Diffuse/direct radiation

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. 

The effect of temperature on the kinetics of the direct radiation method is quite complex. Increase in temperature increases the monomer diffusion rate but also increases transfer and termination reaction rates of the growing chains and reduces the importance of the gel effect. Solubility and radical mobility may also change as the temperature is varied [88,89]. 

As the reflected radiation is emitted from the sample in a random direction, diffusely reflected radiation can be separated from, potentially sensor-blinding, specular reflections. Common techniques are off-angle positioning of the sensor with respect to the position(s) of the illumination source(s) and the use of polarisation filters. Application restrictions apply to optically clear samples with little to no scattering centres, thin samples on an absorbing background and dark samples. In either of these cases, the intensity of radiation diffusely reflected off such samples is frequently insufficient for spectral analysis. While dark objectives remain a problem, thin and/or transparent samples can be measured in transmission or in transflectance. 

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. 

In radiation through the atmosphere, the electromagnetic energy is scattered and absorbed so that part of the radiation is observed to be distributed over the entire sky. The radiation within the sun solid angle is usually considered as direct radiation, but in addition, contains smaller amounts of scattered radiation. The scattering and absorption processes depend both spacially and spectrally upon the atmosphere composition and cloud distribution. It is useful to arbitrarily separate the input radiation to a surface into the direct and diffuse or scattered radiation. Figure 2 indicates the relative spectral magnitudes of each of these components. 

The rate of evaporation of water from such an isolated drop, although governed entirely by diffusion, is not governed entirely by diffusion of water vapor. Latent heat must be supplied during evaporation. The drop cools until this heat is conducted in from the air at a rate equalling the outward diffusion of water vapor. There is no other significant heat source since even if the drop were black, the direct radiation contribution of full sunlight would not be important for drops of this size. 

Equation (8-77) expresses the diffuse radiation leaving 1 which arrives at 2 and which may contribute to a diffuse radiosity of surface 2. The factor 1 - ps represents the fraction absorbed plus the fraction reflected diffusely. The inclusion of this factor is most important because we are considering only diffuse direct exchange, and thus must leave out the specular-reflection contribution... 

The radiation flux af fhe wall of radiation entrance (Figure 22) was determined by actinometric measurements (Zalazar et al., 2005). Additionally, the boundary condition for fhis irradiafed wall (x = 0) was obtained using a lamp model with superficial, diffuse emission (Cassano et al., 1995) considering (i) direct radiation from fhe two lamps and (ii) specularly reflected radiation from fhe reflectors (Brandi et al., 1996). Note that the boundary conditions at the irradiated and opposite walls consider the effect of reflection and refraction at the air-glass and glass-liquid interfaces, as well as the radiation absorption by the glass window at low wavelengths (the details were shown for fhe laboratory reactor). The radiation model also assumes that no radiation arrives from fhe top and bottom reactor walls (x-y plane at z = 0 and z = Zr). 

This effect has been observed [cf. Leighton (149), p. 22] in surface measurements of ultraviolet radiation, where the diffuse sky radiation is equal to or greater than the direct. 

The DVI does not take into account the detailed nature of the process and is based on a fair amount of estimation it can be argued, for instance, that it is not the total amount of dust that matters, but rather the fraction that reaches the stratosphere, particularly as a sulfur compound, and how that content evolves in time also, it is not just the change in direct radiation that has an effect, but the redistribution, in the entire spectrum, of the direct and diffuse radiation, etc. The DVI is an approach to identify and relate the rdevant and measurable parameters it gives a useful insight by associating volcanic and climatic chronologies. In the words of the author this is about as far as one can go towards objectivity in the assessment of past eruptions . In this respect it reflects the state of knowledge on the problem in 1970. 

In practice, for simplicity, surfaces are assumed to reflect in a perfectly specular or diffiise manner. In specular (or mirrorlike) reflection, the angle of reflection equals the angle of incidence of the radiation beam. In diffuse reflection, radiation is reflected equally in all directions, as shown in Fig. 12-32. Reflection... 

The solar energy incident on a surface on earth is considered to consist of direct and diffuse parts. The part of solar radiation that reaches the earth s surface without being scattered or absorbed by the atmosphere is called direct solar radiation G. The scattered radiation is assumed to reach the earth s surface uniformly from all directions and is called diffuse solar radiation G. ... 

The radiation flux incident on a surface from all directions is irradiation O, and for diffusely incident radiation of intensity I, it is expressed as... 

Solar radiation is incident on the from surface of a thin plate with direct and diffuse components of 300 and 250 W/m, respectively. The direct radiation makes a 30° angle with Ihe normal of the surface. The plate surfaces have a solar absorpi i vity of 0.63 and an emissivity of 0.93. The air temperature is 5°C and the convection heat transfer coefficient is 20 W/m °C. The cf-... 

Table 1 shows the annual mean values of daily total horizontal, direct-normal and diffuse solar radiation energy in five locations assumed. The hourly mean values of direct and diffuse components of solar radiation have been estimated from the given monthly mean values of daily total horizontal solar radiation [3]. 

Annual mean values of daily total horizontal, direct-normal and diffuse solar radiation energy. 

Fig. 5.41 Direct and diffuse solar radiation that passes through the atmosphere to the earth s surface (schematic)...

The direct solar radiation and the diffuse sky-radiation are combined under the term global radiation. The global irradiance EG of a horizontal area on the ground is made up of the following parts ... 

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