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Lesson 3.5
Lesson 3.5 |
Lesson 3.5 Radiation Flux & Absorption Overview This lesson deals with the variation in the amount of energy transported by electromagnetic waves as they spread out from their source. On completion of this lesson you should be able to explain the difference between intensity and flux. You should also be able to calculate the variation in flux with distance from a radiating source. MINI LAB
CHOICE OF ACTIVITIES
Solar Water Heaters Many households save a significant amount in potential energy costs by using solar water heaters. Solar water heaters typically consist of a dark colored metal panel that heats up when exposed to the sun. This panel transfers heat to water that circulates through cavities or pipes connected to the panel. The panel is usually enclosed in a shallow box with a transparent cover to prevent cooling of the panel by wind and air currents. Water circulates through the heater to a holding tank either by convection or by means of a small pump. The amount of heat absorbed depends on the area of the panel, the efficiency of the absorber and where it is located. Directly beneath the Sun, the top of the atmosphere receives roughly 1.39 kilowatts of radiant energy. The atmosphere absorbs some of this energy. The maximum amount of solar energy that reaches the earth’s surface at noon in the tropics is in the region of 1000 Watts per square meter. The maximum amount of energy that a panel can absorb depends on the direction and angle at which it is mounted and its distance from the equator. Emission of Thermal Radiation The relationship between temperature and radiant energy was developed and explained by Stephan and Boltzmann in 1884. According to the Stephan-Boltzmann model, the total energy emitted per unit area per second by a black body is given by the equation: Total Energy = s T4 Where: T is the absolute temperature (K) And s is the Stephan-Boltzmann
constant (5.67x10-8 W.m-2 K-4) Radiation Flux Radiation flux is the quantity of radiant energy that is moving (or flowing) through an area of space at right angles to the direction of movement. It is defined in terms of this area. In the S.I system, flux is measured in Watts per square meter (W.m-1 ). Since radiant energy generally spreads out with distance from the emitting source, the flux decreases with distance from the emitter. The area through which energy emitted from a small source can pass increases with the square of the distance from the source. This means that the flux decreases with the square of the distance from the source. Radiation Intensity Radiation intensity is the quantity of radiant energy per unit solid angle. The intensity of radiation from an emitter is a measure of the emitter’s power and is not affected by the distance from the emitter. Radiation intensity is therefore defined in terms of the radiation emitted into a solid angle. A solid angle is a 3-dimensional angle. Solar Radiation The Sun’s surface is
at 5777 K, it’s luminosity is in the region of 3.85
x 1026 Watts 99% of solar energy
reaching the earth falls between 0.15 - 4.0 µm
Photosynthetically
Active Radiation: About 50% of solar energy (including
scattered light) is Photosynthetically Active Radiation (PAR:
0.4 - 0.7 µm). Absorptivity The amount of radiation absorbed by an opaque object depends on the nature of its surface. In general, dark rough surfaces absorb more radiation than light colored, shiny surfaces. The ability of a surface to absorb radiation is measured in terms of its Absorptivity. The Absorptivity is the fraction of radiation that is absorbed. For example, a surface with an Absorptivity of 0.75 will absorb 75% of the radiation reaching that surface. In referring to Absorptivity, the wavelength or range of wavelengths needs to be specified. If a wavelength is not specified, the Absorptivity will be assumed to refer to thermal radiation which is roughly in the range: 0.15 to 4.0 µm When radiation reaches the surface of an object, a fraction of the radiation will be absorbed and the remainder will be reflected. If the object is partially transparent, a fraction of the radiation will be absorbed, a fraction of the radiation will be reflected and the remainder will be transmitted through the object. Reflectivity Reflectivity is the fraction of incident radiation that is reflected by a body. Shiny surfaces and light colored surfaces tend to have relatively high reflectivities and correspondingly low absorptivities. Transmissivity The Transmissivity of an object is the fraction of incident radiation that is transmitted through the object. This depends on the thickness of the material since most transparent materials absorb radiation as it passes through the material. Specific Heat Capacity The specific heat capacity of water is used to estimate the increase in temperature that is produced by absorption of energy. The specific heat capacity of water is roughly 4.18 kJ.kg–1 .K–1 . This means that if one kilogram of water absorbs 1000 Joules of energy, the temperature will be increased by one Kelvin. The specific heat capacity of water at 10 oC is 4.1922 kiloJoules/kg/oC. This changes slightly with temperature. For example: The specific heat capacity of water at 25 oC is 4.1796 kiloJoules/kg/oC. Solar Heating and Re-Radiation The Absorptivity of the absorbing panel in a solar heater will affect the efficiency of the heater. In addition to this, the warm surface of the panel will also re-emit some of the absorbed energy. The net amount of energy that is absorbed by the water will depend on the difference between the amount absorbed and the amount that is re-emitted. The transparent cover over the panel can also play an important part in the design of a solar heater. If it is made from glass, it will block UV and IR radiation from reaching the panel. or this reason, solar panels usually have plastic covers that allow more IR and UV to reach the panel. Plastic surfaces can also be made to have a lower reflectivity than glass and thus minimize the amount of energy lost due to reflection. The primary purpose of the cover is however to prevent cooling of the panel by conduction. Air is a poor conductor but its ability to absorb energy from hot surfaces is increased considerably by movement. By mounting the absorbing panel inside a shallow box with a transparent cover, the air trapped inside helps to insulate the absorber from heat losses due to conduction. Some convective cooling will occur bit this will be minimal due to the low speed of movement of the air.
Review Questions
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