30-12-2010, 05:21 PM
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INTRODUCTION
Solar radiation is the energy given off by the sun in all directions. When this energy reaches the earth's surface, it is called insolation.
Solar radiation gives out light and heat to the earth in the form of electromagnetic waves. It has different wave lengths. Radiation such as ultraviolet radiation, X rays and visible light have short wave lengths. Infrared radiation has a long wave length.
Solar radiation is measured with a pyranometer or pyrheliometer.
The atmosphere affects the amount of solar radiation received. When solar radiation travels through the atmosphere, some of it is absorbed by the atmosphere(16%). Some of it is scattered to space (6%). Some of it is reflected by clouds (28%). About 47% of it reaches the earth's surface.
Almost all of the energy that drives the various systems (climate systems, ecosystems, hydrologic systems, etc.) found on the Earth originates from the sun (Figure 1). Solar energy is created at the core of the sun when hydrogen atoms are fused into helium by nuclear fusion (Figure 2). The core occupies an area from the sun’s center to about a quarter of the star’s radius. At the core, gravity pulls all of the mass of the sun inward and creates intense pressure. This pressure is high enough to force the fusion of atomic masses.
For each second of the solar nuclear fusion process, 700 million tons of hydrogen is converted into the heavier atom helium. Since its formation 4.5 billion years ago, the sun has used up about half of the hydrogen found in its core. The solar nuclear process also creates immense heat that causes atoms to discharge photons. Temperatures at the core are about 15 million degrees Kelvin (15 million degrees C or 27 million degrees F). Each photon that is created travels about one micrometer before being absorbed by an adjacent gas molecule. This absorption then causes the heating of the neighboring atom and it re-emits another photon that again travels a short distance before being absorbed by another atom. This process then repeats itself many times over before the photon can finally be emitted to outer space at the sun’s surface. The last 20% of the journey to the surface the energy is transported more by convection than by radiation. It takes a photon approximately 100,000 years or about 1025 absorptions and re-emissions to make the journey from the core to the sun’s surface. The trip from the sun’s surface to the Earth takes about 8 minutes.
The radiative surface of the sun, or photosphere, has an average temperature of about 5,800 Kelvins. Most of the electromagnetic radiation emitted from the sun's surface lies in the visible band centered at 500 nm (1 nm = 10-9 meters), although the sun also emits significant energy in the ultraviolet and infrared bands, and small amounts of energy in the radio, microwave, X-ray and gamma ray bands. The total quantity of energy emitted from the sun's surface is approximately 63,000,000 Watts per square meter (W/m2 or Wm-2).
The energy emitted by the sun passes through space until it is intercepted by planets, other celestial objects, or interstellar gas and dust. The intensity of solar radiation striking these objects is determined by a physical law known as the Inverse Square Law (Figure 3). This law merely states that the intensity of the radiation emitted from the sun varies with the squared distance from the source. As a result of this law, if the intensity of radiation at a given distance is one unit, at twice the distance the intensity will become only one-quarter. At three times the distance, the intensity will become only one-ninth of its original intensity at a distance of one unit, and so on.
Given the amount of energy radiated by the sun and the average Earth-sun distance of 149.5 million kilometers, the amount of radiation intercepted by the outer limits of the atmosphere can be calculated to be around 1,367 W/m2. Only about 40% of the solar energy intercepted at the top of Earth's atmosphere passes through to the surface. The atmosphere reflects and scatters some of the received visible radiation. Gamma rays, X-rays, and ultraviolet radiation less than 200 nanometers in wavelength are selectively absorbed in the atmosphere by oxygen and nitrogen and turned into heat energy. Most of the solar ultraviolet radiation with a range of wavelengths from 200 to 300 nm is absorbed by the concentration of ozone (O3) gas found in the stratosphere. Infrared solar radiation with wavelengths greater than 700 nm is partially absorbed by carbon dioxide, ozone, and water present in the atmosphere in liquid and vapour forms. Roughly 30% of the sun's visible radiation (wavelengths from 400 nm to 700 nm) is reflected back to space by the atmosphere or the Earth's surface. The reflectivity of the Earth or any body is referred to as its albedo, defined as the ratio of light reflected to the light received from a source, expressed as a number between zero (total absorption) and one (total reflectance).
RADIATION TERMINOLOGY
1. Irradiance (Wm–2): Amount of radiant energy incident on a surface per unit area per unit time.
2. Direct solar irradiance: Solar irradiance on a surface held perpendicular to sun rays and diffuse sky radiation obstructed.
3. Global solar irradiance: Solar irradiance on a horizontal surface due to both direct sun rays and diffuse sky radiation.
4. Diffuse solar irradiance: Solar irradiance on a horizontal surface due to sky radiation only.
5. Reflected solar irradiance: Upward radiant exitance in the short wave range.
6. Net terrestrial radiation: Upward radiant exitance minus downward irradiance in long wave range through a horizontal surface
near earth surface
7. Net total irradiance: Downward irradiance minus Upward radiant exitance in entire spectrum
ENERGY FROM THE SUN
The Earth receives 174 pet watts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet.
Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones. Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C. By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which fossil fuels are derived.
The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one hour than the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass. The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and mined uranium combined.
From the table of resources it would appear that solar, wind or biomass would be sufficient to supply all of our energy needs, however, the increased use of biomass has had a negative effect on global warming and dramatically increased food prices by diverting forests and crops into bio fuel production. As intermittent resources, solar and wind raise other issues.
Solar energy can be harnessed in different levels around the world. Depending on a geographical location the closer to the equator the more "potential" solar energy is available.
SOLAR RADIATION MANAGEMENT
Solar radiation management (SRM) projects are a type of geo engineering which seek to reflect sunlight and thus reduce global warming. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by these gases. Their principle advantage as an approach to geo engineering is the speed with which they can be deployed and become fully active. By comparison, other geo engineering techniques based on greenhouse gas remediation, such as ocean iron fertilization, need to sequester the anthropogenic carbon excess before they can arrest global warming. Solar radiation management projects can therefore be used as a geo engineering 'quick fix' while levels of greenhouse gases can be brought under control by greenhouse gas remediation techniques.
A study by Lenton and Vaughan suggest that marine cloud brightening and stratospheric sulfur aerosols are each capable of reversing the warming effect of a doubling of the level of CO2 in the atmosphere when compared to pre-industrial levels.
SOLAR VARIATION
Solar variation refers here to changes in the amount of total solar radiation and its spectral distribution over multi-annual to multi-millennial time-scales. There are periodic components to these variations, the principal one being the 11-year solar cycle (or sunspot cycle), as well as aperiodic fluctuations. Solar activity has been measured by satellites during recent decades and estimated using 'proxy' variables in prior times. Scientists studying climate change are interested in understanding the effects of variations in the total and spectral solar irradiance on the Earth and its climate.
The variations in total solar irradiance remained at or below the threshold of detectability until the satellite era, although the small fraction in ultra-violet wavelengths varies by a few percent. Total solar output is now measured to vary (over the last three 11-year sunspot cycles) by approximately 0.1% or about 1.3 W/m² peak-to-trough during the 11 year sunspot cycle. The amount of solar radiation received at the outer surface of Earth's atmosphere averages 1366 watts per square meter (W/m²). There are no direct measurements of the longer-term variation and interpretations of proxy measures of variations differ. The intensity of solar radiation reaching the Earth has been relatively constant throughout the last 2000 years, with variations of around 0.1-0.2%. The combination of solar variation and volcanic effects are likely to have contributed to climate change, for example during the Maunder Minimum. Apart from solar brightness variations, more subtle solar magnetic activity influences on climate from cosmic rays or the Sun's ultraviolet radiation cannot be excluded although confirmation is not at hand since physical models for such effects are still too poorly developed.
SOLAR ACTIVITY AND IRRADIANCE
Direct irradiance measurements have only been available during the last three cycles and are based on a composite of many different observing satellites. However, the correlation between irradiance measurements and other proxies of solar activity make it reasonable to estimate past solar activity. Most important among these proxies is the record of sunspot observations that has been recorded since 1610. Since sunspots and associated faculae are directly responsible for small changes in the brightness of the sun, they are closely correlated to changes in solar output. Direct measurements of radio emissions from the Sun at 10.7 cm also provide a proxy of solar activity that can be measured from the ground since the Earth's atmosphere is transparent at this wavelength. Lastly, solar flares are a type of solar activity that can impact human life on Earth by affecting electrical systems, especially satellites. Flares usually occur in the presence of sunspots, and hence the two are correlated, but flares themselves make only tiny perturbations of the solar luminosity.
Recently, it has been claimed that the total solar irradiance is varying in ways that are not duplicated by changes in sunspot observations or radio emissions. However, this conclusion is disputed. Some believe that shifts in irradiance may be the result of calibration problems in the measuring satellites. These speculations also admit the possibility that a small long-term trend might exist in solar irradiance.
