Comparative Multicriteria Analysis of Thermophysical and Corrosion Properties of main Types of Heat Transfer fluids for Solar Water Heating Collectors

This study presents a detailed, multifaceted analysis of the operational, thermophysical, hydraulic, corrosion, and economic characteristics of four different types of heat transfer fluids relevant for use in solar water heating collectors (SWHC): prepared water, the industrial solution EcoSol®, a water-glycol mixture with a glycol mass fraction of 50%, and innovative aqueous nanofluids modified with copper oxide (CuO) and titanium dioxide (TiO₂) nanoparticles. The investigation included a comparison of key parameters such as thermal conductivity, specific heat capacity, dynamic viscosity, limiting phase transition temperatures (freezing/boiling points), specific cost, as well as chemical aggressiveness towards structural materials and corrosion tendency. The results obtained show that the thermal conductivity of nanofluids on average exceeds that of ordinary water by 35–40%, although their specific heat capacity decreases by up to 8%, and viscosity and cost vary widely—from threefold to fifty- or even one hundredfold increases compared to water. Water-glycol mixtures exhibit outstanding frost resistance (t_freeze ≈ –41 °C) and a high boiling point (about 110 °C), but these benefits come at the expense of significantly higher viscosity (2.5–3 mPa·s) and the need for constant control of buffer additives and corrosion inhibitors. The use of EcoSol® solution is justified where it is necessary to strike a balance between moderate frost resistance (up to –30 °C), relatively low cost, and minimal corrosive activity. Water, as before, remains a kind of “gold standard” due to its maximum specific heat capacity (4.19 kJ/(kg·K)), minimal viscosity (about 1 mPa·s), and exceptional economic efficiency (0.5–2 som/l); however, its practical application necessarily requires the implementation of anti-corrosion measures and technologies to prevent freezing in winter. For an objective comparison of the alternatives considered, the multicriteria decision-making (MCDM) methodology was employed, which made it possible to take into account weighting priorities determined by operating conditions. The final conclusions indicate that for regions with a mild climate not subject to significant subzero temperatures, water remains the optimal choice; in severe winter conditions, water-glycol solutions are preferable; in budget systems with moderate requirements for frost resistance, EcoSol® is justified; and nanofluids are advisable only for high-temperature or energy-efficient compact systems where the necessary technical maintenance can be provided and increased costs are acceptable.