Measurement of Solar Radiation


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 solar radiation may be characterized by the measured solar irradiance (power per area at a given moment) (or radiation) and by the solar insolation (the energy per area delivered over a specified time period). 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 irradiance is strongly dependent on location and local weather and varies throughout each day. Solar irradiance 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, these data have been collected for more than forty years.

The two most common sensors used as pyranometers are thermopiles (which measure the heat reaching the surface directly) and reference cells (which measure the number of photons reaching a surface that are successful in delivering an electron to an outside circuit). A thermopile is an array of thermocouples that collectively measure the temperature difference between the irradiated side and the side that is in the dark. Thermopiles are preferred for characterizing the solar resource for solar thermal collectors or for comparing the performance of solar thermal and solar photovoltaic systems. Reference cells are typically silicon solar cells packaged in such a way as to measure the photocurrent. A reference cell may be most useful when constructed to mimic the response of the solar panels being measured and may be fabricated of any semiconductor material with a package (i.e. textured glass) that matches that of the solar panels.  Sensors may vary in their response to different wavelengths of light (especially if a solar cell with a different band gap is used) and to their angular response (the dome on the thermopile shown to the right is designed to accept light equally well from all angles, while a reference cell typically has a higher reflection for light striking at a glancing angle.) Some silicon-based pyranometers use a diffusing dome on the top to change the angular acceptance to be more isotropic, which may be useful for some applications, but using a reference cell that mimics the angular response of the solar panels being studied gives a more precise measurement of the solar resource available to those solar panels.

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 derived from cloud cover and other data taken by satellites. The bulk of the solar resource data available on line have been compiled from satellite data, as discussed later in Chapter 2.

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 irradiance 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