Take advantage of the residual heat of the urban subsoil

Cities have great potential to implement innovative solutions that improve energy efficiency and take advantage of non-conventional energy sources, thus establishing the foundations for a more sustainable urban model.

With this conviction, researchers from the Polytechnic University of Madrid (UPM), in collaboration with the Madrid Subterra Association, in Spain, are promoting the exploration and exploitation of the potential for clean and renewable energy in the urban subsoil of Madrid, as an experience pilot that will then guide similar initiatives in other cities. The main results of this line of research and development obtained so far indicate that energy from residential wastewater can serve to save more than 50% of the heating energy bill of said homes; or that more than a thousand people can be supplied with hot water for a year with the waste heat from small underground metropolitan railway (metro) stations.

It is now widely recognized that carbon dioxide (CO2) emissions can have devastating effects on the environment and people. Human activity, especially the burning of fossil fuels that generate greenhouse gases, is the main cause of this serious threat. This problem is particularly critical in urban areas, where high dependence on fossil fuels makes the population especially vulnerable to the effects of global climate change.

Therefore, taking advantage of the waste heat associated with the underground infrastructure of cities is a good way to help face this challenge.

“By residual heat, we mean heat that is generated and not used, therefore, it is ‘free’ heat that must begin to be used,” says UPM researcher Susana Sánchez Orgaz. This heat is found in the subway, in water pipes and in road traffic tunnels, and can be used to air-condition buildings or electrical rooms in underground infrastructure, as well as to provide domestic hot water in buildings.

Ventilation grille of the Madrid Metro. On these grates you can see a temperature higher than that of the surrounding outside environment. (Photo: UPM)

The analyzes carried out by both entities include the estimation of the energy resource available in Madrid’s underground infrastructure, the application of this resource in the infrastructure itself or in nearby buildings, both public and private, as well as estimates of CO2 emissions savings, without forget the associated costs.

The main conclusion revealed by these analyzes points to the high possibilities of using this residual energy to reduce carbon dioxide emissions in large cities, improving their energy efficiency.

“We have also shown that depending on the particular typology of the tunnel or the metro station this use will be different, in the same way that a solar panel does not produce the same energy in Seville or in Lugo, so this form of energy use must be evaluated for each specific case” conclude the authors of the study.

The study is titled “An Approach to Capturing Residual Energy From the Subway System: Methodological Considerations.” And it has been published in the academic journal Journal of Energy Resources Technology. (Source: UPM)