Converting wave energy into electricity

Capturing wave energy and converting it into electricity is not an easy task, but researchers have developed technology to overcome the problems. Three of the pioneering devices are described here.

TAPCHAN

TAPCHAN is the name of a prototype generator that was installed on a remote Norwegian island in 1985 and has been functioning ever since. The name is an abbreviation of ‘tapered channel’, which describes the basic idea behind the device. TAPCHAN consists of a reservoir built into a cliff a few metres above sea level. Leading into it is a tapered channel – wide at the mouth, which is open to the sea, and becoming narrower as it penetrates the reservoir. Incoming waves increase in height as they move up the channel, eventually overflowing the lip of the channel and pouring into the reservoir. In this way, TAPCHAN converts the kinetic energy of the wave into potential energy, which is subsequently converted into electrical energy by a generator as the water is fed back to the sea through a pipe.

Oscillating water column

Another kind of wave energy converter is known as the oscillating water column (OWC). Like TAPCHAN, this is a fixed device – which means that the housing of the device does not move – located either onshore or fixed to the seabed. It consists of a wedge-shaped chamber that is open to the sea at the bottom. A wave surging into this chamber forces air upwards, which drives a turbine both on its way up (as the wave surges) and on its way down (as the wave recedes). These oscillations give the device its generic name. To take best advantage of this two-way flow, a special kind of turbine (such as the British-designed Wells turbine) is needed.

An Australian scientist claims to have produced an innovative OWC design that greatly improves its performance. Dr Tom Denniss, from Energetech Australia, uses a parabolic wall (shaped like a satellite dish) to focus the energy of an incoming wave. The rushing air is used to drive a special turbine he claims is four to five times as efficient as the Wells turbine. A 200-300 kilowatt prototype is under development and will probably be installed at Wollongong or Newcastle, in New South Wales.

The duck

The ‘duck’ is an example of a floating wave energy converter. It is not fixed to the shore or seabed, relying instead on the ‘nodding’ motion of floats to drive a generator. In fixed devices, the turbine is fixed while the water or air rushes past its blades. Floating devices generate their power by the relative motion of components as they bob up and down in the sea. The duck consists of rows of floats, each generating electricity that is fed ashore by a connecting cable.

One of the advantages of floating devices over fixed devices it that they can be deployed in deeper water, where wave energy is greater (since waves lose energy with decreasing water depth). There is no need for significant earthworks, either, as there is with onshore devices.

Related site

• The Port Kembla project (Energetech Pty Ltd, Australia)

Wave power

As any surfer knows, there’s plenty of energy in a wave. Waves are a form of solar energy – the uneven heating of the Earth by the sun causes air to move. This wind, in turn, transfers some of its energy to the surface layers of water bodies, particularly the ocean, thereby generating waves.

Putting this energy to use has proved a titanic task for scientists. For example, sea water is highly corrosive, so making generators that are sensitive to small undulations in the sea yet strong enough to withstand the inevitable storms has been a major undertaking. But scientists are now confident that many of these difficulties are close to being solved. They have developed an array of potential machines, although few have been tested commercially (Box 1: Converting wave energy into electricity).

The advocates of wave power foresee few environmental side-effects from a large-scale adoption of the technology. There is little potential for pollution – either chemical, visual or noise – and no greenhouse gas emissions. Floating devices are not expected to have any significant impact on the coastal environment, but they could present a hazard to shipping.

Australia has a huge coastline and significant wave energy resources – particularly along the southern coast of the mainland and the west coast of Tasmania. But the potential for wave power to provide a significant amount of our energy needs remains untested

Hot dry rocks – a form of geothermal energy

‘Geothermal’ means heat stored in rock. The best evidence of geothermal activity can be seen in regions close to the boundaries of tectonic plates – such as Japan and New Zealand – where hot springs, volcanoes and geysers are plentiful. These resources are already being used in some countries for heating and electricity generation.

The words ‘Australia’ and ‘geothermal’ are not often closely associated. Australia doesn’t have any active volcanoes and relatively few hot springs or geysers. Yet, according to some Australian scientists, we have some of the best reserves of hot dry rocks in the world, offering prospects for a plentiful supply of energy.

Australia’s hot dry rock resources are found in granite rock layers buried up to several kilometres underground, beneath layers of sedimentary rock. They are hot – up to 300ºC – because of what is known as the radiogenic decay of minerals, in which trace elements in the granite slowly break down, releasing heat as they do.

Australian hot dry rock resources are unusually well suited to extraction because of a combination of three factors:

• Heat is being generated in the crust at more than twice the global average.

• The ‘blankets’ of sedimentary rock above the granite provide excellent insulation but are also of an optimal thickness for heat extraction.

• The hot dry rocks are oriented horizontally, providing good (and relatively cheap) drilling access.

The process of extracting the heat is quite simple. Water is pumped down into the hot granite through a bore-hole that may be several kilometres deep. This helps to open up existing tiny cracks in the granite, increasing the permeability of the rock. The water is converted to steam by the heat and is channelled to the surface through another bore-hole, where it can be used to drive a turbine and thereby generate electricity.

Energy from hot dry rocks is not strictly renewable because the granite mass will eventually cool down. Nevertheless, it produces no greenhouse gases or other pollutants and has a very small ‘footprint’ on the landscape (unlike coal mining, hot dry rock energy requires no large-scale excavations). Some scientists say that Australia has enough hot dry rock resources – particularly in the Hunter Valley near Newcastle and the Eromanga Basin near the South Australia/Queensland border – to provide all our energy needs for centuries. A pilot project in the Hunter Valley is now underway.

Generating new ideas for meeting future energy needs

This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.

Concerns about the greenhouse effect, smog and energy security have led to increasing interest in energy sources such as hot dry rocks, wave power and hydrogen.

The world has changed dramatically over the last 200 years, thanks largely to fossil fuels – coal, oil and natural gas. These have provided us with cheap and convenient energy, which we use to heat and cool our homes and to run our cars, appliances and industries.

But there has been a cost. No city in the world is immune from the polluting effects of fossil fuels, and they contribute vast quantities of greenhouse gases to the atmosphere, something that many scientists believe causes global warming.

So, in the last few decades, scientists have been looking for ways to produce energy without adverse side-effects. Promising renewable energy sources such as wind, direct solar and biomass are dealt with in other Nova topics (see links at the end of this page). Now we'll have a look at hot dry rocks, waves and hydrogen. It may be some years before these energy sources make a big impact but they illustrate the diversity of options that are available.

Renewable Energy

Solar energy technologies, paired with energy conservation, have the potential to meet a large portion of future US energy needs...

Renewable Energy for America's Future

Energy for America's Future

President George W. Bush delivers remarks to the Washington International Renewable Energy Conference 2008 Wednesday, March 5, 2008, at the Washington Convention Center in Washington, D.C. White House photo by Chris Greenberg

President Bush Attends Washington International Renewable Energy Conference 2008

"...[L]et me start first by telling you that America has got to change its habits. We've got to get off oil. And the reason why is, first, oil is -- dependency on oil presents a real challenge to our economy. As economies grow -- and we want all our economies to grow; we want people to be prosperous, we want people who are living in poverty to be able to grow out of poverty. We want there to be general prosperity, but as economies grow, until we change our habits, there is going to be more dependency on oil."

--President George W. Bush, March 5, 2008

Wind Energy Profile: The Big Promise

Theoretically, wind could produce enough energy to meet global demand. In 2006, however, less than one percent of global electricity consumption came from wind. Why such an imbalance?

Worldwide Importance and Future Prospects

Global installed wind capacity in 2006 was around 74 Gigawatts (GW), according to the World Wind Energy Association. This was more than one percent of global electricity consumption, but because installed capacity does not reflect actual production, its contributions to the global energy mix are less than that.

Wind energy capacity is expected to more than double between early 2007 and 2010. Growth will be driven by rapidly developing countries, such as India, Brazil, and China. Several offshore wind parks are being planned in northern Europe and North America. Improving efficiency and falling costs of turbine production and installation will make wind power more price competitive.

Global Resources and Producers

Global land and near-shore wind resources are around 72,000 GW, or five times the world's current energy use, according to a study at Stanford University. But sites convenient for wind power production are limited by factors such as land use for agriculture or living, distance to consumers, and technology. Experts from the Intergovernmental Panel on Climate Change estimate that only four to ten percent of given resources could be used in an economically viable way.

An entirely wind-powered economy is thus not yet possible. Global growth in wind power, however, is still tremendous. In 2005, markets grew by 41 percent. The value of new generating equipment installed in 2006 was about 18 billion euros.

Energy Output

The amount of wind energy generated depends mostly on the size, height, type, and location of a wind turbine. Some small turbines, such as those fixed on a sailboat, can generate as little as a few hundred watts - enough to power a few light bulbs. On the other side of the spectrum are the large, utility-scale turbines like the Vestas V90 that produces 3 MW. According to the manufacturer, these turbines produce in 2-3 hours the electricity that an average European family consumes in one year. The Enercon E126 turbines installed in Germany in late 2007 will produce 6MW each, making it the most powerful turbine on the market.

Most wind power turbines are still installed on land, but the future could lie offshore. Wind speeds over oceans are on average twice as high as over land, making offshore wind parks an interesting alternative, but technically more challenging alternative.