Renewable energy

Encyclopedia : R : RE : REN : Renewable energy

Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, wave power, flowing water (hydropower), biological processes such as anaerobic digestion, and geothermal heat flow. The most common definition is that renewable energy is from an energy resource that is replaced by a natural process at a rate that is equal to or faster than the rate at which that resource is being consumed. Renewable energy is a subset of sustainable energy.

Most renewable forms of energy, other than geothermal and tidal power, ultimately derive from solar energy. Energy from biomass derives from plant material produced by photosynthesis using the power of the sun. Wind energy derives from winds, which are generated by the sun's uneven heating of the atmosphere. Hydropower depends on rain which again depends on sunlight's power to evaporate water.

Even fossil fuels, derive from solar energy, as fossil fuel originates from plant material. However, while theoretically renewable on a very long time-scale, fossil fuels are exploited at rates that may deplete these resources in the near future, and are therefore not considered renewable.

Renewable energy resources may be used directly, or used to create other more convenient forms of energy. Examples of direct use are solar ovens, geothermal heating, and water- and windmills. Examples of indirect use which require energy harvesting are electricity generation through wind turbines or photovoltaic cells (PV cells), or production of fuels such as biogas from anaerobic digestion or ethanol from biomass (see alcohol as a fuel).

Renewable energy development is concerned with the use of renewable energy sources by humans. For aspects of renewable energy use in modern societies see Renewable energy development. Modern interest in renewable energy development is linked to concerns about exhaustion of fossil fuels and environmental, social and political risks of extensive use of fossil fuels and nuclear energy. For a general discussion, see future energy development.

Contents

Modern sources of renewable energy

Wind energy

As the sun heats up the Earth unevenly, winds are formed. The kinetic energy in the wind can be used to run wind turbines, some capable of producing 5 MW of power. The power output is a function of the cube of the wind speed, so such turbines generally require a wind in the range 5.5 m/ (20 km/h), and in practice relatively few land areas have significant prevailing winds. Luckily, offshore or at high altitudes, the winds are much more constant.

There are now many thousands of wind turbines operating in various parts of the world, with utility companies having a total capacity of 59,322 MW"Wind energy is a relatively young but rapidly expanding industry. Over the past decade, global installed capacity has increased from 2,500 megawatts (MW) in 1992 to just over 40,000 MW at the end of 2003, at an annual growth rate of near 30%." [EWEA Executive summary (pdf)] (URL accessed January 30, 2006)"Record year for wind energy : global wind power market increased by 43% in 2005" [Press Release with 2005 statistics (PDF)] (URL accessed February 20, 2006). Capacity in this case means maximum possible output which does not count load factor.

New wind farms and offshore wind parks are being planned and built all over the world. This has been the most rapidly-growing means of electricity generation at the turn of the 21st century and provides a complement to large-scale base-load power stations. Most deployed turbines produce electricity about 25% of the time (load factor 25%), but some reach 35%. The load factor is generally higher in winter. It means that a 5 MW turbine can have average output of 1.7 MW in the best case.

Global winds long-term technical potential is believed to be 5 times current global energy consumption or 40 times current electricity demand. This requires 12.7% of all land area, or that land area with Class 3 or greater potential at a height of 80 meters. It assumes that the land is covered with 6 large wind turbines per square kilometer. Offshore resources experience mean wind speeds of ~90% greater than that of land, so offshore resources could contribute substantially more energy."Offshore stations experience mean wind speeds at 80 m that are ~90% greater than over land on average. [Evaluation of global wind power]
"Overall, the researchers calculated winds at 80 meters [300 feet] traveled over the ocean at approximately 8.6 meters per second and at nearly 4.5 meters per second over land [20 and 10 miles per hour, respectively]." [Global Wind Map Shows Best Wind Farm Locations] (URL accessed January 30, 2006) This number could also increase with higher altitude ground based or airborne wind turbines."High-altitude winds could provide a potentially enormous renewable energy source, and scientists like Roberts believe flying windmills could put an end to dependence on fossil fuels. At 15,000 feet, winds are strong and constant. On the ground, wind is often unreliable -- the biggest problem for ground-based wind turbines." [Windmills in the Sky] (URL accessed January 30, 2006)

Wind strengths vary and thus cannot guarantee continuous power. Some estimates suggest that 1,000 MW of wind generation capacity can be relied on for just 333 MW of continuous power. While this might change as technology evolves, advocates have suggested incorporating wind power with other power sources, or the use of energy storage techniques, with this in mind. It is best used in the context of a system that has significant reserve capacity such as hydro, or reserve load, such as a desalination plant, to mitigate the economic effects of resource variability.

Wind power is renewable and is one of the few energy sources that contributes to greenhouse gas mitigation, because it removes energy directly from the atmosphere without producing net emissions of greenhouse gases such as carbon dioxide and methane (others greenhouse gas mitigating energy sources include solar thermal and ocean thermal).

Water power

Energy in water can be harnessed and used, in the form of motive energy or temperature differences. Since water is about a thousand times heavier than air, even a slow flowing stream of water, or moderate sea swell, can yield great amounts of energy.

There are many forms:

Hydroelectric energy, a term usually reserved for hydroelectric dams.
Tidal power, which captures energy from the tides in vertical direction. Tides come in, raise water levels in a basin, and tides roll out. The water must pass through a turbine to get out of the basin. If the basin is a river delta then silt will block the turbine.
Tidal stream power, Captures a stream of water as it is pushed horizontally around the world by tides.
Wave power, which uses the energy in waves. The waves will usually make large pontoons go up and down in the water, leaving an area with no waves in the "shadow".
Ocean thermal energy conversion (OTEC), which uses the temperature difference between the warmer surface of the ocean and the cool (or cold) lower recesses. To this end, it employs a cyclic heat engine.
Deep lake water cooling, although not technically an energy generation method, can save a lot of energy in summer. It uses submerged pipes as a heat sink for climate control systems. Lake-bottom water is a year-round local constant of about 4 °C.
Blue energy, the reverse of desalination. A difference in salt concentration exists between seawater and river water. This gradient can be utilized to generate electricity by separating positive and negative ions by ion specific membranes. Brackish water is produced. This form of energy is in research, costs are not the issue, tests on pollution of the membrane are in progress. At this moment it is predicted that if everything works out, 1/3 of the electricity needs in the Netherlands can be covered with this system.(2005)

Hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations with the potential for harnessing gravity in this way are either already being exploited or are unavailable for other reasons such as environmental considerations. However, micro hydro may be an option for small scale applications such as single farms, homes or small businesses.

Building a dam often involves flooding large areas of land, this can change habitats so immensely that this risk of endangering local and non local wildlife is great. For example, since damming and redirecting the waters of the Platte River in Nebraska for agricultural and energy use, many native and migratory birds such as the Piping Plover and Sandhill Crane have become increasingly endangered.

Wave and tidal stream power schemes exist but require development capital.

OTEC has not been field tested on a large scale.

Critics of hydroelectric dams state that they may produce significant amounts of carbon dioxide and methane from rotting vegetation. In some cases produce more of these greenhouse gases than power plants running on fossil fuels[link]. Dam failures, while rare, are potentially serious - the Banqiao Dam failure in China killed 171,000 people, many more than the immediate death toll in the Chernobyl disaster.

Solar energy

The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 amperes in full, direct sunlight.

In this context, "solar energy" refers to energy that is directly collected from sunlight. However, most fossil and renewable energy sources are ultimately derived from "solar energy" so some ascribe much broader meanings to the term.

Solar energy can be applied in many ways, including to:

generate electricity using photovoltaic solar cells
generate electricity using concentrated solar power
generate electricity by heating trapped air which rotates turbines in a Solar updraft tower.
heat buildings, directly. Careful positioning of windows and use of brises soleil can maximise inflow of light at the times it is most needed, heating the building while preventing overheating during midday and summer.
heat foodstuffs, through solar ovens.
heat water for domestic consumption and heating using rooftop solar panels.
heat and cool air through use of solar chimneys.

Obviously the sun does not provide constant energy to any spot on the Earth, so its uninterrupted use requires a means for energy storage. This is typically accomplished by battery storage. However, battery storage implies energy losses. Some homeowners use a grid-connected solar system which feeds energy to the grid during the day, and draw energy from the grid at night; this way no energy is expended for storage.

Advantages from solar energy sources include the inexhaustible supply of energy and zero emissions of greenhouse gas and air pollutants. Shortcomings include, depending on application:

intermittency, e.g. that it is not available at night.
Solar panels are in many applications considerably more expensive than alternatives.
The current generated is only of DC type, and must be converted if transmission over the grid is needed.

Geothermal energy

Geothermal energy is energy obtained by tapping the heat of the earth itself, usually from kilometers deep into the Earth's crust. Ultimately, this energy derives from the radioactive decay in the core of the Earth, which heats the Earth from the inside out. This energy can be used in three ways:

Geothermal electricity
Geothermal heating, through deep Earth pipes
Geothermal heating, through a heat pump.

Usually, the term 'geothermal' is reserved for thermal energy from within the Earth.

Geothermal electricity is created by pumping a fluid (oil or water) into the Earth, allowing it to evaporate and using the hot gases vented from the earth's crust to run turbines linked to electrical generators.

The geothermal energy from the core of the Earth is closer to the surface in some areas than in others. Where hot underground steam or water can be tapped and brought to the surface it may be used to generate electricity. Such geothermal power sources exist in certain geologically unstable parts of the world such as Iceland, New Zealand, United States, the Philippines and Italy. The two most prominent areas for this in the United States are in the Yellowstone basin and in northern California. Iceland produced 170 MW geothermal power and heated 86% of all houses in the year 2000 through geothermal energy. Some 8000 MW of capacity is operational in total.

Geothermal heat from the surface of the Earth can be used on most of the globe directly to heat and cool buildings. The temperature of the crust a few feet below the surface is buffered to a constant 7 to 14 °C (45 to 58 °F), so a liquid can be pre-heated or pre-cooled in underground pipelines, providing free cooling in the summer and, via a heat pump, heating in the winter. Other direct uses are in agriculture (greenhouses), aquaculture and industry.

Although geothermal sites are capable of providing heat for many decades, eventually specific locations cool down. Some interpret this as meaning a specific geothermal location can undergo depletion, and question whether geothermal energy is truly renewable.

Small scale geothermal heating can also be used to directly heat buildings: there are many names for this technology including "Ground Source Heat Pump" technology, and "Geoexchange".

Biofuel

Plants use photosynthesis to store solar energy in the form of chemical energy. Biofuel is any fuel that derives from biomass, including living organisms or their metabolic byproducts, such as cow manure.

Typically biofuel is burned to release its stored chemical energy. Research into more efficient methods of converting biofuels and other fuels into electricity utilizing fuel cells is an area of very active work. Biomass, also known as biomatter, can be used directly as fuel or to produce liquid biofuel. Agriculturally produced biomass fuels, such as biodiesel, ethanol and bagasse (often a by-product of sugar cane cultivation) can be burned in internal combustion engines or boilers.

A drawback is that all biomass needs to go through some of these steps: it needs to be grown, collected, dried, fermented and burned. All of these steps require resources and an infrastructure. However, the United States government passed legislation that requires the integration of 7.5 billion U.S. gallons (28,000,000 m³) of ethanol into the gasoline supply experts estimate that six billion dollars of investment will be created along with 200,000 additional jobs in the United States.

Biomatter energy, under the right conditions, is considered to be renewable.

Liquid biofuel

Liquid biofuel is usually bioalcohol such as ethanol and biodiesel and virgin vegetable oils. Biodiesel can be used in modern diesel vehicles with little or no modification to the engine and can be obtained from waste and virgin vegetable and animal oil and fats (lipids). Virgin vegetable oils can be used in modified diesel engines. In fact the Diesel engine was originally designed to run on vegetable oil rather than fossil fuel. A major benefit of biodiesel is lower emissions. The use of biodiesel reduces emission of carbon monoxide and other hydrocarbons by 20 to 40 percent. In some areas corn, sugarbeets, cane and grasses are grown specifically to produce ethanol (also known as alcohol) a liquid which can be used in internal combustion engines and fuel cells. Ethanol is being phased into the current energy infrastructure. E85 is a fuel composed of 85% ethanol and 15% gasoline that is currently being sold to consumers.

The EU plans to add 5% bioethanol to Europe's petrol by 2010. For the UK alone the production would require 12,000 square kilometres of the country's 65,000 square kilometres of arable land assuming that no biofuels are created using waste produces from other agriculture. The supermarket chain Tesco has started adding the 5% bioethanol to the petrol it sells as of January 2006.

In the future, there might be bio-synthetic liquid fuel available. It can be produced by Fishcer-Tropsch processes, also called Biomass-To-Liquids (BTL). Please also see : Fossil fuel beneath. Source: http://www.sunfuel.de and also http://www.senternovem.nl/mmfiles/Status_perspectives_biofuels_EU_2005_tcm24-152475.pdf

Solid biomass

Direct use is usually in the form of combustible solids, either wood, the biogenic portion of municipal solid waste or combustible field crops. Field crops may be grown specifically for combustion or may be used for other purposes, and the processed plant waste then used for combustion. Most sorts of biomatter, including dried manure, can actually be burnt to heat water and to drive turbines.

Sugar cane residue, wheat chaff, corn cobs and other plant matter can be, and is, burnt quite successfully. The process releases no net CO₂.

Solid biomass can also be gasified, and used as described in the next section.

Biogas

Many organic materials can release gases, due to metabolisation of organic matter by bacteria (anaerobic digestion, or fermentation). Landfills actually need to vent this gas (called landfill gas) to prevent dangerous explosions. Animal faeces releases methane under the influence of anaerobic bacteria.

Also, under high pressure, high temperature, anaerobic conditions many organic materials such as wood can be gasified to produce gas. This is often found to be more efficient than direct burning. The gas can then be used to generate electricity and/or heat.

Biogas can easily be produced from current waste streams, such as: paper production, sugar production, sewage, animal waste and so forth. These various waste streams have to be slurried together and allowed to naturally ferment, producing methane gas. This can be done by converting current sewage plants into biogas plants. When a biogas plant has extracted all the methane it can, the remains are sometimes better suitable as fertilizer than the original biomass.

Alternatively biogas can be produced via advanced waste processing systems such as mechanical biological treatment. These systems recover the recyclable elements of household waste and process the biodegradable fraction in anaerobic digesters.

Renewable natural gas is a biogas which has been upgraded to a quality similar to natural gas. By upgrading the quality to that of natural gas, it becomes possible to distribute the gas to the mass market via the existing gas grid.

Small scale energy sources

There are many small scale energy sources that generally cannot be scaled up to industrial size. A short list:

Piezoelectric crystals generate a small voltage whenever they are mechanically deformed. Vibration from engines can stimulate piezoelectric crystals, as can the heels of shoes
Some wristwatches are already powered by kinetics, in this case movement of the arm
Thermoelectric generators produce energy from the heat difference between two objects. This is also used to power a type of wristwatch, as heat energy from the human body is radiated through the watch into the environment.
Special antennae can collect energy from stray radio waves or theoretically even light (EM radiation).

Criticisms

Some critics charge that renewable energy is an arbitrary definition with no bearing on how much an energy source pollutes, how dangerous it is, whether it takes up a large amount of land that could be left wild or farmed for food, whether the source of the renewable energy will last a very long time, or even whether a given energy source produces a net amount of energy.

Supporters respond that most renewable energies are ultimately powered by the Sun, the Earth, or the Moon, so the underlying sources for these energies are expected to last for billions of years. Of course, this does not mean that renewable energy infrastructure, like hydroelectric dams, will last forever. Events like the shifting of riverbeds, or changing weather patterns could potentially alter or even halt the function of hydroelectric dams. But, while this may be a concern in theory, few if any examples of such problems have occurred in modern times. Most renewable energy infrastructure seems to be at least as permanent and relieable as that of fossil fuel energy sources.

While most renewable energy sources do not produce direct pollution, some of the inputs required to produce renewable energy, such as the crops grown to create ethanol or biodiesel, require energy inputs. The exact amount of energy required to grow crops varies widely, since a number of modern farming methods can significantly reduce the amount of energy that must be used. It is also very tricky to account for all energy inputs to biofuels. Opponents of corn ethanol production in the U.S. often quote the work of David Pimentel and Tadeusz Patzek. Pimentel is a retired Entomologist, and Patzek is a Geological Engineer from Berkeley. Both have been exceptionally critical of ethanol and other biofuels. Their studies contend that ethanol, and biofuels in general, are "energy negative," meaning they take more energy to produce than is contained in the final product. However, this does not appear to be the consensus opinion among scientists.

A [report] by the U.S. Department Agriculture compared the methodologies used by a number of researchers on this subject and found that the majority of researchers think the energy balance for ethanol is positive. In fact, a large number of recent studies, including an [article] in the Journal Science offer the consensus opinion that fuels like ethanol are energy positive. Furthermore, it should be pointed out that fossil fuels also require significant energy inputs which have seldom been accounted for in the past.

According to [information] from the American Council for Ethanol, "ethanol has a 125 percent positive energy balance, compared to 85 percent for gasoline." Ultimately, the issue of energy balance may be the wrong thing to worry about, anyway. In the case of gasoline, it is a convenient, portable fuel, even though it is energy negative.

Batteries are also energy negative, since you put more energy into them than you can get out. Nonetheless, they function as a useful energy storage mechanism. Electricity generation often takes 2.5 times more energy to generate than the final product contains, but the key point to understand in all these cases is that forms of energy like electricity are considered higher quality energy than the original energy sources that were used to make them. This is because they can do things like power a light or a computer that the original energy sources they were made from, like coal, cannot do. It is the high quality of this energy that would justify producing it even if it did take more energy than you directly recovered from the final product.

Aesthetics, habitat hazards and land use

Some people dislike the aesthetics of wind turbines or bring up nature conservation issues when it comes to large solar-electric installations outside of cities. Some people try to utilize these renewable technologies in an efficient and aesthetically pleasing way: fixed solar collectors can double as noise barriers along highways, roof-tops are available already and could even be replaced totally by solar collectors, amorphous photovoltaic cells can be used to tint windows and produce energy etc.

Some renewable energy capture systems entail unique environmental problems. For instance, wind turbines can be hazardous to flying birds, while hydroelectric dams can create barriers for migrating fish - a serious problem in the Pacific Northwest that has decimated many salmon populations.

Another problem with many renewables, especially biomass and biofuels, is the large amount of land required, which otherwise could be left as wilderness.

Concentration

Another inherent difficulty with renewables is their variable and diffuse nature (the exception being geothermal energy, which is however only accessible in exceptional locations). Since renewable energy sources are providing relatively low-intensity energy, the new kinds of "power plants" needed to convert the sources into usable energy need to be distributed over large areas.

Electrical power consumption in Western countries averages about 100 watts continuously per person (i.e. about 1 MWh per year). In cloudy Europe this would require about eight square meters of solar panels per person, assuming a below-average solar conversion rate of 12.5%. Systematic electrical generation requires reliable overlapping sources or some means of storage on a reasonable scale (pumped-storage hydro systems, batteries, hydrogen fuel cells, etc). So, because of current costs of such energy storage systems, a stand-alone system is only economic in rare cases, or where a connection to a public grid would be impractical.

Proximity to demand

The geographic diversity of resources is also significant. Some countries and regions have significantly better resources than others in particular RE sectors. Some nations have significant resources at distance from the major population centers where electricity demand exists. Exploiting such resources on a large scale is likely to require considerable investment in transmission and distribution networks as well as in the technology itself.

Rooftop photovoltaic arrays are especially attractive in that most of the power they produce is consumed in the structure on which they are mounted or in other nearby buildings.

Availability

One recurring criticism of renewable sources is their intermittent nature. Sunlight is only available during the day when the sun is well above the horizon when the sky is not cloudy. Wind energy is typically available much less than half the time. Wave energy is continuously available, although wave intensity varies by season. A wave energy scheme installed in Australia is generating electricity with an 80% availability factor.

Issues

Fossil fuels

Renewable energy sources are fundamentally different from fossil fuel or nuclear power plants because the Sun will 'power' these 'power plants' (meaning sunlight, the wind, flowing water, etc.) for the next 4 billion years. They also do not directly produce greenhouse gases and other emissions, as fossil fuel combustion does. Most do not introduce any global new risks such as nuclear waste.

Fossil fuels are not considered a renewable energy source, but are often compared and contrasted with renewables in the context of future energy development.

The traditionally, though not universally, held Western (biogenic) theory postulates that fossil fuels are the altered remnants of ancient plant and animal life deposited in sedimentary rocks. They were formed millions of years ago and have rested underground, mostly dormant, since that time.

In contrast, the Abiogenic petroleum origin theory states that petroleum (or crude oil) is primarily created from non-biological sources of hydrocarbons located deep in the Earth. This view was championed by Fred Hoyle in his book The Unity of the Universe.

Though it is possible to produce complex hydrocarbons artificially by using the Fischer-Tropsch process, this process does not generate energy, and cannot be considered a large scale solution to the energy problem. However, liquid fuels and hydrocarbons are needed, and the Fischer-Tropsch-process can use biomass, hydrogen and oxygen produced with renewable energy, as feedstocks.

The coal industry in the US is publicly claiming coal is renewable energy because the coal was originally biomass. However, the biomass of fossil fuels was produced on the time scale of millions of years through a series of events and it is considered to be a deposit of energy, not an energy flow. Some scientists hold the view that the formation of fossil fuels was a one-time event, made possible by unique conditions during the Devonian period, such as increased oxygen levels and huge swamps.

When the term renewable was introduced, it was a generally held belief that the Earth's sources would be depleted within some 50 years. Since then, large deposits of deep-Earth oil have been found, which has extended this timetable. Because the current rate of consumption exceeds the rate of renewal (if, indeed, there is renewal of fossil fuels), the Earth will eventually run out of fossil fuels (see peak oil).

Transmission

If renewable and distributed generation were to become widespread, electric power transmission and electricity distribution systems might no longer be the main distributors of electrical energy but would operate to balance the electricity needs of local communities. Those with surplus energy would sell to areas needing "top ups". That is, network operation would require a shift from 'passive management' - where generators are hooked up and the system is operated to get electricity 'downstream' to the consumer - to 'active management', wherein generators are spread across a network and inputs and outputs need to be constantly monitored to ensure proper balancing occurs within the system. Some Governments and regulators are moving to address this, though much remains to be done. One potential solution is the increased use of active management of electricity transmission and distribution networks. This will require significant changes in the way that such networks are operated.

However, on a small scale, use of renewable energy that can often be produced "on the spot" lowers the requirements electricity distribution systems have to fulfill. Current systems, while rarely economically efficient, have proven an average household with a solar panel array and energy storage system of the right size needs electricity from outside sources for only a few hours every week. Hence, advocates of renewable energy believe electricity distribution systems will become smaller and easier to manage, rather than the opposite.

Load balancing

A common critisim of renewable power is that generators such as wind turbines or solar arrays are liable to suffer variable output. To handle this characteristic, a more balanced power supply may be obtained if the various renewable sources are interconnected and distributed. Indeed, distribution and redundancy are already features of existing electrical grids. The challenge of variable power supply may be further alleviated by energy storage. For this purpose hydroelectric dams provide an excellent energy storage mechanism. As a case example, Denmark exports wind power to its neighbours during peak periods. Meanwhile, during times of deficiency the power may be imported, particularly from hydroelectric sources. Combined with 'sheddable' electricity loads, real-time pricing and energy stocking, renewable power has the potential to realiably satisfy demand.

Market development of renewable heat energy

Renewable heat is an application of renewable energy, namely the generation of heat from renewable sources. In some cases, contemporary discussion on renewable energy focuses on the generation of electrical, rather than heat, energy. This is despite the fact that many colder countries consume more energy for heating than as electricity. On an annual basis the United Kingdom consumes 350 TWh Department of Trade and Industry report [UK Energy in Brief July 2005] (URL accessed Mar 18, 2006) of electric power, and 840 TWh of gas and other fuels for heating. The residential sector alone consumes a massive 550 TWh of energy for heating, mainly in the form of gas. Department of Trade and Industry, 2005 study on [Renewable Heat] (URL accessed Mar 18, 2006)

Renewable electric power is becoming cheap and convenient enough to place it, in many cases, within reach of the average consumer. By contrast, the market for renewable heat is mostly inaccessible to domestic consumers due to inconvenience of supply, and high capital costs. Heating accounts for a large proportion of energy consumption, however a universally accessible market for renewable heat is yet to emerge. Also see renewable energy development.

Aviation

Kerosene, a non-renewable petroleum-based fuel, is currently considered to be the only fuel practical and economic for commercial jet-engine aviation. Although hydrogen has a high energy density, the need for heavy fuel tanks and fuel-cell stacks rules it out for aircraft. Biodiesel, another candidate aviation fuel, is problematic due its tendency to freeze more readily than kerosene. Smaller piston-engined aircraft are mainly fueled by aviation grade gasoline (avgas) but are increasingly being fueled by ethanol [link] or diesel. Given the proper equipment to prevent fuel gelling, a diesel-powered piston aircraft engine can be powered efficiently by biodiesel.

Historical usage of renewable energy

Throughout history, various forms of renewable and non-renewable energies have been employed.

Wood was the earliest manipulated energy source in human history, being used as a thermal energy source through burning, and it is still important in this context today. Burning wood was important for both cooking and providing heat, enabling human presence in cold climates. Special types of wood cooking, food dehydration and smoke curing, also enabled human societies to safely store perishable foodstuffs through the year. Eventually, it was discovered that partial combustion in the relative absence of oxygen could produce charcoal, which provided a hotter and more compact and portable energy source. However, this was not a more efficient energy source, because it required a large input in wood to create the charcoal.
Animal power for vehicles and mechanical devices was originally produced through animal traction. Animals such as horses and oxen not only provided transportation but also powered mills. Animals are still extensively in use in many parts of the world for these purposes.
Human power for vehicles, mechanical devices and individual non-machine-aided transportation has been employed throughout human history. Slaves have been used for powering boats and powering construction machinery such as that used to build the Egyptian pyramids. Today, slaves have largely been replaced by other sources of power to the degree that the average American accesses the same amount of power that otherwise would require 50 slaves. One of the largest uses of human power today is bicycling, which remains the most energy-efficient means of transportation.
Water power eventually supplanted animal power for mills, wherever the power of falling water in rivers was exploitable . Water power through hydroelectricity continues to be the least expensive method of storing and generating dispatchable energy throughout the world. Historically as well as presently, hydroelectricity provides more renewable energy than any other renewable source.
Animal oil, especially whale oil was long burned as an oil for light.
Wind power has been used for several hundred years. It was originally used via large sail-blade windmills with slow-moving blades, such as those seen in the Netherlands and mentioned in Don Quixote. These large mills usually either pumped water or powered small mills. Newer windmills featured smaller, faster-turning, more compact units with more blades, such as those seen throughout the Great Plains. These were mostly used for pumping water from wells. Recent years have seen the rapid development of wind generation farms by mainstream power companies, using a new generation of large, high wind turbines with two or three immense and relatively slow-moving blades. Today, wind power is the fastest growing energy source in the world.
Solar power as a direct energy source has not been captured by mechanical systems until recent human history, but was captured as an energy source through architecture in certain societies for many centuries. Not until the twentieth century was direct solar input extensively explored via more carefully planned architecture (passive solar) or via heat capture in mechanical systems (active solar) or electrical conversion (photovoltaic). Increasingly today the sun is harnessed for heat and electricity.
Attempts to harness the power of ocean waves appears in drawings and patents back to the 19th century. Modern attempts to capture wave power began in the 1970's by Professor Steven Salter who started the Wave Energy Group at the University of Edinburgh in Scotland. There are several pilot plants generating power into the grid, and many new and curious designs are in various stages of development and testing.

External links

Wikimedia Commons has media related to:

[Special]

Wikinews has news related to:

[Atlas of Renewable Energy (in English, French & German)]
[World Council for Renewable Energy WCRE]
[Surface meteorology and Solar Energy - a renewable energy resource for data and images]
[SPI Foundation for Renewable Solar Energy]
[Renewable Energy Policy Network REN21]
[Renewable Energy - Beginner's tutorial on using renewable fuel in a diesel engine]
[The British Library - finding information on the renewable energy industry]
[Are hybrids really green? (Alternative Energy)] - A blog debate on the merits of alternative energy, specifically in the context of powering a grid for use in hybrid or electric cars

References

[U.S. Energy Information Administration] provides a wide range of statistics and information on the industry.
Boyle, G. (ed.), Renewable Energy: Power for a Sustainable Future. Open University, UK, 1996.
[U.S. DOE Energy Efficiency and Renewable Energy (EERE) Home Page]

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Sustainability and energy development [Edit]
Energy production	Active solar \| Anaerobic digestion \| Biomass \| Blue energy \| Deep lake water cooling \| Distributed generation \| Electricity generation \| Energy tower \| Fuel cell \| Fusion power \| Geothermal power \| Hydroelectricity \| Mechanical biological treatment \| Ocean thermal energy conversion \| Passive solar \| Seasonal thermal store \| Solar cell \| Solar panel \| Solar pond \| Solar power \| Solar power tower \| Solar thermal energy \| Solar tracker \| Solar updraft tower \| Tidal power \| Trombe wall \| Water turbine \| Wave power \| Wind farm \| Wind power \| Wind turbine
Energy development and use	Energy development > Environmental concerns with electricity generation \| Future energy development \| Inertial fusion power plant \| Hydrogen economy \| Hubbert peak \| Renewable energy \| Hypermodernity \| Technological singularity
Energy and sustainability status	Ecosystem services > Kardashev scale \| TPE \| UN Human Development Index \| Value of Earth \| Appropriate technology \| Infrastructural capital
Sustainability	Autonomous building > Ecoforestry \| Ecological economics \| Earth sheltering \| Development economics \| Environmental design \| Exploitation of natural resources \| Green building \| Green chemistry \| Green gross domestic product \| Natural building \| Permaculture \| Self-sufficiency \| Straw-bale construction \| Sustainability \| Sustainable agriculture \| Sustainable design \| Sustainable development \| Sustainable industries \| Sustainable living \| The Natural Step \| Windcatcher
Sustainability management	Commission on Sustainable Development > Human development theory \| Maldevelopment \| Rio Declaration on Environment and Development \| Rocky Mountain Institute \| Sim Van der Ryn \| Underdevelopment \| World Business Council for Sustainable Development \| World Summit on Sustainable Development \| Precautionary principle \| Intermediate Technology Development Group
Energy and conservation	Energy conservation > Energy-efficient landscaping \| Passive house \| Superinsulation \| Voluntary simplicity \| Ecological footprint \| Ecovillage \| Waste \| Zero energy building
Transportation	Battery electric vehicle > Electric vehicle \| Hydrogen car \| Trolleybus
Communication	Wireless Mesh