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The power of body heat

ENVIRONMENT

From a Paris Metro station to a large shopping mall in the United States, human body heat is being tapped to provide warmth in winter.


In the heart of Paris, there’s a seven-storey corner apartment building that for the past five years has been drawing its warmth from human body heat generated from the hustle and bustle of a nearby metro station.

The air temperature inside the metro tunnel is around 10°C higher than outside. That mainly comes from people moving around the station and from the trains, says Genevieve Littot, climate and energy strategist at the social housing construction company Paris Habitat, which designed the system that extracts the heat from the underground and eds it to the apartment building.


“A staircase connects the basement of the building to the metro tunnel,” Littot told BBC Future. “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.”


This waste heat provides up to 35% of the heat needed for the building’s 20 apartments plus a commercial tenant at ground level. Littot adds it’s more efficient than heating apartments individually.


The potential to reduce energy use and therefore impact on the planet is huge — especially considering half the world’s total energy consumption is used to heat homes and other buildings.


One of the most ubiquitous sources of heat inside buildings is, of course, us. And buildings, where — at least pre-covid — 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.


In Sweden, body heat from Stockholm Central station — which had seen about 250,000 people pass through daily, pre-covid — is used to warm the nearby 17-storey Kungsbrohuset building, helping reduce its energy consumption 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.”


(A district heating system uses a number of inputs besides body heat, including geothermal heat, burning of unrefined biomass — wood waste, straw, forestry residues and so on — as well as 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 has been a popular way to heat buildings. Half of Sweden’s entire heating demand in the residential sector in 2017 was met mainly using heat pumps and waste heat in district heating. It was the oil crisis in the 1970s — when wars in the Middle East prompted Arab oil producers to impose an embargo on oil exports, sending oil prices through the roof — that pushed Sweden to be creative and to choose district heating, she says.


In the US, a shopping mall in Minnesota is using body heat to warm the entire building. In fact, it has had no central heating since it opened in 1991, a bold choice given that state’s typical January low is -15.5°C. Instead, the mall captures enough heat from the body warmth of its more than 109,000 on-average daily visitors, its more than 3ha 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, the mall’s communications vice-president. “Many start early in the morning, so by the time most of these employees enter the building between 6-8am, the mall has been warmed to a very pleasant temperature.”


Janson says the future for this type of energy is promising. “Human body warmth might not fulfil 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.”


ABOVE An apartment building in inner-city Paris that gains much of its heat supply from a nearby metro station

PHOTO Paris Habitat


Mostly we see buildings as something designed to suit us, but when we become a part of the design — or at least part of us does — the result is a more energy efficient building, as well as a more comfortable place to be.


September 24, 2020

Tiny wind turbine collects energy while you walk

Meanwhile, Chinese scientists have developed a “tiny wind turbine” that can scavenge energy from the breeze made while you walk. Once placed on your swinging arm, the airflow is enough to generate power, researchers at the Beijing Institute of Nanoenergy and Nanosystems report.


The device comprises two plastic strips in a tube that flutter or clap together. A gentle breeze of 1.6 metres a second (m/s) is enough to power the mini turbine, but performs best when wind velocity is 4–8m/s and the two plastic strips flutter in sync.

The device could power remote sensors, security cameras or even a weather station on a hill that is otherwise difficult to reach, says Richard Cochrane, associate professor of renewable energy from the University of Exeter, who was not involved in the study. To date, it has been able to power up 100 LED lights and temperature sensors, its makers say. It also has a wind-to-energy conversion efficiency of 3.23%, which they claim is better than previously-reported performances on wind energy scavenging.


“We won’t see this innovation replacing the big turbines, but we are seeing increasing numbers of these sort of technologies being used for energy harvesting . . . providing power in places that are otherwise quite hard to get electricity to,” Cochrane adds.

Its makers hope to combine it with small electronic devices such as phones, to provide sustained electric power, and eventually to make the device competitive with traditional wind turbines, where output is heavily dependent on high wind speeds.


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