RF energy harvesting : don’t wait for the Holy Grail to start saving power

Energy harvesting (or energy scavenging) is the process by which energy is derived from external sources, captured, stored and then used to power an electronic device. The energy sources can be as diverse as thermal energy, kinetic energy, electromagnetic energy, solar energy, …

The Holy Grail in energy harvesting is to be able to manufacture and market electronic devices that are able to gather enough energy from their environment to operate completely without a battery. There are already today some products on the market that meet this requirement.

They typically use mechanical harvesting techniques, e.g. batteryless switches powered by the mechanical energy generated when pushing the button. Some experiments have already proven RF energy harvesting to be feasible, for example by scavenging surrounding TV signals. But so far, it hasn’t been anywhere close to a commercial product 100% powered by energy harvesting.

However, it doesn’t mean that there is no concrete realization possible. The gap between the amount of energy gathered from a harvesting source and the energy needed to run an integrated circuit is becoming smaller and smaller. Ultra Low Power design techniques have brought the power consumption of an IC in the same order of magnitude of what can be harvested, at least when it comes down to performing a limited number of operations.

There are lots of example of devices that only need to execute power demanding actions once in a while, and stay in the waiting mode more than 99% of the time. A wireless sensor network is an example of this kind of systems. Most of the time, there is no activity required.

It is only at regular intervals or after a certain event occurs that the sensor nodes send some information. If however more power hungry operations are needed, one could think of a hybrid system where the sensor nodes are powered both by a harvesting source and a battery. In this case, using harvesting results in significant power savings and increases the battery life dramatically.

To develop that kind of systems, there is a need for an energy source that is available virtually everywhere and all the time. RF harvesting is very well suited for this. Electromagnetic energy is everywhere around and can be used at any time provided that the energy level is high enough. On top of this, RF energy can be used as a remote wake-up event.

Dubbed LOTUS, the ANS1801 is a prototype IC developed by AnSem designed to implement RF energy harvesting in radio-wave rich environments. This ultra low power transceiver requires only 10 nJ per bit to transmit an FSK modulated signal.