Renewable energy resources

Introduction
Solar energy
Geothermal energy
Wind energy
Wave energy
Tidal energy
Hydroelectric energy
Energy from biomass
Renewable energy resources in the UK

 

Introduction

Some sources of energy are known as renewable energy resources. This is because, unlike non-renewable energy resources, they will not run out.

Renewable energy resources include solar energy, geothermal energy, energy from the wind or waves, energy from tides and energy from biomass.

Solar energy

What is solar energy?

Every year the Earth receives about 300,000,000,000,000,000,000,000 kJ of energy. This energy drives processes in the atmosphere that cause the wind and waves.

Some energy is absorbed by green plants and used to make food by photosynthesis. So ultimately, the Sun is the source of most energy resources available to us, including fossil fuels.

Scientists also try to use the energy of the Sun directly. This we call solar energy. 'Solar' means 'sun'.

How is solar energy collected?

Solar energy can be used to heat a fluid such as water in solar collector panels. Simple types use flat collector panels mounted on a south-facing roof or wall, each with transparent cover to admit sunlight.

Water circulates through channels or pipes inside each panel. The inside is usually painted black, because black surfaces readily absorb heat. The water is heated, then the hot water is pumped to a heat exchanger that extracts the heat for use within the house.

Solar energy can also be used to generate electricity in photovoltaic (PV) cells. A PV cell may power your calculator. Photovoltaic cells are made of semiconductors, similar to those used to make computer chips.

Until recently these cells were very costly to produce. However, they are still only about 10-15 per cent efficient.

Where can solar energy be collected?

The Sun's energy can be collected anywhere, but more can be gathered in areas on or near the Equator.

In the UK the average amount of solar energy available is about 4,000 kJ per square metre. If less than a quarter of the population had solar panels this would save about 30 per cent of our annual energy demand.

Advantages
Solar energy is renewable, non-polluting and relatively maintenance free.

Disadvantages
There is less available solar energy in areas near the poles of the Earth. Cloud cover can reduce efficiency. PV cells are still quite expensive.

 

Geothermal energy

What is geothermal energy?

Geothermal energy is the natural heat of the Earth's crust. The temperature at the Earth's core is over 70,000°C. The rocks not too far below the surface are also quite hot, perhaps 500°C about 1 km down.

In some areas there are 'hotspots' where the temperature below the surface is higher. This is usually near where the earth's tectonic plates meet. The existence of hot springs, geysers and volcanoes point to evidence of hot rocks below the surface.

How is geothermal energy collected?

In some places there are reservoirs of hot water below the surface that can be tapped to provide energy. The water is pumped to the surface and used to heat buildings.

Where can geothermal energy be collected?

There are not many places that can currently exploit geothermal energy cost effectively. In Tuscany, Italy, a geothermal plant has been operating since the early 1900s. There are also geothermal power stations in the USA, New Zealand and Iceland.

In Southampton (UK) there is a district heating scheme based on geothermal energy. Hot water is pumped up from about 1,800 metres below ground. The water is about 700°C and is used to heat a number of nearby offices and civic buildings.

Advantages
Geothermal energy will not run out. It is renewable. The running costs can be very low. It can be non-polluting (as in Southampton).

Disadvantages
It can only be used in some areas around the world, where the crust is thin and hot rocks are near the surface. Sometimes the hot water that is pumped to the surface contains pollutants such as sulphur.

Huge turbines in a wind farm can generate enough electricity to supply a small town.

Wind energy

What is wind energy?

When the Earth is irradiated by the Sun the ground absorbs some of this radiation. This heated ground warms the air above it. Hot air rises in what are called convection currents.The uneven heating of the earth's surface causes winds.

For example, if the Sun's rays fall on land and sea, the land heats up more quickly. This results in the air above the land moving upwards more quickly than that over the sea (hot air rises).

As a result the colder air over the sea will rush in to fill the gap left by the rising air. It is processes like these that give rise to high and low pressure areas, and thus to winds.

 

How is wind energy collected?

Renewable energy from the wind has been used for centuries to power windmills to mill wheat or pump water. Nowadays, large wind turbines are used to generate electricity.

The blades of these wind turbines are about 30 metres long. Wind turbines are collected together in wind farms.

Where is wind energy collected?

This energy can be harnessed in areas subject to reasonably consistent and strong winds. There may be large areas of flat land or those near coasts that are subject to prevailing winds.

There are wind farms around the world. Because the UK is on the edge of the Atlantic Ocean it has one of the best wind resources in Europe. Offshore wind farms in coastal waters are being developed because winds blowing across the sea are often stronger than those inland.

A 20-turbine wind farm can generate enough electricity (about 1MW) for a small town. Turbines can produce between 500kW and 1MW of electricity.

Advantages
This source of energy is non-polluting and is freely available in many areas. Wind turbines are becoming more efficient. The cost of the electricity they generate is falling.

Disadvantages
To be efficient wind turbines need to be linked together in wind farms, often with about 20 turbines. This looks unsightly, and can be noisy.

The wind farms also need to be sited reasonably close to populations so that the electricity generated can be distributed. Another disadvantage is that winds are intermittent and do not blow all the time.

 

Wave energy

What is wave energy?

Waves are caused by the action of winds on the sea. Waves can be many metres in height and contain a great deal of energy. This energy can be harnessed to drive turbines that generate electricity.

How is wave energy collected?

Wave energy collectors are of two main types. The first type directs waves into man-made channels, where the water passes through a turbine that generates electricity. The second type uses the up and down movement of a wave to push air.

Where is wave energy collected?

Wave energy can be harnessed in coastal areas, close to the shore. There has been one such device working on the island of Islay in Scotland since the early 1990s, producing 75kW of electricity.

Advantages
This is a non-polluting source of energy. Wave turbines are relatively quiet to operate and do not affect wildlife.

Disadvantages
The turbines can be unsightly. Wave heights vary considerably, so they would not produce a constant supply of energy.

 

Tidal energy

What is tidal energy?

Tides are caused by the gravitational pull of the Moon, and to a lesser extent the Sun, on the oceans around the world. The difference between high tide and low tide can be many metres, and the energy of the tidal movement can be used to generate electricity.

How is tidal energy collected?

If, at high tide, water can be trapped behind a barrage and then let out as the tide ebbs, this water can be passed through a turbine that can generate electricity.

Where can tidal energy be used?

Barrages are built in river estuaries that have large tidal ranges, such as the River Severn in the UK.

Advantages
The rise and fall of the tide is constant, and does not depend on the weather. The production of electricity in this way is relatively cheap.

Disadvantages
Present designs do not produce a lot of electricity, and barrages across river estuaries can change the flow of water and, consequently, the habitat for birds and other wildlife.

 

Hydroelectric energy

What is hydroelectric energy?

Flowing rivers have kinetic energy. This energy can be collected and used. Hydroelectric power is generated by the falling of water through a turbine.

How is hydroelectric energy collected?

If a dam is built across the river, water can be allowed to flow in a controlled way through turbines that generate electricity.

A dam across a river can provide a cheap, constant source of hydroelectric power for large communities.

 

Where is hydroelectric energy used?

Hydroelectric power schemes exist in many countries. They can be built in areas where there are fast-flowing rivers. These are often hilly or mountainous regions where rivers flow down steep slopes.

On flatter land rivers flow more slowly, so very large artificial dams have to be built to create reservoirs. The reservoir then provides a 'head' of water that can be directed through a turbine.

Most people live on flatter land, so most hydroelectric schemes require large dams and flood a lot of land.

Advantages
The river flows continually and provides a constant source of energy. Once built, the supply of electricity is relatively cheap.

Disadvantages
A good site for a hydroelectric scheme, such as a mountainous region, is not always near towns. The building of large dams floods large areas and causes damage to existing habitats.

Changing the flow of a river will affect the water supply to lands nearer the sea. This may cause problems of irrigation for crops.

 

Energy from biomass

What is biomass?

Biomass is material from living things. This could be plant material, animal material or even bacteria. Organic matter can be burned to provide heat, or fermented to produce gas.

How is biomass energy collected?

Plant material such as wood or hay can be burned to provide heat to raise steam and so generate electricity in a power station.

Animal waste (e.g. animal slurry from a farm) can be treated to provide gases that can be burned to generate electricity. Landfill sites emit gases (mainly methane) that can also be used to provide energy. Some plant materials such as sugar cane and maize (sweetcorn) can be fermented to produce alcohol. Alcohol can be used in cars as a substitute for petrol.

Crops can be grown as energy crops rather than food crops. Oilseed rape (the fields of yellow flowers you see in the UK in summer) produces oil. About 32 per cent of the seed is oil. After treatment with chemicals it can be used as a fuel in diesel engines, called RME (rape methyl ester).

Where is biomass energy used?

About 200 years ago biomass in the form of wood was the major source of energy. In many parts of the developing world biomass (not always from trees) is still the major source of energy.

In Brazil, large numbers of cars run on alcohol rather than petrol. In the Western world, people are developing ways of using biomass as an alternative to fossil fuels. There is a large biomass plant in Sweden, while in the UK attempts are being made to develop a power station that will run solely on wood from a nearby farm.

Advantages
Biomass is a renewable resource – for example, trees can be replanted or coppiced. Energy can be extracted from wastes. Biomass energy can be used in similar ways to fossil fuels. Sources of biomass are readily available worldwide.

Disadvantages
Biomass energy sources are no more environmentally friendly than fossil fuels because they recycle carbon into the atmosphere when they are burned. Carbon dioxide, which is produced when these fuels are burned, is a major cause of the greenhouse effect.

The following is an extract from a paper by the UK government's Department of the Environment, Transport and the Regions, entitled Energy in Brief, published in December 1999. The full publication can be obtained at: www.environment.detr.gov.uk/des20/chapter0/2.htm#chapter2

Renewable energy resources in the UK

Renewable energy resources	Equivalent amount in oil (thousands of tonnes)
	1990	1996	1997	1998
Active solar heating	6.4	8.6	9.0	9.5
Onshore wind	0.8	41.8	57.2	76.1
Hydroelectric	447.7	289.0	354.8	449.3
Landfill gas	79.8	247.2	316.0	400.8
Sewage sludge digestion	138.2	190.2	191.5	184.1
Wood	174.1	709.7	710.3	710.3
Straw	71.7	71.7	71.7	71.7
Municipal solid waste	160.0	368.7	427.0	567.7
Other biofuels	80.6	175.7	182.1	185.5
Total	1,159.3	2,102.6	2,319.6	2,655.0

Renewable energy resources used in UK between 1990 and 1998, and the equivalent amount of oil.

 

How renewable energy sources are used

Biofuels account for approximately 80 per cent of renewable energy sources, with most of the remainder coming from large-scale hydroelectricity production. Hydropower accounts for 17 per cent and wind power contributes 3 per cent.

Of the 2.66 million tonnes of oil equivalent of primary energy use accounted for by renewables in 1998, 1.73 million tonnes were used to generate electricity and 0.93 million tonnes to generate heat.

Renewable energy use grew by over 4 per cent in 1998 and has more than doubled since 1990. Renewable electricity accounted for 2.5 per cent of UK electricity supplies in 1998, compared to 2 per cent in 1997.