DoubleDip: leveraging thermoelectric harvesting for low power monitoring of sporadic water use

We present DoubleDip, a low power monitoring system for enabling non-intrusive water flow detection. DoubleDip taps into minute thermal gradients in pipes for both replenishing energy reserves and performing low power wakeup. One of the remaining issues with wireless water monitoring in residences and offices is that current solutions require installing sensor nodes with access to electrical wiring or replacing batteries frequently. DoubleDip (DD) significantly extends the lifetime of vibration-based non-intrusive water flow sensors by harvesting thermal energy from hot pipes wherever accessible. DoubleDip requires less than an inch of exposed metal pipe to attach a coupler for gathering sufficient energy to power itself, in some cases, into perpetuity. We observe that water use in homes and offices is incredibly sporadic, making continuous monitoring both impractical and wasteful. Instead, DD puts a thermoelectric harvester into double duty. It uses thermal gradients not only for gathering energy but also for extremely low power (< 1μA) wakeup. In this paper, we describe the DoubleDip design and demonstrate that thermoelectric wakeup is essential for longevity and accuracy. Since DD wakes up from its low power state only when there is a water flow event, it replenishes the energy it uses in sensing and transmitting data by the energy it harvests from the corresponding heat gradient. While DD nodes installed on cold water pipes harvest far less than those installed on hot water pipes, our pilot deployment over four weeks and five locations suggests that thermoelectric wake up is only slightly worse in latency for cold water monitoring and there is sufficient energy harvested from the hot water that it can be shared to extend the lifetime of nearby cold water nodes too.