Optimization of hydronic heating pavement : Analysis of pipe layouts and heat transfer fluids for greater energy and cost efficiency

In dense urban areas, public spaces play an important role as meeting points for the people living there. The attractiveness and safety of these places are crucial for the overall quality of life in cities. During winter, hydronic heating pavement (HHP) systems can improve the usability of these areas by keeping the surfaces free from ice and snow. A more pleasant and accessible environment is created when slippery conditions are reduced. At the same time, the need for snow removal vehicles decreases which minimize the disruption to pedestrian traffic. The HHP system utilizes embedded pipes in the ground with a circulating heat transfer fluid in order to raise ground temperature and prevent snow accumulation. Different pipe patterns and heat transfer fluids affect the overall performance of the HHP system in terms of snow melting efficiency, surface temperature and energy usage. The aim of this study is to investigate the performance of two different pipe patterns, sinusoidal and semi spiral, as well as the effects of different heat transfer fluids. COMSOL Multiphysics was used to create a 3D model and simulate how different pipe patterns and heat transfer fluids influence the performance of the HHP system. The snow was not directly represented in the model, instead it was approximated by calculating the energy required to melt a predefined snow mass based on the surface heat flux delivered by the system. The results show that the sinusoidal pattern produced a more uneven surface temperature distribution compared to that of the semi spiral pattern. When the sinusoidal pattern was used, areas near the inlet showed higher surface temperatures while regions further from the inlet showed lower temperatures due to the cooling of the heat transfer fluid. Unlike the sinusoidal pattern, the semi spiral pattern maintained a more uniform surface temperature across the entire area. This affected snow melting performance, the sinusoidal pattern resulted in uneven snow melting over the surface with a time difference of approximately five minutes. The semi spiral pattern showed even snow melting over the entire surface. The heat transfer fluids evaluated in this study were propylene glycol, Kilfrost Geo, ammonium hydroxide, and water. Water and ammonium hydroxide required the lowest volume flow rates, 0,19 and 0,21 l/s respectively. Propylene glycol and Kilfrost Geo required the highest, 0,23 and 0,30l/s. The required volume flow rate is dependent on the fluids specific heat capacity and density. Kilfrost Geo had the highest pressure drop, at around 140 kPa along the pipe, whereas ammonium hydroxide and water had the lowest around 40 kPa. The pumping cost followed the same trend with Kilfrost Geo being the most expensive with an annual cost of 8800 SEK, and ammonium hydroxide the cheapest, more than five times cheaper with an annual cost of 1600 SEK. However, the pump cost was found to be negligible compared to the total system costs. Kilfrost Geo with the highest annual cost, only accounts for less than 1% of the systems overall operational cost. The choice between the sinusoidal and semi spiral pipe patterns should be guided by the specific need and requirements of the design area. The semi spiral pattern offers a better temperature distribution if uniform snow melting is prioritized. However its higher installation cost must be considered and given that the difference in melting time between the patterns is relatively small, cost efficiency may be prioritized. The selection of heat transfer fluid may be more influenced by environmental aspects as the differences in operational cost were found to be negligible.