## Comparison

The mode of tracking affects the amount of incident radiation falling on the collector surface in proportion to the cosine of the incidence angle. The amount of energy falling on a surface per unit area for the four modes of tracking for the summer and winter solstices and the equinoxes are shown in Table 2.2. This analysis has been performed with the same radiation model used to plot the solar flux figures in this section. Again, the type of the model used here is not important, because it is used for comparison purposes only. The performance of the various modes of tracking is compared to the full tracking, which collects the maximum amount of solar energy, shown as 100% in Table 2.2. From this table it is obvious that the polar and the N-S horizontal modes are the most suitable for one-axis tracking, since their performance is very close to the full tracking, provided that the low winter performance of the latter is not a problem.

### 2.2.2 Sun Path Diagrams

For practical purposes, instead of using the preceding equations, it is convenient to have the sun's path plotted on a horizontal plane, called a sun path

 Tracking mode Solar energy received (kWh/m2) Percentage to full tracking E SS WS E SS WS Full tracking 8.43 1G.6G 5.7G 1GG 1GG 1GG E-W polar 8.43 9.73 5.23 1GG 91.7 91.7 N-S horizontal 7.51 1G.36 4.47 89.1 97.7 6G.9 E-W horizontal 6.22 7.85 4.91 73.8 74.G 86.2 Notes: E = equinoxes, SS = summer solstice, WS = winter solstice.

e)

90

re

80

gr

e (d

70

le

60

gl

50

n

a

e

40

d

tu

30

alt

20

lar ol

10

S

Declination curves in steps of 5°

Latitude = 35"N

Declination curves in steps of 5°

 0° Noon +23 45 Morning hours 11»- Afternoon hours 10 7 jj///y/x -23.45'^^^oiS^y wvo* 1/ §J/ff//j//y/ \VV\\V<18 1 ■ ■ 1 1 1 ■ ■ f

Solar azimuth angle (degree) figure 2.17 Sun path diagram for 35°N latitude.

Solar azimuth angle (degree) figure 2.17 Sun path diagram for 35°N latitude.

diagram, and to use the diagram to find the position of the sun in the sky at any time of the year. As can be seen from Eqs. (2.12) and (2.13), the solar altitude angle, a, and the solar azimuth angle, z, are functions of latitude, L, hour angle, h, and declination, 6. In a two-dimensional plot, only two independent parameters can be used to correlate the other parameters; therefore, it is usual to plot different sun path diagrams for different latitudes. Such diagrams show the complete variations of hour angle and declination for a full year. Figure 2.17 shows the sun path diagram for 35°N latitude. Lines of constant declination are labeled by the value of the angles. Points of constant hour angles are clearly indicated. This figure is used in combination with Figure 2.7 or Eqs. (2.5)-(2.7); i.e., for a day in a year, Figure 2.7 or the equations can be used to estimate declination, which is then entered together with the time of day and converted to solar time using Eq. (2.3) in Figure 2.17 to estimate solar altitude and azimuth angles. It should be noted that Figure 2.17 applies for the Northern Hemisphere. For the Southern Hemisphere, the sign of the declination should be reversed. Figures A3.2 through A3.4 in Appendix 3 show the sun path diagrams for 30°, 40°, and 50°N latitudes.

In the design of many solar energy systems, it is often required to estimate the possibility of the shading of solar collectors or the windows of a building by surrounding structures. To determine the shading, it is necessary to know the shadow cast as a function of time during every day of the year. Although mathematical models can be used for this purpose, a simpler graphical method is presented here, which is suitable for quick, practical applications. This method is usually sufficient, since the objective is usually not to estimate exactly the amount of shading but to determine whether a position suggested for the placement of collectors is suitable or not.

Shadow determination is facilitated by the determination of a surface-oriented solar angle, called the solar profile angle. As shown in Figure 2.18, the solar profile angle, p, is the angle between the normal to a surface and the projection of the sun's rays on a plane normal to the surface. In terms of the solar altitude angle, a, solar azimuth angle, z, and the surface azimuth angle, Zs, the solar profile angle p is given by the equation tan( p) = tan(a) (2.30a)

A simplified equation is obtained when the surface faces due south, i.e., Zs = 0°, given by tan( p) = (2.30b)

The sun path diagram is often very useful in determining the period of the year and hours of day when shading will take place at a particular location. This is illustrated in the following example. Example 2.8

A building is located at 35°N latitude and its side of interest is located 15° east of south. We want to investigate the time of the year that point x on the building will be shaded, as shown in Figure 2.19. Elevation Solution

The upper limit of profile angle for shading point x is 35° and 15° west of true south. This is point A drawn on the sun path diagram, as shown in Figure 2.20. In this case, the solar profile angle is the solar altitude angle. Distance x-B is (72 + 122)1/2 = 13.9 m. For the point B, the altitude angle is tan(a) = 8/13.9 ^ a = 29.9°. Similarly, distance x-C is (42 + 122)1/2 = 12.6 m, and for point C, the altitude angle is tan(a) = 8/12.6 ^ a = 32.4°. Both points are as indicated on the sun path diagram in Figure 2.20.

Latitude = 35"N Declination curves in steps of 5°

Latitude = 35"N Declination curves in steps of 5° Solar azimuth angle (degree) FiGuRE 2.20 Sun path diagram for Example 2.8.

Solar azimuth angle (degree) FiGuRE 2.20 Sun path diagram for Example 2.8.

Therefore, point x on the wall of interest is shaded during the period indicated by the curve BAC in Figure 2.20. It is straightforward to determine the hours that shading occurs, whereas the time of year is determined by the declination.

Solar collectors are usually installed in multi-rows facing the true south. There is, hence, a need to estimate the possibility of shading by the front rows of the second and subsequent rows. The maximum shading, in this case, occurs at local solar noon, and this can easily be estimated by finding the noon altitude, a„, as given by Eq. (2.14) and checking whether the shadow formed shades the second or subsequent collector rows.

Example 2.9

Find the equation to estimate the shading caused by a fin on a window. Solution

The fin and window assembly are shown in Figure 2.21.

Window

Window South

Perpendicular to window FIGuRE 2.21 Fin and window assembly for Example 2.9.

South

From triangle ABC, the sides AB = D, BC = w, and angle A is z - Zs. Therefore, distance w is estimated by w = D tan(z - Zs).

Shadow calculations for overhangs are examined in more detail in Chapter 6, Section 6.2.5.

All substances, solid bodies as well as liquids and gases above the absolute zero temperature, emit energy in the form of electromagnetic waves.

The radiation that is important to solar energy applications is that emitted by the sun within the ultraviolet, visible, and infrared regions. Therefore, the radiation wavelength that is important to solar energy applications is between 0.15 and 3.0 |m. The wavelengths in the visible region lie between 0.38 and 0.72 |im.

This section initially examines issues related to thermal radiation, which includes basic concepts, radiation from real surfaces, and radiation exchanges between two surfaces. This is followed by the variation of extraterrestrial radiation, atmospheric attenuation, terrestrial irradiation, and total radiation received on sloped surfaces. Finally, it briefly describes radiation measuring equipment.

Thermal radiation is a form of energy emission and transmission that depends entirely on the temperature characteristics of the emissive surface. There is no intervening carrier, as in the other modes of heat transmission, i.e., conduction and convection. Thermal radiation is in fact an electromagnetic wave that travels at the speed of light (C = 300,000km/s in a vacuum). This speed is related to the wavelength (X) and frequency (v) of the radiation as given by the equation:

When a beam of thermal radiation is incident on the surface of a body, part of it is reflected away from the surface, part is absorbed by the body, and part is transmitted through the body. The various properties associated with this phenomenon are the fraction of radiation reflected, called reflectivity (p); the fraction of radiation absorbed, called absorptivity (a); and the fraction of radiation transmitted, called transmissivity (t). The three quantities are related by the following equation:

It should be noted that the radiation properties just defined are not only functions of the surface itself but also of the direction and wavelength of the incident radiation. Therefore, Eq. (2.32) is valid for the average properties over the entire wavelength spectrum. The following equation is used to express the dependence of these properties on the wavelength:

where pX = spectral reflectivity. aX = spectral absorptivity. tx = spectral transmissivity.

The angular variation of absorptance for black paint is illustrated in Table 2.3 for incidence angles of 0-90°. The absorptance for diffuse radiation is approximately 0.90 (Lof and Tybout, 1972).

Most solid bodies are opaque, so that t = 0 and p + a = 1. If a body absorbs all the impinging thermal radiation such that t = 0, p = 0, and a = 1,

 Angle of incidence (°) Absorptance 0-30 0.96 30-40 0.95 40-50 0.93 50-60 0.91 60-70 0.88 70-80 0.81 80-90 0.66

regardless of the spectral character or directional preference of the incident radiation, it is called a blackbody. This is a hypothetical idealization that does not exist in reality.

A blackbody is not only a perfect absorber, it is also characterized by an upper limit to the emission of thermal radiation. The energy emitted by a black-body is a function of its temperature and is not evenly distributed over all wavelengths. The rate of energy emission per unit area at a particular wavelength is termed the monochromatic emissive power. Max Planck was the first to derive a functional relation for the monochromatic emissive power of a blackbody in terms of temperature and wavelength. This was done by using the quantum theory, and the resulting equation, called Planck's equation for blackbody radiation, is given by

where

EbX = monochromatic emissive power of a blackbody (W/m2-pm). T = temperature of the body (K). X = wavelength (pm). C = constant = 3.74 X 108 W-pm4/m2. C2 = constant = 1.44 X 104 pm-K.

By differentiating Eq. (2.34) and equating to 0, the wavelength corresponding to the maximum of the distribution can be obtained and is equal to XmaxT = 2897.8 pm-K. This is known as Wien's displacement law. Figure 2.22 shows the spectral radiation distribution for blackbody radiation at three temperature sources. The curves have been obtained by using the Planck's equation. Wavelength, |m

FÍGuRE 2.22 Spectral distribution of blackbody radiation.

Wavelength, |m

FÍGuRE 2.22 Spectral distribution of blackbody radiation.

The total emissive power, Eb, and the monochromatic emissive power, EbX, of a blackbody are related by

Substituting Eq. (2.34) into Eq. (2.35) and performing the integration results in the Stefan-Boltzmann law:

where a = the Stefan-Boltzmann constant = 5.6697 X 10~8 W/m2-K4.

In many cases, it is necessary to know the amount of radiation emitted by a blackbody in a specific wavelength band X1 ^ X2. This is done by modifying Eq. (2.35) as x

Since the value of EbX depends on both X and T, it is better to use both variables as

AT e

Therefore, for the wavelength band of Xi — X2, we get

which results in -b(0 —> XjT) - —b (0 —X2T). Table 2.4 presents a tabulation of —b(0 — XT) as a fraction of the total emissive power, Eb = ctT4, for various values of XT. The values are not rounded, because the original table, suggested by Dunkle (1954), recorded XT in micrometer-degrees Rankine (|m-°R), which were converted to micrometer-Kelvins (|m-K) in Table 2.4.

A blackbody is also a perfect diffuse emitter, so its intensity of radiation, Ib, is a constant in all directions, given by

Of course, real surfaces emit less energy than corresponding blackbodies. The ratio of the total emissive power, E, of a real surface to the total emissive power, Eb, of a blackbody, both at the same temperature, is called the emissivity (e) of a real surface; that is,

The emissivity of a surface is not only a function of surface temperature but depends also on wavelength and direction. In fact, the emissivity given by Eq. (2.38) is the average value over the entire wavelength range in all directions, and it is often referred as the total or hemispherical emissivity. Similar to Eq. (2.38), to express the dependence on wavelength, the monochromatic or spectral emissivity, eX, is defined as the ratio of the monochromatic emissive power, EX, of a real surface to the monochromatic emissive power, EbX, of a blackbody, both at the same wavelength and temperature:

Kirchoff's law of radiation states that, for any surface in thermal equilibrium, monochromatic emissivity is equal to monochromatic absorptivity:

The temperature (T) is used in Eq. (2.40) to emphasize that this equation applies only when the temperatures of the source of the incident radiation and the body itself are the same. It should be noted, therefore, that the emissivity of a body on earth (at normal temperature) cannot be equal to solar radiation (emitted from the sun at T = 5760 K). Equation (2.40) can be generalized as e(T) = a(T) (2.41)

Equation (2.41) relates the total emissivity and absorptivity over the entire wavelength. This generalization, however, is strictly valid only if the incident and emitted radiation have, in addition to the temperature equilibrium at the surfaces, the same spectral distribution. Such conditions are rarely met in real life; to simplify the analysis of radiation problems, however, the assumption that monochromatic properties are constant over all wavelengths is often made. Such a body with these characteristics is called a graybody.

 XT (p,m-K) Eb(G ^ XT)/aT4 XT (p,m-K) Eb(G ^ XT)/aT4 XT (p,m-K) Eb(G ^ XT)/aT4 555.6 1.7GE-GS 4GGG.G G.4SGS5 7444.4 G.S3166 666.7 7.56E-G7 4111.1 G.5GG66 7555.6 G.S369S 777.S 1.G6E-G5 4222.2 G.51974 7666.7 G.S42G9 SSS.9 7.3SE-G5 4333.3 G.53SG9 7777.S G.S4699 1GGG.G 3.21E-G4 4444.4 G.55573 7SSS.9 G.S5171 1111.1 G.GG1G1 4555.6 G.57267 SGGG.G G.S5624 1222.2 G.GG252 4666.7 G.5SS91 Slll.l G.S6G59 1333.3 G.GG531 4777.S G.6G449 S222.2 G.S6477 1444.4 G.GG9S3 4SSS.9 G.61941 S333.3 G.S6SSG 1555.6 G.G1643 5GGG.G G.63371 SSSS.9 G.SS677 1666.7 G.G2537 5111.1 G.6474G 9444.4 G.9G16S 1777.S G.G3677 5222.2 G.66G51 1GGGG.G G.91414 1SSS.9 G.G5G59 5333.3 G.673G5 1G555.6 G.92462 2GGG.G G.G6672 5444.4 G.6S5G6 11111.1 G.93349 2111.1 G.GS496 5555.6 G.69655 11666.7 G.941G4 2222.2 G.1G5G3 5666.7 G.7G754 12222.2 G.94751 2333.3 G.12665 5777.S G.71SG6 12777.S G.953G7 2444.4 G.14953 5SSS.9 G.72S13 13333.3 G.957SS 2555.5 G.17337 6GGG.G G.73777 13SSS.9 G.962G7 2666.7 G.197S9 6111.1 G.747GG 14444.4 G.96572 2777.S G.222S5 6222.1 G.755S3 15GGG.G G.96S92 2SSS.9 G.24SG3 6333.3 G.76429 15555.6 G.97174 3GGG.G G.27322 6444.4 G.7723S 16111.1 G.97423 3111.1 G.29S25 6555.6 G.7SG14 16666.7 G.97644 3222.2 G.323GG 6666.7 G.7S757 22222.2 G.9S915 3333.3 G.34734 6777.S G.79469 22777.S G.99414 3444.4 G.3711S 6SSS.9 G.SG152 33333.3 G.99649 3555.6 G.39445 7GGG.G G.SGSG6 33SSS.9 G.99773 3666.7 G.417GS 7111.1 G.S1433 44444.4 G.99S45 3777.S G.439G5 7222.2 G.S2G35 5GGGG.G G.99SS9 3SSS.9 G.46G31 7333.3 G.S2612 55555.6 G.9991S

Similar to Eq. (2.37) for a real surface, the radiant energy leaving the surface includes its original emission and any reflected rays. The rate of total radiant energy leaving a surface per unit surface area is called the radiosity (J), given by

where

Eb = blackbody emissive power per unit surface area (W/m2). H = irradiation incident on the surface per unit surface area (W/m2). e = emissivity of the surface. p = reflectivity of the surface.

There are two idealized limiting cases of radiation reflection: The reflection is called specular if the reflected ray leaves at an angle with the normal to the surface equal to the angle made by the incident ray, and it is called diffuse if the incident ray is reflected uniformly in all directions. Real surfaces are neither perfectly specular nor perfectly diffuse. Rough industrial surfaces, however, are often considered as diffuse reflectors in engineering calculations.

A real surface is both a diffuse emitter and a diffuse reflector and hence, it has diffuse radiosity; i.e., the intensity of radiation from this surface (I) is constant in all directions. Therefore, the following equation is used for a real surface:

Example 2.10

A glass with transmissivity of 0.92 is used in a certain application for wavelengths 0.3 and 3.0 pm. The glass is opaque to all other wavelengths. Assuming that the sun is a blackbody at 5760K and neglecting atmospheric attenuation, determine the percent of incident solar energy transmitted through the glass. If the interior of the application is assumed to be a blackbody at 373 K, determine the percent of radiation emitted from the interior and transmitted out through the glass.

Solution

For the incoming solar radiation at 5760 K, we have

A1T = 0.3 X 5760 = 1728|m-K A2T = 3 X 5760 = 17280 pm-K

From Table 2.4 by interpolation, we get

oT 4

oT 4

Therefore, the percent of solar radiation incident on the glass in the wavelength range 0.3-3 |m is

aT 4

In addition, the percentage of radiation transmitted through the glass is 0.92 X 94.61 =87.04%.

For the outgoing infrared radiation at 373 K, we have

XjT = 0.3 X 373 = 111.9 |m-K X2T = 3 X 373 = 1119.0^m-K

From Table 2.4, we get

aT 4

oT 4

The percent of outgoing infrared radiation incident on the glass in the wavelength 0.3-3 |im is 0.1%, and the percent of this radiation transmitted out through the glass is only 0.92 X 0.1 = 0.092%. This example, in fact, demonstrates the principle of the greenhouse effect; i.e., once the solar energy is absorbed by the interior objects, it is effectively trapped.

### Example 2.11

A surface has a spectral emissivity of 0.87 at wavelengths less than 1.5 |m, 0.65 at wavelengths between 1.5 and 2.5 |im, and 0.4 at wavelengths longer than 2.5 |im. If the surface is at 1000 K, determine the average emissivity over the entire wavelength and the total emissive power of the surface.

Solution

From the data given, we have

From Table 2.4 by interpolation, we get

and aT 4

oT 4

0.01313

0.16144

Therefore, and

oT 4

The average emissive power over the entire wavelength is given by e = 0.87 X 0.01313 + 0.65 X 0.14831 + 0.4 X 0.83856 = 0.4432 and the total emissive power of the surface is

E = eaT4 = 0.4432 X 5.67 X 10~8 X 10004 = 25129.4W/m2

### 2.3.3 Transparent Plates

When a beam of radiation strikes the surface of a transparent plate at angle 0!, called the incidence angle, as shown in Figure 2.23, part of the incident radiation is reflected and the remainder is refracted, or bent, to angle 02, called the refraction angle, as it passes through the interface. Angle 0! is also equal to the angle at which the beam is specularly reflected from the surface. Angles 0! and 02 are not equal when the density of the plane is different from that of the medium through which the radiation travels. Additionally, refraction causes the transmitted beam to be bent toward the perpendicular to the surface of higher density. The two angles are related by the Snell's law:

sin 01 sin 02

where n! and n2 are the refraction indices and n is the ratio of refraction index for the two media forming the interface. The refraction index is the determining factor for the reflection losses at the interface. A typical value of the refraction index is 1.000 for air, 1.526 for glass, and 1.33 for water.

 Incident beam Reflected beam \01 Medium 1 \ / ni 1 Medium 2 \ "2 l* ? \ ^^^ Refracted beam Transmitted beam

FiGuRE 2.23 Incident and refraction angles for a beam passing from a medium with refraction index n1 to a medium with refraction index n2.

FiGuRE 2.23 Incident and refraction angles for a beam passing from a medium with refraction index n1 to a medium with refraction index n2.

Expressions for perpendicular and parallel components of radiation for smooth surfaces were derived by Fresnel as 