Renewable energy (sources) or RES capture their energy from existing flows of energy, from on-going natural processes, such as sunshine, wind, flowing water, biological processes, and geothermal heat flows. Neither fossil fuels nor nuclear power are considered to be renewable. For a discussion of this, please see the fossil fuels and nuclear energy sections in this article.
Most renewable forms of energy, other than geothermal and tidal power, ultimately come from the Sun. Some forms are stored solar energy such as rainfall and wind power which are considered short-term solar-energy storage, whereas the energy in biomass is accumulated over a period of months, as in straw, or through many years as in wood. Capturing renewable energy by plants, animals and humans does not permanently deplete the resource. Fossil fuels, while theoretically renewable on a very long time-scale, are exploited at rates that may deplete these resources in the near future (see: Hubbert peak).
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, or production of fuels such as ethanol from biomass (see alcohol as a fuel).
A parameter sometimes used in renewable energy is the tonne of oil equivalent (toe). This is equal to 10,000 megacal or 41,868 MJ of energy.
For aspects of renewable energy use in modern societies see Renewable energy development.
The concept of renewable energy was introduced in the 1970s as part of an effort to move beyond nuclear and fossil fuels. The most common definition is that renewable energy is from an energy resource that is replaced rapidly by a natural, ongoing process. Under this definition, neither fossil fuels nor nuclear power are renewable. Some, notably nuclear power and coal advocates, disagree with this definition, arguing that some renewable energy sources can have as great an impact on the environment as energy sources that don't technically meet this definition.
Modern sources of renewable energy
Main article: Solar power
Since most renewable energy is ultimately "solar energy" this term is slightly confusing and used in two different ways: firstly as a synonym for "renewable energies" as a whole and secondly for the energy that is directly collected from sunlight. In this section it is used in the latter category. Solar power can be used to:
Obviously the sun does not provide constant energy to any spot on the Earth, so its use is limited. Solar cells are often used to power batteries, as most other applications would require a secondary energy source, to cope with outages.
Solar power is renewable.
Main article: Wind power
As the 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/s (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 over 47,317MW. 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 is winter.
There is resistance to the establishment of land based wind farms owing initially to perceptions they are noisy and contribute to "visual pollution," i.e., they are considered to be eyesores. Many people also claim that turbines kill birds, and that they in general do little for the environment.
Others have argued that they find the turbines beautiful, that turbines out at sea are invisible to anyone on the shore, that cars kill more birds annually and that turbines are continuing to evolve.
While the winds don't die down when the sun sets, they do die down, and thus cannot be relied upon the generate continuous power. Some calculations suggest that 1000MW of wind generated electricity can replace just 300MW of continuous power. While this might change as technology evolves, advocates have suggested using wind power to pump water into reservoirs (see water power in this article), or power industrial applications that don't depend on a continuous electricity supply, like electrolysis.
Wind power is renewable.
Main article: Geothermal energy
Geothermal energy ultimately comes from radioactive decay in the core of the Earth, which heats the Earth from the inside out, and from the sun, which heats the surface. It 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 the thermal energy from the core of 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 7-14C (45-58F), 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. Others see such an interpretation as an inaccurate usage of the word depletion because the overall supply of geothermal energy on Earth, and its source, remain nearly constant. Geothermal energy depends on local geological instability, which, by definition, is unpredictable, and might stabilise.
The general public considers geothermal energy to be renewable.
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 heavier than air is, even a slow flowing stream of water 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 horizontal direction. Tides come in, raise waterlevels in a basin, and tides roll out. The water must pass through a turbine to get out of the basin.
- Tidal stream power , which does the same vertically, capturing the stream of water as it is moved around the world by the tides.
Wave power, which uses the energy in waves. The waves will usually move large pontoons up and down.
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, not technically an energy generation method, though it 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.
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. Building a dam often involves flooding large areas of land, changing habitats, and while hydroelectric energy produces essentially no carbon dioxide, recent reports have linked hydroelectric power to methane, which forms out of decaying submerged plants which grow in the dried up parts of the basis in times of drought. Methane is a potent greenhouse gas.
The other methods of energy generation (and cooling) have had varying degrees of success in the field. Wave and tidal power prove hard to tap, while OTEC has not been field tested on a large scale.
The general public mostly considers water power energy to be renewable.
Main article: Biofuel
Plants partly use photosynthesis to store solar energy, water and CO2. Biofuel is any fuel that derives from biomass - recently living organisms or their metabolic byproducts, such as manure from cows. It is a renewable energy.
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.
Liquid biofuel is usually bioalcohol such as methanol, ethanol and biodiesel. Biodiesel can be used in modern diesel vehicles with little or no modification and can be obtained from waste and crude vegetable and animal oil and fats (lipids). 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.
The EU plans to add 5% bioethanol to Europe's petrol by 2010. For the UK alone this would require 1.2 million hectares of arable land to be used exclusively for the production of bioethanol. Within its borders, the country only has 6.5 million hectares of arable land. Other, more efficient sources of biofuel, such as palm and soya oil, would probably have a significant negative environmental impact due to habitat damage in the areas in which they are grown.
Direct use is usually in the form of combustible solids, either firewood 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 van also be gasified, and used as described in the next section.
- Main article: biogas
Many organic materials can release gases, due to metabolisation of organic matter by bacteria (fermentation. Landfills actually need to release this 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.
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.
Biomatter energy, under the right conditions, is considered to be renewable.
Small scale energy sources
There are many small scale energy sources that generally cannot be scaled up to industrial size. A short list:
Piezo electric crystals generate a small voltage whenever they are mechanically deformed. Vibration from engines can stimulate piezo electric crystals, as can the heels of shoes
- Some watches are already powered by kinetics, in this case movement of the arm
Electrokinetics generate electricity from the kinetic energy in water that is pumped through tiny channels
- Special antennae can collect energy from stray radio waves or theoretically even light (EM radiation).
Discussion of renewable energy
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 the numbers of many salmon populations. Burning biomass and biofuels causes air pollution similar to that of burning fossil fuels.
Another problem with many renewables, especially biomass and biofuels, is the large amount of land required, which otherwise could be left as wilderness.
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.
To illustrate, note that production of 1000 kWh of electricity per year (a typical per-year-per-capita consumption of electricity in Western countries), in cloudy Europe would require about eight square meters of solar panels (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, future hydrogen fuel cells, etc). So, because of currently-expensive energy storage systems, a small stand-alone system is only economic in rare cases, or in applications where the connection to the global energy network would drive costs up sharply.
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.
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.
Main article: Nuclear power
Nuclear energy is not considered a renewable energy, but is often compared and contrasted with renewables in the context of future energy development.
Nuclear energy comes from fission. In physics, fission is when the nucleus splits into two or more smaller nuclei plus some by-products. Fission releases substantial amounts of energy (the strong nuclear force binding energy).
When the term renewable was introduced (see defining renewable within this article), it was a generally held belief that the Earth's sources of nuclear material would be depleted within some fifty years. Since then, the breeder reactor was invented. By using breeder reactors, which transform materials that are not generally fissile (such as most isotopes of uranium into easily fissile material, such as plutonium and by harvesting nuclear material from mines, seawater and granite, we could theoretically continue to use nuclear power for billions of years, as calculated by Bernard Cohen . However, these calculations have been proven to be faulty. For example, they don't take the natural decay of uranium into account. Moreover, they ignore rising electricity demands. They also ignore alternate fissile materials such as thorium. It is currently impossible to say how much fissile material will be available in the future, or how long it will last when used.
Nuclear fission, the common form of nuclear energy production, releases radiation and large amounts of radioactive material, which most scientists agree must be contained for thousands of years. During this time, the material could leak into the environment and cause devastation to ecosystems.
Nuclear power is not considered to be renewable, because nuclear power plants actually generate energy; they do not tap into an existing flow of energy, but rather contain fissile materials in a reactor and force them to decay, releasing energy in the form of radiation and heat, which is used to generate electricity. Nuclear power generates radioactive waste, which needs to be safely stored. Finding a site that will remain safe for several thousands of years isn't easy. In the U.S., a proposal to store the nation's nuclear waste under Yucca Mountain in Nevada has generated controversy.
Nuclear power is a limited power. Using fissile materials means depleting them, while renewable energy sources such as the wind can in realistic terms be tapped indefinitely, by any number of turbines. Increasing energy output through wind turbines would involve erecting additional wind turbines, while increasing energy output through nuclear power brings the moment of depletion closer.
The amount of uranium in the seas is currently being replenished by rivers through erosion of underground resources at a rate of 32,000 tonnes per year. This could supply about 25 times the current world demand for electricity for billions of years.
Some critics of nuclear energy argue that nuclear energy could lead to the proliferation of nuclear weapons technology, since some nuclear reactors create the materials necessary for these weapons. While it's true that breeder reactors generate large amounts of plutonium for use in nuclear reactors, plutonium is a base component of nuclear fission bombs.
Main article: Fossil fuel
Fossil fuels are not considered a renewable energy source, but are often compared and contrasted with renewables in the context of future energy development.
Fossil fuels are generally thought to be the altered remnants of ancient plant and animal life deposited in sedimentary rocks. They were formed millions of years ago during the Davonian period, and have rested underground, mostly dormant, since that time.
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 scientist 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 (see Defining renewable within this article), it was a generally held belief that the Earth's sources would be depleted within some fifty years. Since then, large deposits of deep-Earth oil have been found, which upset the timetable, there is no denying that eventually Earth will run out of fossil fuels. (See peak oil).
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.
See also: Grid energy storage and the section on energy storage in Energy development.
Historical usage of (renewable) energy
In times when fossil fuels and nuclear power where not available, only renewable energy sources were used (though one might argue that clear cutting to provide firewood is hardly sustainable).
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 [[smoking (food)|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.
Motive 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.
Animal power for mills was eventually supplanted by water power, the power of falling water in rivers, wherever it was exploitable. Direct use of water power for mechanical purposes is today fairly uncommon, but still in use. Originally, water power through hydroelectricity was the most important source of electrical generation throughout society, and is still an important source today. Throughout most of the history of human technology, hydroelectricity has been the only renewable source of electricity generation significantly tapped for the generation of electricity.
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.
Solar power as a direct energy source has been 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.