An efficient and sustainable approach for cooling underground substations

With rapid rates of urbanisation and significant improvements in construction technologies, the number of subsurface infrastructure projects has drastically increased in recent years. In addition to their primary functions, these structures have shown great potential as energy geo-structures, exchanging heat with the ground to heat and cool spaces. Given their large contact area with the ground, energy tunnels are proving to be a sustainable source of thermal energy for effectively heating under- and above-ground spaces. However, their efficiency in cooling-dominated conditions has not yet been adequately studied. This paper tackles one of the key challenges regarding transport tunnels: sustainable cooling of underground substations, by introducing an efficient and costeffective cooling method. The method takes advantage of airflow in the tunnels and relatively stable ground temperatures and involves heat exchangers in the form of water-filled high-density polyethylene (HDPE) pipes being integrated into the tunnel space. The efficiency of the proposed system is numerically assessed by analysing the spatio-temporal variations of temperature in the ground, substation, tunnel air, tunnel structure and heat exchangers caused by continuous heat rejection from the substations. A detailed 3D finite element heat and mass transport model is used, and alternative placements of heat exchangers are investigated. Results show that heat exchangers placed on the tunnel lining, and hence exposed to the tunnel airflow, could efficiently supply a substation's cooling demand, without significantly increasing the temperature of the tunnel air or the ground. The substantial economic benefits of this cooling system compared to a conventional cooling system is also demonstrated.