Direct insolation is the
insolation measured at a given location on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
with a surface element
perpendicular
In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
to the
Sun's rays, excluding
diffuse insolation (the solar radiation that is
scattered or
reflected by atmospheric components in the
sky
The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.
In the field of astronomy, ...
). Direct insolation is equal to the
solar irradiance
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/ ...
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 cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
s, the losses depend on the time of day (length of light's path through the atmosphere depending on the
solar elevation angle
The solar zenith angle is the zenith angle of the sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between th ...
),
cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
,
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, 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
In astronomy, air mass or airmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from below Earth's atmosphere ( Green 1992). It is formulated as the integral of air density along the lig ...
given by
:
with θ being the zenith angle (90° minus the
altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
) of the sun.
For the sun at the
zenith
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" ...
, this gives 947 W/m
2. However, another source states that
direct sunlight
Direct may refer to:
Mathematics
* Directed set, in order theory
* Direct limit of (pre), sheaves
* Direct sum of modules, a construction in abstract algebra which combines several vector spaces
Computing
* Direct access (disambiguation), a ...
under these conditions, with 1367 W/m
2 above the atmosphere, is about 1050 W/m
2, and total insolation about 1120 W/m
2.
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 (W/m
2) or kilowatt-hours per square meter per day (kW·h/(m
2·day)).
: 1 kW·h/(m
2·day) = 1,000 W · 1 hour / ( 1 m
2 · 24 hours) = 41.67 W/m
2
In the case of photovoltaics, average direct insolation is commonly measured in terms of peak direct insolation as kWh/(kWp·y)
(kilowatt hours per year per kilowatt peak rating)
Applications
Since radiation directly from the sun can be focussed with mirrors and lens, it can be applied to concentrated solar thermal (CST) systems. Due to clouds and aerosols, the direct insolation can fluctuate throughout the day, so forecasting the available resource is important in these applications
References
{{reflist
External links
National Science Digital Library - Direct Insolation
Atmospheric radiation
Visibility