What is clearness index?
Clearness index is defined as the ratio of global solar irradiance measured at ground level G and its counterpart estimated at the top of the atmosphere Gext (Liu and Jordan, 1960):(9.3)kt=GGext.
How do you calculate diffuse radiation?
The ratio of the irradiance of a surface at angle to the irradiance of a horizontal surface is therefore ( 1 + cos α ) / 2 = cos 2 ( α / 2 ) . For an inclined plane the factor ( 1 + cos α ) / 2 for diffuse radiation is equivalent to the factor A h / A derived for direct radiation.
How do you find the diffusing fraction?
2.1. Depending on clearness index value, the diffuse fractions (Id/It) are calculated as per Eqs. (6), (7), (8).
What is solar DNI?
Direct Normal Irradiation (DNI) is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky.
What is the role of the clearness index for estimating solar radiation for cloudy day?
The clearer the day, the less cloudy. That is why it makes sense to use the clearness index as an indicator of the fractions of diffuse/direct radiation. The higher the clearness index, the less cloudy is your day, and thus the larger the fraction of beam radiation.
How do you calculate extraterrestrial radiation?
Hourly Extraterrestrial Radiation (Io)
- Hourly Extraterrestrial Radiation (Io) Extraterrestrial radiation is defined as the incidence of solar radiation outside the earth’s.
- atmosphere and is computed with the following equation: Io =
- 12 × 3.6. π
- IscE0 × (
How do you calculate global horizontal irradiance?
Global Horizontal Irradiance (GHI), from the hemisphere above a horizontal plane surface, is a combination of DNI, corrected for the angle of incidence of the beam (θ), and Diffuse Horizontal Irradiance (DHI). GHI = DNI*cosθ + DHI. All these parameters are measured in units of W/m2.
What is direct and diffuse radiation?
In simple terms, direct radiation has a definite direction but diffuse radiation follows a much more scattered, uncertain path. During times when the sky is clear and the sun is very high in the sky, direct radiation is around 85% of the total insolation striking the ground and diffuse radiation accounts for about 15%.
What is DNI and GHI?
Global Horizontal Irradiance (GHI) is the total amount of shortwave radiation received from above by a surface horizontal to the ground. This value is of particular interest to photovoltaic installations and includes both Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI).
What is DNI and Dhi?
Both DHI & DNI are actually components of GHI and mathematically, Global Horizontal Irradiance (GHI) = Direct Normal Irradiance (DNI)* cos(solar zenith angle) + Diffused Horizontal Irradiance (DHI) GTI is acronym for Global Tilted Irradiance which represents irradiation that falls on a tilted surface.
What is cloudiness index?
The clearness index is the ratio of the measured global solar radiation at the Earth’s surface to the extra-terrestrial solar radiation at the top of the atmosphere, while the cloudiness index is the ratio of the diffuse solar radiation to the global solar radiation at the Earth’s surface.
What is the difference between terrestrial and extraterrestrial?
Extraterrestrial refers to any object or being beyond (extra-) the planet Earth (terrestrial). It is derived from the Latin words extra (“outside”, “outwards”) and terrestris (“earthly”, “of or relating to the Earth”). It may be abbreviated as “E.T.”
What is the clearness index?
The clearness index is a dimensionless number between zero and one, indicating the fraction of the solar radiation striking the top of the atmosphere that makes it through the atmosphere to strike the Earth’s surface. The following equation defines the monthly average clearness index:
How does Homer calculate the clearness index?
Based on the value of the average radiation, the month of the year, and the latitude, HOMER calculates the clearness index. This section of the help describes the relationship between the two variables and how HOMER calculates the clearness index from the global horizontal radiation (GHI).
What does indexindex calculation agent mean?
Index Calculation Agent means the Index Calculation Agent as specified in the Index Rules. Index Calculation Agent means the Index Calculation Agent as specified in the Index Rules. ” Index Calculation Day ” has the meaning given to such term in the Index Rules.
What is the clearness index of the atmosphere?
Clearness Index The clearness index is a measure of the clearness of the atmosphere. It is the fraction of the solar radiation that is transmitted through the atmosphere to strike the surface of the Earth. The clearness index can be defined on an instantaneous, hourly, or monthly basis.