The sun (which is, incidentally, only a medium-size star) is larger than any of the planets in our solar system. Its diameter is 1,392,000 kilometers (864,949 miles). In comparison, earth’s diameter is only 12,756 kilometers (7,926 miles). More than one million Earths could fit inside the sun. The large mass of the sun produces an enormous gravitational pull that keeps all the planets of the solar system in their orbits.
The core of the sun is supposedly made of ionized hydrogen and helium nuclei at very high temperatures, of the order of 106 k. Outside the core is the photosphere at about 5500 K and is the main source of radiation. It rotates on its axis in about 25.4 days and has a radius of about 7 x 108 m or 7 x 105 km.
The sun’s output is 2.8 x 1023 kW and the energy reaching the earth is 1.5 x 1018 kWh per year. It emits radiation in the entire electromagnetic spectrum from gamma rays to radio waves. This radiant energy is a combination of energy released by various layers which are at different temperatures. Even the photosphere which is the main source of solar radiation does not have uniform temperature distribution. Thus, the sun is not strictly a black body although it is commonly treated as a black body radiator at about 5500 K.
[A black body is a substance which absorbs all the radiant energy incident on it i.e. there is no reflection or transmission of the energy. In practice, a perfect black body does not exist. For a given temperature, a black body has maximum radiant emission in all wavelengths.]
The earth revolves around the sun in an elliptical orbit with the sun at one of the foci. Therefore, the distance between the sun and the earth changes continually during its revolution around the sun in about 365 days. The average sun – earth distance is 149.6 x 106 km. With this varying distance the radiant energy received by the earth varies by about ± 3 percent. The radiant energy falling on a unit area, termed irradiance, at normal incidence outside the earth’s atmosphere at mean sun-earth distance is termed the solar constant. Its value varies depending upon the actual distance from the sun. The present accepted value of solar constant derived from space-based measurements is about 1367 W/m2.
Due to large distance sun is considered to be a point source and the radiations are considered parallel. Because of tilt of the axis of rotation (by about 23.5º), the angle of incidence of solar rays varies with latitude, season and the time of the day and since the length of the day varies with latitude and season, the amount of radiant energy varies from place to place and over time during a year at the same place.
Interaction of Solar Radiation with the atmosphere
As the solar electromagnetic radiations travel through the space and approach earth, they are modified and attenuated due to various mechanisms in the atmosphere of the earth. The radiant energy from the sun peaks at around 500 nm (1 nanometer is 10-9 meter) and about 98 percent of the energy lies in the wavelength range from 300 nm to 4000 nm. The energy content beyond this limit is too little to effect changes in the daily values. The extremely short wavelengths and very long wavelengths are absorbed/reflected by the upper atmosphere (in thermosphere, mesosphere and upper stratosphere) limiting the available radiant energy to UV, visible and IR wavelengths. [Image at the left comes from: http://www.globalwarmingart.com/images/4/4c/Solar_Spectrum.png%5D
[As per recommended international standard, units are generally expressed in multiples of 1000; for example, kilo, mega, gega, tera, milli, micro, nano, pico, femto, etc]
The ultraviolet radiation, shorter than 240 nm, dissociates molecular oxygen in the mesosphere into atomic oxygen which combines with another oxygen molecule to form ozone in presence of a neutral molecule like nitrogen. The ozone thus formed absorbs ultraviolet radiation in wavelengths shorter than 290 nm and at specific wavelengths in the region 290-400nm. There are weak absorption bands by ozone in the visible spectrum also.
Radiant energy content in the wavelengths longer than 4000 nm or 4 μm (1 micrometer is 10-6 meter) is quite small and most of them are absorbed by carbon dioxide in the atmosphere and water vapor in the troposphere. Thus, the irradiance that reaches the earth’s surface is restricted to wavelength range, 290 nm – 4000 nm. Part of this irradiance, about 31 percent, is reflected back to the space by the atmosphere (including the cloud-cover) and the earth’s surface. The remaining proportion reaches the surface directly after undergoing multiple scattering by air molecules and suspended particles.
The radiant energy absorbed by the earth and the atmosphere is partially re-radiated in the infra-red wavelength region, which controls the entire activity of all living organisms. This radiant energy is also the primary driving force behind the atmospheric heat engine and sustains the atmospheric circulations and the ocean currents. The solar heating of the earth-atmosphere system govern the local weather patterns across the world and leads to formation of biomass, winds, ocean thermal gradients and waves and other geothermal resources which are tapped for generating power.
The earth also emits radiation according to its temperature. Since the temperature of the earth’s surface is considerably low (about 288K, on an average) compared to that of the sun, the radiance from the earth lies in the wavelengths longer than 4 μm – the maximum emittance is around 10μm. This energy is absorbed almost totally by gases like carbon dioxide, ozone, water vapor etc. in the atmosphere. They then reradiate energy in about the same or higher wavelength range both upward and downward. Thus, the major part of the energy emitted by the earth is returned which maintains the temperature of the earth and the atmosphere at an optimum level.
Air mass is the optical path length through the Earth’s atmosphere for light from a celestial source. As it passes through the atmosphere, light is attenuated by scattering and absorption; the more atmosphere through which it passes, the greater the attenuation. Consequently, celestial bodies at the horizon (morning and late afternoon) appear less bright than when at the zenith. An airmass of 1 is looking straight up from sea level at the sun when it is directly overhead. At any location with latitude greater than 23.5 degrees, the sun is never directly overhead and so airmass will be always greater than 1. The number 1.5 has been agreed upon for the STC (Standard Test Condition) for testing solar panels.
Solar irradiance indicates the amount of solar power incident on a unit area and is typically expressed in watts per square meter (W/m2) or kilowatts per square meter (kW/m2). Irradiance is measured through an instrument called ‘pyranometer,’ which displays the instantaneous power available from the Sun.
As mentioned earlier, at the earth’s outer atmosphere, the solar energy incident on a 1 square meter surface oriented normal to the sun’s rays is about 1367 W/m2 and called the solar constant. This is attenuated by the atmosphere and the peak solar insolation on a surface oriented normal to the sun on a clear day is of the order of 1000 W/m2.
This irradiance of 1000 W/m2 corresponds to Standard Testing Conditions (STC) and is called “peak sun” or “1 sun”. If the incident radiation is concentrated 10 times using a lens or a mirror assembly and the incident power increases to 10,000 W/m2, then the irradiance is called “10 Suns.”
Insolation is the amount of solar irradiance that is incident on a fixed area over a period of time, and hence is a unit of energy. It is typically expressed in watt-hours per square meter per day (Wh/m2/day) or kilowatt-hours per square meter per day (kWh/m2/day) or even (kWh/m2/year) for a particular location, orientation and tilt of a surface.
Since 1000 W/m2 is “1 sun”, one hour of this ideal irradiance produces 1,000 watt-hours per square meter (1 kWh/m2). This is also known as “1 sun hour.” Colorful maps of solar potential display solar energy in kWh/m2/day, which is equivalent to the number of full sun hours per day. This is a useful parameter for sizing solar panels in the PV systems. More “sun hours” of insolation a location receives the more attractive it is for producing solar power.
Solar insolation at any place depends upon several factors such as latitude and longitude of the location; local climatic factors like the altitude and atmospheric conditions such as cloud cover, smog, pollutants and humidity; time of the day, time of the year, angle of tilt and collector design.
[Always Remember: 1 Sun Hour = 1 kWh/(m2/hour) and 1 kWh is one unit of electricity. So, if a location has average sunshine of 6 kWh/m2/day a 1kW panels implies a potential of 6kWh or 6 units of electricity generation everyday.]
Types of Insolation: GHI, GTI, and DNI
Global Horizontal Insolation (GHI): It is the solar insolation received by a fixed flat horizontal surface.
Global Tilt Insolation (GTI): The fixed solar panel or collector is generally inclined at an angle roughly equal to the latitude of its location (facing south in India or any place in the northern hemisphere) to maximize the annual insolation received. The insolation received by such an oriented surface is called the Global Tilt Insolation (GTI).
Direct Normal Insolation (DNI): Many solar technologies prefer tracking the Sun so that the collector surface always faces the sun in order to maximize the irradiance and insolation received. Tracking of the Sun becomes necessary when higher concentrations of light are required to be focused at the appropriate collector location. The insolation received by any such surface that is constantly facing the Sun, i.e. ‘normal’ to the Sun, is called Direct Normal Insolation (DNI).
DNI is of prime importance for various concentrated solar photovoltaic and thermal technologies; it is only the normal component (DNI) of solar radiation that is effectively concentrated.