Generating new ideas for meeting future energy needs

Key text

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.

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.

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

Hydrogen

You don't have to be a rocket scientist to see the potential benefits of hydrogen as a fuel. But, actually, it helps – today's rockets and space-shuttles are all powered by hydrogen. Many experts are predicting that this, the most simple and most common of all the elements, will revolutionise the energy sector.

Hydrogen is not so much an energy source as an energy carrier. It exists on Earth in its free form (H ₂ ) in only minute quantities, so we need to manufacture it from materials such as water and hydrocarbons. This requires energy – and you don't get out more than you put in. So what's so great about it?

Hydrogen's usefulness lies in its ability to store energy at high densities and to produce it on demand – much like petrol and natural gas do today. It can be used to generate electricity at times when primary energy sources (like wind, wave or solar) are producing insufficient power to meet demand. Conversely, such energy sources can be used to produce hydrogen when they are generating more electricity than is required by the grid. It can be used in much the same way as petrol, providing fuel for cars and aeroplanes. And it can provide power for fuel cells that can be used much like batteries and recharged at will.

Significant quantities of hydrogen are produced and consumed each year, mostly by the chemical and petroleum industries (eg, in the production of methanol from natural gas and in the manufacture of ammonia), but hardly any is used as fuel. The most common way of producing it at the moment is from natural gas (mostly methane, CH₄ ) using a process called steam reforming.

It can also be produced by splitting water (H₂ O) into its constituent parts – hydrogen and oxygen. A common way of doing this is by the process of electrolysis. If the electricity is generated by a renewable energy source, this process causes almost no pollution.

But there are some other rather interesting ways to produce hydrogen. For example, it can be derived from biomass by processes called gasification and pyrolysis.

Another intriguing technique involves the use of certain strains of algae and bacteria, which can produce hydrogen from water as a by-product of photosynthesis, using solar energy. The current efficiency of this process is quite low but, with the aid of genetic engineering, scientists hope to achieve significant advances in the next decade or so.

A novel way of producing hydrogen using water and solar energy is called photoelectrochemical (PEC) technology. This combines a photovoltaic cell, which produces electricity when exposed to sunlight, and an electrolyser to convert water directly to hydrogen and oxygen. Again, the technology is not yet perfected. One of the problems is to find a photovoltaic cell that isn't corroded by the electrolytes in solution but is still cheap enough to be competitive with alternative techniques and fuels.

The widespread adoption of hydrogen as a transport and industrial fuel would clean up our cities in dramatic fashion. The extraction of energy from hydrogen is simply a reversal of the electrolysis process, so the only chemical output would be water. In fact, the waste product from the hydrogen fuel cells used on board space flights also serves as drinking water for the crew. If you can imagine a traffic jam in which the only fumes are a little water vapour then you might start to appreciate what a hydrogen fuel economy could mean for our quality of life.

Change will take time

In Australia, about 9 per cent of Australia's electricity is generated using renewable resources (mostly hydroelectricity), and the federal government wants this to increase to 11 per cent by 2010.

Effecting a change will take time – time for alternative fuel technologies to develop so that they are competitively priced and capable of providing substantial amounts of energy. We must also develop the necessary infrastructure, so that when consumers buy a hydrogen-powered car they can refuel it in Cloncurry, Oodnadatta or downtown Sydney, just as they can now with a petrol-powered car.

Related Nova topics:

• Harnessing direct solar energy – a progress report

• Wind power gathers speed

• Biomass – the growing energy resource

• Fuelling the 21st century

• Which way ahead for hydrogen cars?

Box 1. Converting wave energy into electricity

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)

• Oceanlinx (Australia)

Activities

• Power for a sustainable future (Queensland Department of Education, Australia)
  • How different communities use natural resources to make energy
  • Predicting the impact of renewable energies
  • Evaluating the environmental impact of a renewable energy project
  • The future of transport

• NSW Country Areas Program (Australia)
  • Alternate energy – a web quest in which students research alternative energy sources with a focus on their local area.

• The Helix teacher’s guide (CSIRO, Australia)
  • The future of energy in Australia – what would you choose? – students research, design and market an energy solution for two fictitious Australian communities.

• Origin Energy (Australia)
  • Energy efficiency calculator – students calculate energy usage and costs in their home.

• Energy Quest (California Energy Commission, USA)
  • Splitting water into hydrogen and oxygen
  • Experiment with water to produce energy – shows how to build a small water turbine and a water wheel.

• Science upd8 (UK)
  • Hot rocks – students look at the relationship between geology and geothermal energy, and how geothermal power stations might affect local communities.

Further reading

About the House
June 2009, pages 36-39
Go with the flow (by Simon Grose)
Looks at wave energy trials around Australia.

RTD Info
February 2006, pages 30-33
The wild card of distributed production
Looks at the role of renewable energy sources in the move away from centralised production of power in Europe.

March 2009, pages 50-55
The power of renewables (by Matthew Wald)
Clearly describes and evaluates a range of renewable energy options with descriptive diagrams for each technology.

Useful sites

Three fact sheets:

• Geothermal energy
  http://www.epa.qld.gov.au/register/p00395aa.pdf

• Ocean energy
  http://www.epa.qld.gov.au/register/p00398aa.pdf

• Hydro
  http://www.epa.qld.gov.au/register/p00397aa.pdf

Geothermal energy project (Geoscience Australia)

Describes a project to locate, describe and provide information on Australia’s geothermal resources. The outreach and education section includes a series of clear fact sheets on geothermal energy use in Australia.
http://www.ga.gov.au/minerals/research/national/geothermal/index.jsp#

Map of operating renewable energy generators in Australia (Australian Government Department of the Environment, Water, Heritage and the Arts)

Provides maps of proposed and operational renewable energy generators across Australia.
http://www.agso.gov.au/renewable/

Australian Broadcasting Corporation

• Wave power (In Depth, 23 February 2009)
  Explains the potential of wave power in Australia.
  http://www.abc.net.au/science/articles/2009/02/23/2498704.htm

• UK microgen (Catalyst, 22 May 2008)
  Discusses the use decentralised power in London to help reduce carbon emissions.
  http://www.abc.net.au/catalyst/stories/2244790.htm

• Solar hydrogen power (Innovations, 14 May 2007)
  Looks at the generation of hydrogen fuel from sunlight and sea water.
  http://www.abc.net.au/ra/innovations/stories/s1913712.htm

• Hot rock energy (Earthbeat, 19 June 2004)
  Describes current research into geothermal power in Australia.
  http://www.abc.net.au/rn/science/earth/stories/s1135215.htm

Hot dry rock geothermal energy (Geodynamics Limited, Australia)

Uses diagrams to explain how hot dry rock geothermal energy is produced, and compares it with conventional geothermal energy. More information is available by clicking on ‘HDR explained’.
http://www.geodynamics.com.au/IRM/content/02_hotdryrock/02.html

Mother Earth – a source of sustainable energy? (Australian Academy of Technological Sciences and Engineering)

Dr Neil Williams, Executive Director of the Geoscience Australia, discusses geothermal energy.
http://www.atse.org.au/index.php?sectionid=542

Alternative energy latest news (Alternative Energy, USA)

Provides the latest information on various forms of energy.
http://www.alternative-energy-news.info/

The hydrogen economy (Physics Today, USA)

Describes the basic research that is required in materials and design to make hydrogen-based energy a viable proposition.
http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_57/iss_12/39_1.shtml

Hydrogen Energy Center (USA)

Presents the advantages hydrogen offers as an alternative fuel.
http://www.hydrogenenergycenter.org/

Glossary

electrolyte. A substance that produces ions (particles with an electric charge) when dissolved in water. The resulting solution (which can also be referred to as an electrolyte) conducts electricity.

fuel cell. A device that converts energy from chemical reactions directly into electrical energy. The simplest fuel cell 'burns' hydrogen in a flameless chemical reaction to produce electricity. In order to 'burn' the hydrogen a fuel cell needs a source of oxygen and this is usually obtained from air. The only by-product from this type of fuel cell is water.

For more information about fuel cells see Fuelling the 21st century (Nova: Science in the news, Australian Academy of Science).

gasification. A process that exposes a solid fuel to heat in the presence of limited oxygen to produce a gaseous fuel. This fuel contains hydrogen but also other gases such as carbon monoxide, carbon dioxide, nitrogen, and methane. Under suitable circumstances, gasification can produce synthesis gas, a mixture of just hydrogen and carbon monoxide.

genetic engineering. A set of procedures whereby a specific piece of DNA can be excised from a chromosome and inserted into the DNA of a chromosome of a different organism.

greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia).

hydrocarbon. Compound containing only the two elements, carbon and hydrogen.

kilowatt, megawatt, gigawatt. The basic unit of power (the rate at which energy is used) in the metric system is the watt (W); a kilowatt is 1000 watts. A watt is a very small amount of power and in most mechanical applications we count power in kilowatts. A kilowatt is about equal to the heat energy put out by a single bar radiator, and is also about equal to the power expended by a person running up stairs. A car engine typically produces 50 to 100 kilowatts.

photosynthesis. The biochemical process in which green plants (and some microorganisms) use energy from light to synthesise carbohydrates from carbon dioxide and water. Photosynthesis can be shown as:

CO₂ + H₂O + energy® [CH₂O] + O₂

photovoltaic (PV) cells. Also known as solar cells. A photovoltaic cell is made of thin wafers of two slightly different types of silicon. One, doped with tiny quantities of boron, is called P-type (P for positive) and contains positively charged 'holes', which are missing electrons. (Electrons are negatively charged particles that orbit the nuclei of atoms.) The other type of silicon is doped with small amounts of phosphorus and is called N-type (N for negative). It contains extra electrons. Putting these two thin P and N materials together produces a junction which, when exposed to light, will produce a movement of electrons – and that constitutes an electric current. Photovoltaic cells thus convert light energy into electrical energy.

plate tectonics. The theory that the Earth's surface is made up of huge plates that have moved very slowly during geological history, and continue to move, thus changing the position of continent and oceans. The plates are about 100 kilometres thick and move at a rate of about 1-12 centimetres per year.

potential energy and kinetic energy. Potential energy is stored or supressed energy. For example, the wound-up spring of a toy has potential energy. Kinetic energy is the energy associated with a moving object (energy of motion). In the example of the toy, potential energy is converted into kinetic energy when the toy is set running.

For more information see Energy basics (Nova: Science in the news, Australian Academy of Science).

pyrolysis. A process which involves heating biomass to drive off the volatile matter, leaving behind the black residue we know as charcoal. More sophisticated pyrolysis techniques have been developed recently to collect volatiles – gaseous compounds – that are otherwise lost to the system. The collected volatiles produce a gas rich in hydrogen and carbon monoxide.

turbine. A device in which a stream of water or gas turns a bladed wheel, converting the kinetic energy of the fluid flow into mechanical energy available from the turbine shaft. The earliest turbines were water wheels. Now, steam turbines are driven by jets of high-temperature steam; gas turbines are driven by burning fuel vapour; and wind turbines use the power of moving air.