The evolution of energy harvesting technology
The ability to harvest energy from the environment is increasingly opening up design opportunities in the Internet of Things and wearable technology
Harvesting energy from the environment to power sensors and electronics has been a ‘holy grail’ for system developers for many years. With the emergence of the Internet of Things and the Smart Home, being able to place sensors and actuators anywhere without having to worry about power and data links provides dramatically more flexibility.
While battery-backed wireless sensor nodes have been able to do this already, harvesting solar, thermal or even vibrational or RF energy extends the time the node can operate without having to change the battery. In systems with thousands of nodes this is a vital requirement, and good design of the energy harvesting power system can extend or even eliminate the battery replacement cycle and save considerable amounts of operating expenses.
Several factors have come together in the last few years to make energy harvesting a viable prospect. The march of Moore’s Law and semiconductor process technology has seen the power requirements of sensors, microcontrollers and wireless transceivers plummet. The latest devices such as the Leopard Gecko microcontrollers from Silicon Labs or the STM320 transceivers from EnOcean have been designed to have power requirements in the microamp range and still deliver the processing performance and data links that are needed for the Smart Home. This allows solar panels to deliver sufficient power from indoor lighting, for example.
Image - Silicon Labs’ Leopard Gecko used in a low energy fitness tracker
The efficiency of the energy harvesting sources has also improved. The latest solar panels are seeing efficiencies up in the 15 to 20% range, allowing either for more power for local processing or more power-hungry sensors, or for smaller cells for less obtrusive wireless nodes with development kits such as those from Silicon Labs.
Harvesting energy from temperature differences has also improved over the years. This is particularly useful for sensors and actuators at heating elements such as radiators, linking to the latest smart thermostats such as Nest. As thin film deposition process technology has become more affordable, this is being used to build energy harvesting engines that use the Peltier Effect. These are now able to use a temperature difference of a few degrees to power temperature sensors and actuators.
Image - Silicon Labs’ energy harvesting reference kit
Another factor that has improved the adoption of energy harvesting technologies is the improvement in power management devices. Energy harvesting provides a mixture of very low levels of energy and large bursts, and many traditional power management chips cannot cope with this. A new generation of power management chips can handle the low currents and wide ranges of the energy input, as well as manage the battery sub-system.
Improving standards and interoperability for ultra-low power wireless links is also helping the adoption and development of energy harvesting. The EnOcean Alliance now has over 350 companies supplying all kinds of energy harvesting equipment. This ranges from kinetic sources for switches that use a piezo electric crystal to generate a flash of power to send a signal, to solar powered proximity and occupation sensors and thermally powered heating and window controllers. The savings from installing such self-powered systems is recouped in a matter of months by reducing energy bills, and the data that is collected can be analysed in the cloud to further optimise the operation of buildings both large and small. For the EnOcean Alliance, this is enabled by a very low power wireless protocol that links all the different devices and has been released as an open standard.
EnOcean Alliance energy harvesting applications
There are also other alliances and industry groups, particularly in building automation, that are taking advantage of energy harvesting technologies and ensuring that products from different manufacturers are interoperable. Open protocol groups such as Thread and Alljoyn are also aiming to provide an open environment so that smart devices, many of which will use energy harvesting, can connect easily and securely.
Energy harvesting is also getting more personal. As wearable systems such as smart watches become more popular, so there is increasing development for personal area network (PANs) around the body. These can generate power from the heat or the movement of the body, using those piezoelectric crystals in shoes or even in clothing to generate power. Researchers have shown that metallic fibres can be woven into cloth that generates power as the person moves around. This can then be used for sensors around the body, to recharge personal electronics such as watches and phones, or even to power LEDs that are sewn into the clothing to change its colour or spell out messages.
By reducing or eliminating the need to charge batteries, energy harvesting technologies are opening up new areas of design. From smart sensors and actuators that can be placed anywhere, to power from walking or clothing, there are opportunities for developers to provide power and intelligence in new places.