Solar Radiation for Energy: A Primer and Sources of Data
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Solar radiation is a general term for the electromagnetic radiation emitted by the sun. This radiation can be captured and converted here on the Earth to useful forms of energy, such as heat and electricity, using a variety of technologies. The technical feasibility and economical operation of these technologies at any specific location is very dependent on the nature of the solar resource. The amount of solar energy shining on a building also affects the amount of energy required to heat and cool the interior of the building.
Solar radiation has a spectral, or wavelength, distribution from short wavelength radiation (gama and X-rays) to long wavelength radiation (long radio waves). The different regions of the solar spectrum can be described by the range of their wavelengths. The combined radiation in the wavelength region from 280 nanometers (nm; a nanometer is one billionth of one meter) to 4,000 nm is called the broadband, or total, solar radiation. About 99 percent of solar radiation is contained in the wavelength region from 300 nm to 3,000 nm. The region of the spectrum that is visible to humans (sunlight) extends from about 390 nm (ultraviolet) to 780 nm (near-infrared), and makes up only about 10 percent of the total solar spectrum. However, it is the most practically useful part of the spectrum for humans (and most other life on the planet). This is because the wavelengths of the solar spectrum also correspond to different energy levels. Short-wavelength radiation has a higher energy level than long-wavelength radiation.
The rate at which solar radiation strikes earth's upper atmosphere is expressed as the "solar constant." This is the average amount of energy received in a unit of time on a unit of area perpendicular to the sun's direction at the mean distance of the earth from the sun: 92,960,000 miles (149,604,970 kilometers). While the distance between the earth and the sun varies as the earth moves around the sun on its elliptical orbit, the variation in the distance does not have a significant effect on the amount of solar radiation reaching the earth. (The earth is closest to the sun in late December/early January, and farthest from the sun in late June/early July.) The average intensity of solar radiation reaching the upper atmosphere is about 1,367 watts per square meter (W/m2) or 434 British Thermal Units (Btu) per square foot.
The amount of this energy that reaches any one "spot" on the earth's surface will vary according to atmospheric and meteorological (weather) conditions, the latitude and altitude of the spot, and local landscape features that may block the sun at different times of the day.
As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by air molecules, water vapor, clouds, dust, and pollutants from power plants, forest fires, and volcanoes. This is called diffuse solar radiation. The solar radiation that reaches the surface of the earth without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10 percent on clear, dry days, and by 100 percent during periods of thick clouds.
The daily rotation of the earth and its seasonal movement on its axis has significant implications for practical use of solar energy. For any spot on the earth's surface, the amount of energy it receives will vary on an hourly, daily, and seasonal basis. It is the angle of the sun's position in the sky relative to a point on the earth's surface that determines the intensity of sunlight reaching that spot. The lower the sun is in the sky, the more of the earth's atmosphere that the sunlight passes through before it reaches the surface, and the more it is diffused.
Direct solar radiation is generally most intense at any one spot on the surface of the Earth at solar noon, since it is most perpendicular in the sky, and has the least amount of the atmosphere to travel through. For locations at and north of 23.5 degrees north latitude, it is most intense at solar noon on June 21st (the summer solstice). At that time, the sun is at the highest point in the sky that it will reach during the year, and it is at this point that sunlight passes through the least amount of the earth's atmosphere. The summer solstice is also the longest day of the year. For these same locations, the shortest day of the year, and the day when sunlight is the least intense is December 21st, the winter solstice. (The opposite is true for locations in the southern hemisphere.) Higher latitudes have more hours of sunlight in the summer and less hours of sunlight in the winter, relative to lower latitudes. For a point on the equator, the sun will be most intense around March and September 20th and 21st (the spring and vernal equinoxes) as these are the days when the sun is directly overhead.
Solar collectors can be positioned to maximize the amount of solar energy that they receive on a daily and seasonal basis. In general, the optimum orientation of a solar collector is directly true south (in the northern hemisphere; true north in the southern hemisphere). However, local landscape features, such as trees, buildings, and hills or mountains may shade a solar collector during different times of the day during different seasons. Local weather conditions, such as typically foggy mornings or cloudy afternoons, may also affect the optimum orientation. In these situations, the orientation may be east or west of south to optimize solar energy reception daily and/or seasonally.
The angle of a solar collector relative to sun's position in the sky also greatly affects the amount of solar energy it receives. For example, a flat, horizontal surface facing true south in Topeka, Kansas (at 39 degrees North latitude), with total exposure to the sun all day throughout the year, will receive an annual average of 4.3 kilowatt-hours (kWh), or 12,969 Btu, per square meter (10.76 square feet) per day, while a vertical surface will receive 3.3 kWh (10,239 Btu) per square meter per day. In July, the horizontal surface will receive 6.6 kWh (22,526 Btu) per square meter per day and the vertical surface will receive 2.6 kWh (8,874 Btu), because the sun is higher in the sky in the summer and strikes the horizontal surface more directly. When the sun is lower in the sky in December, the horizontal surface will receive 1.9 kWh (6,485 Btu) per day, while the vertical surface will receive 3.4 kWh (11,604 Btu).
For solar collectors where the angle is fixed (such as low-temperature, flat-plate water or space heaters) an optimum angle must be selected. The general rule of thumb is that for maximum solar reception during winter months, the optimum angle is the angle of latitude plus 15 degrees. For maximum solar reception in summer months, the optimum angle is the angle of latitude minus 15 degrees.
Some types of solar energy conversion systems use a tracking device that positions a solar collector to face the sun. A tracking system is necessary for solar energy systems that concentrate sunlight onto an absorber. Concentrating solar collectors use primarily direct beam solar radiation. Flat-plate collectors (in fixed positions and on trackers) can use direct beam and diffuse radiation, and even radiation reflected off the ground or surrounding objects and fixed reflectors.
