REST2 Model

REST2 is a state-of-the-art, high-performance radiative model to predict cloudless-sky broadband irradiance, illuminance and photosynthetically active radiation (PAR) from atmospheric data. Its derivation uses the same two-band scheme as in the previous CPCR2 model, but with numerous improvements. Great attention is devoted to precisely account for the effect of aerosols, in particular.

The REST2 model uses the following inputs:
  • Site coordinates (latitude, longitude, elevation, time zone)
  • Station pressure (calculated from site elevation and latitude if unknown)
  • Precipitable water (estimated from temperature and relative humidity if unknown)
  • Angstrom turbidity coefficient
  • Angstrom wavelength exponent
  • Aerosol single-scattering albedo (defaulted if unknown)
  • Total columnar ozone amount
  • Total columnar nitrogen dioxide amount (defaulted if unknown)
  • Ground albedo
  • Solar zenith angle (calculated from date and time if unknown).

REST2 is based on a parameterization of a large number of SMARTS simulations. It is considerably easier and faster to use than SMARTS, and is therefore particularly recommended when only broadband (rather than spectral) fluxes are to be obtained. It can be combined with other models to evaluate fluxes under cloudy conditions, or on tilted collectors.

A detailed validation analysis has shown that the model predictions are within instrumental uncertainty (about 5% or better) when the inputs are known with good accuracy.

A presentation illustrating the main features and derivation of the model, as well as an assessment of its accuracy was made during the ASES Solar 2004 Conference. An edited version is available here.

A scientific paper providing all the necessary calculation details and a rigorous performance evaluation is available here.

A 2010 ASES conference paper presented new comparative validation results showing that REST2's accuracy is unsurpassed in this category of radiative models.

Two peer-reviewed papers with expanded results and intercomparison of the performance of various radiative models commonly used in various solar applications are available here and here.

Some sample validation satterplots for direct irradiance appear below, for three sites in highly different climatic areas.

The REST2 code is continuously improved to respond to the most critical applications, such as CSP or CPV. The code is proprietary, but interested parties can benefit from it through a proper consulting agreement.