Subway tunnel heat-exchangers could heat and cool thousands of nearby apartments

In Numerical investigation of the convection heat transfer driven by airflows in underground tunnels (Sci-Hub mirror), a group of engineers from L'Ecole Polytechnique Fédérale de Lausanne and the Swiss Federal Institute of Technology propose that low-cost heat-exchangers placed in subway tunnels could be used to heat and cool homes essentially for free (the system would last 50-100 years, and the pumps would need replacing every 25 years). Read the rest