What is the clear sky index?

The clear sky index, which is the ratio of measured irradiance at ground level to estimated irradiance in clear sky conditions at ground level, is also utilized if a clear sky model and information on atmospheric input parameters are available.

What is clear sky insolation clearness index?

The Insolation Clearness Index is the fraction representing clearness of the atmosphere; the all sky insolation that is transmitted through the atmosphere to strike the surface of the Earth divided by the average of top of the atmosphere total solar irradiance incident.

What is cloudiness index?

The ratio of the diffuse to global solar radiation is known as the cloudiness index of the atmosphere.

What is the bird clear sky model?

The Bird Clear Sky Model, authored by Richard Bird, is a broadband algorithm that produces estimates of clear sky direct beam, hemispherical diffuse, and total hemispherical solar radiation on a horizontal surface.

What is DHI in solar?

DHI characterizes the amount of energy coming from the sky a er being scattered in the atmosphere. DHI is measured by a pyranometer shaded from the direct sunlight. GHI is the amount of solar radiation received per unit area by a horizontal surface from the hemisphere above.

What is the difference between a clear sky and the Sun?

Clear sky has less than 30 % cloud cover, or none. This sky is most likely to be combined with sun. Partly cloudy sky has between 30 % and 70 % cloud cover. This sky can be combined with sun in some cases.

What is the role of the clearness index for estimating solar radiation for cloudy day?

The clearness index is the measured global horizontal radiation divided by the extraterrestrial radiation. It basically tells you how cloudy that moment is. If you know the clearness index, then you can calculate the extraterrestrial radiation (it does not depend on the weather) and thus calculate the solar radiation.

What is ground reflectance?

The ground reflectance (also called albedo) is the fraction of solar radiation incident on the ground that is reflected. A typical value for grass-covered areas is 20%. Snow-covered areas may have a reflectance as high as 70%.

What causes low cloud cover?

Observations have shown, however, that warmer temperatures seems to create less dense, low-level clouds instead. The evidence we have so far suggests that this effect occurs because, as temperature increases, the air near the surface becomes drier, causing the cloud base to rise and reducing the cloud layer thickness.

What is DNI GHI and Dhi?

The global horizontal irradiance (GHI) falling onto the Earth’s surface consists of the diffuse horizontal irradiance (DHI) from the sky and the direct normal irradiance (DNI) from the sun. GHI = DHI + DNI*cos(θ) where θ is the solar zenith angle (vertically above the location is 0°, horizontal is 90°).

What is the Clear-Sky index?

The clear-sky index Kc is the ratio of the irradiance E, or irradiation #, received at ground to the irradiance Ec/ear, or irradiation Hc/ear, received at ground in clear-sky conditions, provided Eciear or Hc/ear is not null: The clear-sky index is sometimes called cloud modification factor.

What is clear-sky performance?

They defined a system’s clear-sky performance as the 80th percentile of the system’s performance at the same time of day on the previous 15 days. This allowed them to identify outages, partial shading and cloud occurrence within the fleet.

What is the normal K PV during a clear-sky period?

During clear-sky periods, the K PV values are approximately one, as is expected, with small variations occurring due to system level variations, possibly from inefficiencies in the modules, inverters or wiring (see Fig. 5 ). Fig. 3. Clear-sky K PV calculations.

Why do completely clear-sky conditions matter?

Completely clear-sky conditions, as depicted in Fig. 3, provide a unique opportunity to identify the sources of deviations from theoretical clear-sky performance that are not due to clouds. The most obvious example is the sharp drop in power production at higher zenith angles for stations 3 and 4 in the morning, and station 5 in the evening.