Energy tunnels allow the harvesting of untapped heat at shallow depths in the underground to meet the thermal energy requirements of buildings and infrastructures over large areas. Such heat can derive from two sources: the ground surrounding energy tunnels and the air circulating in the environment internal to these tunnels. To date, various investigations have addressed the role of ground characteristics on the heat exchange potential of energy tunnels, as they significantly influence the amount of geothermal energy that these geostructures can harvest. Despite the comparable role of airflow characteristics on the amount of aerothermal energy that energy tunnels can harvest, no extensive analysis of this problem has been reported before this study. To fill in this knowledge gap, this paper investigates the heat exchange potential of energy tunnels for a broad range of internal airflow characteristics. From this perspective, the work specifically provides: (i) the first charts, validated against representative experimental evidence, summarizing the thermal power that energy tunnels can harvest per unit surface for different convection heat transfer coefficients and temperatures associated with internal airflows, as well as undisturbed temperatures and effective thermal conductivities of the ground; and (ii) the analysis of an energy tunnel at the regional scale, based on the application of the developed charts and the use of a large hydrogeological dataset. Based on the results of this study, it is concluded that the internal airflow characteristics significantly influence the harvesting of aerothermal energy through energy tunnels. Together with the ground characteristics, which can markedly vary along energy tunnel applications at the regional scale, internal airflow characteristics rule the thermal power that can be harvested through such heat exchangers, deserving thorough quantifications for any adequate energy performance assessment. (c) 2020 Elsevier Ltd. All rights reserved.
