All Internet of Things devices require power to track the assets in which they are installed. But a major challenge in deploying wireless sensor networks is the limited length of time during which the sensors can do their jobs.

Using batteries in sensors brings major drawbacks. First, there’s only a finite amount of energy available in the typical battery.

In addition, the cost of replacing batteries in thousands of devices, especially those in out-of-the-way or hazardous locations, can make deploying wireless sensors inconvenient and cost prohibitive, according to Ahmad Salman, assistant professor, School of Integrated Sciences at James Madison University and an IoT device  expert.

“When you’re trying to deploy wireless sensor networks that would track certain birds in the jungle, you expect to deploy these and leave them for five years to gather as much data as you can from that span. You’re certainly not going to go and charge the batteries or [replace them] every once in a while,” he said.

The answer is to charge these batteries  by enabling sensors to harvest energy from the surrounding environment, said Jerry Luo, a lecturer in energy storage and energy harvesting at Cranfield University in the United Kingdom.

“That [energy harvesting] means we don’t need to send someone to replace the battery or charge the battery because the sensors will be able to operate by themselves and for a period of time that is long enough to collect the data,” he said.

The process of collecting energy from outside sources—such as light, heat and vibrations—is to power IoT devices is called energy harvesting.

These sources power IoT devices differently. Let’s look at these methods in turn.

Solar Energy

In many cases, solar energy is the No. 1 option unless the sensor is underground or in a location without sunlight, Luo said.

A rechargeable battery combined with a solar panel is sufficient enough to power an IoT device indefinitely, said Shams Kanji, manager of technical sales engineering at the Morey Corp.

In these applications, the IoT device is configured to read the sensors and typically transmit data every two to four hours, he said. The device configuration is smart enough to scale the data reporting based on the stored power. 

In most outdoor locations, the rechargeable batteries can store enough energy when there is sunlight to keep things running when darkness falls and in bad weather.

“Solar panels and rechargeable batteries work for most environments and geographical regions but not all, for example: Alaska in the winter months,” Kanji said.

Mechanical Energy

Mechancial energy is another energy source to power IoT.

“Machinery that can be deployed in power plants produce their own vibrations, and the vibrations from that heavy machinery produce a lot of energy,” Salman said . “These can produce up to 150 microwatts per centimeter square that can be used. So, if you have IoT devices that are on manufacturing belts or something like that, you can power these completely from the vibrations that are being produced by the machine that they’re monitoring.” 

A piezoelectric (an electric charge that accumulates in certain solid materials in response to stress) or electromagnetic device extracts the energy from sensors mounted to the piece of vibrating machinery and converts it into electrical energy to power the battery.

“The energy output for this kind of energy harvester is not really much, but for sensors it is usually sufficient because in many cases the sensor doesn’t need to send the data all the time,” Luo said. “They can send the data for a period of time, and then they have intervals between sending the data to the collection point.”

The piezoelectric effect allows you to harvest energy from anything that moves—even people, said Richard Soley, executive director of the Industrial Internet Consortium. 

“You can harvest power just from shoes moving up and down – not a lot of power, just a tiny amount,” he said. 

Energy generated by this kind of walking can be converted into electrical energy to charge wearable IoT devices, such as a smartwatch.

Tidal Motion

Energy harvesting technology can also harness the power of the oceans’ tides and convert their movement into electrical power with a piezoelectric approach, Soley said. 

Researchers have explored the use of tidal motion to generate electricity in California and Costa Rica, among other locations, he added.

A piezoelectric device built into the underwater pumps of an offshore oil rig could harvest energy from the motion of the surrounding tides to power IoT sensors to monitor pump status and send the data back to a remote worker. 

Thermoelectric Energy

Thermoelectric energy harvesting uses semiconductor devices to extract energy from temperature differences in natural and man-made environments. The thermoelectric generators convert the heat flow to electrical power.

“Semiconductor devices can harvest the waste heat from the hot surface and convert it into electrical power,” Luo said. “When there’s a temperature difference, there’s a hot site and there’s a cold site, that’s known as a temperature difference.”

When there are no vibrations, mechanical energy and solar power, thermoelectric can be used if there’s a hot surface because the semiconductor devices can harvest the waste heat from the hot surface and convert it into electrical power, he said.

“This might be feasible on an oil or gas pipeline,” he said. “On a pipeline they usually have some waste heat, but they usually don’t have much vibration and the pipeline is usually underground or it’s not exposed to the sun. So they can use the thermal energy on the surface of the pipe to provide the electrical energy.”

One drawback to using thermoelectrical energy is the expense.  

“So there are people trying to increase the efficiency and lower the costs for the thermoelectric material,” Luo said.

Advanced Primary Battery Technology

Although not quite an alternative energy source, newer primary battery technologies, such as lithium thionyl chloride, manganese dioxide, sulfur dioxide and silver oxide batteries, have come a long way in minimizing the leakage current and allowing operation at temperature extremes, Kanji said. 

When practical, these batteries are more reliable than solar charging because they can predict the exact number of transmit cycles based on battery capacity and power consumption for each data reporting cycle, he said. 

“The leakage current for battery type is specified and can be used in calculating the service life,” Kanji said. “The power level can also be monitored during each report to remedy any excessive power consumption by sending corrections to the device over the air. With longer transmit cycles, these batteries can provide power to the IoT device for up to 12 years, which is far longer than [the device’s] specified design life.”

Conclusion

Although it’s more complicated to implement energy harvesting technologies to power IoT devices than it is to use batteries, converting energy from the devices’ immediate surroundings may enable them to be powered perpetually and be maintenance free.