Introduction:
Renewable
energy is generally defined as energy that comes from resources which are
naturally replenished on a human timescale such as sunlight, wind, rain, tides,
waves and geothermal heat. Renewable energy replaces conventional fuels in four
distinct areas: electricity generation, hot water/space heating, motor fuels,
and rural (off-grid) energy services.
About 16%
of global final energy consumption presently comes from renewable resources,
with 10% of all energy from traditional biomass, mainly used for heating, and
3.4% from hydroelectricity. New renewables (small hydro, modern biomass, wind,
solar, geothermal, and biofuels) account for another 3% and are growing
rapidly. At the national level, at least 30 nations around the world already
have renewable energy contributing more than 20% of energy supply. National
renewable energy markets are projected to continue to grow strongly in the
coming decade and beyond.[6] Wind power, for example, is growing at the rate of
30% annually, with a worldwide installed capacity of 282,482 megawatts (MW) at
the end of 2012.
Renewable
energy resources exist over wide geographical areas, in contrast to other
energy sources, which are concentrated in a limited number of countries. Rapid
deployment of renewable energy and energy efficiency is resulting in
significant energy security, climate change mitigation, and economic benefits. In
international public opinion surveys there is strong support for promoting renewable
sources such as solar power and wind power.
While many
renewable energy projects are large-scale, renewable technologies are also
suited to rural and remote areas and developing countries, where energy is
often crucial in human development. United Nations' Secretary-General Ban
Ki-moon has said that renewable energy has the ability to lift the poorest
nations to new levels of prosperity.
Overview
Renewable energy flows involve natural
phenomena such as sunlight, wind, tides, plant growth, and geothermal heat, as
the International Energy Agency explains:
Renewable
energy is derived from natural processes that are replenished constantly. In
its various forms, it derives directly from the sun, or from heat generated
deep within the earth. Included in the definition is electricity and heat
generated from solar, wind, ocean, hydropower, biomass, geothermal resources,
and biofuels and hydrogen derived from renewable resources.
Wind power
is growing at the rate of 30% annually, with a worldwide installed capacity of
282,482 megawatts (MW) at the end of 2012, and is widely used in Europe, Asia,
and the United States. At the end of 2012 the photovoltaic (PV) capacity
worldwide was 100,000 MW, and PV power stations are popular in Germany and
Italy. Solar thermal power stations operate in the USA and Spain, and the
largest of these is the 354 MW SEGS power plant in the Mojave Desert. The
world's largest geothermal power installation is The Geysers in California,
with a rated capacity of 750 MW. Brazil has one of the largest renewable energy
programs 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.
As of 2011,
small solar PV systems provide electricity to a few million households, and
micro-hydro configured into mini-grids serves many more. Over 44 million
households use biogas made in household-scale digesters for lighting and/or
cooking and more than 166 million households rely on a new generation of
more-efficient biomass cook stoves. United Nations' Secretary-General Ban
Ki-moon has said that renewable energy has the ability to lift the poorest
nations to new levels of prosperity.
Renewable
energy resources and significant opportunities for energy efficiency exist over
wide geographical areas, in contrast to other energy sources, which are
concentrated in a limited number of countries. Rapid deployment of renewable
energy and energy efficiency, and technological diversification of energy
sources, would result in significant energy security and economic benefits.
Renewable
energy replaces conventional fuels in four distinct areas:
electricity
generation, hot water/space heating, motor fuels, and rural (off-grid) energy
services:
Power generation. Renewable
energy provides 19% of electricity generation worldwide. Renewable power
generators are spread across many countries, and wind power alone already
provides a significant share of electricity in some areas: for example, 14% in
the U.S. state of Iowa, 40% in the northern German state of Schleswig-Holstein,
and 49% in Denmark. Some countries get most of their power from renewables,
including Iceland (100%), Norway (98%), Brazil (86%), Austria (62%), New Zealand
(65%), and Sweden (54%).
Heating.
Solar hot water makes an important contribution to renewable heat in many
countries, most notably in China, which now has 70% of the global total (180
GWth). Most of these systems are installed on multi-family apartment buildings
and meet a portion of the hot water needs of an estimated 50–60 million
households in China. Worldwide, total installed solar water heating systems
meet a portion of the water heating needs of over 70 million households. The
use of biomass for heating continues to grow as well. In Sweden, national use
of biomass energy has surpassed that of oil. Direct geothermal for heating is
also growing rapidly.
Transport fuels. Renewable
biofuels have contributed to a significant decline in oil consumption in the
United States since 2006.The 93 billion liters of biofuels produced worldwide
in 2009 displaced the equivalent of an estimated 68 billion liters of gasoline,
equal to about 5% of world gasoline production.
History
Prior to
the development of coal in the mid 19th century, nearly all energy used was
renewable. Almost without a doubt the oldest known use of renewable energy, in
the form of traditional biomass to fuel fires, dates from 790,000 years ago.
Use of biomass for fire did not become commonplace until many hundreds of thousands
of years later, sometime between 200,000 and 400,000 years ago.
Probably
the second oldest usage of renewable energy is harnessing the wind in order to
drive ships over water. This practice can be traced back some 7000 years, to
ships on the Nile.
Moving into
the time of recorded history, the primary sources of traditional renewable
energy were human labor, animal power, water power, and wind, in grain crushing
windmills, and firewood, a traditional biomass. A graph of energy use in the
United States up until 1900 shows oil and natural gas with about the same
importance in 1900 as wind and solar played in 2010.
By 1873,
concerns of running out of coal prompted experiments with using solar energy. Development
of solar engines continued until the outbreak of World War 1. The importance of
solar energy was recognized in a 1911
Scientific
American article: "in the far distant future, natural fuels having
been exhausted [solar power] will remain as the only means of existence of the
human race".
The theory of
peak oil was published in 1956. In the 1970s environmentalists promoted the
development of renewable energy both as a replacement for the eventual
depletion of oil, as well as for an escape from dependence on oil, and the
first electricity generating wind turbines appeared. Solar had long been used
for heating and cooling, but solar panels were too costly to build solar farms
until 1980.
Mainstream renewable technologies
Wind power
The
Shepherds Flat Wind Farm is a 845 megawatt (MW) wind farm in the U.S. state of
Oregon. Airflows
can be used to run wind turbines. Modern utility-scale wind turbines range from
around 600 kW to 5 MW of rated power, although turbines with rated output of
1.5–3 MW have become the most common for commercial use; the power available
from the wind is a function of the cube of the wind speed, so as wind speed
increases, power output increases dramatically up to the maximum output for the
particular turbine. Areas where winds are stronger and more constant, such as
offshore and high altitude sites are preferred locations for wind farms.
Typical capacity factors are 20-40%, with values at the upper end of the range
in particularly favorable sites.
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,
assuming all practical barriers needed were overcome. This would require wind
turbines to be installed over large areas, particularly in areas of higher wind
resources, such as offshore. As offshore wind speeds average ~90% greater than
that of land, so offshore resources can contribute substantially more energy than
land stationed turbines.
Hydropower
Energy in
water 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 is a term usually reserved for
large-scale hydroelectric dams. The largest of which is the Three Gorges Dam in
China and a smaller example is the Akosombo Dam in Ghana.
·
Micro hydro systems are hydroelectric power
installations that typically produce up to 100 kW of power. They are often used
in water rich areas as a remote-area power supply (RAPS).
·
Run-of-the-river hydroelectricity systems derive
kinetic energy from rivers and oceans without the creation of a large
reservoir.
Hydropower
is produced in 150 countries, with the Asia-Pacific region generating 32
percent of global hydropower in 2010. China is the largest hydroelectricity
producer, with 721 terawatt-hours of production in 2010, representing around 17
percent of domestic electricity use. There are now three hydroelectricity
plants larger than 10 GW: the Three Gorges Dam in China, Itaipu Dam across the
Brazil/Paraguay border, and Guri Dam in Venezuela.
Solar energy
Solar
energy, radiant light and heat from the sun, is harnessed using a range of
ever-evolving technologies such as solar heating, solar photovoltaic, solar
thermal electricity, solar architecture and artificial photosynthesis.
Solar
technologies are broadly characterized as either passive solar or active solar
depending on the way they capture, convert and distribute solar energy. Active
solar techniques include the use of photovoltaic panels and solar thermal
collectors to harness the energy. Passive solar techniques include orienting a
building to the Sun, selecting materials with favorable thermal mass or light
dispersing properties, and designing spaces that naturally circulate air.
Solar power
is the conversion of sunlight into electricity, either directly using
photovoltaic (PV), or indirectly using concentrated solar power (CSP).
Concentrated solar power systems use lenses or mirrors and tracking systems to
focus a large area of sunlight into a small beam. Commercial concentrated solar
power plants were first developed in the 1980s. Photovoltaic convert light into
electric current using the photoelectric effect. Photovoltaic are an important
and relatively inexpensive source of electrical energy where grid power is
inconvenient, unreasonably expensive to connect, or simply unavailable.
However, as the cost of solar electricity is falling, solar power is also
increasingly being used even in grid-connected situations as a way to feed low-carbon
energy into the grid.
In 2011,
the International Energy Agency said that "the development of affordable,
inexhaustible and clean solar energy technologies will have huge longer-term
benefits. It will increase countries’ energy security through reliance on an
indigenous, inexhaustible and mostly import-independent resource, enhance
sustainability, reduce pollution, lower the costs of mitigating climate change,
and keep fossil fuel prices lower than otherwise. These advantages are global.
Hence the additional costs of the incentives for early deployment should be
considered learning investments; they must be wisely spent and need to be
widely shared".
Biomass
Biomass is
biological material derived from living, or recently living organisms. It most
often refers to plants or plant-derived materials which are specifically called
lignocelluloses biomass. As an energy source, biomass can either be used
directly via combustion to produce heat, or indirectly after converting it to
various forms of biofuels. Conversion of biomass to biofuels can be achieved by
different methods which are broadly classified into: thermal, chemical, and
biochemical methods.
Wood
remains the largest biomass energy source today;[36] examples include forest
residues (such as dead trees, branches and tree stumps), yard clippings, wood
chips and even municipal solid waste. In the second sense, biomass includes
plant or animal matter that can be converted into fibers or other industrial
chemicals, including biofuels. Industrial biomass can be grown from numerous
types of plants, including miscanthus, switch grass, hemp, corn, poplar,
willow, sorghum, sugarcane, bamboo, and a variety of tree species, ranging from
eucalyptus to oil palm (palm oil).
Plant
energy is produced by crops specifically grown for use as fuel that offer high
biomass output per hectare with low input energy. Some examples of these plants
are wheat, which typically yield 7.5–8 tons (tones?) of grain per hectare, and
straw, which typically yield 3.5–5 tons (tones?) per hectare in the UK. The
grain can be used for liquid transportation fuels while the straw can be burned
to produce heat or electricity. Plant biomass can also be degraded from
cellulose to glucose through a series of chemical treatments, and the resulting
sugar can then be used as a first generation biofuels.
Biomass can
be converted to other usable forms of energy like methane gas or transportation
fuels like ethanol and biodiesel. Rotting garbage, and agricultural and human
waste, all release methane gas—also called "landfill gas" or
"biogas." Crops, such as corn and sugar cane, can be fermented to
produce the transportation fuel, ethanol. Biodiesel, another transportation
fuel, can be produced from left-over food products like vegetable oils and
animal fats. Also, biomass to liquids (BTLs) and cellulosic ethanol are still
under research.
There is a
great deal of research involving algal, or algae-derived, biomass due to the
fact that it’s a non-food resource and can be produced at rates 5 to 10 times
those of other types of land-based agriculture, such as corn and soy. Once
harvested, it can be fermented to produce biofuels such as ethanol, butanol,
and methane, as well as biodiesel and hydrogen.
The biomass
used for electricity generation varies by region. Forest by-products, such as
wood residues, are common in the United States. Agricultural waste is common in
Mauritius (sugar cane residue) and Southeast Asia (rice husks). Animal
husbandry residues, such as poultry litter, are common in the UK.
Geothermal energy
Geothermal
energy is from thermal energy generated and stored in the Earth. Thermal energy
is the energy that determines the temperature of matter. Earth's geothermal
energy originates from the original formation of the planet (20%) and from
radioactive decay of minerals (80%). The geothermal gradient, which is the
difference in temperature between the core of the planet and its surface,
drives a continuous conduction of thermal energy in the form of heat from the
core to the surface. The adjective geothermal originates from the Greek roots
geo, meaning earth, and thermos, meaning heat.
The heat
that is used for geothermal energy can be from deep within the Earth, all the
way down to Earth’s core – 4,000 miles (6,400 km) down. At the core, temperatures
may reach over 9,000 °F (5,000 °C). Heat conducts from the core to surrounding
rock. Extremely high temperature and pressure cause some rock to melt, which is
commonly known as magma. Magma convicts upward since it is lighter than the
solid rock. This magma then heats rock and water in the crust, sometimes up to
700 °F (371 °C).
From hot
springs, geothermal energy has been used for bathing since Paleolithic times
and for space heating since ancient Roman times, but it is now better known for
electricity generation.
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