Energy harvesting for wireless sensor networks
Without wires, wireless sensor networks directly don’t receive power for operation; so researchers are investigating energy harvesting methods. Here’s a look at this latest method for powering these networks.
The most important feature of wireless sensor networks (WSNs) is the elimination of wires – communication and power. Without wiring requirements, they can be deployed in a range of remote monitoring applications where running wires is prohibitive or impossible. The removal of wires also reduces the cost per measurement point required for monitoring existing systems.
WSN systems have been varying power needs from a few microwatts of power in sleep mode to 100 milliwatts in transmit mode. Thus these systems require a means to store energy. Wireless protocols and several energy-storage technologies are proven and available from multiple vendors, although as with all technology, expect and plan for evolution and change. The scale of technology maturation is broad.
Advantages and disadvantages of batteriesBatteries are the most commonly used energy-storage medium for WSNs. They are mature technology with an easily accessible distribution channel. Power and voltage availability are well defined, and the ranges of operating temperature are clear. The two prominent, available battery technologies are the traditional alkaline battery and in recent years, the lithium iron disulfide (Li/FeS2) batteries with extended operating temperatures, capacities and shelf lives.With new low-power WSN systems, nodes can operate for up to 3 years at a 1-minute sample interval on four AA batteries. For certain applications, such as machine condition or structural health monitoring, the goal of continuous power for the operating life of the system has generated significant interest and investment in energy harvesting research.
Future WSN and energy harvesting system architecture In discussions of future architectures for WSNs, engineers and researchers bring up energy harvesting transducers. How the transducers and power electronics will be packaged is still unclear. Future architectures often include batteries and other forms of energy storage.
Supercapacitors One technology under investigation and used in some early products are supercapacitors, also known as ultracapacitors. Supercapacitors are commonly used in hybrid electric vehicles (HEVs), and store energy by physically separating positive and negative charges, unlike batteries that rely on chemical reactions to separate charges. Supercapacitors store electric charges similar to static electricity with a balloon. Because the process is highly reversible and can be cycled millions of times to extend life, they have the advantages of longevity and quick charge and discharge rates. They currently do not offer the same energy storage density as batteries.
Supercapacitors also require additional electronic control and switching circuitry. In future WSN architectures, a supercapacitor might be used for energy storage and connected to specific power electronics. The power could come from multiple sources, each requiring a unique transducer to provide a power source for WSN nodes.
Solar energy harvestingAs an energy source for WSNs, solar panels are the most mature technology and supported by developed markets. Complete systems with solar panels, inverters, batteries, and mechanical enclosures are available from several vendors including SunWize, Mr. Solar, and Powerup, to name a few. Stores including Lowe’s and Home Depot also provide solar panels for outdoor lighting and other applications that can be converted for use with WSN systems. The factors to consider when selecting a solar panel include:
Solar radiation or insulation of installed location: typically specified using zones which correspond to the average solar intensity of a region (highest solar intensity zones are closest to the equator)
Voltage range of a WSN: typically 12 or 24 volts are available from solar vendors
Daily load requirements: measured in amp/hour/day
Mechanical enclosure and mounting accessories.
Thermoelectric energy harvestingAnother potential energy source is derived from thermal differences in the environment. Thermoelectric generators (TEGs) convert temperature differences across dissimilar materials into an electrical potential or voltage. Based on the Seebeck effect, the voltage generated is directly proportional to the number of leg pairs (N) times the Seebeck coefficient (α), times the temperature difference (ΔT) between hot and cold side.
Voltage = N x α x ΔT
By depositing metals with low-thermal conductivity like bismuth (Bi2Te3) onto the silicon substrate of silicon wafers a TEG can extract potential energy from large differences in temperature. According to data from Micropelt, a supplier of TEG-sbased energy harvesting technology, this type of technology can generate ~ 0.5 mW with a temperature difference of 10 °K up to 1.5 mW with a temperature difference of 20 °K.Vibration energy harvestingThis method can use one of two types of technologies: electromagnetic induction where energy is harvested from the magnetic field produced within an AC inductive motor and piezoelectric energy harvesting from mechanical vibration.
Electromagnetic inductionEnergy is harvested from the metal windings within the rotor and stator of AC inductive motors. The actual transducer is a magnetised mass of several grams connected to a specifically designed and tuned spring. This spring is tuned to harvest energy from the resonate frequency produced from AC current.
In the US, line voltage alternates at 60 hertz (Hz) producing a resonance frequency at 120 Hz. In Europe, where the line supply is 50 Hz, the frequency of interest is 100 Hz. An example of this technique is the PMG energy harvester from Perpetuum. According to data from Perpetuum, a 450-gram device can produce at least 0.3 mW on most AC motors and when tuned properly can reach a maximum of 50 mW.
Piezoelectric energy harvestingEnergy is harvested from mechanical vibration using the piezoelectric effect, which converts mechanical strain from a vibrating mass into electrical current. This strain can come from different sources, including low-frequency seismic vibrations, acoustic noise, and most commonly, vibration from rotating equipment.
One viable application is generating power from large rotating equipment. When the piezoelectric energy harvester is tuned to the appropriate resonance frequency of a specific vibration source, power in the mW range is possible. Data provided by AdaptivEnergy with their “joule thief” technology shows that a range of 0.05 to 0.15 mW is possible with vibration in the 0.025 to 0.075 gm/s. These results are encouraging. Technology should continue to improve the efficiency of energy harvesting techniques.
The promise growsHarvesting energy from the environment offers an exciting promise for long-term WSN deployments. Different sources of energy suit specific areas of applications. For example: solar harvesters suit outdoor remote applications where sunlight is readily available.
Vibration energy harvesting technology works well in industrial settings with large AC motor drives and other rotating equipment.
If anyone wants to deploy WSN systems today, the best approach is to select hardware and software platforms from established measurement and automation vendors. They have software architectures to abstract changes in networking protocols and hardware product roadmap to adopt the compelling new power management and energy harvesting technologies, whether from batteries or a combination of emerging energy harvesting and storage technologies.