So, how close are we to successfully harvesting the heat we all radiate? Could our bodies one day be an infinite source of low-cost energy – similar to the millions of human batteries enslaved by evil machines in the Matrix? Or is reality a little more complicated? 

The average human body produces around 100 watts of power while it’s at rest, reaching 300-400 watts during exercise. That’s the equivalent of 2,000 kcal consumed in a day, or an outdoor LED floodlight left on for 24 hours. 

We use most of this energy for vital functions like our heartbeat, muscle movement, and digestion. Similar to the floodlight, 80 per cent of it is produced as heat – much of it ‘wasted’ through the skin.

Many innovators have dreamt of tapping into this wastage, and there have been a few exciting successes. They suggest that capturing even just a few watts of power could play a small but important role in creating a more sustainable future.

When calculating the heat load for new urban spaces, it makes sense to take into account pedestrians' body temperature and movements. 

Indeed, if one person can create enough heat to keep your train seat warm, imagine how much of it the Mall of America's 40 million yearly visitors generate. The biggest shopping mall in the US maintains a comfortable 21 degrees Celsius at any given time, despite Minnesota’s arctic winters and having no central heating. This is mostly due to the clever use of thermal energy produced by the hordes of shoppers crisscrossing the venue in search of bargains. 

Stockholm’s Central Station – Sweden’s busiest transport hub – took this idea one step further. Aside from shoring up the station's temperature, heat exchangers convert some of the excess body heat of its 250,000 daily commuters' into warm water. This water is then piped to a nearby office block for heating purposes.

Stockholm's large-scale example can't be replicated everywhere, because transferring low-temperature heat across long distances might use more energy than it creates. But heat exchangers can offer a sustainable solution as long as buildings are located close by, and there's enough footfall for creating excess warmth in the first instance.

While body heat might uniquely contribute to warming up buildings and urban spaces, its utilisation doesn’t end there – body heat also shows a lot of promise in small-scale applications like wearable technology. 

Thermoelectricity uses a thin conductive material that generates electricity by exploiting the temperature difference between its two sides. It can power a device as long as the outside temperature is lower than that of your skin.

Several ultra-low-power wearables like fitness trackers and medical devices that use this technology are already commercially available. But we’re still some way off from running larger and more power-hungry applications like smartphones on body heat alone. 

Nevertheless, we see rapid advancements in the development of thermal harvesters and nanofibre textiles. This could soon see us wear flexible materials and smart garments that use body temperature as their only source of energy.

Body heat isn’t yet a major source of renewable energy. But the use of heat exchangers in urban spaces, along with advancements in ultra-low-power wearable electronics, suggest that it is turning into an exciting niche. 

It’s also a helpful reminder of the direct impact we can have as individuals when it comes to cutting greenhouses. In the new energy world, our power is the driving force. 

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