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| Fig. 1: Pounds of CO2 emitted per passenger mile, by mode of transportation, in the US in 2019. [4] (Courtesy of the CBO) |
In an era where climate change and environmental sustainability have emerged as paramount global concerns, the exploration of energy-efficient and environmentally friendly mass transit systems has taken center stage in urban planning and development. Among the myriad options available, metro rail systems have gained widespread recognition for their potential to reduce carbon emissions and alleviate the traffic congestion that plagues many metropolitan areas. This report delves into the intricate details of energy usage within metro rail systems, focusing on the vibrant and bustling city of Madrid, Spain, as a compelling case study. Madrid's metro system is renowned for its extensive coverage and efficiency, making it an ideal subject for understanding the complexities of energy consumption and the implications for sustainable urban mobility. In the following sections of this report, we will explore how metro systems use energy, look at estimates for the number of passenger miles it delivers, and energy-saving initiatives of Madrid's metro rail system.
Metro rail systems are a marvel of engineering and urban planning, designed to efficiently transport millions of passengers daily while minimizing energy consumption and environmental impact. These systems typically consist of a network of underground and elevated railway tracks, electrified using a third rail or overhead wires to power the trains. In the case of Madrid's metro, which spans over 294 kilometers and features 302 stations, the trains are predominantly electrically powered. [1] Each train car is equipped with electric motors that drive the wheels, and the electricity needed to propel the trains is primarily drawn from the local power grid. Madrid's metro, for instance, consumes an estimated 562 GWh (gigawatt-hours) of electricity annually, equivalent to the power consumption of approximately 110,000 households for a year, or 2.02 × 1015 Joules. [2]
Metro rail systems operate through a complex interplay of mechanics and energy utilization. These systems typically comprise electric trains running on dedicated tracks, and understanding how energy drives these systems is integral to their sustainability. In Madrid's metro, electricity plays a pivotal role, primarily supplied through overhead wires or third rails. The electric motors, located on the train cars' undercarriages, are the workhorses of the system. They convert the electrical energy received from the overhead wires or third rail into mechanical energy, driving the train's wheels and propelling it forward.
As the metro train sets into motion, energy consumption becomes particularly pronounced during acceleration, which requires a substantial power input. The energy required for these dynamic movements depends on factors like passenger load, track conditions, and the network's topography. However, one of the noteworthy features of metro rail systems is their regenerative braking systems. These systems come into play when the train decelerates or brakes, recapturing kinetic energy and converting it back into electricity. [3] This reclaimed energy can then be used to power other parts of the train or returned to the grid, significantly enhancing the system's overall energy efficiency.
In addition to propulsion, energy is expended on various accessories and services critical for passenger safety and comfort. Stations and trains necessitate electrical power for lighting, air conditioning, information systems, and other amenities. These electrical loads, while essential, contribute to the overall energy consumption of the metro system. Understanding this intricate balance between energy supply, propulsion, and auxiliary systems is key to optimizing the energy efficiency of metro rail networks like Madrid's, ultimately contributing to more sustainable urban transportation solutions.
Evaluating the relative carbon efficiency of different transportation modes involves considering metrics that quantify their environmental impact. Metro rail systems, such as Madrid's, demonstrate commendable carbon efficiency, emitting an average of approximately 0.17 lbs of CO2 equivalent per passenger-kilometer. This efficiency stems from their reliance on electric power, which can be sourced from low-carbon or renewable energy, significantly reducing their carbon footprint. In contrast, personal vehicles, particularly those powered by gasoline, tend to be less carbon-efficient, emitting approximately 0.47 lbs of CO2 equivalent per passenger-kilometer. The higher emissions in this case result from the direct combustion of fossil fuels. Electric buses and trams, like metro systems, exhibit strong carbon efficiency when powered by clean electricity sources, typically emitting less than 0.35 lbs of CO2 equivalent per passenger-kilometer. (See Fig. 1.)
In summary, metro rail systems, as exemplified by Madrid's case study, offer a pragmatic solution to foster sustainability dand environmentally responsible urban development. As cities expand and grapple with the pressing challenges of traffic congestion and emissions, investments in energy-efficient metro rail networks hold the promise of curbing carbon footprints and enhancing urban living standards. The lessons drawn from these systems extend beyond Madrid's borders, providing valuable insights and inspiration for cities globally as they endeavor to forge cleaner, more sustainable paths in urban transportation.
© Sebastian Aguirre. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All dother rights, including commercial rights, are reserved to the author.
[1] "Madrid Metro Map," Metro de Madrid, September 2023.
[2] "Sustainability Report 2022," Metro de Madrid, 2022.
[3] "This is Light Rail," Transportation Research Board, U.S. National Research Council, E-Circular E-C033, July 2001.
[4] "Emissions of Carbon Dioxide in the Transportation Sector," U.S. Congressional Budget Office, December 2022, p. 7.