In design of PV systems, it is essential to know the amount of sunlight available at a specific location at a particular time. The two common measures which describe solar radiation are the solar radiance and solar insolation.
The solar radiance is the instantaneous power density in the units of kilowatt per square meter. The solar radiance range varies throughout the day from zero kW/m2 at night to a maximum of nearly 1 kW/m2. The solar radiance depends strongly on the location and weather. Solar radiance measurement comprise of global and direct radiation measurements taken occasionally throughout a day.
Ways of measurement of Solar Radiation
The measurements are taken using either a pyranometer for measurement of global radiation and/or a pyrheliometer for measurement of direct radiation. A less accurate –yet less expensive- alternative method for measurement of solar radiation, is using a sunshine recorder.
A pyranometer is a device used for measuring solar irradiance on a planar surface by measuring the solar radiation flux density ( in units of W/m2) from the above hemisphere within a range of wavelength from 0.3 μm to 3 μm. The name pyranometer originates from the Greek words meaning fire, and above sky.
The sunshine recorders (know also by Campbell-Stokes recorders), measure the number of hours in the day during which the sunshine level is above a threshold (typically 200 mW/cm2). Data collected in this way can be used for determining the solar insolation by comparison of the measured number of sunshine hours to the calculated ones by including several correction factors.
A final way to estimate solar insolation is the cloud cover data obtained from available satellite images.
Solar irradiance is most commonly measured, a more common form of radiation data for use in designing systems is the solar insolation. The solar insolation is defined as the total amount of solar energy striking a particular position during a specific time period, often in units of kWh/(m2 day).
While the units of both solar insolation and solar irradiance is power density, solar insolation is really different than solar irradiance as the solar insolation is the instantaneous solar irradiance averaged over a specific time period. Solar insolation data is used usually for designing simple PV systems while solar radiance is used for analysis of more complicated PV system performance, which calculates the performance of the system at each point in the day. Solar insolation is sometimes expressed in units of MJ/m2 per year.
Solar radiation for a specific location can be measured in multiple ways including:
Location’s typical mean year data
Location’s average daily, monthly or yearly solar insolation data
Global isoflux contours for either a year, part of a year or a particular month