Radiator, diffuse

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. 

Gebhart B. Surface temperature calculations in radiant surroundings of arbitrary complexity—for gray, Diffuse Radiation. Int.. Heat Mitss Transfer, vol. 3, no. 4, 19iil. 

Differential pressure cells 237, 239 Diffuse radiation 439 Diffusion 59, 573... 

Compared to a °Co-7 source, the electron accelerator yields an accurately focusable and constant radiation. The °Co-7 source always creates a diffuse radiation, the energy of which decreases with time according to the half-life of the material [40]. 

Martin et al. [102] reported a study in which LIBS was applied for the first time to wood-based materials where preservatives containing metals had to be determined. They applied PLS-1 block and PLS-2 block (because of the interdependence of the analytes) to multiplicative scattered-corrected data (a data pretreatment option of most use when diffuse radiation is employed to obtain spectra). They authors studied the loadings of a PCA decomposition to identify the main chemical features that grouped samples. Unfortunately, they did not extend the study to the PLS factors. However, they analysed the regression coefficients to determine the most important variables for some predictive models. 

Figure 9 shows the spectra obtained from the application of the action spectrum for the global, diffuse and direct irradiances for the same case. It can be seen the change in the spectra and the increase of the importance of diffuse irradiance relative to global. In this context, it can be expected that UV diffuse radiation have an important role in the erithema formation. This result supports the idea that shadow is not enough to avoid a sunburn and that additional protection measures must be taken in order to avoid also the exposition to diffuse radiation (sunscreens, clothes, etc). 

The term Qsh is the net solar radiant energy absorption rate on the basin bottom. It is equivalent to total radiation incident on the basin cover minus reflection from the cover, the water surface, and the basin bottom, and minus loss due to structural shadowing. Its determination from Weather Bureau records of total daily radiation on a horizontal surface is complicated by many factors such as variation in angle of incidence, and resulting transmissivity of cover, hourly and seasonally, intensity change due to cloudiness, and different properties of direct and diffuse radiations. Detailed explanation of these meteorological and optical calculations is beyond the scope of this paper, but may be found in the literature (6). 

The reactions are fast. Activation energies greater than 5kcal.mole-1 are unusual (and not well established). Observed chemiluminescence is often that of the alkali metal atom (D-line radiation), although for some cases (Na reactions with inorganic polyhalides), diffuse radiation from a molecular product has been observed. 

Henyey L, Greenstein J. Diffuse radiation in the galaxy. Astrophysical Journal 1941, 93, 70-83. 

Perez R., Stewart R., Arbogast C., Seals R., Scott J., An anisotropic hourly diffuse radiation model for sloping surfaces description, performance validation, site dependency evaluation. Solar Energy 1986 36 481M97. 

Low concentration ratio 3.5-7.5 is attractive due to collection of diffused radiation in the range of aperture angle and possibility of passive cooling. All these costs can be decreased by a factor 1.5 if we apply solar PV receiver with better efficiency and module power output 150 Wp/ m2. 

Although the relation is derived for black surfaces, it holds for other surfaces also as long as diffuse radiation is involved. 

All the preceding discussions have considered radiation exchange between diffuse surfaces. In fact, the radiation shape factors defined by Eq. (8-21) hold only for diffuse radiation because the radiation was assumed to have no preferred direction in the derivation of this relation. In this section we extend the analysis to take into account some simple geometries containing surfaces that may have a specular type of reflection. No real surface is completely diffuse or completely specular. We shall assume, however, that all the surfaces to be considered emit radiation diffusely but that they may reflect radiation partly in a specular manner and partly in a diffuse manner. We therefore take the reflectivity to be the sum of a specular component and a diffuse component ... 

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... 

Now consider the radiation leaving 2 which arrives at 1. There is a direct diffuse radiation of... 

Part of the diffuse radiation from 2 is specularly reflected in 3 and strikes 1. This specularly reflected radiation acts like diffuse energy coming from the image surface 2(3). Thus we may write... 

It is also necessary to take sky radiation into account, that is, sunlight scattered by the atmosphere and reflected diffusely and which reaches all surfaces of the tank, including those not hit by direct sunlight because they are in shadow. This diffuse radiation Gs varies greatly but is generally small, between about 2.2 Btu/(h)(ft2) (6.93 W/m2) on a clear day and 44.2 Btu/(h)(ft2) (139 W/m2) on a cloudy day. For the day as described, assume that Gs is 25 Btu/(h)(ft2). This value must be added to all surfaces, including those in shadow. 

A second characteristic of UV solar radiation is fhat, even for very clear afmospheres, if is composed in similar amounfs of both beam and diffuse radiation (Hulstrom et al., 1985). The first is defined as the radiation arriving directly from the sun, while the second is the solar radiation that has been scattered by gases and aerosols after entering the earth s atmosphere. This second t)/pe of radiafion reaches fhe ground in a more or less diffuse manner that is, with similar intensity from all directions in the sky. In this respect, the situation encountered in solar photocatalytic reactors is quite different from fhe one encounfered in solar fhermal collectors. The latter are able to use the whole solar spectrum, and in that case diffuse radiation accounts for a much smaller fraction of fhe global irradiance. 

Concentrating collectors are only able to use a limited fraction of fhe diffuse solar radiation. This utilizable fraction can be estimated as the inverse of the concentration ratio of fhe collecfor (1/Cr) (Rabl, 1985). Because of this, parabolic trough photocatalytic reactors, which have a concentration ratio around 15 or higher, miss practically all diffuse radiation. This amounts to losing around half of fhe available UV solar irradiance. 

When comparing different solar collectors it is important to take into account the different quantities of radiation collected in each case. For a given receiver, the radiative power collected increases with aperture size (i.e., with concentration). However, this is not a linear effect and, for instance, a CPC with a two suns concentration ratio provides twice the aperture area of a nonconcentrating CPC. Nevertheless the power received by the tubular absorber is not doubled, because the former CPC misses around half of the diffuse radiation. Other important consideration is the fact that concentrating reactors are faster simply because they collect more radiation, which is associated with larger collector area. 

It is important to note that the accumulated available energy does not take into account the actual energy collected by each reactor. In fact, the actual collected energy is a function of concentration ratio, as concentrating collectors are able to capture only a 1 /Cr fraction of the diffuse radiation. In order to reflect this, another quantity can be defined called the accumulated collected energy... 

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. 

Petroncelli P., Fiocco G. and Mugnai A., Annual variation of the effects of diffuse radiation on the photodissociation of ozone. Pageoph., 118, 20-34 (1980). 

---