Abstract
This paper reports the design of a photovoltaic energy harvesting device used as a telemetry node in wireless sensor networks. The device draws power from the small solar cell, stores it into the primary energy buffer and backup supercapacitor, collects measured data from various sensors and transmits them over low power radio link at 868 MHz. Its design ensures reliable cold booting under very poor illumination conditions (down to 20 lx). The solar cell also enables indirect illumination level detection for the subcircuit that manages stored energy (day/night detector). The device is allowed to draw power from the backup supercapacitor only when it is not possible to gather enough energy from the solar cell during the sleep period. Short lasting and sudden drops of the illumination do not activate the backup power supply. A wireless sensor node design is adjusted to the proposed photovoltaic harvesting circuitry, so the overall power consumption in the sleep mode is less than 25 μW. Also, due to adaptive power consumption, proposed device topology ensures its autonomy time in the total darkness of 81 h. The device has been produced using commercially available components enabling versatile telemetric functionality by the implementation of different sensors.
Highlights
Constant reduction in power consumption of microcontrollers and sensors has led to the appearance of various energy harvesting systems [1]
wireless sensor networks (WSNs) node was acquiring temperature and humidity data from a sensor and transmitting a packet consisting of 12 bytes every 200 s
The device was characterized by using an incandescent light bulb with the nominal power of 21 W as a light source
Summary
Constant reduction in power consumption of microcontrollers and sensors has led to the appearance of various energy harvesting systems [1]. There are commercial ICs dedicated to indoor light harvesting which operate with low input/start–up voltage, implement MPPT technique and have low quiescent current [28] These ICs can not perform telemetric and large storage charging activities simultaneously. Appropriate power management circuit can obtain extended node autonomy due to the lower discharging current [36], or by forcing use of the ambient energy as long as it is possible [37] Another approach is to optimize the storage capacity to satisfy the energy neutral operation condition [31]. Detector forces power supply from the primary storage as long as the illumination level is high enough to allow at least one operating cycle of the WSN node.
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