T2.2

• The density of power radiated from the sun at the outer atmosphere is 1.373kW/m^2
• Part of the solar energy is absorbed and scattered by the earth's atmosphere
• Final incident sunlight on the earth's surface has a peak density of 1kW/m^2 at noon in the tropics
• Photovoltaic (PV) technology converts solar energy into usable electrical form
• A solar cell converts light to electricity through the photovoltaic effect
• Solar cells are made from semiconductors such as silicon in thin slices about ¼ mm thick
• Output voltages of commercial silicon cells and modules range from 0.5–0.7 V
• Efficiencies of silicon cells for converting light into electric power range from 12%–16%
• Manufacturers quote cell output for a sunlight intensity of 1kW/m^2
• Solar cells are encapsulated between sheets of glass, metal, or polymer foils for protection
• Several cells are interconnected into a PV module to obtain desired output power and voltage
• A simplified equivalent circuit of a solar cell consists of a current source in parallel with a diode
• Semiconductor materials suitable for solar cells include monocrystalline Si cells, polycrystalline Si cells, and amorphous Si cells
• Important points on a solar cell's current–voltage curve are open circuit voltage (Voc) and short-circuit current (Isc)
• To maximize power output, steps are taken to maximize open-circuit voltage, short-circuit current, and fill factor (FF)
• Fill factor (FF) for a silicon solar cell is typically 0.6–0.8
• An array is a collection of panels grouped together on a support structure
• The current-voltage characteristics of solar cells consist of a constant-voltage segment and a constant-current segment
• The maximum power condition of a solar cell occurs at the knee of the characteristic where the two segments meet
• Factors influencing the electrical design of a solar array include sun intensity, sun angle, load matching for maximum power, and operating temperature
• The magnitude of the photocurrent is maximum under a full bright sun (1.0 sun)
• On a partially sunny day, the photocurrent diminishes in direct proportion to the sun intensity
• Short-circuit current decreases significantly on a cloudy day
• Open-circuit voltage reduction is small on a cloudy day
• Photo conversion efficiency of the cell is insensitive to solar radiation in the practical working range
• The cell output current is given by I = Io * cosθ, where Io is the current with normal sun and θ is the angle of the sun line measured from the normal
• Cosine law holds well for sun angles ranging from 0 to about 50°
• Electrical output deviates significantly from the cosine law beyond 50°
• Array may get partially shadowed due to a structure interfering with the sun line
• Shadowed cell cannot produce power and acts as a load, producing local I^2R loss and heat
• Short-circuit current of the cell increases with increasing temperature
• Open-circuit voltage decreases with increasing temperature
• PV module can produce up to 80% of its full sun power on a partly cloudy day
• PV module can produce about 30% power even with heavy clouds on an extremely overcast day
• More energy is collected by the end of the day if the PV module is installed on a sun tracker
• Two types of sun trackers: One-axis tracker and Two-axis tracker
• Simple passive trackers based on the differential heating of fluids by the Sun and associated piston movements or gravitational forces are popular
• Maximum power is extracted from the module when the load resistance matches with the source
• Three electrical methods of extracting peak power from a PV source: small signal current injection, voltage adjustment based on power change, and using the ratio of voltage at the maximum power point to the open-circuit voltage
• PV power system components include a structure to mount the array, a sun tracker, various sensors, power electronic components, and an inverter if the load is AC