Solar air heating system with low environmental impact materials: Mathematical model and optothermal characterization

[Display omitted] •An eco-technological device for heating with solar thermal use was developed.•The developed device uses low-cost materials with low environmental impact.•A mathematical model was developed for solar air heater.•The photothermal properties of the studied materials were evaluated.•The device can be used in various fluid heating processes. The present study shows a solar air heating system, made up of a thermal accumulator of limestone and gravel, as well as aggregates of soot from forest biomass as a photothermal material for solar absorption. This research is divided into three stages: (a) the construction of the solar air heating system with soot-gravel-limestone composite materials (b) the optothermal characterization of the system: using UV–VIS-NIR spectrophotometry to estimate solar absorptivity, scanning electron microscopy to analyze the morphology of materials, thermographic analysis, and experimental evaluation with solar irradiance to determine the thermal efficiency and to carry out an exergy analysis (c) the development of a mathematical model to infer the behavior of the system, allowing the theoretical variation of the materials and properties involved to optimize the construction and characterization process. The results show that soot, as a photothermic material, increases the solar absorption capacity of thermal accumulators of gravel and limestone above 90%, and contributes to reaching maximum accumulated temperatures above 60℃. In this system, the cooling curve is slow and the temperature gradient at the end of the solar day exceeds the ambient temperature by more than 15 °C. The mathematical model is functional and had an approximate error of 4% with respect to the experimental analysis. This proposal with low environmental impact and low cost materials has favorable photothermic properties for its implementation in more complex solar thermal technologies.