3D numerical modelling and validation of an asphalt solar collector

The extensive use of fossil fuels to produce energy is a big problem today which causes many environmental hazards due to pollution and global warming. Therefore, the need for energy resources that has minimal impact on climate change is growing faster. Many renewable energy sources are proposing to produce thermal energy from solar energy, among which harvesting solar energy by means of an asphalt collector is drawing attention due to its huge potential. Asphalt solar collector has received special attention in the last few years due to the suitable heat absorbing properties of asphalt surfaces as well as the millions of kilometers of road built worldwide with potential energy harvesting capabilities.

Researchers at University of Cantabria in Spain: Alejandro Alonso-Estebanez, Pablo Pascual-Muñoz, Jose Luis Sampedro-Garcia, and Daniel Castro-Fresno developed a laboratory scale asphalt solar collector formed by different slabs which is characterized by applying numerical techniques. The thermal performance of the asphalt solar collector was determined for three values of heat exchange fluid flow rate. Their research work is now published in journal, Applied Thermal Engineering.

The authors used Computational Fluid Dynamic model to analyze the thermal response of the collector for the different parameters which include flow rate, solar irradiance, size and thickness. An unsteady 3D model was also used which simultaneously resolves the heat transfer process near the surface of the collector and also within the collector by applying Finite Volume Method.

A 2×2 model is presented which consist of four slabs. The overall dimensions of slabs makes a total area 0.22m² of the collector. A U-shaped copper pipe was embedded at a depth of 25mm from the surface of slabs. The water flows through the pipe removing the energy collected by asphalt slabs. Asphalt mixture contains conventional binder and a continuous fraction of limestone with 8 mm maximum aggregate size. Four bulbs having radiation spectrum similar to sun was used for each slab in 2×2 manner. For the measurement of irradiance on the surface of the collector, a pyranometer was used. Three different water flow rates of 1.0, 1.5 and 2.0L/min were considered and duration of tests was set to 8h.

The authors achieved similar results in their experimental and numerical methods with errors under 10% for the temperature of gradient, heat collection and thermal performance per unit of water flow rate. They also found that solar radiation reaching the surface, the heat exchange fluid flow rate and size are key parameters in designing the asphalt solar collector. Moreover, the thickness of slabs for storing surplus energy has not been proved to have a significant influence on their thermal performance, at least for the range studied.

 The Computational Fluid Dynamic model developed by scientists of the GITECO research group of the University of Cantabria will enable researchers to further refine the results to make the solar harvesting technique practical for domestic as well as industrial purpose and help to provide a cost-effective heating system without using fossil fuels.