[Editor's Note: This was originally posted on the Embedded Master]
The lower limit of power consumption of embedded processors continues to drop; however, there is a point where parts that operate with even smaller amounts of energy is equivalent to operating on no power. Today’s lowest power parts, such as the ones I discussed in earlier posts in this low power series, are at the edge of this point because designers are beginning to be able to pair them with energy harvesters that are able to pull more energy from the ambient environment than the application needs to operate for an indefinite period of time.
The practical limit for low power operation as “no power operation” will be at that point where harvesting the ambient energy is sufficient to allow a system to operate continuously. There are currently no systems that operate at this lower limit yet. Additional energy efficiency after this point becomes an opportunity to add more processing features to the system, analogous to how higher clock frequencies and parallel computing engines enable today’s high-end systems to take on more capabilities.
Energy harvesting is a process that enables a system to capture, store, and operate off of the ambient energy from the surrounding environment. Ambient energy can be harvested in many forms; the most commonly tapped forms at this time are thermal, light, vibration, and RF (radio frequency). I will explore the companies providing methods for harvesting these types of energy in later posts. Essentially, these types of systems harvest “free energy” from sources that we currently are not able to tap for any other work.
Currently, batteries are able to provide a reliable and cost effective source for low power systems, and they usually enjoy a cost advantage over the various energy harvesting methods. Despite this cost advantage, there are usage scenarios, namely those cases where changing batteries are impractical, costly, dangerous, or even impossible, that make using an energy harvester a more practical approach. Examples scenarios include implantable medical devices; surveillance and security equipment, as well as buildings and structures with smart sensors distributed throughout them.
The first requirement of energy harvesting systems is that they must be able to extract more energy from the environment than the amount of energy the collection and storage components consume. Storage options include batteries, capacitors, and thin-film technologies. Follow-up posts in this series will explore the cost and efficiency challenges facing the types of transducers available to extract the ambient energy as well as the challenges facing the energy storage technologies.
The second requirement of energy harvesting systems is that they must be able to monitor their own energy storage and adjust their operation to avoid starving their energy storage so that the energy collection components can still operate when there is energy available to harvest. Designers of these types of system need to be able to view energy as a variable resource and design their systems to scale with the inevitable fluctuations of energy availability so that the system can remain operational despite periods of “starvation.”
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Hi Robert,
Interesting post.
I attended the “New Horizons in Medical Electronics” panel last week at ESC, and asked the panel whether
a) increases in efficiency for energy harvesting methods, or
b) improvements in energy efficiency for ultra-low power devices
was the primary “nut to crack” to enable proliferation of such devices. They answered emphatically “the latter”.
Mir Imran also had an interesting comment, however. While it may seem appealing to avoid surgery to replace batteries in implantable devices, the truth is that by the time the battery needs to be replaced (~7 years), the user/patient may prefer to upgrade to new technology anyway – providing a convenient opportunity for battery replacement.
Of course, I wonder how this dynamic would change if the ultra-low power devices were actually programmable – e.g. an ASIP – enabling a degree of “field-upgradability”.
Something to think about.