SolarPhotovoltaic system complexity varies from as simple as pocket calculators andas complicated as space station power suppliers specially, because of thephenomenon of photovoltaic effect, the conversion from solar energy to directcurrent electricity in certain types of semiconductors. To understand themechanism of solar cells one needs to have knowledge of photons and solarradiation, semiconductor structure, conversion of solar energy and lightenergy, chemical and electrical energy.Thischapter will be focusing on the mechanism within the solar photovoltaic cellprinciples and its conversion of energy to produce electricity, the structureof solar cell will also be covered. 2.1 Photovoltaic Theoretical Background In semiconductor materials, conduction is dependent onthe energy band called band gap.
There exist one excited band and one lower band.In the normal state, exited band contains minimum number of electrons and lowerband which is also called valence band contains maximum possible number ofelectrons, when energized, electrons move from valence band to excited statewhere they become free to move hence resulting in easy conduction of thematerial. The free electrons in the conduction band are separated from thelower state is measured in electron volts (eV).
This energy in diodes isprovided by electricity to make the diode conduct and is quite opposite in thephotovoltaic cell in which it is provided by photons (particles of light). Whenelectrons gain enough energy in the valence band they move up to the conductionband and when the move from conduction band to the valence band the lose energyequivalent to the difference of energy band. Process of movement of electronsfrom valence band to conduction band is called excitation as it gains energyand when electrons move back from conduction band to valence band is calledrecombination as it losses energy.
Figure 1. Excitement energy levels inintrinsic and doped semiconductor. Modified from Nipun (2015) 1 2.1.1 Dopingit is the process to make semiconductormaterials to conduct in more proper way, by adding some impurities. Assemiconductor materials from fourth group are mixed with 3rd and 5thgroup elements making P-type and N-type semiconductors respectively, figure 1shows N-type, a semiconductor doped with 5th group element hasexcessive number of electrons in the conduction band, and P-type having minimumnumber of electrons there.2.
1.2 PN-JunctionWhenboth N-type and P-type elements are combined, electrons and holes diffuse atthe center making a region called depletion region when electrons from N-typeburst into the P-type and hole from P-type to the N-type. This process is wellexplained in the figure 2.
Figure 2. Depletion region and electricfield E created by diffusions of electrons and holes in P-N junction. Modifiedfrom Wikimedia (2016) 3. With the creation of depletion region,it becomes hard to conduct, in photovoltaic cell when exposed to light,receives energy that is needed to make it conduct and depletion region alsoknown as PN-Junction contracts as show in next figure. P – type N – type E photons load I Figure3. Occurrence of electric current I when an external circuit was connected to aP-N junction. Modified from Apec (2007) 4. In photovoltaic cell, asthe light energy is received at PN-Junction, current starts flowing in thereverse direction, which is called photovoltaic effect which manipulates thesolar power using semiconductor properties.
2.1.3 Solar Cell As the generic semiconductor doping is done by addingimpurities into the 4th group of elements of 3rd and 5thgroup elements. But famous solar cell materials are Silicon (Si), Galliumarsenide (GaAs), Cadmium selenide (CdSe) have spectrum properties according tothe wavelengths of the solar radiation received on the surface of earth. Bydoping the different elements and making multiple junctions increased range ofsun light can be used to make solar cell more efficient.
72 93 44 34 79 18 23 18 17 11 9 76 8 67 8 61 7 13 5 51 0 1000 2000 3000 4000 5000 6000 7000 8000 0 0.5 1 1.5 2 2.5 InSb InAs Ge GaSb Si InP GaAs CdTe CdSe GaP Wavelength nm Band Gaps eV Figure 4. Band gap levels of typicalsemiconducting materials and corresponding light’s wavelength values. Datagathered from Georgia State University 5 and Michigan State University 6. Amongthe materials shown about, most famous is Silicon for its availability, foundfrom the silica sand and purified either in form of monocrystalline orpolycrystalline. Si for its cheapness is mostly used and its pn-junctionbehavior as well as it is conducted at mid-range radiation from sun.
c-Si solarcells have efficiency averaging over 15%. Which means they are capable ofconverting over 15% of solar energy in electrical. Typically, each solar cellis made up of a square having an area of approximately 1 cm2 producespower of about 1.5 W of the voltage 0.
6 V, which is too below the leastrequired to operate for applications. Therefore, they are combined in series tomake a proper standard of voltage. Different sizes and standards are designedby manufacturers according to the applications.
Each photovoltaic systemcontains Solar module which is heart of solar PV system.2.1.4 Solar Photovoltaic System Basically, solar system contains,· Photovoltaic cell· Battery backup· Control unit orsolar module· Load· Main supply unit PV cell Battery backup Load Control Unit Main supply Figure5.TypicalBlock diagram of solar Photovoltaicsystem The PV cell is the main component ofthe system which converts solar energy into electrical energy, which is usuallya series of solar modules mounted on solar panels for larger output power. Itconverts light into direct current, DC power which can be utilized directly forlow power applications and even can be utilized for high power applications by usinginverter and storing the power into the battery for backup. Due to the serial connection ofphotodiodes, if any of the module is stops conducting in case it is shaded andother is producing energy, power will start flowing within the shaded solar moduleand causing over heat and may damage the solar panel, so for its safety thisproblem is prevented by using bypass diodes, even blocking diode is used sothat battery does not drain its power when solar panel is shaded.
The solar panel is supported by amechanical structure or placed such a way that it receives maximum possibleradiations coming onto the surface of earth, or a solar tracking system isdesigned. 2.2 Solar Module’s Performance and SolarTracking System 2.2.
1 Performance by Fixed Mounting For PV module which catches solar radiation, solarradiation that is reached on the surface of earth has three components.· Direct beam, thatreaches on the surface without scattering· Diffuse radiation,that scatters while entering earth’s atmosphere· Radiation thatreflects from surface of earthSolarradiation reaching on the surface of earth consist of 80-90% component ofdirect beam in sunny weather. It is the major source of energy for PV modules,so it needs to be aligned with the direct beam of solar radiation for maximumpossible duration. This process can be well defined by assuming an angle j,which is the angle between solar radiation’s direct beam and surface of PVmodule. The area that collects the solar radiation direction beam is proportionalto the sine(j), hence the power collected will be calculated as;P= P(max) * sin(j) (1)WhereP(max) is maximum power achieved by PV module when angle between surface of PVmodule and direct beam radiation is 90°, and the loss of power can becalculated as using equation (1),P(loss)= = 1 – sin(j) (2)According to equation (2), as the solar direct beamgoes parallel to surface of the PV module, P is lost directly, as the radiationis more misaligned with direct beam the more energy is lost. This relationshipis illustrated in figure 6. Figure 6.
Relationship between power loss andmisalignment of direct beamThe graph of figure 6 shows power loss in accordancewith misalignment if the angle of misaligned is assumed to be i, then theequation (1) and equation (2) will be function of cosine. In that manner as theangle of cosine increases the power loss increases. As it can be seen that ifthe angle of misalignment is 30° then the power loss is about 15%. Power lossincreases gradually if the angle of misalignment is further increased. By somecalculation we can obtain the values of output power of PV module on certainlocation of the earth, let’s say sun rises at 05:30 and sets at 19:30,calculating the output power from the solar power calculator database online weget a graph assuming weather is sunny.
By using equation (1) and giving time ofsunrise and sunset for a particular location we get approximation of outputpower of a fixed mounted PV module. Figure7. Approximation of power output (red line) compared to maximum output (blueline) for a fix mounted solar module. Data gathered from NOAA Solar Calculator7. It can be seen in figure 7, that from 11:00 to 16:00,power output is merely over 80% from a fixed mounted PV module. Which is only 5hours of the day.
For the typical 12 hours of summer day in Pakistan, 5 hoursof 80% efficiency from a PV module is certainly not a good way to gather solarenergy.However, figure 7 does not reflect the whole story, isbased on assumption but somehow it does explain that fixed mounted Solar Panelis not a solution for maximum power conversion of solar energy despite in theideal weather conditions.In reality, there are multiple problems that causedsolar radiations fade that are cloud shades, seasonal angles changes andlimited time of daylight. In spite of mechanical stability and simplicity fixedmounted solar panel can not exploit maximum possible conversion. Therefore, notsuitable for important projects and high capacity. Thereis a need of better solutions for mounting the system.
We will be subsequentlydiscussing about current solutions of solar tracker for better harvesting ofsolar power. This subtopic contains useful information for the design andcreation of the solar tracking system in this project. 2.2.2 Enhancement Using Tracking Systems As discussed above, fixedmounted panels have many power losses for the working of PV module.
Solartracker systems are solution decreasing the angle of misalignment between solarradiation’s direct beam and panel. Using different means of mechanicalstructures, solar trackers can rotate the panels to the optimized positionthroughout the whole operation of the system. Comparing solar tracker systemwith fixed mounted panel below, Advantages Disadvantages Higher overall efficiency Higher accuracy Longer active functioning time Better lifetime for solar cells Applicability for different applications More complicated design Higher cost Worse tolerance against weather condition Consumption of energy (active trackers) Table1.
Advantages and disadvantages of trackers over fixed mounts Solartrackers can be categorized in two types, one being the active and other ispassive.Passivetrackers are designed by flued that vaporizes and expands along both ways whenheated by sun. Flued is filled in canisters connected to a tube along bothsides. Different expansions of the flued according the heat of the sunaccording to its position occur. Balancing weight of the solar panel on bothsides, causing it rotate towards the direction of the sun.
Passive trackers areless favorable and efficient because of its complex design and low accuracy.Active trackers on the other hand, utilize motorsystem to control the movement of panels in single or dual-axis, by observingthe Sun’s position using photo sensors. The operation of this type of trackersis managed by controller or computer. Active trackers normally cost more to thesystem, but provide the best accuracy and efficiency compared to the othersolutions. A dual-axis tracker (DAT) can provide additional 40% of solar energyover the year, compared to normal fixed mounting system. 2.3.2 ActiveSolar Trackers Amongthe introduced solar tracking systems, active solar tracker is the chosen topicof research for this project, because of its extensive utilization ofelectrical and electronic knowledges.
It is also the most implemented solutionfor capturing the sunlight of PV systems. Together with better manufacturingtechnologies of PV materials, enhancing the operation of active solar trackersis the most efficient way to better exploit the immense energy amount of theSun. Basedon rotation of solar modules, active solar trackers can be categorized into twomain types: single-axis and dual-axis. In single-axis trackers (SAT), solar PVpanels are rotated about a single axis that normally aligns with the North meridian.SATs can be configured in a number or ways according to the position of theaxis with respect to the ground: · Tilted single-axis tracker (TSAT) – Horizontal single-axis tracker (HSAT) – Vertical single-axis tracker(VSAT). SATs allow the solarmodules to rotate between east-west directions according to the Sun’spositions. SATs provide reasonably good balance between flexibility, simplicityand performance. Different configurations of SAT are illustrated in figure 8.
Figure 8. Typicalconfigurations for active solar tracking systems: (1) TSAT (2) HSAT (3) VSAT(4) TTDAT (5) HDAT (6) AADAT. Reprinted from Juda (2013) 8