Little bit of Introduction about Solar cell...

Every metal has its own band gap that describes how strong the electrons are bonded to the atoms. For semiconductors, such as silicon, the band gap refers to the energy difference between the valence band and the conduction band. When a negative electron is excited, it leaves behind a void which is called a positive hole. The presence of a missing covalent bond allows the bonded electrons of neighbouring atoms to jump into the hole, leaving another hole behind. Because of this, holes also move through the lattice. When photons are absorbed in the semiconductor, it can be said they create mobile electron-hole pairs.

The History Highlight of Solar Sells (Photovoltaic Cells)

1839 – The photovoltaic effect was discovered by Alexandre-Edmond Becquerel, who was a French physicist. This was “the beginning” of the solar cell technology. Becquerel's experiment was done by illuminating two electrodes with different types of light. The electrodes were coated by light sensitive materials, AgCl or AgBr, and carried out in a black box surrounded by an acid solution. The electricity increased when the light intensity increased.

1873 – The photo conductivity of an element, selenium, was discovered by Willoughby Smith, who was an English electrical engineer.

1876 – Selenium produces electrical current when it is exposed to sun light. William Grylls Adams and Richard Evans Day proved that it is possible to convert solar energy into electricity directly, without any moving parts or heat. The solar cell was very inefficient, and it couldn't be used to run any electrical equipment.

1883 – A description of the first solar cells made from selenium wafer were made by Charles Fritts.

1894 – Charles Fritts constructed what was probably the first true solar cell. He coated a semiconductor material (selenium) with an extremely thin layer of gold. The efficiency were only about 1%, so it couldn't be used as energy supply, but were later used as light sensors.

1904 – A German physicist, Wilhelm Ludwig Franz Hallwachs, discovered that a combination of copper and cuprous oxide was photosensitive.

1905 – Albert Einstein published his paper about the photoelectric effect. There he claimed that light consists of “packets” or quanta of energy, which we now call photons. This energy varies only with its frequency (electromagnetic waves, or the “color of the light”). This theory was very simple, but revolutionary, and it explained very well the absorption of the photons regarding to the frequency of the light.

1914 – Goldman and Brodsky noted that it existed a barrier layer in photovoltaic devices.

1916 – Robert Andrews Millikan provided experimental proof of the photoelectric effect. He was an American experimental physicist who later won the Nobel Prize for his work on the photoelectric effect and for his measurement of the charge of the electron.

1918 – Jan Czochralski, a Polish chemist, developed a way to grow single-crystal silicon. This increased the efficiency of the silicon-based cells considerably.

1923 – Albert Einstein received the Nobel Prize for his theories explaining the photoelectric effect, which he published 18 years earlier.

1930s – Walter Schottky, Neville Mott and some others developed a theory of metal-semiconductor barrier layers.

1932 – Audobert and Stora discover the photovoltaic effect in cadmium sulfide (CdS).

1950s – Bell Labs produce solar cells for space activities.

1951 – A grown p-n junction enabled the production of a single-crystal cell of germanium.

1953 – Dr. Dan Trivich of Wayne State University makes the first theoretical calculations of the efficiencies of various materials of different band-gap widths based on the spectrum of the sun

1954 – Three researchers, Gerald Pearson, Daryl Chapin and Calvin Fuller, at Bell Laboratories discovered a silicon solar cell, which was the first material to directly convert enough sunlight into electricity to run electrical devices. The efficiency of the silicon solar cell, which Bell Labs produced, were 4%, which later increased to 11%. The cells were made by hand and cost $1000 per watt.

1954 – A cadmium sulphide p-n junction was produced with an efficiency of 6%

1958 – Hoffman Electronics achieved 9% efficient PV cells.

1958 – The first PV-powered satellite, Vanguard I, was launched. The solar panel had an area of 100cm² and delivered an effect of approximately 0.1W. The satellite power system operated for 8 years, and is the world's oldest satellite still in orbit (2007).

1958 – Ted Mandelkorn of U.S. Signal Corps Laboratories fabricates n-on-p (negative layer on positive layer) silicon photovoltaic cells,

1959 – Hoffman Electronics achieved 10% efficient commercially available PV cells and demonstrated the use of a grid contact to significantly reduce series resistance.

1959 – Explorer-6 was launched with a PV array of 9600 cells, each only 1 cm x 2 cm.

1960 – Hoffman Electronics achieved 14% efficient PV cells.

1962 – The Telstar communications satellite, launched by Bell Labs, is initial powered (14W) by solar cells.

1963 – A Japanese electronics manufacturer, Sharp Corporation, produces a viable photovoltaic module of silicon solar cells.

1970 – First highly effective GaAs heterostructure solar cells are created by Zhores Alferov (a Russian physicist) and his team in the USSR.

1972 – The Institute of Energy Conversion is established at the University of Delaware to perform research and development on thin-film photovoltaic and solar thermal systems, becoming the world’s first laboratory dedicated to photovoltaic research and development.

1976 – David Carlson and Christopher Wronski of RCA Laboratories produced the first amorphous silicon photovoltaic cells, which could be less expensive to manufacture than crystalline silicon devices. The efficiency was of 1.1%.

1980 – At the University of Delaware, the first thin-film solar cell exceeds 10% efficiency. It's made of copper sulfide (Cu2S) and cadmium sulfide (CdS).

1981 – Paul MacCready builds the first solar-powered aircraft, the Solar Challenger, and flies it from France to England across the English Channel. The aircraft had over 16,000 solar cells mounted on its wings, which produced a power of 3kW.

1982 – Hans Tholstrup, an Australian, drives the first solar-powered car, the Quiet Achiever, 4,000km between Sydney and Perth in 20 days. That was 10 days faster than the first gasoline-powered car to do so. The maximum speed was 72 km/h, and the average speed was 24 km/h.

1984 – The IEEE Morris N. Liebmann Memorial Award was presented to Drs. David E. Carlson and Christopher R. Wronski at the 17th Photovoltaic Specialists Conference, "for crucial contributions to the use of amorphous silicon in low-cost, high-performance photovoltaic solar cells."

1985 – The University of South Wales breaks the 20% efficiency barrier for silicon solar cells under one sun conditions.

1989 – Reflective solar concentrators are first used with solar cells.

1991 – Efficient Photo electrochemical cells (PEC) are developed. Each cell consists of a semiconducting photo anode and a metal cathode immersed in an electrolyte. The Dye-sensitized solar cell (DSC), also called Grätzel cells, is invented. It was a new class of low-class DSC.

1992 – University of South Florida develops a 15.9% efficient thin-film photovoltaic cell made of cadmium telluride, breaking the 15% barrier for the first time for this technology.

1994 – The National Renewable Energy Laboratory develops a solar cell, made from gallium indium phosphide and gallium arsenide that becomes the first one to exceed 30% conversion efficiency.

1996 – Renewable Energy Corporation (REC), a Norwegian solar energy company established.

1996 – EPFL, the Swiss Federal Institute of Technology in Lausanne, achieves 11% efficiency with the DSCs.

1999 – Spectrolab, Inc. and the National Renewable Energy Laboratory develop a photovoltaic solar cell that converts 32.3 percent of the sunlight that hits it into electricity. The high conversion efficiency was achieved by combining three layers of photovoltaic materials into a single solar cell. The cell performed most efficiently when it received sunlight concentrated to 50 times normal. To use such cells in practical applications, the cell is mounted in a device that uses lenses or mirrors to concentrate sunlight onto the cell. Such “concentrator” systems are mounted on tracking systems that keep them pointed toward the sun.

1999 – The National Renewable Energy Laboratory achieves a new efficiency record for thin-film photovoltaic solar cells. The new measurement is of 18.8 percent efficiency.

2000 – Two new thin-film solar modules, developed by BP Solarex, break previous performance records. The company’s 0.5-square-meter module achieves 10.8 % conversion efficiency—the highest in the world for thin-film modules of its kind. And its 0.9-square-meter module achieved 10.6% conversion efficiency and a power output of 91.5 watts — the highest power output for any thin-film module in the world.

2001 – TerraSun LLC developes a method of using holographic films to concentrate sunlight onto a solar cell

2003 – REC Solar started production.

2007 – The University of Delaware achieves a 42.8% efficiency solar cell technology.

The photovoltaic effect

Photovoltaic is a term in solar technology that describe a solar cells ability to convert light from the sun directly into electric power. When photons in the sun light collide with the silicon solar cell, one of three things can happen:

• The photon can be reflected at the surface of the silicon
• The photon can be absorbed by the silicon
• The photon can pass right through the silicon

As the photons hits the atoms in the silicon, the energy is absorbed by the electrons and excited into a higher state of energy. When these free electrons flow through the material, electricity arises.

Notice this is the case for semiconductors in general. Silicon does not have three valence and conduction bands.

The holes move to the negative layer of the cell, and the negative excited electrons move to the positive layer. This will be described by the p-n junction technique later. When placing a circuit between the two layers, a path of continuous flow of electrons is established.

Due to the concept conservation of energy, the excited electrons cannot have greater nor less energy than that of the incident rays from the sun. Photons with less energy than the energy gap will go straight through the semiconductor and no electrons will be excited. Photons with greater energy than the energy gap will be absorbed, but the difference in energy between the photons and the energy gap is converted into heat by lattice vibration.

HOW SOLAR CELLS WORK

A solar cell, sometimes called a photovoltaic cell, is a device that converts light energy into electrical energy. A single solar cell creates a very small amount of energy (about .6 volts DC) so they are usually grouped together in an integrated electrical panel called a solar panel. Sunlight is a somewhat diffuse form of energy and only a portion of the light captured by a solar cell is converted into electricity.  The current generation of solar cells converts only 12 to 15 per cent of the sun's light into electricity.  However in recent years there have been significant advances in their design.  Some new cells on the market now are around 20% efficient and some laboratory prototypes are reaching as high as  30 per cent.  Given this it is likely that their efficiency will continue to improve over time.

Theory Behind Solar Cells

A solar cell is based upon the "photovoltaic effect" discovered in 1839 by Edmund Becquerel, a French physicist.  In his experiments he found that certain materials would produce small amounts of electric current when exposed to sunlight.  Sunlight is made up of packets of energy called photons.  When the photons strike the semi-conductor layer (usually silicon) of a solar cell a portion of the photons are absorbed by the material rather than bouncing off of it or going through the material.  When a photon is absorbed the energy of that photon is transferred to an electron in an atom of the cell causing the electron to escape from its normal position.  This creates, in essence, a hole in the atom.  This hole will attract another electron from a nearby atom now creating yet another whole, which in turn is again filled by an electron from another atom.  This hole filling process is repeated a few zillion times and voila, an electric current is formed.

Structure of a Solar Cell

A typical solar cell is a multi-layered material. Let's review what the layers are:

Cover Glass - this is a clear glass layer that provides outer protection from the elements.

Transparent Adhesive - this holds the glass to the rest of the solar cell.

Anti-reflective Coating - this substance is designed to prevent the light that strikes the cell from bouncing off so that the maximum energy is absorbed into the cell.

Front Contact - transmits the electric current.

N-Type Semiconductor Layer - This is a thin layer of silicon which has been doped with phosphorous.

P-Type Semiconductor Layer - This is a thin layer of silicon which has been doped with boron.

Back Contact - transmits the electric current.

Types of Solar Cells

Because of the extensive research being done on solar energy there are now many types of solar cells on the market.  All of them follow the principles described when it comes to generating an electric current.  However, many different approaches are now used to create the structures in order to reduce the costs of production. These approaches involve a tradeoff between lower manufacturing costs versus lower efficiency in converting sunlight to electricity.  The three most common approaches are summarized below: 

Monocrystalline Silicon - This type of solar cell uses a single layer of silicon for the semi-conductor. In order to produce this type of silicon it must be extremely pure which means it is the most expensive type of solar cell to produce.

Polycrystalline Silicon - To make polycrystalline silicon cells liquid silicon is poured into blocks that are subsequently sawed into plates. This type of approach produces some degree of degradation of the silicon crystals which makes them less efficient.  However, this type of approach is easier and cheaper to manufacture.

Amorphous Thin Film Silicon - This type of solar cell uses layers of semiconductor that are only a few micrometers thick (about 1/100th the thickness of a human hair).  This lower the material cost but makes it even less efficient than the other types of silicon. However, because it is so thin this type of cell has the advantage that it can be placed on a wide variety of flexible materials in order to make things like solar shingles or roof tiles.

Another way of looking at solar cells is in terms of the types of materials they are made with.  While silicon is the most commonly used crystal a number of other materials can be used as well.  These include the following:

gallium arsenide

copper indium diselenide

cadmium telluride

Different types of substances perform better under certain light conditions.  Some cells perform better outdoors (i.e., optimized for sunlight), while others perform better indoors (optimized for fluorescent light).

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A Bit of an Introduction about Geo-Thermal Energy

Introduction of Geo-Thermal Energy

Geothermal energy is energy extracted from heat stored in the earth. This geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar energy absorbed at the surface. It has been used for space heating and bathing since ancient times, but is now known for both heating as well as for generating electricity.

Geothermal power is cost effective, reliable, and environmentally friendly, but has previously been geographically limited to areas near tectonic plate boundaries. Recent technological advances have significantly expanded the range and size of viable resources, especially for direct applications such as home heating.

Geo-thermal Potential

Geothermal energy has shown signs of considerable growth over the last few years. Global geothermal installed capacity (for electricity) has escalated from 7,972 MWe in 2000 to around 9,700 MWe in the year 2007 (generating about 0.3% of global electricity demand) and is expected to reach around 13,600 MWe by 2012.

The US continues to be the world leader in terms of total installed capacity of geothermal energy and the generation of electric power from geothermal energy.

By mid 2008, worldwide installed capacity of geothermal energy for electricity generation had crossed the 10 GW mark. Worldwide, about 30 GW of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications. If heat recovered by ground source heat pumps is included, the non-electric use of geothermal energy is estimated at more than 100 GWt (gigawatts of thermal power) and is used commercially in over 70 countries.

Geothermal (ground-source) heat pumps (GHPs) have become a major growth area of geothermal energy use in the United States, Canada and Europe. The number of GHPs has steadily increased over the past 10 years. By 2008, an estimated 800,000 equivalent 12 kW (3.4 ton) units have been installed in the United States and about 50,000 in Canada.

Geo-thermal Locations

Geothermal energy supplies more than 10,000 MW to 24 countries worldwide and now produces enough electricity to meet the needs of 60 million people. The Philippines, which generates 23% of its electricity from geothermal energy, is the world’s second biggest producer behind the U.S. Geothermal energy has helped developing countries such as Indonesia, the Philippines, Guatemala, Costa Rica, and Mexico. The benefits of geothermal projects can preserve the cleanliness of developing countries seeking energy and economic independence, and it can provide a local source of electricity in remote locations, thus raising the quality of life. 

Iceland is widely considered the success story of the geothermal community. The country of just over 300,000 people is now fully powered by renewable forms of energy, with 17% of electricity and 87% of heating needs provided by geothermal energy. Iceland has been expanding its geothermal power production largely to meet growing industrial and commercial energy demand. In 2004, Iceland was reported to have generated 1465 gigawatt-hours (GWh) from geothermal resources; geothermal production is reached 3000 GWh in 2009. 

According to some experts, the most likely value for the technical potential of geothermal resources suitable for electricity generation is 240 GWe (This is about 5% of total global installed capacity for electricity in 2008). Theoretical considerations, based on the conditions in Iceland and the USA, reveal that the magnitude of hidden resources is expected to be 5-10 times larger than the estimate of identified resources. If this is the case for other parts of the world, the upper limit for electricity generation from geothermal resources is in the range of 1-2 TWe.

Prominent countries worldwide with geothermal potential:

• Russia 
• Japan 
• Eastern China 
• Himalayan Geothermal Belt 
• The Philippines 
• Indonesia 
• New Zealand 
• Canada 
• United States 
• Mexico 
• Central American Volcanic Belt 
• Andean Volcanic Belt 
• The Caribbean 
• Iceland and other Atlantic Islands 
• Northern Europe 
• Eastern Europe 
• Italy 
• Eastern and Southern Mediterranean 
• East Africa Rift System

Geothermal Energy - How it works

There are three main types of geothermal energy in use currently:

• Direct Use Heating Systems – these use hot water from springs or reservoirs near the earth’s surface.
• Electricity from Geothermal Energy – Electricity generation in power plants require water or steam at very high temperature. Geothermal power plants are generally built where geothermal reservoirs are located within a mile or two of the surface. Thus, these plants use the geothermal heat for generating steam that run a turbine to produce electricity.
• Geothermal Heat Pumps – These heat pumps use stable temperatures under the ground to heat and cool buildings.

Applications of Geothermal Energy

Geothermal Electricity Production

Geothermal Electricity: This geothermal power plant generates electricity for the Imperial Valley in California.

This geothermal power plant generates electricity for the Imperial Valley in California. Credit: Warren Gretz

Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found a couple of miles or more below the Earth's surface. There are three types of geothermal power plants: dry steam, flash steam, and binary cycle.

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.

Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Binary cycle power plants operate on water at lower temperatures of about 225°-360°F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas, possibly even as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.

In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

Geothermal Direct Use

Geothermal Direct Use: Geothermally heated waters allow alligators to thrive on a farm in Colorado, where temperatures can drop below freezing.

Geothermally heated waters allow alligators to thrive on a farm in Colorado, where temperatures can drop below freezing. Credit: Warren Gretz

When a person takes a hot bath, the heat from the water will usually warm up the entire bathroom. Geothermal reservoirs of hot water, which are found a couple of miles or more beneath the Earth's surface, can also be used to provide heat directly. This is called the direct use of geothermal energy.

Geothermal direct use dates back thousands of years, when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, hot springs are still used as spas. But there are now more sophisticated ways of using this geothermal resource.

In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system - piping, a heat exchanger, and controls - delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface.

Geothermal hot water can be used for many applications that require heat. Its current uses include heating buildings (either individually or whole towns), raising plants in greenhouses, drying crops, heating water at fish farms, and several industrial processes, such as pasteurizing milk. With some applications, researchers are exploring ways to effectively use the geothermal fluid for generating electricity as well.

In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

Indian Geo-thermal Energy Program

Potential

It has been estimated from geological, geochemical, shallow geophysical and shallow drilling data it is estimated that India has about 10000 MWe of geothermal power potential that can be harnessed for various purposes.^[iv]

Rocks covered on the surface of India ranging in age from more than 4500 million years to the present day and distributed in different geographical units. The rocks comprise of Archean, Proterozoic, the marine and continental Palaeozoic, Mesozoic, Teritary, Quaternary etc., More than 300 hot spring locations have been identified by Geological survey of India (Thussu, 2000). The surface temperature of the hot springs ranges from 35 C to as much as 98 C. These hot springs have been grouped together and termed as different geothermal provinces based on their occurrence in specific geotectonic regions, geological and strutural regions such as occurrence in orogenic belt regions, structural grabens, deep fault zones, active volcanic regions etc., Different orogenic regions are – Himalayan geothermal province, Naga-Lushai geothermal province, Andaman-Nicobar Islands geothermal province and non-orogenic regions are – Cambay graben, Son-Narmada-Tapi graben, west coast, Damodar valley, Mahanadi valley, Godavari valley etc.

Potential Sites:

•  Puga Valley (J&K)
•  Tatapani (Chhattisgarh)
•  Godavari Basin Manikaran (Himachal Pradesh)
•  Bakreshwar (West Bengal)
•  Tuwa (Gujarat)
•  Unai (Maharashtra)
•  Jalgaon (Maharashtra)

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Renewable Energy...An Introduction

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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