Absorptivities for solar radiation

The atmosphere absorbs radiation quite selectively in narrow-wavelength bands. The absorption for solar radiation occurs in entirely different bands from the absorption of the radiation from the earth because of the different spectrums for the two types of radiation. In Fig. 8-65 we see the approximate spectrums for solar and earth radiation with some important absorption bands... 

Tie and Mo powders were sprayed by the plasma twin-torch process to form an FGM layer over a substrate to be used as an emitter in a thermoionic energy converter. The sprayed TiC coatings showed high absorptance for solar radiation. As compared to Tie monolayer coating and Tie/Mo two-layer coating formed on Mo substrate, FGM coatings performed better in a thermal cycling test. 

The roof of Prob. 14.2 is painted with an aluminum paint, which has an emissivity of 0.9 and an absorptivity for solar radiation of 0.5. What is the equilibrium temperature of the painted roof ... 

A nonconcentrating solar heat collector consists of a transparent cover, an absorber, tubes or ducts for the heat transport fluid, and an insulated case that limits the thermal losses to the environment. The collector cover serves several important functions it permits the passage of the solar radiation, prevents loss of heat, and protects the absorber from mechanical and weathering damage. The cover is selected for the highest transmittance of solar radiation and the lowest transmittance of the infi ared reradiated from the absorber. Polymeric materials have a low absorptivity for solar radiation and the absorptance can be decreased, compared to glass, by using thinner sheets or films. 

In 1817, Josef Fraunhofer (1787-1826) studied the spectrum of solar radiation, observing a continuous spectrum with numerous dark lines. Fraunhofer labeled the most prominent of the dark lines with letters. In 1859, Gustav Kirchhoff (1824-1887) showed that the D line in the solar spectrum was due to the absorption of solar radiation by sodium atoms. The wavelength of the sodium D line is 589 nm. What are the frequency and the wavenumber for this line ... 

Most ozone is formed near the equator, where solar radiation is greatest, and transported toward the poles by normal circulation patterns in the stratosphere. Consequendy, the concentration is minimum at the equator and maximum for most of the year at the north pole and about 60°S latitude. The equihbrium ozone concentration also varies with altitude the maximum occurs at about 25 km at the equator and 15—20 km at or near the poles. It also varies seasonally, daily, as well as interaimuaHy. Absorption of solar radiation (200—300 nm) by ozone and heat Hberated in ozone formation and destmction together create a warm layer in the upper atmosphere at 40—50 km, which helps to maintain thermal equihbrium on earth. 

Table 31.1 Coefficients ol absorption (ol solar radiation) and emission (natural radiation) for a metallic surface located outdoors...

Obtain an expression for the absorptivity of solar radiation as a function of surface temperature and compare the absorptivity and emissivity at 300, 400, and 1000 K. 

The total emissivity of concrete a( 330 K is 0.89, whilst the total absorptivity of solar radiation (sun temperature --- 5500 K) at this temperature is 0.60. Use the data of Problem 9.31 for aluminium. 

There is also a significant correlation between temperature fluctuations in the lower stratosphere and fluctuations in total ozone. There are two sources of this correlation, radiative and dynamical (McCormack and Hood, 1994). Thus, increased ozone leads to increased absorption of solar radiation and increased heating. In addition, dynamical effects associated with vertical and meridional air motions also give a positive correlation between ozone and stratospheric temperature. For example, Randel and Cobb (1994) analyzed total column 03 and temperatures in the lower stratosphere from 1979 to 1992. Correlations between 03... 

While there thus appears to be evidence for apparent excess absorption of solar radiation by clouds, there is substantial controversy over whether this is indeed true absorption or whether there is some other explanation for the discrepancies (e.g., see Stephens and... 

The necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

The peak of the O2 photodissociation occurs in the stratosphere (near 35 km for an overhead sun) where the total number of 02 molecules pho-todissociated is of the order of 107 cm-3 sec-1. Below the ozone peak (<25 km) the photodissociation rate decreases rapidly, particularly when the solar zenith angle increases. Below 20 km, the atomic oxygen production becomes very small and there is no atomic oxygen production in the troposphere by the 02 photodissociation. The ozone photodissociation is the result of the absorption of solar radiation in the visible and the ultraviolet ... 

Historically, the photochemistry of oxygen and ozone has been of considerable interest because of the importance of oxygen and ozone in the atmosphere. In 1881 Hartley (39) postulated that ozone was a normal constituent of the upper atmosphere and that absorption of solar radiation by ozone could account for the limitation of the solar spectrum at about 3000 A. These speculations were not definitely corroborated until 1913 by Fabry and Buisson (26). The formation of ozone from oxygen in ultraviolet light in the region 1400-1900 A. was first reported by Lenard (53) in 1900 and afterwards confirmed by Goldstein (34) in 1903. 

Fig. 3. The absorption spectrum for solar radiation in the Earth s atmosphere 19X On the ordinate is plotted the altitude at which the radiation intensity is reduced by a factor e 1 from its unattenuated value. The species predominantly responsible for the absorption in the various wavelength ranges are as indicated. The wavelength of the H Lyman-a radiation closely coincides with a window in the O2 absorption spectrum...

Since radiative recombination is tied on one side to the absorption coefficient, which should be as large as possible to facilitate the absorption of solar radiation, and on the other side to the difference between the Fermi energies fc — fV) which is the free energy per electron-hole pair and should also be as large as possible, radiative recombination is quite unavoidable. On the contrary, in a solar cell, which does not emit photons under open-circuit conditions, non-radiative recombination is dominant and causes the difference between the Fermi energies pc — fv to be too small for a sizeable emission according to (4.52). In an optimal situation, all recombination is radiative. The efficiency for this situation is the maximum efficiency a 2-band solar cell can have [6,8-10]. 

A positive temperature dependence of t] is a remarkable peculiarity of solar cells, which is not observed for any kind of inorganic solar cell [157]. It is important to note that heating the cells to such elevated temperatures as Tmax may be achieved merely by the absorption of solar radiation, i.e., without any additional heating. 

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. 

We have already described the radiation spectrum of the sun and noted that the major portion of solar energy is concentrated in the short-wavelength region. It was also noted that as a consequence of this spectrum, real surfaces may exhibit substantially different absorption properties for solar radiation than for long-wavelength earthbound radiation. 

Calculate the absorption rate for solar radiation on bright fine sand for a solar altitude angle of 50° and a turbidity factor of 3.5. 

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