Renewable energy

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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.

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.[1]

For aspects of renewable energy use in modern societies see Renewable energy development. For a general discussion, see future energy development.

The most common definition is that renewable energy is from an energy resource that is replaced rapidly by a natural process such as power generated from the sun or from the wind.

Modern sources of renewable energy

Geothermal energy

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 present consumption of Geothermal energy does not in any way threaten or diminish the quality of life for future generations, consequently, it is considered a renewable energy source.

Solar energy

File:Solar panels on yacht at sea.jpg
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

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. Some homeowners use a solar system which sells energy to the grid during the day, and draw energy from the grid at night; this is to everyone's advantage, since power demand for air conditioning is highest during the day.

Wind energy

Main article: Wind power

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/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 [2]. Capacity probably means maximum possible output which does not count load factor. It is worth noticing that this number suggests a much higher real percentage of power supply than it really has.

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 5MW turbine can have average output of 1,7MW 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.[3][4]. This number could also increase with higher altitude ground based or airborne wind turbines [5].

There is resistance to the establishment of land based wind farms owing initially to perceptions that 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.

Wind strengths vary and thus cannot guarantee continuous power. Some estimates suggest that 1,000MW of wind generation capacity can be relied on for just 333MW 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.

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 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 pushed around the world by the tides.
  • Wave power, which uses the energy in waves. The waves will usually make large pontoons go up and down in the water.
  • 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 use photosynthesis to store solar energy in the form of chemical energy. 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.

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.

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

Liquid biofuel

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 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.

Solid biomass

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 can 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.

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. We just need to convert current sewage plants to biogas plants, build more locally centered smaller biogas plants and plan for the future. Biogas production has the capacity to provide us with about half of our energy needs, either burned for electrical productions or piped into current gas lines for use. It just has to be done and made a priority. Besides, when a plant has extracted all the methane it can, we are left with a better fertilizer for our farms than we started with.

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).


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, although it causes a lower greenhouse effect since the carbon placed in the atmosphere was already there before the plants were grown, rather than being "new" carbon from 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.

Electrical power consumption in Western countries averages about 100 watts per person (i.e. about 1000 kWh per year). In cloudy Europe this 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, 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.


One recurring criticism of renewable sources is their intermittant nature. Solar insolation, for example can only be expected to be available during the day (50% of the time). Wind energy is somewhat more available, while geothermal and wave energy are available all of the time, although the intensity of the waves varies season to season. A wave energy scheme installed in Australia is generating electricity with an 80% availability factor.

Fossil fuels

Main article: Fossil fuel

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.

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 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).


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.

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.
  • 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 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.
  • 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


External links


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