Renewable energy

Renewable energy is energy which can be replenished at the same rate it is used. Renewable energy sources contribute approximately 25% of human energy use worldwide. The prime source of renewable energy is solar radiation, i.e. sunlight. The Earth-Atmosphere system supports approximately 5.4 x 10²⁴ joules per year in the solar radiation cycle (Sorensen, 2004).

Mankind's traditional uses of wind, water, and solar power are widespread in developed and developing countries; but the mass production of electricity using renewable energy sources has become popular only recently, reflecting the major threats of climate change due to pollution, concerns about the exhaustion of fossil fuels, and the environmental, social and political risks of fossil fuels and nuclear power. Many countries and organizations promote renewable energies through taxes and subsidies. Varying definitions of the term renewable energy have been adopted to define eligibility under these policies.

Renewable energy sources

The Earth-Atmosphere system is in equilibrium such that heat radiation into space is equal to incoming solar radiation, the resulting level of energy within the Earth-Atmosphere system can roughly be described as the Earth's "climate". The hydrosphere (water) absorbs a major fraction of the incoming radiation. Most radiation is absorbed at low latitudes around the equator, but this energy is dissipated around the globe in the form of winds and ocean currents. Wave motion may play a role in the process of transferring mechanical energy between the atmosphere and the ocean through wind stress (Sorensen, 2004).

Mass production of electricity from renewable energy flows requires technology that harnesses natural phenomena such as sunlight, wind, the movement of water, and geothermal heat. Power sourced this way may be intermittent, and may not be of a specification or quality necessary for the grid.

Wind power

Example of a traditional windmill

Main article: Wind power

Kinetic energy in airflows can be used to run wind turbines; some are capable of producing 5 MW of power, but the most cost effective are currently 500 kW - 1.5 MW (James + James 1999). The power output of a turbine is a function of the cube of the wind speed, so high-power output can be achieved as wind speed increases, shutting off at extreme wind speeds to prevent damage. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations for wind farms.

Wind is the fastest growing of the renewable energy sectors. Over the past decade, global installed capacity increased from 2,500 MW in 1992 to just over 40,000 MW at the end of 2003, at an annual growth rate of near 30%, (capacity in this case means maximum possible output which does not count load factor). Most deployed turbines in the EU produce electricity about 25% of the time (load factor 25%)[1], but under favourable wind regimes some reach 35% or higher. The load factor is generally higher in winter. It would mean that a typical 5 MW turbine in the EU would have an average output of 1.7 MW.

Globally, the long-term technical potential of wind energy is believed to be 5 times current global energy consumption or 40 times current electricity demand. This would require covering 12.7% of all land area with wind turbines, or that land area with Class 3 or greater potential at a height of 80 meters. This land would have to be 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. This number could also increase with higher altitude ground based or airborne wind turbines.

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 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.

Water power

Main article: 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 more dense than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy.

There are many forms:

• Hydroelectric energy is a term usually reserved for hydroelectric dams.
• Tidal power 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 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) 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 is 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, and tests on pollution of the membrane are in progress. At this moment it is predicted that if everything works out, 33% of the electricity needs in the Netherlands could 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, which 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.

The reservoir created for hydroelectric dams may produce significant amounts of carbon dioxide and methane from rotting vegetation. In some cases they produce more of these greenhouse gases than power plants running on fossil fuels. They also affect water quality, creating large amounts of stagnant water without oxygen in the reservoir, and excessive air bubbles in the water downstream from the dam, both of which kill marine life. Dam failures, while rare, are potentially serious — the Banqiao Dam failure in China killed 171,000 people and left millions homeless, many more than the death toll from the Chernobyl disaster. Though the dams can be built stronger, at greater cost, they are still prone to sabotage and terrorism.

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

OTEC has not been field tested on a large scale.

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.

Main article: Solar power

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 Space Solar Power Satellite in geostationary orbit and beaming it down via microwaves
• 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 or air for domestic hot water and space heating needs using solar-thermal panels.
• heat and cool air through use of solar chimneys.

The sun does not provide constant energy to any spot on the Earth, so its uninterrupted use on Earth 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 that feeds energy to the grid during the day and draw energy from the grid at night; this way no energy is expended for storage. Batteries provide direct current (DC), whereas most household appliances run off alternating current (AC). Conversion from DC to AC leads to some energy loss.

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:

• Economic competitiveness with conventional energy conversion
• Intermittency; it is not available at night or during heavy cloud cover.
• For photovoltaics (solar-electric), the current generated is only of DC type, and must be converted if transmission over the standard AC grid is needed.

Geothermal energy

Main article: 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 with the use of Geothermal Systems. 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

Main article: 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.

Biogas is a biofuel produced through the intermediary stage of anaerobic digestion. Biogas consists mainly (45–90%) biologically produced methane.

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, cornstalks, sugarbeets, sugar cane, and switchgrasses 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 13,000 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 February 2006.

In the future, there might be bio-synthetic liquid fuel available. It can be produced by the Fischer-Tropsch process, also called Biomass-To-Liquids (BTL).

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. Whether this process releases net CO₂ emissions is still up for debate in the scientific community.

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

Biogas

Main articles: Biogas and Anaerobic digestion

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 feces 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

Criticism of renewable concept

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.

While most renewable energy sources do not produce pollution directly, it is often produced indirectly or during construction. 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, a retired Entomologist, and Tadeusz Patzek, 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.

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." As far as dealing with peak oil is concerned, this is an apples to oranges comparison, because gasoline comes as a portion of depletable crude oil, while ethanol is supposedly a sustainable alternative. If crude oil with a 10:1 energy balance is used to make gasoline with an 85% energy balance (simplifying to assume all energy is convertible into gasoline or waste heat), one gets 8.5 units of energy for every 1 they put into gasoline-powered oil wells and refineries. In this manner, ethanol proponents faced with these facts have come to argue that corn ethanol production may be a more efficient way of using crude oil than refining it. The issue of energy balance is important for any major energy source, but ultimately other factors come into play as well. Corn ethanol, for example, could not create energy independence for current markets because there is not enough arable land to provide equivalent ethanol as we use imported gasoline.

Batteries, while a source of power, are energy negative, since more energy is put into them during charging than can be taken out by discharging. They function solely as an energy storage or energy carrier mechanism. As much as 4/5 of the energy taken from a fuel is wasted in order to create electrical energy, due to inefficiencies, some of which it is not physically possible to improve. (Fossil fuel and nuclear power plants are rated in MWth and MWe, for instance; the thermal and electrical power outputs. The thermal output is always higher than the electrical output; much of the thermal energy is wasted.) Despite these inefficiencies, electrical energy is still generated, because it is more useful (powering a light or a computer) than the original energy sources that were used to create it. It is the high quality of this energy that would justify producing it even if it did take more energy than directly recovered from the final product.

Aesthetics and habitat hazards

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. Methods and opportunities exist to deploy these renewable technologies in an efficient and aesthetically pleasing way: fixed solar collectors can double as noise barriers along highways; tremendous roadway, parking lot, and roof-top area is available already (and rooftops 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 actually create environmental problems. For instance, older wind turbines can be hazardous to flying birds and hydroelectric dams create barriers for migrating fish. This latter example exists in the Pacific Northwest where salmon populations have been affected.

Land usage

Another problem with many renewable energy sources, particularly biomass and biofuels, is the large amount of land required to harvest energy, which otherwise could be left as wilderness. In general, renewables face inherent difficulty with 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. However, this criticism must also take into consideration the land area required by non-renewable energy sources, such as vast strip-mined areas and slag mountains for coal, safety zones around nuclear plants, and hundreds of square miles being strip-mined for oil sands, for example.

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 (a square 9 feet on a side), assuming a median solar conversion rate of 12.5%. (about half the theoretical maximum efficiency for crystal silicon ). This assumes no technological energy efficiency gains or conservation measures. If high reliability is required, systematic electrical generation requires overlapping sources or some means of storage on a reasonable scale. Available storage options include pumped-storage hydro systems, batteries, hydrogen fuel cells, etc. Initial investments in such energy storage systems can be high, although the costs can recovered in the long-term. Such solutions may be the only alternative where 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.

In certain cases, for people that live in big houses, rooftop photovoltaic arrays may be 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 available only 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 generates electricity with an 80% availability factor.

Issues

Fossil fuels

Main article: Fossil fuel

Renewable energy sources are fundamentally different from fossil fuels, because the Sun, Earth, or Moon power these 'power plants' (meaning sunlight, the wind, flowing water, etc.) for billions of years. They do not produce as many greenhouse gases and other pollution as fossil fuel combustion.

When the term renewable was introduced in the early 1970s, it was a generally held belief that the Earth's sources would be depleted within some 50 years:

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. Although this process may continue today, it is extremely slow, and produces a negligible amount of these resources compared to consumption by humans. 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). Fossil fuels are therefore not considered a renewable energy source, but are often compared and contrasted with renewables in the context of future energy development.

Since then, large deposits of deep-Earth oil have been found, which has extended this timetable.

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.

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 net energy, and is not a solution to the energy problem; it is mainly useful for storing energy, and converting energy from alternative sources to provide power for equipment that can only use hydrocarbons. For instance, in a hypothetical world that used only electrical energy generated from renewables, jet fuel would still be needed because of its high energy density, and would be generated artificially by this process. The Fischer-Tropsch-process can use biomass, hydrogen and oxygen produced with renewable energy, as feedstocks.

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 and storage

A common criticism 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 example, pumped-storage hydroelectricity provides a popular energy storage mechanism. Also there are other means for energy storage.

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 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.

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, it has very high volume even in liquid form so the need for huge fuel tanks or heavy fuel-cell stacks makes it impractical 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 [2] or diesel. Given the proper equipment to prevent fuel gelling, a diesel-powered piston aircraft engine can be powered efficiently by biodiesel.

Nuclear power

Main article: Nuclear power

Because nuclear power is not renewed from an external energy source, it does not meet the conventional definition of renewable energy. 'Renewable', as a term in modern usage, was coined during the energy crisis of the 1970s and was clearly meant to exclude nuclear power . Inclusion of nuclear power under the "renewable energy" umbrella may render nuclear power projects eligible for development aid under various jurisdictions. Arguments in favor of including nuclear power under renewable energy title are based on the potentially large amount of raw materials that may become available to fuel nuclear fission. In 1983 the physicist Bernard Cohen calculated the useful lifetime of nuclear power in the billions of years — longer than the life of the sun itself, remarking that this should qualify it as a renewable resource. Accidents notwithstanding, and ignoring decommissioning issues, the relatively little direct pollution from nuclear power plants while productive is also often cited (see Sustainable energy).

Nuclear has been referred to as "renewable" by the President of the United States George W. Bush and United Kingdom politician Lord David Sainsbury..

No legislative body has yet included nuclear energy under any legal definition of "renewable energy sources" for provision of development support (see: Renewable energy development). Similarly, statutory and scientific definitions of renewable energies by-and-large exclude nuclear energy. In England and Wales there is a Non-Fossil Fuel Obligation , which provides support for renewable energy. Nuclear power production is promoted indirectly, by exclusion from this obligation.

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. Human beings placed in forced bondage or indentured servitude have been used for powering boats and powering construction machinery such as that used to build the Egyptian pyramids. Today, human power has largely been replaced by other sources of power to the degree that the average American accesses the same amount of power that otherwise would have required 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.

See also

For renewable energy use in current societies, see Renewable energy development.

For a discussion of future options, see Future energy development.

• List of energy topics
• Biodiesel
• Biofuel
• Biomass
• Non food crops
• Butanol from Clostridium acetobutylicum
• Energy conservation
• Ethanol fuel in Brazil
• Ethanol fuel
• Hydrogen economy
• Oil phase-out in Sweden
• Peak Oil
• Renewable energy development
• Mitigation of global warming
• Mechanical Biological Treatment
• Renewable energy in the European Union
• Soft energy path
• Solar chimney
• Sustainable energy
• Tidal power
• Wave Power
• Wind power

External links

• National Wind Watch — US grassroots coalition for presenting all of the facts about industrial-scale wind energy
• National Non-Food Crops Centre: UK independent information source on biofuels and other renewable crop-based technologies
• ESRU University of Strathclyde, Scotland — Library of MSc Theses on broad ranging renewable energy research topics
• EnergyHack — Blog and news site about alternative energy.
• Atlas of Renewable Energy (in English, French & German)
• Latest research on Renewable Energy from ScienceDaily
• Green-e: Certified Renewable Energy
• The Renewable Planet — renewable energy map
• Renewable Energy — Beginner's tutorial on using renewable fuel in a diesel engine
• Ressourceo : compare ecology
• World Council for Renewable Energy WCRE
• Surface meteorology and Solar Energy — a renewable energy resource for data and images
• Renewable Energy Policy Network REN21
• The British Library — finding information on the renewable energy industry
• RenewNews — See what's happening in the world of renewable energy in language of regular people.
• CleanEnergyTalk — A community dedicated to the education about renewable energy and it's future in society.
• Renew-o-pedia! — A community maintained wiki encyclopedia all about renewable energy.
• reegle Information Gateway for Renewable Energy and Energy Efficiency
• Wind Power South Africa Small home based wind powered generators
• Database of State Incentives for Renewable Energy — Summary of U.S. federal and state renewable energy programs

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