Chapter 3 Solar Radiation

Solar 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 heat and electricity, using a variety of technologies. The technical feasibility and economical operation of these technologies at a specific location depends on the available solar radiation or solar resource.

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03.2 Extraterrestrial Radiation and Solar Constant

Measurements indicate that solar energy flux received outside the earth's atmosphere is fairly constant at mean distance of the earth from the sun. This is known as Solar Constant ISC. Solar Constant is defined as the energy from the sun per unit time, received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth sun distance (1.5 x 10 11 m) outside of the atmosphere. Its value is 1353 W/m: with an 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. the radiation that would be received in the absence of atmosphere. The extraterrestrial solar spectrum 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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  y   p   o   C    t   o    N   o    D Figure 10: Earth’s Revolution around the Sun

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

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The infrared region for which the A is more than 0.7 mm. This fraction accounts for about 46% of the total irradiance. Extraterrestrial radiation varies due to two reasons Variation of radiation emitted by the sun Variation of earth sun distance, this can cause a change of ±3%. Extraterrestrial radiation at any time of the year is given by I on = I sc (1 + 0.033cos 360n / 365) Where, is the solar constant and is the extraterrestrial radiation measured on a plane normal to the radiation on 'n' day of the year counted from January 1st as n=l.

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03.3 Components of Radiation

During its travel through earth's atmosphere, a part of the solar radiation undergoes changes in its direction while the rest reaches the earth's surface without any change of direction. The former fraction is known as diffuse radiation while the latter is known as direct 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 can be focused using optical devices. Even on clear days there is some diffuse radiation. The ratio between beam irradiance and 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 solar radiation that reaches any one "spot" on the Earth's surface varies according to these factors: Geographic location Time of day Season Local landscape Local weather. Because the Earth is round, the sun strikes the surface at different angles ranging from 0º (just above the horizon) to 90º (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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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 of the 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 during part 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 southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.

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The 23.5º tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days 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. Days and 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 solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more solar energy in June than they do in December. The rotation of the Earth is responsible for hourly variations in sunlight. In the early morning 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, the greatest amount of solar energy reaches a solar collector around solar noon.

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Fig 11: Earth’s Energy Budget

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03.5 Diffuse and Direct Solar Radiation [11]

As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by the following: Air molecules Water vapor Clouds Dust Pollutants Forest fires Volcanoes.

This is called diffuse solar radiation. The solar radiation that reaches the Earth's surface without 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 reduce direct 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 times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total 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 as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).Radiation data for solar water heating and space heating systems are usually represented in British thermal units per square foot (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 wavelength range within the broad band range of 0.20 to 4.0 µm (microns). Terrestrial radiation is a term used to describe infrared radiation emitted from the atmosphere. The following is a list of the components of solar and terrestrial radiation and their approximate wavelength ranges:

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Ultraviolet: 0.20 - 0.39 µm Visible: 0.39 - 0.78 µm Near-Infrared: 0.78 - 4.00 µm Infrared: 4.00 - 100.00 µm

Approximately 99% of solar, or short-wave, radiation at the earth's surface is contained in the region from 0.3 to 3.0 µm while most of terrestrial, or long-wave, radiation is contained in the region from 3.5 to 50 µm. Outside the earth's atmosphere, solar radiation has an intensity of approximately 1370 watts/meter2. This is the value at mean earth-sun distance at the top of the atmosphere and 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 many locations. 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 biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time. Historically, solar measurements have been taken with horizontal instruments over the complete day. In the Northern US, this results in early summer values 4-6 times greater than early winter values. In the South, differences would be 2-3 times greater. This is due, in part, to the weather and, to a larger degree, the sun angle and the length of daylight.

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03.6 The solar radiation spectrum The electromagnetic radiation emitted by the sun shows a wide range of wavelengths. It can be divided into two major regions with respect to the capability of ionizing atoms in radiation-absorbing matter: ionizing radiation (X-rays and gamma-rays) and no ionizing radiation (UVR, visible light and infrared radiation). Fortunately, the highly injurious ionizing radiation does not penetrate the earth's atmosphere. Solar radiation is commonly divided into various regions or bands on the basis of wavelength (Table 5). Ultraviolet radiation is that part of the electromagnetic spectrum between 100 and 400 nm. It is, in turn, divided rather arbitrarily from the viewpoint of its biological effects into three major components (Fig. 12).

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

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

03.7 Transmission through different media [12]

Solar energy impinging upon a transparent medium or target is partly reflected and partly absorbed; the remainder is transmitted. The relative values are dependent upon the 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 and thickness of the glass, the angle of incidence, and the specific wavelength bands of radiation. Ordinary glass of the soda-lime-silica type (window or plate glass) can transmit more than 90% of the incident radiation in the UV-A and visible regions of the spectrum, provided the Fe2O3 content is lower than 0.035%; if it is higher, the transmittance is somewhat decreased. Increased thickness of glass diminishes

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transmittance. The transmittance is uniform at a high level for angles of incidence ranging 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 maximum transmission 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 to 0.65 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; Kreidl and Rood 1965; Weast 1972).

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Transparent plastic materials such as Lucite and Plexiglas are good transmitters in the UV and visible ranges of the spectrum (Dietz 1963). Translucent materials such as polyethylene can also transmit the germicidal components of sunlight (Fujioka and Narikawa 1982). Solar energy passing through water is also attenuated by reflection and absorption. The proportion of transmitted sunlight in water depends on water depth; turbidity caused by organic and inorganic particles in suspension; optical properties as modified by the presence in solution of light-absorbing substances such as colouring materials, mineral salts, and humates; and wavelength of the incident radiation. Up to 10% of the solar UV-B intensity at the surface of clear seawater may penetrate to a depth of 15 m (Calkins 1974), inactivating Escherichia coli to a depth of 4 m (Gameson and Saxon 1967). The exponential attenuation values of UVR (200-400 nm) in distilled water are less than in seawater and range from 10/m at 200 nm to a minimum of 0.05/m at 375 nm. Values rise sharply in the visible and infrared regions of the spectrum, showing that 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 the surface of tertiary sewage lagoons, for example, the solar UV-B intensity drops exponentially 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 is caused by the dissolved humic substances, whereas the suspended particles absorb and scatter UVR and protect bacteria during UV disinfection (Qualls et al. 1983). Textiles used in clothing are not necessarily complete absorbers of natural UVR and may give a false sense of security against sunburn and skin cancer. The average white shirts worn by men may transmit 20% of the solar UVR, whereas lighter weaves favoured 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 and increased with the intensity of UVR (Hutchinson and Hall 1984). The depths of penetration 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 and Olishifski 1983). [

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03.8 World distribution of solar radiation [12] Solar radiation is unevenly distributed throughout the world because of such variables as solar altitude, which is associated with latitude and season, and atmospheric conditions, which are determined by cloud coverage and degree of pollution. The following guidelines are useful for the broad identification of the geographic areas with favourable solar energy conditions in the Northern Hemisphere based on the collection of the direct component of sunlight. Similar conditions apply for the Southern Hemisphere (Acra et al. 1984).

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The most favourable belt (15-35° N) encompasses many of the developing nations in northern Africa and southern parts of Asia. It has over 3 000 h/year of sunshine and limited cloud coverage. More than 90% of the incident solar radiation comes as direct radiation. The moderately favourable belt (0-15° N), or equatorial belt, has high atmospheric humidity and cloudiness that tend to increase the proportion of the scattered radiation. The global solar intensity is almost uniform throughout the year as the seasonal variations 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 significantly increased because of the higher latitudes and lower solar altitude. In addition, cloudiness and atmospheric pollution are important factors that tend to reduce sharply the solar radiation intensity. However, regions beyond 45° N have less favourable conditions 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 and diffuse radiations) and solar UVR impinging on a horizontal plane at ground level are available (Landsberg 1961; Schulze 1970; WHO 1979). A set of values for average daily influx of solar UVR as a function of wavelength, latitude, and time of year have also been published (Johnson et al. 1976). The tabulated data pertain to sea level and clearsky conditions and are distributed at intervals of latitude from 0 to 65° N and S for selected wavelengths from 285 to 340 nm. The calculated values for the erythemal effect corresponding to 307 and 314 nm have been included for comparison. These data indicate that for all UVR wavelengths from 285 to 340 nm, the solar UVR flux decreases as the latitude increases. Assuming cloudless conditions, the solar UVR intensity at sea level is expected theoretically to be significantly greater at the equator than at the higher latitudes. In addition, at each latitude, the maximum intensity would be reached in summer; the minimum, in winter. A similar pattern will be followed by the erythemalresponse wavelengths of 307 and 314 nm. The variation with latitude or season in the calculated 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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Fig. 15: Seasonal and annual variations in relative solar UV-A radiation at 357deg;N (based on Johnson et al. 1976).

These data (Johnson et al. 1976) were transcribed into values relative to those pertaining to the equator. Relative values (ratios) for the solar UV-A component at 340 nm as a function of latitude are shown in Fig. 2. The important inferences include the following: The latitudinal trends vary with season and solar angle, attaining a minimum declination in December (winter solstice) and a maximum inclination in June (summer solstice); The trends for the other months fall within these two limits; The annual mean values show a declining trend.

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These phenomena are dictated by the solar angle and its variation as a function of latitude and month (Fig. 3). Also illustrated are the seasonal variations in the angles of the earth's tilt and of the sun's altitude at noon, indicating the minimum and maximum levels attained worldwide and at 34° N in Beirut. Computations are based on the following relationships (Michels 1979): At equinoxes: solar altitude = 90° - latitude At solstices: solar altitude = (90° - latitude) + 23.5° This means that, for any latitude, the relative intensity in each month is appreciably greater for the longer UV-A wavelengths than it is for the shorter ones (Fig. 4).

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