In this paper, we considered the possibility of using electricity harvested from the microwave field leaked from commercial microwave ovens. Our experimental results showed that the leakage received by a dipole antenna was about 0 dBm (1 mW) at a point 5 cm in front of the door. A rectenna consisting of a dipole antenna and charge pump can convert the leaked microwave energy into a DC current. When a microwave oven is operated for 2 min, 9.98 mJ of energy was harvested. We demonstrated that this energy is sufficient for powering a digital cooking timer to count down for 3 min and beep for 2.5 s. The operation of other kitchen devices was also demonstrated.

Energy harvesting technology is one of the key technologies for the long-term operation of wireless sensor networks. However, power supply from such energy harvesting sources is not as stable as batteries. In order to compensate for the spatial and temporal variations of the energy harvesting sources, an energy storage device is used. Rechargeable batteries are commonly used in wireless sensor nodes. However, batteries can be charged and discharged for only a limited number of cycles. When the harvested energy is small, capacitors are better suited for energy storage because they can be charged and discharged quickly and efficiently. Moreover, the charge-discharge capability of capacitors does not degrade. The only critical problem of capacitors is that their energy leakage is too large to ignore. In this paper, we propose a capacitor leakage aware duty cycle control for energy harvesting wireless sensor networks. Our proposed method focuses on capacitor leakage reduction, which must be considered in a capacitor driven system but has not been considered to date. Our method maximizes the energy usage without degrading system availability by applying optimal control using multiple evaluations of the energy neutral operation and capacitor leakage.