Module 5 Solar Power Generators, principle and operations.pdf
- 1. PHOTOVOLTAICS • Photovoltaic systems convert sunlight directly into electricity, and are potentially one of the most useful of the renewable energy technologies. • Also known as solar cells, PV systems are already an important part of our lives. The simplest systems power many of the small calculators and wrist watches we use everyday. • The photovoltaic cell is the basic building block of a PV system. Individual cells can vary in sizes from about 1cm to about 10 cm across. Most cells are made with silicon today. Silicon must be purified– this is one of the biggest expenses in the production of solar cells. • A slab (or wafer) of pure silicon is used to make a PV cell. The top of the slab is very thinly diffused with an “n” dopant, such as phosphorous. On the base of the slab, a small amount of a “p” dopant, typically boron, is diffused. The boron side of the slab is 1,000 times thicker than the phosphorous side. Dopants are similar in atomic structure to the primary material. The phosphorous has one more electron in its outer shell than silicon, and the boron has one less. These dopants help create the electric field that motivates the energetic electrons out of the cell created when light strikes the PV cell.
- 2. • When both sides of the silicon slab are doped, there is a negative charge in the p-type section of the junction and a positive charge in the n-type section of the junction due to movement of the electrons and “holes” at the junction of the two types of materials. This imbalance in electrical charge at the p-n junction produces an electric field between the p-type and n-type. • If the PV cell is placed in the sun, photons of light strike the electrons in the p-n junction and energize them, knocking them free of their atoms. These electrons are attracted to the positive charge in the n-type silicon and repelled by the negative charge in the p-type silicon. Most photon-electron collisions actually occur in the silicon base.
- 3. • A conducting wire connects the p-type silicon to an external load such as a light or battery, and then back to the n-type silicon, forming a complete circuit. As the free electrons are pushed into the n-type silicon they repel each other because they are of like charge. The wire provides a path for the electrons to move away from each other. This flow of electrons is an electric current that can power a load, such as a calculator or other device, as it travels through the circuit from the n-type to the p-type. • In addition to the semi-conducting materials, solar cells consist of a top metallic grid or other electrical contact to collect electrons from the semi-conductor and transfer them to the external load, and a back contact layer to complete the electrical circuit. • It is this flow of electrons that produces electrical current • The first PV cells were converting light to electricity at 1 to 2 percent efficiency. Today’s PV devices convert up to 17 percent of the radiant energy that strikes them into electric energy.
- 4. • One PV cell only produces 1 or 2 watts of electricity, which isn't enough power for most applications. • To increase power, groups of solar cells are electrically connected and packaged into weather-tight modules and arrays to provide useful output voltages and currents for a specific power output. • A PV System typically consists of 3 basic components. • PV cells - Electricity is generated by PV cells, the smallest unit of a PV system • Modules - PV cells are wired together to form modules which are usually a sealed, or encapsulated, unit of convenient size for handling. • Arrays – Groups of panels make up an array.
- 5. PV Types • Single-crystal silicon • 15–18% efficient, typically • expensive to make (grown as big crystal) • Poly-crystalline silicon • 12–16% efficient, slowly improving • cheaper to make (cast in ingots) • Amorphous silicon (non-crystalline) • 4–8% efficient • cheapest per Watt • called “thin film”, easily deposited on a wide range of surface types
- 6. • Clean • Sustainable • Free • Provide electricity to remote places • Less efficient and costly equipment • Part Time • Reliability Depends On Location • Environmental Impact of PV Cell Production Disadvantages Advantages
- 7. Photovoltaic Energy Conversion: Working principle • Solar energy is considered one of the most promising energy sources due to its infinite power. • Thus, modern solar technologies have been penetrating the market at faster rates • photovoltaic (PV) technology that has the greatest impact, not because of the amount of electricity it produces but because PV cells – working silently, not polluting – can generate electricity wherever sun shines, even in places where no other form of electricity can be obtained • more than 95% of these cells have power conversion efficiency about 17%, whereas solar cells developed over the last decade in laboratory environment have efficiency as high as 31%
- 8. PHOTOVOLTAICS • The photovoltaic cell is the basic building block of a PV system. Individual cells can vary in sizes from about 1cm to about 10 cm across. Most cells are made with silicon • A slab (or wafer) of pure silicon is used to make a PV cell. The top of the slab is very thinly diffused with an “n” dopant, such as phosphorous. On the base of the slab, a small amount of a “p” dopant, typically boron, is diffused. The boron side of the slab is 1,000 times thicker than the phosphorous side. • Dopants are similar in atomic structure to the primary material. The phosphorous has one more electron in its outer shell than silicon, and the boron has one less. • These dopants help create the electric field that motivates the energetic electrons out of the cell created when light strikes the PV cell.
- 9. • When both sides of the silicon slab are doped, there is a negative charge in the p-type section of the junction and a positive charge in the n- type section of the junction due to movement of the electrons and “holes” at the junction of the two types of materials. This imbalance in electrical charge at the p-n junction produces an electric field between the p-type and n- type. • If the PV cell is placed in the sun, photons of light strike the electrons in the p-n junction and energize them, knocking them free of their atoms. These electrons are attracted to the positive charge in the n-type silicon and repelled by the negative charge in the p-type silicon. Most photon-electron collisions actually occur in the silicon base.
- 10. • A conducting wire connects the p-type silicon to an external load such as a light or battery, and then back to the n-type silicon, forming a complete circuit. As the free electrons are pushed into the n-type silicon they repel each other because they are of like charge. • In addition to the semi-conducting materials, solar cells consist of a top metallic grid or other electrical contact to collect electrons from the semi-conductor and transfer them to the external load, and a back contact layer to complete the electrical circuit. • It is this flow of electrons that produces electrical current • Each cell produces approx. 0.5V (for Silicon). Voltage across solar cell depends on design and materials of the cell. Current depends on incident solar irradiance and cell area • This current fluctuates since the path of the sun varies with hour and season. • The elevation angle of the sun (θelevation sun ) is expressed in degrees above the horizon. • Azimuth angle (θ azimuth sun ) of the sun is expressed in degrees from true north. • Zenith angle (θ zenith sun ) of the sun equals 90 degrees less than the elevation angle of the sun, θ zenith sun = 90◦ - θelevation sun
- 11. • Solar cells produce direct current (DC), therefore they are only directly used for DC equipment. If alternating current (AC) is needed for AC equipment or backup energy is needed, solar photovoltaic systems require other components in addition to solar modules. These components are specially designed to integrate into solar PV systems. • The components of a solar photovoltaic system are: 1. Solar Module -- the essential component of any solar PV system that converts sunlight directly into DC electricity. 2. Solar Charge Controller -- regulates voltage and current from solar arrays, charges the battery, prevents battery from overcharging and also performs controlled over discharges. 3.Battery -- stores current electricity produced from solar arrays for use when sunlight is not available. 4. Inverter -- a critical component of any solar PV system that converts DC power into AC power. 5. Lightning protection -- prevents electrical equipment from damage caused by lightning or induction of high voltage surge. It is required for the large size and critical solar PV systems, which include grounding. Solar PV System
- 12. Net metering • A PV system produces DC- current. • The DC current goes into an Inverter where it becomes AC current. • An inverter is connected to your home’s or building’s electric system, and to your meter. • You use all electricity needed, while all excess electricity goes into the grid. • Electricity that goes into the grid is purchased from you by your utility company through Net Metering, usually at retail price. Net metering is an agreement that allows the solar PV system owner to sell excess solar energy to the utility company or buy deficit energy from the utility company using a meter to track this energy exchange.
- 13. Photovoltaic Systems’ Components • PV systems are classified into two major classes: • i) grid-connected ii) stand-alone • Grid-connected PV System • PV arrays connected to the grid through a power conditioning unit and are designed to operate in parallel with the electric utility grid • power conditioning unit include the MPPT, the inverter, the grid interface as well as the control system needed for efficient system performance • systems that interact with the utility power grid and have no battery backup capability, • systems that interact and include battery backup
- 15. Types of PV Power Systems • Photovoltaic power systems can be classified as: • Stand-alone PV systems. • Hybrid PV systems. • Grid-connected PV systems.
- 16. Stand-alone PV Systems • Stand-alone PV systems, are used in remote areas with no access to a utility grid. • Stand-alone PV energy system requires storage to meet the energy demand during periods of low solar irradiation and night time. • Batteries for PV system • Types of batteries are available such as the lead acid, nickel– cadmium, lithium, zinc bromide, zinc chloride, sodium sulfur, nickel– hydrogen, redox, and vanadium batteries. • The provision of cost-effective electrical energy storage remains one of the major challenges
- 17. Hybrid Energy Systems • Hybrid energy systems generate AC electricity by combining RES such as PV array with an inverter, which can operate alternately or in parallel with a conventional engine driven generator. • They can be classified according to their configuration as • Series hybrid energy systems. • Switched hybrid energy systems. • Parallel hybrid energy systems. • The parallel hybrid systems can be further divided to • DC coupling. • AC coupling.