Exploitation of drainage water heat: A novel solution experimented at the Brenner Base Tunnel

•This paper illustrates a new system to exploit drainage water heat from tunnels.•The heat exchange rate between the prototype and the drainage water is quantified.•The conducted tests over the prototype verified the effective exploitable heat.•The approach was implemented during the construction of the Brenner Base Tunnel.•A generalization of the design method to other underground situations is proposed.