The quest for smarter cities and ultimate sustainability has led European researchers to fabricate an advanced energy harvesting technology that can serve as an independent power source for sensor networks.
Wireless sensor nodes or WSN hold powerful technological potentials with manifold applications ranging from weather forecasting to crime control. However, the power management in this technology has remained nearly intractable, until now.
Like most electronic devices, wireless sensor nodes have mostly been powered via the use of batteries and connection to power grids. While battery power sources are limited by their battery life, sourcing power from grids requires extensive hardware installations to transmit power from the grid to a WSN facility.
Hence the two power management solutions are fraught with numerous pitfalls. However, with the advanced energy harvesting technological innovation, sensors and WSN can be imbued with capacities to harness energy directly from environmental mediums like the sun, ambiance, waves, and vibrations. This enables the installation of WSN with maximal self-sufficiency and without environmental concerns, in just about any location.
Module Designs for Energy Harvesting Power Sources
The innovations in energy harvesting power sources are on the cusps of incorporating multiple energy harvesting circuitry in single highly efficient modular designs of wireless sensor nodes. In general, the power requirement for operating sensors is very low, since the operations entail very low duty cycles of once every 15-30 minutes.
However, a typical sensor comes with 3.7 V, 1200mAh battery, and draws down 25 mA per second of active mode, 100 μA per 10 minutes of sleep mode. As such, wireless sensor nodes with batteries as the power source are usually not expected to last longer than a year, and that implies that the sustainability of the power grid power management solution for WSN isn’t quite viable.
However, one innovative way around this limitation is the technology for harnessing energy mediums such as the sun and vibrations that show an appreciable presence in the environment where the wireless sensors are set up.
And to bypass the limitations imposed by the diurnal variations of solar energy, these innovative energy harvesting power sources use solar energy to trickle charge the supercap of solar energy harvesters with high Fill Factor (FF). This modular design minimizes the size, cost, and rating of the solar PV cells, making for a highly efficient harvesting of solar energy by wireless sensors.
The table below presents the maximum harvestable power of one prototype of this circuitry design under various operating conditions. This prototype comes with 8 cells each with an open cell voltage of 0.5 V, arranged in a series array. The solar cell’s total open circuit voltage is 4 V and its short circuit current is 2.5 mA at 1000 W/m2 insolation.
The voltage of a completely charged supercapacitor of this prototype (with open circuit current of 3.3V) will begin to fizzle out when an outage occurs during the active mode. However, once the sensor goes into sleep mode, the supercap begins to recharge and regains its max voltage of 3.3 V before another duty cycle of the wireless sensor commences.
With this solar cell, the sensor can be operated every two minutes even during power outage. Furthermore, this energy harvesting power source enables a wireless sensor to start from a completely discharged state in a timeframe ranging from 14min in high insolation conditions (at 1000 W/m2) to 45 min in average insolation conditions (at 500 W/m2), which is an ideal operational condition for wireless sensor nodes.
Note also that this prototype of advanced energy harvesting power sources offers a means to trickle charge the ultra-capacitor of the sensors to make for additional power supply, making wireless sensors more cost-effective.
The difference between these advanced solar energy harvesting power sources and batteries is quite dramatic. While batteries provide an invariable power supply at all times, these energy harvesting technologies allow sensors to use energy when the energy is available; and at the same time, the power harvesters save energy for periods of energy shortfalls.
WSN is already a transformative utility that is increasingly offering prospective software solutions in a number of industries. It is now being made even more self-sustainable through the use of advanced energy harvesting power sources that make sensors even more reliable.