Effect of Mechanical Ventilation on the Thermal Offset of Pyranometers during Cloud-Free Summer Conditions

Thermal offset is a significant source of uncertainty for solar radiation measurements. This study assesses the influence of mechanical ventilation on the daytime thermal offset of pyranometers. Toward this goal, an intensive unprecedented campaign of measurements was conducted in Badajoz, Spain, during four selected summer days under cloud-free conditions, covering a large range of solar zenith angle, irradiance, and temperature. Three leading manufacturers participated in the campaign, providing secondary standard pyranometers and compatible ventilation units. The thermal offset was experimentally measured following the capping methodology. A total of 372 capping events were conducted, the largest number ever reported in the literature. Each pyranometer was tested under different operational conditions (with/without ventilation and measuring global/diffuse irradiance). Results show that mechanical ventilation generally reduces the thermal offset. The magnitude of this reduction is different for each pyranometer model and depends on whether the instrument is shadowed (for measuring diffuse irradiance) or not (for measuring global irradiance). Mechanical ventilation tends to homogenize the temperature around the pyranometer and therefore reduces the impact of environmental conditions on the thermal offset. CMP11 and SPP pyranometers show notable tendencies in the thermal offset even when mechanical ventilation is applied. The Dutton et al. model, which aimed to correct the daytime thermal offset, is evaluated. Results show this model performs well for the SPP pyranometer but underestimates the absolute value of thermal offset for the CMP11 and SR20 pyranometers.