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This paper focuses on an analysis of selected control methods in solar heating systems. Proportional, ON-OFF, and new proposed IPC control methods were tested. Experimental tests were conducted under natural conditions using the author’s method of clustering measurement days. In the form of thermal energy gains in the storage tanks, the results for all tested control methods are presented. The ON-OFF control method is suitable for low variability of solar radiation conditions but is ineffective under dynamic solar radiation conditions. The proportional controllers collect thermal energy under high variability solar radiation effectively, but they tend to cause thermal drifts from the system under high heat load. The proposed IPC control method prevents the waste of heat energy and makes more efficient use of the high and dynamic solar radiation. In conclusion, energy gains depend more on the level of solar radiation and less on its variability. However, the variability of solar radiation makes control in solar systems more challenging, and it is one of the factors that should determine the control strategy. The novelty of this work is based on an extension of the control algorithm by adding the temperature at the entry and exit of the solar coil. This makes it possible to eliminate thermal drift and observe the intensity of heat transfer to the water in the tank.

Work was carried out to diagnose irregularities in the operation of a vacuum-tube solar collector. Experimental investigations of the collector were carried out at the solar collector field test stand in the Laboratory of Solar Collectors at the Koszalin University of Technology. The scope of the work included the following: research on thermal efficiency characteristics, research on the temperature distribution on the solar collector manifold, and research on the geometric structure of the heat pipe and filling with the working medium. Based on the diagnostic tests carried out, the occurrence of incorrect sedimentation in the condenser on the neck of the heat pipe (44%) and an incorrect amount of working medium (66%) were found. The results show that the functioning of the heat pipe significantly depends on its geometric structure and the amount of working medium. Any irregularity at the production stage contributes to a reduction in the thermal efficiency of the solar collector. The results of the experimental research presented in this paper can be used in the diagnostic analyses of solar systems as well as in the organisation of the manufacturing processes of these systems.

This article presents the results of numerical and experimental studies on the impacts of the selected geometrical features of liquid solar collectors on their thermal efficiency. The experiments were carried out while meeting the requirements of the ISO 9806:2017 standard. Selected changes in the geometrical features were analysed by using fully functional prototypes of modified solar collectors. The correlations between the design and performance properties of the solar collectors were determined in accordance with the changes in the shape of the thermal efficiency η(T*m) curve.