Solar Radiation
solar radiation definition
How much solar radiation reaches the earth
●The energy reaching the earth is 1.5×1018KWH/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 year) also
affects the amount of solar energy received by the place.
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 1m2
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 '.
● The graph shown gives the amount of power present
in different wavelengths of radiation.
● It can be seen from the graph that 50% of solar energy
is in the form of thermal energy .
Solar PV captures the energy in visible region. Solar
thermal captures energy in infrared region.
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 1kW/m2.
●The worldwide average daily value of solar irradiance on optimally oriented surfaces is approximately 5 kWh/m2 or 5 PSH. Solar irradiance is denoted by ' H 'Radiation Measurement
●We know that the atmosphere is made up of ions and other particles including clouds.
●when the incident radiation passes through the atmosphere, some radiation penetrates and falls directly on to the panel, some radiation diffuses in atmosphere and travels to the panel and some radiation gets reflected from the surroundings of the panel and reaches the panel, the effect being called albedo effect.
●It becomes extremely important to know the amount of energy that has reached the panel through all the paths.
●There are several factors on which this energy is dependent. They are as follows:
● Latitude and longitude of the geographical location.
●Climatic conditions such as presence of clouds, water vapor etc.
●Time of the year.
●Angle of tilt.
● Collector design.
Now, let us see how we make use of this information in calculating the solar energy available at the panel. The steps are as follows:
● Find the sun position with respect to the location. This is a function of latitude (φ), hour angle (ω) and declination angle (δ)
(SunPosition =f(φ ,ϖ ,δh
● Find the available solar energy or irradiance with no atmosphere, Ho. This is a function of sun position.
Ho=f (SunPosition)
atmospheric effects, HOA. This is a function of Ho and clearness index KT
HOA = KT HO
●Find the actual solar energy available at the panel, Ht. this is a function of HOA and the tilt factor RD.
Ht = R DHOA
● All the above mentioned steps can be written as an algorithm so that the moment available data is fed, the actual solar energy available at the panel can be calculated instantly. The algorithm would involve the following equations:
Insolation at any location
We need to develop an algorithm, which calculates insolation (Ht) in kWh/m2 at any place, once weinput the following parameters:
▪Day of the year (N)
▪ Tilt angle (β)
▪ Angle of declination (δ)
▪ Clearness Index (KT)
▪ Reflection co-efficient (varies from 0.2 to 0.7)
ISC = mean solar constant = 1.37 kW/m2
φ = latitude in degrees/radians
δ = declination angle in degrees/radians
▪ The next parameter that needs to be known is KT, the clearness index.
▪ It is one of the most important and difficult factors to be determined since it depends on atmospheric conditions such as absorption, pressure, cloud-cover at the place etc., which are not constant at a given place.
▪KT was initially modeled using linear polynomial regression and multiple regression techniques.
▪ Since the results obtained with these models were not very accurate, a model was developed using Fourier series techniques of curve fitting since KT is a periodic function of period one year.
▪ We have seen earlier that the irradiance in kWh/m2 can be calculated for any location by inputting latitude of the location, declination angle, day number of the year for a given tilt angleusing algorithm.
▪ The plot of irradiance as a function of the year is shown in the following figure
solar radiation curve
▪ We can see that the level varies with the day of the year. It may reach a peak at some day of the year and reach a bottom on some other day of the year. The peaks and valleys are the direct result of the amount of irradiance reaching the earth. This is a graph that gives us the irradiance level over one year period.
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