Harvesting Ambient Energy in Wireless Sensor

Energy is the greatest problem faced by wireless sensor networks. When a sensor is depleted of energy, it can no longer fulfill its role unless the source of energy is replenished. Harvesting ambient energy is increasingly becoming one of the more attractive means for powering wireless sensor nodes. With it, it is possible to have self-powered, zero maintenance sensors. In nowadays, there are a number of ambient energy can be converted into adequate power for sensor nodes, include solar, wind, water and thermal energy. Harvesting energy for low-power (and possibly embedded) devices like wireless sensors presents a new challenge as the energy harvesting device has to be comparable in size (i.e. small enough) with the sensors. There are complex tradeoffs to be considered when designing energy harvesting circuits for WSNs arising from the interaction of various factors like the characteristics of the energy sources, energy storage device(s) used, power management functionality of the nodes and protocols, and the applications’ requirements. Currently, the main sources of ambient energy considered suitable for use with WSNs are solar, mechanical (vibration or strain) and thermal energy.

A power efficient RF transceiver is a critical component of the wireless sensor’s design. Just as with the MCU, it has a significant impact on battery life with a low power sleep mode, low RX power, programmable TX power, and wakeup timer capability.

Integrated transceivers are commonly available today that include all of the essential RF circuitry – filters, amplifiers, mixers, modulator/demodulator, etc. in a single small package. While critically is important to WSAN node design, the many subtleties of RF circuit design can make this portion of the design complex and difficult. Fortunately, IC and module suppliers offer products that can greatly simplify the task.

The chip industry is progressively offering more integrated SoC devices optimized for WSAN applications. These typically integrate low power MCUs with standards based RF communications devices. There are some early hints of an emerging trend toward offering devices pre-programmed with ROM-based protocol stacks to further simplify the software development task. Module suppliers take this even further, offering complete wireless modules that include the MCU, radio, protocol stack, and in many cases even the antenna, in a small integrated module that is already tested and certified to FCC/ETSI requirements.

Modules offer a much quicker path to market, and potentially large cost savings compared to the expense of an in-house custom design (assuming RF design and test capabilities even exist in-house). With complete modules priced between $10 and $20, and the underlying component bill of materials accounting for two-thirds of that, the make vs. buy cost analysis for a custom design may not point to an economic benefit until volumes exceed 50 k units.

With no ubiquitous WSAN protocol, one must choose from a confounding array of protocol options. Some, like ZigBee, WirelessHART and 6LoWPAN are more widely adopted for certain types of applications. Fortunately, component suppliers today support a variety of options with pre-tested software stacks for ultra-low-power MCUs, sophisticated standards-compliant RF ICs, and even fully integrated and pre-certified off-the-shelf modules designed for WSAN applications.