The buildings heated by human warmth

From a stuffy metro station in Paris, to a large shopping mall in the United States, human body heat is helping to lessen the need to burn fossil fuels for warmth.

This seven-storey corner building’s milky white facade blends in well with its neighbours on Rue de Beaubourg, in the heart of Paris. The area is known for its more conspicuous occupants – just a short stroll away is the Centre Pompidou, a contemporary museum and an extravagant 20th-Century architectural statement.

By comparison, the building at number two Rue de Beaubourg looks modest, but it is perhaps even more unusual in its design, though you wouldn’t know it from the outside. Since 2015, the building has been drawing its warmth from the hustle and bustle of human body heat in a nearby metro station.

The air temperature inside the metro tunnel is around 10C (18F) higher than outdoors. This heat mainly come from human bodies moving around the station and the heat generated by the trains, says Genevieve Littot, climate and energy strategist at the social housing construction company Paris Habitat, which designed the heat extraction system.

“A staircase connects the basement of the building to the metro tunnel,” says Littot. “The installation extracts warm air from the metro tunnel through the existing passageway, as the warm air passes through a heat exchanger to produce hot water, which is used for space heating.”

The Paris project is hardly alone in this regard. Different innovative projects with energy saving designs are emerging around the world to mitigate carbon emissions.

One of the most ubiquitous sources of heat inside buildings is, of course, the human body. And buildings where, at least in ordinary times, people gather in large numbers have the greatest potential to put that stuffy, human-warmed air to good use. Bustling train stations in particular have proven to be a popular place to experiment with harnessing human body heat.

Sweden has made a name for capturing the body heat in its busy Stockholm Central station – about 250,000 people passed through it each day, pre-Covid-19. That heat is used to warm a 17-storey building named Kungsbrohuset nearby, helping to reduce energy consumption of the building by up to 10%.

“We take in seawater to cool the ventilation in Kungsbrohuset and the Stockholm Central Station,” says Roger Björk, technical manager at Folksam, which owns Kungsbrohuset. “When the water returns, it is pretty hot [warmed by body heat]. Then we recycle the water to generate heat in our district heating system.”

The district heating system utilises a number of other sustainable sources besides body heat, including geothermal heat, burning of unrefined biomass – waste wood, straw, forestry residues and so on – and surplus heat from industrial buildings. That heat is then distributed to homes and buildings across the country through underground pipes.

Ulla Janson, senior lecturer at the division of building services at Lund University, says district heating systems have been a powerful way to heat buildings up. Half of Sweden’s entire heating demand in the residential sector in 2017 was satisfied mainly through use of heat pumps and utilising waste heat in district heating.

It was an energy crisis in the 1970s – when wars in the Middle East prompted Arab oil producers to impose an embargo on oil exports to the US and the Netherlands, sending oil prices through the roof – that pushed Sweden to be creative, says Janson. “District heating is a part of the Swedish way of a strong state intervention, where the system turns out to be a natural part of developing a city.”

Train stations are not the only public spaces to make use of their many human occupants for heating. A pioneering mall in the Minnesota in the US also depends partly on the heat from human body warmth to heat up the building. The Mall of America has had no central heating since its opening in 1991 – a bold choice given that state’s typical January low is -15.5C (4.1F).

Instead, the mall captures enough heat from the body warmth of its over 109,000 average daily visitors, more than eight acres of skylights and the heat from thousands of lights and fixtures to stay at a comfortable temperature through the winter.

“On a given day, there could be between 8,000 and 12,000 employees in the building,” says Dan Jasper, communications vice president of Mall of America. “Many start early in the morning, so by the time most of these employees entered the building between 6-8am, the mall has been warmed to a very pleasant temperature.”

Designs that rely on trapping incidental sources of heat are put to the test as the thermal demands of a building become more exacting. A hospital, for example, is a big challenge for thermal regulation as there are many people moving around and plenty of energy-intensive instruments producing heat, but it needs to be distributed precisely to ensure a constant comfortable and safe temperature, especially for vulnerable patients.

But a six-storey hospital in Frankfurt, Germany, the Klinikum Frankfurt Hoechst, has nevertheless embraced this approach.

Over 1,000 triple-glazed windows were installed in the building, the first hospital to use such a design. The energy consumption in the hospital, which has about 1,000 beds, is three to four times higher than that of a residential building of comparable size, due to the higher energy demand from hospital equipment.

Patient rooms have to remain at 22C (71.6F), which is enabled by improved thermal insulation. As the triple-glazed windows keep cold air outside from infiltrating into the building, it reduces the energy demand needed to heat up the building. The hospital also uses a ventilation system that preheats fresh air before it flows into the room to maintain indoor temperature and prevent unpleasant odours.

While large, busy buildings like hospitals or stations have the benefit of many bodies to warm them, individual dwellings with just a few occupants can also benefit from body heat. This form of human-heated design dates back decades. The German architect Wolfgang Feist built the first building in 1990 with what he called a “passive house” design, which seeks to drastically reduce heat loss.

“In the late 1970s, we realised that more than a third of all energy consumed in Europe was just used for heating buildings. At the physics department where I was doing research, we knew very well that better insulation could save almost all of that,” says Feist.

Buildings with such design lay heavy focus on thermal insulation, including an airtight building envelope, double or triple-glazing, a ventilation system with heat recovery and avoiding something called a “thermal bridge”. A thermal bridge is the area in a building envelop that has a higher heat conductivity than surrounding materials and can allow warmth to leak out of a house.

By keeping as much warmth as possible trapped inside, it can reduce the heating demand in the building, which can then be met through “passive” sources, such as solar radiation and heat from people inside and technical appliances.

Body heat is just one of many elements of passive-house design, but it is an important one. “My colleague from Denmark once made a joke out of that: ‘The Weather Channel said colder in the weekend. Shall we invite some friends to keep the house warm?’,” laughs Feist.

The Passive House Institute, which Feist founded, claims that such buildings consume about 90% less heating energy than conventional buildings and 75% less energy than the average new construction. Feist estimates that the additional cost for such design amounts up to 8% for a single-family passive house.

Since its origins in the 1990s, many countries have since adopted passive-house design. The institute recorded 25,000 certified units registered worldwide as of January 2020.

Passive-house design, Janson says, is nothing more than a way of building energy efficient buildings with a good indoor climate all year round.

“The [passive] house design is for a good indoor climate all year round, otherwise it is not a passive house. It works as a thermos, just as good for keeping the heat out as keeping the heat inside,” she explains.

Although body heat has been an innovative and sustainable source of energy, it has its limitations.

Littot from Paris Habitat says the biggest challenge for projects seeking to use heat from infrastructure like train stations is finding the space. “This type of project would be especially suited for some new builds near a metro line extension, where the initial planning could integrate with the metro recovery solution from the start,” she says. But such forward planning is rare, and her company has not been able to find another feasible opportunity to do a similar project in France.

Location aside, human body heat cannot be the sole heating source of a building. “I don’t think body heat would be a major source of heat [in a building], but it has to be combined with that from electrical appliances, cooking, refrigerators and so on,” says Leon Glicksman, professor of building technology and mechanical engineering at the Massachusetts Institute of Technology, in Boston, Massachusetts.

Energy-saving building design is not always appreciated, especially in countries without a mature green awareness. “More people build houses in green standards, but there is not a general agreement to how much energy this can save. [Thus,] there’s still a reluctance in employing such design,” Glicksman says.

“Some people are very frugal when it comes to energy, while others use a very large amount. Even if we design very energy efficient buildings, if people don’t know how to use it, it won’t be effective.”

Glicksman suggests that governments can set energy saving standards, and build efficient and affordable demonstrations that are monitored to show their capabilities in saving energy.

Despite the hurdles, Janson says the future for this type of energy is promising. “Human body warmth might not fulfill the total energy demand for space heating, but it is a good contribution and is often used in a heat exchanger to pre-heat the supply air or re-used in a heat pump,” she says.

“The heat from humans is always included when doing an energy simulation of a building, so the future is already here.”

We might normally see buildings as something designed to suit us, but when we become a part of the design the result is a more sustainable building, as well as a more comfortable place to be.