Second law performance of TiO₂ nanofluid-enhanced panel radiators modeled by response surface methodology

Panel radiators are the most used end-user units in building heating systems. However, the thermodynamic optimization of panel radiators from the perspective of second-law analysis has not yet been comprehensively studied. This study presents a comprehensive experimental investigation of the entropy production rate, exergy destruction rate, and second-law efficiency of a Type 22 panel radiator operated with water-based TiO₂ nanofluids. The experiments were conducted at two volumetric concentrations (0.2% and 0.4%), five inlet temperatures (30–70 °C), and five volumetric flow rates (3–15 lpm), yielding 75 different experimental configurations under laminar flow conditions. The results show that entropy production and exergy loss are primarily dependent on the inlet temperature. The maximum values for these parameters are approximately 0.80 W/K and 20 W, respectively, at 70 °C and 15 lpm. The second-law efficiency ranges from 64% to 69% under all conditions; a statistical sensitivity analysis revealed that flow rate is the primary determinant of this efficiency. It was found that the addition of TiO₂ nanoparticles to the system has a statistically significant but quantitatively limited effect on all three thermodynamic metrics examined.This study is the first statistically designed second-law optimization study conducted on nanofluid-based panel radiators and provides a statistically validated framework for the thermodynamic optimization of building heating systems. These findings demonstrate that second-law performance is largely governed by thermal driving force and provide a new physical explanation for why dilute nanofluids offer limited practical benefits in natural convection heating systems.