Environmental considerations

Describe about  photovoltaic cells?

The importance of sustainability:

Sustainability is   the meeting of the needs of the present generation without compromising the ability of future generations to meet theirs. The sustainability has many aspects like environmental, social, and economical..                 

Photovoltaic (PV) is a renewable energy technology that converts solar radiation directly into electricity. Solar energy is abundantly available; the Earth receives enough solar energy every hour to meet the world’s annual energy needs (Archer and Hill, 2001)

.The electrical power which has been produced from PV is less attractive than the electrical power produced by modern technology because of its long payback time. But nowadays incase of mixing of the energy for future, PV is most important because of the rising energy cost and drooping production cost ((Archer and Hill, 2001). So the sustainability of PV power is important. The sun is the renewable source of a photovoltaic cell using ultra thin high efficiency wafers in crystalline solar cell; reduce the carbon footprints of crystalline solar cell. Though recent studies have shown that the life cycle green house gas emission of PV systems as low as 37 g CO₂ –eq/KWH. It is 4% low compared to coal based power plants.

Photovoltaic solar cell converts the sunlight to electricity. Photovoltaic cell uses array to capture the solar energy and convert into electricity. The general theory behind the PV cell is that the solar cell absorbs the incoming solar radiation. The electrons of the solar cell are promoted by the energy that is stored in the photon. Solar radiation represents such an infinite source of energy for the Earth. The sun delivers 1.2 × 1014 kW energy on the Earth, which is about 10.000 times more than the present energy consumption (Ho, 2007). The energy that the Earth receives from the sun in just one hour is equal to the total amount of energy consumed by humans in one year. So photovoltaic cell is being considered as the significant contributor to renewable energy.

The crystalline silicon cells in photovoltaic cell are most expensive, so instead of these cells thin film cells are more affordable. But its efficiency lies below 10 percent. Nanocrystalline dye cell can also be used with 5 percent efficiency. These are the innovation of solar technology that made the system more affordable.

1. The photovoltaic energy is used in MSU sustainable house.

The solar cell is called photovoltaic cell. These cells are found in solar powered calculator or on satellite for creating the electrical energy. These cells convert light energy to electrical energy directly .These cells are made up of silicon. A silicon atom consists of 14 electrons which are arranged in 3 general electron shells. The first 2 cell which are nearest to the nucleus, are full. The outer shell half full comprises 4 electrons. Silicon can share its electron with its neighbor where in case of crystalline silicon; other silicon atoms provide electrons to share.

The mathematical parameters used to evaluate the performance and efficiency of the various types of the various types of Photovoltaic cell

Fundamentals of photovoltaic cell

The important parameter of the PV performance is called the Fill Factor (FF). It is a term that describes how the curve fills the rectangle that is defined by (V _oc) and (I_sc). It gives an indication of the quality of a cell’s semiconductor junction and measures of how well a solar cell is able to collect the carriers generated by light (Hajizadeh et al. 2011). It is defined as:

                   FF  (1)=V_mp I_mp/V_∞I_{sc                   (1)}

 After a simple manipulation, one gets the equation:

              V _oc  I _SC  FF = V _mp I _mp = Pmax       (2)

It can be easily observed that FF is always less than unity and differs from material to material. The closer the value of the fill factor is to unity, the better the operation of the PV cell. The PV efficiency (η) is defined as the  ratio of the maximum output power P _mp to the solar power received by the cell surface area (A):

          η= FFI _scV _sc / GA

A five parameter photovoltaic model is adapted in this study to determine the module parameters. In the present work, the five parameter model is used to simulate the characteristic of thin film solar cells at different weather conditions.

In addition identify what type of voltage is produced and the wattage ,how the power is regulated to battery or more importantly to an inverter to mains power.

A single PV cell produces about 1 to 2 watts of electricity; an amount that is quite insignificant compared to what is required by most electrical equipment.

Two or more PV Cells are built to produce a PV Module to provide higher wattages as required. For instance, a PV module producing 50 watts may comprise of at least 25 of 2 Watts output PV cells.

To meet the the electrical need of a home or an industrial setting, PV Modules are assembled together to form a PV Array that meets the total energy requirement.

A PVC system design begins with determining the total energy requirement for a facility to be powered. Next the number of solar panel units required and other components of the PVC (description below) are determined. A basic PVC system comprises of the following:

• Solar panels– collects visible light from Sun and converts it to electricity (direct current (DC));
• Batteries (rechargeable)– Store solar energy up to provide electricity for sun-down periods (nights and cloudy days). They must be able to discharge and recharge. Rechargeable batteries are a little more expensive than the disposable batteries. Without batteries a PVC system can only provide electricity when it is sunny.
• Charge Controller- feeds electricity from the solar panel to the batteries in a manner that prevents the solar panel from overcharging the batteries.
• Power Inverter– converts the low-voltage direct currents (DC) from  the battery to high-voltage alternating current (AC)

How A Pv Cell Is Made And How It Works

What is the PV slab made of:

The slab or wafer is mainly made of silicon. On the top of the slab there is very thinly phosphorous is diffused and on the base of the slab, boron is diffused, . The boron side of the slab is 1,000 times thicker than the phosphorous side and the phosphorous has one more electron in its outer shell than silicon, and the boron has one less.

What chemical bonding takes place on the surface:

The photovoltaic cell is essentially a diode with a large surface and there is covalent bond.

What are the dopants and what part do they play in the design process:

In a crystalline silicon PV cell, p-type silicon must contact n-type silicon to create the built-in electrical field. The process of doping, which is used to create these materials, introduces an atom of another element into silicon crystal to alter its electrical properties. The dopant, which is the introduced element, has either three or five valence electrons—which is one less or one more than silicon’s four. The most common method of doping is to coat a layer of silicon material with phosphorus and then heat the surface

Mathematical Parameters used to evaluate PV performance

How does boron affect the electron flow

 A thicker, boron doped bottom layer contains holes, or absences of electrons, that also can move freely. In effect, precise manufacturing has instilled an electronic imbalance between the two layers.

What is meant by p-type and n-type and the electrical charge between them:

P-type (or positive) semiconductors have an abundance of positively charged holes, and n-type (or negative) semiconductors have an abundance of negatively charged electrons. When n- and p-type silicon layers contact, excess electrons move from the n-type side to the p-type side. The result is a buildup of positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side(Nadeau,2011).

Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet. That  is called thep/n junction. The electrical field causes the electrons to move from the semiconductor toward the negative surface, making them available for the electrical circuit. At the same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.

How is the electrical current being produced.

Same as above

How does light get converted into electrical energy:

In a crystal, the bonds between the silicon atoms are made of electrons that are shared between all of the atoms of the crystals. When the light gets absorbed, one electron in one of the bond gets excited up to a higher energy level and can move around more freely than its boundation phase. Then we can get current. A photon comes in and activates the electron on to high energy level.

What is photoelectronic effect:

Some materials which show the photo electronic effect ,they can absorb the photons of the light and release electrons because of this effect.

Where was it discovered and by whom:

The photo electronic effect was first noticed by French physicist, Edmund Bequerel, in 1939.

Why non reflective coatings used:

The non reflective coatings are used in silicon cells because these coatings allow entering the highest amount of light in the cell.

Explain how electrons are knocked loose from the atoms in the semiconductor material:

The solar cell which is also called photovoltaic cell is mainly composed of semiconductor. When the light strikes the cell, the semiconductor absorbs the portion of light, so the electrons of the semiconductors are knocked loose. For this, electrons can flow freely and can create a current.

What does the band gap control in a PV cell:

Band gap is called energy gap. In this gap no electron states can exist. The band gap is the energy difference between the top of the valence bond and the bottom of conduction bond in insulator and semiconductor. The energy is equivalent to the energy which is needed to free an outer shell electron from its orbit. For this reason electrons can move freely within the solid material. Band gap determines the electrical conductivity of the solid. The semi conductor has small band gap whether the insulator has large band gap.

Illustrate and explain the different each substrate used to construct a the PV cell:

The majority of multi-junction cells have used three layers . However, the triple junction cells require the use of semiconductors that can be tuned to specific frequencies, it is made of gallium arsenide (GaAs) compounds, often germanium for the bottom-, GaAs for the middle-, and GaInP2(Gallium Indium Phosphate) for the top-cell.

Gallium arsenide substrate:

Dual junction cells can be made on Gallium arsenide wafers. Alloys of Indium gallium phosphide in the range .In.5Ga.5P through In.53Ga.47P serve as the high band gap alloy.

Germanium substrate:

Triple junction cells consisting of Indium gallium phosphide, Gallium arsenide or Indium gallium arsenide and Germanium can be made of germanium wafers. Early cells used straight gallium arsenide in the middle junction.            

Indium phosphide substrate:

Indium phosphide may be used as a substrate to fabricate cells with band gaps between 1.35eV and 0.74eV. Indium Phosphide has a band gap of 1.35eV. Indium gallium arsenide(In0.53Ga0.47As) is lattice matched to Indium Phosphide with a band gap of 0.74eV.

Indium Gallium Nitride:

Indium gallium nitride (InGaN, InxGa1-xN) is a semiconductor material made of a mix of gallium nitride (GaN) and indium nitride (InN).

Conclusion

Conclusion The deployment of solar technologies for energy production at a large scale requires the involvement of both political and economical players, but also further improvements in the conversion efficiency and reduction of manufacturing cost. A large ongoing research effort aims to find innovative solutions to overcome these barriers. In the last decade, photovoltaic technologies have experienced an astonishing evolution that led to the increase of the efficiency of crystal-silicon solar cells up to 25% and of thin-film devices up to 19%. Recently, nano-technology, innovative deposition and growth techniques, and novel materials opened routes for reaching higher performances (multijunction devices and other 3rd generation photovoltaics) and for developing very low-cost devices such as organic-based PVs. All these technologies face comparable fundamental issues related to the steps involved in the conversion of photon energy into electricity: photon absorption, charge carrier generation, charge separation, and charge transport. Both fundamental research and technical development are critical requirements for these technologies to become more efficient, stable, and reliable. Solar thermal systems are at the demonstration stage and some installations are already operational. Their ability to overcome the intermittency problem using hybridization and thermal storage renders these technologies particularly suitable for large-scale electricity production. Direct production of chemicals fuels, and particularly hydrogen, from solar energy is a promising alternative to using fossil fuels for the development of a sustainable carbon-free fuel economy (Collins, 2005). Thermochemical and biological conversion processes are GCEP Solar Energy Technology Assessment – Summer 2006 37 promising technologies with potential for high efficiency. However, only a few thermochemical processes have been investigated to date and biological systems require more understating of genetics and biological conversion to become efficient and stable (Collins, 2005). Solar energy has a large potential to be a major fraction of a future carbon-free energy portfolio, but technological advances and breakthroughs are necessary to overcome low conversion efficiency and high cost of presently available systems.

References

Archer, M., & Hill, R. (2001). Clean Electricity from Photovoltaics. Singapore: World Scientific Publishing Company.

Collins, R. (2005). Amorphous and nanocrystalline silicon science and technology–2005. Warrendale, Pa.: Materials Research Society.

Hajizadeh, A., Tesfahunegn, S., & Undeland, T. (2011). Intelligent control of hybrid photo voltaic/fuel cell/energy storage power generation system. J. Renewable Sustainable Energy, 3(4), 043112. doi:10.1063/1.3618743

Ho, P. (2007). e-e correlated intense-field multiple ionization as a completely classical photo-electric effect.

Nadeau, N. (2011). The Green Revolution. Hobart, NY: Hatherleigh Press.

Swatantra Prakash Singh, S. (2013). High Efficient Solar Photo Voltaic Cell. IOSR-JEEE, 4(6), 49-52. doi:10.9790/1676-0464952