Estimation of Global Solar Radiation Flux Density in Brazil from a Single Measurement at Solar Noon

The calculated values of solar energy potential permit the evaluation of the maximum possible performance of energy capture systems. This is very important in engineering projects designed to make use of solar energy in its various forms, including capture by the biomass, photoelectricity for small potentials, direct capture by collectors, irrigation of crops, agricultural potential production, agroclimatic zoning and resources for varied thermodynamic cycles. The aim of this study was to develop and appraise a mathematical model for evaluation of solar energy potential with radiometric data collected in the cities of Botucatu, Piracicaba and São Paulo, State of São Paulo, Brazil, as a function of a single measurement of solar irradiance at solar noon. The performance of the estimation model of maximum energy input was examined using determination coefficients R2 and Willmottʹs index of agreement d applied to estimated and observed data at the sites in study. A new way of estimating the a and b parameters of the Angström equation is proposed, which is irrespective of daily integrations and does not require a large number of radiometric measurements, one such measurement taken at solar noon being sufficient. The results presented show that the proposed methodology was feasible for the evaluation of the Angström equation parameters, besides eliminating the multiple error sources produced by the conventional methodology, which is largely used in Brazil from data measured by bimetallic actinograph. The latter can present restrictions, such as lack of calibration, reading failures and integration errors of radiometric curves especially on cloudy days. For the climatic conditions of the studied sites, it is concluded that it is possible to estimate accurately the maximum daily solar energy input. The estimation of the global irradiance by the proposed method generated values equivalent to those obtained using measurements of the duration of bright sunshine for Botucatu, SP and even more exact ones for Piracicaba, SP as well as values extremely close to the real conditions measured in São Paulo, capital, Brazil.