Ice free roads using hydronic heating pavement with low temperature: Thermal properties of asphalt concretes and numerical simulations
Sammanfattning: A traditional method to mitigate the slippery conditions of a road is to spread out salt and sand on the road surface. However, salting causes corrosion on the road infrastructures, damage to surrounding vegetation and salification of fresh water. Hence, there is a need for alternative solutions to mitigate the slippery conditions. A renewable alternative is to use a Hydronic Heating Pavement (HHP). The HHP system consists of embedded pipes in the road. A fluid as thermal energy carrier circulates through the pipes. During sunny days, when the road surface is warm, the energy is harvested and saved in seasonal thermal energy storages. During cold days, the warm fluid from the storage is pumped back to the pipes to increase the surface temperature.
The aim of this study is to investigate the performance of the HHP system for harvesting energy from the road surface during summer and anti-icing the road surface during winter. In the HHP system, the main part of the heat transfer occurs between the embedded pipes and the road surface. Hence, it is of importance to determine the thermal properties of the road materials. The thermal properties of a few Swedish typical asphalt concretes, used to construct the asphalt road pavements, were experimentally measured by the Transient Plane Source (TPS) method. The accuracy of the measurements of the TPS method was examined using different sensor sizes. Moreover, in order to investigate the effects of the different design parameters of asphalt concrete such as the types of aggregates on the thermal properties, a numerical model of asphalt concrete was developed. Comparing the obtained thermal properties by the numerical model and the experimental measurements exhibited that the relative error between two methods is in the range of 2% to 10%.
Furthermore, in order to investigate the performance of the HHP system, a two-dimensional numerical model of the HHP system was developed based on the Finite Element Method (FEM). The developed numerical model was validated by two cases: (i) for the road without pipes, using a one year measured data and (ii) for the road with the embedded pipes, using analytical solutions. The validation results for the road without pipes showed that the annual mean difference of the temperature at the depth of 10 cm from the road surface is 0.1°C with the standard deviation of 1.15°C between the measured data and the numerically predicted temperature. The validation results for the road with the embedded pipes showed that the maximum relative error of the thermal resistance between the pipe and surface is less than 5% between the obtained results from the numerical model and the analytical solution.
In order to investigate the harvesting and anti-icing performance of the HHP system, the climate data were selected from Östersund in middle of Sweden, where there is an ongoing test site project to construct the HHP system in 2017. It was assumed that when the road surface temperature was lower than 0°C, the heating was started to keep the surface temperature higher than the dew point temperature. The heating was stopped when the air temperature was below -12°C. Based on the climate data, 90% of the slippery conditions on the road surface, due to condensation, occurred when the air temperature was above -12°C. Furthermore, the air temperature was above 8°C during 70% of the warm days (from the first of May to the end of September). The air temperature of 8°C was taken into account to start harvesting energy from the road surface. The results showed that by maintaining constant fluid temperature of 6°C through the pipes, 100 mm distance between the pipes and 3.5 m width of the road, the annual required energy for anti-icing the road surface is 356 kWh/(m·year) and the annual harvested energy from the road surface was 1,047 kWh/(m·year). Enhancing the thermal conductivity of road layers improves the harvesting and anti-icing performances of the HHP system.
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