Measurement of Solar Radiation

pyranometer

The photograph at left shows equipment for solar irradiance measurements. (Photograph from David Pearsons) via NREL information exchange.

 

In PV system design it is essential to know the amount of sunlight available at a particular location at a given time. The two common methods which characterise solar radiation are the solar radiance (or radiation) and solar insolation. The solar radiance is an instantaneous power density in units of kW/m2. The solar radiance varies throughout the day from 0 kW/m2 at night to a maximum of about 1 kW/m2. The solar radiance is strongly dependant on location and local weather. Solar radiance measurements consist of global and/or direct radiation measurements taken periodically throughout the day. The measurements are taken using either a pyranometer (measuring global radiation) and/or a pyrheliometer (measuring direct radiation). In well established locations, this data has been collected for more than twenty years.

An alternative method of measuring solar radiation, which is less accurate but also less expensive, is using a sunshine recorder. These sunshine recorders (also known as Campbell-Stokes recorders), measure the number of hours in the day during which the sunshine is above a certain level (typically 200 mW/cm2). Data collected in this way can be used to determine the solar insolation by comparing the measured number of sunshine hours to those based on calculations and including several correction factors.

A final method to estimate solar insolation is cloud cover data taken from existing satellite images.

While solar irradiance is most commonly measured, a more common form of radiation data used in system design is the solar insolation. The solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of kWh/(m2 day). While the units of solar insolation and solar irradiance are both a power density (for solar insolation the "hours" in the numerator are a time measurement as is the "day" in the denominator), solar insolation is quite different than the solar irradiance as the solar insolation is the instantaneous solar irradiance averaged over a given time period. Solar insolation data is commonly used for simple PV system design while solar radiance is used in more complicated PV system performance which calculates the system performance at each point in the day. Solar insolation can also be expressed in units of MJ/m2 per year and other units and conversions are given in the units page.

Solar radiation for a particular location can be given in several ways including:

  • Typical mean year data for a particular location
  • Average daily, monthly or yearly solar insolation for a given location
  • Global isoflux contours either for a full year, a quarter year or a particular month
  • Sunshine hours data
  • Solar Insolation Based on Satellite Cloud-Cover Data
  • Calculations of Solar Radiation