Generation of electricity through thermal and pressure-driven airflows in road tunnels and ventilation shafts

Abstract Background Tunnels are an important part of an efficient transport network in mountain regions as well as in densely populated urban areas. However, tunnel operation requires considerable amounts of energy. This results in high operating costs and in a significant CO 2 footprint. Owing to the high energy costs and the need to reduce the CO 2 footprint, alternative options for providing electricity need to be considered. This paper assesses the possibility of operating existing tunnel ventilation systems in reverse to generate electricity when thermal or pressure-driven air flows are present. In order to put such considerations on a sound basis, the Austrian Science Funds initiated a research project. The work presented here is a feasibility study designed to provide basic information for future, more detailed, assessments of the idea. Results An analysis was carried out for standard 2-lane road tunnels with various lengths and meteorological pressure differences. As a first step in evaluating the potential of the, pressure differences between portals, or between the foot and the top of ventilation shafts, were recorded in 28 Austrian road tunnels. Tunnels that exhibited a relatively high meteorologically-induced pressure difference were then selected for further consideration. After taking account of relevant aerodynamic conditions and expected efficiencies in the production chain, the annual potential for electricity generation was estimated. It is found that the direct use of existing fans as turbines for generating electricity is not suitable. This is mainly due to the different requirements in these two operating modes. The powers that could be generated by using the fans as turbines in pressure-driven or buoyancy-driven mode are orders of magnitude smaller than those required for handling fires in ventilation mode. However, if the effort required to install additional turbines in parallel with the ventilation fans would be acceptable, there could be useful potential in tunnels with tall shafts. Conclusions If this idea is to be pursued further, a detailed study tailored to the specific application will be required, taking into account not only technical feasibility, but also cost-benefit aspects.