|Five Megawatt Offshore Wind
Turbines Are Now Being Installed
Editor’s note: Terawatts of energy bombard earth daily via the sun’s rays, but competitively converting renewable solar energy into usable energy, electricity in particular, remains a formidable hurdle. When evaluating totally renewable sources of energy for their economic viability, the world’s oceans beckon as an alternative quietly emerging, especially in Europe, as a replacement to fossil fuels that could take hold before solar solutions. Ocean winds blow harder and with more reliable consistency than wind on land, which more than offsets the greater cost of building windmills offshore. While winds are in many respects indirectly derived from solar energy, the world’s oceans also contribute massive amounts of renewable energy that is gravitationally derived through the interplay of the earth and the moon. The energy from ocean waves and tidal streams, along with ocean-based wind energy, make the world’s oceans a source of renewable energy that may in the next few decades greatly outstrip solar energy as the economical alternative of choice.
Options for exploiting the energy available from the world’s oceans include offshore wind, wave and tidal stream energy. Offshore wind is by far the closest to commercial exploitation, but the range of possibilities is surprisingly broad.
Offshore wind is set for rapid development and could become fully commercial in 15-20 years. However, both wave and tidal stream energy face one of the so-called ‘valleys of death’ in the development of successful innovation – that between the prototype and wide utilisation. Some, perhaps many, of the current companies and designs will fall by the wayside. Nevertheless, wave and tidal hold considerable promise for the longer term.
Extracting clean and economically viable energy from the world’s oceans has fascinated researchers and engineers for centuries. The first patent on a wave energy device was taken out in 1799, and more than 300 such devices have been patented since. Commercial application has been limited to a small number of devices that use wave energy to power navigation buoys. However, concerns over climate change may fuel progress. All of the marine renewables offer energy with low environmental impact and near-zero emissions.
While estimates of resources available depend upon assumptions about technology and the availability of suitable sites, all options are able, in principle, to provide large amounts of electricity. Global resources have not been mapped in detail, but studies of EU and UK resources provide an indication of the scale of the potential. Offshore wind could provide 900 terawatt-hours (TWh) per year in Western Europe, and wave 50-700 TWh/year in UK waters alone. Tidal resources appear more modest: 48 TWh has been identified in EU waters, but at present only limited sites have been considered. (UK electricity consumption is around 380 TWh/year.)
Here is an assessment of the economic prospects for deriving energy from ocean wind, waves, and tides.:
OCEAN WIND ENERGY
|Maritime Denmark is a World Leader in
Low-Cost, Large-Scale Wind Energy
Offshore wind has benefited from progress made onshore over the last 20 years. Costs have fallen dramatically – in good locations it is almost competitive with conventional energy sources. Turbines have grown from less than 100 kilowatts (kW) to 1 megawatt (MW) or more. Offshore wind farms under development feature turbines of 2 MW, and 3-5 MW turbines are widely predicted to become available soon. Large turbines are essential if offshore wind is to deliver energy at acceptable cost, and this is one reason why the stage is set for developments offshore.
Offshore wind involves additional costs in installation, cabling and maintenance, with existing plants generating at 4.5-7 US cents per kWh – twice the cost of developments on land. However, wind regimes are typically more stable offshore and the absence of noise constraints means turbines can spin faster, which raises efficiency and reduces costs. Current developments in turbine design, experience in installation and operation, and economies of scale as the industry expands, suggest costs will fall. Costs halved between Denmark’s Vindby development in 1991 and its recent Horns Rev project. A recent study for the UK government concluded that offshore wind costs are likely to fall to 3-5 US cents per kWh by 2020. Although less than 100 MW of offshore wind is currently installed worldwide, this is expected to grow to 3 gigawatts (GW) by 2005 because individual offshore wind farms can be large. Several developments of 100-500 MW are being built.
OCEAN WAVE ENERGY
Power generation using wave energy is at a much earlier stage of
development. Wave energy offers more predictable outputs than wind, but in early 2003
there was only around one megawatt of generating capacity installed worldwide, all of it essentially with demonstration prototypes. Proposed projects are likely to take this to about 6 MW over the next few years. The wave industry is characterised by a wide variety of novel devices
and a large number of small firms. Devices can be classified by generic technology type, though there is some overlap:
Types of Wave Energy Devices:
|Inexhaustible Electricity from Wave Energy
Can Already Cost Under $.06 per Kilowatt Hour
* Pneumatic devices, such as the oscillating water column (OWC), use wave motion to compress and decompress air, and drive a turbine.
* Float-based devices utilise a buoyant float moving with the waves, reacting against a sea bed anchor in order to harness energy.
* Spillover devices utilise wave height to replenish a reservoir of seawater, which runs a turbine.
* Raft-type devices use the relative motion of adjacent rafts or pontoons to harness wave energy.
* Moving-body devices articulate in the water, inducing motion, which may be used to drive a hydraulic motor.
Commercial-scale wave energy is yet to become a reality and as such empirical evidence on costs is limited. Of those devices that have been deployed (for the most part near-shore and shoreline OWC devices), costs are in the region of 6-8 US cents per kWh. Three designs – the Limpet, Osprey and Pelamis – have secured support from the Irish and Scottish renewables schemes – though supplementary investment has also been required (for example, EU grants). The other devices are still at the research stage, though some are much closer to commercial deployment than others. (Float based devices are already in use for niche applications such as navigation buoys.) The Osprey is designed to provide a mounting platform for wind turbines and hence offers the prospect of the first hybrid wind-wave device. Hybrids have the potential to improve the utilisation of sub-sea power connections and to raise the ratio of output to construction cost.
OCEAN TIDE ENERGY
|A Seabed-Mounted Tidal Energy
System is now being tested off
the U.K.’s Shetland Islands
Tidal stream devices extract energy from the diurnal flow of tidal currents (caused by the gravitational pull of the moon). Unlike wind and wave power, tidal streams offer entirely predictable output. However, as the lunar cycle is of around 25 hours’ duration, the timing of peak outputs differs by around an hour each day and tidal energy cannot be guaranteed at times of peak demand.
Typically, tidal turbines, similar in appearance to wind turbines, are mounted on the seabed. They are designed to exploit the higher energy density, but lower velocity, of tidal flows compared to wind. Tidal stream differs from established technology for exploiting tidal energy (eg estuarine tidal barrages, such as the 240 MW barrage operating in France) in that tidal flows are not captured and controlled by means of a large dam-like structure. Rather, tidal turbines operate in the free flow of the tides, meaning that large construction costs and disruption of estuarine ecosystems associated with barrages may be avoided. However, as tidal streams are a diffuse form of energy and the purpose of the barrage is to concentrate tidal flow, this also means that large numbers of turbines, spread over relatively large areas of seabed, are required if significant amounts of energy are to be extracted.
Until recently, the diffuse nature of the resource, combined with the relatively high costs of engineering and installing turbines able to withstand the rigours of the sea, confined tidal stream to university laboratories. However, several large grid-connected demonstration projects are expected to enter the water in the near future. Tidal stream is thus a few years behind wave energy.
|Seabed-Mounted Tidal Energy
Lowering A Tidal Propeller
Marine Current Turbines
Marine Current Turbines is about to field test a submerged 300 kW tidal turbine off Devon in the United Kingdom, and a seabed-mounted system called Stingray is being tested off the Shetlands. Both have EU and UK government funding. A novel device called the Rochester Venturi, which uses tidal flow to draw a working fluid through turbines mounted onshore and hence has no moving parts under water, is also expected to enter large-scale demonstration soon. The manufacturers of all these devices expect to deliver
energy at a cost of 10-14 US cents per kWh, falling to below 6 US cents as
experience grows and technologies mature
About the Author:
Gordon Feller is the CEO of Urban Age Institute (www.UrbanAge.org). During the past twenty years he has authored more than 500 magazine articles, journal articles or newspaper articles on the profound changes underway in politics, economics, and ecology – with a special emphasis on sustainable development. Gordon is the editor of Urban Age Magazine, a unique quarterly which serves as a global resource and which was founded in 1990. He can be reached at GordonFeller@UrbanAge.org and he is available for speaking to your organization about the issues raised in this and his other numerous articles published in EcoWorld.