Numerical study of the thermal performance of three different energy micropiles in hot climates

Energy micropiles have recently emerged as a viable solution to the provision of clean and renewable geothermal energy especially in densely populated urban areas. This innovative technology has been found to be advantageous over piles primarily because of their small sizes and ease of installation. However, their small size also poses a challenge when energy exchange is concerned as only limited heat exchanging pipe configurations can fit in the micropile section. Moreover, the available heat exchange area is small compared to piles. In this paper, a numerical study was conducted to analyze three types of energy micropiles with different pipe configurations and structural materials in summer conditions. The results show that the coaxial energy micropiles provided the best thermal performance compared to the grouted U-pipe micropile. More specifically, thermal extraction rates of about 59.5 W/m, 55.9 W/m and 26.5 W/m were obtained with the steel coaxial, grouted coaxial and grouted U-pipe energy micropiles, respectively, at the end of the 60-day period. Additionally, the impact of the energy micropiles on the ground temperature was observed up to 4 m from the pile centre, where the ground increased by a maximum of about 1 °C, 0.73 °C and 0.5 °C with the grouted U-pipe, steel coaxial and grouted coaxial micropile respectively. This highlights the need to sufficiently space out energy micropiles. Finally, an intermittent mode of operation was found to enhance the thermal performance by allowing the ground to recover its initial temperature. • Energy micropiles provide renewable heating and cooling for dense urban areas. • Coaxial pipes provide higher thermal power compared to U-pipes. • Flow conditions and storage capacity considerably impact the thermal performance. • Ground temperature changes are observed up to 4 m from micropile centre. • Intermittent operation improves thermal efficiency of energy micropiles.