Author: Tasneem Kaawash – Lebanon – PROMPT! Cohort #1

Introduction

Every human emits energy constantly — could our own bodies help solve the energy crisis? Energy dominates almost every political, economic, and scientific debate; the stretching connections between countries, the booming population, and the ever-growing business world, manifested an energy crisis. According to the International Energy Agency (IEA), emerging markets will drive 85% of the increase in electricity demand by 2040 (IEA, 2024). In response, scientists and governments (and energy enthusiasts like me) started a hunt for new energy sources. 

Currently, the majority of the world’s energy is generated from burning fossil fuels, however, fossil fuels have become harder and harder to find. Research shows that we will run out of them by 2052 (Kuo, 2019). Consequently, cleaner sources of energy like solar panels, wind turbines, and EV batteries are gaining ground. Between 2015 and 2024, annual electricity capacity of renewables increased by 140%. In the same period, fossil fuels electricity capacity only increased by around 16% (IEA, 2024).

This ongoing transition proved that we can successfully culminate energy that is cheap, reliable, and environmentally friendly. But what if the next renewable source isn’t outside us — but within us? In this review, we will discuss one of the most exciting engineering projects: generating energy from the human body using thermoelectric generators!

Generating Energy From The Human Body

For centuries, we have sought to use all forms of energy around us, but never within us. It does sound like science fiction, but scientists are already testing it! The human body continuously generates considerable energy, much of which is wasted. In 1996, Starner calculated that a 68 kg adult male could generate up to 100 W of power through body movement (Starner, 1996).  Human energy stands out from other renewables, like the sun and wind, because it is independent of location and climate, which makes it more consistent, reliable, and stable. Harvesting human energy can make sustainable energy cheaper and more accessible to developing countries while reducing the reliance on batteries & other non-degradable sources of power. 

Energy generation from the human body is divided into two categories: parasitic human power generation (PHPG), which captures energy as the human body functions during normal daily activities like collecting sweat while exercising to power a health device; and active human power generation (AHPG), which focuses on generating energy as an activity in itself like using a bike to generate mechanical energy (McCleary, 2009). Currently, researchers are focusing on PHPG as a primary criterion to sustainably meet the energy needs of the growing market for wearable technologies and portable devices. By simply wearing a device and going on with your day, you can generate clean energy — your body becomes your own battery! Other criteria include flexibility, wearability, and noninterference with daily activities (Liu et al., 2021).

Consequently, generating energy becomes more challenging because the proportion of passive energy that can be utilized is small. Nevertheless, it provides new possibilities for people to replace low-power electronic components, like periodic battery replacement, cumbersome daily charging, and even surgical replacement of implanted electronic medical devices. 

In order to understand how we can utilize the energy of the human body, it is important to understand the kinds of energy released. We can classify the energy released into three categories: chemical, mechanical, and thermal. First of all, we have chemical energy. Chemical energy is a very promising source of energy. It utilizes chemical reactions as well as the transfer of electrons in chemical substances between the generator and the body. Devices with such technologies are usually referred to as biofuel cells. Depending on the catalysts used, there are two types: enzymatic fuel cells (EFCs), which are based on oxidation and reduction reactions, and microbial fuel cells (MFCs), which use living microorganisms (Zou, Bo and Li, 2021). One device that harnesses such technology is the sweat-powered biofuel watch from Singapore’s Nanyang Technological University (Nanyang Technological University, 2021). The biofuel cells from Nanyang Technological University ^[]

Secondly, comes mechanical energy, which is generated by the movement of the muscles. Mechanical energy harvesting is not new. With a simple circuit of magnets, wires, and a bike, you can generate electricity. However, scientists are elevating mechanical energy by looking for ways to utilize the passive mechanical energy. Such technologies utilize piezoelectric nanogenerators (PENG) and triboelectric nanogenerators (TENG). In 1999, Bradbury Face patented his invention for shoes that utilize piezoelectric technologies (Bradbury, 2006). By walking or running, the shoes apply a first force deforming a piezoelectric actuator, thereby generating electrical energy. However, it will not be the focus of this discussion, since it is already well-documented and far less puzzling compared to thermal energy. 

Bradburry’s peizoelectric shoes^[]

Our subject of discussion, thermal energy, is the most abundant form of energy our bodies produce; the average human body produces about 100 watts of heat at rest, with heat production increasing to 300-400 watts during vigorous exercise (Freudenberg Sealing Technologies, 2023). Presently, the harvesting of thermal energy from the human body mainly depends on the thermoelectric effect and the pyroelectric effect, which respectively correspond to two types of energy harvesters which are thermoelectric generators (TEG) and pyroelectric generators (PEG). In 2013, Ann Makosinski invented an LED flashlight that harnesses thermoelectric generators to collect heat from the hand in order to power the flashlight (Let’s Talk Science, 2019). Despite the considerable potential of thermal energy cultivation, it also presents underlying challenges that make it a particularly unique and fascinating source of energy.Makosinski’s flashlight ^[]

Thermal Energy Generation

In 2024, researchers from the University of Washington demonstrated a stretchy thermoelectric wearable that powered an LED light using only energy from the user’s body—no battery required (Taguchi and McQuate, 2024)! This challenges the common belief that human energy harvesters can’t fully replace batteries in wearables and highlights emerging opportunities for innovation in this area.

If one could capture passive human energy with a device of a conversion efficiency of ~1%, the power generated would be ~0.6-1.8 W, which is enough to power many wearable or implanted sensors (Nozariasbmarz et al., 2020). However, this would require covering the entire body with heat harvesting devices, which do not align with passive energy generation. Therefore, it is more practical to cover only a small part of the body with TEG, maximize efficiency, and minimize the load power for wearable systems. If we were able to do so, and every person in the world wore this kind of device (~8,245,000,000 people at the time of calculation), we would generate 14,841,000,000 W. This is equivalent to burning 53,400 tons of coal per day, which would release 133,500 tons of CO2. This demonstrates the promise and brilliance of thermoelectric energy generation of the body. 

Thermoelectricity

As discussed, generating power from human body heat utilizes two types of generators: thermoelectric generators (TEG), which make use of the Seebeck effect and the Peltier effect, and pyroelectric generators (PEG), which convert temperature fluctuations into electrical energy to generate alternating current. 

Pyroelectric generators (PEG) function through the spontaneous re-orientation of electric dipoles in polar materials due to time-dependent temperature fluctuations, producing an alternating current. These generators rely on polarized crystals, which really need a reason to generate electricity, making them very sensitive. So, when exposed to thermal change, then generate voltage. However, they lose current as temperature stabilizes, a key limitation given that the human body typically maintains a constant temperature. Due to this reliance on temperature fluctuations and suitability for low-temperature environments, PEGs are less relevant for harvesting human body heat and will not be discussed further.

Thermoelectricity enables direct conversion of heat into electricity through the Seebeck effect, making it relevant for harnessing human body heat. We focus exclusively on this effect, as it forms the basis of power generation from body heat rather than the reverse Peltier effect. The Seebeck effect was discovered by Thomas Seebeck, who discovered that metals that are good at conducting electricity are just as good at conducting heat. So, he conducted an experiment in which he joined two metals together by tying both of their ends (which we now call a thermocouple) and exposed one joint to a hot surface and the other to a cold surface, generating an electrical current. The greater the difference between the temperatures, the greater the voltage produced. This is due to the concentration of energy on the hot side of the circuit due to thermal induction. And since mother nature likes to reach the lowest energy state, it forces electrons to migrate from the hotter side to the colder side to reduce the imbalance. The movement of the electrons leads to the generation of a potential difference. 

In his experiment, he used conductors, which allowed electrons to move freely; however, today in industrial usage of the Seebeck effect, we use semiconductors, which don’t conduct electricity as good as conductors in low temperatures. However, they are more controllable, which is very important. The most used semiconductors include lead telluride, silicon Germanium, bismuth telluride, and antimony telluride. They are efficient due to their metalloid nature and electron configurations, which give them a suitable band gap for controlling electrical conductivity.The Seebeck experiment ^[]

Thermoelectricity is very reliable. NASA uses thermocouples in a round series where decaying plutonium-238 is centered. Plutonium acts as the hot side, whereas the environment on the planets they visit acts as the colder side. Voyager 1, which was sent to space 47 years ago, is still working with no issues (NASA, 2024). What makes it especially reliable for the human body is the body’s ability to maintain a constant temperature, which contributes to its low maintenance and longevity. Other advantages include low cost and scalability. 

Thermoelectricity is very reliable. NASA uses thermocouples in a round series where decaying plutonium-238 is centered. Plutonium acts as the hot side, whereas the environment on the planets they visit acts as the colder side. Voyager 1, which was sent to space 47 years ago, is still working with no issues (NASA, 2024). What makes it especially reliable for the human body is the body’s ability to maintain a constant temperature, which contributes to its low maintenance and longevity. Other advantages include low cost and scalability.  

Efficiency is the main challenge. Thermoelectric body energy harvesting generates only milliwatts to about one watt due to low power density and efficiency. This may be improved with energy boosters. In fact, boosters play an essential element in the engineering of thermoelectric generators. They are power converters that boost unregulated input voltage to a higher output voltage. Speaking of engineering, it is essential to talk about the circuits used in such systems. They start with an energy harvester, which in this case is a TEG harvester. The harvester sends the energy collected through a rectification circuit, then to a power booster, then to a voltage regulator. 

Applications

The most important factors of engineering a thermal energy harvester are size, position, type, and efficiency of the harvesters. Numerous experiments involving thermoelectricity have been conducted in the past. For example, in 1998, Seiko, a brand renowned for its watch innovations, launched Thermic, a quartz watch that generates electricity to power itself from the temperature difference between the wearer’s body and the surrounding environment. It is able to run for approximately 10 months on full charge. This early prototype is made up of a dedicated microcomputer and a recharging circuit. However, it is not only reliant on energy from the human body, as it utilizes a second lithium-ion battery (Kable, 2020). Building on this, many engineers are working on smart watches that are 100% energy-reliant on the body. Seiko’s thermic watch ^[]

Continuing this trend of innovation, in 2024, Northwest scientists created stretchy fabric that converts body heat into electricity, with no battery required, using 3d printable materials. They used rigid thermoelectric semiconductors, 3D-printed composites, and liquid metal traces. Its stretchy and flexible design allows us to use it for a wide range of applications.Northwest’s stretchy fabric ^[]In addition to wearable fabrics, other ongoing projects build on past successes, such as thermoelectric generators fabricated into clothes. A lot of companies and colleges are working on it, most prominently from North Carolina State University has developed a new wearable thermoelectric generator to convert body heat into electricity. This new lightweight wearable thermoelectric generator can effectively generate above 20 μW/cm2 power. This produces far more electricity than compared to previous heat harvesting technologies. 

Conclusion

I believe that powering devices from human energy is the next big revolution in clean energy. Harnessing body heat for even the smallest wearables holds the potential to save vast amounts of electricity and transform the way we power our devices. These advances not only pave the way for a more sustainable future but also help bridge the technological divide between developed and developing countries. By focusing on thermoelectric solutions, we can guide the expanding market toward clean energy while improving access to electricity in underserved regions. However, progress in this field is slowed by a lack of funding and initial resources. It is crucial that we prioritize investment and innovation in thermoelectric technology. While this review has focused on thermoelectricity, we must also continue to explore other methods of harvesting energy from the human body. By supporting research and development across all avenues, we can accelerate the transition to sustainable energy solutions for wearables worldwide. This is just the beginning. The next breakthrough project could come from a lab, a company, or even a student project.

References

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Nanyang Technological University (NTU) (2021) ‘Stretchable battery that is powered by sweat’, NTU News. Available at: https://www.ntu.edu.sg/news/detail/stretchable-battery-that-is-powered-by-sweat

Bradbury, F. (2006) Footwear incorporating piezoelectric energy harvesting system. United States Patent Application US20060021261A1, 2 February. Available at: https://patents.google.com/patent/US20060021261A1/en

Freudenberg Sealing Technologies (2023) ‘Human Power Plant’, Freudenberg Sealing Technologies News Stories, 16 August. Available at: https://www.fst.com/news-stories/2023/human-power-plant/

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Taguchi, K. and McQuate, S. (2024) ‘UW researchers develop a stretchable, wearable device that lights up an LED using only the warmth of your skin’, UW News, 10 September. Available at: https://www.washington.edu/news/2024/09/10/uw-researchers-develop-a-stretchable-wearable-device-that-lights-up-an-led-using-only-the-warmth-of-your-skin/

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