Renewable energy sources are energy sources that are continually replenished. These include energy from water, wind, the sun, geothermal sources, and biomass sources such as energy crops. In contrast, fuels such as coal, oil, and natural gas are non-renewable. Once a deposit of these fuels is depleted it cannot be replenished – a replacement deposit must be found instead. Both renewable and non-renewable energy sources are used to generate electricity, power vehicles, and provide heating, cooling, and light.

Renewable sources of energy vary widely in their cost-effectiveness and in their availability across the United States. Although water, wind, and other renewables may appear free, their cost comes in collecting, harnessing, and transporting the energy so that it can do useful work. For example, to utilize energy from water, a dam must be built along with electric generators and transmission lines.

Renewables themselves are non-polluting, while the structures built to harness them can have positive or negative environmental impacts. For example, dams may affect fish migration but may also create wildlife habitat.

    back to top   Hydropower  

An impoundment hydropower plant dams water in a reservoir.Hydropower refers to using water to generate electricity. Water is the most common renewable source of energy in the United States today.

Many hydroelectric power plants use a dam on a river to store water. Water released from behind the dam flows through a turbine, spinning it, which then turns a generator to produce electricity. Electricity generated this way is known as hydroelectricity, and it accounts for about 7% of the electricity used by the nation. Hydroelectric power doesn't necessarily require a large dam – some hydroelectric power plants just use a small canal to channel the river water through a turbine. A small or micro-hydroelectric power system can produce enough electricity for a home, farm, or ranch.

 

The Tazimina project in Alaska is an example of a diversion hydropower plant. No dam was required.Dam sites for hydropower plants are limited both by available rivers and by competing uses for those rivers, such as recreation, tourism, industry, and human settlements. Because of such limitations, water power could never generate all the electricity used in the United States. In addition, environmental impacts are considered when locating dams.

While all hydroelectric dams have some environmental impact, the impacts vary widely, and current regulations and policies attempt to address environmental concerns. A dam may either create a reservoir or may be a run-of-river project that does not store large amounts of water but

simply takes advantage of a river's natural flow. A dam that Fish ladder.creates a reservoir may flood a large area upstream, and can change flow patterns and impact flooding downstream with resulting environmental consequences, either positive or negative. Fish migration, which has long been a concern associated with dams, is often addressed with fish ladders and other structures to ensure the successful movement of fish both upstream and downstream.

In addition to power, dams often provide other benefits such as recreation opportunities on upstream reservoirs, habitat for a wide variety of aquatic and terrestrial species, diversion of water for irrigation, and control of destructive flooding and environmental damage downstream.

Hydropower is one of the least expensive sources of electricity and areas with good sources of hydropower tend to attract industries with large needs for electricity. Major hydroelectric dams in the United States are found in the Northwest, the Tennessee Valley, and on the Colorado

River.  

Click image for larger view.

Existing hydroelectric plants (yellow) and potential high head/low power energy sites (orange) in the conterminous United States. Purple represents areas excluded from hydropower development due to Federal statutes and policies. Source: Water Energy Resources of the United States with Emphasis on Low Head/Low Power Resources (p. 47), U.S. Department of Energy   back to top   Wind Power  

Turbines at Martinsdale Hutterite Colony.For hundreds of years, humans have used wind to pump water or grind grain, usually with small windmills. Large, modern wind turbines are used to generate electricity, either for individual use or for contribution to a utility power grid. Wind turbines usually have two or three blades and, because winds above the ground tend to be faster and less turbulent than those near the surface, the turbines are mounted on tall towers to capture the most energy. As the blades turn, the central shaft spins a generator to make electricity.

In recent years, wind has become an increasingly attractive source of renewable energy – wind energy is the world's fastest-growing energy technology. Wind turbines placed at sites with strong, steady winds can economically generate electricity without producing pollutants. The power in wind increases rapidly with its speed, which means that locating windmills in areas of strong winds is critical. The strongest winds in the United States tend to be in Alaska, the western United States, and the Appalachians. Wind power currently supplies about 1% of United States electricity needs, but capacity is expanding rapidly. Although wind will contribute more to the United States electric supply in the future, like hydropower it cannot be expected to supply all of our electric needs.

Click image for larger view.

United States wind resource map.While wind power helps the environment by producing electricity without producing pollution, there can be negative environmental impacts of wind power generation, including wildlife deaths. However, recent studies suggest that the number of birds and bats killed by collision with wind turbines is far lower than the number killed by collisions with other tall structures such as buildings. Appropriate siting of wind farms and individual turbines can reduce the impact on wildlife. Noise, which was a problem with older turbine designs, has mostly been eliminated through improved engineering.

  back to top   Solar Power  

Annual average daily solar radiation per month, using a flat-plate collector facing south at a fixed tilt equal to the latitude of the site. Capturing the maximum amount of solar radiation throughout the year can be achieved using a tilt angle approximately equal to the site's latitude.Solar technologies use the sun's energy to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. Despite sunlight's significant potential for supplying energy, solar power provides less than 1% of U.S. energy needs. This percentage is expected to increase with the development of new and more efficient solar technologies.

Different types of solar collectors are used to meet different energy needs. Passive solar building designs capture the sun's heat to provide space heating and light. Photovoltaic cells convert sunlight directly to electricity. Concentrating solar power systems focus sunlight with mirrors to create a high-intensity heat source, which then produces steam or mechanical power to run a generator that creates electricity. Flat-plate collectors absorb the sun's heat directly into water or other fluids to provide hot water or space heating. And solar process heating and cooling systems

use specialized solar collectors and chemical processes to meet large-scale hot water and heating and cooling needs.

Solar technologies produce few negative environmental impacts during collector operation. However, there are environmental concerns associated with the production of collectors and storage devices. In addition, cost is a great drawback to solar power. Although sunlight is free, solar cells and the equipment needed to convert their direct-current output to alternating current for use in a house is expensive. Electricity generated by solar cells is still more than twice as expensive as electricity from fossil fuels. Part of the problem with cost is that solar cells can

The parabolic troughs that make up this concentrating solar power system generate power from the sun on a large scale in California.only operate during daylight hours. In contrast, a coal or natural gas plant can run around the clock, which means the cost for building the plant can be spread over many more hours of use.

Around the United States, available sunlight varies considerably as a result of differences in cloud cover and latitude, and also varies with the seasons. In the summer, longer daylight hours and a higher sun angle provide more solar power, compared to the winter when the sun is up for fewer hours and at a lower position in the sky. These variations must be taken into consideration when planning solar collection facilities.

  back to top   Geothermal Power  

Geothermal power plant at The Geysers, California.Geothermal power uses the natural sources of heat inside the Earth to produce heat or electricity. Currently, most geothermal power is generated using steam or hot water from underground. Geothermal power generation produces few emissions and the power source is continuously available.

There are three geothermal technologies currently in use in the United States: direct-use systems, use of deep reservoirs to generate electricity, and geothermal heat pumps.

In direct-use geothermal systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system—piping, a heat exchanger, and controls—delivers the heat directly for its intended use. A disposal system

then either injects the cooled water underground or disposes of it in a surface storage pond. Geothermal hot water is used for heating buildings, raising plants in greenhouses, drying crops, heating water for fish farms, or for industrial processes, at hundreds of sites around the country. Geothermal reservoirs appropriate for direct-use systems are widespread throughout the western United States.

Geothermal power plants convert hydrothermal fluids (hot water or steam) to electricity. The oldest type of geothermal power plant uses steam, accessed through deep wells, to directly drive a turbine to produce electricity. Flash steam plants are the most common type of geothermal power plants in operation today. They use extremely hot water (above 300 degrees F (149 degrees C)), which is pumped under high pressure to the generation equipment at the surface.

The hot Estimated subterranean temperatures at a depth of 6 kilometers.water is vaporized and the vapor in turn drives turbines to generate electricity. Binary-cycle geothermal power plants use moderate-temperature water (100-300 degrees F (38-149 degrees C)). The water is used to vaporize a second fluid that has a much lower boiling point than water. The vapor from this second fluid is then used to drive the turbines to produce electricity. California, Hawaii, Nevada, and Utah currently have operating geothermal power plants.

Geothermal heat pumps are used for space heating and cooling as well as water heating, for residential and commercial applications. The technology relies on the fact that beneath the surface, the Earth remains at a relatively constant temperature throughout the year, warmer than the air above it during the winter and cooler in the summer. A geothermal heat pump takes advantage of this by transferring heat, stored in the ground, into a building during the winter, and transferring it out of the building and back into the ground during the summer. The heat pump consists of a series of pipes, buried in the ground near a building to be conditioned or where water is to be heated. Fluid is circulated through the pipes to either absorb heat from the ground or distribute heat to the ground. Geothermal heat pumps can be used in most areas of the United States.

While geothermal energy use is efficient, reliable, and environmentally friendly, it currently meets less than 1% of U.S. power needs.

  back to top   Biomass Power  

McNeil Generating Station, Burlington, Vermont, the country's only utility-owned and operated wood-fired power plant.Biomass power is power obtained from the energy in plants and plant-derived materials, such as food crops, grassy and woody plants, residues from agriculture or forestry, and the organic component of municipal and industrial wastes. Biomass power provides two valuable services: it is the second most important source of renewable energy in the United States and it is an important part of our waste management infrastructure. In the future, farms cultivating high-yielding energy crops (such as trees and grasses) will significantly expand our supply of biomass. These energy crops, coupled with high-efficiency conversion technologies, can supplement our consumption of fossil fuels and help us respond to global climate change concerns.

Wood has been used for energy longer than any other biomass source and today is still the largest biomass energy resource. The largest source of energy from wood is pulping liquor or "black liquor," a waste product from processes of the pulp, paper, and paperboard industry. Biomass energy can also be derived from waste and from alcohol fuels. Waste energy is the second-largest source of biomass energy. The main contributors of waste energy are municipal solid waste, manufacturing waste, and landfill gas.

Biomass can be used for direct heating (such as burning wood in a fireplace or wood stove), for generating electricity, or can be converted directly into liquid fuels to meet transportation energy needs.

Truck unloading wood chips that will fuel the Tracy Biomas Plant, Tracy, California.Electricity generated from biomass is also called biopower. Biopower facilities use many different technologies; the most common is burning of wood or other biomass feedstocks to produce steam which then is used to drive turbines and produce electricity. Some generators use a mix of biomass and fossil fuels to generate electricity, while others burn methane, a product of the natural decay of organic materials. In the United States, the pulp and paper industries are major

producers of biopower, using residues from paper production to produce electricity for industrial plant use.

Biomass power is close to a carbon-neutral electric power generation option — biomass absorbs carbon dioxide from the atmosphere during its growth and then emits an equal amount of carbon dioxide when it is processed to generate electricity. Thus, biomass fuels "recycle" atmospheric carbon, and may reduce global warming impacts. Biopower facilities produce fewer other pollutants than equivalent fossil fuel power facilities.

Biofuels are liquid fuels produced from plants. The two most common types of biofuels are ethanol and biodiesel. Ethanol is an alcohol, the same as in beer and wine. It is made by fermenting any biomass high in carbohydrates through a process similar to beer brewing. The majority of ethanol produced in the United States is made from corn. Current research is exploring ways to efficiently convert cellulose (agricultural waste, forest residue, municipal solid waste, and energy crops) to ethanol. Ethanol is mostly used as a fuel additive for vehicles to increase octane and cut down carbon monoxide and other smog-causing emissions. Biodiesel is made by processing vegetable oil, animal fat, or recycled cooking grease with alcohol or other chemicals. It can be used as an additive (typically 20%) to reduce vehicle emissions or in its pure form as a renewable alternative fuel for diesel engines.

 

Biomass and biofuels resource potential in the conterminous United States.  

Because biomass power is produced from plant sources, it can potentially be produced almost anywhere in the United States.

While biomass is a renewable energy resource, it can have both negative and positive environmental impacts. It may reduce emissions and pollutants, but factory farming of biomass crops can reduce biodiversity and negatively impact wildlife habitat. Municipal solid waste may contain toxins which could cause pollution if it is used as a biomass feedstock. As with other

renewable resources, use of appropriate technology will promote the most positive environmental impacts.

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.

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

DisadvantagesThere 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.

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

DisadvantagesIt 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.

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.

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

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.

AdvantagesThis 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.

DisadvantagesTo 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.

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

DisadvantagesThe 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.

AdvantagesThe 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.

DisadvantagesPresent 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.

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

DisadvantagesA 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.

AdvantagesBiomass 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.

DisadvantagesBiomass 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.