Futuristic cityscape powered by underground subway heat.

Underground Heat: How Subway Systems Can Power Our Cities Sustainably

Reese Marlowe in Climate & Environment July 2025 • 4 min read.

"Discover innovative ways urban environments are leveraging subway systems to recover heat energy, reducing carbon emissions and creating sustainable energy solutions."

Cities around the world are facing increasing pressure to reduce their carbon footprint and transition to more sustainable energy sources. One unexpected solution lies beneath our feet: the vast network of underground railways. These subway systems, traditionally seen as consumers of energy, are now being recognized for their potential to generate and recover heat energy, offering a unique opportunity to power our cities more sustainably.

The European Council set a target in March 2007 to reduce greenhouse gas emissions by 20% by 2020, with the UK Government pushing further to achieve an 80% reduction by 2050 compared to 1990 levels. A significant portion of fossil fuels are used for low-temperature heating in buildings, making the decarbonization of the heat sector essential to achieving these goals. Cities across Europe are now focusing on reducing heating-related carbon emissions.

London, for example, aims to generate 25% of its heat and power through local, decentralized energy systems by 2025. Renewable energy opportunities, including secondary sources like sewers, electricity cable tunnels, and underground railways (URs), are gaining traction. These urban infrastructures, often located near high heat demand areas, could provide year-round heat supplies. By harnessing the latent energy of our existing subway systems, we can make significant strides toward a greener, more sustainable future.

Tapping into the Underground Heat Source: How Does It Work?

Futuristic cityscape powered by underground subway heat.

Underground railways generate substantial heat due to train operations, braking systems, and ventilation equipment. This heat, often released into the surrounding soil, can be captured and repurposed using various techniques. Ground Source Heat Pumps (GSHPs) are at the forefront of this innovative approach, offering an efficient method to extract and utilize this surplus heat energy.

GSHPs can be strategically installed near underground railway tunnels to harness the thermal energy. These systems circulate a fluid through a network of pipes, absorbing heat from the ground and transferring it to buildings for heating or cooling. By integrating GSHPs with underground railways, cities can reduce their reliance on traditional energy sources and lower carbon emissions.

Here are a few techniques for heat recovery from URs:

Embedded Tunnel Liner Heat Exchangers: These are ideal for newly constructed tunnels, involving closed-loop water-filled pipework within the tunnel segments to extract heat.
Ventilation System Heat Recovery: This method uses heat exchangers built into railway ventilation shafts to capture heat from the air. A water circuit then transfers the heat to a heat pump, which can connect to nearby buildings or district heating systems.
Ground Source Heat Pumps (GSHP): Extracts heat from the soil surrounding UR tunnels and can then be used by the city.

The potential benefits extend beyond reducing carbon emissions. Recovering heat from underground railways can also improve the efficiency and reliability of subway operations. By removing excess heat, these systems can help regulate tunnel temperatures, reduce equipment strain, and enhance overall performance.

A Sustainable Future Below Our Feet

The integration of underground railways with ground source heat pumps represents a significant step towards creating more sustainable and resilient urban environments. By recognizing the potential of these subterranean networks as sources of renewable energy, cities can unlock new opportunities to reduce their carbon footprint, improve energy efficiency, and create a greener future for all. As technology advances and infrastructure evolves, expect to see more cities tapping into this readily available heat source beneath our feet.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

Everything You Need To Know

How can subway systems contribute to urban sustainability?

Subway systems, traditionally seen as energy consumers, possess the capability to generate and recover heat energy. This recovered heat can be utilized as a sustainable energy source for cities, aiding in the reduction of their carbon footprint. Techniques like Ground Source Heat Pumps, Embedded Tunnel Liner Heat Exchangers and Ventilation System Heat Recovery make this possible. While the text outlines several pathways, it does not talk about the specific cost or challenges in implementing these different systems, or compare their efficacies. These implementations contributes to the global drive for greener energy.

What is the aim of the European Council regarding greenhouse gas emissions, and how do underground railways play a role in achieving this goal?

The European Council set a target to reduce greenhouse gas emissions by 20% by 2020, with the UK aiming for an 80% reduction by 2050 compared to 1990 levels. Underground railways (URs) contribute by offering renewable energy opportunities, specifically by capturing and repurposing heat generated from train operations, braking systems, and ventilation equipment. Although the text mentions the European Council target, it does not detail specific policies or incentives implemented to encourage the adoption of heat recovery systems in URs, or how these policies correlate to achieving those overall environmental goals. London, for instance, aims to generate 25% of its heat and power through local, decentralized energy systems by 2025.

What are Ground Source Heat Pumps (GSHPs), and how are they used in conjunction with underground railways?

Ground Source Heat Pumps (GSHPs) are efficient systems used to extract and utilize surplus heat energy. Near underground railway tunnels, GSHPs circulate a fluid through pipes, absorbing heat from the ground and transferring it to buildings for heating or cooling. This integration reduces reliance on traditional energy sources and lowers carbon emissions. The text does not mention the specific types of GSHPs that are most effective in these environments, or the maintenance and operational costs associated with these systems, which would be critical for practical implementation considerations.

Besides reducing carbon emissions, what other benefits can be derived from recovering heat from underground railways?

Recovering heat from underground railways can improve the efficiency and reliability of subway operations. Removing excess heat helps regulate tunnel temperatures, which reduces equipment strain and enhances overall performance. The discussion could be expanded to discuss the impacts on passenger comfort and safety, or the potential to integrate these heat recovery systems with smart city infrastructure for more holistic urban energy management.

Can you explain the different methods for heat recovery from Underground Railways (URs)?

There are a few techniques for heat recovery from URs: Embedded Tunnel Liner Heat Exchangers (ideal for new tunnels using water-filled pipework within tunnel segments), Ventilation System Heat Recovery (using heat exchangers in ventilation shafts to capture heat and transfer it via a water circuit to heat pumps), and Ground Source Heat Pumps (GSHP) which extracts heat from the soil surrounding UR tunnels. This heat can be used by the city. However, the explanation does not include a comparison of the costs, benefits, and practical implementation challenges of each method. Also, a real-world case study demonstrating the application and effectiveness of these technologies would enhance understanding.