Energy Havesting for Wearables

by Florian Schumacher

July 2, 2014

Energy Harvesting for Wearables

One of the main difficulties with wearables is how to provide enough energy for the electronics to run over a reasonable amount of time without making the battery too large or the device bulky. In an ideal world, users would not have to charge their wearables at all. But considering the wearables of today even increasing the battery life by a few days would be a huge improvement.

The term energy harvesting summarizes several different approaches which might lead us a step closer to such an ideal world. Instead of charging wearables with some sort of cable, new wearables could produce the energy they need from the light, heat or vibration in their surroundings. That might sound like science fiction, but energy-harvesting wearables have been around for many years. Automatic watches have been converting the energy from arm movements for a while now and Seiko has even invented an electromagnetic generator that powers its quartz watches by its wearers’ physical movements.

For today’s era of wearables that rely heavily on sensors, computing power and communication technology a simple approach to energy no longer is sufficient. However, there is a whole range of new technologies on the rise that could make devices more independent from power outlets. When it comes to energy harvesting for wearables, science and industry are currently focusing on the following technologies:

Solar cells not only make sense on rooftops or in big solar parks, soon much smaller versions could provide enough energy to power wearable devices. Solar watches that function without batteries have been around for years, and Energy Bionics recently invented a watch that can produce enough energy to additionally be able to charge your mobile phone or other devices. One of the problems solar cells on wearables have is that they only work if directly exposed to the light and stop producing energy as soon as they are covered up, for example, under a sleeve. This, however, makes solar energy harvesting an interesting option for smart cloths, where flexible cells could be woven straight into the textile. Traditional solar cells are designed to absorb sunlight, which is much brighter than the indoor light sources in modern working environments. To address this issue, new materials for solar cells are being developed that are capable of producing energy indoors and with a much higher efficiency.

Thermoelectric harvesting transforms heat into electric energy using a physical principle called the Seebeck effect. Peltier elements, a certain pair of semiconductors, produce an electrical current whenever a temperature gradient occurs, so whenever one side is warmer than the other. For wearables, the human body, which is constantly emitting heat, might be used as the hot side of the equation while the surroundings can pose the cooler side that is needed for thermoelectric harvesting. The amount of energy that can be produced depends on the delta between the high and low temperatures. Peltier elements could deliver a comparably high amount of energy, making them interesting for devices worn in direct contact with the skin and with a high energy demand. One of the major benefits of thermoelectric harvesting is that the energy is always available, both indoors and outdoors, day and night.

Piezoelectric harvesting converts mechanical energy from vibrations or shocks into electrical energy. In piezoelectric elements, the piezo effect generates a small electrical current whenever the element is manipulated by mechanical forces. For energy harvesting in wearables, the piezoelectric elements are often designed to produce energy with the vibrations that occur when walking, breathing or moving your hands. Piezoelectric harvesting generates comparably small amounts of energy, which limits the technology to applications with low power demands and to body areas continuously in motion. Scientists are also working on polymeric piezoelectric fibres which are flexible, strong, breathable and could be integrated into textiles, allowing for a whole new range of health monitoring and other applications.

 Optimizing Energy Storage and Consumption in Wearables

When it comes to new types of wearables that can run with less energy or completely without charging, energy harvesting is only one side of the story. Storing energy is another area with plenty of room for improvement – here, supercapacitors and graphene show lots of potential. Today’s wonder material graphene might significantly improve the efficiency of batteries and capacitors, thus improving wearables’ overall performance. And structural capacitors might manage turning the casing and other wearable components into an energy store, thus reducing the amount of space needed for a separate battery.

Another way to either improve battery life or become fully independent from charging batteries is to reduce the amount of energy it takes to operate sensors, chips and communication systems. While the smartphone’s success has lead to powerful and energy-efficient processors for mobile devices, activity trackers, smart watches and other smart devices, wearables’ processors generally have a much lower demand for computational power and energy supply. Big chipset vendors such as Intel are working to address the issue by integrating processors, memory and communication into a single chip, which would reduce the typical energy loss found in most companies’ regular setup.

Choosing the most efficient network technology could provide additional opportunities to reduce energy consumption. Therefore, wearables might soon be equipped with several different wireless technologies, such as LTE, WiFi and Bluetooth simultaneously, in order to be able to pick the most energy-efficient and readily available solution depending on the given situation. Glimpsing into the future with a whole array of sensors and devices worn in different body areas, increasing the efficiency of communication within the so called „body area network“, offers huge opportunities for energy saving. Companies such as EnOcean have developed optimized protocols that allow for much shorter data telegrams compared to the IPv6 formats, which leads to a significantly lower power consumption for the same amount of information.

All these different improvements can be used to provide truly self-powering wearables or to increase the performance of wearables with high energy demands due to complex systems for sensing, processing or displaying information. Combined with wireless energy supplies for inductive charging, consumers might soon experience significant improvements in the maintenance of their wearables, which in return would push the boundaries of acceptance for wearable technologies into broader markets. Innovative energy solutions will also be featured as one of the key areas at the upcoming Wearable Technologies Conference on July 8th and 9th in San Francisco. Make sure to check the agenda and take a chance to get in touch with the leaders in the wearable technologies industries.