Hand fans might be the only way to cool off in subway stations. Robert Caplin/Bloomberg via Getty Images

If the sweat running down the back of your knee hadn’t made you aware, the Regional Plan Association wanted to remind you last week that it is very hot in the subway. It’s not exactly news that the subway gets hot in the summer. But the RPA’s press release wanted to point out that it’s likely only going to get worse as climate change causes more severe heat waves.

On August 9, the outside temperature was 86 degrees, but the RPA, an urban planning nonprofit agency focused on the tristate area, diagnosed the downtown 4/5/6 platform at Union Square with a fever of 104. Most of the sixteen platforms it measured that day had temperatures above 90 degrees.

The average temperature in New York increased 3.4 degrees from 1990 to 2013, the RPA notes. “If we don’t tackle the issue of heat in the subway, the public health impacts will continue to worsen as our planet and our city warms.” The design and age of the city’s hottest stations make traditional options — improved ventilation or climate control — too costly to consider for now. So, is there anything the MTA can actually do to cool down the platforms?

First, it’s worth noting not all subway platforms are swelteringly hot. In July 2015, WNYC measured the temperature at 103 platforms during that summer’s second heat wave; many stations were hotter than the ambient air temperature at that time of 96 degrees, but some were cooler. For example, the Brooklyn G train stops WNYC measured — Nassau Avenue, Greenpoint Avenue, and Metropolitan Avenue — all had temperatures below 80 degrees. Cortlandt Street and Rector Street were downright pleasant at 74 and 71 degrees, respectively. Not coincidentally, comparatively few trains run through these stations.

Generally speaking, the more trains that run through a station, the hotter the station will be. In part, this happens because a train’s air-conditioning unit expels heat. But a lot of heat also comes from a train’s brakes. A tremendous amount of energy is needed to slow down a train of ten cars, which can weigh between 700,000 and 900,000 pounds, depending on the age of the car. A lot of that brake energy gets turned into heat, which is released as trains approach and enter stations. And the more times a train stops, the more heat it creates, so the increasing number of delays and starting and stopping in tunnels over the last several years has further exacerbated the problem.

Fortunately, technology exists that can help mitigate the brake heat problem. Anyone who has ever paid attention to the dashboard of a hybrid car might recognize regenerative braking, a process whereby some of the kinetic energy created by braking is captured and then returned into an active circuit for reuse. Not only does regenerative braking reduce the amount of heat, but it also lowers energy costs. For example, in 2015 London officials discovered via a five-week trial that regenerative energy could lower the London Underground’s energy bill by 5 percent. Studies also suggest that this braking system could lower the Underground’s ambient and platform temperatures by 50 percent (a much lower percentage of the Underground’s cars are air-conditioned, as the U.K.’s average high temperature in the summer is in the low 70s. The Underground’s stations are notoriously hot, although that is caused more by a lack of ventilation in stations that are buried deep).

The MTA is familiar with regenerative braking. In 2007, then–MTA CEO Elliot Sander created the Blue Ribbon Commission on Sustainability and the MTA, which was expected to offer suggestions on how to reduce the authority’s carbon footprint and promote green practices in anticipated future climate change–based challenges. In the final report that was released in 2009, two of the commission’s key suggestions were to implement regenerative braking and to reduce the weight of approximately 50 percent of subway cars (the remaining cars were too old to be used with the technology). Although nothing written in the report mentioned regenerative braking in the context of hot stations, one estimate said that in the most aggressive scenario the MTA could regenerate 11.25 kilowatt-hours every time a ten-car train stopped. (For context: The energy captured from just eighty of those stops, or two round-trip 7 train runs, could power the average U.S. household for one month.) The commission estimated such a program would cost $805 million over a forty-year period — mostly in battery replacement costs — which is less than the MTA is now spending to “enhance” twenty subway stations.

But implementing regenerative braking has never yielded such benefits in New York. According to an RPA report issued in June, which echoed the Blue Ribbon Commission’s 2009 call for regenerative braking, the MTA’s current power system doesn’t allow the energy created from braking to be put back into the third rail. The Paris Metro, by contrast, has been using some form of regenerative braking since 1977, and today the line consumes 30 percent less traction energy thanks to the technology, the RPA reported.

Although 57 percent of subway cars are equipped with the necessary technology to implement this style of braking, “most of the regenerative energy is currently being wasted,” as it simply turns into heat, Ahmed Mohamed, a City College electrical engineering professor, wrote in a report last year after conducting a study that was funded by a $200,000 grant from Con Edison to examine the issue. (Mohamed did not respond to an inquiry from the Voice.)

The MTA’s official position on hot subway stations is that it can’t install air-conditioning in the stations and is more focused on improving system reliability so that passengers aren’t waiting on platforms for so long. “Climate control didn’t exist when the subway system was built more than a century ago, and the air-conditioning units on trains discharge a lot of heat into tunnels and stations,” MTA spokesman Andrei Berman told the Voice. “We’re working hard to reduce delays so we can get our customers off the platforms and on their way in an air-conditioned car.” The MTA did not respond to follow-up questions regarding specific technologies or approaches, such as the possible use of regenerative braking, or something as simple as putting more electric fans on the platforms.

A potentially key bit of technology that has been discussed often by the transit authority in regard to reducing delays could also help cool stations: communications-based train control, or CBTC. Not only do its supporters say CBTC can help trains run reliably and on time, but the software that controls the trains can help them brake more efficiently, reducing the number of stops and starts that generate so much heat. But the MTA is averaging roughly ten years per line to install CBTC. New York City Transit president Andy Byford is currently looking for funding for his plan to install CBTC across the majority of the system by 2030.

Christopher Jones, the senior vice president and chief planner at RPA, says tells the Voice that there’s “probably not a lot of things you can do in the short term” to cool down stations, regenerative braking included, and the authority must focus on long-term fixes.

As Jones alludes, this is just another issue where the MTA’s previous long-term planning has failed New Yorkers in the present time. As Paris’s and London’s systems demonstrate, regenerative braking and CBTC — which have been widely deployed in both cities — are neither new nor strange technologies. Like plans to fix the elevators in stations, and plans to fix the damage caused by Hurricane Sandy, cooling subway platforms is yet another issue that requires prioritization and money. Jones expressed the need for the MTA to make this and other resiliency issues a priority in the next capital plan, along with all the plan’s other priorities. In the meantime, maybe it can plug in some more fans to push all that hot air around.