Renewable energy (sources) or RES includes all sources of energy that are captured from on-going natural processes, such as solar power, wind power, water flow in streams (hydro power), biomass, biodiesel and geothermal heat flows. Most renewable forms of energy, other than geothermal and tidal power, 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 watermill and windmills. Examples of indirect use are electricity generation through wind turbines or photovoltaic cells, or production of fuels such as ethanol from biomass (see alcohol as a fuel).
For aspects of renewable energy use in modern societies see Renewable energy development.
Other definitions of renewable energy sources
Is nuclear energy renewable?
Sources of nuclear energy on Earth are very large, which makes this resource similar to renewable resources in size. In present-day nuclear reactors fissile uranium can be used, which is an exhaustible resource on the order of a few hundred years. Only by transformation of nonfissile isotopes of uranium and thorium in breeder reactors does nuclear fission become a long-term resource. With these reactors, use of very diluted uranium resources becomes feasible. Uranium can be extracted from seawater and granite with a positive life-cycle energy balance. The amount of uranium in the seas is replenished by rivers through erosion of underground resources at a rate of 32,000 tonnes per year.  (http://www-formal.stanford.edu/jmc/progress/cohen.html) According to Bernard Cohen, this could provide 25 times the world's present electricity usage.  (http://www.phyast.pitt.edu/~blc/book/chapter13.html)
Estimates of the lifespan of breeder reactor power vary from millions of years to billions, even lasting longer than the sun itself.  (http://www-formal.stanford.edu/jmc/progress/cohen.html) As recorded civilization is only thousands of years old, even the conservative estimates indicate effectively inexhaustible energy.
Critics point out this proposed definition ignores the environmentally harmful nuclear waste that is produced and ignores the environmental impact from a single significant radiation leakage accident (see Yucca Mountain). Other critics point to the possible proliferation of nuclear weapons technology as a consequence of widespread nuclear power technology, since some nuclear reactors create the materials necessary for these weapons.
Man-made nuclear fusion of light elements, such as hydrogen, is not yet practical except for destructive purposes (hydrogen bomb).
Renewable energy use should not harm the environment
The most common definition of renewable energy, used in legal documents and in the political discourse on sustainable energy, include only those sources that do not harm the environment during use. For example, large hydro power plants are excluded. (see:WWF Gold Standard (http://www.panda.org/climate/goldstandard/))
Pros and cons of renewable energy
Renewable energy sources are fundamentally different from fossil fuel or nuclear power plants because of their widespread occurrence and abundance - the sun will 'power' these 'power plants' (meaning sunlight, the wind, flowing water, etc.) for the next 4 billion years. The primary advantage of many renewable energy sources are their lack of greenhouse gas and other emissions in comparison with fossil fuel combustion. Some renewable sources do not emit any additional carbon dioxide and do not introduce any new risks such as nuclear waste. In fact, most biomass actively sequesters carbon dioxide while growing.
A visible disadvantage of renewables is their visual impact on local environments. 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.
Another inherent difficulty with renewables is their variable and diffuse nature (with the exception being geothermal energy, which is however only accessible where the earth's crust is thin, such as near hot springs and natural geysers). 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 make the phrases 'low-intensity' and 'large area' easier to understand, note that in order to produce 1000 kWh of electricity per year (a typical per-year-per-capita consumption of electricity in Western countries), a home owner in cloudy Europe needs to use eight square meters of solar panels (assuming a below-average energy efficiency 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.
If renewable and distributed generation were to become widespread, electric power transmission and electricity distribution systems would 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.
Renewable energy history
The original energy source for all human activity was the sun via growing plants. Solar energy's main human application throughout most of history has thus been in agriculture and forestry, via photosynthesis.
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.
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.
Modern renewable energy technologies
Use of several types of renewable energy, including the following, is boosted by new technologies:
Of course there are some smaller-scale applications as well:
- Piezo electric crystals embedded in the sole of a shoe can yield a small amount of energy with each step. Vibration from engines can stimulate piezo electric crystals.
- Some watches are already powered by movement of the arm.
- Special antennae can collect energy from stray radio waves or even light (EM radiation).
Renewables as solar energy
Most renewable energy sources can trace their roots to solar energy, with the exception of geothermal and tidal power -- yet even these can be attributed to the sun's gravity. For example, wind is caused by the sun heating the earth unevenly. Hot air is less dense, so it rises, causing cooler air to move in to replace it. Hydroelectric power can be ultimately traced to the sun too. When the sun evaporates water in the ocean, the vapor forms clouds which later fall on mountains as rain which is routed through turbines to generate electricity. The transformation goes from solar energy to potential energy to kinetic energy to electric energy.
Solar energy per se
Since most renewable energy is "Solar Energy" this term is slightly confusing and used in two different ways: firstly as a synonym for "renewable energies" as a whole (like in the political slogan "Solar not nuclear") and secondly for the energy that is directly collected from solar radiation. In this section it is used in the latter category.
There are actually two separate approaches to solar energy, termed active solar and passive solar.
Solar electrical energy
For electricity generation, ground-based solar power has serious limitations because of its diffuse and intermittent nature. First, ground-based solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 20%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (14% - 18%), though increased efficiency or lower production costs have been the subject of much research over several decades.
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.
Two methods of converting the Sun's radiant energy to electricity are the focus of attention. The better-known method uses sunlight acting on photovoltaic (PV) cells to produce electricity. This has many applications in satellites, small devices and lights, grid-free applications, earthbound signaling and communication equipment, such as remote area telecommunications equipment. Sales of solar PV modules are increasing strongly as their efficiency increases and price diminishes. But the high cost per unit of electricity still rules out most uses.
Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and the USA. Japan has 150 MWe installed. A large solar PV plant was planned for Crete. In 2001 the world total for PV electricity was less than 1000 MWe with Japan as the world's leading producer. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the Sun's rays during the day.
Alternatively, many individuals have installed small-scale PV arrays for domestic consumption. Some, particularly in isolated areas, are totally disconnected from the main power grid, and rely on a surplus of generation capacity combined with batteries and/or a fossil fuel generator to cover periods when the cells are not operating. Others in more settled areas remain connected to the grid, using the grid to obtain electricity when solar cells are not providing power, and selling their surplus back to the grid. This works reasonably well in many climates, as the peak time for energy consumption is on hot, sunny days where air conditioners are running and solar cells produce their maximum power output. Many U.S. states have passed "net metering" laws, requiring electrical utilities to buy the locally-generated electricity for price comparable to that sold to the household. Photovoltaic generation is still considerably more expensive for the consumer than grid electricity unless the usage site is sufficiently isolated, in which case photovoltaics become the less expensive.
Centralization and decentralization
Frequently renewable electricity sources are disadvantaged by regulation of the electricity supply industry which favors 'traditional' large-scale generators over smaller-scale and more distributed generating sources.
Solar thermal electric energy
The second method for utilizing solar energy is solar thermal. A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the resulting heat then being used to drive turbines. The concentrator is usually a long mirrored parabolic trough oriented north-south, which tilts, tracking the Sun's path through the day. A black absorber tube is located at the focal point and converts the solar radiation to heat (about 400°C) which is transferred into a fluid such as synthetic oil. The oil can be used to heat buildings or water, or it can be used to drive a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which ensures full-time energy output. The gas generates about a quarter of the overall power output and keeps the system warm overnight. Over 800 MWe capacity worldwide has supplied about 80% of the total solar electricity to the mid-1990s.
One proposal for a solar electrical plant is the solar tower, in which a large area of land would be covered by a greenhouse made of something as simple as transparent foil, with a tall lightweight tower in the center, which could also be composed largely of foil. The heated air would rush to and up the center tower, spinning a turbine. A system of water pipes placed throughout the greenhouse would allow the capture of excess thermal energy, to be released throughout the night and thus providing 24-hour power production. A 200 MWe tower is proposed near Mildura, Australia.
Solar thermal energy
Solar energy need not be converted to electricity for use. Many of the world's energy needs are simply for heat; space heating, water heating, process water heating, oven heating, and so forth. The main role of solar energy in the future may be that of direct heating. Much of society's energy need is for heat below 60°C (140°F) - e.g. in hot water systems. A lot more, particularly in industry, is for heat in the range 60 - 110°C. Together these may account for a significant proportion of primary energy use in industrialized nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.
Solar water heating
Domestic solar hot water systems were once common in Florida until they were displaced by highly-advertised natural gas. Such systems are today common in the hotter areas of Australia, and simply consist of a network of dark-colored pipes running beneath a window of heat-trapping glass. They typically have a backup electric or gas heating unit for cloudy days. Such systems can actually be justified purely on economic grounds, particularly in some remoter areas of Australia where electricity is expensive.
Solar heat pumps
With adequate insulation, heat pumps utilizing the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than energy needed to run a compressor. Eventually, up to ten percent of the total primary energy need in industrialized countries may be supplied by direct solar thermal techniques, and to some extent this will substitute for base-load electrical energy.
Large scale solar thermal power plants, as mentioned before, can be used to heat buildings, but on a smaller scale solar ovens can be used on sunny days. Such an oven or solar furnace uses mirrors or a large lens to focus the Sun's rays onto a baking tray or black pot which heats up as it would in a standard oven.
Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Generator units of more than 1 MWe are now functioning in several countries. The power output is a function of the cube of the wind speed, so such turbines require a wind in the range 3 to 25 m/s (11 - 90 km/h), and in practice relatively few land areas have significant prevailing winds. Like solar, wind power requires alternative power sources to cope with calmer periods.
There are now many thousands of wind turbines operating in various parts of the world, with utility companies having a total capacity of over 39,000 MWe of which Europe accounts for 75% (ultimo 2003). Additional wind power is generated by private windmills both on-grid and off-grid. Germany is the leading producer of wind generated electricity with over 14,600 MWe in 2003. In 2003 the U.S.A. produced over 6,300 MWe of wind energy, second only to Germany.
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. Denmark generates over 10% of its electricity with wind turbines, whereas wind turbines account for 0.4% of the total electricity production on a global scale (ultimo 2002). The most economical and practical size of commercial wind turbines seems to be around 600 kWe to 1 MWe, grouped into large wind farms. Most turbines operate at about 25% load factor over the course of a year, but some reach 35%.
Bird kills and migratory disruption
Nothing comes without a price, and along with the growth of large-scale on- and off-shore wind farms, problems have been identified of wind turbines killing birds or interfering with their large-scale migratory routes.
It is hoped that, armed with this knowledge, planners of present and future wind farm projects will research and avoid important avian migratory routes. Designers of turbines can try to make them less lethal, for example by reducing the speed of the blades and by increasing their visability to birds.
Geothermal electricity is created by hot gases vented from the fissures in the earth's crust. A wheel is turned by the pressure of the gases. The wheel turns the dynamo on the generator, which makes electricity.
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 have potential in certain parts of the world such as New Zealand, United States, 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 MWe geothermal power and heated 86% of all houses in the year 2000. Some 8000 MWe of capacity is operating over all.
There are also prospects in certain other areas for pumping water underground to very hot regions of the Earth's crust and using the steam thus produced for electricity generation. An Australian startup company, Geodynamics, is currently using this technology in a commercial plant in the Cooper Basin region of South Australia (2004).
Energy inherent in water can be harnessed and used, in the forms of kinetic energy or temperature differences.
This type of energy harnesses what happens to water when it is pumped through tiny channels. See electrokinetics (water).
Hydroelectric energy produces essentially no carbon dioxide, in contrast to burning fossil fuels or gas, and so is not a significant contributor to global warming. Hydroelectric power from potential energy of rivers, now supplies about 715,000 MWe or 19% of world electricity. Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it is so readily stopped and started. It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations.
The chief advantage of hydrosystems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilization of stored water is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.
Harnessing the tides in a bay or estuary has been achieved in France (since 1966), Canada and Russia, and could be achieved in certain other areas where there is a large tidal range. The trapped water can be used to turn turbines as it is released through the tidal barrage in either direction. Worldwide this technology appears to have little potential, largely due to environmental constraints. See: tidal power.
Tidal stream power
A relatively new technology development, tidal stream generators draw energy from underwater currents in much the same way that wind generators are powered by the wind. The much higher density of water means that there is the potential for a single generator to provide significant levels of power. Tidal stream technology is at the very early stages of development though and will require significantly more research before it becomes a significant contributor to electrical generation needs.
Several prototypes have however shown some promise. For example, in the UK in 2003, a 300kW Seaflow marine current propeller type turbine was tested off the north coast of Devon, and a 150kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Fransisco Bay.
The Canadian company Blue Energy has plans for installing very large arrays tidal current devices mounted in what they call a 'tidal fence' in various locations around the world, based on a vertical axis turbine design.
Harnessing power from wave motion is a possibility which might yield much more energy than tides. The feasibility of this has been investigated, particularly in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity for delivery to shore. Numerous practical problems have frustrated progress.
A 100-400 kW prototype shore based wave power generator is being constructed at Port Kembla in Australia, due for completion in January, 2005. The energy of waves crashing against the shore is absorbed by an air driven generator and converted to electricity. For countries with large coastlines and rough sea conditions the energy density of breaking waves offers the possibility of generating electricity in utility volumes. Excess capacity in periods of rough sea could be used to generate renewable Hydrogen.
Ocean thermal energy conversion
Ocean thermal energy conversion is a relatively unproven technology, though it was first used by the French engineer Jacques Arsene d'Arsonval in 1881. The difference in temperature between water near the surface and deeper water can be as much as 20°C. The warm water is used to make a liquid such as ammonia evaporate, causing it to expand. The expanding gas forces its way through turbines, after which it is condensed using the colder water and the cycle can begin again.
Deep lake water cooling
Deep lake water cooling is the use of cold water piped from a lake bottom and used for cooling. Energy measures work or heat exchange; although this technology doesn't generate energy that can do work, water-cooling is a form of heat exchange. That is, this technology is an efficient, renewable substitute for expensive air conditioning which requires expensive, peak demand electrical generation which, typically uses fossil fuels. Like geothermal energy and unlike many other forms of renewable energy, water-cooling taps a reliable supply because lake-bottom water is a year-round constant 4 °C.
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 byproduct of sugar cane cultivation) can be burned in internal combustion engines or boilers.
Liquid biofuel (biodiesel or bioalcohol)
Liquid biofuel is usually bioalcohols -like methanol and ethanol- or 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.
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. Plants partly use photosynthesis to store solar energy, water and CO2. Sugar cane residue, wheat chaff, corn cobs and other plant matter can be, and is, burnt quite successfully. The process releases no net CO2.
Animal feces (manure) release methane under the influence of anaerobic bacteria which can also be used to generate electricity. See biogas.
Renewable energy storage systems
One of the great problems with renewable energy, as mentioned above, is transporting it in time or space. Since most renewable energy sources are periodic, storage for off-generation times is important, and storage for powering transportation is also a critical issue.
Hydrogen fuel cells
Hydrogen as a fuel has been touted lately as a solution in our energy dilemmas. However, the idea that hydrogen is a renewable energy source is a misunderstanding. Hydrogen is not an energy source, but a portable energy storage method, because it must be manufactured by other energy sources in order to be used. However, as a storage medium, it may be a significant factor in using renewable energies. It is widely seen as a possible fuel for hydrogen cars, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, in the same way the human body burns fuel. Making hydrogen requires either reforming natural gas (methane) with steam, or, for a renewable and more ecologic source, the electrolysis of water into hydrogen and oxygen. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.
With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilized fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.
Nuclear advocates note that using nuclear power to manufacture hydrogen would help solve plant inefficiencies. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean far better efficiency for the nuclear power plants.
About 50 kWh (1.8 MJ) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial. ( At $0.10/kWh this means hydrogen costs $5 a kilogram for the electricity, equivalent to $5 a US gallon for gasoline if you use in a Fuel Cell vehicle, plus electrolyzer plant costs which will not be small.)
Other renewable energy storage systems
Sun, wind, tides and waves cannot be controlled to provide directly either reliably continuous base-load power, because of their periodic natures, or peak-load power when it is needed. In practical terms, without proper energy storage methods these sources are therefore limited to some twenty percent of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for fossil fuels or nuclear power, however important they may become in particular areas with favorable conditions. If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying base-load energy demand would be much greater.
Pumped water storage
Already in some places pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours this water can be used for hydroelectric generation. However, relatively few places have the scope for pumped-storage dams close to where the power is needed.
Many "off-the-grid" domestic systems rely on battery storage, but means of storing large amounts of electricity as such in giant batteries or by other means have not yet been put to general use. Batteries are generally expensive, have maintenance problems, and have limited lifespans. One possible technology for large-scale storage are large-scale flow batteries. NAS batteries could also be inexpensive to implement on a large scale and have been used for grid storage in Japan.
Electrical grid storage
One of the most important storage methods advocated by the renewable energy community is to rethink the whole way that we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Solar electric generation is a daylight process, whereas most homes have their peak energy requirements at night. Domestic solar generation can thus feed electricity into the grid during grid peaking times during the day, and domestic systems can then draw power from the grid during the night when overall grid loads are down. This results in using the power grid as a domestic energy storage system, and relies on 'net metering', where electrical companies can only charge for the amount of electricity used in the home that is in excess of the electricity generated and fed back into the grid. Many states now have net metering laws.
Today's peak-load equipment could also be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plants more effectively so that the economic value of that wind contribution is greatly increased.
Simple physics is the basis of this storage method. A heavy rotating disc is accelerated by an electric engine which acts as a generator on reversal, slowing down the disc and producing electricity. Electricity is stored as the kinetic energy of the disc. Friction must be kept to a minimum to prolong the storage time. This is achieved by placing the flywheel in a vacuum and using magnetic bearings, making the method expensive. Larger flywheel speeds allow greater storage capacity but require ultra strong materials such as carbon nanotubes to resist the centrifugal forces (or rather, to provide centripetal forces).
Compressed air storage
Another method is to use excess electricity to compress air, which is usually stored in an old mine or some other kind of geological feature. And when electricity demand is high, use the compressed air to run an engine and generate electricity. Projects of this type have been tried fairly successfully in Alabama and in Germany.
Ton petroleum equivalent
A parameter used in renewable energy is the ton petroleum equivalent (TPE), this is, 10,800 Mcal, this is, 45,217.44 M joules.
- Genome News Network (GNN) Energy News (http://www.GenomeNewsNetwork.org/categories/index/energy.php) Collection of articles about how advances in genomics is leading to advances in energy production.
- Centre for alternative energy(European) (http://www.cat.org.uk/)
- Carbon Activism for Beginners (http://www.activistmagazine.com/index.php?option=content&task=view&id=120)
- Renewable Energy Media Analysis (http://www.ecoresearch.net/election2004/report/sentence?s=1) — US Election 2004 Web Monitor
- U.S. Energy Information Administration (http://eia.doe.gov/) 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.