Hot Air, Smart Cities

Hi everyone,

Cities are full of invisible systems, such as pipes under our feet, wires behind the walls, and energy, often in the form of heat, escaping into the air without a second thought.

But what if that heat could be captured and reused? What if your subway ride helped warm your neighbour’s apartment? Or the exhaust from a data centre powered hot showers down the street?

This week, I’m exploring one of the most overlooked resources in the modern city: waste heat, and the role it can play in a more circular, climate-smart urban future.

Where Heat Goes to Die

Every day, our cities throw away staggering amounts of heat from subways, server rooms, factories, and sewage plants. It’s called waste heat, and most of it quite literally goes up in smoke.

The International Energy Agency estimates that up to 60% of global energy becomes waste heat, and in urban settings, a huge portion is considered “low-grade”, where it’s warm, but not hot enough to be easily reused.

In Europe alone, more heat is wasted than is used to heat buildings. That’s a wild stat, and one that’s not just about inefficiency, but about missed opportunity.

Turning Waste into Warmth

That’s starting to change. Around the world, cities are learning how to recapture this lost energy and put it to work.

In Paris, heat from metro tunnels now warms over 20 public housing buildings. In Stockholm, more than 30% of district heating comes from treated wastewater. And in Helsinki, excess heat from data centres is piped directly into homes, creating a feedback loop that makes the city warmer without burning more fuel.

The tech isn’t complicated: heat exchangers, underground piping, and smart heat pumps do the job. But the concept is powerful; let’s reuse what we already have.

The Circular Shift

These projects are part of a larger movement to shift toward circular cities. Where energy, water, and materials cycle through systems instead of being discarded at the end of a process.

Waste heat is a perfect example of the way this can run. It’s an inevitable byproduct of industries and infrastructure, so why not turn it into a localised asset?

The EU has invested heavily in this space, supporting urban pilots through programs like RUSEHEAT. And globally, the concept of one system feeding another is gaining traction.

Not So Hot (Yet)

The path to scalable heat reuse isn’t simple.

Many cities lack the infrastructure, especially district heating networks. Retrofitting old buildings is costly and time-consuming. Heat doesn’t travel far efficiently, so location matters. And regulatory questions, like who owns the heat, or whether it can be sold, can stall even the best intentions and ideas.

Even when captured, low-grade heat often needs a boost from electricity to become usable. But that challenge is shrinking as heat pump efficiency improves.

What makes waste heat so compelling is its abundance around the world. It’s already all around us; we just haven’t been tapping into it.

In 2023, McKinsey named waste heat recovery one of the top 10 scalable energy strategies for cities. And as energy prices rise and grids strain under climate pressure, making smarter use of existing energy is going to become less of a novelty and more of a necessity.

Ultimately, this is a mindset shift. Cities don’t just need to build more infrastructure; they also need to notice what’s already available and redirect the flow.

Catch you next week,
James