Solar energy
Nature photosynthesis.
Fossil fuels.
Coal.
Petroleum and natural gas.
Global warming.
Renewable energy.
Global energy demand.
Trillion watt or TW.
Clean energy.
Solar energy capacity.
Photosynthesis.
Passive solar energy.
Photodetectors.
Photoelectric effect.
Photovoltaic panel.
Einstein-photon.
Silicon wafers.
Photovoltaic cell.
First viable photovoltaic cell.
Solar panel cost.
Insolation.
Tilting and tracking.
Shading.
Potential capacity.
Efficiency.
Multiple layers.
Recombination.
Gallium arsenide.
Competing technologies.
O LEDs, TFT.
Band gap energy.
ETA cells.
More than one approach.
Solar-way to go forward.
Nature already uses solar energy as a renewable resource.
Photosynthesis is the means by which nature captures solar energy.
Photosynthesis captures 90 terawatts, or 90 TW of energy per year .
This is 6 times the current power consumption of all human beings.
Photosynthesis is the source of carbon in all organic compounds, within every living organisms.
It converts 115 billion tons of carbon into biomass every year.
Photosynthetic organisms are photoautotrophs.
Photoautotrophs are capable of synthesising their own food, from inorganic substances,
using light as source of energy.
Green plants and photosynthetic bacteria are photoautotrophs.
Green plants are repositories of energy.
It can be consumed as food.
What is not consumed as food is often preserved long after the plant has died.
When we burn a log of wood, it is this repository of stored energy, that we are tapping into.
The same is true when we burn a lump of coal.
The energy in a lump of coal is much older.
Coal, petroleum and natural gas are fossil fuels.
These fuels are the fossil remains of dead plants and animals.
The stores of cellular energy, built up while they were alive, remain intact as fossils fuels today.
These organisms typically expired millions of years ago.
In some cases it could be more than 650 million years ago.
Because they were once living organisms, fossil fuels contain high percentages of carbon.
They were formed from the remains of organisms, fossilised by exposure over millions of years,
to heat and pressure with in the Earth's crust.
Coal is the most abundant of fossil fuels.
Trees that lived 300 million years ago during the carboniferous period,
formed dense forests in low lying wetlands, which covered most of the planet.
The wide shallow seas which existed during this period provided the ideal conditions for coal to form.
Flooding caused these forest to get buried under the soil.
A combination of mud and acidic water prevented decomposition.
The trees are covered with increasing layers of soil, sinking deeper and deeper into the Earth.
Over a period of time all the carbon was trapped in immense peat bogs.
This was further buried by further sediments.
Over millions of years, the pressure and temperature increased.
The vegetation slowly became converted to coal, in a process called carbonisation.
Nature took million of years to cook the coal that we have now.
If we use it up, we have to wait for millions of more years, for more coal.
Petroleum and natural gas are formed by remains of algae and zooplankton that lived in prehistoric oceans.
When these creatures died, their bodies settled in large quantities at the bottom of seas and lakes.
Oxygen was scarce there.
This lack of oxygen prevented normal decomposition from taking place.
This preserved the organic matter, which like coal, was buried under layers of sediment.
Over millions of years heat and pressure built up, causing a chemical transformation into a waxy material called kerogen.
This was subjected to further heat and pressure, which turned it into liquid and gaseous hydrocarbons.
This process is called catagenesis.
We call these hydrocarbon products, as petroleum and natural gas.
What was cooked by nature over millions of years, will only last another century or so.
It is glaring obvious that we need to find new sources of energy, for our existence.
Fossil fuels pose an additional danger.
Burning fossil fuels releases carbon dioxide.
This results in polluting the atmosphere with 22 billion tons, of carbon dioxide every year.
Plants and some natural processes absorb carbon dioxide.
But we release double the amount of carbon dioxide then what plants and natural process can absorb.
This means there is a net increase of 11 billion tons of atmospheric carbon dioxide every year.
Carbon dioxide, is one of the main greenhouse gases which contributes to global warming.
Estimates of how much coal, gas, and petroleum are left in the planet, may vary a little.
But certainly we do not have enough to last for centuries.
If we have to think beyond the lifetime of our grandchildren, we need to think of alternative sources of energy.
It makes sense to think of renewable energy, for a sustainable future of mankind.
Renewable energy could be solar, wind, hydro, etc,.
The most promising renewable energy source, is solar energy.
Our energy consumption is growing year by year.
In 2000 the world consumed 13 terawatt or TW of energy per year.
The U.S. accounted for 25 % of this.
In 2012 the global energy consumption went up to 17 terawatt.
87% of this energy comes from fossil fuels.
The global demand for energy is expected to reach 28 terawatt by 2050.
It would be interesting to visualise the amount of energy we consume.
1 watt is about the energy required to operate a smartphone.
1000 watts or a Kilowatt is a kind of energy required by a electric kettle.
1000 kilowatt is a megawatt or MW.
A megawatt is the amount of energy required to power a jet engine.
1000 megawatts is a gigawatt or GW.
A gigawatt is the electricity output, of an average nuclear power plant.
A thousand gigawatt is a trillion watt or TW.
The demand for energy should be differentiated from the demand for electricity.
Electricity is a form of energy which can be transported, but cannot be stored.
We need fuel to generate electricity.
We can generate electricity by burning coal and converting the heat to mechanical energy via turbines.
Whatever fuel we use, coal, gas, oil, or even nuclear, the conversion rate is only about 33% efficient.
To generate one Terawatt of electricity we would need thousand nuclear power plants.
This is 2 and half times the number of nuclear plants in operation today.
If we have to reduce carbon dioxide emissions, to acceptable levels, we need to generate 10 to 20 terawatt of clean energy.
The sun radiates more energy to Earth in one year,
than the energy consumed by all humanity in its history,
and all the energy it will consume for maybe 1000 more years.
It is no wonder that the Sun was worshipped as a God, by most civilisations in the planet.
The Sun is the only the renewable resource which has the capacity,
to satisfy our global carbon free target of 10 to 20 terawatts of energy by 2050.
The Earth receives 174 petawatts of solar energy.
A petawatt is a quadrillion watts or 10 to the power of 15 watts.
A petawatt is also 1000 terawatts.
Obviously the Sun radiates enough energy for us to use,
provided we can harness it.
A third of the energy radiated from the Sun, is reflected back into space.
The rest is absorbed by oceans, land, and clouds in the atmosphere.
89 petawatts of solar energy is soaked up by the Earth annually.
Considering all the practicalities involved, 600 teraWatts of solar energy,
is available for onshore solar power generation.
If we have solar farms which run at 10% efficiency, it would be reasonable to assume,
that we could produce 60 terawatts of terrestrial solar energy.
The amount of energy available from solar power is enormous.
Photosynthesis captures solar energy.
90 terawatts of energy is captured by photosynthesis.
Natural photosynthesis is pretty inefficient.
It requires vast areas of land for the unit of energy produced.
It is not practical to produce energy use needs from photosynthesis.
Solar energy capture can be passive.
Buildings can be oriented, so that they capture the optimal amount of sunlight.
We can use materials in buildings that absorb heat, or disperse light.
We can design interiors so that, air is circulated naturally.
The now familiar method, to harvest solar energy,
is to use photovoltaic or PV panels, and solar thermal collectors.
Solar panels work by exploiting the photoelectric effect, and the photovoltaic effect.
These two processes are related but fundamentally different.
Some materials known as photodetectors, or sensitive to light,
and other forms of electromagnetic energy.
Various forms of silicon, or compounds such as cadmium telluride are examples of photodetectors.
When they are exposed to light, a electric current is created.
This is the photoelectric effect, which can be caused by exposure to electromagnetic radiation.
Light is composed of tiny packets of energy, called photons.
Each photons contains a different amount of energy depending on its wavelength.
This is what gives the light spectrum, its array of different colours.
The photoelectric effect occurs when photons are absorbed from the visible ultraviolet parts of the spectrum.
When photons strike the material in a photovoltaic panel, some are reflected,
some pass right through, and some are absorbed.
It is the absorbed photons that produce electricity.
They transfer their energy to the negatively charged electrons,
which are orbiting the nucleus of the atoms of the semiconductor.
This allows the electron to escape from their natural position, leaving a 'hole' behind them.
This hole behaves like a positively charged particle.
These liberated electrons and holes, head off to separate electrodes, generating a voltage.
The photovoltaic effect, differs from the photoelectric effect, because the electrons are not emitted,
but transferred between different quantum bands in the material.
Together these escaping electrons form an electric current.
This can be used to create a build up of voltage between two electrodes,
and then driven through an external load.
Devices using the photovoltaic effect are usually called solar cells.
When light falls on these cells it creates an electric current,
as escaping electrons and the holes they leave behind, are swept in opposite directions,
because the depleted semiconductor junction now has its own built-in electric field.
The photovoltaic effect was first observed by Alexandre Edmond in 1839.
The underlying process wasn't fully explained, until 1905,
when Einstein worked out how the photoelectric effect operated.
The relationship between light and electricity was recognised in the 18th century.
It was discovered that light could produce an electric charge.
Working out the fundamental details had to wait till Einstein explained it.
His paper contained a simple description of ' light quanta' or photons.
This was used to explain the photoelectric effect and associated phenomena.
The premise that quanta of light are discretely absorbed, ultimately leads to all the features,
and the characteristic frequencies.
It wasn't until the arrival of quantum mechanics,
and the principle of particle-wave duality, that photovoltaic evolved into a field of research.
Einstein was awarded a nobel prize for his paper, in 1921.
The modern field of photovoltaics is devoted to the practical application of the physical phenomenon.
This is to produce energy from light.
It is achieved through the use of cells made of a photovoltaic material.
Typically these are pure crystals of silicon sliced into wafers.
These wafers are capable of absorbing light to generate the electrons and holes, they leave behind in pairs.
In some cases a quantum variant of the two, in a bound state, is called as a exciton.
These various type of charge carriers are first separated out and then extracted to an external circuit.
There is a simple way to understand how photovoltaic cells work.
We can think of a LED torch running in reverse.
In a LED the charge is stored in a battery, which provides a current of charged electrons.
When the electric current passes through an LED,
the electrons become excited and briefly jump up a quantum level.
When they jump back down, the pent -up excitement is released as a photon of light.
A photovoltaic cell does exactly the opposite.
It absorbs the photons of light at various wavelengths,
that excite the electrons in the material, allowing them to escape.
Electrons are negatively charged,
so the hole they leave behind, in their parent atoms become positively charged.
This charge separation and now be collected as a current.
There are several photovoltaic materials.
Each one works with a narrow range of the spectrum.
The intensity of the light source has a huge bearing on the current output.
The first truly viable photovoltaic cell was developed in Bell laboratories in 1954.
This was able to convert sunlight into electricity with an efficiency of 6%.
Though this was a considerable improvement over previous efforts,
it was still prohibitively expensive.
It costs $250 to produce 1 watt.
This was at a time coal fired plants could produce electricity at $2 a watt.
In 1958 U.S. scientist were searching for ways to extend the life cycle of satellites.
They were very expensive to launch, but a very limited lifetime, due to limited battery power.
The original sputnik-1 was a football size metal orb.
It beeped at regular intervals.
Its battery ran out of power after 22 days in orbit.
Scientists thought that solar panels could be a cost effective means of extending mission times.
Solar panel turned out to be a huge success.
Vanguard one, the fourth artificial satellite to be launched, and the first to be solar powered,
was communicating with Earth for more than 6 years, in 1964.
For two decades solar power remained exclusively space age technology.
Space agencies were willing to pay large amounts for the best possible science.
There was very little incentive for manufacturers to invest in less efficient technology, for other domestic markets.
The demand for transistors increased the availability of large single crystals of silicon.
This brought down the price of silicon production, and solar power to dollar hundred per watt.
The solar cells used by satellites were based on standard semiconductor production processes.
They were in effect the same silicon wafer used by the computer industry to make transistors.
For the purpose of generating solar power they were vastly over engineered.
A breakthrough occurred that silicon rejected as substandard for transistor production,
would be perfectly adequate for solar cells.
By making panels from the industries waste materials,
his team was able to achieve a five fold reduction in price.
Today solar energy can produce electricity for a wholesale cost, well under dollar 3 per watt.
The costs have reduced so much, that the unit price of the solar panel,
is less than that of the ancillary equipment.
Over the years there have been a number of developers that have enabled us,
to extract energy via the photovoltaic effect with much more efficiency.
Individual solar cells produce very low voltage.
Several cells are wired in series inside a weather proof container, to make a photovoltaic module.
This is what we call as a solar panel.
One module produces a maximum of 400 watts.
Most often it produces less than 100 watts.
Individual panels are grouped together to form a solar array.
The energy generated by an array can be used directly at source, or fed into the grid.
It can also be stored in a battery.
The electricity produced by solar panels is direct current.
This must be converted into alternative current using an inverter.
Solar panels produce most electricity when they are directly facing the sun.
There location on Earth also affects there productivity.
During cloudless days on the equator, the sun's rays,
give a maximum strength of 2400 kilowatts per meter squared of solar radiation per day.
The amount of solar energy reaching a particular area is called insolation.
In more temperate climes, the sun radiates much less energy, usually between 700 to 1000 kilowatt per meter square.
The solar farm is essentially a photovoltaic power station,
that can generate electricity on a utility scale.
The first solar farm was setup in Germany.
It had an area of 110 hectares .
It generated 52 thousand megawatt hour of electricity in 2011.
A plant setup in California in 2015 with an area of 260 hectares.
It generated thousand one hundred gigawatt hour of electricity annually.
The variation is due to climate and location.
In Germany each megawatt capacity produces .6 gigawatt hour in a year.
In California the output is more than double , at 1.4 gigawatt per year.
At latitudes away for the equators, PV arrays can be engineered so that they tract the sun through the day.
They can also tilt at different angles during different seasons.
This ensures that they collect the maximum amount of energy.
This kind of engineering can increase the amount of solar energy captured by as much as 45%.
Most panels are fixed, and their installation is set to provide the optimal output,
during the annual period of peak electricity demand.
This is a reason in northern europe, the panels tend to be on south facing roofs.
Almost all solar arrays producing more than one megawatt using tilting and tracking mechanism.
Solar trees are artificial solar arrays that mimic the looks of a tree.
They provide shade during the day, and act as street lights during the night.
This kind of setup will be useful for remote communities, where it is the sole source of electricity.
Solar trees can produce 50% more power than a flat layout during winter.
During other seasons it can produce 20% more electricity.
The cells themselves are extremely sensitive to shading.
Shading causes the output to drop dramatically.
This is due to the fact that shaded areas act as a short circuit, with electrons reversing into them,
rather than flowing through them.
Instead of adding power to the cell, the shaded area absorbs power and converts it to heat.
Sunlight can also be absorbed by dust, snow or other materials on the surface of the module.
If this accumulates it can have a big impact on productivity.
In Europe and northern latitudes, solar panels have average efficiency of 15%.
The best commercial models have an efficiency of 22%.
The average amount of usable daily insolation in Europe is 5 hours.
The typical solar array located here produces between .75 and 1 KW hour, per meter squared, per day.
The same solar array in the Sahara desert, in ideal conditions would provide 8.3 KW hour, per meter squared, per day.
The Sahara covers more than 9 million square kilometers.
It is a barren wilderness with very little fauna and flora.
Covering just 1% of the Sahara with solar panels, would provide as much electricity,
as all of the world's power stations combined.
This gives an idea of the potential capacity of solar power.
There are practical problems in making the Sahara as a global power house.
To start with the power has to be transported to where it is needed.
If electricity has to be transported for 1000s of kilometers the cables need to be very thick, like a tree trunk.
Power losses also would be massive.
This would make the whole idea impractical.
An important factor is the theoretical limit to how much power he can produce from a solar cell.
This is called the Shockley-Queisser limit, which is the conversion efficiency limit 33.7%.
Current technology is approaching this limit.
The best modern mono crystalline units available today,
convert sunlight into electricity at an efficiency of 22%.
This shortfall is due to the practicality of design.
Some factors for the inefficiency are:
Light reflecting off the glass.
The shading of the thin wires on the surface of the silicon.
Need for equipment for tilting and tracking.
The solutions to further increase the efficiency are expensive.
There are applications in which the Shockley-Queisser limit can be overcome.
One technology is to employ multiple layers of semiconductors.
In this system cells are stacked in series,
so that the radiation passing through the first layer,
can be absorbed by the subsequent layer.
This makes possible to capture light across the whole solar spectrum,
which would be more effective.
Theoretically if we construct a cell with an infinite number of layers,
we could capture 86% of the light that hits it.
The practical limit is much less.
These kind of cells are very expensive to produce.
To make them stack up commercially also requires the construction of a massive array of mirrors,
to concentrate the light.
There is still a lot more we can do with single layer cells.
The objective would be to find a way of making ultra cheap, single junction cells.
The price needs to be lower by a factor of 25 to 100.
This would make solar extremely competitive compared to other sources of power.
The challenge is that if we use cheap materials, that is those with smaller crystal grains,
then the boundaries between the grains promote a process called 'recombination' .
When a energetic photon has been absorbed by a semiconductor, an electron/electron-hole pair is created.
Sometimes the pair inadvertently recombine before they can be drawn off to an external circuit.
The more of these 'recombination sites' there are , the less efficient the cell is at producing electricity.
There are other potentially promising technologies.
They use thin films on flexible substrates,
comprising a sandwich of metallic electrode, semiconductor, and a upper most transparent electrode.
As of now crystalline silicon version are more efficient.
Thin film technology has failed to deliver cheap solar power.
Crystalline solar power don't need to be sandwiched between two panes of glass,
which reduces weight and cost.
The solar to electricity conversion rate is 2 to 3% better than thin film.
Gallium arsenide multijunction cells were originally developed for use in outer space.
Their use in terrestrial concentrators could provide the lowest cost in terms of dollars per kilowatt hour.
These cells consists of multiple thin films produced using a highly complex process called,
metal organic vapour phase epitaxy.
This grows crystalline layers to create complex semiconductor multilayer structures.
They absorb merely all of the solar spectrum.
This generates as much electricity as possible.
These are the most efficient solar cells produced as of now.
They have an efficiency of 43.5% .
This technology is currently being used to power the Mars Rover.
As a whole, the market for all PV solar modules is experiencing incredible growth.
Increasing energy demand, security of supply, raising fossil fuel prices,
and concerns over global warming is contributing to the rise and demand.
Now that solar power caused almost the same as other forms, demand will increase rapidly.
The competing technologies are,
Thin film chalcopyrite, crystalline-Si, Thin film amorphous-Si, dye-synthesised solar cells,
organic PV, concentrated PV, and PV systems.
There is more than enough sunlight to produce all the electricity we need.
At present less than half percent of all electricity production uses solar radiation.
Currently crystalline silicon is increasing in popularity.
There is a growing interest in the development of PV based on organic materials, or OPV.
They can in principle be produced at a extremely low cost, over vast areas, using an in-solution based process,
requiring only a small energy input.
The Noble Prize, in the year 2000 for chemistry was given to scientists, who paved the way,
for the development of organic light emitting diodes, or O LEDs,
and organic thin film transistors or TFTs.
This led to the production of brand new range of flexible electronic devices.
They were created by printing directly onto conducting plastic films.
The organic materials used in most cases are polymers.
Several manufactures brought OLED technology to the market with display devices on consumer electronics,
TV, and computer displays.
Devices like kindle use TFT technology, printed from organic materials for displays.
A PV device is effectively an LED working backwards.
Soon Organic PVs or OPVs started to get developed.
The first generation had a single layer of organic semiconductor sandwiched between the pair of electrodes.
The second generation had a pair of organic semiconductors, an electron donor and an electron acceptor, in a bilayer.
The current was generated from the interface.
Their power was limited by the size of the bilayer.
When a photon excites the polymer and liberates an electron to leave a hole behind,
the exciton produced can move only 10 nanometers before the hole and electron combine to liberate heat.
To seperate them and generate electricity there must be a interface between the two plastic donors and acceptors with in 10 nanometers.
The best morphology for this is a set of interpenetrating hairs, which are 10000 times thinner than human hairs.
The construction methodology leading the way currently, is called bulk hetero- junction .
Research is ongoing for this technology.
There is another set of technologies based on dye-synthesised solar cells.
Here a metal organic dye captures the light rays and generates an electron, with the charge separation process taking place,
with a titanium dioxide semiconductor.
This research team used titanium dioxide in the form of nanoparticles.
These particles have a very large surface area per unit mass.
This can go upto 300 meter squared per gram of material.
20 grams of titanium dioxide gives a surface area bigger than a football field.
This leads to highly efficient devices.
The whole cell comprises of a liquid or solid electrolyte over a layer of dye-coated titanium dioxide,
that is structured like a carpet of tiny fibres.
They can be made very thin and flexible through techniques like screen printing.
Another research team employed low band gap semi-conductor crystals, that are so small they found,
quantum dot, instead of the organic dyes are light absorbers.
The quantum dots have a number of unique features.
Sunlight has photons of a wide range of energies.
Only a certain amount of energy is required to liberate an electron/hole pair.
This is the 'band gap energy' of a material.
Some photons won't have enough energy to liberate an electron/hole pair.
They will simply pass through the cell as if it was transparent.
Others have too much energy.
In this case the extra energy is lost.
These two effects account for the loss of upto 70% of radiation energy.
The minute size of the quantum dots means that the band gap can be tuned,
by changing the particle size.
Another way to create more energy is simply to remove a layer of the semiconductor.
This stripped down version of a normal PV cell has been found to boost energy output.
These cells are very inexpensive to make and convert just under 11% of light to electricity.
These cells use an observer layer which is just a few nanometers thick.
They are called extremely thin absorbers or ETAs.
Liberated electrons travel to their electrode via a scaffold of semi-porous semiconductors of titanium dioxide.
The holes travel through conductors.
Researchers have been developing ETA cells that replace the titanium dioxide scaffold with a semiconductor made of Perovskite.
The cells combine the efficiency of silicon with the low cost of thin film.
This achieved 15% of efficiency.
Researchers are hoping to raise the efficiency levels comparable to 29% achieved by crystalline gallium arsenide cells,
which are very expensive.
Researchers are also trying to glaze large buildings using Perovskite cells, instead of glass.
Research is ongoing in OPV technology, and the more promising technologies will get commercialised.
Solar energy is theoretically a perfect solution for generating electricity.
There are some practical problems, which prevent it from becoming a universal solution for our energy needs.
We will not be able to generate solar energy in a centralised place.
The sites for generation should be distributed widely.
Even a solar cells become very inexpensive, it would not solve the problem of producing,
the amount of carbon free energy we need by 2050.
Assuming a mean efficiency of 10% conversion, putting a solar energy convertor onto the roof of every home in the US,
will generate only 0.25 tera watts of power.
Covering every roof in the world will not get us anywhere near the 10 to 20 tera watts we need.
To get that kind of power we require more than one approach for the production of energy.
However positive the outlook for photovoltaic technology, we should remember,
that the technology is for producing electricity.
At present there is no cost effective way to store it.
Not all solar arrays are connected to the central grid.
Solar energy production is intermittent in the isolated systems.
Stand alone systems vary widely in size.
Small systems could power a watch or calculator.
Large arrays can power entire communities.
Sunlight is intermittent, but our energy needs are continuous.
Solar energy has conversion efficiency issues.
In spite of all these issues, solar is the way to go forward.
Solar is a far more efficient means of generating electricity than wind, water, or biomass.
It is far more versatile.
Realising solar energy's potential will involve a significant reduction in cost,
together with a massive increase in manufacturing capacity.
OPV is one emerging technology that promises to deliver on both counts.
Luminescent solar concentrator or LSC is another promising technology.
The biggest challenge facing solar energy is to find a cost effective way of storing and transporting electricity.