Measurement of solar radiation angular distribution with a sunshape profiling irradiometer

  • Ragini Kalapatapu

Student thesis: Master's Thesis


Measurement and analysis of the angular distribution of solar radiation is a key problem in Solar Resource Assessment. Existing high resolution, circumsolar and sky-mapping instruments are too expensive and labor intensive for long term or remote deployment. This thesis presents a novel instrument called the Sunshape Profiling Irradiometer (SPI) and an inverse model for the determination of the solar radiance profile and the circumsolar ratio (CSR) for a location. A prototype has been built from a stepping-motor-based Rotating Shadowband Irradiometer with mechanical modifications that are novel yet simple. An optical slit receiver is used to best resolve brightness change during progressive occultation of the circumsolar region by the shadow band. For the prototype a Licor's receiver is narrowed to a slit, cut on the axis line. A trough signal trajectory is obtained when the band crosses the sun and the amount of rounding of the corners of the trough is related to the CSR. A family of normalized simulated signals corresponding to different CSRs generated by integrating Buie's equations is used to find the best match of the normalized measured profile from the SPI to get a good estimate of the CSR. Results of the obtained CSRs are validated with the broadband CSRs given by the Sun and Aureole Measurement System (SAM). Buie's model is observed to work well only for clear sky conditions and the assumed angular extent of the sun aureole is small for regions that have large aerosol concentration, relative humidity and hazy skies like Abu Dhabi. Hence, a more complete and rigorous model based on direct inversion of the SPI signal is proposed to obtain the actual bright¬ness profiles for any location. The instrument can be used for climate research ( as well as improved resource characterization for applications that need an accurate circumsolar and/or sky model such as concentrating solar power (CSP, CPV) and day lighting.
Date of Award2012
Original languageAmerican English
SupervisorPeter Armstrong (Supervisor)


  • Solar radiation
  • Sunshine

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