A foundational concept of wireless sensor networks is that they are not tied down by wires, neither for data transmission nor power transmission. The traditional approach to providing power to sensors is with batteries, but batteries eventually die, hence the search for a renewable power source. Although solar and wind were early contenders in this search, it was only a matter of time before researchers started looking to harness all of the electromagnetic radiation that permeates our environment.

This paper starts by taking specifications from commercially available radio frequency power harvesting equipment to build a MATLAB simulation. The authors show how energy can be transferred between nodes and how a multihop energy transfer mechanism could be more efficient than one using a single hop, in some cases extending the range covered by a charging node. I did appreciate that the authors then built a working prototype network where the wireless sensor nodes would charge each other by sharing power transmitted from a charging node.

Talk to a layperson about “power over WiFi” (a not entirely inappropriate name for this line of research), and most will start envisioning a utopia where home theater speakers can be placed in a room without the need to run any wires, mobile phones never run out of power, and, in the extreme, we celebrate the death of the plug and socket. Is this future possible? Looking at the numbers in the paper does cast a shadow over the dream. Distances between the nodes are still less than one meter and it still costs thousands of joules to transfer a single millijoule over two hops. Dream we can, but for now we would have to live inside a microwave oven for a sufficiently high energy density.