Methodology for partial vacuum pressure and heat losses analysis of parabolic troughs receivers by infrared radiometry

Loss of vacuum due to incoming gases (hydrogen or air) in evacuated receiver tubes of parabolic-through solar collectors (PTC) is one of the most common failures in large solar power plants and it has a significant impact on the thermal performance of the solar field. The loss of vacuum is translated in increasing heat losses and at certain point, damaged receivers must be replaced, but first they must be identified among all receiver pipes installed in a large solar field, which counts thousands units. Several efforts have been done to develop a methodology to locate receivers which loss completely the vacuum. These methodologies are based on the glass envelope temperature, the so called surface temperature method. In this work, the surface temperature method has been extended to determine partial vacuum pressure of receiver tubes not completely damaged and to determine its influence in their thermal performance. Results obtained using air as the incoming gas demonstrate that the methodology presented allows distinguishing partial vacuum pressure among 10⁻⁴, 10⁻², 10⁰, 10², 10³ mbar in the receiver cover annulus but it does require accurate measurements that must be carefully addressed as described in this work. Further tests with this methodology may be also applied to determine the partial vacuum pressure of hydrogen in the annulus of this type of solar receivers.