Direct insolation


Direct insolation is the solar insolation measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation. Direct insolation is equal to the solar irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. While the solar irradiance above the atmosphere varies with the Earth-Sun distance and solar cycles, the losses depend on the time of day, cloud cover, moisture content, and other impurities.

Simplified formula

A simple formula gives the approximate level of direct insolation when there are no clouds:
where AM is the airmass given by
with θ being the zenith angle of the sun.
For the sun at the zenith, this gives 947 W/m2. However, another source states that direct sunlight under these conditions, with 1367 W/m2 above the atmosphere, is about 1050 W/m2, and total insolation about 1120 W/m2.

Average direct insolation

For practical purposes, a time-average of the direct insolation over the course of the year is commonly used. This averaging takes into account the absence of sunlight during the night, increased scatter in the morning and evening hours, average effects of cloud cover and smog, as well as seasonal variations of the mid-day solar elevation.

Units of measurement

Direct insolation is measured in or kilowatt-hours per square meter per day.
In the case of photovoltaics, average direct insolation is commonly measured in terms of peak direct insolation as kWh/

Applications

Since radiation directly from the sun can be focussed with mirrors and lens, it can be applied to concentrated solar thermal systems. Due to clouds and aerosols, the direct insolation can fluctuate throughout the day, so forecasting the available resource is important in these applications