5.5 Cont.. Renewable energy.
Multiple layers.
Recombination.
Gallium arsenide.
Competing technologies.
O LEDs, TFT.
Band gap energy.
ETA cells.
More than one approach.
Solar-way to go forward.
Wind power.
Potential capacity.
Usable energy.
Several sizes.
Unpredictable.
Research.
Capacity.
Hydroelectric projects.
Small scale hydro.
Hydro electric capacity.
Wave energy.
Biomass.
Geothermal energy.
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.
Wind power is another important source of solar energy.
Wind power can complement solar energy.
Wind power has been with us for a long time.
Sailing ships and wind mills were in use 5000 years back.
Wind power is effectively a secondary source of solar power.
Wind is caused by rising hot air that is replaced by cooler air drawn in near the surface.
Gliders for example, take advantage of this energy to keep them airborne.
In 2014 the wind power capacity in the world was only 319 giga watts.
This is one percent of our annual energy needs.
There is probably enough economically extractable power from the wind.
Simulations imply that there is a potential capacity between 18 to 68 TeraWatts of mechanical wind power,
that can be extracted from the boundary layer of the atmosphere using a wind turbine.
The most common wind turbine is the mast design.
These have the turbine situated in front of the mast to cope with the turbulence.
One of the key things to understand about wind, is the amount of energy that wind speed can generate,
is not one to one.
The energy generated increases by the cube of the wind speed.
If we double the speed of the wind,we can get 8 times the energy.
Even a small difference in the wind speed can have a huge impact on the amount of energy,
a wind turbine can generate.
The taller the tower the greater the energy generation potential.
The flow of air over the rotor allows the wind turbine to extract energy from the wind by slowing it down.
The theoretical maximum amount of energy that can be extracted is 59%.
This is known as the Betz limit.
If the wind turbine was 100% efficient, it would have removed all the energy from the wind.
The wind would have stopped.
In practice rotor efficiency is between 35 and 45% .
If we take transmission, generation and storage into account, we are looking at a usable energy,
which is between 10% and 30% of the original energy available in the wind.
Turbines are available in several sizes.
The smallest 50 Watt turbines could be used to power a boat or a caravan.
Larger domestic models can produce 1 KiloWatt.
It can supply a single household, and feed any excess energy to the grid.
The largest turbine can produce 7 MegaWatt.
These megaliths are situated out at sea.
The world's tallest windmill, is located in Germany and is 205 meters high.
The largest swept area turbine in Spain has blades 128 meters long.
The most common ones are 1.5 MegaWatt turbines.
They are used in onshore wind farms.
Wind turbines, whatever the size, are a remarkable feat of engineering.
The 1.5 megawatt turbines are 18 meters tall and weigh 52 tons.
Wind turbines are intrinsically unstable.
They are embedded in a 450 tonne concrete base, reinforced with 26 tonnes of steel.
Wind is unpredictable, it is not always available.
We cannot store wind.
We need to make a breakthrough, in energy storage, to make windmills the primary source of renewable energy.
Practically we must mix it with other non-renewables.
Denmark produces 21% of its electricity from wind.
On certain days without wind it can drop to 1%.
The grid has to adapt to this by installing capacitors, and bank of batteries.
Offshore windmills have several advantages.
The wind is much stronger, and blows more consistently.
The offshore electrical power potential of wind is also larger,
than 2 Terawatt terrestrial potential.
Countries with a coast line could locate windmills offshore.
There are engineering challenges in locating windmills offshore.
A windmill looks sedate when rotating at 22 rpm.
The tips of the blades at this speed is 200 miles per hour.
A typical farm will have a turbine spacing of 6 to 10 times the blade diameter.
The speeds involved place severe constraint on their design.
Wind turbines are expensive because they use a lot of carbon fiber composite in their blades.
They also use copper wire and rare-earth magnets in their dynamos.
They also have complex rotor shafts and gearboxes.
Marine turbines are larger .
Research is ongoing to develop even better windmills.
Wind tunnels are used to test them.
This helps to do vibration analysis and control.
This helps to model and design complex structures.
Engineers have developed a unique magnetic force/ torque transmission system.
This reduces maintenance and produces more energy.
It is immune to the kind of wear and tear experienced by mechanical gears.
Mathematicians are helping to develop high frequency radar systems,
that can measure surface currents and wave direction.
This provides more accurate wind measurements.
Long distance health monitoring will identify problems as they arise and reduce maintenance costs.
Structures are being designed to be self healing.
Offshore turbines have their own challenges.
Transmission loss is one of them.
To generate 10 terawatts of power would require 4 million turbines.
Such a large number would interfere with atmospheric circulation.
It is likely that removing 50% of the energy from the atmosphere will have an impact on the weather.
We can possible aim for the capacity of 2 terawatts of terrestrial wind power, and 2 terawatts of offshore wind power.
More than half of the current installed capacity of renewable energy is made up of large hydro electric systems.
1/6th of this is generated from biomass.
Biomass is mainly wood used for domestic heating.
Like wind hydro electricity is powered by sunlight.
Water is evaporated from the sea by the sun.
It falls as rain on land.
It flows back to the sea through rivers, under the effect of gravity.
Kinetic energy is extracted from the water as it makes its way back to the sea, and converted to electricity.
Hydroelectric projects have 2 major purposes.
They generate electricity.
They manage water for irrigation, drinking, and for domestic purposes.
The Hoover dam in the U.S. has a power capacity of 2 gigawatts.
It has produced an average of 4.2 terawatt hours per year, since its completion in 1936.
The primary purpose is to control the supply of water to 400000 hectares prime agricultural land.
The money generated from the energy supplied has more than covered the cost of the dam, and its upkeep.
The project has been producing more than its cost for 50 years.
Hydro electric's greatest potential is in pumped storage.
At times of excess supply, water is pumped to an upper reservoir, as a store of potential energy.
During periods of heavy demand it creates power by flowing through turbines to a lower reservoir.
The 3 Gorges dam on the Yangtze river in China is the biggest installed hydro electric scheme in the world.
It has a capacity of 22.5 gigawatts.
It generates 80 terawatt hour of electricity per year.
The Itaipu dam on the border of Brazil and Paraguay, has a capacity of 14 gigawatts.
It generates 90 terawatt hour of electricity per year.
It supplied 90% of Paraguay's electricity, and 15% of Brazil's electricity.
The construction of the 3 Gorges dam caused severe environmental and cultural damage.
It displayed 1.3 million people.
Unique ecosystems along with rare species were destroyed.
The Akosombo dam on the Volta river, is a glaring example of exploitation by commercial interests.
Its construction resulted in creating the World's biggest man-made lake.
It had a area of 8500 square kilometers, which is 3.6% of Ghana's land.
It was not built for the benefit of Ghanaian citizens, but to provide electricity,
for a American owned aluminium company.
About 80,000 people were forcibly relocated for its construction.
Some of the best agricultural land was flooded.
The weeds in the newly formed lake provided a perfect breeding ground for black flies and mosquitos.
These were carriers of bilharzia and malaria.
Water borne diseases rose to a epidemic level, due to the dam.
Hydro electric plants have several advantages.
But we need to be conscious of potential negative factors.
Small scale hydro electric facilities are also a possible renewable resource.
100 gigawatt of small scale hydro electric infrastructure is already installed.
70% of this is in China.
These systems use river flows, with no lake storage.
They can generate power upto 10 megawatt.
They can be standalone to serve remote areas,
or they can be connected to the grid.
They have very little environmental impact.
They provide high levels of return on investment.
There is scope for tapping the hydro electric capacity around the world.
The total hydrological energy potential of the Earth is 4.6 Terawatts.
The total extractable energy is estimated at 1.5 Terawatt.
We need to be conscious of habitable and agricultural land lost to large scale hydro electric plants.
However, we could pursue small hydro electric projects.
A much less developed form of hydro energy is wave power.
Waves are generated by the wind passing over the surface of the sea,
and by tides caused by the sun and moon's gravitational pull.
There is realistic around 2 terawatts of extrable wave energy.
The main stumbling block is cost.
Sea water is highly corrosive.
Oceans are hostile environments.
Power plants must be robust to work in harsh conditions.
In spite of the constraints there is considerable potential for the wave energy.
The most optimistic estimates for hydro electricity, wave and wind,
provide us with a combined potential of 7.5 terawatts of energy.
Biomass cannot make up the shortfall of energy required.
Biomass is the energy captured from the sun by plants, by photosynthesis.
It is renewable, but it is very inefficient.
In optimal conditions less than 1% of the recoverable sunlight is stored as energy.
To produce 20 terawatts of electrical energy, we would need to cover 31% of the Earth's total land area,
in farms devoted exclusively for the production of biomass.
The total amount of land with crop potential is estimated at 2.45 into 10 to the power of 13 meter squared.
This is about 24.5 million square kilometers.
One third of that is currently under cultivation.
Additional land will be required to feed a population of 10 billion people.
We will need at least an additional 4 million square kilometers by 2050, for food production.
This leaves us with 13 million square kilometers for biomass production.
We can practically produce 8.5 tonnes of dry biomass per hectare.
The entire area is covered in biomass farms, the energy produced will be 7 to 12 terawatts.
The higher figure is based on assumption that yield can be doubled through future biotechnology breakthroughs.
However there is not enough fresh water t0 support agriculture, on such a scale.
Another potential source for energy is geothermal energy.
One example, is the project at Beppu in Japan.
Beppu is in a volcanic region where superheated water from deep within the Earth,
escapes to the surface to form hot springs.
These feed public baths, which is a great tourist attraction.
They also supply domestic heat and generate electricity.
Geothermal power plants use the superheated water to drive the turbines.
No fuel is used to heat the water.
The process of extracting geothermal energy from hot water springs is well understood.
Unfortunately, there is very little recoverable energy worldwide.
Geothermal potential is estimated to be about 10 gigawatts.
It is more complicated to extract geothermal energy from the Earth's core.
In volcanic regions the heat and energy contained below the Earth's crust is,
relatively close to the surface.
In principle it can be used to heat water to provide electrical energy.
In Russia a binary cycle system has been developed.
It uses cooler geothermal source, together with a heat exchanger.
This removes warm water from the ground and sends colder water back down.
This technology can be used only in places where geothermal energy is close to the surface.
These regions are also geologically unstable places.
They are prone to tremors, Earthquakes and volcanic erruptions .
The engineering challenges are considerable.
Intervention can have negative concequences.
A geothermal project in Switzerland was cancelled because it caused several mild tremors.
Iceland generates around 400 megawatts of electricity from its geothermal sources.
All the domestic heating is produced in this way.
But it is very small country with a population of 300 thousand people.
Geothermal energy has a limited potential as of now, as a source of renewal energy.
There are good reasons to be optimistic about renewable energy.
It is not very clear that solar and other renewable energies can meet all our energy demands by 2050.
We need to think about methods to consume less energy.
We also need to give importance to energy utilisation efficiency.
Much more research is required to harness the full potential of renewable energies.
Notes:
India renewable energy
Wind power - 34 GW - 49%
Solar -22 GW - 31%
Biomass - 9 GW - 12%
Mini hydel - 4 GW - 6%
Solid waste - .14 GW - .2%
Power generation
Coal - 197 GW - 57%
Major Hydel - 45 GW - 13%
Renewable energy - 69 GW - 20%
Gas - 24 GW - 7%
Diesel - .8 GW - .2%
Atomic - 7 GW - 2%
Total power - 343 GW