of solar energy. It is estimated that 1-2% of the solar radiation that reaches the earth is converted to wind energy. In general, wind results from an unequal heating of different parts of the earth, causing cooler, dense air to circulate to replace warmer light air. While some of the sun’s energy is absorbed directly by the air, most of the energy in the wind is first absorbed by the surface of the earth and then transferred to the air by convection.
Wind energy is recognised worldwide as a proven technology to meet increasing electricity demands in a sustainable and clean way. Offshore wind energy has the added attraction that it has minimal environmental effects and, broadly speaking, the best resources are reasonably well located relative to the centres of electricity demand. Moreover, higher wind speeds at sea mean an increased energy production, as energy output is a function of the cube of the wind speed. Average offshore wind energy increase ranges from 10-20%.
It is expected that an important part of the future expansion of wind energy utilisation at least in Europe will come from offshore sites. The first large offshore wind farms are currently in the planning phase in several countries in Europe. However, the economic viability of offshore wind farms depends on the favourable wind conditions compared to sites on land. The higher energy yield has to compensate the additional installation and maintenance cost. For project planning and siting, especially for large projects, a reliable prediction of the wind resource is therefore crucial.
While the global wind-generation market is growing at a 28% annual clip, it relies overwhelmingly on fickle government subsidies.Vestas has 40% of the market, almost three times that of GE Wind Energy, who bought the assets of defunct Enron. The Denmark firm supplies 20% of the electricity used by the country’s households. The penetration of wind energy in the U.S. remains low — less than 1% of American consumption — but wind players think the long-term opportunities are huge. see also Windpower.org
Where is the best wind?
Areas that typically experience high marine current flows are in narrow straits, between islands and around headlands. Entrances to lochs, bays and large harbours often also have high marine current flows (EECA,1996). Generally the resource is largest where the water depth is relatively shallow and a good tidal range exists. In particular, large marine current flows exist where there is a significant phase difference between the tides that flow on either side of large islands.
There are many sites world-wide with velocities of 5 knots (2.5 m/s) and greater. Countries with an exceptionally high resource include the UK (E&PDC, 1993), Ireland, Italy, the Philippines, Japan and parts of the United States. Few studies have been carried out to determine the total global marine current resource, although it is estimated to exceed 450 GW (Blue Energy, 2000).
In the US, the Florida Current and the Gulf Stream are reasonably swift and continuous currents moving close to shore in areas where there is a demand for power. If ocean currents are developed as energy sources, these currents are among the most likely. But most of the wind-driven oceanic currents generally move too slowly and are found too far from where the power is needed. Here is a map of all major known ocean currents.
What is the impact on the environment?
The environmental impact of offshore wind farms is considerably reduced compared with those onshore; both noise and visual impact are unlikely to be issues, but there are still some considerations. For example, there could be an environmental impact from carrying out work offshore, such as localised disturbance of the seabed.
Studies on existing projects have shown that some foundations can act as artificial reefs with a resultant increase in fish populations from the new food supply. It has been suggested that the noise from the turbine travel underwater and disturb sea life. Nonetheless ships, boats and engines have been a fact of life for over a hundred years.
What are the anticipated costs of offshore wind energy?
Current estimates based partly on European experience since 1991, indicate offshore wind energy costs of under 6 cents per kWh. Capital costs are around 30-50% higher than onshore, due to larger machine size and the costs of transporting and installing at sea. This is partially offset by higher energy
yields – as much as 30%. However, as happened onshore, these prices are expected to drop as technology improves and more experience is gained.
Wind resources up to 40 kilometers from shore are currently considered economically feasible according to studies in Denmark, with the key factor being water depth.
What is involved in building offshore wind farms?
Most developments will be installed on either gravity foundations or sited on steel monopiles. Gravity foundations are concrete structures which settle and are stabilised by sand or water and the turbine tower fits into them. Monopiles are long, steel tubes which are hammered, drilled or vibrated into the sea bed until secure and then platforms and towers are installed on top.
Although it would be technically feasible to mount wind turbines on floating structures, studies have shown that it would be very expensive to do this. However, technical developments may make floating offshore wind farms economically feasible in the future.
Why offshore wind energy?
There are several factors which suggest the development of an offshore wind energy industry. The resource is extremely large, the energy costs, although initially higher than for onshore, are cheaper than other renewable technologies and the risks are low, as several demonstration projects elsewhere have shown. Many people, while agreeing that wind turbines are a useful strategy, are not happy to see them in their area. This is the NIMBY principle – not in my back yard. Siting wind turbines at sea will reduce the constraints that can be found on land, such as the visual impact and planning challenges.
- Greater distance will reduce visual impact from land
- Opportunity to apply new technologies
- Similar issues on the potential impact on fish and mollusc stocks, bird life and seabed sediment,
- Navigation and fishing issues may be greater
- Water may be deeper
- Weather and the sea state may be rougher
- Economics may dictate larger turbines with limited proven performance
- Installation will be more difficult and costly
- Connection costs will be greater
- Maintenance will be more difficult and costly
- Wind farms will have to be larger to provide economies of scale to cover these costs
- Investment and risk will be greater
America currently uses some 95 Quad – – one quad is equivalent to 1 quadrillion BTU or a one followed by 15 zeros. Some estimate that we could generate 100 Quad if we deployed anywhere from 3-10 million wind machines, (on Alaskan coastal plain?) depending on the size of the machines used. The electricity produced by these machines would be converted to hydrogen, which in turn can be stored and shipped via pipeline, tanker or cryogenic bulk carrier.
The technology is well developed but off-shore wind is expensive because of construction costs and bringing the power to grid.
Disadvantages: Wind is not predictable so other forms of power must be available to make up any shortfall. Harry Braun, Hydrogen News, proposes: The cost of electricity is a major factor in hydrogen production costs. Although any solar energy option can generate the electricity needed for hydrogen production, the cost of electricity generated from photovoltaic solar cells is approximately 10-times more expensive than the electricity generated from megawatt-scale wind machines. State-of-the-art wind systems, which have an installed capital cost of approximately $1,000 per kW and a 35% capacity factor, are able to generate electricity for approximately 4-cents per kWh. If the wind systems are mass-produced like automobiles for large-scale hydrogen production, their capital costs will be expected to drop to well below $300/kW, which will reduce the cost of electricity to 1 or 2-cents per kilowatt hour (kWh).
There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today’s peak-load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plant more effectively so that the economic value of that wind contribution is greatly increased.
View a video explaining offshore wind energy:Source: www.oceanenergycouncil.com