Solar radiation diffuse

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. 

In thermal building-dynamics simulation codes, outdoor conditions are mostly input by the so-called weather data file, containing (usually hourly) data for air temperature, wind speed and direction, air humidity, and global and diffuse solar radiation on horizontal surfaces. 

A solar pond does not concentrate solar radiation, hut collects solar energy in the pond s water by absorbing both the direct and diffuse components of sunlight. Solar ponds contain salt in high concentrations near the bottom, with decreasing concentrations closer to the surface. This variation in concentration, known as a salt-density gradient, suppresses the natural tendency of hot water to rise, thus... 

Bruntz, S. M., W. S. Cleveland, B. Kleiner, and J. L. Warner. The dependence of ambient ozone on solar radiation, wind, temperature, and mixing hei t, pp. 125-128. In Preprints. Symposium on Atmospheric Diffusion and Air Pollution of the American Meteorological Society, Santa Barbara, California, September 9-13, 1974. Boston American Meteorological Society, 1974. 

Photochemistry. Both CFC13 and CF2C12 arc used as aerosol propellants and refrigerants in large quantities. They arc chemically inert in the troposphere. However, when they diffuse into the stratosphere they are photodissociatcd by solar radiation to produce Cl atoms. The Cl atoms so formed would catalytically destroy 03 in the stratosphere (see Section VIII -2, p. 350). 

Concentration profiles of H2, He, CH4, and NH3 at 150 K and with the < tldy diffusion coefficient K = 3 x I05 cm2 sec-1 have been calculated by Strobel (943) and are given in Fig. VIII—15. Because of the presence of CH4 ibove the layer of NH3, solar radiation of wavelengths only above about 1600 A is effective in photodissociating NHj. 

Modelling of solar radiation components (diffuse and direct) in clear sky conditions is necessary in many solar energy applications (systems design and simulation, radiometers accuracy control procedures, quality control of data, recovering of observational missing data, etc) as well as in routine procedures in engineering. 

Ozone is a form of oxygen. Ozone is present in the upper atmosphere and it prevents dangerous solar radiation from reaching the Earth s surface. Some of the chemicals that diffuse into the upper atmosphere decompose ozone. Chemicals that have this effect are methane (CH4), chloromethane (CH3C/) and an oxide of nitrogen (N02). 

Collares-Pereira M., Rabl A., The average distribution of solar radiation correlations between diffuse and hemispherical and between daily and hourly insolation values. Solar Energy 1979 22 155-164. 

Liu Byh, Jordan R. C., The interrelationship and characteristic distribution of direct, diffuse and total solar radiation. Solar Energy 1960 4 1-9. 

Pyrheliometer is an instrument that measures only the direct radiation from the solar disk itself, without bouncing off clouds or the atmosphere. Concentrating solar collectors utilize only this part of the total solar radiation. Its measurement gives an indication of the clearness of the sky. The normal incident pyrheliometer (NIP) measures this form of radiation. A tracker, called the equatorial mount, is used to keep the NIP pointed at the sun. The difference between the NIP and the total pyranometer readings is referred to as the diffuse solar radiation. 

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