Effects of Natural Ventilation and Solar Radiation on the Performance of Pyrgeometers

Abstract

In this work the authors present the results of field experiments carried out in Almería (36.83°N, 2.42°W), a seashore location in southeastern Spain, in order to evaluate the performance of Eppley precision infrared radiometer (PIR) pyrgeometers. The authors estimate the systematic errors in the measurements of downward longwave radiation caused by solar heating of the pyrgeometer?s dome. Pyrgeometer measurements have been obtained in a series of experiments in which the dome of the pyrgeometer has been, alternately, exposed to the solar beam and shaded by a disk. These measurements have been completed with solar direct irradiance and wind velocity measurements. This study confirms previous assessments about the magnitude of this effect and its possible estimation in terms of global horizontal solar irradiance. Additionally, the authors have quantified the influence of natural ventilation on the solar heating effect. The experiments have confirmed the reduction of the solar heating effect with an increase of natural ventilation rates on the pyrgeometer. Nevertheless, this reduction reaches a limit, indicating that the effect cannot be fully eliminated, as has been already pointed out for mechanically driven ventilated pyrgeometers. A formula for the correction of the solar heating effect considering the wind velocity influence is proposed. It estimates the necessary correction as a function of solar irradiance and wind velocity, thus allowing the suppression of systematic errors, which could represent up to +47 W m?2 for the worst situation (no wind, high irradiation), and providing experimental measurements that are affected by a random error of about ±5 W m?2.