No more batteries: Breaking the hype of energy harvesting
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.