Modern renewable energy technologies

Use of several types of renewable energy, including the following, is boosted by new technologies:

• 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 internal combustion engines can stimulate piezo electric crystals.
• Some watches are already powered by movement of the arm.
• Special antenna can collect energy from stray radio waves or even light (Electromagnetic radiation).

Renewables as solar energy

Most renewable energy sources can trace their roots to solar energy, with the exception of geothermal energy|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.

Image:Solar_panels_on_yacht_at_sea.jpg|thumb|right|300px|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 United States|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 Battery (electricity)|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.

Solar ovens

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 energy

Wind generator|Wind turbines have been used for household electricity generation in conjunction with battery (electricity)|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 energy

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 National Park|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).

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), 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.

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.

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 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 mechanical work| 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 good|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 °Celsius|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 maize|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|CO₂. Sugar cane residue, wheat chaff, maize|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.