Converting kinetic energy and heat flux are possible electrical energy sources for users to supply power to their wearable devices.
The integration of electronics has generated many applications and multifunctional scenarios by offering new control opportunities to improve our living environment. Technological advances are travelling at a high rate, and the possibilities offered by wearable devices embrace the field of medicine. Health monitoring wearables can allow a firm control of vital signs, all in real time, offering industry experts the possibility to access monitoring data through the cloud.
Wearables prove to be a fertile ground for the use of energy harvesting techniques, where one can exploit the kinetic energy of the wearer to produce electricity and directly recharge the battery of the worn device.
Figure 1: A wearable device built around TI's CC2541 SoC.
Thermal energy harvesting
A heat flux can be converted into electrical energy by using a thermoelectric generator (TEG), whose core is a thermopile. From the theory of thermodynamics, the heat flux on a human skin cannot be effectively converted into electrical energy, even if a human being generates on average more than 100W. If we suppose a low conversion of about 1 to 2%, the amount of available power is sufficient to operate a low power wearable device. The thermal circuit of a TEG wearable placed in direct contact with the skin can be described by a thermal resistance of the body and that of the environment. These resistors are connected in series and represent the thermal resistance of the thermoelectric generator.
We constantly produce heat as a side effect of our metabolism. However, only part of this heat is dissipated in the environment as a flow of heat and infrared radiation, the remainder being rejected in the form of water vapour. What's more, only a small fraction of the heat flow can be collected and stored as energy. The magnitude of the voltage generated between the two layers depends on the material and the temperature, following a linear relationship as a function of the Seebeck coefficient S.
Figure 2: Example of a block diagram for a pacemaker with thermal energy harvesting.
The energy optimisation, as we have seen, necessarily brings with it the need for accurate choices, not only of the various components, but also of a power supply and an intelligent management system, capable of supplying power only when necessary.
There are many aspects to consider when designing a low power system: power consumption, required cycles, voltage and total power consumed. All design scenarios will require careful planning.
First published by EE Times Europe.