Energy is the primary and most universal measure of all kinds of work by human beings and nature. Everything what happens in the world is the expression of flow of energy in one of its forms. Energy crisis is due to the two reasons; firstly that the population of the world has increased rapidly and secondly the standard of living of human beings has increased.
Many people today are concerned for the future of the planet. Conventional energy technologies are widely recognized as a major cause of environmental destruction - both in terms of depletion of natural resources and pollution. PV and other renewable energy technologies are gaining acceptance as a way of maintaining and improving living standards without harming the environment. More and more energy utilities are responding to the wishes of consumers by including PV in their supply mix.
If present trend continues, the world in the year 2000 A.D. will be more crowded than that of today. The conventional sources of energy are depleting and may be exhausted by the end of century or beginning of the next century. Solar energy and other non-conventional energy sources are the sources; those are to be utilized in future.
Even though renewable options are not likely to supply a substantial amount of energy to developing countries over the short term, they do have some advantages:
Renewable energy is an indigenous resource available in considerable quantities to all developing nations and capable, in principle of having a significant local, regional or national economic impact.
Several renewable options are financially and economically competitive for certain applications, such as in remote locations.
Rapid scientific and technological advantages are expected to expand the economic range of renewable energy applications over the next 8-10 years, making it imperative for international decision makers and planners to keep abreast of these developments.
Solar energy is the source of all life on Earth. Without it, we would not be here today living on such a rich and diverse planet. Most of the energy available to us radiates from the Sun. It provides us with food energy through plant photosynthesis and provides the heat that we need to survive. Trapped solar energy is released when we burn fossil fuel reserves and the sun drives the earth's weather systems which provide renewable forms of energy like wind, solar and wave power. It is now widely recognized that utilizing the sun's natural energy can offer real alternatives to burning finite resources of fossil fuels or endangering future generations by relying on dangerous technologies such as nuclear power.
Solar energy has long been used for space heating utilizing passive solar design, and for water heating through the use of solar water panels but one of the most exciting areas of development has come in the form of the photovoltaic cell which can convert the sun's energy directly into electricity. Solar energy has following features.
In half an hour enough of the sun's energy reaches the Earth's surface to meet the World's energy demand for a year.
The sun produces 400,000,000,000,000,000,000,000,000 watts of power. That's 400 x 1012 TW.
The World's average energy consumption is around 14 TW!
Just one square cm of the Sun's surface burns with the brightness of 232,500 candles.
All the Earth's oil, coal and wood supplies would fuel the Sun for only a few days.
1.2 Sources of Renewable Energy:
The renewable sources of energy mainly include solar, wind, hydel, biomass, geothermal and ocean energies.
Solar energy or the energy from the sun is the most sought after. It is a non-polluting and constant source of energy. Equipment designed for this purpose can be used for many functions, and hence it is also cost effective. Equipment to harness solar energy can be built both for domestic and industrial purposes. Though availability of solar energy depends on the availability of sunlight at all times, it is highly beneficial in the sense that it causes no pollution at all.
Wind energy is another good source of renewable energy. This is particularly feasible in regions like coastlines and high altitude areas which receive strong wind currents throughout the year. Energy produced from a single windmill can be used for domestic purposes. A number of windmills coupled together help to generate energy on a large scale.
Water energy is available in a number of forms. For instance, the water which is stored in dams can be released from a great height and made to flow through turbines, hence converting potential energy of falling water into kinetic energy. The oceans also possess a high amount of energy, either as wave energy present in the waves or as energy which can be generated due to difference in temperatures of different regions of the ocean.
Biomass, which is nothing but human and animal bi-products, can be used to generate energy. This is a very useful method, as it helps to cleanse the environment of the unwanted waste matter and also provides a non-polluting source of energy. This method has come into limelight due to problem of waste disposal being faced in many of the major towns and cities. Apart from this, there is also the geothermal energy which can be obtained from heat energy present either in the earth's layers or on the earth's surface.
Renewable Energy Scenario In India
India is blessed with an abundance of sunlight, water and biomass. Vigorous efforts during the past two decades are now bearing fruit as people in all walks of life are more aware of the benefits of renewable energy, especially decentralized energy where required in villages and in urban or semi-urban centers. India has the world's largest programmer for renewable energy.
Government created the Department of Non-conventional Energy Sources (DNES) in 1982. In 1992 a full fledged Ministry of Non-conventional Energy Sources was established under the overall charge of the Prime Minister.
The range of its activities cover
Promotion of renewable energy technologies,
Create an environment conducive to promote renewable energy technologies,
Create an environment conducive for their commercialization,
Renewable energy resource assessment,
Research and development,
Demonstration,
Extension,
Production of biogas units, solar thermal devices, solar photovoltaic's, cookstoves, wind energy and small hydropower units.
1.3.1 Wind Energy
India now ranks as a "wind superpower" with an installed wind power capacity of 1167 MW and about 5 billion units of electricity have been fed to the national grid so far.
In progress are wind resource assessment programmed, wind monitoring, wind mapping, covering 800 stations in 24 states with 193 wind monitoring stations in operations. Altogether 13 states of India have a net potential of about 45000 MW.
1.3.2 Solar Energy
Solar water heaters have proved the most popular so far and solar photovoltaic's for decentralized power supply are fast becoming popular in rural and remote areas. More than 700000 PV systems generating 44 MW have been installed all over India. Under the water pumping programmed more than 3000 systems have been installed so far and the market for solar lighting and solar pumping is far from saturated. Solar drying is one area which offers very good prospects in food, agricultural and chemical products drying applications.
1.3.2.1 Annual insolation
With about 301 clear sunny days in a year, India's theoretical solar power reception, just on its land area, is about 5Ph/year (i.e. = 5000 trillion kWh/yr ~ 600TW). The daily average solar energy incident over India varies from 4 to 7 kWh/m2 with about 2,300-3,200 sunshine hours per year, depending upon location. This is far more than current total energy consumption. For example, even assuming 10% conversion efficiency for PV modules, it will still be thousand times greater than the likely electricity demand in India by the year 2015.
1.3.2.2 Installed capacity
The amount of solar energy produced in India is merely 0.5% compared to other energy resources. The Grid-interactive solar power as of June 2007 was merely 2.12 MW. Government-funded solar energy in India only accounted for approximately 6.4 megawatt-years of power as of 2005.
The amount of solar energy produced in India is merely 0.4% compared to other energy resources. The Grid-interactive solar power as of June 2007 was merely 2.12 MW. Government-funded solar energy in India only accounted for approximately 6.4 megawatt-years of power as of 2005.However, as of October 2009, India is currently ranked number one along with the United States in terms of installed Solar Power generation capacity.
Number of solar street lighting systems : 55,795
Number of home lighting systems : 342,607
Solar lanterns : 560,295
Solar photovoltaic power plants : 1566 kW
Solar water heating systems : 140 km2 of collector area
Box-type solar cookers : 575,000
Solar photovoltaic pumps : 6,818
1.4 TOTAL INSTALLED CAPACITY AT A GLANCE "ALL INDIA"
As on 31-05-2010
Table:1.1-Total Installed Capacity (Sector Wise)
Sector
MW
% age
State Sector
80,525.12
52.5
Central Sector
50,992.63
34.0
Private Sector
29,834.05
13.5
Total
1,61,351.80
Table: 1.2-Total Installed Capacity (Fuel Wise)
FUEL
MW
% age
Thermal
103448.98
64.6
Hydro (Renewable)
36,913.40
24.7
Nuclear
4,560.00
2.9
R.E.S.(MNRE)
16,429.42
7.7
Total
1,61,351.80
Table: 1.3-All India Region wise Generating Installed Capacity (MW)
S. No.
REGION
THERMAL
NUCLEAR
HYDRO
(RENEWABLE)
R.E.S. @
(M.N.R.E.)
TOTAL
1
Northern
25101.25
1620
13310.75
2690.62
42722.62
2
Western
36551.79
1840
7447.5
4849.93
50689.22
3
Southern
23654.6
1100
11157.03
8329.67
44241.3
4
Eastern
17102.58
0
3882.12
334.91
21319.61
5
N.Eastern
968.74
0
1116
218.19
2302.93
6
Islands
70.02
0
0
6.1
76.12
7
All India
103448.98
4560
36913.4
16429.42
161351.8
RES- renewable Energy Sources includes Small Hydro Project (SHP), Biomass Gas (BG), Biomass Power (BP), Urban and Industrial Waste Power (U & I), And Wind Energy.
The installed capacity figures as on 31.03.10 in respect of RES is based on statement dated 28.05.10 received on 28.05.10 from ministry of Renewable Energy (MNRE) where cumulative grid interactive power installed capacity has been indicated as 16817.04 MW.
(MW)
Fig. 1.1-All India Generating Installed Capacity (MW) as on 31.05.10
A capacity of about 5,531 MW grid-interactive power generations from various renewable energy sources has been installed up to 31.1.2010 against a target of 12,300 MW for 11th Five Year Plan.
Table 1.4 Total installed capacity of non-conventional energy sources
Non-Conventional Energy Sources
Installed Capacity(MW)
Target(11th Plan)
(MW)
Wind Power
3,857
9,000
Small Hydro Power
619.53
1,400
Biomass power
322
500
Biogas Cogeneration
704.20
1200
Urban &Industrial waste-to-energy
20.10
79
A total grid-interactive renewable power generation capacity of 15,789MW has been installed in the country as on 31.1.2010. A total amount of Rs.390.78 crore has been released under various programmes for promotion of renewable energy sources in the country during FY 2009-10 till February, 2010.
1.5 Renewable Energy Scenario of Rajasthan
The total installed capacity of renewable energy source in Rajasthan is 738.5 MW. It includes wind and solar energy. Rajasthan is blessed with an abundance of sunlight and wind.
WIND-
Gurgaon-headquartered Gujarat Fluorochemicals Ltd is in an advanced stage of commissioning a large wind farm in Jodhpur district of Rajasthan. Out of the total 31.5 mw capacity, 12 mw had been completed so far. The remaining capacity would come on line shortly. For the INOX Group Company, this would be the largest wind farm. In 2006-07, GFL commissioned a 23.1-mw wind power project at Gudhe village near Panchgani in Satara district of Maharashtra. Both the wind farms will be grid-connected and will earn carbon credits for the company. In an independent development, cement major ACC Ltd has proposed to set up a new wind power project in Rajasthan with a capacity of around 11 mw. Expected to cost around Rs 60 crore, the wind farm will meet the power requirements of the company's Lakheri cement unit where capacity was raised from 0.9 million tpa to 1.5 million tpa through a modernisation plan. For ACC, this would be the second wind power project after the 9-mw farm at Udayathoor in Tirunelvelli district of Tamil Nadu.
Rajasthan is emerging as an important destination for new wind farms, although it is currently not amongst the top five states in terms of installed capacity. As of 2007 end, this northern state had a total of 496 mw, accounting for a 6.3 per cent share in India's total capacity.
SOLAR-
To demonstrate and commercialize solar thermal technology in India, Ministry of Non-Conventional Energy Sources (MNES) is promoting megawatt scale projects such as the proposed solar thermal plant at Mathania in Rajasthan.
Mathania is about 30 Kms from Jodhpur. Solar radiation available in this regions is of the order of 6-6 .4 K wh/m2/ day which makes it a suitable location for setting up of large Central Power Station based on Solar Energy. The Main objectives of the project are -
To demonstrate that operational viability of solar thermal power generation
Support solar power technology development and
Help reduce greenhouse gas (GHG) global emissions in the longer term
This project has been envisaged as the first step in a long term program or promoting solar thermal power in Rajasthan which would lead to development of similar systems in the country.
Reliance to set up solar power project in Rajasthan
Reliance Industries is setting up its first solar power project of 5 MW capacity at Khimsar in Nagaur district of Rajasthan.
1.6 Executive Summary of CEA Report (2009)
Worldwide capacity reached 1, 59,213 MW, out of which 38,312 MW were added.
Wind power showed a growth rate of 31, 7 %, the highest rate since 2001.
The trend continued that wind capacity doubles every three years.
All wind turbines installed by the end of 2009 worldwide are generating 340 TWh per annum, equivalent to the total electricity demand of Italy, the seventh largest economy of the world, and equaling 2 % of global electricity consumption.
The wind sector in 2009 had a turnover of 50 billion!
The wind sector employed 5, 50,000 persons worldwide. In the year 2012, the wind industry is expected for the first time to offer 1 million jobs.
China continued its role as the locomotive of the international wind industry and added 13,800 MW within one year - as thebiggest market for new turbines -, more than doubling the installations for the fourth year in a row.
The USA maintained its number one position in terms of total installed capacity and China became number two in total capacity, only slightly ahead of Germany, both of them with around 26,000 Megawatt of wind capacity installed.
Asia accounted for the largest share of new installations (40, 4 %), followed by North America (28, 4 %) and Europe fell back to the third place (27, 3 %).
Latin America showed encouraging growth and more than doubled its installations, mainly due to Brazil and Mexico.
A total wind capacity of 200,000 Megawatt will be exceeded within the year 2010.
Based on accelerated development and further improved policies, WWEA increases its predictions and sees a global capacity of 1,900,000 Megawatt as possible by the year 2020.
The Ministry of New and Renewable Energy (MNRE) has fixed a target of 10,500 MW between 2007-12, but an additional generation capacity of only about 6,000 MW might be available for commercial use by 2012.
CHAPTER-2
SOLAR POWER ASPECT
2.1 History
The Sun (or Sol) is the star at the center of our Solar system. Earth orbits the Sun, as do many other bodies, including other planets, asteroids, eteoroids, comets and dust. Its heat and light support almost all life on Earth. The Sun is a ball of plasma with a mass of about 2Ã-1030 kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 5 billion years old, and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will become a white dwarf. Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 106 K when its visible surface (the photosphere) has a temperature of just 6,000 K.
Figure 2.1: General View of Sun
The diameter of the Sun is 1,400,000 km (840,000 miles) which is more than 100 times the diameter of the Earth. Its mass is more than 300,000 times that of the Earth.
The Sun is classified as a main sequence star, which means it is in a state of "hydrostatic balance", neither contracting nor expanding, and is generating its energy through nuclear fusion of hydrogen nuclei into helium. The Sun has a spectral class of G2V, with the G2 meaning that its color is yellow and its spectrum contains spectral lines of ionized and neutral metals as well as very weak hydrogen lines, and the V signifying that it, like most stars, is a "dwarf" star on the main sequence.
The Sun has a predicted main sequence lifetime of about 10 billion years. Its current age is thought to be about 4.5 billion years, a figure which is determined using computer models of stellar evolution, and nucleocosmochronology. The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 million years. The orbital speed is 217 km/s, equivalent to one light year every 1400 years, and one AU every 8 days.The astronomical symbol for the Sun is a circle with a point at its centre.
2.2 Sun-As an Integrated Source of Energy
Sunlight that is, light radiated from the surface of the Sun is thought to be the main source of energy near the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. It is about 1370 watts per square meter of area. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere, so that less power arrives at the surface closer to 1000 watts per directly exposed square meter in clear conditions. This energy can be harnessed through several natural and synthetic processes. Photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or do other useful work. The energy stored in petroleum is thought to have been converted from sunlight by photosynthesis in the distant past.
Every day the sun radiates, or sends out, an enormous amount of energy. The sun radiates more energy in one second than people have used since the beginning of time! Where does the energy come from that constantly radiates from the sun? It comes from within the sun itself. Like other stars, the sun is a big ball of gases--mostly hydrogen and helium atoms.
The hydrogen atoms in the sun's core combine to form helium and generate energy in a process called nuclear fusion. During nuclear fusion, the sun's extremely high pressure and temperature cause hydrogen atoms to come apart and their nuclei (the central cores of the atoms) to fuse or combine. Four hydrogen nuclei fuse to become one helium atom. But the helium atom contains less mass than the four hydrogen atoms that fused.
Some matter is lost during nuclear fusion. The lost matter is emitted into space as radiant energy. It takes millions of years for the energy in the sun's core to make its way to the solar surface, and then just a little over eight minutes to travel the 93 million miles to earth. The solar energy travels to the earth at a speed of 186,000 miles per second, the speed of light.Only a small portion of the energy radiated by the sun into space strikes the earth, one part in two billion. Yet this amount of energy is enormous. Every day enough energy strikes the United States to supply the nation's energy needs for one and a half years!
Where does all this energy go? About 15 percent of the sun's energy that hits the earth is reflected back into space. Another 30 percent is used to evaporate water, which, lifted into the atmosphere, produces rainfall. Solar energy also is absorbed by plants, the land, and the oceans. The rest could be used to supply our energy needs.
Figure 2.2: Solar energy
The earth receives more energy from the Sun in just one hour than the world's population uses in a whole year. The total solar energy flux intercepted by the earth on any particular day is 4.2 X 1018 Watt hours or 1.5 X 1022 Joules (or 6.26 X 1020 Joules per hour).
2.3 Solar Radiation
Sunlight comes in many colours, combining low-energy infrared photons (1.1 eV) with high-energy ultraviolet photons (3.5 eV) and all the visible-light photons between.
The graph below shows the spectrum of the solar energy impinging on a plane, directly facing the sun, outside the Earth's atmosphere at the Earth's mean distance from the Sun. The area under the curve represents the total energy in the spectrum. Known as the "Solar Constant" G0, it is equal to 1367 Watts per square metre (W/m2).
Irradiance and Insolation.
Total solar irradiance is defined as the the amount of radiant energy emitted by the Sun over all wavelengths, not just visible light, falling each second on a 1 square metre perpendicular plane outside Earth's atmosphere at a given distance from the Sun. It is roughly constant, fluctuating by only a few parts per thousand from day to day.On the outer surface of the Earth's atmosphere the irradiance is known as the solar constant and is equal to about 1367 Watts per square meter.
The amount of solar energy that actually passes through the atmosphere and strikes a given area on the Earth over a specific time varies with latitude and with the seasons as well as the weather and is known as the insolation (incident solar radiation).
When he Sun is directly overhead the insolation, that is the incident energy arriving on a surface on the ground perpendicular to the Sun's rays, is typically 1000 Watts per square metre. This is due to the absorption of the Sun's energy by the Earth's atmosphere which dissipates about 25% to 30% of the radiant energy. Insolation increases with altitude. The terms "irradiance" and "insolation" are often used interchangeably to mean the same thing.
CHAPTER-3
Solar Cell or PV Cell
A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified. Assemblies of cells are used to make solar panels, solar modules, or photovoltaic arrays. Photovoltaic is the field of technology and research related to the application of solar cells in producing electricity for practical use. The energy generated this way is an example of solar energy (also called solar power).
3.1 History of solar cells
The term "photovoltaic" comes from the Greekφῶς (phÅs) meaning "light", and "voltaic", meaning electric, from the name of the Italian physicist Volta, after whom a unit of electro-motive force, the volt, is named. The term "photo-voltaic" has been in use in English since 1849.
The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel. However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Subsequently Russian physicist Aleksandr Stoletov built the first solar cell based on the outer photoelectric effect (discovered by Heinrich Hertz in 1887). Albert Einstein explained the photoelectric effect in 1905 for which he received the Nobel prize in Physics in 1921. Russell Ohl patented the modern junction semiconductor solar cell in 1946, which was discovered while working on the series of advances that would lead to the transistor.
3.2 Photovoltic Principle
3.2.1 Introduction:
The physical phenomenon responsible for converting light into electricity - the photovoltaic effect - was first observed by a French physicist, Edmund Becquerel, in 1839. He noted that a voltage appeared when one of two identical electrodes in a weak conducting solution was illuminated. The photovoltaic effect can be described simply as follows: Light, which is a form of energy, enters a photovoltaic (PV) cell and transfers enough energy to cause the freeing of electrons. A built-in potential barrier in the cell acts on these electrons to produce a voltage which can be used to drive a current through an electric circuit. The first cells were made from selenium during the last century with only 1 - 2% conversion efficiency. Since then, significant research has been done in this field. Quantum mechanics, developed during the 1920s and 930s, laid the theoretical foundation for our present understanding of PV. However, a major step forward in solar-cell technology was done during the 1940s and early 1950s when a method called the Czochralski method was developed for producing highly pure crystalline silicon. Other important triggers for the PV industry were the space programs started in the 1950s and also the development of the transistor industry. Transistors and PV cells are made from similar materials, and many of their working principles are determined by the same physical mechanisms.
3.2.2 Cell Structure:
The basic element in the photovoltaic module is the solar cell which absorbs sunlight and converts it directly into electricity. Figure 3.1 shows the basic structure of a PV cell.
Fig 3.1(a): Cell Structure of PV Cell
Fig 3.1(b) PV Cell Connection
The solar cell consists of a thin piece of semiconductor material, which in most cases is silicon. A semiconductor is an element, whose electrical properties lie between those of conductors and insulators, making it only marginally conductive for electricity. Through a process called "doping" a very small amount of impurities is added to the semiconductor, thus creating two different layers called n-type and p-type layers. A n-type material has an increased number of electrons in the conduction band (n=negative) whereas the p-type material has vacancies of electrons (p=positive). Typically, phosphorus is used to create the n type layer and silicon doped with boron makes the p-type layer. Between these two layers a p-n junction is created which is of great importance for the function of the solar cell.
The light passes through a "window layer" which is thin and therefore absorbs only a small fraction of it. The major part of the light is absorbed in the absorber layer where it creates free electrons that can flow through a wire connected to both sides of the cell. In order to do so, a built-in electrical field is needed. This field is formed along the zone or junction between the two layers of n- and p-type silicon. The current produced by the cell is proportional to the amount of incident light (the number of photons entering the cell). Therefore, current increases with the cell area as well as with the light intensity. The voltage, on the other hand, depends on the material used. A silicon cell produces about 0.5 V regardless of cell area.
Fig 3.2 Voltage v/s Current Characteristic
3.3. Characteristics of the Photovoltaic Cell
3.3.1 Photocurrent and quantum efficiency
The photocurrent generated by a solar cell under illumination at short circuit is dependent on the incident light. To relate the photocurrent density, J, to the incident spectrum we need the cell's quantum efficiency, (QE).
QE(E) is the probability that an incident photon of energy E will deliver one electron to the external circuit. Then Z J = q b (E) QE(E)d E
where b (E) is the incident spectral photon ux density, the number of photons of energy in the range E to E +dE which are incident on unit area in unit time and q is the electronic charge. QE depends upon the absorption coefficient of the solar cell material, the efficiency of charge separation and the efficiency of charge collection in the device but does not depend on the incident spectrum. It is therefore a key quantity in describing solar cell performance under different conditions. Figure 3.6 shows a typical QE spectrum in comparison with the spectrum of solar photons.
Figure 3.3 Voltage Current Curves of a conventional Battery and a Solar Cell
QE and spectrum can be given as functions of either photon energy or wavelength, Energy is a more convenient parameter for the physics of solar cells and it will be used in this book. The relationship between E and is defined by
E = hc/ λ
where h is Planck's constant and c the speed of light in vacuum. A convenient rule for converting between photon energies, in electron-Volts, and wavelengths, in nm, is E eV = 1240(λ=nm).
Figure 3.4 Quantum Effect of GaAs Cell Compared to Solar Spectrum
3.3.2. Dark Current and Open Circuit Voltage
When a load is present, a potential difference developed between the terminals of the cell. This potential difference generates a current which acts in the opposite direction to the photocurrent, and the net current is reduced from its short circuit value. This reverse current is usually called the dark current in analogy with the current Idark(V ) which flows across the device dark under an applied voltage, or bias, V in the dark. Most solar cells behave like a diode in the dark, admitting a much larger current under forward bias (V > 0) than under reverse bias (V < 0). This rectifying behaviour is a feature of photovoltaic devices, since an asymmetric junction is needed to achieve charge separation. For an ideal diode the dark current density
Jdark (V ) varies like
Jdark(V) = Jo (eqv kBT - 1)........................... (3.1)
Where Jo is a constant, kB is Boltzmann's constant and T is temperature in degrees Kelvin.
The overall current voltage response of the cell, its current voltage characteristic, can be approximated as the sum of the short circuit photocurrent and the dark current. This step is known as the superposition approximation. Although the reverse current which flows in response to voltage in an illuminated cell is not formally equal to the current which flows in the dark, the approximation is reasonable for many photovoltaic materials and will be used for the present discussion. The sign convention for current and voltage in photovoltaics is such that the photocurrent is positive. This is the opposite to the usual convention for electronic devices. With this sign convention the net current density in the cell is
J(V) = Jsc - Jdark (V)............................. (3.2)
which becomes, for an ideal diode
J = Jsc - Jo (eqV kBT - 1)......................... (3.3)
When the contacts are isolated, the potential difference has its maximum value, the open circuit voltage Voc. This is equivalent to the condition when the dark current and short circuit photocurrent exactly cancel out.
................... (3.4)
Figure 3.5 Current Voltage Characteristic of Ideal Diode in the Light and Dark
Figure 3.5 shows that the current voltage product is positive, and the cell generates power, when the voltage is between 0 and Voc. At V < 0, the Illuminated device acts as a photo detector, consuming power to generate a photocurrent which is light dependent but bias independent. At V > Voc, the device again consumes power. This is the regime where light emitting diodes operate.
3.4 Types of Photovoltic Cells
3.4.1 Crystalline Solar Cells
The most commonly used cell material is silicon. PV cells made of single-crystal silicon (often called mono crystalline cells) are available on the market today with efficiencies close to 20%. Laboratory cells are close to the theoretical efficiency limits of silicon (29%). Polycrystalline silicon is easier to produce and therefore cheaper. It is widely used, since its efficiency is only a little lower than the single crystal cell efficiency. Gallium arsenide (GaAs) is another single-crystal material suitable for high efficiency solar cells. The cost of this material is considerably higher than silicon which restricts the use of GaAs cells to concentrator and space applications.
3.4.2 Mono-Crystalline
Mono-crystalline cells are made from polished, wafer thin slices of single crystals of silicon. Originally, all PV cells were made in this way. These tend to be expensive, as the crystals take time to grow, but it does produce the most efficient cells (~16%). These cells have a life span of about 40 years with most manufacturers guaranteeing output for 10 years.
Figure 3.6: Mono-Crystalline Solar Cells
3.4.3 Polycrystalline Solar Cells
Polycrystalline cells are made from slices of ingots cast from raw silicon crystals giving the characteristic flaked appearance. This method gives a better surface coverage than mono-crystalline cells and is cheaper than growing large crystals. These cells have the second highest efficiency (~12%) and are fast becoming the most popular form of cell. They also have as long a life span as mono-crystalline cells with similar performance guarantees.
Figure 3.7: Polycrystalline PV Cell
3.4.4 Amorphous Solar Cells
Amorphous cells are made by spraying the silicon directly onto glass or ceramic in layers. These types of cells are the cheapest to produce but contain impurities, so the overall conversion of light to electricity is low (~5%). They also have a much shorter life span than others and are usually guaranteed for 6 years. Recent developments in this field are constantly improving performance.
Figure 3.8: Amorphous PV Cell
3.5 Module Model
For the majority of applications multiple solar cells need to be connected in series or in parallel to produce enough voltage and power. Individual cells are usually connected into a series string of cells (typically 36 or 72) to achieve the desired output voltage. The complete assembly is usually referred to as a module and manufacturers basically sell modules to customers. The modules serves another function of protecting individual cells from water, dust etc. as the solar cells are placed into an encapsulation of single or double at glasses.
Figure: 3.9 Structure of a PV module with 36 cells connected in series
Within a module the different cells are connected electrically in series or in parallel although most modules have a series connection. Figure 3.3 shows a typical connection of how 36 cells are connected in series. In a series connection the same current flows through all the cells and the voltage at the module terminals is the sum of the individual voltages of each cell. It is therefore, very critical for the cells to be well matched in the series string so that all cells operate at the maximum power points. When modules are connected in parallel the current will be the sum of the individual cell currents and the output voltage will equal that of a single cell.
3.6 Array Model
An array is a structure that consists of a number of PV modules, mounted on the same plane with electrical connections to provide enough electrical power for a given application. Arrays range in power capacity from a few hundred watts to hundreds of kilowatts. The connection of modules in an array is similar to the connection of cells in a single module. To increase the voltage, modules are connected in series and to increase the current they are connected in parallel. Matching is again very important for the overall performance of the array. The structure of an array is shown in
Figure 3.10: Structure of PV Array
Figure 3.10, which has 4 parallel connections of 4 module strings connected in series. The voltages for n modules in series are given as:
For an array to perform well all the modules must not be shaded otherwise it will act as a load resulting in heat that may cause damage. Bypass diodes are usually used to avoid damage although they result in further increase in cost. Integration of bypass diodes in some large modules during manufacturing is not uncommon and reduces the extra wiring required. It must be pointed out though that it becomes very difficult to replace the diode if it fails.
3.7 Applications and implementations
Solar cells are often electrically connected and encapsulated as a module. Photovoltaic modules often have a sheet of glass on the front (sun up) side, allowing light to pass while protecting the semiconductor wafers from the elements (rain, hail, etc.). Solar cells are also usually connected in series in modules, creating an additive voltage. Connecting cells in parallel will yield a higher current. Modules are then interconnected, in series or parallel, or both, to create an array with the desired peak DC voltage and current.
The power output of a solar array is measured in watts or kilowatts. In order to calculate the typical energy needs of the application, a measurement in watt-hours, kilowatt-hours or kilowatt-hours per day is often used. A common rule of thumb is that average power is equal to 20% of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.8 kWh per day (24 hours x 1 kW x 20% = 4.8 kWh)
To make practical use of the solar-generated energy, the electricity is most often fed into the electricity grid using inverters (grid-connected photovoltaic systems); in stand-alone systems, batteries are used to store the energy that is not needed immediately.
Solar cells can also be applied to other electronics devices to make it self-power sustainable in the sun. There are solar cell phone chargers, solar bike light and solar camping lanterns that people can adopt for daily use.
CHAPTER 4
PHOTOVOLTIC (PV) SYSTEMS
4.1 General
Photovoltaic systems are composed of interconnected components designed to accomplish ranging from powering a small device to feeding electricity into the main distribution grid. Photovoltaic systems are classified as shown in figure 4.1. The two main general classification according to the figure are ther stand-alone and grid connected systems
Figure 4.1: Classification of the PV system
To cater for different load patterns, storage elements are generally used and most systems currently use batteries for storage.
Photovoltaic (PV) systems are of a modular nature. Solar cells can be connected in series or parallel in virtually any number and combination. Therefore, PV systems may be realized in an extraordinary broad range of power: from milliwatt systems in watches or calculators to megawatt systems for central power production. Building power supply systems are usually in the range of several kilowatts of nominal power. There are two basically different PV systems: those with a connection to an (available) electric grid and remote or "stand-alone" systems. While in the first case the grid serves as an ideal storage component and ensures system reliability, the stand-alone systems require a storage battery. This battery serves as a buffer between the fluctuating power generated by the PV cells and the load. In order to ensure continuous power supply, even under extreme conditions, a back-up generator is often also installed. Building-integrated PV systems have an economical advantage over conventional PV generator systems: The PV modules serve for multiple purposes. They are part of the building envelope, ideally replacing conventional facade or roof material. Modem commercial building facades often cost as much as a PV facade which means immediate or short-term payback for the PV system. Depending on the type of integration, the PV modules may also provide shading or noise protection. Here again, the costs for replaced conventional means for these purposes may be deducted from the initial PV costs.
4.2 Grid Connected or On Grid PV Power System
An on-grid solar PV system essentially uses the existing commercial utility system for power and does not store electrical power. A grid connected solar PV system is shown in Figure 4.1. A solar PV system is installed into the electrical system of a home or facility for use during daylight hours or when grid power is down. It also works the other way, when the solar PV system does not produce enough electricity, it can draw power from the grid. When using the solar PV system, if more electricity is produced than what is needed the excess can be put back on the grid.
This is done automatically through a device that monitors the available power and switches between solar and grid power. A second utility meter can be added to keep track of how much electricity has been put back on the grid. Advantages of grid interconnection include having uninterrupted access to standard utility power and avoiding the cost of a battery back- up system. A disadvantage is the utility interconnection fee, reliability of solar components, and the initial cost of the solar PV system.
Figure 4.2: Grid Connected Solar Photo Voltaic (PV)System
4.3 Stand Alone or Off Grid PV Power System
4.3.1 Without Storage System
Stand-alone PV systems are designed to operate independent of the electric utility grid, and are generally designed and sized to supply certain DC and/or AC electrical loads. Stand-alone systems may be powered by a PV array only, or may use wind, an engine-generator or utility power as a backup power source in what is called a PV-hybrid system. The simplest type of stand-alone PV system is a direct-coupled system, where the DC output of a PV module or array is directly connected to a DC load.
Figure 4.3: Direct Coupled stand alone Solar Photo Voltaic (PV) System
Since there is no electrical energy storage (batteries) in direct-coupled systems, the load only operates during sunlight hours, making these designs suitable for common applications such as ventilation fans, water pumps, and small circulation pumps for solar thermal water heating systems.
4.3.2 With Storage System
In many stand-alone PV systems, batteries are used for energy storage. Below is a diagram of a typical stand-alone PV system with battery storage powering DC and AC loads.
Figure 4.4: Standalone Solar Photovolatic (PV) System with Battery Storage
PV systems are most effective at remote sites off the electrical grid, especially in locations where the access is possible by air only, e.g. in alpine regions. Their high reliability and low servicing requirements make them ideally suited for applications at (for parts of the year) unattended sites. The costs for a PV system compete in this case against the cost for a grid connection or other possible ways of remote energy supply. As stated above, a storage battery is needed. Excess energy produced during times with no or low loads charges the battery, while at times with no or too low solar radiation the loads are met by discharging it.
4.4 Hybrid or Direct Use PV Power System
A combination of an on-grid and off-grid solar PV system has the advantages of both. A hybrid system is connected to the utility grid in case of poor weather or night use, but also has a battery bank to store electricity in case utility grid power is lost. The design and installation of hybrid systems is more complicated and expensive, but they are the most effective in providing constant, reliable electricity. Figure 4.9 depicts a hybrid solar PV system.
Figure 4.5: Hybrid Solar Photovolatic System
There are applications where the load matches the available radiation exactly. This eliminates the need for any electricity storage and backup. A typical example is the electricity supply for a circulation pump in a thermal collector system.
CHAPTER 5
MODELLING & SIMULATION OF 300W STAND-ALONE PV SYSTEM
5.1 Overview
This report present a simulink model of a standalone photovoltaic system with a battery bank. The model of the PV system is made up by blocks in order to facilitate the modelling of other structures of the PV systems.
Many photovoltaic system operate in stand-alone mode. Such system consist of a PV generator, energy storage element like battery AC & DC consumers and elements for power conditioning as shown in fig: 5.1
Figure 5.1: Elementary scheme of the component of a Stand-alone PV system
A photovoltaic (PV) generator can contain several Arrays. Each array is composed of several modules, while each module is composed of several solar cells. Battery bank stores energy when the supplied by the PV module exceeds load demand and releases it back when PV supply is insufficient. The load for the stand-alone PV system can be of many types, both DC (television, lightning) and AC (electric motors, heaters, etc.). Power conditioning system provides an interface between all the element of PV system, giving protection and control. The most frequently encountered element of the power conditioning system are blocking diodes, charge controllers and DC to AC converters.
