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Renewabl energy is energy generated from natural resources (eg sunlight, wind, rain, tides and geothermal heat) which are ‘renewable’ (ie naturally replenished). Some renewable-energy technologies are criticized for being intermittent or unsightly, yet the renewable-energy market continues to grow and renewable energy supplies are of ever increasing environmental and economic importance in all countries (Twiddell and Weir, 2006).
In 2006, about 18% of global final energy consumption came from renewables (13% from traditional biomass such as wood-burning, and hydroelectricty was the next largest renewable source), providing 3% of global energy consumption and 15% of global electricity generation (REN21, 2008).
A wide range of renewable energy technologies are established commercially and recognised as growth industries by most governments. UN and other international agencies have large programmes to encourage such technologies. Wind power is growing at an annual rate of 30 %, with a worldwide installed capacity of 121,000 MW in 2008 (REN21, 2009) and is widely used in European countries and the USA. The annual manufacturing output of the photovoltaics industry reached 6,900 MW in 2008 (REN21, 2009), and photovoltaic power stations are popular in Germany and Spain). Solar thermal power stations operate in the USA and Spain. Brazil has one of the largest renewable energy programmes in the world, involving production of ethanol fuel from sugar cane, and ethanol now provides 18 % of the country's automotive fuel. Ethanol fuel is also widely available in the USA.
While most renewable energy projects and production is large-scale, renewable technologies are also suited to small off-grid applications, sometimes in rural and remote areas, where energy is often crucial in human development.(ref 9).Kenya has the world's highest household solar ownership rate with roughly 30,000 small (20–100 watt) solar power systems sold per year (UNDP/UNDESA.WEC, 2001).
Climate change concerns, coupled with high oil prices, peak oil, and increasing government support, are driving increasing renewable-energy legislation, incentives and commercialisation (UNEP 2007b). New government spending, regulation and policies should help the industry weather the 2009 economic crisis better than many other sectors (Makower et al., 2009).
Box 4.3: Source of renewable energy
Windpower
Globally, the long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand. This could require large amounts of land to be used for wind turbines, particularly in areas of higher wind resources. Offshore wind speeds tend to be some 90% greater than those on land, so offshore resources could contribute substantially more energy (Archer and Jacobsen 2005). [link to references] This number could also increase with higher altitude ground-based or airborne wind turbines.
Water energy
Energy in water (in the form of kinetic energy, temperature differences or salinity gradients) can be harnessed and used. Since water is about 800 times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy. There are many forms of water energy:
- Hydroelectric energy - a term usually reserved for large-scale hydroelectric dams.
- Micro (hydropower) systems – typically installations delivering <100 kW of power, often used in water-rich areas as a remote area power supply (RAPS).
- Damless hydro systems – do not useg a dam.
- Ocean and sea energy, marine current and tidal power;
- Ocean thermal energy conversion (OTEC) - uses a cyclic heat engine to exploit the temperature difference between the warmer surface of the ocean and the colder lower recesses (not yet been field-tested on a large scale).
- Wave power - use floating or neutrally buoyant structures which move relative to one another or to a fixed point.
- Osmotic power or salinity gradient power - energy retrieved from the difference in the salt concentration between seawater and river water. Reverse electrodialysis (PRO) is in the research and testing phase.
- Vortex power is generated by placing obstacles in rivers in order to cause the formation of vortices which can then be tapped for energy.
Geothermal energy
Energy obtained by tapping the heat of the earth itself - from kilometers deep in some places or from just a few meters via a geothermal heat pump (a central heating and/or cooling system that pumps heat to or from the ground - using the earth as a heat source in winter or a heat sink in summer). Geothermal power station are expensive to build, but operating costs are low. Three types of power plants are used to generate power from geothermal energy:
- Dry steam plants take steam out of fractures in the ground and use it to directly drive a turbine;
- Flash plants take hot water, usually at temperatures over 200 °C, out of the ground, allow it to boil as it rises to the surface, and then separate the steam to run through a turbine;
- Binary plants allow the hot water to flow through heat exchangers, boiling an organic fluid that spins the turbine.
The condensed steam and remaining geothermal fluid from all three types of plants are injected back into the hot rock to pick up more heat.
New technology is being explored to generate geothermal energy from underground, hot, radiogenic, granites rocks. Holes at least 3 km deep are drilled into the earth. Some of these holes pump water into the earth, while other holes pump hot water out.
Solar energy
Solar energy usually refers to energy collected from sunlight. It can be applied in many ways, including to generate electricity by using: photovoltaic solar cells; systems that concentrate solar power (eg mirrors, lenses); heating trapped air (which rotates turbines in a solar updraft tower); generating hydrogen using photoelectrochemical cells; heating water or air for domestic hot water and space heating needs using solar-thermal panels; heating buildings, directly, through passive solar building design; and solar air conditioning.
Biofuel
Biofuel (in solid, liquid or gaseous form) is obtained from living, dead or plant-derived materials. It is dereived from the biomatter produced by plants through photosynthesis. Agriculturally produced biomass fuels (agrofuels – produced from particular crops), such as biodiesel, ethanol, and bagasse (often a by-product of sugar cane cultivation), can be burned in internal combustion engines or boilers. Finding more efficient methods of converting biofuels and other fuels into electricity utilizing fuel cells is an area of active research. Biofuel has the added advantage of biosequestration of atmospheric C02 to remediate greenhouse gas and climate change problems. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking. Biofuel industries are expanding in Europe, Asia and America.
Solid biomass
Solid biomass is most commonly used directly as a combustible fuel. It includes, for example, wood fuel (used for millennia), coal, the biogenic portion of municipal solid waste, the unused portion of field crops (eg wheat chaff, corn cobs).
Biogas
Biogas can easily be produced from current waste streams, such as paper production, sugar production, sewage, animal waste (eg cow dung), etc. These are slurried together and allowed to naturally ferment, producing methane gas. Sewage plants can also be converted into biogas plants. After extraction of methane the residues can be used as fertilizer. Biogas can also be produced via advanced waste processing systems such as mechanical biological treatment which recover the recyclable elements of household waste and process the biodegradable fraction in anaerobic digesters. Renwable natural gas is a biogas which has been upgraded to a quality similar to natural gas., making it possible to distribute it to the mass market via existing gas grids |
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