Solar Radiation and Measurement

Solar radiation is a term used to describe 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

• 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

The output of sun is 2.8× 10²³ KW. The energy reaching the earth is 1.5 × 10¹⁸ KWH/year. When light travels from outer space to earth, solar energy is lost because of following reasons

• Scattering : The rays collide with particles present in atmosphere
• Absorption: Because of water vapor there is absorption.
• Cloud cover: The light rays are diffused because of clouds.
• Reflection: When the light rays hit the mountains present on the earth surface there is reflection.
• Climate: Latitude of the location, day (time in the yea r) also affects the amount of solar energy received by the place.

The above mentioned factors determine the amount of power falling on the surface.

Insolation – It is a quantity indicating the amount of incident solar power on a unit surface, commonly expressed in units of kW / m². At the earth’s outer atmosphere, the solar insolation on a 1 m² surface oriented normal to the sun’s rays is called SOLAR CONSTANT and its value is 1.37 kW / m². Due to atmospheric effects, the peak solar insolation incident on a terrestrial surface oriented normal to the sun at noon on a clear day is on the order of 1 kW/ m². A solar insolation level of 1 kW / m² is often called PEAK SUN. Solar insolation is denoted by ‘ I ‘.

Spectral Distribution of Extraterrestrial Radiation is the radiation that would have been received by the earth in the absence of the atmosphere.

Irradiance – It is an amount of solar energy received on a unit surface expressed in units of kWh/m². Solar irradiance is essentially the solar insolation (power) integrated with respect to time. When solar irradiance data is represented on an average daily basis, the value is often called PEAK SUN HOURS (PSH) and can be thought of as the number of equivalent hours/day that solar insolation is at its peak level of 1 kW /m². The worldwide average d a ily value of solar irradiance on optimally oriented surfaces is approximately 5 kWh/ m² or 5 PSH. Solar irradiance is denoted by ‘ H ‘ . Now we know the definition of two basic term s commonly used in design of a photovoltaic systems. Of course, these terms are often used interchangeably. Hence, one has to be careful in looking at the unit that has been used. In designing a photovoltaic system , it is important to know the amount of insolation available to us at a given time so that the power can be captured using solar panels and convert it into electricity. Depending on the requirement, the size of the panel can be then designed.

Solar Radiation – Solar energy received at the Earth’s surface can be separated into two basic components: direct solar energy and diffuse solar energy. Direct solar energy is the energy arriving at the Earth’s surface with the Sun’s beam. The Sun’s beam is quite intense, and hence has also been described a ‘shadow producing’ radiation.

Diffuse solar energy is the result of the atmosphere attenuating, or reducing the magnitude of the Sun’s beam. Some of the energy removed from the beam is redirected or scattered towards the ground – the rate at which this energy falls on a unit horizontal surface per second is called the diffuse solar irradiance.

The remaining energy from the beam is either scattered back into space, or absorbed by the atmosphere. Absorption only occurs at specific wavelengths, for example, UVB solar energy is absorbed by ozone in the stratosphere. Scattering occurs at all wavelengths; hence the mechanism by which solar energy is scattered from water droplets and ice particles makes possible those majestic satellite pictures of clouds. The combination of both forms of solar energy incident on a horizontal plane at the Earth’s surface is referred to as global solar energy and all three quantities (specifically their rate or irradiance) are linked mathematically by the following expression

Eg = Ed + Eb cos(z)

where: Eg = global irradiance on a horizontal surface, Ed = diffuse irradiance, Eb = direct beam irradiance on a surface perpendicular to the direct beam, z = Sun’s zenith angle. By measuring the three components independently, a useful quality assurance test is immediately available by comparing the measured quantity with that calculated from the other two.

Radiation Units – Radiation quantities are generally expressed in terms of either irradiance or radiant exposure. Irradiance is a measure of the rate of energy received per unit area, and has units of watts per square metre (W/m2), where 1 watt (W) is equal to 1 Joule (J) per second. Radiant exposure is a time integral (or sum) of irradiance. Thus a 1 minute radiant exposure is a measure of the energy received per square metre over a period of 1 minute. Therefore a 1-minute radiant exposure = mean irradiance (W/m2) x 60(s), and has units of joule(s) per square metre (J/m2). A half-hour radiant exposure would then be the sum of 30 one-minute (or 1800 one-second) radiant exposures. For example: a mean irradiance of 500 W/m2 over 1 minute yields a radiant exposure of 30 000 J/m2 or 30 KJ/m2. The output of the Bureau of Meteorology’s computer model, which estimates the daily global solar exposure from satellite data, provides irradiance integrated over a period of a day i.e. radiant or global exposure, with units of megajoule(s) per square metre. In terms of remote sensing by satellite, radiance refers to energy received by a satellite sensor and is the rate of energy received per unit area per unit of solid angle (with units of watt(s) per square metre per steradian).

Direct Solar Irradiance – Direct solar irradiance (also referred to as direct normal irradiance) is a measure of the rate of solar energy arriving at the Earth’s surface from the Sun’s direct beam, on a plane perpendicular to the beam, and is usually measured by a pyrheliometer mounted on a solar tracker. The tracker ensures that the Sun’s beam is always directed into the instrument’s field of view during the day. The pyrheliometer has a field of view of 5° . In order to use this measurement for comparison with global and diffuse irradiances, it is necessary to obtain the horizontal component of the direct solar irradiance. This is achieved by multiplying the direct solar irradiance by the cosine of the Sun’s zenith angle.