AY Honors/Renewable Energy/Answer Key

Renewable energy is energy from a source which can be managed so that it is not subject to depletion in a human timescale . Sources include the sun's rays, wind, waves, rivers, tides, biomass, and geothermal. Renewable energy does not include energy sources which are dependent upon limited resources, such as fossil fuels and nuclear fission power.

General Information

Most renewable forms of energy, other than geothermal, are in fact stored solar energy. Water power and wind power represent very short-term solar storage, while biomass represents slightly longer-term storage, but still on a very human time-scale, and so renewable within that human time-scale. Fossil fuels, on the other hand, while still stored solar energy, have taken millions of years to form, and so do not meet the definition of renewable.

Renewable energy resources may be used directly as energy sources, or used to create other forms of energy for use. Examples of direct use are solar ovens, geothermal heat pumps, and mechanical windmills. Examples of indirect use in creating other energy sources are electricity generation through wind generators or photovoltaic cells, or production of fuels such as ethanol from biomass (see alcohol as a fuel).

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 'powerplants' (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 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 centres 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. 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

Firewood 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 traction

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.

Water power

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

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

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.

Earth power

Geothermal electricity is created by hot gases vented from the fissures in the earth's crust. A wheel, like the kinds used in hydroelectric plants and wind mill generators, is turned by the pressure of the gases. The wheel turns the dynamo on the generator, which makes electricity.

The earth is a giant spinning magnet generator with two poles, one positive and the other negative. Enormous amouts of electricity are created by the earth every second, though diffuse and spread out. Telluric currents are part of this energy. This is why lightning is attracted to the earth. The earth's magnetic field is created by this massive amount of electricity generated by the earth. There is more than enough electricity in the earth to power every household in the world for free and with no byproduct pollution. It would use up much less that 1% of the total electricity that the earth contains. And each time the earth spins, it regenerates all of its electricity.

But how can this immense power be tapped and concentrated for home use? Inventor Nicola Tesla realized the earth's potential. From 1901 to 1917, he built Wardenclyffe Tower, also called the 'Tower of Power'. It's purppose was to tap into the earth's natural electricity through a 300 foot shaft into the earth, hold, concentrate, and broadcast the electricity wireless to households all over the world for free. No one would ever have to pay electric bills again. It couls also transmit radio and phone for free - no more phone bills. His inventions worked, but the project was abandoned because it was too expensive and he ran out of funding. Tesla went on to invent many household electric appliances that people take for granted, included in his 700 patents. Some wonder if a cheap portable non-wireless version of Tesla's tower could be invented for home use.

The renewable energy movement

Renewable energy as an issue was virtually unheard-of before the middle of the twentieth century. There were experimentations with passive solar energy, including daylighting, in the early part of the twentieth century, but little beyond what had actually been practiced as a matter of course in some locales for hundreds of years. The renewable energy movement gained awareness, credence and strength with the great burgeoning of interest in environmental affairs in the mid-1900s, which in turn was largely due to Rachel Carson's 'Silent Spring'.

The first US politician to focus significantly on solar energy was Jimmy Carter, in response to the long term consequences of the 1973 energy crisis. No president since has paid much attention to renewable energy.

Renewable energy today

Around 80% of energy requirements in western industrial societies are focused around heating or cooling buildings and powering the vehicles that ensure mobility (cars, trains, airplanes). However, most uses of renewable power focus on electricity generation.

Geothermal heat pumps (also called ground-source heat pumps) are a means of extracting heat in the winter or cold in the summer from the earth to heat or cool buildings.

Modern sources of renewable energy

There are several types of renewable energy, including the following:

• Solar power.
• Wind power.
• Geothermal energy.
• Electrokinetic energy.
• Hydroelectricity.
• Energy from biomass, also called biomatter energy

Smaller-scale sources

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 radiowaves 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.

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.

System problems with solar electric

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. 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". 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.

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 centre, which could also be composed largely of foil. The heated air would rush to and up the centre 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.

Solar ovens

Large scale solar thermal powerplants, 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 energy

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 windpower 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 windturbines, whereas windturbines 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%.

Geothermal energy

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, proposes to build a commercial plant in the Cooper Basin region of South Australia using this technology by 2004.

Water power

Energy inherent in water can be harnessed and used, in the forms of kinetic energy or temperature differences.

Electrokinetic energy

This type of energy harnesses what happens to water when it is pumped through tiny channels. See electrokinetics (water).

Hydroelectric energy

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.

Tidal power

Harnessing the tides in a bay or estuary has been achieved in France (since 1966) 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.

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.

Wave power

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 electricty in utility volumes. Excess capacity in periods of rough sea could be used to generate renewable Hydrogen.

OTEC

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

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.

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. 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.

Biogas

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. (Could someone explain why this is important? .)( At 10c/kWHr this means Hydrogen costs $5 a kilogram for the electricity, equivelent to $5 a gallon for gasoline if you use in a Fuel Cell vehicle, plus electrolyser plant costs which will not be small.) I think it is really telling me that you can store a lot of electricity in a kg of H2. Perhaps a rough electricity to hydrogen to electricity round trip figure would be more helpful? I'm guessing 60%. ( About 60% each way would be closer, giving 36% for the round trip)

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.

Battery storage

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 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.

Flywheel storage

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.

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.

Renewable energy use by nation

Iceland is a world leader in renewable energy due to its abundant hydro- and geothermal energy sources. Over 99% of the country's electricity is from renewable sources and most of its urban household heating is geothermal. Denmark was the initial leader in wind energy generation and remains the nation which produces the highest per capita levels of electricity production from wind. Germany began to build up its wind capacity in earnest from the mid 1990's with the application of generous subsidies and cheap loans and now has over one third of all the wind generation capacity in the world. Spain was also a latecomer to wind energy generation, but in 2002 overtook the US to become the nation with the second highest level of installed wind energy capacity. Israel is also notable as much of its household hot water is heated by solar means. These countries' successes are at least partly based on their geographical advantages, though it is worth noting that Germany does not have particularly good wind resources (much worse for example than the UK, where policies have led to much less success) and other factors have thus played an important part in its commitment to wind and other renewables.

Leading countries by renewable electricity production, (2000)

	Hydro	Geothermal	Wind	PV Solar
1.	Canada	U.S.	Germany	Japan
2.	U.S.	Philippines	U.S.	Germany
3.	Brazil	Italy	Spain	U.S.
4.	China	Mexico	Denmark	India
5.	Russia	Indonesia	India	Australia

Share of the total power consumption in EU-countries that are renewable.

	1985	1990	1991	1992	1993	1994
EUR-15	5,61	5,13	4,92	5,16	5,28	5,37
Belgium	1,04	1,01	1,01	0,96	0,84	0,80
Denmark	4,48	6,32	6,38	6,80	7,03	6,49
Germany	2,09	2,06	1,61	1,73	1,75	1,79
Greece	8,77	7,14	7,63	7,13	7,33	7,16
Spain	8,83	6,70	6,56	5,73	6,49	6,50
France	7,24	6,34	6,75	7,54	7,32	7,98
Ireland	1,75	1,65	1,68	1,59	1,59	1,63
Italy	5,60	4,64	5,16	5,19	5,34	5,50
Luxembourg	1,28	1,21	1,14	1,26	1,21	1,34
The Netherlands	1,36	1,35	1,35	1,37	1,38	1,43
Austria	24,23	22,81	20,99	23,39	24,23	23,71
Portugal	25,07	17,45	17,03	13,88	15,98	16,61
Finland	18,29	16,71	17,02	18,10	18,48	18,28
Sweden	24,36	24,86	22,98	26,53	27,31	24,04
United Kingdom	0,47	0,49	0,48	0,56	0,54	0,65

Table from [1]

Renewable energy controversies

As with anything, even renewable energy generates controversies.

Lack of motivation for funding

Research and development in renewable energies has been severely hampered in many nations due to the allocation of only a tiny fraction of energy R&D budgets, with conventional energy sources getting the lion's share.

Centralization versus decentralization

Frequently renewable electricity sources will be disadvantaged by regulation of the electricity supply industry which favors 'traditional' large-scale generators over smaller-scale and more distributed generating sources. 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". 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.

The nuclear "renewable" claim

Some nuclear advocates claim that nuclear energy should be regarded as renewable energy.

Arguments often put forward include:

• Nuclear energy does not contribute to global warming.
  • Evaporative cooling has a minor effect by introducing additional water vapor into the atmosphere, along with the heat production of the process. However, both of these are insignificant compared to geothermal events such as volcanoes.
• Fast breeder reactors can produce more fuel than they consume.
  • Breeder reactors consume uranium or thorium to produce fissile fuel, so this particular argument is a misunderstanding of the basic processes involved.
• Uranium and thorium, being radioactive, are not resources that are essential in the long-term in the way that, say, oil is.
• Nuclear waste, since it will eventually become less radioactive than the original ore bodies, is not a long-term problem.

This claim is rejected by most renewable energy advocates, primarily because of concerns over pollution caused by failure to store the waste material correctly, and the dangers involved in operation of plants (see Chernobyl and Sellafield).

That

1. nuclear power uses a depleting resource (uranium or thorium)
2. the half-life of uranium 238 is 4.5 billion years
3. the decay of the waste to a safe level may take three thousand years or longer (depending on the technology used)

are widely accepted arguments that fission power cannot be included in such a classification.

Similar arguments have been applied against proposed nuclear fusion power stations using deuterium as fuel. However, the expected by-products are entirely different to those of nuclear fission, so they need to be re-examined somewhat.

Renewable Energy Support Mechanisms

Different countries and territories have employed a range of different mechanisms to encourage increases in renewable energy capacity within their borders. These have applied across a range of different technologies. Their remains considerable discussion amongst politicians, academics and other actors as to the most appropriate mechanism - or combination of mechanisms - for achieving renewable energy policy goals.

Tariff Mechanisms

In a tariff mechanism, the government fixes a price for every unit of electricity produced from any technology which it classifies as renewable. This price is typically greater than the market price for electricity available in that territory and thus enables generators to operate economically. Different tariff levels may be set for different technologies. A Government may provide the subsidy from its own funds or may compel utility companies to purchase the electricity thus produced, passing the costs on to its consumers. Network supply companies are compelled to accept all electricity from specified technologies. There are a number of advantages and disadvantages with the use of the mechanism.

Advantages

Risk Reduction: Generators receive a fixed price for a fixed period, thus reducing volume and price risk to investors. Generators are also not subject to balancing risk as network companies are compelled to take all electricity. There is also reduced regulatory risk in comparison with other mechanisms in that once a plant is operational it is effectively guaranteed a price for a fixed period into the future.

Dynamic Efficiency: Different technologies develop at different rates. If a government wishes to support a new technology it can provide a tariff specific to that technology and thus encourage it to move closer to market. The balance of evidence suggests that this provides long term benefits in terms of developing more competitive technologies.

Proven Capability: Tariff Mechanisms have been widely applied in Germany, Denmark and Spain. Their employment has led to significant increases in renewable electricity generating capacity, particularly of wind energy.

Disadvantages

Expensive: The fixed price over time means that it is difficult to pass on the benefits of increased technological efficiency to consumers. Instead benefits accrue at the level of the generating plant owner, who may be able to access high rates of return. One possible solution is through degression, that is, lowering the tariff rate over time. Reductions in the tariff must be transparent to ensure investor uncertainty is minimised, and there is no guarantee that reductions will match the actual improvements in the technology.

Unpredictable: Whilst tariff mechanisms fix the price available to renewable energy generators, the level of capacity is subject to the market, that is, there is no way of predicting how many investors will be attracted to generation by the price available. This means it is not possible to predict the overall costs of the mechanism in either the short or long term. This can be unattractive to government and consumers/taxpayers.

Network Balancing: Distribution network operators are compelled to accept all electricity from renewable generators, regardless of the demand for electricity at the time of generation. This can lead to network balancing issues, and these tend to increase with the level of intermittent generation on the network. This leads to increasing potential for technological problems and for increased costs to the network operator.

Market Prioritisation: Compelling network operators to accept all renewable generation means that electricity from renewables ia always the first to be bought. This effectively interferes with any open market for general electricity generation, and impacts on the ability of 'traditional' generators to compete in the electricity sector. This can be problematic where Governments are committed to maximising competition in markets.

Quota Mechanisms

A quota mechanism, also known as a Renewable Portfolio Standard (RPS), sees a government place an obligation on either an electricity supply company or on consumers (albeit usually manifested through their supply company) to source a specified fraction of their electricity from renewable energy sources. Companies which fail to meet the obligation are required to pay a penalty price for every unit of electricity by which they fall short of their obligation. The mechanism acts to create a market for electricity, allowing competition amongst renewable generators to meet the needs of that market. The underlying theory is that competition in this market place will drive down the costs of supplying renewable electricity and thus minimise the costs to the consumer of meeting renewable energy targets.

The market allows a government to set the capacity that is required, and allows the market place to set the cost. The level at which the penalty price is set allows the government to place an upper limit on the total costs to the consumer.

The mechanism is in place in a number of US states as well as the United Kingdom, Italy and Belgium amongst other European countries. In the US quota mechanisms applied at the State level are often assisted by the intermittent application of a federally mandated production tax credit.

Advantages

Generators must compete for contracts, and thus there is pressure to drive down costs within a mechanism that can pass these costs on to the consumer. Theoretically, this means that the costs to the consumer are minimised.

Disadvantages

Increased risk: In practice, generators are vulnerable to considerable risks, including risk relating to volume (there being no guarantee that they will be able to sell all of their electricity), price (as this depends on the market for both electricity and tradable certificates), balancing cost (a function of generator intermittency) and regulation (i.e. the withdrawal of political support effectively ends the market completely). The effect of this is to raise the cost of capital and thus the overall cost of generation, resulting in an increase in the cost to the consumer.

Dynamic efficiency: The mechanism tends to support only those technologies which are close to market when it is introduced. Technologies outside the mechanism are likely to become less and less competitive, and thus never be developed. This may prove to be more costly in the long term. One solution is to provide additional support outside the mechanism, however, this has the disadvantage of raising the overall costs of supporting renewables.

Contract Bidding Mechanisms

Government places an obligation on supply companies to accept electricity from renewable energy generators who have been awarded contracts by government. Generators win these contracts by taking part in a competitive bidding process organised by the government or a specified agent of the government. Historcially, competition has usually takes place within technology 'bands', that is, such that competing bids are only compared between generators employing the same technology. For example, wind generator against wind generator. Essentially, this means there are usually different competitions going on at the same time for each technology. Generally, the lowest bids are awarded contracts, provided they meet any criteria set down by the government as part of the competitive process. Examples of such a mechanism in practice include the UK's Non-Fossil Fuel Obligation (NFFO), Ireland's Alternative Energy Requirement (AER) and the French EOLE.

Production Tax Credits

Production tax credits support the introduction of renewables by allowing companies which invest in renewables to write off this investment against other investments they make. A PTC can be used as the central mechanism for the support of renewables as part of a national or regional mechanism, or it can be used in support of another mechanisms, such as a quota mechanism. Production tax credits have been supplied at the federal level in the US; they have tended to be most effective in States which also provide some other form of support, most notably a quota mechanism.

Advantages

Have proven to stimulate capacity alongside quota mechanisms. They may provide a useful way to bring stability to generators when used in this way, reducing uncertainty and thus capital costs.

Disadvantages

In practice they tend to be be vulnerable to political conflict regarding their maintenance in the long term.

Tax credits tend to restrict investment in renewables to large companies with significant portfolios against which they can write off the tax credits they earn.

See also

• United Kingdom Climate Change Programme

External links

• Genome News Network (GNN) Energy News Collection of articles about how advances in genomics is leading to advances in energy production.
• Centre for alternative energy(European)
• Carbon Activism for Beginners

References

• U.S. Energy Information Administration provides lots of statistics and information on the industry.
• Boyle, G. (ed.), Renewable Energy: Power for a Sustainable Future. Open University, UK, 1996.

cy:Egni cynaliadwy de:Regenerative Energie es:energía renovable fr:énergie renouvelable nl:Duurzame energie [[pl:Odnawialne %BCr%F3d%B3a energii]]