Thermal radiation solar

Thermal effects depend on radiation intensity and duration of radiation exposure. American Petroleum Institute s Recommended Practice 521 (1982) reviews the effects of thermal radiation on people. In Table 6.5, data on time to reach pain threshold are given. As a point of comparison, the solar radiation intensity on a clear, hot summer day is about 1 kW/m (317 Btu/hr/ft ). Criteria for thermal damage are shown in Table 6.6 (CCPS, 1989) and Figure 6.10 (Hymes 1983). 

To measure the efficiency of a whole window, special testing takes into account all heat transfer from conduction, convection, and radiation. Certain values are used to represent the thermal and solar efficiency of high-performance windows by measuring reduced thermal heat loss (measured by the U-... 

Studies carried out on Earth, for example, by the NASA infrared telescope on Mauna Kea (Hawaii), showed albedo variations which indicated the presence of holes in the Titanian cloud formations (Griffith, 1993). It is, however, still unclear as to whether these inhomogeneities result from differences in the surface composition. Lorenz et al. (1997) reported large variations in Titan s atmosphere due to photochemical processes. The methane contained in the dense nitrogen atmosphere is decomposed by solar and thermal radiation, and its content may be replenished from methane lakes or from clathrates. 

Since the earth has temperature, it emits radiant energy called thermal radiation or planetary infrared radiation. Measurements by satellites show an average radiant emission from the earth of about 240 watts per square meter. This is equivalent to the radiation that a black body would emit if its temperature is at -19°C (-3°F). This is also the same energy rate as the solar constant averaged over the earth s surface minus the 30% reflected radiation. This shows that the amount of radiation emitted by the earth is closely balanced by the amount of solar energy absorbed and since the earth is in this state of balance, its temperature will change relatively slowly from year to year. 

Figure 4 illustrates the results of this type of analysis for a deep-basin solar still in the San Diego area. Thermal radiation from basin to cover is the largest loss, followed by reflection of solar radiation from the cover and air convection inside the still. Solar utilization efficiency is the height of the lowest curve as a fraction of the height of the top curve, ranging from about 30% in January to 50% through the summer months. 

Radiation from salt water surface to the wetted cover is the largest loss, but there seems to be no assurance that it can be reduced. There is some evidence that microscopic roughening of the underside of the cover may render that surface reflective for the bulk of the long wave radiation (peaking at about 8 to 10 microns) from the water surface, without appreciably reducing its transparency for short wave solar radiation. However, even a thin film of condensate on the cover is an effective absorber for thermal radiation, so the benefit of a thermally reflective cover may not be realized in ordinary basin-type stills. 

Another approach to radiation loss reduction might be the alteration of the salt water surface in some manner to lower its emissivity for thermal radiation. If a transparent thin liquid film or porous solid film of low thermal emissivity, permeable to water vapor, could be floated on the salt water, solar energy could continue to be absorbed on the basin bottom, water would vaporize, but thermal radiation loss would be reduced. Whether materials with these properties can be found and successfully utilized remains to be seen. 

The surface of an outer space station receives solar radiation at a rate of 1.2 kW/m2. The surface has an absorptivity of a = 0.75 for solar radiation and an emissivity of e = 0.86. There are no heat losses into the space station. However, heat is dissipated by thermal radiation into the space at absolute zero. Determine the equilibrium temperature of the surface. 

Solar radiation is a form of thermal radiation having a particular wavelength distribution. Its intensity is strongly dependent on atmospheric conditions, time of year, and the angle of incidence for the sun s rays on the surface of the earth. At the outer limit of the atmosphere the total solar irradiation when the earth is at its mean distance from the sun is 1395 W/m2. This number is called the solar constant and is subject to modification upon collection of more precise experimental data. 

The equivalent solar temperature for thermal radiation is therefore about 5800 K (10,000°R). 

Table 8-3 Comparisons of Absorptivities of Various Surfaces to Solar and Low-Temperature Thermal Radiation as Compiled from Ref. 14.

Gubareff, G. G., J. E. Janssen, and R. H. Torborg Thermal Radiation Properties Survey, 2d ed., Minneapolis Honeywell Regulator Co., Minneapolis, Minn., 1960. Threlkeld, J. L., and R. C. Jordan Direct Solar Radiation Available on Clear Days, ASHAE Trans., vol. 64, pp. 45-56, 1958. 

In the lower panels of the figure, properties of each pane of a double pane structure are shown. Low IR transmittance (lower left) of the outer pane keeps thermal infrared solar radiation from entering the building. The broad transmittance band of the inner layer and low inside reflectance of the inner pane allows thermal infrared energy to escape. This structure reduces the cooling load in a sunny environment. 

Thermally assisted solar electrolysis consists of (i) light harvesting, (ii) spectral resolution of thermal (sub bandgap) and electronic (super-bandgap) radiation, the latter of which (iiia) drives photovoltaic or photoelectrochemical charge transfer V(iH2o) while the former (iiib) elevates water to temperature T, and pressure, p ... 

Besides the absorption of the various components of solar irradiation, additional infrared (IR), or thermal, radiation is also absorbed by a leaf (see Eq. 7.2 and Fig. 7-1). Any object with a temperature above 0 K ( absolute zero ) emits such thermal radiation, including a leaf s surroundings as well as the sky (see Fig. 6-11). The peak in the spectral distribution of thermal radiation can be described by Wien s displacement law, which states that the wavelength for maximum emission of energy, A,max, times the surface temperature of the emitting body, T, equals 2.90 x 106 nm K (Eq. 4.4b). Because the temperature of the surroundings is generally near 290 K, A,max for radiation from them is close to... 

Passive optical techniques exploit the natural illumination in the environment (e.g., thermal radiation from the elements in the scene, the Sun, the cloud itself) to replace the active laser beam. In the infrared spectral region, where CW agents have the most characteristic spectral properties, solar radiation contributes little compared with thermal self-emission. As long as the suspect cloud or surface is not in complete thermal equilibrium with the environment, that is, as long as there is a temperature difference between the target and elements in the scene, there are measurable spectral differences between emissions from elements (pixels) on and off the target. 

Thermal radiation is the subject of the fifth chapter. It differs from many other presentations in so far as the physical quantities needed for the quantitative description of the directional and wavelength dependency of radiation are extensively presented first. Only after a strict formulation of Kirchhoff s law, the ideal radiator, the black body, is introduced. After this follows a discussion of the material laws of real radiators. Solar radiation and heat transfer by radiation are considered as the main applications. An introduction to gas radiation, important technically for combustion chambers and furnaces, is the final part of this chapter. 

Certain gaseous components of the atmosphere, called greenhouse gases, transmit the visible portion of solar radiation but absorb specific spectral bands of thermal radiation emitted by the Earth. The theory is that terrain absorbs radiation, heats up, and emits... 

---