Radiation, solar, ground reflection

Ground Reflection - Solar radiation reflected from the ground onto a solar collector. 

FIGURE 11.31 Radiaiion fluxes at the buildirtg facade the solar radiation components (direct or beam, diffuse, and reflected radiation from the ground or other buildings) and the components of the radiation back from the building facade (reflected solar and thermal infrared radiation from the building envelope). 

T he total or global solar radiation has a direct part (beam radiation) and a diffuse part (Fig. 11.31). In the simulation, solar radiation input values must be converted to radiation values for each surface of the building. For nonhorizontal surfaces, the diffuse radiation is composed of (a) the contribution from the diffuse sky and (b) reflections from the ground. The diffuse sky radiation is not uniform. It is composed of three parts, referred to as isotropic, circumsolar, and horizontal brightening. Several diffuse sky models are available. Depending on the model used, discrepancies for the boundary conditions may occur with the same basic set of solar radiation data, thus leading to differences in the simulation results. 

The amount of solar radiation that reaches any point on the ground is extremely variable. As it passes through the atmosphere, 25 to 50 percent of the incident energy is lost due to reflection, scattering nr absorption. Even on a cloud-free day about 30 percent is lost, and only 70 percent of 1,367 W/nf, or 960 W/m, is available at the earth s surface. One must also take into account the earth s rotation and the resultant day-night (diurnal) cycle. If the sun shines 50 percent of the time (twelve hours per day, every day) on a one square meter surface, that surface receives no more than (960 W/m ) X (12 hours/day) X (365 days/year) =... 

The magnitude of the solar heating is indicated by the so-called solar constant. In space, at the radius of the earth s orbit, the solar constant is about 443 Btu/(h)(ft2) (1396 W/m2). However, solar radiation is attenuated by passage through the atmosphere it is also reflected diffusely by the atmosphere, which itself varies greatly in composition. Table 7.2 provides representative values of the solar constant for use at ground level, as well as of the apparent daytime temperature of the sky for radiation purposes. 

An alternate technique uses the back-scattered solar ultraviolet (BUV) radiation reflected into space and measured outside the atmosphere from an orbiting satellite. The vertical distribution of ozone within this total column can be measured with the ground-based Dobson instrument through the umkehr technique, which depends upon the variation in ultraviolet penetration versus solar zenith angle over a period of several hours. The vertical distribution of ozone can also be determined from the wavelength dependence of the BUV signal. 

Evaporation models for boiling pools require definition of the leak rate and pool area (for spills onto land), wind velocity, ambient temperature, pool temperature, ground density, specific heat, and thermal conductivity. Radiation parameters (c.g., incoming solar heat flux, pool reflectivity, and emissivity) are also needed if solar radiation is a significant factor. Most of these data arc readily available, but soil characteristics are quite variable. 

In the calculation for atmospheric photodissociation reactions, how to calculate the effective solar intensity is a major issue, because not only direct irradiation from the sun, but light from all directions reflected and scattered by the ground surface, clouds, atmospheric molecules, and aerosols can contribute to photolysis. Furthermore, in the troposphere for example, only solar radiation that has not been absorbed by atmospheric molecules in the higher atmosphere, the stratosphere and above, can cause photolytic reactions. The spherically integrated solar intensity after considering those many atmospheric processes is called the actinic flux F (X) (photons cm s ), which means solar irradiation valid for photochemical effect. In atmospheric chemistry, jp is often used instead of kp for representing photolysis rate constant. Photodissociation rate constant in the atmosphere can be expressed using these parameters as... 

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