Low Power Design: Energy Harvesting

Friday, March 25th, 2011 by Robert Cravotta

In an online course about the Fundamentals of Low Power Design I proposed a spectrum of six categories of applications that identify the different design considerations for low power design for embedded developers. The spectrum of low power applications I propose are:

1) Energy harvesting

2) Disposable or limited use

3) Replaceable energy storage

4) Mobile

5) Tethered with passive cooling

6) Tethered with active cooling

This article focuses on the characteristics that affect energy harvesting applications. I will publish future articles that will focus on the characteristics of the other categories.

Energy harvesting designs represent the extreme low end of low power design spectrum. In an earlier article I identified some common forms of energy harvesting that are publicly available and the magnitude (typically in the μW to mW range) of the energy that are typically available for harvesting.

Energy harvesting designs are ideal for tasks that take place in locations that are difficult to deliver power. Examples include remote sensors, such as might reside in a manufacturing building where the quantity of devices might make performing regular battery replacements infeasible. Also, many of the sensors may be in locations that are difficult or dangerous for an operator to reach. For this reason, energy harvesting systems usually run autonomously, and they spend the majority of their time in a sleep state. Energy harvesting designs often trade-off computation capabilities to fit within a small energy budget because the source of energy is intermittent and/or not guaranteed on a demand basis.

Energy harvesting systems consist of a number of subsystems that work together to provide energy to the electronics of the system. The energy harvester is the subsystem that interfaces with the energy source and converts it into usable and storable electricity. Common types of energy harvesters are able to extract energy from ambient light, vibration, thermal differentials, as well as ambient RF energy.

The rate of energy captured from the environment by the energy harvester may not be sufficient to allow the system to operate; rather, the output of the energy harvester feeds into an energy storage and power management controller that conditions and stores the captured energy in an energy bladder, buffer, capacitor, or battery. Then, when the system is in an awake state, it is drawing energy from the storage module.

The asymmetry between the rate of collecting energy and consuming energy necessitates that the functions the system needs to perform are only executed on a periodic basis that allows enough new energy to be captured and stored between operating cycles. Microcontrollers that support low operating or active power consumption, as well as the capability to quickly switch between the on and off state are key considerations for energy harvesting applications.

A consideration that makes energy harvesting designs different from the other categories in the low power spectrum is that the harvested energy must undergo a transformation to be usable by the electronics. This is in contrast to systems that can recharge their energy storage – these systems receive electricity directly in quantities that support operating the system and recharging the energy storage module.

If the available electricity ever becomes insufficient to operate the energy harvesting module, the module may not be able to capture and transform ambient energy even when there is enough energy in the environment. This key condition for operating means the decision for when and how the system will turn on and off must take extra precautions to avoid drawing too much energy during operation or it will risk starving the system into an unrecoverable condition.

Energy harvesting applications are still an emerging application space. As the cost continues to decrease and the efficiency of the harvesting modules continues to improve, more applications will make sense to pursue in an analogous fashion that microcontrollers have been replacing mechanical controls within systems for the past few decades.

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