From Subways to City Buildings: Decarbonizing New York City Through Its First Transit-Heat Recovery System
Nine seniors dedicated their capstone project to developing a sustainable plan for harvesting wasted thermal energy from the subway system to provide hot water to local buildings
According to the New York City government, the single largest source of the city’s greenhouse gas emissions is not car traffic or the subway system: it’s city buildings, which generate approximately two-thirds of the city’s total carbon emissions.
One major culprit of these emissions is boiler systems, which burn fuel oil to supply heat and hot water to the buildings’ occupants. When burned, these fuel oils create carbon dioxide, which is then released into the atmosphere. In excess, carbon dioxide contributes to global warming and climate change.
To address the problem, the city passed Local Law 97, which imposes increasing fines every five years on most buildings larger than 25,000 square feet that generate carbon emissions above a certain threshold. Going into effect in 2024, these annual limits aim to reduce greenhouse gas emissions from city buildings by 80% by 2050.
At the same time, anyone who has spent even just minutes in a New York City subway station knows that these predominantly un-air-conditioned stations get hot.
The combination of thousands of travelers’ body heat, friction generated by the subway trains’ movement, and the electricity needed to run the trains’ motors, braking systems, onboard air conditioning units and other electrical components can routinely push the temperature in some stations to above 90 degrees Fahrenheit — even in wintertime.
“All of that electricity makes a lot of heat as a byproduct,” explained Stevens Institute of Technology fifth-year mechanical engineering major Drew Maggio ’23.
Solutions to both problems came together through the 2022-2023 senior capstone project, “NYC Subway Heat Recovery.”
Sponsored by heating, ventilating and air conditioning efficiency company Highmark, an interdisciplinary team of nine seniors developed a plan and proof of concept for building New York City's first transit-heat recovery system by siphoning heat from the city’s subway stations and channeling it for use in the hot water systems of nearby buildings. The project was developed with assistance from the Metropolitan Transportation Authority (MTA), Equity Residential, and the American Society of Heating, Refrigerating and Air-Conditioning Engineers.
By harvesting the untapped energy generated by the subway system, the plan would improve energy inefficiencies in city buildings and reduce carbon emission rates while simultaneously creating a more comfortable environment on subway platforms — helping building owners, subway riders, and current and future generations of city residents alike.
Team lead Maggio sees the project as an effort to rethink the city’s resources to benefit individuals, businesses and the environment in a holistic, collaborative way.
“People are spending hundreds of thousands of dollars and ruining the environment to heat up hot water, and, at the same time, there's somebody across the street who's exhausting twice as much heat as that first building needs and just getting rid of it,” he said.
A lightbulb moment
The NYC Subway Heat Recovery project was initially inspired by a comment made by Highmark Vice President of Engineering Joey Schmitz, with whom Maggio had worked while an intern at the company in the summer of 2022.
Schmitz, Maggio and others were brainstorming how to keep the evaporator coils on air-source heat pumps from freezing over with extended use. Air-source heat pumps, which extract heat from the air and transfer it for use elsewhere, are a popular alternative to fossil-fueled heating technologies.
“Eventually Joey says, ‘It's always hot as hell in the subway. Why don't we put the pumps in the subway, and then they’ll never freeze, even in the winter?’” explained Maggio.
“We all kind of chuckled, and then the room fell silent. And we were all like, ‘That might actually work.’”
With Highmark unable to pursue the project further at the time, Maggio proposed that he pick up the idea for his senior capstone project. The London Underground had implemented a similar project and is now providing heat to more than 1,000 homes using excess thermal energy from subway stations.
“A typical senior design project is allocated $700 from the mechanical engineering department,” said advisor and Teaching Associate Professor Kevin Connington. “But Drew secured external funding of $10,000 from [Highmark].”
“I remember the CEO looking at me and being like, ‘Well, you have the $10,000. Put a team together, and make something happen,’” said Maggio. “Before I knew it, there were nine people, including myself, ready to work on this.”
Finding a building in a haystack
The biggest challenge of the two-semester project, said Maggio, was also one of its first steps: identifying a location for developing their proposed system.
With help from the MTA, which oversees New York City’s public transportation, the students narrowed down the list of 472 possible subway stations to only those already equipped with cooling infrastructure. Such cooling equipment is stored on the rooftop levels of MTA ancillary buildings, which are disguised to look like regular city buildings from the outside.
The team then manually cross-referenced city building efficiency ratings and locations with MTA ancillary building locations to pinpoint residential buildings with poor energy efficiency located in close proximity to an appropriate subway station.
Fortuitously, the owners of one of those buildings — 303 East 83rd Street — were already in talks with Highmark for an emissions reduction solution.
303 East 83rd Street is a residential building located directly across the street from an MTA ancillary building that serves the 86th Street subway station. (The 86th Street station runs underground beneath Second Avenue from 82nd Street to 89th Street.)
Under its current boiler system, 303 East 83rd Street generates 336 tons of carbon dioxide above the Local Law 97 threshold for 2024, generating annual emissions penalties for building owners Equity Residential starting at approximately $90,000.
With fines increasing every five years, said Maggio, “by 2030 and on, you're looking at upwards of $300,000 yearly.”
Focused on hot water specifically, the team’s solution sits as the middle step of what would ultimately be a three-part decarbonization proposal for eliminating the boiler system at 303 East 83rd Street. (The first step would focus on space heating and cooling, the second on hot water production and the third on an electric backup system.)
But developing solutions for real-world problems also requires facing real-world limitations. Taller and thinner than most Manhattan residential buildings, 303 East 83rd Street’s roof was too small to hold all the equipment needed to implement traditional air-source heat pumps, and retrofitting modifications would have proved impractical and prohibitively expensive. Additionally, the MTA requested that the solution not require any new equipment be installed in publicly visible spaces.
So rather than developing a solution that relied directly on the subway system’s hot air, the team pivoted to focusing on that air indirectly by tapping into the subway station’s ancillary air-heated hot water system.
Transferring energy from subways to city buildings
Part of the Second Avenue subway lies 100 feet below ground-level — too deep to ventilate its subway stations via simple sidewalk grates. Instead, fresher, cooler air for these stations is generated hydronically (using water) by pulling heat from station air and circulating it through water loop systems located in MTA ancillary buildings.
Hot air in the subway station is blown over a cooling coil though which cold water flows. Heat energy from the hot air is absorbed by the water inside that coil while the air in the station is cooled.
Similar to how a refrigerator works, a water-to-water heat pump pulls this now-warmed water in from the subway station’s cooling coil and uses its heat to boil a refrigerant liquid into a gas. This refrigerant gas is then compressed to a higher pressure, which allows the gas to condense at a higher temperature than when it was boiled. When the refrigerant condenses, its heat energy is transferred to water flowing through a hot water loop system of pipes that circulates between the subway station and the MTA ancillary building.
This water — now heated — is piped to an evaporative cooling tower on the ancillary building’s roof, where it is misted into the air to cool down. The now-cooled water is then collected, filtered and pumped back to the subway station, where it is ready to pick up heat from the station’s air via the heat pump once again, and the process repeats.
Taking advantage of this existing infrastructure, the seniors’ solution proposed installing underground pipes that connect the existing supply and return pipes of the hot water loop in the basement of the MTA ancillary building to a new heat pump and water loop system located in the basement of 303 East 83rd Street.
“The MTA is already doing all the hard work of capturing heat from the air,” said Maggio. “Our plan is to put a valve on the pipes that are coming up from the subway station and run it underneath Second Avenue.”
Air-heated water from the MTA hot water loop would be diverted to two heat pumps and storage tanks at 303 East 83rd Street running in parallel in two distinct loops, with one water loop used for primary storage and one used for maintaining system temperature. A series of heat pumps raise the water to the desired temperature, while a heat exchanger transfers heat from the MTA water to new, fresh water coming in from the city’s domestic water supply. Using water-to-water heat pumps, the system can remove the same amount of heat that a cooling tower would, but produce a smaller volume of water at a much higher temperature.
Once heated, the domestic hot water would be kept in insulated storage tanks and dispensed to the building’s sinks and showers as needed. By carefully sizing all necessary equipment, the team’s solution would require minimal to no alteration of the MTA’s cooling operations.
Integrating both public and private infrastructure into a mutually beneficial collaboration, their solution would allow city buildings within half a mile of a subway station to reduce their carbon footprint year-round and could easily be adapted for use in other major metropolitan areas.
A multifaceted business solution
Composed of nine students majoring in mechanical engineering, civil engineering and engineering management, the NYC Subway Heat Recovery team was more than twice the size of a typical senior design team. But the team’s unusual size and range of expertise made it possible to turn what would have been a single engineering solution into a full-blown product package.
“Having a large team was instrumental in being able to do everything that we did, including so many things that some people might not consider engineering,” said Maggio.
In addition to the engineering system design and drawings (with accompanying demolition, new construction and outdoor piping plans), the students built an animated schematic and a scale model demonstrating the functionality of the proposed multi-building heat pump system. The model was designed so Highmark could demonstrate the heat transfer process at trade shows and to prospective clients.
Another major resource the seniors developed was an interactive “opportunity map” to assist building owners considering pursuing sustainable energy projects. The interactive tool calculates Local Law 97 fines for a specific building based on its current energy usage and emissions, location, building size and occupancy, domestic hot water heating loads, and space heating loads.
“It will also tell you for any building in New York City where the nearest MTA installation is, estimate the cost of installing piping for something that far away and estimate the Local Law 97 savings from implementing our solution,” said Maggio.
The students also developed a comprehensive website of resources, including an easy-to-use guide to help building owners assess whether they might qualify for state funding to develop multi-building heat energy improvement projects.
Although the coordination of tasks was challenging with such a large team, said fifth-year mechanical engineering major Erik Cederstav ’23, the depth and variety of the project’s opportunities proved beneficial. Cederstav, whose major concentration was in sustainable energy, worked on technical research and design, model design and construction, and creation of the engineering drawings.
“This experience was excellent in terms of learning how to get an idea off the ground,” he said. “The specifics of the project changed a lot through the course of the year, which taught us to be flexible in the ‘startup’ mindset.”
Advocacy, impact, and a larger vision
Internal and external feedback for the project and team was overwhelmingly positive.
"As an advisor, this team made my job easy, and any success is due to their efforts,” Connington said. “They performed research with an independence beyond their years and delivered a proof-of-concept study and model that will provide immediate real-world returns for their sponsor and will play a central role in actualizing the decarbonization of New York City.”
Maggio returned to Highmark in June 2023 for a full-time position as a sales engineer. He hopes to continue developing the subway heat recovery project that he began as a student into a viable commercial solution.
In fact, with untold scores of wasted heat being generated, he said, New York City is full of untapped opportunities for sustainable energy just waiting to be captured. Bridging the gap between those generating excess energy and those who need it, he said, could not only benefit the earth as a whole: it could also create a new revenue stream.
“You know the Red Lobster with the big vents on the side that makes Times Square on 42nd Street smell like fish? There's a lot of hot air coming out of there. Or all those massive computers running simulations of the stock market? Those make a lot of heat. You could definitely be offsetting your building emissions by harnessing that heat — and if you've already done all the hard work of offsetting your emissions and you still have all this excess heat, you could sell it to your neighbor.”
To reduce its reliance on fossil fuels long-term, Maggio said, the city needs to reinvent itself from individuals acting independently into an interconnected, district-wide thermal distribution network.
“The big idea is rethinking how we see energy around us — seeing these buildings as energy envelopes,” he said. “Anytime you have energy coming in, you want to make sure you're getting it from someplace responsibly. Anytime you have energy going out, you want to make sure you've used as much of it as you can. We need New York City to rethink how they heat and cool their buildings and try to get everyone to work together.”
“I'm only starting to realize now how big of a paradigm shift that's going to end up being,” he added.
Cederstav, who has joined engineering consulting firm Kimley-Horn, noted the importance of government and local action to making these kinds of paradigm shifts possible. He will work full-time on an electrical design/consulting team that focuses on solar photovoltaic energy projects (which convert sunlight into electricity).
“This project likely wouldn't have been feasible without several government programs and policies that have been recently implemented,” Cederstav said. “For better or worse, it seems that it takes an authoritative entity to motivate people and businesses to implement carbon reduction strategies on a large scale.”
“For people who care not only about climate change, but about keeping their local environment clean, healthy and pleasant, encouraging local and state governments to implement pro-environment policies is just as important as personal choices,” he added. “Many people tend to underestimate the impact of local advocacy.”
That kind of broader mindset, said Maggio, would also benefit his chosen industry as a whole.
“As engineers, we’ve all kind of lost sight of what we're really trying to do,” he said. “I don't care how fancy your boilers are or how many LED light bulbs you have. If there's a free, renewable heat source right next to your building and you’re using fossil fuels instead, your building, in the grand scheme of things, is not that efficient.”