|Fig. 1: This shows body heat. (Source: Wikimedia Commons)|
The first law of thermodynamics simply states that energy can neither be created nor destroyed: it is the conservation of energy.  Thus, power generation processes and energy sources involve the conversion of energy from one form to another, rather than the creation of energy from nothing. Metabolism is one example of the first law of thermodynamics in action in the human body: the conversion of food into energy, which is then utilized by the body to perform activities. On a similar note and adaptation of the first law of thermodynamics, human body heat can be converted into useful energy.
The average human, at rest, produces around 100 watts of power.  Over periods of a few minutes, humans can comfortably sustain 300-400 watts; and in the case of very short bursts of energy, such as sprinting, some humans can output over 2,000 watts.  The bulk of this energy is required for important tasks, such as pumping your heart and flexing your muscles, but a lot of it is wasted, primarily as heat.  Almost all of this wasted energy could be captured and turned into electricity. Furthermore, this process could then augment or completely replace reliance on batteries.
Battery technology is accompanied by a huge limitation when it comes to modern technology. If batteries could be removed from the equation, it would suddenly be possible to have wearable computers: computers wrapped around your wrist, embedded in your shoes, or woven into your clothes. To do this, only a few watts of power from the human body would need to be captured; a negligible amount that would probably have zero effect on the body.
The idea of transforming human body heat into electricity has been an ongoing process for scientists for years. In Sweden, for example, Stockholm Central Station uses heat exchanges to convert commuter body heat into hot water, which is then piped to an office building next door: an approach that can easily be replicated in shopping malls and supermarkets around the world.  Researchers have been attempting ways to power small devices, such as cellphones and laptops, when there is no conventional and accessible energy sources. At the University of Wisconsin, research engineers have created a shoe that utilizes reverse electrowetting to produce as much as a kilowatt of energy, just by simply taking a walk. 
While it is easy to capture body heat on a grand scale, such as in Sweden, there is still no easy way to harvest large amounts of waste heat on a local, wearable scale. It seems as if human batteries are a feasible goal. It is just a matter of time and research. Ambient energy is driven predominantly by researches seeking more eco-friendly solutions and by corporations who wish to cut costs. In the not-so-distant future, we will very likely see cordless electronic devices in hospitals that sense and report vital signs of patients. However, true to its nature, it is more than likely that the use of human energy will be used to power technological innovations in the process.
© Lo'eau LaBonta. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 H. C. Van Ness, Understanding Thermodynamics (Dover, 1983)
 O. C. Ozcanli, "Turning Body Heat Into Electricity," Forbes, 8 Jun 10.
 T. Starner, "Human-Powered Wearable Computing," IBM Systems J. 35, 618 (1996).
 X Hinchey, "Harvesting Body Heat to Warm Buildings," BBC News, 9 Jan 11.
 "Harvesting Energy From Humans," Popular Science, 29 Jan 09.