  A Wireless Power Transfer Network (WPTN) aims to support devices with
cable-less energy on-demand. Unfortunately, wireless power transfer
itself-especially through radio frequency radiation rectification-is fairly
inefficient due to decaying power with distance, antenna polarization, etc.
Consequently, idle charging needs to be minimized to reduce already large costs
of providing energy to the receivers and at the same time reduce the carbon
footprint of WPTNs. In turn, energy saving in a WPTN can be boosted by simply
switching off the energy transmitter when the received energy is too weak for
rectification. Therefore in this paper we propose, and experimentally evaluate,
two "green" protocols for the control plane of static charger/mobile receiver
WPTN aimed at optimizing the charger workflow to make WPTN green. Those
protocols are: 'beaconing', where receivers advertise their presence to WPTN,
and 'probing' exploiting the receiver feedback from WTPN on the level of
received energy. We demonstrate that both protocols reduce the unnecessary WTPN
uptime, however trading it for the reduced energy provision, compared to the
base case of 'WPTN charger always on'. For example, our system (in our
experiments) saves at most approx. 80% of energy and increases 5.5 times the
efficiency with only approx. 17% less energy possibly harvested.
