In Poland, the most commonly used solar fluid in heating installations with solar collectors are 35–50% water propylene glycol solutions or ethyl. These mixtures (with ennobling additives) effectively protect the solar installation against the effects of freezing of the working medium at low outside temperatures prevailing in winter. These solutions, compared to water, are characterized by higher viscosity and lower values of the specific heat in the range of typical working temperatures of the working fluid in the solar circuit, have a higher price per unit volume, and may cause poisoning due to accidental ingestion. It is possible in solar installations operating also in winter use of water only, without the addition of glycols, but then equipment is necessary the automatic control system controlling the installation with the protection function of anti-freeze. The aim of this study is to review selected examples of solar installations in which water was used as the solar fluid, proving the disadvantage of using glycol.

Promotion of renewable energy sources and their ecological advantages in comparison to fossil fuels together with continual technological development results in the expansion of the solar collectors market even in countries where solar radiation is unfavorable for solar systems. In this paper, the analysis of the employment of plate and tube collectors in a residential buildings located in Poland was conducted. The paper shows the results of preliminary studies of a water solar collector system installed in Bialystok University of Technology, Poland. The efficiency of the system in the winter season was analyzed, based on data garthered in December 2019. Results showed that a water solar installation in the north-Europe countries, such as Poland, need additional energy supply during cold months for anti-frozen protection. The daily average of additional energy consumption can be twice higher than solar contribution for extreme conditions as studied in this paper.