Chapter 3 Solar Radiation

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Study of Solar Module & Its Efficient Use in Bangladesh

22Solar Radiation

Chapter 03

SOLAR RADIATION

03.1 Solar Radiation Basics

Solar radiation is a general term for the electromagnetic radiation emitted by the sun.We can capture and convert solar radiation into useful forms of energy, such as heatand electricity, using a variety of technologies. The technical feasibility and economicaloperation of these technologies at a specific location depends on the available solarradiation or solar resource.

03.2 Extraterrestrial Radiation and Solar Constant

Measurements indicate that solar energy flux received outside the earth's atmosphere isfairly constant at mean distance of the earth from the sun. This is known as SolarConstant ISC.

Solar Constant is defined as the energy from the sun per unit time, received on a unitarea of surface perpendicular to the direction of propagation of the radiation at meanearth sun distance (1.5 x 1011 m) outside of the atmosphere. Its value is 1353 W/m: withan estimated error of 1.59%. The World Radiation Center (WRC) has adopted a value

of 1368 W/m2 [10] with an uncertainty of 1.09%.

It is also important to know the spectral distribution of extraterrestrial radiation, i.e. theradiation that would be received in the absence of atmosphere. The extraterrestrial solarspectrum at mean earth sun distance can be divided into following three main regions,usually divided into wave bands ( 1 micron = 1m = 10-6 mm = 10-6 meter ).


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23Solar Radiation

Figure 10: Earths Revolution around the Sun

The ultraviolet region for which is less than 0.4 mm and which accounts forabout 9% of the total irradiance.The visible region for which the is between 0.4 and 0.7 mm and which accountsfor about 45% of the irradiance.


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24Solar Radiation

The infrared region for which the A is more than 0.7 mm. This fraction accountsfor about 46% of the total irradiance.

Extraterrestrial radiation varies due to two reasons

Variation of radiation emitted by the sunVariation of earth sun distance, this can cause a change of 3%.

Extraterrestrial radiation at any time of the year is given byI on = I sc (1 + 0.033cos 360n / 365)

Where, is the solar constant and is the extraterrestrial radiation measured on a planenormal to the radiation on 'n' day of the year counted from January 1st as n=l.

03.3 Components of Radiation

During its travel through earth's atmosphere, a part of the solar radiation undergoeschanges in its direction while the rest reaches the earth's surface without any change ofdirection. The former fraction is known as diffuse radiation while the latter is known asdirect or beam radiation. Total solar radiation is the sum of these two components.Beam radiation differs from diffuse radiation in the sense that only beam radiation canbe focused using optical devices.

Even on clear days there is some diffuse radiation. The ratio between beam irradianceand total irradiance varies from about 0.9 on a clear day to zero on a completely

overcast day.

03.4 Basic Principles [11]

Every location on Earth receives sunlight at least part of the year. The amount of solarradiation that reaches any one "spot" on the Earth's surface varies according to thesefactors:

Geographic location

Time of daySeasonLocal landscapeLocal weather.

Because the Earth is round, the sun strikes the surface at different angles ranging from0 (just above the horizon) to 90 (directly overhead). When the sun's rays are vertical,the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the


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25Solar Radiation

longer they travel through the atmosphere, becoming more scattered and diffuse.Because the Earth is round, the frigid polar regions never get a high sun, and because ofthe tilted axis of rotation, these areas receive no sun at all during part of the year.The Earth revolves around the sun in an elliptical orbit and is closer to the sun duringpart of the year. When the sun is nearer the Earth, the Earth's surface receives a little

more solar energy. The Earth is nearer the sun when it's summer in the southernhemisphere and winter in the northern hemisphere. However the presence of vastoceans moderates the hotter summers and colder winters one would expect to see in thesouthern hemisphere as a result of this difference.

The 23.5 tilt in the Earth's axis of rotation is a more significant factor in determining theamount of sunlight striking the Earth at a particular location. Tilting results in longerdays in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal)equinox and longer days in the southern hemisphere during the other six months. Daysand nights are both exactly 12 hours long on the equinoxes, which occur each year on or

around March 23 and September 22.

Countries like the United States, which lie in the middle latitudes, receive more solarenergy in the summer not only because days are longer, but also because the sun isnearly overhead. The sun's rays are far more slanted during the shorter days of thewinter months. Cities like Denver, Colorado, (near 40 latitude) receive nearly threetimes more solar energy in June than they do in December.

The rotation of the Earth is responsible for hourly variations in sunlight. In the earlymorning and late afternoon, the sun is low in the sky. Its rays travel further through the

atmosphere than at noon when the sun is at its highest point. On a clear day, thegreatest amount of solar energy reaches a solar collector around solar noon.


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26Solar Radiation

Fig 11: Earths Energy Budget

Fig 11: Earths Energy Budget

03.5 Diffuse and Direct Solar Radiation [11]

As sunlight passes through the atmosphere, some of it is absorbed, scattered, andreflected by the following:

Air moleculesWater vaporCloudsDustPollutantsForest firesVolcanoes.

This is called diffuse solar radiation. The solar radiation that reaches the Earth's surfacewithout being diffused is called direct beam solar radiation. The sum of the diffuse and

direct solar radiation is called global solar radiation. Atmospheric conditions can reducedirect beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.Scientists measure the amount of sunlight falling on specific locations at different timesof the year. They then estimate the amount of sunlight falling on regions at the samelatitude with similar climates. Measurements of solar energy are typically expressed astotal radiation on a horizontal surface or as total radiation on a surface tracking the sun.


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Radiation data for solar electric (photovoltaic) systems are often represented askilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may alsobe expressed as watts per square meter (W/m2).Radiation data for solar water heatingand space heating systems are usually represented in British thermal units per squarefoot (Btu/ft2).

Solar radiation describes the visible and near-visible (ultraviolet and near-infrared)radiation emitted from the sun. The different regions are described by their wavelengthrange within the broad band range of 0.20 to 4.0 m (microns). Terrestrial radiation is aterm used to describe infrared radiation emitted from the atmosphere. The following isa list of the components of solar and terrestrial radiation and their approximatewavelength ranges:

Ultraviolet: 0.20 - 0.39 mVisible: 0.39 - 0.78 m

Near-Infrared: 0.78 - 4.00 mInfrared: 4.00 - 100.00 m

Approximately 99% of solar, or short-wave, radiation at the earth's surface is containedin the region from 0.3 to 3.0 m while most of terrestrial, or long-wave, radiation iscontained in the region from 3.5 to 50 m.

Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370watts/meter2. This is the value at mean earth-sun distance at the top of the atmosphereand is referred to as the Solar Constant. On the surface of the earth on a clear day, at

noon, the direct beam radiation will be approximately 1000 watts/meter2 for manylocations.

The availability of energy is affected by location (including latitude and elevation),season, and time of day. All of which can be readily determined. However, the biggestfactors affecting the available energy are cloud cover and other meteorologicalconditions which vary with location and time.

Historically, solar measurements have been taken with horizontal instruments over thecomplete day. In the Northern US, this results in early summer values 4-6 times greaterthan early winter values. In the South, differences would be 2-3 times greater. This isdue, in part, to the weather and, to a larger degree, the sun angle and the length ofdaylight.


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03.6 The solar radiation spectrum

The electromagnetic radiation emitted by the sun shows a wide range of wavelengths. Itcan be divided into two major regions with respect to the capability of ionizing atoms inradiation-absorbing matter: ionizing radiation (X-rays and gamma-rays) and no

ionizing radiation (UVR, visible light and infrared radiation). Fortunately, the highlyinjurious ionizing radiation does not penetrate the earth's atmosphere.Solar radiation is commonly divided into various regions or bands on the basis ofwavelength (Table 5). Ultraviolet radiation is that part of the electromagnetic spectrumbetween 100 and 400 nm. It is, in turn, divided rather arbitrarily from the viewpoint ofits biological effects into three major components (Fig. 12).


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Table 5: Spectral bands of incoming solar energy and atmospheric effects

Fig. 12: Spectra of no ionizing solar radiation (A) and ultraviolet radiation (B)showing main radiation bands, their nomenclature, and approximate wavelengthlimits. Other synonyms: UV-A, black light; UV-B, sunburn or erythemal radiation;UV-C, germicidal radiation (compiled from WHO 1979; Parmeggiani 1983; Harvey etal. 1984).

03.7 Transmission through different media [12]

Solar energy impinging upon a transparent medium or target is partly reflected andpartly absorbed; the remainder is transmitted. The relative values are dependent uponthe optical properties of the transparent object and the solar spectrum (Dietz 1963).

Transmission of the incident solar energy through glass is a function of the type andthickness of the glass, the angle of incidence, and the specific wavelength bands ofradiation. Ordinary glass of the soda-lime-silica type (window or plate glass) cantransmit more than 90% of the incident radiation in the UV-A and visible regions of thespectrum, provided the Fe2O3 content is lower than 0.035%; if it is higher, thetransmittance is somewhat decreased. Increased thickness of glass diminishes


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30Solar Radiation

transmittance. The transmittance is uniform at a high level for angles of incidenceranging from 0 to 40 and drops sharply as the angle approaches 90 (Dietz 1963).Ordinary glass is opaque to radiation in the UV-B and UV-C regions; Pyrex glass(borosilicate type) is opaque to radiation in the UV-B band and attains a maximumtransmission level at 340 nm and beyond (Acra et al. 1984). The coefficient of

transparency for borosilicate glass, 1.0 cm in thickness, is 0.08 at 310 nm, rises sharply to0.65 at 330 nm, and attains a peak level of 0.95-0.99 from 360 to 500 nm (Weast 1972).The transmission properties of Pyrex are exceeded only by quartz (Dietz 1963; Kreidland Rood 1965; Weast 1972).

Transparent plastic materials such as Lucite and Plexiglas are good transmitters in theUV and visible ranges of the spectrum (Dietz 1963). Translucent materials such aspolyethylene can also transmit the germicidal components of sunlight (Fujioka andNarikawa 1982). Solar energy passing through water is also attenuated by reflection andabsorption. The proportion of transmitted sunlight in water depends on water depth;

turbidity caused by organic and inorganic particles in suspension; optical properties asmodified by the presence in solution of light-absorbing substances such as colouringmaterials, mineral salts, and humates; and wavelength of the incident radiation. Up to10% of the solar UV-B intensity at the surface of clear seawater may penetrate to a depthof 15 m (Calkins 1974), inactivating Escherichia coli to a depth of 4 m (Gameson andSaxon 1967). The exponential attenuation values of UVR (200-400 nm) in distilled waterare less than in seawater and range from 10/m at 200 nm to a minimum of 0.05/m at375 nm. Values rise sharply in the visible and infrared regions of the spectrum, showingthat solar UV-A has a greater penetration power in water than UV-B or visible light(Stewart and Hopfield 1965). The absorption of UVR (210-300 nm) by materials in

natural water seems to be related to chemical oxygen demand (Ogura 1969). At thesurface of tertiary sewage lagoons, for example, the solar UV-B intensity dropsexponentially to 20% at a depth of 10 cm, 3% at 20 cm, 0.6% at 30 cm, and 0.1 % at 40 cm(Moeller and Calkins 1980). Most of the UV-B absorbance in wastewater effluents iscaused by the dissolved humic substances, whereas the suspended particles absorb andscatter UVR and protect bacteria during UV disinfection (Qualls et al. 1983).

Textiles used in clothing are not necessarily complete absorbers of natural UVR andmay give a false sense of security against sunburn and skin cancer. The average whiteshirts worn by men may transmit 20% of the solar UVR, whereas lighter weavesfavoured by women may allow up to 50% to penetrate to the covered skin (WHO 1979).Transmission of UVR through various samples of fabrics ranges from 64% for 100%nylon to 5% for black cotton, the values being reduced by thickness and dyes andincreased with the intensity of UVR (Hutchinson and Hall 1984). The depths ofpenetration of UVR and visible light into the human skin are as follows: 0.01-0.1 mm for[

UV-B, 0.1-1.0 mm for UV-A, and 1.0-10.0 mm for the visible spectrum (Largent andOlishifski 1983).


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03.8 World distribution of solar radiation [12]

Solar radiation is unevenly distributed throughout the world because of such variablesas solar altitude, which is associated with latitude and season, and atmosphericconditions, which are determined by cloud coverage and degree of pollution. The

following guidelines are useful for the broad identification of the geographic areas withfavourable solar energy conditions in the Northern Hemisphere based on the collectionof the direct component of sunlight. Similar conditions apply for the SouthernHemisphere (Acra et al. 1984).

The most favourable belt (15-35 N) encompasses many of the developing nations innorthern Africa and southern parts of Asia. It has over 3 000 h/year of sunshine andlimited cloud coverage. More than 90% of the incident solar radiation comes as directradiation.

The moderately favourable belt (0-15 N), or equatorial belt, has high atmospherichumidity and cloudiness that tend to increase the proportion of the scattered radiation.The global solar intensity is almost uniform throughout the year as the seasonalvariations are only slight. Sunshine is estimated at 2 500 h/year.

In the less favourable belt (35-45 N), the scattering of the solar radiation is significantlyincreased because of the higher latitudes and lower solar altitude. In addition,cloudiness and atmospheric pollution are important factors that tend to reduce sharplythe solar radiation intensity. However, regions beyond 45 N have less favourableconditions for the use of direct solar radiation. This is because almost half of it is in the

form of scattered radiation, which is more difficult to collect for use. This limitation,however, does not strictly apply to the potentials for solar UVR applications.World maps illustrating the isolines of the mean global solar radiation (both direct anddiffuse radiations) and solar UVR impinging on a horizontal plane at ground level areavailable (Landsberg 1961; Schulze 1970; WHO 1979). A set of values for average dailyinflux of solar UVR as a function of wavelength, latitude, and time of year have alsobeen published (Johnson et al. 1976). The tabulated data pertain to sea level and clear-sky conditions and are distributed at intervals of latitude from 0 to 65 N and S forselected wavelengths from 285 to 340 nm. The calculated values for the erythemal effectcorresponding to 307 and 314 nm have been included for comparison. These dataindicate that for all UVR wavelengths from 285 to 340 nm, the solar UVR flux decreasesas the latitude increases. Assuming cloudless conditions, the solar UVR intensity at sealevel is expected theoretically to be significantly greater at the equator than at the higherlatitudes. In addition, at each latitude, the maximum intensity would be reached insummer; the minimum, in winter. A similar pattern will be followed by the erythemal-response wavelengths of 307 and 314 nm. The variation with latitude or season in thecalculated influx is much sharper for shorter wavelengths.


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Fig. 13: Seasonal and annual variations in relative solar UV-A radiation (340 nm) for

different latitudes (based on Johnson et al. 1976).

Fig. 14: Variations in angles of solar tilt and altitude worldwide (A) and for Beirut(B), as a function of time of year.


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These phenomena are dictated by the solar angle and its variation as a function oflatitude and month (Fig. 3). Also illustrated are the seasonal variations in the angles ofthe earth's tilt and of the sun's altitude at noon, indicating the minimum and maximumlevels attained worldwide and at 34 N in Beirut. Computations are based on thefollowing relationships (Michels 1979):

At equinoxes: solar altitude = 90 - latitudeAt solstices: solar altitude = (90 - latitude) + 23.5This means that, for any latitude, the relative intensity in each month is appreciablygreater for the longer UV-A wavelengths than it is for the shorter ones (Fig. 4).

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Chapter 3 Solar Radiation