The third wireless wave is here and is about to transform our lives. The first two world-changing wireless waves were cell phones and wireless Internet. This new wave is the “Internet of Things”–a cornucopia of wireless sense and control networks, connecting all kinds of equipment in our homes from freezers to light switches, from consumer electronics (like TVs and DVD players) and remote controls to sensors, for detection or protection, and to central door and window locking mechanisms (like those in our cars).
Unfortunately, all these sensors and controls need power and will require significant quantities of batteries. This, in turn, generates a plethora of environmental concerns surrounding the manufacture and disposal of batteries–think heavy metals and toxic chemicals. In addition, a serious maintenance problem needs to be solved: no one wants to be continuously exchanging and recharging batteries.
Energy harvesting (or scavenging) is an exciting technology development that could end our battery addiction. Instead of relying on batteries, or even power from the grid, this next generation of wireless devices can be powered by energy that's available in the environment.
Unfortunately, there's a lot of hype around energy harvesting that must be separated from the reality. Energy harvesters can usually be divided in two categories: the tricklers and the bursters . The most common energy trickler is the solar cell, where certain varieties of solar cells can pull energy from the limited amount of light that's available indoors.
To be usable for sense and control networks, it's important that the energy be stored until required and that the use of that energy is controlled. For example, a task such as data transmission is only started when there's enough energy to complete the task. Solar cells have a big advantage in that they're relatively inexpensive. However, they have a serious drawback: they only work when there's light. A reliable network needs to be independent from ever-changing and uncontrollable lighting conditions.
Another interesting trickler energy harvester is a Peltier element that taps into the energy stream that comes into existence when two sides of one object have a different temperature, such as a wall of a house. Usually a temperature difference of five degrees Celsius gives enough usable energy. Unfortunately, like solar, the dependability of Peltier elements is limited because temperature differences usually can't be controlled, and, contrary to solar cells, Peltier elements are relatively expensive.
The most common burster is the dynamo , a sort of reverse step engine, where instead of energy being used to create motion, motion is used to create energy. This form of energy creation is quite crude, because in most cases, once the process is set into motion, the amount of energy that becomes available is way beyond the requirements of a sensor network and is wasted. In addition, this type of energy harvesting is quite expensive.
Another energy burster is the piezo element , where energy is created from mechanical torsion. The amounts of energy generated by this solution can be relatively large. However, piezo elements are sensitive to wear and tear, and designing them to be reliable and durable makes them also quite expensive.
Energy harvesting is still not ready for prime time. There are major challenges to overcome before it can be viewed as a realistic option. Aside from solar cells, no cheap and high-volume solutions are in sight. So far, solar cells are the only energy harvesters being produced in high volume and at the low cost required by sense and control networks. However, to overcome solar's shortcomings, small batteries are still required to store power during dark times. The good news is that the batteries' need for heavy metals and toxic chemicals is somewhat reduced.
One of the most interesting developments in this area is an emerging technology that harvests energy from trees (by putting an element between the tree and the ground and tapping energy from the difference in the electrical charge–and available 24/7. It would be a nice step forward to be able to generate power from houseplants. You could eliminate the need to change the batteries in some of household controls simply by watering your plants.
Eliminating all batteries through energy harvesting is still a dream, but as the technology improves and costs drop, this dream of “No Batteries Needed” is coming closer. In the meantime, reliable ultra low-power networks that are so highly energy efficient that they can operate off of a single-cell battery for the life of the device are already are rolling out.
Cees Links is the CEO of GreenPeak Technologies, a fabless semiconductor company with a strong focus on wireless and batteryless technology, targeting communication devices working on ambient energy. Links holds a masters in applied mathematics and a BSEE from the Twente University of Technology in Enschede, The Netherlands.