Introduction

PROJECT REPORT ON Solar Energy Plant

INTRODUCTION All forms of energy on the earth are derived from the sun. However, the more conventional forms of energy, the fossil fuels received their solar energy input eons ago and posses the energy in a greatly concentrated form. These concentrated solar energy sources are being used as such at a rapid rate that they will be depleted in not too distant in future.

There are four primary sources of energy viz. petroleum, natural gas, coal and wood. Except wood, all these common sources have finite supply. These Fossil fuel reserves are limited. In addition, when burnt, these add to global warming, air pollution and acid rain.

Another source of energy is nuclear power system. The new capacity installation decisions today are becoming complicated in many parts of the world because of difficulty in finding sites for new generation and transmission facilities of any kind. In the U.S.A., no nuclear power plants have been ordered since 1978. Given the potential for cost overruns, safety related design changes during the construction, and local opposition to new plants, most utility executives suggest that none will be ordered in the foreseeable future. Assuming that no new nuclear plants are built, and that the existing plants are not relicensed at the expiration of their 40-year terms, the nuclear power output is expected to decline sharply after 2010. This decline must be replaced by other means. With gas prices expected to rise in the long run, utilities are projected to turn increasingly to coal for base load-power generation. The U.S.A. has enormous reserves of coal, equivalent to more than 250 years of use at current level. However, that will need clean coal burning technologies that are fully acceptable to the public.

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An alternative to the nuclear and fossil fuel power is renewable energy technologies (hydro, wind, solar, biomass, geothermal, and ocean). Large scale hydroelectric projects have

become increasingly difficult to carry through in recent years because of competing use of land and water.

Table 1: Status of Conventional and Renewable Power Sources

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Conventional

Renewable

Coal, nuclear, oil, and natural gas

Wind, solar, biomass geothermal, and ocean

Fully matured technologies

Rapidly developing technologies

Numerous tax and investment subsidies embedded in national economies

Some tax credits and grants available from some federal and/or state governments

Accepted in society under the ‘grandfather clause’ as necessary evil

Being accepted on its own merit, even with limited valuation of their environmental and other social benefits

SECTION Ⅱ SUN The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers (about 109 Earths) and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three-quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others. The Sun is a fairly ordinary star which formed in an open star cluster about 4.65 billion years ago.

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The Sun, like all other stars, is a nuclear fusion reactor. Every second our Sun changes 4 billion kilograms of protons into light energy. So far the Sun has only used up half its hydrogen fuel, so it should last for at least another 4 billion years before it turns into a red giant, and destroys the inner planets including the Earth.

The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering. Its stellar classification, based on spectral class, is G2V, and is informally designated a yellow star, because the majority of its radiation is in the yellow-green portion of the visible spectrum. In this spectral class label, G2 indicates its surface temperature of approximately 5,778 K (5,505 °C; 9,941 °F), and V (Roman five) indicates that the Sun, like most stars, is a main sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the sun fuses 430– 600 million tons of hydrogen each second.

The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though this varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather.

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Energy from the Sun [12] The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet.

Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 째C. By photosynthesis green plants convert solar energy into chemical energy, which produces

food,

wood

and

the

from fossil

biomass which fuels

are

derived.

Figure 1: Breakdown of the incoming solar energy [19] The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass. The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth's nonrenewable resources of coal, oil, natural gas, and mined uranium combined.

Yearly Solar fluxes & Human Energy Consumption [12]

Solar Wind Biomass Primary energy use (2005) Electricity (2005)

3,850,000 EJ 2,250 EJ 3,000 EJ 487 EJ 56.7 EJ

From the table of resources it would appear that solar, wind or biomass would be sufficient to supply all of our energy needs, however, the increased use of biomass has had a negative effect on global warming and dramatically increased food prices by diverting forests and crops into biofuel production. As intermittent resources, solar and wind raise other issues.

Solar energy can be harnessed in different levels around the world. Depending on a geographical location the closer to the equator the more "potential" solar energy is available.

Solar energy distribution can be shown in following diagram

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Utilization of solar energy Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar-powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.

Solar powered electrical generation relies on heat engines and photovoltaics. Solar energy's uses are limited only by human ingenuity. A partial list of solar applications includes space heating and cooling through solar architecture, potable water via distillation and disinfection, daylighting, solar hot water, solar cooking, and high temperature process heat for industrial purposes.To harvest the solar energy, the most common way is to use solar panels.

Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

There are main two ways we can produce electricity from the sun [20]: Photovoltaic Electricity – This method uses photovoltaic cells that absorb the direct sunlight just like the solar cells you see on some calculators.

1.

Solar Thermal Electricity – This also uses a solar collector: it has a mirrored surface that reflects the sunlight onto a receiver that heats up a liquid. This heated liquid is used to make steam that produces electricity.

SECTION â…˘ 2.

PHOTOVOLTAIC SYSTEM Photovoltaic System The most useful way of harnessing solar energy is by directly converting it into electricity by means of solar photovoltaic cells. A photovoltaic cell is a type of photoelectric cell that uses the photovoltaic effect to generate electrical energy using the potential difference that arises between materials when the surface of the cell is exposed to electromagnetic radiation. A photovoltaic cell is commonly used for detecting radiation (for example, infrared detectors), measurement of light intensity (such as in measuring optical density), chemical processes (for example, spectrophotometry), and for conversion of light energy to electricity in conversion photovoltaic cells (when only the solar light is converted they are called solar cells). The photovoltaic cell was developed in 1954 at Bell Laboratories.

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When sunshine is incident on solar cells they generate DC electricity without the involvement of any generators. In this system of energy conversion there is direct conversion of solar radiation in electricity.

The photo voltaic effect is defined as the generation of an electromotive force as a result of the absorption of ionizing radiation. Energy conversion device which are used to convert sunlight to electricity by the use of the photo voltaic effect are called solar cells.

In recent years photo-voltaic (PV) power generation has been receiving considerable attention as one of the more promising energy alternative.

The reason for this rising

interests lies in PV’s direct conversion of sunlight to electricity, the non-polluting nature of the PV conversion process, and PV’s non-dependence of fossil and nuclear fuels.

To date, the wide spread use of PV generation has been hampered by economic factors.

The solar cells should be distinguished from devices usually referred to as “photo cells” which detect light intensity by use of the photo conductivity characteristics of materials.

‘Photocell’s are very sensitive to light, since their conductivities may change by many order of magnitude with small variations of light intensity. These photocells are typically used for light meters in cameras.

The photo voltaic effect can be observed in all most any junction materials that have different electrical characteristics but the best performance has been from cells using semiconductor materials. Essentially all of the solar cells used for both space and terrestrial applications have been made of the semiconductor materials silicon. Futures cells may use materials as the semiconductor like Galium Arsenide, Copper Sulphate, Cadsulphide etc.

The photovoltaic (PV) power technology uses semiconductor cells (wafers), generally several square centimeters in size. From the solid-state physics point of view, the cell is basically a large area p-n diode with the junction positioned close to the top surface. The cell converts the sunlight into direct current electricity. Numerous cells are assembled in a module to generate required power.

The PV cell manufacturing process is energy intensive. Every square centimeter cell area consumes a few kWh before it faces the sun and produces the first kWh of energy. However, the manufacturing energy consumption is steadily declining with continuous implementation of new production processes.

The present PV energy cost is still higher than the price the utility customers pay in most countries. For that reason, the PV applications have been limited to remote locations not connected to the national grid lines in Bangladesh. With the declining prices, the market of new modules has been growing at more than a 15 percent annual rate during the last five years.

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Advantages of PV Energy Conversion [1] Photo voltaic solar energy conversion is one of the most attractive non-conventional energy sources of proven reliability from the micro to mega watt level. Its advantages are:

1. Solar power is highly modular in that their capacity can be increased incrementally to match with gradual load growth.

2. Direct room temperature conversion of solar radiation to electricity to a simple solid state device, 3. Absence of moving parts.

4. Ability to function unattended for long period as evidence in space programme.

5. The maintenance cost is low as they are used to operate.

6. Their construction lead time is significantly shorter than those of the conventional plants, thus reducing the financial and regulatory risks.

7. They do not create pollution.

8. They have a long effective life, and 9. They are highly reliable.

Some disadvantages of such systems are [1]: 1. Distributed nature of solar energy, 2. Absence of energy storage.

3. Relatively high capital cost.

While the first this disadvantages can be partly overcome by concentration, the second is inherent disadvantage overcome in PV systems by use of conventional storage batteries.

A PV (photo-voltaic) system consists of: I. II. III. IV.

Solar cell array.

Load leveler.

Storage system.

Tracking system (where necessary).

Thus the solar cell is only a part though the most important part of PV systems.

In actual usage, the solar cells are interconnected in certain series/parallel combinations to form modules. These modules are hermetically sealed for protection against corrosion, moisture, pollution and weathering. A combination of suitable modules constitutes an array.

One square meter of a fixed array kept facing south yields nearly 0.5 kWh of electrical energy on a normal sunny day. If the orientation of the array is adjusted to face the sun’s rays at any time, the output can increase by 30 percent. Solar PV system can produce an output only if sunlight is present. If it is required to be used during non-sunshine hours, a suitable system of storage batteries will be required.

Semiconductor Principles The conductivities of solid materials vary over an enormous range. The semi conductors are class of materials with electric conductivity somewhere between metals and insulators. The distinguishing features of conductors, insulators and semiconductors are best explained in terms of what is called band theory or energy bans concept of materials.

There exist energy bands in which electrons are allowed to exist and other bands in which electrons are not allowed to exist within the system. The electrons are constantly seeking the lower energy levels but are constantly being exited to higher states by interactions such as with photons. The band representation indicates only were electrons are allowed to exist and does not indicate the energy the electron usually have.

The distribution of electrons in the outermost or highest energy band determines most of the electrical and thermal properties of the material. This is similar to the outermost electron in an atom, the valance electron that mostly determine the atoms chemical characteristics. The highest occupied band corresponds to the ground state of the outer most valence electrons and is thus called valence band.

If a material has an outer most allowed energy band partially filled, an external applied electrical field can shift the occupation of the energy level and cause a current to flow. If a band of allowed energy states is completely empty, there can, of course, be no contribution to an electric current in that band. Similarly, if a band is completely filled, there can be no contribution to an electric current by that band. [1]

CONDUCTION Electrons (-ve charge

CONDUCTION BAND (EMPTY) FORBIDDEN ENERGY GAP

HOLES (+ve charge carrier)

VELENCE BAND (FULL)

a) Intrinsic insulator or 0째C

b) Intrinsic semiconductor at T >

semiconductor at 0째C

PARTIALLY FILLED BAND ENERGY FERMY

ELECTRONS (FREE TO MOVE)

c) Metal or good conductor at 0째K

d) conductor at T째K

Figure 2: The band configuration. [1] In an insulator the valence band is completely filled. In that case the conduction band is so large that valence electrons can accept no energy from applied field. Semi conductors are similar to insulator in band configuration except that in semi conductors the forbidden energy gap is much narrower. If an electron is exited across the forbidden gap it leaves a vacancy in the valence band which is referred to as a hole. An electron near a hole can jump into fill that vacancy, leaving a new hole which in turn be filled by another electron and so on. Current is

then actually carried by electrons moving in relays but it can equally be pictured as a flow of positively charged hole moving in opposite direction. Thus conduction is by both electrons and holes. When conduction of current is only for the excited electrons the materials is called an intrinsic semi conductors.

Most commercially used semi conductors are made with intentionally added impurities to give desired characteristics. By adding small amount of impurities called dopants into semi conductors crystals it is possible to create a controllable number of allowed energy levels within the forbidden energy gap. When the conduction in a semi conductor is due to impurities the materials is called extrinsic semi conductor. [1]

S

S S

S

S

S

S S

S

S

S

S

S

S S

S

S

Fig 3.a: Normal silicon bonding impurity(n-type)

S

Fig

S S

S

S

P

S

S S

B S

3.b:

S

S

Fig 3.c : Deficient electron impurity(p-type)

Excess

electron

Figure schematically shows impurities atoms in a silicon crystal structure. a) shows the normal bond that exist by the shared electron in a perfect silicon crystal. Substituting a phosphorus atom as shown in (b) makes an extra electron available. Since this extra electron held in position only by coulomb attraction to the phosphorus nucleus, it can be removed and made a conduction electron with for less energy than required to remove in silicon. In a similar way, an impurity such as Boron shown in fig (c) has deficiency of one electron that a silicon neighbor would like to share. A valence electron can jump into this location with for less energy than would be required to jump the entire energy gap. [1] Semiconductor diodes A modern semiconductor diode is made of a crystal of semiconductor like silicon that has impurities added to it to create a region on one side that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called p-type semiconductor. The diode's terminals are attached to each of these regions. The boundary within the crystal between these two regions, called a PN junction, is where the action of the diode takes place. The crystal conducts conventional current in a direction from the ptype side (called the anode) to the n-type side (called the cathode), but not in the opposite direction.

The photo voltaic cell is such a semiconductor where electric conductivity can controlled by doping. In the dark this act as diode but in the presence of sun light or voltage from outside the PV cell act as semicondutor. [12] Fig 4: Diode sign [12]

Photo-Voltaic Principles [1] When the suns rays i.e. the suns photons flux, strike a PN junction semi conductor junction, they help generate electron-hole pairs; so that they cause electrons to be raised to the conduction band in the N material and holes to be moved to the valence band in the P material when connected to a load, electron will thus diffuse from n to p across the junction, thus creating a electric current to the loads and hence electric power, which is a function of the photon flux.

The electric field in most solar cells is provided by a junction of materials which have different electrical properties.

To obtain a useful power output from photon interaction in a semiconductor, three processes are required: 1. The photon has to be absorbed in the active part of the materials and result in electrons being excited to a higher energy potential.

2. The electrode hole charge created by the absorption must be physically separated and moved to the edge of the cell.

3. The charge carriers must be removed from the cell and delivered to a useful load before they loose their extra potential.

For completing the above processes, a solar cell consists of : (a) Semi conductor in which electron hole pairs are created by absorption of incident solar radiation, (b) Region containing a drift field for charge separation, and (c) Charge collecting front and back electrodes.

The photo voltaic effect can be described easily for p-n junction in a semiconductor. In an intrinsic semiconductor such as silicon, each one of the four valence electrons of the material atom is tied in a chemical bond, and there are no free electron at absolute zero. If a piece of such a material is doped on one side by a five valence electron material such as arsenic or phosporus, there will be an excess of electron in that side, becoming an n-type semiconductor. The excess electron will be practically free to move in the semiconductor lattice. When the other side of the same piece is dopped by three valence electron material such as boron, there will be deficiency of electrons leading to a p-type semi conductor. This deficiency is expressed in terms of excess of holes free to move in the lattice. Such a piece of semiconductor, with one side of the p-type and other of the n-type is called a p-n junction.

In this junction after the photons are absorbed, the free electron of n-side will tend to flow to the p-side, and the holes of the p-side will tend to flow to the n-region to compensate for their respective deficiencies. This diffusion will create an electric field E f from the n-region to the p-region. This field will increase until it reaches the equilibrium for V e the sum of the diffusion potential for holes and electrons.

sun

Solar radiation n- Electrode n-type surface layer

P – Type (base)

P-electrodes

Fig 5: A typical n on p photovoltaic.

If the electrical contacts are made with the two semiconductor materials and the contacts are connected through an external electrical conductor, the free electrons will flow from the ntype material through the conductor to the p-type material. Here the free electrons will enter the holes and become bound electron, thus both free electrons and holes be removed. The flow of the electrons through the external conductor constitutes an electric current which will continue as long as more free electrons and holes are being formed by the solar radiation.

This is the basis of photo-voltaic conversion that is, the conversion of solar energy to electrical energy. The combination of n-type and p-type semiconductors thus constitute PV cell or solar cell.

The PV Cell [3] The physics of the PV cell is very similar to the classical p-n junction diode (Figure 6). When light is absorbed by the junction, the energy of the absorbed photons is transferred to the electron system of the material, resulting in the creation of charge carriers that are separated at the junction. The charge carriers may be electron-ion pairs in a liquid electrolyte, or electron hole pairs in a solid semiconducting material. The charge carriers in the junction region create a potential gradient, get accelerated under the electric field and circulate as the current through an external circuit. The current squared times the resistance of the circuit is the power converted into electricity. The remaining power of the photon elevates the temperature of the cell.

Figure 6: Photovoltaic effect converts the photon energy into voltage across the p-n junction. [3]

The origin of the photovoltaic potential is the difference in the chemical potential, called the Fermi level, of the electrons in the two isolated materials. When they are joined, the junction approaches a new thermodynamic equilibrium. Such equilibrium can be achieved only when the Fermi level is equal in the two materials. This occurs by the flow of electrons from one material to the other until a voltage difference is established between the two materials which have the potential just equal to the initial difference of the Fermi level. This potential drives the photocurrent Figure 7 shows the basic cell construction. For collecting the photocurrent, the metallic contacts are provided on both sides of the junction to collect electrical current induced by the impinging photons on one side. Conducting foil (solder) contact is provided over the bottom (dark) surface and on one edge of the top (illuminated) surface. Thin conducting mesh on the remaining top surface collects the current and lets the light through. The spacing of the conducting fibers in the mesh is a matter of compromise between maximizing the electrical conductance and minimizing the blockage of the light. In addition to the basic elements, several enhancement features are also included in the construction. For example, the front face of the cell has anti-reflective coating to absorb as much light as possible by minimizing the reflection. The mechanical protection is provided by the coverglass applied with a transparent adhesive.

Figure 7: Basic construction of PV cell with performance enhancing features coating and cover glass protection).

PV Solar cell type [21] 1. Single-Crystalline Silicon 2. Polycrystalline and Semicrystalline 3. Cadmium sulfide solar cell 4. Gallium arsenide multijunction 5. Amorphous Silicon 6.

Copper-Indium Selenide

7. Organic/polymer solar cells 8. Nanocrystalline solar cells

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(current collecting mesh, anti-reflective

9. Thin-film solar cells

Crystalline silicon cell The single crystal silicon is the widely available cell material, and has been the workhorse of the industry. By far, the most prevalent bulk material for solar cells is crystalline silicon (abbreviated as a group as c-Si), also known as "solar grade silicon". Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

1. Monocrystalline silicon (c-Si): Monocrystalline silicon often made using the Czochralski process. Single-crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the four corners of the cells.

In the most common method of producing this material, the silicon raw material is first melted and purified in a

crucible. A seed crystal is

then placed in the liquid silicon and drawn at a slow constant rate. This results in a solid, single-crystal cylindrical ingot (Figure 8). The manufacturing process is slow and energy intensive, resulting in high raw material cost presently at $25 to $30 per pound. The ingot is sliced using a diamond saw into 200 to 400 Îźm (0.005 to 0.010 inch) thick wafers. The wafers are further cut into rectangular cells to maximize the number of cells that can be mounted together on a rectangular panel.

Unfortunately, almost half of the expensive silicon ingot is wasted in slicing ingot and forming square cells. The material waste can be minimized by making the full size round cells from round ingots. Using such cells would be economical where the panel space is not at a premium. Another way to minimize the waste is to grow crystals on ribbons. [3] Fig 8: Single-crystal ingot-making by Czochralski process.

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2. Poly- or multicrystalline silicon (poly-Si or mc-Si)

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: Poly or multi crystalline

made from cast square ingots — large blocks of molten silicon carefully cooled and solidified. Poly-Si cells are less expensive to produce than single crystal silicon cells, but are less efficient. US DOE data shows that there were a higher number of multicrystalline sales than monocrystalline silicon sales.

Advantages of multicrystalline silicon over monocrystalline silicon produced by Czochralski are lower capital cost, higher throughput and higher tolerance to poor feedstock quality.

The technique involves controllably solidifying molten silicon in a suitable container to give ingots with large columnar grains generally growing from the bottom of the crucible upwards.

3. Ribbon silicon

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: Ribbon silicon is a type of multicrystalline silicon: it is formed by

drawing flat thin films from molten silicon and results in a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots. Some U.S. companies have set up plants to draw pv ribbons, which are then cut by laser to reduce

Silicon processing – recent development [12] One way of reducing the cost is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO 2) to produce silicon is a very high energy process - at current efficiencies, it takes one to two years for a conventional solar cell to generate as much energy as was used to make the silicon it contains. More energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole.

The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 °C. In this process, known as carbothermic reduction, each tonne of silicon (metallurgical grade, about 98% pure) is produced with the emission of about 1.5 tonnes of carbon dioxide.

Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 °C). While this new process is in principle the same as the FFC Cambridge Process which was first discovered in late 1996, the interesting laboratory finding is that such electrolytic silicon is in the form of porous silicon which turns readily into a fine powder, with a particle size of a few micrometres, and may therefore offer new opportunities for development of solar cell technologies.

Another approach is also to reduce the amount of silicon used and thus cost, is by micromachining wafers into very thin, virtually transparent layers that could be used as transparent architectural coverings. The technique involves taking a silicon wafer, typically 1 to 2 mm thick, and making a multitude of parallel, transverse slices across the wafer, creating a large number of slivers that have a thickness of 50 micrometres and a width equal to the thickness of the original wafer.

These slices are rotated 90 degrees, so that the surfaces corresponding to the faces of the original wafer become the edges of the slivers. The result is to convert, for example, a 150 mm diameter, 2 mm-thick wafer having an exposed silicon surface area of about 175 cm2 per side into about 1000 slivers having dimensions of 100 mm × 2 mm × 0.1 mm, yielding a total exposed silicon surface area of about 2000 cm2 per side. As a result of this rotation, the electrical doping and contacts that were on the face of the wafer are located at the edges of the sliver, rather than at the front and rear as in the case of conventional wafer cells. This has the interesting effect of making the cell sensitive from both the front and rear of the cell (a property known as bifaciality). Using this technique, one silicon wafer is enough to build a 140 watt panel, compared to about 60 wafers needed for conventional modules of same power output.

waste.

Gallium arsenide multi junction [12]

Multi-junction photovoltaic cells are a sub-class of solar cell developed for higher efficiency. These multi-junction cells consist of multiple thin films produced using molecular beam epitaxy and / or metal organic vapour phase epitaxy. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly all of the solar spectrum, thus generating electricity from as much of the solar energy as possible.

In short, in the multijunction structure, several layers each capture part of the sunlight passing through the cell. These layers allow the cell to capture more of the solar spectrum and convert it into electricity.

High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP 2.

GaAs based multijunction devices are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under "500-sun" solar concentration and laboratory conditions. This technology is currently being utilized in the Mars rover missions.

Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge pn junctions, are seeing demand rapidly rise. In just the past 12 months (12/2006 - 12/2007), the cost of 4N gallium metal has risen from about $350 per kg to $680 per kg. Additionally, germanium metal prices have risen substantially to $1000–$1200 per kg this year. Those materials include gallium (4N, 6N and 7N Ga), arsenic (4N, 6N and 7N) and germanium, pyrolitic boron nitride (pBN) crucibles for growing crystals, and boron oxide, these products are critical to the entire substrate manufacturing industry.

Triple-junction GaAs solar cells were also being used as the power source of the Dutch fourtime World Solar Challenge winners Nuna in 2005 and 2007, and also by the Dutch solar cars Solutra (2005) and Twente One (2007).

The Dutch Radboud University Nijmegen set the record for thin film solar cell efficiency using a single junction GaAs to 25.8% in August 2008 using only 4 Âľm thick GaAs layer which can be transferred from a wafer base to glass or plastic film.

In a single band gap solar cell, efficiency is limited due to the inability to efficiently convert the broad range of energy that photons possess in the solar spectrum. Photons below the band gap of the cell material are lost; they either pass through the cell or are converted to only heat within the material. Energy in the photons above the band gap energy is also lost, since only the energy necessary to generate the hole-electron pair is utilized, and the remaining energy is converted into heat.

By utilizing multiple junctions with several band gaps, different portions of the solar spectrum may be converted by each junction at a greater efficiency.

Copper-Indium Selenide [12] The materials based on CuInSe 2 that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for thin film solar cell application because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device.

CIS is an abbreviation for general chalcopyrite films of copper indium selenide (CuInSe2), CIGS mentioned below is a variation of CIS. CIS films (no Ga) achieved greater than 14% efficiency. However, manufacturing costs of CIS solar cells at present are high when compared with amorphous silicon solar cells but continuing work is leading to more costeffective production processes. The first large-scale production of CIS modules was started in 2006 in Germany by W端rth Solar. Manufacturing techniques vary and include the use of Ultrasonic Nozzles for material deposition. Electro-Plating in other efficient technology to apply the CI(G)S layer.

When gallium is substituted for some of the indium in CIS, the material is referred to as CIGS, or copper indium/gallium diselenide, a solid mixture of the semiconductors CuInSe 2 and CuGaSe2, often abbreviated by the chemical formula CuIn xGa(1-x)Se2. Unlike the conventional silicon based solar cell, which can be modelled as a simple p-n junction, these cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as of March 2008 was 19.9% with CIGS absorber layer. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light or by using multi-junction tandem solar cells. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage, but decreasing the short circuit current. In another point of view, gallium is added to replace indium due to gallium's relative availability to indium. Approximately 70% of indium currently produced is used by the flat-screen monitor industry. However, the atomic ratio for Ga in the >19% efficient CIGS solar cells is ~7%, which corresponds to a bandgap of ~1.15 eV. CIGS solar cells with higher Ga amounts have lower efficiency. For example, CGS solar cells (which have a bandgap of ~1.7 eV have a record efficiency of 9.5% for pure CGS and 10.2% for surface-modified CGS. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium. Producing 2 GW of CIGS cells (roughly the amount of silicon cells produced in 2006) would use about 10% of the indium produced in 2004. For comparison, silicon solar cells used up 33% of the world's electronic grade silicon production in 2006.

Se allows for better uniformity across the layer and so the number of recombination sites in the film are reduced which benefits the quantum efficiency and thus the conversion efficiency

Organic/polymer solar cells [12] Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes and fullerene derivatives such as PCBM. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials. However, it improved quickly in the last few years and the highest NREL (National Renewable Energy Laboratory) certified efficiency has reached 6.77%. In addition, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built-in electric field of a PN junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.

The invention of conductive polymers (for which Alan Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded a Nobel prize) may lead to the development of much cheaper cells that are based on inexpensive plastics. However, organic solar cells generally suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable. The bonds in the polymers, are always susceptible to breaking up when radiated with shorter wavelengths.

Additionally, the conjugated double bond systems in the polymers which carry the charge, react more readily with light and oxygen. So most conductive polymers, being highly unsaturated and reactive, are highly sensitive to atmospheric moisture and oxidation, making commercial applications difficult. [12]

Nanocrystalline solar cells [12] These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption). Using nanocrystals allows one to design architectures on the length scale of nanometers, the typical exciton diffusion length. In particular, single-nanocrystal ('channel') devices, an array of single p-n junctions between the electrodes and separated by a period of about a diffusion length, represent a new architecture for solar cells and potentially high efficiency.

Experimental non-silicon solar panels can be made of quantum heterostructures, e.g. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot modified photovoltaics may be able to achieve up to 42% energy conversion efficiency due to multiple exciton generation (MEG)

Thin-film solar cells [12] Thin-film photovoltaic cells can use less than 1% of the expensive raw material (silicon or other light absorbers) compared to wafer-based solar cells, leading to a significant price drop per Watt peak capacity. There are many research groups around the world actively researching different thin-film approaches and/or materials. However, it remains to be seen if these solutions can achieve a similar market penetration as traditional bulk silicon solar modules.

One particularly promising technology is crystalline silicon thin films on glass substrates. This technology combines the advantages of crystalline silicon as a solar cell material (abundance, non-toxicity, high efficiency, long-term stability) with the cost savings of using a thin-film approach.

Another interesting aspect of thin-film solar cells is the possibility to deposit the cells on all kind of materials, including flexible substrates (PET for example), which opens a new dimension for new applications.

In 2002, the highest reported efficiency for thin film solar cells based on CdTe is 18%, which was achieved by research at Sheffield Hallam University, although this has not been confirmed by an external test laboratory.

The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide thin films, also known as CIGS NREL has since developed a robot that builds and analyzes the efficiency of thin-film solar cells with the goal of increasing the efficiency by testing the cells in different situations.

These CIGS films have been grown by physical vapour deposition in a three-stage coevaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again.

Thin film solar has approximately 15% marketshare; the other 85% is crystalline silicon.

Most of the commercial production of thin film solar is CdTe with an efficiency of 11%.

As of 28th of April 2010, ZSW in Stuttgart have released a statement claiming they have created CIGS-based cells with a new record 20.1% efficiency.

Solar Fibers [16] This is also a photovoltaic device (like the solar cells), only it does not use silicon. Rather, it has a solar tape that is made with titanium dioxide. This tape could actually be combined with building materials or even clothing.

Photovoltaic modules and arrays A photovoltaic module or photovoltaic panel is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. The photovoltaic module, known more commonly as the solar panel, is then used as a component in a larger photovoltaic system to offer electricity for commercial and residential applications.

Because a single photovoltaic module can only produce a limited amount of power, many installations contain several modules or panels and this is known as a photovoltaic array.

[12]

In actual usage, the solar cells are interconnected in certain series/parallel combinations to form modules.

These modules are hermetically sealed for protection against corrosion,

moisture, pollution and weathering. A combination of suitable modules constitutes an array.

One square metre of fixed array kept facing south yields nearly 0.5 kWh of electrical energy on a normal sunny day if the orientation of the array is adjusted to face the sun’s rays at any time, the output can increase by 30 per cent. Solar PV system can produce an output only if sunlight is present. If it is required to be used during non sunshine hours, a suitable system of storage batteries will be required.

Figure 9: Several pv cells make a module and several modules make an array.

[3]

There may be tracking arrays or modules or fixed arrays. A tracking array is defined as one which is always kept mechanically perpendicular to the sun-array line so that all times it intercepts the maximum insolation. Such array must be physically movable by a suitable prime mover and are generally considerably more complex than fixed arrays. A fixed

array

is usually oriented east west and tilted up at an angle approximately equal to the latitude of the site (tilt=latitude angle). Fixed arrays are mechanically simpler than tracking array s.

Thus the array designs fall in two broad classes : (1) Flat-plate Arrays: Wherein solar cells are attached with a suitable adhesive to some kind of substrate structure usually semi-rigid to prevent cells being cracked.

This technology springs from the space-related photo voltaic

technology, and many such arrays have been built in various power sizes.

(2) Concentrating Arrays: Wherein suitable optics e.g. Fresnel lenses, parabolic mirrors, compound parabolic concentrators (CPC), and others, are combined with photo voltaic cells in any array fashion. This technology is relatively new to photo voltaic in terms of hardware development, and comparatively fewer such arrays have actually been built. [3] The individual cells, of size 10cm Ă— 10cm, are then connected into modules of about 30 cells. Each module usually has three to five columns of cells in series. Such an arrangement produces an e.m.f. of about 15V, which is safe and convenient for charging 12 volt batteries.

Such batteries are best if especially designed for photo voltaic use.

Equivalent circuit of a solar cell To understand the electronic behavior of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behavior is well known. An ideal solar cell may be modeled by a current source in parallel with a diode; in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. [23] The resulting equivalent circuit of a solar cell is shown on the left.

Also shown, on the right, is the schematic representation of a solar cell for use in circuit diagrams.

Figure 10: Equivalent circuit of a solar cell.

equation [12]

Characteristic

From the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor:

I = IL − ID − ISH where 

I = output current (amperes)



IL = photogenerated current (amperes)



ID = diode current (amperes)



ISH = shunt current (amperes).

The current through these elements is governed by the voltage across them:

Vj = V + IRS where 

Vj = voltage across both diode and resistor RSH (volts)



V = voltage across the output terminals (volts)



I = output current (amperes)



RS = series resistance (Ω).

By the Shockley diode equation, the current diverted through the diode is:

where 

I0 = reverse saturation current (amperes)



n = diode ideality factor (1 for an ideal diode)



q = elementary charge



k = Boltzmann's constant



T = absolute temperature



At 25°C,

volts.

By Ohm's law, the current diverted through the shunt resistor is:

where 

RSH = shunt resistance (Ω).

Substituting these into the first equation produces the characteristic equation of a solar cell, which relates solar cell parameters to the output current and voltage:

An alternative derivation produces an equation similar in appearance, but with V on the lefthand side. The two alternatives are identities; that is, they yield precisely the same results.

In principle, given a particular operating voltage V the equation may be solved to determine the operating current I at that voltage. However, because the equation involves I on both sides in a transcendental function the equation has no general analytical solution. However, even without a solution it is physically instructive. Furthermore, it is easily solved using numerical methods. (A general analytical solution to the equation is possible using Lambert's W function, but since Lambert's W generally itself must be solved numerically this is a technicality.)

Since the parameters I0, n, RS, and RSH cannot be measured directly, the most common application of the characteristic equation is nonlinear regression to extract the values of these parameters on the basis of their combined effect on solar cell behavior.

Open-circuit voltage and short-circuit current

[12]

When the cell is operated at open circuit, I = 0 and the voltage across the output terminals is defined as the open-circuit voltage. Assuming the shunt resistance is high enough to neglect the final term of the characteristic equation, the open-circuit voltage VOC is:

Similarly, when the cell is operated at short circuit, V = 0 and the current I through the terminals is defined as the short-circuit current. It can be shown that for a high-quality solar cell (low RS and I0, and high RSH) the short-circuit current ISC is:

Effect of physical size [12] The values of I0, RS, and RSH are dependent upon the physical size of the solar cell. In comparing otherwise identical cells, a cell with twice the surface area of another will, in principle, have double the I0 because it has twice the junction area across which current can leak. It will also have half the RS and RSH because it has twice the cross-sectional area through which current can flow. For this reason, the characteristic equation is frequently written in terms of current density, or current produced per unit cell area:

Where

J = current density (amperes/cm2)  JL = photogenerated current density (amperes/cm2)  J0 = reverse saturation current density (amperes/cm2)  rS = specific series resistance (Ω-cm2)  rSH = specific shunt resistance (Ω-cm2).

This formulation has several advantages. One is that since cell characteristics are referenced 

to a common cross-sectional area they may be compared for cells of different physical dimensions. While this is of limited benefit in a manufacturing setting, where all cells tend to be the same size, it is useful in research and in comparing cells between manufacturers.

Another advantage is that the density equation naturally scales the parameter values to similar orders of magnitude, which can make numerical extraction of them simpler and more accurate even with naive solution methods.

There are practical limitations of this formulation. For instance, certain parasitic effects grow in importance as cell sizes shrink and can affect the extracted parameter values.

Recombination and contamination of the junction tend to be greatest at the perimeter of the cell, so very small cells may exhibit higher values of J0 or lower values of RSH than larger cells that are otherwise identical. In such cases, comparisons between cells must be made cautiously and with these effects in mind.

This approach should only be used for comparing solar cells with comparable layout. For instance, a comparison between primarily quadratical solar cells like typical crystalline silicon solar cells and narrow but long solar cells like typical thin film solar cells can lead to wrong assumptions caused by the different kinds of current paths and therefore the influence of for instance a distributed series resistance rS.

Cell temperature [12]

Temperature affects the characteristic equation in two ways: directly, via T in the exponential term, and indirectly via its effect

on I0 (strictly

speaking,

temperature affects all of the terms, but these two far more significantly than

the

others).

While

increasing T reduces the magnitude of the exponent in the characteristic equation, the value of I0 increases exponentially with T. The net effect is to

reduce VOC (the

open-circuit

voltage) linearly with increasing

Fig 11 :

Effect of

temperature on the current-voltage characteristics of a solar cell temperature. The magnitude of this reduction is inversely proportional to VOC; that is, cells with higher values of VOC suffer smaller reductions in voltage with increasing temperature.

For most crystalline silicon solar cells the reduction is about 0.50%/°C, though the rate for the highest-efficiency crystalline silicon cells is around 0.35%/°C. By way of comparison, the rate for amorphous silicon solar cells is 0.20-0.30%/°C, depending on how the cell is made.

The amount of photo generated current IL increases slightly with increasing temperature because of an increase in the number of thermally generated carriers in the cell. This effect is slight, however: about 0.065%/°C for crystalline silicon cells and 0.09% for amorphous silicon cells.

The overall effect of temperature on cell efficiency can be computed using these factors in combination with the characteristic equation. However, since the change in voltage is much stronger than the change in current, the overall effect on efficiency tends to be similar to that on voltage. Most crystalline silicon solar cells decline in efficiency by 0.50%/°C and most amorphous cells decline by 0.15-0.25%/°C. The figure above shows I-V curves that might typically be seen for a crystalline silicon solar cell at various temperatures.

Series resistance [12]

As series resistance increases, the voltage drop between the junction voltage and the terminal voltage becomes greater for the same flow of current. The result is that the current-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal

voltage V and

a

slight

reduction in ISC, the short-circuit current.

Very high values of RS will also produce a significant

reduction

in ISC;

in

these

regimes, series resistance dominates and the behavior of the solar cell resembles that of a resistor.

Fig 12:

Effect

of

series resistance on the I-V characteristics of a solar cell These effects are shown for crystalline silicon solar cells in the I-V curves displayed in the figure to the right.

Losses caused by series resistance are in a first approximation given by P loss=VRsI=I2RS and increase quadratically with (photo-)current. Series resistance losses are therefore most important at high illumination intensities.

Shunt resistance [12] As shunt resistance decreases, the current diverted through the shunt resistor increases for a given level of junction voltage. The result is that the voltage-controlled portion of the I-V curve begins to sag toward the origin, producing a significant decrease in the terminal current I and a slight reduction in VOC. Very low values of RSH will produce a significant reduction in VOC. Much as in the case of a high series resistance, a badly shunted solar cell will take on operating characteristics similar to those of a resistor. These effects are shown for crystalline silicon solar cells in the I-V curves displayed in the figure.

Fig 13: Effect of shunt resistance on the current–voltage characteristics of a solar cell Reverse saturation current [12] If one assumes infinite shunt resistance, the characteristic equation can be solved for VOC:

Thus, an increase in I0 produces a reduction in VOC proportional to the inverse of the logarithm of the increase. This explains mathematically the reason for the reduction in VOC that accompanies increases in temperature described above. The effect of reverse saturation current on the I-V curve of a crystalline silicon solar cell is shown in the figure to the right. Physically, reverse saturation current is a measure of the "leakage" of carriers across the p-n junction in reverse bias. This leakage is a result of carrier recombination in the neutral regions on either side of the junction.

Fig 14: Effect of reverse saturation current on the current-voltage characteristics of a solar cell Ideality factor [12] The ideality factor (also called the emissivity factor) is a fitting parameter that describes how closely the diode's behavior matches that

predicted

by

theory,

which

assumes the p-n junction of the diode is

an

infinite

plane

and

no the

recombination

occurs

within

space-charge

region.

A

perfect

match to theory is indicated when n

= 1. When recombination in the spacecharge

region

dominate

other

recombination, however, n = 2. The effect

of

changing

ideality

factor

independently of all other parameters is shown for a crystalline silicon solar

Fig 15: Effect of ideality factor on the I-V

characteristics of a solar cell cell in the I-V curves displayed in the figure to the right.

Most solar cells, which are quite large compared to conventional diodes, well approximate an infinite plane and will usually exhibit near-ideal behavior under Standard Test

Condition (n ≈ 1). Under certain operating conditions, however, device operation may be dominated by recombination in the space-charge region. This is characterized by a significant increase in I0 as well as an increase in ideality factor to n ≈ 2. The latter tends to increase solar cell output voltage while the former acts to erode it. The net effect, therefore, is a combination of the increase in voltage shown for increasing n in the figure to the right and the decrease in voltage shown for increasing I0 in the figure above. Typically, I0 is the more significant factor and the result is a reduction in voltage.

A basic photo-voltaic system for Power Generation [24] Photovoltaic arrays can be used to provide electricity for a wide variety of applications, many of which would make a meaningful contribution to economic development. There are two main types of systems.

(A) Stand-alone in which the photo voltaic array is the principal or only source of energy.

Energy is stored often in batteries, for periods when there is insufficient solar

radiation. There may also be a back-up power supply such as an engine-generator set.

These 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. They may be powered by a solar array only, or may use wind, an engine-generator, or utility power as an auxiliary power

source

in

what

is

called

a

solar-hybrid

system.

The simplest type of stand-alone system is a direct-coupled system, where the DC output of a solar module or array is directly connected to a DC load.

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. Matching the impedance of the electrical load to the maximum power output of the PV array is a critical part of designing well-performing direct-coupled system.

For certain loads such as positive-displacement water pumps, a type of electronic DC-DC converter, called a maximum power point tracker (MPPT), is used between the array and load

to

help

better

utilize

the

available

array

maximum

power

output.

In many stand-alone solar power systems, batteries are used for energy storage. The following diagram is of a typical stand-alone system powering DC and AC loads.

Fig 16: Stand alone system powering DC and AC loads.

[24]

This is how a typical solar hybrid system might be configured.

Fig 17: Solar hybrid system [24] (B) Grid-connected, where the load is connected to both a photovoltaic power system and an electricity grid. In periods when there is sufficient solar radiation, the array powers the load, otherwise the grid is used. In some cases, any surplus electricity produced by the array (i.e. when the output exceeds the load) is fed back into the grid. This type includes large MW-sized systems.

These are designed to operate in parallel with, and interconnected, with the electric utility grid. The primary component in grid-connected systems is the inverter, or power conditioning unit (PCU). The PCU converts the DC power produced by the solar array into AC power consistent with the voltage and power quality requirements of the utility grid, and

automatically stops supplying power to the grid when the utility grid is not energized. A bidirectional interface is made between the solar power system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the solar power system to either supply on-site electrical loads or to back-feed the grid when the solar power system output is greater than the onsite load demand.

Figure 18: A bi-directional interface that uses both

electrical utility supply and solar panel electricity.

[24]

At night and during other periods when the

electrical

loads

greater than the solar power system output,

the

required by the loads is received from the

electric utility. This safety

balance

of

are power

feature is required in all grid-connected systems, and ensures that the system will not continue to operate and feed back into the utility grid when the grid is down for service or repair.

Solar power systems are able to operate normally in grid-connected mode and still operate critical loads when utility service is disrupted, providing that battery storage is used. This type of system is popular for homeowners and small businesses where a critical backup power supply is required for critical loads such as refrigeration, water pumps, lighting, and other necessities. Under normal circumstances, the system operates in grid-connected mode, serving the on-site loads or sending excess power back onto the grid while keeping the battery fully charged. In the event the grid becomes de-energized, control circuitry in the inverter opens the connection with the utility through a bus transfer mechanism, and operates the inverter from the battery to supply power to the dedicated loads only. In this configuration, the critical loads must be supplied from a dedicated sub panel. The diagram below shows how a solar power system might be configured to operate normally in gridconnected

mode and also

power critical

loads from a

battery

bank

when the grid

is

de-

energized.

Figure 19: Grid connected mode and power critical loads from battery bank.

[24]

Large grid systems are not yet appropriate for most developing countries, but work on these is of considerable importance because their construction contributes to reducing the cost of photo-voltaic modules.

Basic photovoltaic system A basic photovoltaic system integrated with the utility grid is shown in previous section, It permits solarly generated electric power to be delivered to a local load. It consists of mainly [1]

: (i) Solar array, large or small, which converts the insolation to useful DC electrical power. (ii) A Blocking Diode which lets the array – generated power flow only towards the

battery or grid. Without a blocking diode the battery would discharge back through the solar array during times of no insolation. (iii) Battery storage, in which that solarly generated electric energy may be stored.

The simplest means of storage on a small or moderated scale is in electric storage batteries, especially as solar cells produce the direct electric current required for battery charging.

The stored energy can then be delivered as electricity upon discharge. The

common lead acid storage batteries, such as are used in automobiles, are not ideal for this purpose, but they are probably the best presently available. Extensive research in progress should lead to the development of more suitable batteries.

Because of the large variation in solar irradiance, both diurnally and seasonally, batteries are used for energy storage. They also act as buffer between the solar array and the load. The battery supplies energy to the load during periods of little or no solar irradiance and stories energy from the array during periods of high irradiance. This enables the systems to meet momentary peak power demands and to maintain stable voltage to the load. The two types of battery that have been used for photovoltaic systems are lead-acid and nickel-cadmium. Due to higher costs, lower energy efficiency and limited upper operating temperature (40oC), nickel-cadmium batteries have been employed in relatively few systems. Lead acid batteries a reavailable in many capacity sizes and types, ranging from car batteries for starting, lighting and ignition (SLI to sealed float-service batteries.

Because a low discharge rate is needed for photovoltaic applications, float-service batteries which use pure lead or calcium alloy grids are preferable. The life-time of these batteries often exceeds 5 years, so that they are better suited for systems in remote locations.

A possible alternative is to use the direct current from solar cells to decompose water (by electrolysis) into hydrogen and oxygen gases. These gases would be stored in a suitable form and utilized as needed to generate electricity in a fuel cell.

Table 2: Average Cell Voltage During Discharge in Various Rechargeable Batteries Electrochemistry

Cell Voltage

Remark

Lead-acid

2.0

Least cost technology

Nickel-cadmium

1.2

Exhibits memory effect

Nickel-metal hydride

1.2

Temperature sensitive

Lithium-ion

3.4

Safe, contains no metallic lithium

Lithium-polymer

3.0

Contains metallic lithium

Lead-acid batteries [12] Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery.

Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large power-to-weight ratio. These features, along with their low cost, make them attractive for use as photovoltaic electricity storage device.

• •

Open-circuit (quiescent) at full charge: 12.6 V to 12.8 V (2.10-2.13V per cell) Open-circuit at full discharge: 11.8 V to 12.0 V In the charged state, each cell contains electrodes of elemental lead (Pb) and Lead(IV) Oxide (PbO2) in an electrolyte of approximately 33.5% v/v (4.2 Molar) sulfuric acid (H2SO4).

In the discharged state both electrodes turn into lead(II) sulfate (PbSO4) and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. The chemical reactions are (discharged to charged): Anode (oxidation): PbSO4(s) + 5 H2O (l) ∏ PbO2(s) + 3H3O+ (aq) + HSO4-(aq) + 2e- ; ϵº = 1.685 V Cathode (reduction): PbSO4(s) + H3O+(aq) + 2e- ∏ Pb(s) + HSO4-(aq) + H2O(l) ; ϵº=-0.356V Because of the open cells with liquid electrolyte in most lead-acid batteries, overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, forming an explosive mix. The acid electrolyte is also corrosive.

Practical cells are usually not made with pure lead but have small amounts of antimony, tin, calcium or selenium alloyed in the plate material to add strength and simplify manufacture.

Nickel-cadmium battery [12] The nickel-cadmium battery (commonly abbreviated NiCd or NiCad) is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. Nickel-cadmium cells have a nominal cell potential of 1.2 V. 12 V NiCd batteries are made up of 10 cells connected in series.

The primary trade-off with NiCd batteries is their higher cost and the use of cadmium. They are more costly than lead-acid batteries because nickel and cadmium are more costly materials.

One of the NiCd's biggest disadvantages is that the battery exhibits a very marked negative temperature coefficient.

This means that as the cell temperature rises, the internal resistance falls. This can pose considerable charging problems.

The chemical reactions in a NiCd battery during discharge are: Cd + 2OH- ɹ Cd(OH)2 + + 2eAt the cadmium electrode, and 2NiO(OH)+ 2H2O+ 2e- ɹ 2Ni(OH)2 + 2OHAt the nickel electrode. The net reaction during discharge is 2NiO(OH) + Cd + 2H2O ɹ 2Ni(OH)2 + Cd(OH)2 .

During recharge, the reactions go in the reverse direction.

(iv)

Inverter/converter, usually solid state which converts the battery bus

voltage to AC of frequency and phase to match that needed to integrate with utility grid.

Thus it is typically a DC, AC inverter. It may also contain a suitable output step up transformer, perhaps some filtering and power factor correction circuits and perhaps some power conditioning, i.e. circuitry to initiate battery charging and to prevent overcharging.

Power conditioning may be shown as a separate system functional block. This block may also be used in the figure shown to function as a rectifier to charge the battery from utility feeder when needed and when no insolation was present.

Inverters are the devices usually solid state, which change the array DC output to AC of suitable voltage, frequency and phase to lead photovoltaically generated power into the power grid or local load. These functional blocks are sometimes referred to as power conditioning. [3]

Figure 20: DC to three-phase AC inverter circuit.

[3]

A general type of inverter circuit which is found best suitable for the utility applications is shown in Fig. 20. The current can be used in two modes: (1) As an inverter changing DC to AC or (2) As a rectifier changing AC to DC, thus charging the battery.

The line-commutated inverter must be connected to the AC system into which they feed power. The design method is matured and has been extensively used in the high-voltage DC transmission line inverters. Such inverters are simple and inexpensive and can be designed in any size. The disadvantage is that they act as a sink of reactive power and generate high content of harmonics.

It is clear that the system photo voltaic offers the option of DC power, AC power, hydrogen and oxygen fuels in either gas or liquid forms from which electricity can be generated. (v)

[3]

Power conditioner. Because the voltage output of the photovoltaic array

varies with insolation and temperature, systems with battery storage require voltage or shunt regular to prevent excessive overcharging of the battery. Further controls are used, as required, to prevent discharge or to ensure that the array is operating at its maximum power point. The technology and design of power conditioners is well developed and system tests in a variety of environments have not revealed any serious problem. (vi)

[3]

Appropriate switches and circuit breakers, to permit isolating parts of

the system, as the battery. One would also want to include breakers and fusing protection (not shown) between the inverter output and the utility grid to protect both the photovoltaic system and the grid.

[3]

Solar Energy Systems wiring diagram examples [25] These system sizes are based on 100 watt solar panels and 5 hours of average daily sunshine. This is an example of a typical 2 KiloWatt system.

Figure 21: A sample wiring of a solar panel. [25]

Building integrated PV (BIPV) system [12] Building-integrated photovoltaics (BIPV) are photovoltaic materials that are used to replace conventional building materials in parts of the building envelope such as the roof, skylights, or facades. They are increasingly being incorporated into the construction of new buildings as a principal or ancillary source of electrical power, although existing buildings may be retrofitted with BIPV modules as well. The advantage of integrated photovoltaics over more common non-integrated systems is that the initial cost can be offset by reducing the amount spent on building materials and labor that would normally be used to construct the part of the building that the BIPV modules replace. These advantages make BIPV one of the fastest growing segments of the photovoltaic industry In new markets, the near-term potentially large application of the PV technology is for cladding buildings to power air-conditioning and lighting loads. One of the attractive features of the PV system is that its power output matches very well with the peak load demand. It produces more power on a sunny summer day when the air-conditioning load strains the grid lines.

Develop and manufacture is low cost, easy to install, pre-engineered Building Integrated Photovoltaic (BIPV) modules. Such modules made in shingles and panels can replace traditional roofs and walls. The building owners have to pay only the incremental cost of these components. The land is paid for, the support structure is already in there, the building is already wired, and developers may finance the BIPV as part of their overall project. The major advantage of the BIPV system is that it produces power at the point of consumption Building-Integrated Photovoltaic modules are available in several forms.

• Flat roofs: The most widely installed to date is a thin film solar cell integrated to a flexible polymer roofing membrane.

•

Pitched roofs:

Modules shaped like multiple roof tiles. Solar shingles are modules

designed to look and act like regular shingles, while incorporating a flexible thin film cell.

It extends normal roof life by protecting insulation and membranes from ultraviolet rays and water degradation. It does this by eliminating condensation because the dew point is kept above the roofing membrane.

•

Facade: Facades can be installed on existing buildings, giving old buildings a whole new look. These modules are mounted on the facade of the building, over the existing structure, which can increase the appeal of the building and its resale value.

•

Glazing: (Semi) transparent modules can be used to replace a number of architectural elements commonly made with glass or similar materials, such as windows and skylights.

Transparent solar panels use a tin oxide coating on the inner surface of the glass panes to conduct current out of the cell. The cell contains titanium oxide that is coated with a photoelectric dye.

SECTION â…Ł SOLAR THERMAL SYSTEM

Solar thermal system There are two principle forms of energy into which solar radiation can be converted for practical applications. One is the heat and other is the electricity. Heat is obtained when the solar radiation is absorbed in the black surface. The heat may then be used in various ways, which may be divided broadly in two classes.

Direct application like water heating, drying, distillation and cooking etc. and application involving the second law of thermodynamics like production of mechanical power and refrigeration etc. This mechanical power production system is called the solar thermal power production system. As far as the conversion of solar energy into electrical is concerned it can either be done by solar thermal power production route or solar radiation can be directly converted into electrical power. In direct conversion of solar energy one may employ photovoltaic, thermoelectric, thermionic and photo chemicals.

Solar thermal power generation employs power cycles which are broadly classified as low, medium and high temperature cycles. Low temperatures generally use flat-plate collectors so that maximum temperature are limited to about 100˚C. [1] Table 3: Different Cycle working temperatures Power cycles

Working temperature

Low temperature cycle

Less than 100˚C

Medium temperature cycle

150˚C - 300˚C

High temperature cycle

More than 300˚C

For the low and medium temperature ranges, the thermodynamics cycles preferred is the Rankine cycle, the Brayton and Stirling cycle are also being considered.

Solar energy can be converted to thermal energy by flat-plate or focusing collectors. That thermal energy can, then, in principle be used to drive a heat engines thus converting solar energy into mechanical energy. The basic problem arises from the temperature limitations of solar collectors; The solar energy supply system works best at low temperature, while the heat engines is most efficient with energy input at high temperatures . The problem is particularly acute with flat-plate collector, which are now limited by practical considerations to delivery of energy perhaps 100˚C above ambient temperatures; this, in turn, results in lower thermal efficiencies for the heat engine. Focusing collector which would deliver energy at high temperature, is not yet practical device. Under these circumstances, the projected costs of mechanical or electrical energy from solar energy have not appeared favorable. The

spectrum of solar thermal power system technologies range from non-tracking, low temperature energy conversion subsystems to point focusing collector based on high temperature system providing high density flux to an efficient power conversion system.

There exist two major categories of thermal collector and power generation system [1]: 1. Centralized system (Tower concept) 2. Distributed system (Farm concept) The distributed system category has a number of variants and is generally differentiated from centralized system as follows: a) The energy from each of discrete number of receivers is summed in either thermal or electrical form, and b) The receivers of a distributed system can be generally be moved in accordance with the sun tracking mechanism adopted.

The solar farm is primarily intended for the decentralized supply of process heat and electricity in the range of a few hundred to a few thousand kilowatts respectively. There exists an aerial layout of sun tracking concentrating collectors. Here a number of smaller heat engine units are associated with each collector unit at the focal point and the total outputs are combined.

In the tower concept a field of Heliostats which follow the direction of sun, concentrates direct incident solar radiation onto a central receiver mounted on the top of the tower. A heat transfer fluid removes heat from the receiver and in some cases carries it to another heat storage unit. This heat source is used to operate an electrical power plant.

The temperatures developed in the various concepts depend on the concentration ratio and the temperature of working fluid depends on the number tracking degrees of freedom of the concentrators. The advantage of the farm concept over the centralized concept is that it embodies the modularity aspect and so there is possibility of approaching cost reduction due to mass production of many identical units. It also offers advantage of maintenance and overhauling.

Solar thermal power can also be generated by utilizing parabolic trough systems (Cylindrical parabolic collectors). These collectors consist of a trough with a parabolic cross section which focuses solar radiation onto a absorber pipe placed at the focus of parabola. By means of circulating fluid, heat is removed from the hot absorber pipe which is suitably

painted either by a ordinary black paint or by selected paint and may be enclosed in a transparent evacuated tube to reduce heat loss by convection. In this case the range of fluid temperature may go up to 400ËšC. This hot fluid is then used for generation of power or for process heat. Generally organic Rankine cycle engine are used for production of power generation.

[1]

The solar tower concentrator system, the incoming solar radiation is focused to a central received or a boiler mounted on a tall tower using a thousands of plane reflector which are steerable about two axis and are called heliostat.

The solar thermal energy is collected by concentrators. Three alternative [3] configurations of the concentrators are shown in Figure 22. Their main features and applications are as follows:

a) Parabolic Trough The parabolic trough system is by far the most commercially matured of the three technologies.

It focuses the sunlight on a glass-encapsulated tube running along the focal line of the collector.

The tube carries heat absorbing liquid, usually oil, which in turn, heats water to generate steam. More than 350 MW of parabolic trough capacity is operating in the California Mojave Desert and is connected to the Southern California Edison’s utility grid. This is more than 90 percent of the world’s solar thermal capacity at present.

[3]

b) Central Receiver In the central receiver system, an array of field mirrors focus the sunlight on the central receiver mounted on a tower. To focus the sun on the central receiver at all times, each heliostat is mounted on the dual-axis sun tracker to seek position in the sky that is midway between the receiver and the sun. Compared to the parabolic trough, this technology produces higher

Fig 22: Alternative thermal energy collectors.

concentration, and hence, higher temperature

working medium, usually a salt.

Consequently, it yields higher Carnot efficiency, and is well suited for utility scale power plants in tens or hundreds of megawatt capacity.

[3]

c) Parabolic Dish The parabolic dish tracks the sun to focus heat, which drives a sterling heat enginegenerator unit. This technology has applications in relatively small capacity (tens of kW) due the size of available engines and wind loads on the dish collectors. Because of their small size, it is more modular than other solar thermal power systems, and can be assembled in a few hundred kW to few MW capacities. This technology is particularly attractive for small stand-alone remote applications.

[3]

Table 4: Comparison of Alternative Solar Thermal Power System Technologies Technology

Solar Concentration (x Suns)

Operating Temperature

Thermodynamic Cycle Efficiency

on the Hot Side Parabolic Trough

100

300–500°C

Low

1000

500–1000°C

Moderate

3000

800–1200°C

High

Receiver Central Receiver Power Tower Dish Receiver with Engine

Principle of Solar Thermal Power Generation In a solar thermal power production system the energy is first collected b using a solar pond, a flat-plate collector or a focusing collector. This energy is used to increase the internal energy or temperature of a fluid. This fluid may be directly used in any of the common or in known cycle such as Rankine, Brayton or Stirling or passes through a heat exchanger to heat a secondary fluid (working fluid) which is being used in the cycle to produce mechanical power from which electrical power can be produced easily.

[1]

The following three systems are discussed in the following section: (1) Low temperature cycles using flat plate collector (2) Concentrating collector for medium and high temperature cycle.

(3) Power tower concept.

Low temperature system The flat plate collector and solar pond are classified as low temperature collectors, because the temperature achieved is of the order of 60˚C to 100˚C, with collection efficiency 30 to 50%. If Rankine cycle solar thermal power production system is employed, since the temperature of the fluid (water) is usually below 100˚C (with solar pond the maximum temperature is limited approximately 80˚C) and it is not possible to generate steam with flat-plate collector, so this cannot be used directly to run the prime mover. Therefore some other organic fluid is used (Freon group etc) which evaporates at low temperature and high pressure by absorbing the heat from the heated water. The vapour formed can be used to run a turbine or engine which may generate power, which will be light the group of houses for rural areas and for irrigation purposes.

[1]

sufficient

to

Figure 23: Schematic diagram of Low temperature solar power plant

[1]

Concentrating collector for medium and high temperature cycle Cylindrical parabolic concentrating collectors (line focus system) give a temperature range of 250˚C to 700˚C with efficiency of 50% to 70%. High temperature collectors such as paraboloid type concentrators consists of many flat mirrors give a temperature range of 600 - 2000˚C with an efficiency of 60 – 75%.

A parabolic cylindrical concentrator for medium temperature system is shown in the figure 24.

Fig 24: Basic geometry of parabolic cylindrical concentrator

[1]

Its consists of a parabolic cylinder reflector to concentrate sun light on to collecting pipe within a pyrex or glass envelop. A selecting coating of suitable material is applied to pipe to minimize infra-red emission. The space between the transparent tube surrounding the pipe can be evacuated to reduce convection heat loss. Proper sun tracking arrangement is made so that maximum sunlight is focused on the reflector.

The line focus system, also called the trough system, uses concentrators in the form of long troughs of cylindrical or parabolic cross sections, which are lined with the mirrors to collect and concentrate the suns radiation on to a focal linear conduit through which the primary coolant flows. Because of their geometry such troughs are usually made to track the sun in only one plane, by being rotated about their focal line. Thus, other than, solar noon, they receives sun rays that get more inclined with respect to their projected surface as the sun deviates from the solar noon. They, therefore, usually operate in lower temperatures ranges of about 90˚C to 315˚C. Line focus system are thus believed are suitable only for small electric generating systems for which thermal efficiency is not of prime importance and for other applications such as driving irrigation pumps, providing industrial process heat, space heating and cooling and other industrial applications but not for large scale electric generation.

[1]

Fig 24: Power generation by using solar collector or focal concentrator.

[26]

Concentrating collectors use reflective surfaces to focus the sun’s rays onto a receiver or absorber where the solar energy heats a circulating fluid. The hot fluid can then be used directly for an industrial process, to power a turbine for mechanical work or to generate electricity.

Thermal storage systems accumulate thermal energy to be used during cloudy weather or at night. Currently, Storage system capacities range from “buffer” storage for short intervals such as during cloud passage, to as long as six continuous hours.

Table 5: The power generation by this method follows following steps – Stage Description These solar collectors with curved reflectors cover large areas and are 1 used to capture and concentrate sunlight.

That sunlight then heats a synthetic oil called therminol, which then heats 2 water to create steam.

The steam is piped to an onsite turbine-generator to produce electricity.

The steam is then condensed and reused as the cycle begins again. After 3 the electricity leaves the generator, it goes to a transformer so that the voltage can be increased, allowing the power to be transmitted long distances via the transmission system.

On cloudy days, the plant has a supplementary natural gas boiler. The 4 plant can burn natural gas to heat the water, creating steam to generate electricity.

Tower power plant (Central Receiver system) [1] In this system the incoming solar radiation is focused to a central receiver or a boiler mounted on a tall tower using thousands of plane reflectors which are steerable about two axes and are called heliostat.

A schematic of an electric power plant making use of this idea is shown in the figure 25. As shown in it an assembly of separate flat mirrors is oriented in such a way that all the incident light beams are reflected towards the same point, the concentration factor achieved is roughly equal to the number of mirrors.

The mirrors are installed on the ground and are oriented so as to reflect the direct beam radiation into an absorber or receiver (boiler) which is mounted on the top of a tower located near the center of the field of mirrors to produce high temperature. The factor make it possible to position the boiler in field of view of all mirrors of at all hours of the day. Beam radiations incident on the boiler absorbed by black pipe in which working fluid circulate and is heated. The hot working fluid is allowed to drive a turbine and produce electrical energy.

The turbine which is coupled to an alternator produces electrical energy. As in any thermodynamic conversion, the heat sink is provided. A suitable heat storage is also provided to supply the heat energy during the periods of cloudiness.

The system should incorporate storage for night time and cloudy periods. The receiver output is made greater than that required by steam cycle and the excess output during the periods of greatest solar incidence is bypassed to a thermal storage system. During periods of low or no solar incidence, the feed water is shunted to the storage system, instead of to the receiver, where it vaporizes for use in the turbine. Proper valving in the system allows operation in either mode.

The generation costs, per unit of electrical energy, decreases with increasing the plant capacity. But increasing the size there is a decrease in the average effectiveness of the heliostats because of the greater distance from the central receiver. As far as can be estimated as present the optimum electrical capacity of central receiver solar thermal electric plant, from the economic stand point might be in the range of 50 to 300 MW. The height of tower might be about 300 m and the total land area of the plant roughly 325 hectares.

Figure 25: Schematic of a solar thermal central receiver system

power

plant

The system can be

sub divided into

following

systems,

sub-

namely: a) The heliostat b) Tower with central receiver c) Heat transport subsystem d) Heat storage or thermal storage device e) Heat coversion subsystem

The heliostat [1] The heliostats are reflecting mirrors that are steerable so that they can reflect the sun’s rays on the central receiver at almost all times during the day light hours. The heliostat is composed of a reflective surface or mirror, mirror support structure, pedestal, foundation, and control drive mechanism. Current designs have total reflective area between 40 and 70m2. Ideally, the surface should be slightly parabolic with the focal length equal to the distance from the surface to the receiver but because that distance is too long, spherical or even flat surfaces offer good performance.

There are two type of reflecting surface: glass and plastic. Heliostats that use glass are much further developed at this time then plastic once mainly because of their reflectance and strength. The plastic heliostat, although lower in reflectance and strength in than the glass heliostat, promises lower costs, lower mass of the reflector surface and hence lower mass and cost of the support structure and drive mechanism.

The flat mirror surface can be manufactured by metallization of float glass or flexible plastic sheets. Float glass metallized with silver or aluminum provides reflectivities of 93% and 82% to 86% respectively, subject to cleaning.

Receiver subsystem [1] The central receiver at the top of the tower has a heat absorbing surface (e.g, panels coated with a heat-absorbing material) by which the hear-transport fluid is heated. Two basic receiver configurations have been proposed; they are the Cavity and External receiver types.

In the cavity type, pipes line the interior of the cavity; the solar radiation reflected by the heliostats enters through an aperture at the bottom of the cavity. On the other hand, in the external receiver type the absorber surfaces are on the exterior of a roughly cylindrical structure. A tube-panel arrangement within the cavity is concave toward the heliostat field, while the external receiver has the coolant-tube panels lining the outside of the receiver and they are slightly convex towards the heliostat field for large plants. The panels are flat for small plants.

Heat losses by radiation and convection are generally less for the cavity than for the external receiver configuration, but focusing may be more critical because, in order to be absorbed the radiation must enter through the cavity aperture. Furthermore, the external receiver has the advantage of simplicity, modular panel construction, easier access for maintenance, and a large absorber area. A large receiver has low spillage. This is the energy reflected by the heliostats but not intercepted by the receiver heat transport fluid. The reflected rays may miss the receiver altogether or fall outside an aperture (*in a cavity receiver). Spillage may be caused by heliostat tracking errors caused by control system errors, wind effects steering backlash, etc. Spillage in normally less than 5 percent in a well designed system. A large receiver suffers from large convection and radiation losses. A cavity receiver has fewer reflection losses than an external receiver as well as less convection because the heat transfer surfaces are protected (a cavity approximates a block body). The radiation entering through the aperture is repeatedly reflected within the cavity with very little of it finding its way back out. The radiation is thus trapped inside, and the result is an absorptivity nearly equal to unity. But cavity receiver does however have greater conduction losses because of its greater size and complexity. Cavity receivers are therefore more efficient than external receivers but they are much larger, heavier, and more costly than external receivers.

Heat-transport subsystem [1]

Two basic thermodynamic conversion methods, i.e. the Rankine and Brayton cycles, are used for the subsystem that converts heat into electrical power. Water is a convenient heat transport fluid, and in the form of steam it is the working fluid in the majority of existing turbines for generating electricity. Hence, water will be used, at least in the earliest solar thermal electric plants. Liquid water (i.e. turbine condensate) under pressure enters the receiver, absorb heat energy and leaves as superheated steam; typical steam conditions might be a temperature of 500 0C and a pressure of about 100 atm (10 MPa). The steam is piped to ground level where it drives a conventional turbine generator system. As in all steam-electric plants, heat is rejected to the condenser cooling water.

In more advanced central receiver systems, the heat transport fluid might be liquid sodium or a molten mixture of salts at about atmospheric pressure. At the bottom of the tower the hightemperature liquid is circulated through a heat-exchanger, where the heat would be transferred to water to generate s team at a high temperature and pressure. Thus, a high efficiency of conversion into electricity could be achieved without the need for high pressure in the receiver. Another possibility is to use a gas as the heat transport medium and also as working fluid in a gas turbine. In the Brayton cycle gas turbine cycle, a gas is compressed and heated and then expanded through a turbine, which is coupled to the generator. The exhaust gases may serve to preheat the compressed gas a regenerator or recuperator. Air, helium, argon or other gases can be used.

Heat transfer oils for solar central receiver systems as the heat transport medium suggested are Therminol-66 and Carolina HT-43, which have an operating range of -7 to 315 0C. Oils are also used to transport receiver heat to a steam generator at the plant side. Oils have the advantage of low-corrosion characteristics with most materials and can be selected for low vapour pressures. They are however, flammable, can ca use flow problems at low temperatures and usually suffer pyrolitic damage, i.e. they decompose under hightemperature conditions, called pyrolitic decomposition.

Heat rejection during the thermodynamic conversion presents a special problem in solar power generation. It is estimate that the cooling water requirement of a 100 MW plant would amount to 30,000 m3 per hour unless cooling towers are used. It is probable that solar power plants using direct sunlight concentrators will be installed mostly in deserts or semi-arid lands, where the need of large amount of cooling water will restrict the choice of sites.

Thermal storage subsystem [1] The purpose of the thermal storage subsystem is to store solar heat energy absorbed in the receiver for use at a later time. As in all the power systems attention must be paid to the energy storage problem. It seems necessary to provide thermal storage, at least for short periods, so as to avoid the loss of time which would otherwise accrue after an occasional shutdown, due to lack of sun shine while the plant reached its operating temperature again.

Short term storage of heat can be provided by fire-bricks, ceramic oxides (MgO), fused salts

(NaNO3) melts at 2600C : Hitec a fused salt mixture is stable upto 540 0C), sulphur (liquid between 1130 and 44440C and 14000C) or metals such as mercury (liquid between-39 0C and 3570C), lithium (liquid between 1800C and 14000C, sodium (liquid between 980C and 88800C). Some organic materials are also suitable upto 300 0C. Current work is focused on storage by rocks, eutectic salts, and some synthetic organic materials of low vapour pressure, such as Gilotherm. The choice of a conventional storage material is determined by its energy density, thermal conductivity, corrosion characteristics, cost and convenience of use, as well as by the operating temperature of the working fluid. The storage space must be well insulated against heat losses.

A simple example of the integration of thermal storage into a solar thermal power plant with steam water as the working fluid is outlined in fig 26.

Figure

26:

Schematic

of

a

solar-

power plant and power generation using

thermal central receiver system thermal storage [1].

High temperature steam in excess of the immediate demand is diverted to a heat exchanger within the storage medium contained in a well insulated tank. Heat is transferred to storage and the coolant fluid is returned to the central receiver. When heat is required, the stored heat is used to generate steam for turbine.

There are two types of thermal storage that are mainly considered with solar systems. These are (1) single tank, or thermocline and (2) dual tank, or hot-cold systems.

The single-tank or thermocline system is shown schematically in Fig. 26. Storage takes place by circulating some of the hot primary coolant through the storage medium and returning the cooled primary coolant from the bottom of the storage tank back to the receiver for reheating. Heat extraction during times of need is accomplished by reversing the process: cold primary coolant from the power plant is heated by the storage medium, drawn

from the top of the tank to the power system, and then returned to the bottom of the tank.

The thermal gradient of hot at top cold at bottom maintains stratification, allowing the hot fluid to remain afloat on the top, and gives the tank thermal stability. In addition, a solid storage medium of low thermal conductivity and high volumetric heat capacity, such as rock, is used to help impede mixing of hot and cold fluids. A solid storage medium is a necessity in case the primary coolant is a gas (such as air or helium) because of the low heat capacity of gases. A porous solid makes a good storage medium for gases.

Cost Effectiveness [1] The installed capital costs of solar thermal system generating electric power are power are about 3 to 5 times larger than the present cost of fossil fuel systems, but the running costs are obviously very low for solar system as the ‘fuel’ is free. Several groups in U.S.A. have evaluated relative costs of electric power from solar energy and it is concluded that the tower power plant is likely to be the most economical in the 60 to 1000 MWe region. Although parabolic cylinder or segmental collector system may turn out to be costly for transporting the heat from absorber to the engine they may be attractive for small and medium power installations. The cost effectiveness of such solar installations would improve considerably if they are used directly for production of process steam or hot water and for refrigeration and air conditioning. One should consider hybrid systems also where auxiliary fossil fuel fired boilers can be incorporated along with solar concentrator plants such that when sunshine fails, the conventional fuel source takes over. In such hybrid systems capital costs on thermal storage can be cut down significantly. It should be noted, however, that many of the present cost evaluations are quite subjective. A clearer picture would emerge in the coming few years during which operating experience with prototypes and pilot schemes of several different types of concentrators would have been accumulated.

Table 6: Major solar thermal projects and programs

SECTION Ⅴ GLOBAL SITUATION AND BANGLADESH

Global Solar Energy Situation There is a looming energy crisis world-wide. It arises not only from shrinking reserves of fossil fuels and the public concern on the continued use of fossil fuel for energy generation, but also from ageing nuclear power plants (in the developed countries) which are going to cease operation in a relatively near future. There is a global realization that fossil fuel usage must be reduced drastically in order to arrest green house gas (mainly CO2) emission to the atmosphere, which causes global warming. In fact, this aspect of global warming, rather than the imminent shortage of fossil fuel, that is propelling all industrialized countries, in the West as well as in the East, into taking urgent actions now. Commercial nuclear power all over the world is undergoing an unprecedented revival. But some countries, such as Germany, Italy and few others, are reluctant to jump into the nuclear bandwagon and, instead, concentrating on research and development of alternative sources of energy, particularly the solar energy.

Even when countries are expanding nuclear capabilities, they are also undertaking development of alternative energies such as solar power, wind power, geothermal power etc. as part of energy mix. Out of these alternative sources, solar power seems very promising.

The following diagram, Figure 27, shows the worldwide growth of solar photovoltaic power from year 2000 to 2008. But even this fantastic growth is dwarfed by the phenomenal growth that is anticipated from year 2009 to 2020, which is shown in Figure 28. It may be noted that another major use of solar energy in the form of solar heating has not been addressed in this article, as it is not particularly relevant in the context of Bangladesh.

However, should solar heating becomes important to Bangladesh, it can be addressed quite easily in future.

[11]

Figure 27: Growth of Solar Power Worldwide from 2000 to 2008. [11]

Figure 28: Anticipated Growth of Solar Power Worldwide from 2009 to 2020.

[11]

Asian tiger-economy countries like China and India are going flat-out for solar energies with particular emphasis on Photo Voltaic (PV) energies. Figure 3 below shows the worldwide

production of PV cells from 1990 to 2008. Admittedly, these quoted PV production capacities are somewhat lower than those shown in Figures 27 and 28, due primarily to the fact that these figures are based on government official reports (JRC PV Status Report 2009), not industrial output. However, what this Figure shows is that in 2008 China became the global leading producer of solar cells with an annual production of about 2.4 GW, followed by Europe with 1.9 GW, Japan with 1.2 GW and Taiwan with 0.8 GW. If this trend continues, China will be producing nearly third of all solar cells world wide by 2012.

Figure 29: PV Production World Wide from 1990 to 2008.

[11]

[11]

Bangladesh present electrical situation At present the power demand in Bangladesh is about 6000MW, whereas the generation ranges only 35004200MW. The generation capacity is 4300MW. But peak demand is estimated to exceed 6000 MW. As a result of power shortage causes excessive load shading.

Bangladesh relies heavily on fossil fuels for its energy especially on gas resources. But the present proven reserve would be depleted by 2015. Coal is still the major fuel for power generation. Bangladesh has sufficient high quality coal resources. But the coal mining is not started. Exploration and development of natural gas resource has almost reduced to zero.

Also the exploration of coal continues to remain uncertain. The shortage of power can be met by renewable energy which is abundant in nature. In Bangladesh the development of renewable energy is insufficient. The one and only hydro power plant is in Karnaphuli can generate 230MW. The coastal areas and the north-eastern regions contain areas with high wind, and small-scale wind energy conversion system could be built only on that area.

Grameen Sakti (GS) is working on solar home system. But GS’s solar program mainly targets those areas, which have no access to conventional electricity and a little chance of getting connected to the grid within 5 to 10 years.

Table 6: PRODUCTION AND VALUE OF ENERGY [14] Sl. No.

Particulars

Year

Year

2007-08

2008-09

% change over previous year

1

Installed Capacity in MW (As of June)

5,307

5,719

7.76

2

Generation Capacity (Derated)

4,791

5,166

7.83

3

Max.Capability Available in MW

4,130

4,162

0.78

4

Net

26,533

6.34

Generation 24,952

(BPDB+IPP+Rental+REB) GWh 5

Maximum demand served in MW

4,130

4,162

0.78

6

Cost of fuel/kWh in Taka

1.05

1.11

5.74

7

Per capita generation in kWh

176.87

183.26

3.61

An ordinary house in Dhaka, may consume around 12kWh/d with a peak demand of nearly 1.1403kW. Meeting such a load by only solar energy source is not practical. A set of energy consumption data for a typical grid-connected house in Dhaka’s was collected. For a typical day energy consumption is higher in the evening hours

[4]

(Fig. 30).

On the other hand, summer months especially the month of April the power consumption is highest.

Figure 30: Energy consumption pattern.

[4]

Today, Solar Home Systems (SHS) are gradually becoming popular in the rural areas in Bangladesh. But in cities, where the power supply is insufficient, fluctuating and failure is a regular event, grid-connected PV system can be a good power source if installed on the roof-tops of the building. The power produced by the roof-top grid-connected PV system can be used to supply local loads, with the excess energy fed into the local grid for use by other customers. At night, the local loads are simply supplied by the grid power. If the PV system is large enough, it can supply more energy into the grid than is used by local loads. Instead of receiving a bill every month from the utility supply office, the owner of the system would then

be

able

to

earn

money

by

generating

surplus

electricity.

Grid-connected PV power systems are being installed in cities in different countries of the world. Government policies are being framed to encourage and popularize this system by providing necessary regulations and incentives in many developed and developing countries.

From the gradual decrease of prices and increased rate of installation of the systems in the cities all over the world it can be easily comprehended that this system will become an important

source

of

electricity

in

a

very

short

time

in

the

urban

areas.

Roof-top grid-connected PV systems are also being installed in our neighboring countries like India, Thailand and Indonesia. The future of PV-grid electricity in Bangladesh is also very bright

as

we

have

bright

sun

light

throughout

the

year.

[9]

The installed system was run for several days in different weather conditions and the performance was found to be quite satisfactory.

The system was run for several days in different weather conditions. Efficiency, Input and Output power of, the system were found to be as follows

[2]

:

路

Efficiency of the inverter: 90-93%

路

Input power (DC): 1000-1300 W (on a typical semi-cloudy day)

路

Output power (AC): 672-1120W

To understand the financial viability of the system, a preliminary economic analysis of the 1.1kW roof-top grid-connected PV systems along with various sizes (Table-7) has been made.

[2]

Table 7: Economic analysis of the 1.1kW roof-top grid-connected PV systems

In the analysis standard methods of economics have been utilized considering various factors, viz., capital cost, life-cycle of the system, interest rate, inflation rate, operation and maintenance cost with and without net metering benefit. The above estimation was made by considering an average demand of 3000kWh for a four-member family. It is also seen from the table that a system of 2kW power for a single house-hold can produce surplus energy

that

can

be

fed

to

the

national

grid.

[2]

For 0% to 10% interest rates and 10% depreciation the unit-price of electricity with and without net-metering facilities will be respectively Tk.4.85-15.14 and Tk.4.85-15.14 only. As the system size becomes larger, the unit-price with net-metering decreases rapidly. The unitprice of electricity for the 1.1kW system at the above interest rates and depreciation is from 6.18

to

19.32

taka

only.

[2]

At present Bangladesh is going through severe electricity crisis. In this situation, this system can be a good alternative small-scale power source on the roof-top of the building in the cities that does not require any fuel. It is observed from the preliminary economic analysis that the system would be financially feasible if subsidy is given and net-metering regulation is framed by the government. Moreover, the impact of the system on the environment friendly issue should be considered as the system does not pollute the environment at all.

From the performance study it is also found that the system works efficiently. For emergency power supply of multistoried building Rajuk should frame some incentive based building-acts to encourage the integration of solar PV system as a part of future design and implementation.

[2]

Solar home system (SHS):

The Solar Home Systems (SHS) are gradually gaining popularity in the rural areas of Bangladesh, where grid power has not reached yet. Performance of a solar PV system varies with a number of factors like geographical location, climatic condition.

More than 3,15,000 units of Solar Energy System have been installed in remote rural areas of Bangladesh by December 2009. The combined power generation capacity is approximately 63 Mega Watt per day. About 13,000 Solar Energy Systems are installed per month. People are saving money that they had to spend for traditional fuels e.g. Kerosene and also helping the ecology to improve by emitting less green house gases like carbon-dioxide in the atmosphere.

[4]

Figure 31: Typical residential grid connected PV system

[4]

The optimized result contains a PV panel of 2.5KW, 10 batteries for storage and a 3kW converter. The initial capital cost is $28900. The power production from the PV panel is 68% whereas only 32% is purchased from the grid. The excess electricity and unmet load is zero.

The generated power by the PV is delivered to load is 53% and 47% is for sell back to the grid.

[4]

The initial capital cost for the renewable system is very high. The project life time is 20 years. A part of the initial capital cost is possible to recover from the power sell back to the grid. As 68% of the load demand is possible to meet by the PV generation, only 32% load demand needs to fulfil from the grid. Other than the initial cost, there are some constrains of solar systems. First, PV produces power intermittently because it works only when the sun is shining. This is not a problem for PV systems connected to the utility grid, because

any additional electricity required is automatically delivered to the consumer by the utility.

In case of off-grid or stand-alone PV systems, batteries can be purchased to store the excess energy for later use. PV-generated electricity is usually more expensive than conventional utility-supplied electricity. Therefore, if the consumer lives near the existing power lines, a solar rebate program and net metering can help to make PV home system more affordable. The customer is billed for the net electricity purchased from the utility over the entire billing period that is, the difference between the electricity coming from the power grid and the electricity generated by the PV system. In more than 35 states, customers who own PV systems can benefit from laws and regulations that require “net� electric meter reading.

[2]

Finally, unlike the electricity purchased monthly from a utility, PV power requires a high initial investment.

Tax incentives may include a sales tax exemption on the PV system

purchase, a property tax exemption,

or state personal income-tax credits, all of which

provide an economic benefit to consumers by lowering high capital costs.

Solar energy system has been installed for BTS, Grameen Phone. Grameen Shakti has successfully installed four solar systems for towers, each 6.50 Kilo Watt capacity at different locations as pilot project. There are more than 22,000 mobile phone towers in Bangladesh.

In future, there is a good possibility for Grameen Shakti to power those towers with solar systems.

[6]

Economics of Small SHS Depending on system capacity (10 Wp to 20 Wp), the price of the system varies in the range of Tk. 6,500- 13,000 and monthly installment fee of Tk. 175- 350. Households’ average fuel (i.e. kerosene) requirement was found to be 5-6 liter per month and they used to spend Tk. 200-250 on an average in each month as fuel cost. Therefore users of small SSHS are able to provide installment fee in place of fuel cost i.e. kerosene. Furthermore, proposed buy-down grant of Tk. 1250 and capacity development grant of Tk. 400 for per SSHS can play a key role in keeping the price of small SHS within the limit of low income group people.

[6]

At present, the solar home systems are not cost competitive for personal level against conventional fossil fuel based grid interfaced power sources because of the initial capital cost. However, to fulfill the basic need for the consumer and improvements in alternative energy technologies bear good potential for widespread use of such systems. The proposed system feasibility may be a cost issue in respect of Bangladesh; however, it is possible to overcome by introducing some incentives offered by the government and utility companies.

It can also be implemented in commercial building, telecommunication sector and water pumping for irrigation.

RENEWABLE ENERGY POLICY OF BANGLADESH, October 2002 [13] Some of solar energy related points are given here 5.2 100% depreciation in the first year for solar photovoltaic, solar thermal projects and 100% depreciation in five (5) years for wind, biomass, geothermal, tidal and small hydro projects.

5.3 The sponsors will be allowed to import plant and equipment (listed in Annex-1) without payment of customs duties, VAT (Value Added Tax) and any other surcharges as well as import permit fee provided that the equipment is not manufactured or produced locally.

LIST OF EQUIPMENT (Annex - 1) Solar Solar Photovoltaic Cells / Panel / Array Solar Pyranometer Solar Pyrheliometer Solar Pathfinder Solar PV IV-curve tester Solar Inclinometer Solar A/C D/C converters Solar Fans Solar Lanterns Solar bulb and Fluorescent tube light Solar dryer Solar water heater Solar cooker

Conclusion The main source of electricity in Bangladesh is Natural gas (about 82.69%, in the fiscal year 2008-09 its value was 4542MW) [14]. Natural gas produce the heat require to drive the turbine which produces electricity. Natural gas reserve is reducing day after day. To reduce the consumption, Government closed the production of some industry (Ghorasal fertilizer, polash fertilizer etc). But the reserve is very low, a alternative should employed. Solar energy is a very good option.

Bangladesh is a country with enough solar radiation to provide potential for sustaining SHS. From this radiation using the current available technology full demand of electricity can be overcome. But both the PV system and thermal system is very costly. This cost is high for personal level so government should take steps to setup solar energy plant.

At present, the solar home systems are not cost competitive against conventional fossil fuel based grid interfaced power sources because of the initial capital cost. However, to fulfil the basic need for the consumer and improvements in alternative energy technologies bear good potential for widespread use of such systems.

The proposed system feasibility may be a cost issue in respect of Bangladesh; however, it is possible to overcome by introducing some incentives offered by the government and utility companies. It can also be implemented in commercial building, telecommunication sector and water pumping for irrigation.

The Government of the people’s republic of Bangladesh is trying to meet the national electricity demand through various ways including installing Solar system. PV Solar energy conversion is only renewable energy source currently in operation in our country.

Solar thermal system is currently is popular technology for producing electricity in megawatt scale.

At latest technology it is equivalent to nuclear plant (Mojave solar park - 220,000 megawatts per year) without the radioactive dangers or the giant cooling towers to clog up the skyline. It is costly but in 10 years the cost can be recovered. (It doesn’t require any fuel!). So government should think about it.

If we can produce solar cell in our country the PV system cost will become 60% of current cost. Some organization in private level already started assembling of solar panel. But the Government should take some step toward solar cell production inside the country.

References 1. Solar energy utilization, G.D.Rai, Khanna Publishers, 1997.

2. Design Considerations for Solar PV Home Systems in Bangladesh M. Rezwan Khan, Department of EEE, BUET,Dhaka 1000, Bangladesh and M. Fayyaz Khan, Department of EEE, IUT, Gazipur, Bangladesh.

3. Wind and solar power system, Mukund R. Patel, CRC press (New York) LLC, 1999.

4.

A Feasibility Study of an On-grid Solar Home System in Bangladesh, Sanjida Moury, R. Ahshan, Faculty of Engineering and Applied Science, Memorial University of Newfoundland.

5.

Solar energy Fundamentals and modeling techniques, Zekei Sen, Springer, 2008.

6. Grameen Shakti's renewable energy role, Abser kamal, The Daily Star, Dhaka, Wednesday 24, 2010.

7.

Promoting solar PV for poverty reduction in Bangladesh, by Dipal C. Barua, MD, Grameen Shakti,2002.

8. Grid-connected solar power shines at DU campus, Dr. Rezaul Karim Mazumder, Professor at the depmnt of APECE, University of Dhaka.

9. Roof top Grid-connected PV power system in Bangladesh, RERC, DU. 10. The Case for Solar Energy in Bangladesh, Dr. A. Rahman, CRadP FNucI. 11. www.energybangla.com 12. www.wikipedia.org 13. Renewable energy policy of Bangladesh, Ministry of information, Oct 2002 14. Bangladesh Power Development Board Annual Report-10 (FOR YEAR 2008-09) 15. http://www.solarpowerfor.us/blog/category/solar-energy/ 16. www.renewableenergyworld.com/rea/news/article 17. http://www.concentratingsolarpower.com/ 18. www.thedailystar.net 19. Introduction to solar energy, Fernando O. Paras, Jr, AMD-IAE, CEAT, 2007, Presentation. p.14 20. www.inhabitat.com 21. Solar electricity, abu ishaq, 1988 22. Crystalline silicon solar cells, Martin A. Green, Photovoltaic special research centre, Australia.

23. Solar Electricity: Engineering of Photovoltaic Systems, Eduardo Lorenzo, G. Araujo, R. Zilles, PROGENSA, p78 24. http://www.daviddarling.info/encyclopedia/ 25. http://www.freesunpower.com/solarhome.php 26. http://jagadees.files.wordpress.com/2008/05/acciona-energia-solar-011.jpg