Well, there are four main strategies Australia could adopt, which involve:

• continuing to use fossil fuels, while developing the use of low emission fossil fuel technologies;
• encouraging the use of low-emission renewable sources of energy such as solar, wind and geothermal power;
• improving the energy efficiency of Australian households and businesses and managing energy demand (Box 1: Reducing emissions by increasing energy efficiency) and;
• placing a price on carbon through an emissions trading scheme (ETS).

The introduction of an ETS such as the one in place in the European Union places a cap on greenhouse gas emissions, with companies that emit significant amounts of greenhouse gases having to buy permits to do so (Box 2: How an emissions trading scheme works).

Reducing emissions from fossil fuels

Around ninety-four per cent of Australia’s energy requirements come from burning fossil fuels such as coal, oil and natural gas (Box 3: Electricity generation).

Related site: Low emission energy future A movie and interactive resource on low emission energy. The movie includes a good introduction to low emission fossil fuel technologies. (Queensland Resources Council, Australia)

Although burning fossil fuels releases greenhouses gases (mainly carbon dioxide or CO₂) Australia is likely to continue relying on them for energy, at least in some capacity, because of their availability and relatively low cost.

Developing low emission fossil fuel technologies (sometimes referred to as 'clean coal' technologies) is seen as one way to lower emissions, while still using our abundant fossil fuel resources. Several methods are being investigated or trialled around Australia, with the technology still developing and not yet competitive with conventional power generation.

Some of the technologies being investigated for coal in Australia include:

Integrated gasification combined cycle (IGCC)

The integrated gasification combined cycle is a two-step process designed to reduce CO₂ emissions from a coal-fired power station.

The first stage involves turning coal into a gas, in what’s called coal gasification. This involves feeding the coal into a heated, pressurised container with steam and oxygen. The resulting synthesis gas, or ‘syngas’ mixture consists of mainly hydrogen (H₂) and carbon monoxide (CO). This gas mixture is reacted with additional water to convert it into a mixture of additional hydrogen and carbon dioxide (CO₂). In a process known as pre-combustion carbon capture, the CO₂ can be removed, compressed to a liquid and stored, usually deep underground.

The second stage of the process involves burning the remaining hydrogen in a gas turbine that turns a generator to produce electricity. The resulting hot exhaust (mainly steam) is then used to make more steam that then turns a steam turbine producing more electricity. The dual use of a gas and steam turbine, gives the process its 'combined cycle' name.

Integrated gasification combined cycle (IGCC) with carbon capture and storage Coal converted to synthesis gas (mostly carbon monoxide and hydrogen) Particulates removed Carbon monoxide and water converted to carbon dioxide and additional hydrogen Carbon dioxide separated Carbon dioxide transported by pipeline for storage (eg, underground) Cleaned gas used  to generate electricity Hot exhaust used to produce steam. Steam used to generate more electricity. (Image: adapted with permission from Taylorville Energy Center)

The basic IGCC process without carbon capture and storage (CCS) is more efficient and produces fewer carbon emissions than conventional coal-fired power stations. With the additional reaction to produce CO₂ and then remove and store it, the efficiency of the process reduces. However, it can reduce CO₂ emissions to the atmosphere by up to 90 per cent. An added advantage of the IGCC/CCS method is that it produces hydrogen, which can be used to provide low emission energy in stationary fuel cells or as a fuel for hydrogen powered vehicles.

IGCC has yet to be proven on a commercial scale in Australia and costs will initially be higher compared to conventional coal-fired power stations. The introduction of a cost on carbon emissions, by an ETS would help to make the technology more competitive.

Underground coal gasification (UCG) is another way of turning coal into a gas, but this time it’s done underground.

A bore hole is drilled down into the coal seam and by feeding oxygen or air down to the coal, it is ignited and burned underground. The resulting syngas is then piped to the surface through another bore hole where it is used to produce electricity using a combined cycle gas power station.

It is claimed that UCG extracts a lot of energy for a relatively small footprint (compared to coal and coal seam gas), but its environmental impact on groundwater and the surrounding environment is being investigated. The process does produce CO₂ that will need to be captured and stored appropriately to qualify as a low emission technology.

The underground coal gasification process burns coal underground to produce syngas. (Image: courtesy of Linc Energy Ltd)

In conventional coal-fired power stations, coal is burned in air. With oxyfuel combustion, instead of using air, the coal is burned in oxygen and recycled flue gas. This  produces a flue gas that is mainly water and CO₂. The water is removed by cooling the flue gas, and the remaining CO₂ is then compressed into a liquid which can be stored eg, underground (see Carbon capture and storage below). This technology is being tested at the Callide oxyfuel project at Biloela in central Queensland.

The advantage of this method is that it can be fitted to existing coal-fired power stations. By producing flue gas that has a high concentration of CO₂, it is easier to capture the CO₂. As with IGCC, oxyfuel combustion is still being developed for commercial scale operations.

Reducing emissions with gas

Both natural gas and coal seam gas are mostly methane (a greenhouse gas, CH₄) and, strangely enough, can play a key role in generating electricity with fewer greenhouse gas emissions. A power station fired by gas produces about half the greenhouse gas emissions of a normal coal-fired power station, because methane contains less carbon per unit of energy than coal. Australia is well placed to use gas technology having large gas reserves eg, in the Carnarvon Basin in Western Australia (natural gas) and the Surat and Bowen basins (coal seam methane) in Queensland.

Low-emission gas technologies being investigated in Australia include:

Coal seam methane gas (CSG) is found associated with underground coal deposits. Traditionally, coal mines have vented the gas into the atmosphere because it poses a health and safety risk during mining. But methane is a powerful greenhouse gas in the atmosphere.

The methane in some underground coal deposits is now being collected and used to generate electricity by burning it in gas turbines. There is particular potential for coal seam gas as a source of energy in New South Wales and Queensland with nearly six per cent of Queensland's energy coming from this source. The Dawson mine in Queensland, for example, is one of the world’s largest producers of coal mine methane power.

To extract coal seam gas, water is pumped up from the coal seam to lower the pressure and encourage the gas to flow to the surface. The environmental impact of the large volumes of water with varying salinity levels that are produced, as well as the effect of operations on groundwater, need to be considered when designing CSG plants.

As with IGCC, combined cycle gas-fired power stations burn gas in a turbine to produce electricity, then use the waste heat to generate steam that turns a steam turbine to generate more electricity. These power stations, such as Pelican Point power station just outside Adelaide, operate more efficiently than older coal-fired stations, and with fewer greenhouse gas emissions.

Carbon capture and storage

The Australian Government and industry are supporting the development of commercial-sized carbon capture and storage (CCS) projects, where CO₂ emissions from burning fossil fuels are captured and stored deep underground. The CO₂ can be collected at either the pre-combustion stage (as in IGCC) or the post-combustion stage.

Processes for carbon dioxide capture (©CO2CRC 2008) (Click on image for a larger version)

The post-combustion capture process involves burning fossil fuels in a power station and then collecting the CO₂ from the flue gases. The CO₂ can then be liquefied, transported and stored underground (geosequestration). Australia has established a demonstration geosequestration plant near Warrnambool, in south-west Victoria. The viability of CCS technology on a commercial scale still remains to be proven.

Renewable energy sources

Renewable energy sources are those that don’t run out or are quickly replenished such as solar, wind, geothermal and ocean energy.

Greenhouse gas emissions and cost of producing electricity using selected technologies. (Sources: National Academy of Sciences 2009; CSIRO 2006. Life-cycle emissions, approximate only, varies widely depending on technology and site).

Although we don’t yet make the most of it, Australia receives more sunshine that almost any other country. However, under the government’s $1.6 billion Solar Flagships Program, four large solar power stations (with a total capacity of an average coal-fired power station) are due to be constructed by 2015 to demonstrate the feasibility of solar technology.

Solar energy can be generated from solar thermal energy or by photovoltaic cells. Solar thermal systems use the sun’s energy to heat a liquid like water or oil. These can be small rooftop systems or, on a larger scale, collections of mirrors that focus the Sun’s rays to heat water to steam, to generate electricity. Photovoltaic solar energy involves flat panels that turn sunlight directly into electricity.

The main disadvantages of solar energy are cost and variability of supply. Obviously, the Sun doesn't shine at night and the radiation we receive varies throughout the day and with cloud cover. The development of technology to store the heat from solar thermal systems, then use it to generate electricity at night, may make the supply of energy from these systems less variable.

Wind power

Wind power, where wind drives giant turbines to generate electricity with minimal greenhouse gas emissions, is a rapidly growing form of energy production in Australia. Most wind power sites are currently in South Australia, Victoria and Western Australia.

Although one of the more promising renewable technologies, issues with wind power include aesthetics of the turbines, bird strikes, noise and variability of supply with wind conditions.

Geothermal energy

Geothermal hot rock energy is created from the hot granite rocks several kilometres below the Earth’s surface. Water circulated through the fractured rocks, is heated and then pumped back to the surface to generate electricity with few emissions.

Although Australia has enormous geothermal resources and a growing interest in using them, the remote location of most of the resources can be an issue and the technology is still developing. There is currently only one small, operating geothermal power station in Australia, at Birdsville in Queensland.

Bioenergy

Bioenergy is a renewable form of low-emission energy made from biomass (organic or vegetable matter). Probably the best known form is bagasse, which is crushed sugar cane waste. Bagasse has been used as a fuel to generate electricity for sugar mills in Queensland for more than 100 years. Biomass can also come from other crops and waste materials. Some refuse stations capture methane gas produced from decomposing rubbish; the gas can then be burned to produce energy. Competition from biomass crops for farming land and water resources has prompted research into alternative sources of biomass, such as algae.

Ocean power

Another form of energy being developed is ocean power, which can either use rising and falling tides, or the surging power of waves, to turn turbines to make electricity.

Despite having a long coastline and some of the best wave energy in the world, ocean power is a largely untapped energy source in Australia, with significant technical challenges. The location, high cost and amount of energy generated has attracted limited investment to date. There is one wave-powered pilot plant operated by Oceanlinx at Port Kembla in NSW, while other ocean-powered generators are being developed in Western Australia, Victoria and Queensland.

Hydroelectricity

Hydroelectricity schemes, which use flowing water to turn turbines to generate electricity, are an established technology currently providing 17 per cent of Australia’s renewable energy supplies. There are large hydroelectric schemes operating in Tasmania, the Snowy Mountains in New South Wales, north-east Victoria, Queensland and the Ord River in Western Australia.

Although hydroelectricity is a relatively inexpensive, low-emission form of energy, schemes across Australia have been affected by drought conditions, and there can be concerns regarding the damming of rivers to supply sufficient quantities of water. Because of Australia’s relatively flat landscape and uncertain water supply there is little scope for the development of additional hydroelectric generation.

Hydrogen

Researchers in Australia and many other countries are looking to hydrogen to replace fossil fuels for powering turbines, fuel cells and vehicle engines.

Hydrogen is the most abundant element on Earth but is mostly combined with other elements, such as oxygen, in the form of water. It is an energy carrier, rather than a primary energy source such as oil. So it has to be produced from other sources. Most hydrogen is currently produced from fossil fuels, principally natural gas. Hydrogen can also be produced from renewable energy sources but these aren't cost-competitive. Most of the hydrogen produced in Australia is used in the petrochemical and fertilizer industries, not for power generation.

Uranium

Putting it all together

With a growing demand for energy, there is no simple way of achieving Australia's goal of reducing carbon pollution by up to 25 per cent of 2000 levels by 2020. Like other countries, we are likely to continue using coal for energy, to some degree, into the future. The development of low emission fossil fuel technologies, increasing the use of gas and alternative energy sources, and adopting more energy efficiency measures, are all expected to play an important role in reducing Australia’s emissions from energy.

Box 1 | Reducing emissions by increasing energy efficiency

Saving energy begins at home …

There’s an old saying that ‘charity begins at home’. But maybe that should be modernised to include ‘… and so does saving energy’.

Becoming more energy efficient around your home needn’t be expensive – in fact, it can save you money on your electricity and gas bills. It can be as easy as turning off the light when you leave a room, or closing doors and windows if the heater or air-conditioner is on.

Other simple things you can do to save energy include:

• Turning the heating down (and maybe wearing warmer clothes!) in winter. And on hot days, only setting your air conditioner to about 24 degrees with curtains, windows and doors closed to keep the heat out.
• Turning off unused appliances and computer equipment at the power point (so the electric clock and standby mode don’t use power).
• Washing your clothes using a cold water cycle. Heating water makes about a quarter of household emissions.
• Drying your clothes outside (or on an inside line in winter), rather than using a tumble dryer.

Other things you can do include:

• Installing insulation and draft proofing in your house.
• Installing more efficient lights, such as compact fluorescent lamps (CFLs) – which use about a quarter of the energy of old-fashioned incandescent bulbs.
• Buying appliances, such as fridges, with higher energy ‘star’ ratings.
• Using solar energy eg, photovoltaic panels to generate electricity or installing a gas-boosted solar hot water heater.

… and can continue at work

Saving energy might start at home, but businesses are making big changes to save energy and reduce CO₂ emissions.

You can see some of their efforts just by walking down the street. There’s a new generation of ‘green’ office buildings – such as Sydney’s 30 The Bond and Melbourne’s Council House 2 – that incorporate better design, more sustainable construction methods and achieve ongoing energy savings.

And it’s not just new office buildings that are becoming more energy efficient. Businesses including the mining, agricultural and industrial sectors are also looking at ways of saving energy with initiatives such as:

• Undertaking energy audits to see where energy is being used – and what reductions can be made.
• Installing more efficient lighting systems and turning off equipment overnight and on weekends.
• Using more efficient equipment, such as the haul trucks and excavators now in use at Rio Tinto coal mines, which can each save up to 50 litres of fuel an hour.

Related sites

• LivingGreener.gov.au (Australian Government)
• Saving energy
• Save energy

• Queensland Resources Council
  • Energy efficiency in homes
  • Energy efficiency in industry

• The CSIRO home energy saving handbook (CSIRO, Australia)

Box 2 | How an emissions trading scheme works

A ‘cap and trade’ emissions trading scheme, like the one operating in the European Union, places a limit (cap) on the amount of greenhouse gases emitted emitted by a country or region each year, with that limit usually lowered in subsequent years.

Under an emissions trading scheme, businesses – such as coal-fired power stations, transport firms, manufacturing and mining companies – must have a permit for every tonne of greenhouse gas produced over certain levels as part of their operations. But only enough permits are made available to equal the emissions cap, ensuring that total emissions are kept below the target level.

Permits can be traded between companies. For some companies it is cheaper to develop low emission technologies than to buy permits for any extra emissions. Others may buy permits to cover their extra emissions from businesses with surplus permits.

By making emissions cost money, the aim is to encourage businesses to develop and switch to cleaner, more energy efficient technologies.

In addition to trading actual emissions permits, emissions trading schemes can also allow for reductions in emissions to be traded. Reductions can be through implementation of cleaner technologies in developing countries or through the removal of carbon eg, by planting trees. A company can buy these carbon credits to cover some of their own emissions.

Related sites

• The EU Emissions Trading System (European Commission)

• Understanding the Carbon Pollution Reduction Scheme (Queensland Resources Council)

• Carbon currency – the credits and debits of carbon emissions trading (Nova: Science in the news, Australian Academy of Science)

• Carbon Pollution Reduction Scheme (Australian Government Department of Climate Change)

• Emissions - who is trading what? (Parliament of Australia)

Box 3 | Electricity generation

Most electricity today is generated using heat. In a coal-fired power station, for example, coal is burnt to heat water, which turns to steam, which is used to spin turbines.

The shaft of a turbine is attached to a device for producing electricity called a generator; it consists of large magnets attached to the shaft inside a coil of copper wire. The rotating turbine turns the generator shaft and its attached magnets; as they rotate the magnets cause a flow of electrons – an electrical current – in the wire coil by a process known as electromagnetic induction. Electrical current can also be produced if, instead of moving the magnets in the coil, the coil is rotated inside large magnets. The coil is connected at both ends to an electrical circuit; the electrical current flows from the generator coil through the electricity grid to light and heat our houses, work our appliances and run factories.

Power plants that use uranium, natural gas or oil operate in a similar way to coal-fired power stations. These systems all rely on the use of steam or another gas to turn the blades of a turbine. (Wind and hydroelectricity also work by turning turbines.)

Related sites

• Coal fired power station (Queensland Resources Council, Australia)

• How electricity is made (CS Energy, Australia)

• So how does an electricity generator work? (ActewAGL, Australia)

• How generators work (Wisconsin Valley Improvement Company, USA)

Activities

• Queensland Resources Council (Australia)
• Low emission energy sources – An introduction – a 14 lesson unit of work on low emission sources of energy. Students learn about fossil fuels, renewable energy and low emission energy technology through class discussion, video and internet research. They then prepare group presentations on a low emission source of energy.
• Renewable sources of energy: Investigation – students complete a worksheet to evaluate major sources of alternative energy available in Australia and internationally using internet resources.
• Future energy options: Business, industry and individuals – students complete a worksheet to evaluate energy efficiency options using internet resources.
• Low emission technologies – a PowerPoint presentation for teaching students about carbon capture and storage.
• Energy – a PowerPoint presentation for teaching students about the renewable and non-renewable sources of energy
• Understanding the Carbon Pollution Reduction Scheme – a PowerPoint presentation for teaching students about the Carbon Pollution Reduction Scheme.
• Coal fired energy plant – students arrange the steps in producing electricity from a power plant.

• Power for a sustainable future: Activity sheets (Queensland Department of Education, Australia)
• Activity sheets – links to a large number of activities on sustainable energy including: Predicting the impact of renewable energies, Being more energy-efficient in our homes, Decreasing use of non-renewable energy and Obtaining and using renewable and non-renewable energy.

• The Helix Teacher’s guide (CSIRO, Australia)
• The future of energy in Australia – what would you choose? (No. 121, August 2008, pages 2-5) – students research, design and market an energy solution for two fictitious Australian communities.

• Origin Energy (Australia)
• Energy in your home – students calculate energy use and costs in their home using the online calculator. They then explore ways to improve energy efficiency through an interactive ‘house’.

• Visy Enviromaniacs, (Australia)
  • Energise the Earth – students recognise where energy is lost from the home in this simple interactive game.

Activity 1. Low emission energy technologies: a worksheet

1. Copy the graphic organiser below onto a piece of A3 paper and use it to summarise information about low emission energy technologies. Divide your information into fossil fuels, renewables and ‘other’ low emission energy technologies.

<pdf version

Although renewable sources of energy are playing an increasing role in Australia, in the short-term, fossil fuels will most likely continue as a source of energy due to their availability, reliability as an energy source and relatively low cost. But at what cost to the environment?

Developing low emission fossil fuel technologies is seen as a way to reduce carbon emissions while continuing to use Australia’s abundant fossil fuel resources.

2. Evaluate the use of low emission fossil fuel technologies to reduce carbon emissions in Australia by completing the PMI table below. What are the pluses, minuses and interesting aspects of these technologies? (Information on low emission fossil fuel technologies can be found in Australia’s low emission energy future, Clean technologies for fossil fuels and Carbon capture and storage.)

PMI Table: Low emission fossil fuel technologies in Australia

Further reading

Useful sites

Note: Selected resources on specific low emission technologies are available in the feature article.

Coal and low emission technology (Oresome Resources, Queensland Resources Council, Australia)

Provides links to a range of resources on low emission energy technologies including an interactive presentation (with fact sheets) and the downloadable movie Low emission energy future. Low emission technologies is a PowerPoint presentation covering the need for low emission energy and the technology behind carbon capture and storage.
http://www.oresomeresources.com/resources_listing/category/coal_low_emission_technology/section/resources

Technology topics: Energy (Nova: Science in the news, Australian Academy of Science)

Provides links to Nova topics on energy, including a number on alternative sources of energy.
http://www.science.org.au/nova/tech-energy.htm

Australian Government Department of Climate Change

Includes climate change information, as well as government initiatives and things the community can do to address climate change.
http://www.climatechange.gov.au/

Fact sheets (Resource Centre, Clean Energy Council, Australia)

Provides a series of fact sheets on renewable sources of energy.
http://cleanenergycouncil.org.au/cec/resourcecentre/factsheets.html

Australian Bureau of Agricultural and Resource Economics (Australian Government)

• Technology: Toward a low emissions future
  A detailed report which includes a chapter on energy efficient and low emission technologies.
  http://www.abareconomics.com/publications_html/climate/climate_07/technology.pdf

• Energy in Australia 2009
  Provides detailed information and statistics on all aspects of energy production and use in Australia.
  http://www.abareconomics.com/interactive/09_auEnergy

Fact Sheets (Energy Supply Association of Australia)

Provides a series of fact sheets on energy in Australia, including The energy industry, Emissions trading and Electricity generation technologies.
http://www.esaa.com.au/fact-sheets.html

The heat is on (CSIRO, Australia)

An extensive report covering the current status of energy in Australia and future scenarios for the energy industry.
http://earthhour.ice4.interactiveinvestor.com.au/CSIRO0702/The Heat Is On Report/EN/body.aspx?z=1&p=-1&v=1&uid=

Department of the Environment, Water, Heritage and the Arts (Australian Government)

• Renewable energy
  Links to government programs and resources for renewable energy.
  http://www.environment.gov.au/settlements/renewable/

• Map of operating renewable energy generators in Australia
  Provides an interactive map of renewable energy generators in Australia. Either current or proposed power generators can be displayed.
  http://www.agso.gov.au/renewable/

• Energy efficiency
  Links to government programs and resources for energy efficiency.
  http://www.environment.gov.au/settlements/energyefficiency/index.html

A clean energy future for Australia (Energy Strategies for the Clean Energy Future Group, Australia)

A detailed report on Australia’s energy future. Includes chapters on energy efficiency, fossil fuel technologies and renewable energy.
http://www.enerstrat.com.au/index.php/publications/a_clean_energy_future_for_australia/

Sustainable energy – without the hot air (United Kingdom)

An on-line book which presents balanced information on energy consumption in Britain and evaluates alternative sources of energy and energy efficiency measures.
http://www.withouthotair.com

Coal gas (Catalyst, 22 October 2009, Australian Broadcasting Corporation)

Provides a 12 minute video and transcript of a program on extraction of coal gas for energy via underground coal gasification or coal seam gas.
http://www.abc.net.au/catalyst/stories/2721216.htm

Glossary

biomass. A general term for material from living things such as plants, animals, fungi, bacteria and food waste. Taken together, the Earth's biomass represents an enormous store of energy. Since biomass can be regrown, it is a potentially renewable resource. For more information see Biomass – the growing energy resource (Nova: Science in the news).

combined cycle. Power plants using that use a combined cycle burn gas to generate electricity, then use the waste heat produced to generate more electricity via a steam turbine. For more information see Energy sources: gas (Ergon Energy).

electromagnetic induction.The production of electrical current in a conductor due to a changing magnetic field (moving magnet) or to the movement of a conductor through a magnetic field. For more information see Faraday's magnetic field induction experiment (Florida State University, USA).

fuel cell. A cell that converts energy from chemical reactions directly into electrical energy. A simple fuel cell 'burns' hydrogen in a flameless chemical reaction with oxygen to produce electricity. For more information see Fuelling the 21st century (Nova: Science in the news).

geosequestration. The capture and long-term underground storage of carbon dioxide. Carbon dioxide produced by coal-fired electricity stations and other industrial sources is compressed to form a liquid and injected into deep underground geological formations. Possible storage sites include saline aquifers, coal seams, and used oil and gas reservoirs. For more information see About geosequestration? (Cooperative Research Centre for Greenhouse Gas Technologies).

Kyoto protocol. An international agreement associated with the UN Framework Convention on Climate Change that commits participating countries to reduce their greenhouse gas emissions relative to 1990 levels over the period 2008-2012. The Kyoto Protocol was adopted in Kyoto, Japan, in 1997.

life-cycle emissions. Emissions of greenhouse gases throughout the full life-cycle of the generating system. Emissions are not only from the generation of electricity, but also from the production of the energy and materials needed to build, maintain and operate the plant, as well as from disposal of wastes and decommissioning.

low emission fossil fuel technologies. Technologies aimed at reducing the environmental impact of burning fossil fuels for power generation. These technologies have recently focused on reducing carbon dioxide emissions. However, they can also include measures to reduce other pollutants, such as sulfur dioxide and particulate matter, as well as more efficient technologies. For more information see Clean technologies for fossil fuels (Nova: Science in the news).

photovoltaic cells. Also known as solar cells. A photovoltaic cell is made of thin wafers of two slightly different types of silicon. One, containing 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 has 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.

renewable energy. Used to describe energy sources that are replenished fast enough that they can be used by humans more or less indefinitely, provided the rate of use is not too great. Examples include biomass, wood, wind, falling water and sunlight. Compare non-renewable.

solar thermal. Describes an energy system that harnesses solar energy to produce heat energy. Small flat plate systems are often used to heat water for household or commercial use. Larger high-temperature systems use reflective mirrors or dishes to focus the energy. The heat may then be used to convert a liquid into a vapour, which turns a turbine to produce electricity.