Why are New York’s subways so hot?
And can we do anything to cool them down?
Ah, subways. When you live in New York, commuting by subway is not a choice, but a way of life. It’s an obligatory love-hate relationship with a sprawling, complex, and aging1 marvel. During the summer, though, the balance tips decidedly toward hate. The platforms are unbearably hot, turning a daily commute into a descent into a subterranean oven.
How hot are they, really?
I was surprised to find a lack of recent, publicly available data on subway platform temperatures2. While historical measurements from the Regional Planning Association do exist, they aren’t very extensive. This data gap sent me on a short but sweaty quest… As any person with a thermometer would do, I decided to take my own measurements. I focused on morning and evening, since the RPA measurements were mostly at midday, and was especially curious to see if the temperature varied much with cooler outdoor temperatures.
The table below compiles my readings with some that I found online345. I acknowledge that this isn’t the most rigorous sample size, but I do think it illustrates that there is a problem.
Why are they hot?
When I searched for a quantitative analysis of subway heat sources, I was again met with limited success. While many qualitative descriptions exist, I could find only one study (from 2003!) that produced a mathematical model for heat transfer in subway systems6. The energy balance equations in the paper are a great starting point, but they don’t directly apply to our modern system. With that in mind, here is a breakdown of what makes our subways so hot.
According to the study, the top contributor of heat (85%) is the braking mechanism. The friction to stop the train converts its kinetic energy to thermal energy. If you would, picture a subway system that operates perfectly—no delays, trains always on time. In this (hard-to-imagine) scenario, trains would only need to brake when they stop at a platform. But, as you know, this does not describe our system. When lines become congested due to delays, trains are forced into a cycle of stopping and starting. This means more braking, which means more heat.
Heat loss from the train cars themselves contribute the next highest amount of heat (13% according to the study). Inside the cars, heat is generated by body heat and lighting. I would theorize that this share may be much higher today, as the air conditioning vents hot air directly onto the platform. This makes me want to build my own model for heat transfer in NYC subways specifically…
On the platform, the sources of heat are similar to those inside the train—lighting and the body heat from millions of commuters. These are compounded by the heat generated by signaling technology and displays.
There aren’t many places for this heat to go. There are some ventilation shafts in the tunnels, but there is a lack of effective airflow in most stations themselves. So the heat builds up in the tunnels, warming up the surrounding thermal mass (rock and earth). This creates another cycle where the tunnels and platforms become heat reservoirs, trapping the thermal energy and making it even hotter. The heat has nowhere to escape, and so, it heats you.
What can we do about it?
I’ll explore this question in more detail in future writing, but I do have a few ideas based on my research thus far.
The first approach is to reduce the amount of heat generated in the first place. This would require prioritizing functional improvements to the system as a whole. A more efficient, on-time subway network would naturally generate less heat, as trains would spend less time in the vicious cycle of stopping and starting. By investing in modern signaling technology and operational improvements, we can create a system that is not only more reliable for riders but is also less taxing on its own infrastructure.
The second approach, a prospect I find extremely exciting, is to leverage the heat that is being generated. Regenerative braking technology can be used to convert the kinetic energy of a braking train into electrical energy, capturing it in a battery to power the trains or other parts of the subway system. Additionally, thermal networks can be built to capture the excess heat and distribute it to heat water and spaces aboveground. Imagine a system where the heat from your commute helps to warm a nearby apartment building! This is the kind of thinking that can transform a liability into a resource, and a problem into a solution.
Did you know that most of the subways were built before 1940? Crazy, right? More on that in a future post.
I look forward to this project being completed! https://www.newyorklab.co/