Photothermal and thermography techniques applied in the characterization of the thermophysical properties of solar absorbers: A review

Abstract

The efficient generation of renewable energies from solar radiation implies the precise knowledge of the thermophysical properties of the materials and systems involved in such applications, this knowledge is fundamental, since it allows us to evaluate and optimize the performance of materials and methods. One of the great challenges for the efficient use of solar energy is the study of the phenomena involved in the interaction of radiation with matter, and the conversion of energy absorbed into heat. Photothermal and thermographic techniques are ideal methodologies to study these phenomena, since they are non-contact techniques (NCT) and, therefore, can be applied in a wide range of operating temperatures and environments. In general, photothermal techniques consist in illuminating the surface of the sample with a laser light beam modulated or pulsed, therefore the sample is heated and emits infrared radiation IR (among other types of energy). The variations of the IR energy are related to the thermophysical properties of the specimens photothermal techniques are based on the detection of these variations. Some examples of photothermal methodologies are the photothermal radiometry pulsed (PPTR) and modulated (PTR), the photothermal reflectometry (RM) and the thermal wave resonant cavity (TWRC). Infrared thermography is a powerful method of non-contact measurement, using this technique thermal images or thermograms are obtained, which are a representation of the temperature distribution on the surface of an object or a scene. The thermophysical properties of materials they are obtained through the analysis of amplitude and phase signals, as well as thermograms. Reflectance spectroscopy is a powerful tool for characterizing the optical properties (solar absorbance and thermal emittance) of a great diversity of materials applied in solar technology. The analysis can be done in two wavelength ranges of interest: Ultraviolet/visible/near infrared and medium infrared, where solar spectrum radiation wavelengths and infrared emission range are located respectively. The methodologies mentioned have been applied in the characterization of materials at room temperature, however, many processes involved in the new applications of solar energy occur at high temperature. It has also been shown that thermophysical mechanisms related to the conversion of energy that occur at room temperature are not dominant in high temperature regimes therefore, the measurement of the conversion, absorption, emission and energy exchange processes to high temperature, they involve new technological challenges. In this work, we present a summary about the photothermal, thermographic and reflectance spectroscopy techniques, used in the evaluation of the thermophysical properties of solar absorbers applied in low temperature solar systems, as well as high temperature concentrate solar power (CSP) systems.