WEEK 03: GENERATION: ALTERNATE SOURCES: SOLAR ENERGY
Solar Constant, ISC is the rate at which the sun's energy irradiates a unit area of surface normal to the sun's rays, in the space outside the earth's atmosphere at the earth's mean distance from the sun 1.5 × 108 km. ± 3%
This constant is approximately:
428 Btu / ft² - hr.
1353 W / m²
4871 kJ / m² - hr.
1.940 cal / cm² - min
The surface temperature of the sun is about 5,762°K. Radiation emitted is approximately 7% UV at a wavelength < 0.3 mm. Of this light, 47% is visible at 0.38 - 0.78 mm, while 46% is infrared at > 0.78 mm.
Average radiation is about 1,500 / Btu / ft² / day. Hindrance is due to the atmosphere, absorption, scattering, the earth's rotation and the earth's tilt.
Flat Plate Collector, heat either a liquid or gas using an absorber plate, a blackened or specially treated metal with a cover, which permits the relatively short wavelength radiation from the sun to enter the collector but absorb the relatively longer thermal radiation emitted by the absorber plate (greehouse effect) and reduces the ehat loss by convection tot e surrounding atmospehere. The number of covers is dependent on the desired operating temperature of the collector. Two covers would generate greater than 82.2°C (180°F), one cover would generate less than 82.2°C, while when omitted, the collector is used as swimming pool heaters.
Collector Efficiency, is the ratio of energy collected tot hat incident energy to the collector. Efficiency decreases as the temperature difference between collector and sorounding air decreases. Collector temperature increases as collector surface area increases. Typically, cells are a few centimeters square producing a little over 0.4 volts. THe larger the cell the more power they produce. Cells are joined together to make a module and then modules joined to make whole panels. One square meter of a solar panel can produce 1.4 to 21.kW. A good absorber heat is also a good emitter. Usually plates are made of selective surfaces, a material with high absorptance for sunlight but a low radiation emittance.
Solar-Thermal Conversion Plants are of three types, based on the thermodynamic cycles employed: Rankine-cycle, using vapor, Brayton-cycle, using gas, and the Stirling-cycle, using high-efficiency fluids.
Thermal Efficiency is the ratio of useful work done to the heat supplied: h = 1 - (TL / TH), where h = thermal efficency, TL = absolute temperature of sink, °C + 273.15º, and TH = absolute temperature of source.
Classification of Solar Power Plants, aside from the cycle in which they operate, solar power plants are also classified according to their construction
1. Central Receiver System, where solar energy is transferred optically from the individual collector to a single receiver, e.g., a boiler for Rankine cycle. The system generates approximately superheated steam at 537.7#176;C (1,000#176;F)
1a. Solar Furnace, is an arragement of mirrors in a parabolic arc that reflect the sun's rays toward a central point called the focus of a parabola. The Solar Furnace at Odeillo-Font-Romeau in France uses 63 steerable heliostats (mirrors) to reflect the sun's rays on to the parabolic wall -- itself made from 9500 mirrors. The furnace, the tower in front of the parabola, can reach 3800°;C and make enough superheated steam to generate 1000kW of electricity. [TechnoQuest, 96]
2. Distributed System, where solar energy is transported from individual solar collectors by heated fluid flowing through pipes to a central boiler. Also known as a solar energy complex.
Solar energy complex is made of troughs of mirror which track the sun and reflect its rays on to oil-filled tubes running up the center of the troughs. Heated to nearly 400°C, the oil is then used to superheat steam to drive turbines.
Collectors can be of two types: concentrating type, either parabolic troughs or paraboloidal dishes; or the more popular, photovoltaic cells.