Powering indoor sensing with airflows: A trinity of energy harvesting, synchronous duty-cycling, and sensing

Whereas a lot of efforts have been put on energy conservation in wireless sensor networks, the limited lifetime of these systems still hampers their practical deployments. This situation is further exacerbated indoors, as conventional energy harvesting (e.g., solar) ceases to work. To enable longlived indoor sensing, we report in this paper a self-sustaining sensing system that draws energy from indoor environments, adapts its duty-cycle to the harvested energy, and pays back the environment by enhancing the awareness of the indoor microclimate through an \energy-free" sensing. First of all, given the pervasive operation of heating, ven- tilation and air conditioning (HVAC) systems indoors, our system harvests energy from air ow introduced by the HVAC systems to power each sensor node. Secondly, as the harvested power is tiny (only of hundreds of W), an extremely low but synchronous duty-cycle has to be applied whereas the system gets no energy surplus to support existing synchronization schemes. So we d sign two complementary synchronization schemes that cost virtually no energy. Finally, we exploit the feature of our harvester to sense the air ow speed (which can be used to infer the indoor microclimate) in an energy-free manner. To our knowledge, this is the first indoor wireless sensing system that encapsulates energy harvesting, network operating, and sensing all together.