Improving the quasi-dynamic test method for low-temperature solar thermal collectors

Abstract

Solar thermal energy systems (STES) are an alternative to fossil fuels for heat production in the residential and industrial sectors, contributing to the reduction of greenhouse gas emissions. Solar thermal collectors, which are responsible for capturing and transferring solar energy to a fluid, are the central components of these systems. To promote the use of STES, it is essential to ensure the quality and thermal performance of collectors, which is achieved through standardized procedures such as those described in ISO 9806:2017, one of the most widely used standards worldwide for this purpose. In Uruguay, the promotion of STES was declared a national interest in 2009, and in 2013, an agreement was signed between the Ministry of Industry, Energy and Mining (MIEM) and the University of the Republic (UdelaR) to create the Solar Heaters Test Bank (BECS, from its name in Spanish). Its purpose is to carry out thermal performance tests on solar thermal collectors and factory-made solar water heaters in accordance with ISO standards, with the aim of homologation, supporting innovation by national companies, and promoting research. For these tests, the ISO 9806:2017 standard is used for solar collectors, which contemplates two methodologies for thermal performance test: steady state testing (SST) and quasi-dynamic testing (QDT). This thesis aims to enhance the thermal performance testing procedures established in ISO 9806:2017 for low-temperature collectors, particularly the QDT method, consolidating the BECS infrastructure as a recognized research platform. These improvements enhance the reliability and applicability of the methodology, providing a more robust framework for evaluating the thermal performance of low-temperature solar collectors. Specifically, the primary contributions were the improved experimental characterization of the incidence angle modifier for the direct and diffuse components, and the enhanced modelling of transient effects in the QDT method. A more detailed description of these contributions is provided below. With respect to the beam component of the IAM, a new model was developed and validated for flat plate (FPC) and heat pipe evacuated tube (ETCHP) collectors. This model can therefore be applied to uniaxial and biaxial IAM collectors. It demonstrated greater accuracy and consistency with the SST method than existing models. For the diffuse component, two alternative models were proposed: one based on global isotropic diffuse irradiance (an extension of the SST approach) and the other distinguishing between contributions from the sky and the ground. In both cases, the associated parameters are estimated by integrating the IAM for the direct component. Both models showed better agreement with the SST, the former being preferred for its simplicity, as the latter requires an additional measurement related to the diffuse irradiance reflected by the ground. In addition, transient modeling was improved by implementing a Dynamic Parameter Identification (DPI) algorithm, which outperformed the traditional Multiple Linear Regression (MLR) method in accuracy and demonstrated a lower sensitivity to averaging time. Both diffuse IAM models and the DPI procedure showed advantages for the two technologies addressed in this thesis, FPC and ETC-HP, although the improvements were more significant in the latter case. Finally, the thesis includes secondary scientific contributions, which are described below. To facilitate the implementation of the aforementioned primary contribution, a MATLAB tool was developed to integrate all the proposed models. Additionally, improvements were made to the measurement of diffuse solar irradiance on an inclined plane, which is necessary for the implementation of the QDT method. To this end, a new correction model for horizontal diffuse irradiance measurements with a shadow band was developed, alongside experimental characterization of the albedo of the BECS environment, and a comparison of different models. Although the thesis focuses primarily on the QDT method, it also proposes an enhancement to the SST method through a refined procedure for converting SST parameters into QDT parameters. This new procedure incorporates the effect of the diffuse fraction into the standard conversion process, thus improving the consistency between the two methods.