Surrounded by trees as I am, I was fascinated by the article "Preventing Forest Fires With Tree Power: Sensor System Runs On Electricity Generated By Trees" that ran in Science Daily. Put simply, by plugging wires into trees, the researchers can power a wireless node equipped with temperature and humidity sensors and use it to monitor for forest fires and provide an alarm if one is spotted. Genius!
Not that the system is quite that straightforward—the electricity produced by the tree is used to provide a trickle charge to a conventional battery, which then provides the power to transmit sensor readings a couple of times a day. (While I knew that you can get electricity from a lemon, I hadn't realized that you can do the same to a tree.) That said, the system is delectably elegant.
At Sensors Expo in June of this year, I sat in on Randy Frank's all-day Energy Harvesting Symposium and one of the recurring themes was the need to evaluate the environment of your particular application to figure out what sources of "waste" energy would be the best fit for your power requirements. If you're monitoring vibrating machinery, for instance, then those vibrations can provide a reliable source to vibration-based energy harvesting devices. If you're somewhere sunny, maybe solar is a better choice. Tapping into the local vegetation hadn't occurred to me, but really, why the heck not?
As with anything sensor-related, fundamentally it's a question of assessing an application's particular environmental and performance needs. I suspect that we're continuing the trend toward more customized (and customizable) sensing systems. That's reflected in the electronic components used in sensors (and used to integrate sensors into larger systems). For instance, microcontrollers and data converters that provide a range of capabilities from which to select those best-suited for your application and data acquisition equipment, which is increasingly modular. And as sensors expand into more markets and many more niche applications, all of which will have slightly different environmental and performance constraints (because that's the nature of sensor applications), expect the need for sensor adjustability to grow. The interest and work in energy harvesting highlights yet another avenue for customization: how to power your devices.