Solar Radiations and its Importance in selecting the Solar Technology

Out of all the things in our solar system, Sun is the most important for our life on Earth. It would not be wrong to say that Earth will be lifeless without Sun. It is Sun which is responsible for major phenomenon that play huge role in our survival on this planet.It produces light and heat through its radiations which reach our planet on continuous basis. As the worlds’ energy generation focus is shifting towards utilization of renewable resources, Sun is the core reason behind the successful extraction of energy through Photo-voltaic (PV) and Concentrated Solar technology.

Radiations from the Sun are emitted continuously and they constantly reach the Earth. Solar Irradiance reaching on the outer of the Earth’s atmosphere is calculated to be 1367W/sq.m. This is also known as Solar Constant. It is important to understand these energy carrying radiations which enter into the atmosphere of Earth. Some reach directly to point of target while others reach through various channels while others are reflected or absorbed in the atmosphere. These radiations may follow different paths up to the target (PV module or a Parabolic Trough collectors/ mirrors). Figure 1 shows the approximate estimation of various paths followed by radiation after they enter the atmosphere.

Radiation Paths:

Radiations from the Sun upon entering Earth’s atmosphere follow different paths depending upon the weather conditions.  Each path is mentioned below. Figure 2 shows paths of Solar radiations upon entering:

• Radiations directly reach the Earth’s surface.
• Radiations are diffused in the atmosphere due to dust, clouds, and molecules. Although these radiations are diffused / scattered yet they still make it to the ground and help in energization of the PV Panel.
• Radiations are reflected of the Earth’s surface, clouds or atmosphere.
• Solar radiations also get absorbed / scattered by the atmosphere and do not reach the surface of the Earth.

Terminologies for Solar Irradiance:

Terminologies used for Solar Power plants whether Photovoltaic (PV) or Thermal; Concentrating Solar Power (CSP) installations are mentioned below:

 1.     Diffused Horizontal Irradiance (DIF or DHI)

It is defined as the radiations reaching the surface of earth after being scattered or diffused by air, dirt or other molecules in the atmosphere i.e clouds. Values of this radiations are generally less than 600kWh/sq.m/ year and depends upon the areas air profile.

2.     Reflected Irradiance

It is defined as radiation reaching the targeted surfaces via reflection of direct beam radiations from the surface of the earth. This radiation increases the diffused component and hence the GHI.

 (Thiscan be observed by covering any PV Module directly from the sunlight but exposing it to reflections of sunlight. The albedo value also known as reflection coefficient varies for different surfaces. Normally it is taken as 0.20 for plain areas. For areas with heavy snowfall a higher albedo value is considered for simulation purposes. According to the research carried out in University of Mauritius, if a PV module is placed in a location having ‘dark’ surroundings, its output power will be much less than if it was placed in another location having this time ‘brighter’ surroundings. Ideally, PV modules and arrays should be installed in ‘bright’ areas so that more light is incident on them thus increasing the output power considerably).

  3.     Point of Array (PoA) Irradiance

It is defined as the amount of radiations received by a fixed tilted surface.

 4.     Direct Normal irradiance  (DNI) also known as beam irradiance

It is defined as the radiation incident directly onto a surface which is kept perpendicular to the surface. For a country with good solar insolation level, values of this type of radiations are generally above 1800kWh/ sq.m/year

 5.     Global Horizontal Irradiance (GHI)

It is the total radiation reaching a horizontal surface on the earth. It consists diffused , reflected and direct radiations.

All the above types are inter related through the following formula:

GHI = DNI x cos θ DHI

θ = Zenith Angle of the sun with the Horizontal surface at the Earth. It is 90° - elevation angle of the Sun. At, zenith angle of 0 degree, the DNI will be approximately equal to GHI. DHI includes the reflected radiation component as well. Value of GHI for a country with healthy solar insolation profile ranges between 1800-2500kWh/sqm/year.

Units

Solar Irradiance units are W/sq.m. However, if integrated over a period of time the unit becomes kWh/sq.m.  The daily, monthly and yearly global horizontal irradiation has the unit of kWh/sq.m. Another commonly used unit is MJ/sq.m.

Factors effecting Solar Irradiance:

• Output of Sun
• Distance between Earth and Sun (vary throughout the year)
• Water molecules in air
• Clouds
• Pollution in Air
• Time of Day
• Location on the Globe
• Seasons

Importance of Solar Irradiance for PV and CSP Technology :

Out of all various irradiance terminologies mentioned above, the two most important are DNI and GHI due to higher values and their direct influence on selection of Solar Technology.

Global Horizontal Irradiance is the shortwave radiation (consisting of huge amount of energy) from the sun which reaches at a surface horizontal to the surface of the Earth. It accounts for all the radiations mentioned above. It is the sum of diffused and the direct normal irradiations reaching the horizontal surface.

For the Photo voltaic (PV) Solar Plants, Global Horizontal Irradiance (GHI) component of solar radiation is an important factor to be considered. Photo voltaic phenomena is the science of production of electric current and voltages in a material due to its exposure to light. Therefore, detailed investigation of diffused and other components of solar radiation and their variability during different weather conditions helps in assessing the solar resource of the area chosen for installing the PV plant. Additionally, type of PV installation i.e., fixed tilted or tracking, is also selected based on the GHI of the area. Assuming a fixed horizontal PV module, the daily GHI value will always be greater than the daily DNI value. However, for the tracking type PV installations, DNI is influential as the movement of PV panel is adjusted in line with the Sun throughout the day.

On the other hand, Concentrating Solar Plants (CSP) utilizes the thermal energy of the the direct radiation from the sun. CSPs collect the direct radiations and uses the parabolic mirrors to concentrate the reflection of the beam onto a pipe for heating the fluid inside. Another technology of this domain is the linear fresnel reflector which reflect the solar radiation towards a suspended receiver tube. Hence, CSPs requires DNI throughout the day for a higher energy output. It is generally assumed that CSP systems are economically viable only for locations with direct normal irradiations above 1800kWh/sqm annually. Moreover, it requires tracking systems which track the suns movement throughout the day inorder to obtain suns maximum beam radiations. The diffused radiations are not significant in case of CSPs as Thermal energy is the main working force in such power plants and diffused radiation energy density considerably decreases along its path.

Table 2 shows the maximum DNI values of areas within the countries. These areas are ideally suited for installation of Concentrated Solar Power Plants.GHI is observed and assessed prior to installation of PV power plants do to its dependence on localized factors such as albedo coefficient, aerosol properties of surrounding atmosphere and weather conditions.

Above two figures have been extracted from Solar GIS website. The figures show average annual GHI and DNI solar maps for each of the countries. The figures are presented here only for comparison purposes. It can be observed that all the countries under observation have a lighter shade for DNI as compared to GHI. Moreover, the higher DNI values for any country is more localized, distributed as compared to the GHI values for the same country which have more wide spread. This indicates that the selections of sites for PV power plants installations are easier due to more options available in terms of land. As for CSPs, possibility exists that the investors may be limited to a specific area for its development. According to reports, by the end of 2016 , 4.815GW capacity of CSPs were installed compared to approximately 75GW capacity of PV plants .