The concept of energy harvesting is not new, but utilizing the solution in real products has its challenges. What is energy harvesting, why is it difficult to implement, how will it affect Internet of Things (IoT) devices, and what implications will it bring?
What is Energy Harvesting?
Energy harvesting is the mechanism by which a device can absorb local, naturally occurring energy, convert it into useful energy, and then use that energy to perform an action. In the case of electrical devices, an energy harvester can convert environmental energy into electricity, which can further be used in low-powered electrical devices. There are a large number of sources of naturally occurring energy which can be used to generate electricity, including solar, wind, vibration, magnetic, thermal, and radio.
Examples of Energy Harvesting Techniques
Extracting energy from the surrounding environment is not always an easy task, and is often very limited. So, how can naturally occurring energy sources be converted into electricity?
Solar
The most obvious example of naturally occurring energy is sunlight; the use of solar cells can convert the light directly into electricity with the use of the photovoltaic effect, and this can charge either a capacitor or battery to further operate a device. However, such a source of energy often requires a large amount of surface space to maximize the energy intake, and often only works in the presence of direct light.
Wind
Wind is another obvious example of naturally occurring energy, but like solar, it has its issues. Converting wind into electricity is done with the use of a dynamo connected to a rotor, and as wind passes through the rotor the dynamo shaft is turned, which results in the generating of electricity. Such a system requires the energy harvester to be placed in the presence of wind, and wind is an irregular source of energy.
Thermoelectric
A thermoelectric system can generate electricity when there is a large difference in temperature between two points. A basic example of a thermoelectric system would be a Peltier cooler with the “hot” in contact with a warm object, while the “cool” side is in contact with a cool object. The result of this is referred to as the “Peltier Effect”, whereby the Peltier cooler generates electricity.
Electromechanical
An electromechanical energy harvester is one that converts mechanical energy into electrical energy. This can be done using a wide range of different methods, but the piezo effect is the most common. A piezo material is one that deflects in a specific direction when an electrical current is applied across it, and generates a current when the material undergoes mechanical deformation. Thus, a piezo element can be mounted to a vibrating object, and the element will produce an electrical current.
Radio
A radio energy harvester is one that converts radio waves directly into electricity. Such energy harvesters are the rarer type (as radio waves are incredibly weak), and generate very little power. However, if placed within a strong enough radio field, these can be used to power devices. One classic example are old-fashioned crystal radios, which do not require a power source to operate (only a long antenna and connection to ground).
Why is Energy Harvesting Difficult?
One of the largest challenges to energy harvesting is finding a reliable source of energy that can provide enough power for the targeted application. For example, solar and wind can both provide decent quantities of power, but the sun does not always shine, and wind is incredibly unpredictable. Even if both are used concurrently, it is possible to have cloudy, still days where such an energy system would generate next to no power.
Other sources of natural energy, such as mechanical and thermal, are also incredibly unreliable, and trying to generate enough energy to run even a basic sensor is a demanding task. Another issue faced by energy harvesting systems is the power demand from the vast majority of devices; finding an IoT system that utilizes Wi-Fi and consumes less than 100mW when active is difficult, to say the least.
What’s Changing, and How is Technology Helping?
While energy harvesting devices are not practical for currently established devices, technological improvements are bringing out devices that may be able to operate entirely on energy harvesters.
The first improvement in technology that dramatically helps is the reduction of transistor size. Generally, the reduction of transistors helps to fit more onto a chip; however, the reduction of transistors also reduces the power consumption of each transistor. Thus, devices utilizing smaller transistors not only become more powerful, they consume less power per transistor, and therefore have potentially lower power requirements.
The second improvement in technology is the use of intelligent power designs that are either incredibly efficient (such as specialized DC-DC converters), or utilize wake-up systems that only consume power when required. The result of intelligent power design are devices that can operate on not only smaller instantaneous power sources, but consume less total energy (thus requiring less energy per burst of operation).
The third factor that is helping to improve energy harvesters, and the devices that are to be powered by them, is the increasing commercial demand for remote devices that require no internal batteries. Some devices, such as door bells, may need to be connected to a network, but are only required to be on when someone presses the door bell itself. The switch is replaced with a piezo energy harvester, and the result is that upon being pressed, the device powers up, sends a message to an alarm system, and then powers down.
How Would Energy Harvesters Change the IoT?
If reliable energy harvesting techniques can be incorporated into IoT technologies, the result would be monumental. Since IoT technologies focus on small sensors with networking capabilities, large scale deployments could quickly be made with no need for finding power sources. This allows for devices to be mounted in almost all places, and never require battery changes or charging.
Energy harvesting in IoT devices could also allow for easy installation of smart homes; the lack of power or communication wires allows an IoT devices to be quickly added to a home network. However, the device capabilities would depend on the energy sources available to it, thus devices such as cameras and audio monitors are unlikely. Such IoT devices may come in the form of smart buttons that perform specific actions upon being pressed (such as ordering more items, opening doors and windows, or setting environmental controls).
Energy Harvesting Myths
While energy harvesting may seem like a golden technology that could solve many issues currently faced by IoT devices (and more), it is important that designers understand the different between energy harvesters and energy stealers. Many so called energy harvesters are actually energy stealers, and can result in increased greenhouse emissions.
Piezo devices that produce electricity from mechanical energy are a good example, and work well in switch applications (such as a door bell). However, those that are inserted into roads to absorb vibrational energy from cars can increase the resistance of the road to the car which, in turn, results in increased fuel consumption of the car.
Radio harvesters that absorb radio waves essentially reduce the strength of the signal, and thus the overall number of devices that can use the network. While it may be minimal in many cases, millions of sensors will result in a net loss of radiative ability, and therefore require the radio source to output more power to reach the same range and signal quality.
Electromechanical devices that couple to mains power supplies draw energy from the grid via capacitive coupling. Thus, power sources connected to the grid are required to output more power to make up the loss caused by the energy harvesters. Again, while individual energy harvesters may be insignificant, many millions could produce a sizeable load.
Is Energy Harvesting Practical?
As current technology stands, energy harvesters are more of a novelty than something that can be mass produced and made practical to modern products. The use of NFC technologies allows for scannable devices that only activate when near a reader, and these have been used for the better part of two decades, but IoT devices that can absorb their environmental energy are few and far between.
Another issue with these designs is that they can be near impossible to locate if they only operate when needed (i.e. deep sleep devices), and these could then bring in a whole new level of concern: privacy. Devices that spend most of their time absorbing energy, then operate in a quick burst, would not only be near impossible to locate, but could also be used in espionage and spying. A cellular IoT device could spend 23 hours charging, and use a 1-minute window to connect to a cellular network, listen in using a microphone, upload the content, and then shut down. While the 1 minute window is small, the sensor could be made to activate upon detecting a door open, a computer booting, or the presence of a NFC tag.
Conclusion
Energy harvesters have the ability to create highly practical IoT devices that do not require replaceable power sources or wires. Such devices would be easily installed into difficult environments, and the ability to use wireless technologies allows for mass deployments.
