This is where energy harvesting comes into play. Energy is plentiful in our environment in form of light, movement, temperature differences, electric charges, electromagnetic waves and more. The challenge is to convert the different forms of ambient energy into a direct current that can be used to power wearable devices. A very interesting form of conversion is from thermal to electric energy. As our metabolism is a constant heat source, temperature differences are almost always present between our body and the ambient. Thermoelectric generators (TEG) translate these temperature differences to a direct current. TEGs can be extremely small, lightweight and robust and do not contain any moving parts or require any maintenance. This makes them perfect candidates for wearable applications.
If the mean produced energy of a body powered harvester exceeds the power consumption of a wearable, the device does not need to be recharged as long as the user is wearing it. This would provide an unprecedent freedom from the power grid and allow users to simply forget about recharging and energy availability. This freedom would increase the comfort and convenience for most users, but it might be of vital importance in applications that are related to health, emergency communication or the use in remote environments.
Rechargeable batteries with high energy density can represent a significant cost factor for low-cost wearables. Current energy harvesting solutions usually surpass the cost per volume in direct comparison with batteries. However, this is mostly driven by quantities. Thermoelectric generators can be produced using highly parallel microfabrication processes. Production in quantities of only a fraction of that of batteries would send the prices for individual modules tumbling. In contrast to the deteriorating capacity of batteries with an increasing number of charge cycles, the stable performance of TEG over a lifetime might make them also a cost-efficient alternative. Eventually, the individual business case will decide if the use of energy harvesters can save expenses additional to providing energy autonomy.
Thermoelectric generators and other energy harvesting techniques are the way to go when it comes to powering wearables. Provided that the energy production matches the power budget of the wearable, devices will not need to be recharged as longer as the wearer is using them. This will free the user from the energy grid. Depending on the business case, energy harvesting might not only be an extremely useful feature but also a cost-efficient way to power wearables.