Solar flux/radiation

The sun radiates approximately as a blackbody, with an effective temperature of about 6000 K. The total solar flux is 3.9 x 10 W. Using Wien s law, it has been found that the frequency of maximum solar radiation intensity is 6.3 x 10 s (X = 0.48 /rm), which is in the visible part of the spectrum 99% of solar radiation occurs between the frequencies of 7.5 X 10 s (X = 4/um) and 2 x 10 s (X = 0.15/um) and about 50% in the visible region between 4.3 x 10 s (X = 0.7 /rm) and 7.5 X 10 s (X = 0.4 /Ltm). The intensity of this energy flux at the distance of the earth is about 1400 W m on an area normal to a beam of solar radiation. This value is called the solar constant. Due to the eccentricity of the earth s orbit as it revolves around the sun once a year, the earth is closer to the sun in January (perihelion) than in July (aphelion). This results in about a 7% difference in radiant flux at the outer limits of the atmosphere between these two times. 

Specific solar radiation conditions are defined by the air mass (AM) value. The spectral distribution and total flux of radiation outside the Earth s atmosphere, similar to the radiation of a black body of 5,900 K, has been defined as AM-0. The AM-1 and AM-1.5 are defined as the path length of the solar light relative to a vertical position of the Sun above the terrestrial absorber, which is at the equator when the incidence of sunlight is vertical (90°) and 41.8°, respectively. The AM-1.5 conditions are achieved when the solar flux is 982 Wm2. However, for convenience purpose the flux of the standardized AM-1.5 spectrum has been corrected to 1,000 Wm2. 

The solar flux can be calculated via Stefan s law from the observed surface temperature of the Sun, and the level of radiation at a known distance is calculated via the inverse square law (Figure 7.6). 

Consider the amount of radiation arriving on the surface of the Earth at a distance of 1 AU or 1.5 x 1011 m. The total flux of the Sun is distributed evenly over a sphere of radius at the distance of the planet, d. From the luminosity calculation of the Sun, F, the solar flux at the surface of Earth, FEarth, is F/47t(1.5 x 1011)2 = 1370 Wm-2 from the least-square law of radiation discussed in Example 2.4 (Equation 2.4). Substituting this into Equation 7.6 with the estimate of the albedo listed in Table 7.2 gives a surface temperature for Earth of 256 K. 

Of the solar flux of 1 368 W/m2 reaching the upper atmosphere, 23% is absorbed by the atmosphere and 25% is reflected back into space. Earth absorbs 48% of the solar flux and reflects 4%. Radiation reaching Earth should be just enough to keep the surface temperature at 254 K, which would not support life as we know it. Why does the average temperature of Earth s surface stay at a comfortable 287 K ... 

The temperature of the upper atmosphere, and hence its density, varies with the intensity of solar ultraviolet radiation and this, in turn, varies with the sunspot cycle and with solar activity in general. The solar radionoise flux is a convenient index of solar activity, since it can be monitored at the earth s surface. The minimum nighttime temperature of the upper atmosphere above 300 kilometers has been expressed in terms of the 27-day average of the solar radio-noise flux at 8-ccntimctcr wavelength. This varies from about 600 K near the minimum of the sunspot cycle to about 1400 K near the maximum of file cycle. The maximum daytime temperature is about one-third larger than tile nighttime minimum. 

The value of 10 is determined by molecular and particulate (cloud and aerosol) scattering, and surface reflection. A small fraction of the molecular scattering is the non-conservative Rotational Raman scattering (RRS) that partially fills the solar Fraunhofer lines in the scattered radiation, creating what is commonly known as the Ring effect [15] As a result, the ratio Iq/F, where F is the extraterrestrial solar flux, contains structure that is correlated with solar Fraunhofer lines. By separating these effects, one can write... 

Solar systems are subjected to a unique set of conditions that may alter their stability and, hence, their performance and life-cycle costs. These conditions include UV radiation, temperature, atmospheric gases and pollutants, the diurnal and annual thermal cycles, and, in concentrating systems, a high-intensity solar flux. In addition, condensation and evaporation of water, rain, hall, dust, wind, thermal expansion mismatches, etc., may impose additional problems for the performance of a solar system. These conditions and problems must be considered not only individually, but also for synergistic degradative effects that may result from their collective action on any part of the system. Since these degradative effects may also reduce the system or component performance, protective encapsulation of sensitive materials from the hostile terrestrial environment is required to provide component durability. 

Calculate the radiation equilibrium temperature for a plate exposed to a solar flux of 700 W/m2 and a surrounding temperature of 25°C if the surface is coated with (a) white paint or (h) flat black lacquer. Neglect convection. 

A cast iron plate is placed in an environment at 25°C and exposed to a solar flux of 800 W/m2. Calculate the radiation equilibrium temperature of the plate neglecting convection. 

In Section III the solar-radiation intensity and its transmission through the troposphere are discussed. The solar flux is tabulated as a function of wavelength, and its dependence on azimuthal angle and total ozone column density is discussed. 

The total solar flux at a given altitude and its wavelength distributions are very important photochemical parameters in the troposphere. Of particular importance is the solar flux in the region 2850 to 3250 A. This ultraviolet radiation, indirectly the major source of radicals in the troposphere, is very sensitive to fluctuations in the total ozone column density. 

R Because the plant is spherical, the total solar radiation absorbed equals its projected area (jrr2) times the solar flux density perpendicular to the solar beam (1000 W m-2) times the absorbance (0.30). To obtain the average solar radiation absorbed over the plant surface, we divide by the plant s surface area (47rr2). Thus... 

The effects of Hz on the calculated vertical distribution of O2 for preindustrial levels of CO2 (280 ppmv) for three different values of solar ultraviolet radiation (UV) 1,300, and the T-Tauri flux, are shown in Figure 8 (for Hz = 17 ppmv) and Figure 9 (for Ha = 10 ). We found higher O2 levels for lower values of Hz. This is not surprising since higher values of Ha result in the more rapid loss of O2 due to the reformation of HaO (reaction (7)). For H2 = 17 ppmv, the surface O2 mixing ratio increased from 10 to 10" as the solar UV flux was increased. For Ha = 10 the surface O2 mixing ratio increased from 10 to almost 10" for the same increase in the UV flux. 

As we noted in Section 4.01.1, the ability of the troposphere to chemically transform and remove trace gases depends on complex chemistry driven by the relatively small flux of energetic solar UV radiation that penetrates through the stratospheric O3 layer (Levy, 1971 Chameides and Walker, 1973 Crutzen, 1979 Ehhalt et al., 1991 Logan et al, 1981 Ehhalt, 1999 Crutzen and Zimmerman, 1991). This chemistry is also driven by emissions of NO, CO, and hydrocarbons and leads to the production of O3, which is one of the important indicators of the oxidizing power of the atmosphere. But the most important oxidizer is the hydroxyl free radical (OH), and a key measure of the capacity of the atmosphere to oxidize trace gases injected into it is the local concentration of hydroxyl radicals. 

Solar radiation is incident at a rate of 950 W/m , and the absorber plate, which can be considered to be black, is maintained at 340 K by the cooling water. Delectnine (n) the solar flux incident on the absorber plate, b) Ihe transmissivity of the glass cover for radiation emitted by the absorber plate, and (r) the rate of heat transfer to the cooling water if Ihe glass cover temperature is also 340 K. 

The first 50 mm OD quartz reactor with pyrex flow straightner was wrapped with electrical heater tape and installed behind a large water cooled shield. The shield had a 10 cm diameter circular hole located in it for admission of the solar flux into the reactor. Behind the reactor, a water cooled semi-circular (cross section) cylindrical backplate prevented radiation from entering the work area through the back of the reactor. Unfortunately, the first reactor cracked at the pyrex/quartz joint when the upper portion of the reactor was accidentaly heated to a temperature in excess of its 100°C design operating temperature. 

Before the second 50 mm OD quartz reactor arrived from Princeton, several exploratory experiments were performed using a 50 mm OD vycor tube with rubber corks and steel inlets for the biomass and steam, and a gas exit. The reactor functioned well when exposed to the solar flux (1000 w/cm ) and some semi-quantitative data was obtained however because the rubber corks partially melted under exposure to the radiation flux carried up and down the walls of the vycor tube by total internal reflection, little significance could be attached to the data. 

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