Comparison of the Efficiency of Solar Collectors in Terms of the Working Medium—Review of Selected Technical Solutions

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.