Solar Radiation

 What is an example of solar radiation?

 

What are the effects of solar radiation?

 solar radiation definition

Does Solar give off radiation?  
How much solar radiation reaches the earth

●The output of sun is 2.8×1023KW. 
●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 day.
●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)

 

●Find the solar energy available on horizontal surface with 
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:

 

where is the reflection factor which ran ges between 0.2 to 0.7

 

Ht  = KT *RD *HO           kWh/m2 /day

 

This algorithm can be translated into any of the programming languages like C, C++ or MATLAB. 

Entering the known parameters, it becomes convenient to find out the solar energy available at any geographical 

location. 

 

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)

▪ Latitude of the location (φ)
 ▪ Tilt angle (β)

▪ Angle of declination (δ)

 ▪ Clearness Index (KT)

▪ Reflection co-efficient (varies from 0.2 to 0.7)

 Following flow chart gives an idea for developing the algorithm:

We can see that once δ, N and φ are input, Extra Terrestrial Radiation (ETR) can be determined. Δ, the declination angle of the sun is assumed to be the same every year and δ = 0 in March 21st.

ISC = mean solar constant = 1.37 kW/m2
φ = latitude in degrees/radians

δ = declination angle in degrees/radians 

ωsr =hour angle at sunrise in degrees/radians =cos-1(-tanφtan δ)
▪ The next parameter that needs to be known is KT, the clearness index. 

solar radiation w/m2

▪ 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.

 However, a model for KT could be developed based on the irradiance level (H) measured at different places and using the relationship KT = H/ETR.
▪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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