Design optimization of road thermal collectors: A numerical and experimental study in the Mediterranean

Roads, car parks, and airport runways constitute some of the largest, but least utilized, solar thermal surfaces in urban areas. Roads thermal collectors can exploit this resource to reduce the effects of urban heat islands and decarbonize low-temperature heat demand by powering building-level heat pump systems. This study presents a two-dimensional numerical model using the finite element method to simulate collector performance. It introduces a novel approach to calculate convective heat transfer coefficients based on wind speed and atmospheric stability, validated against experimental data from an 80 m2 prototype at the University of Palermo, Italy. The prototype features heat exchanger tubes 14.5 cm below the asphalt within a thermally conductive concrete layer, mirroring urban road construction to reduce costs and maintenance. A 50-cm lightweight insulating concrete layer was also included to boost efficiency. The model enabled parametric analyses of 18 collector configurations, varying tube spacing, insulation thickness, and concrete conductivity. Results show that while an insulating layer increases peak thermal output, it does not significantly improve seasonal energy collection. Reducing tube spacing, however, enhances both peak output and total energy harvested. An optimized design with 20 cm tube spacing and no insulating layer is projected to achieve 320 kWh/m2 annually in Palermo, with a 25 % seasonal solar-to-thermal conversion efficiency, assuming a 20 °C inlet water temperature.