Energy can be found everywhere – in the movement of doors and windows or machine components, the vibration of motors, changing temperature or variances in luminance level. These energy sources, which usually remain unused, can be tapped by means of energy harvesting to power electronic devices and transmit wireless signals. This technology is just starting to unfold its potential. The improvement of components and system design setup will open up new applications, particularly in the field of the Internet of Things (IoT).
Wireless energy harvesting technology is already well established in the building automation sector, and has bridged control of lighting, HVAC, and other elements of building technology to smart home, smart metering, and energy management systems. From these industries, platform-based approaches to wireless energy harvesting have evolved that can be transferred to many other applications where data capture and processing are combined with wireless communications. Such platforms incorporate energy converters, wireless transmitters, energy-efficient radio protocols, and development tools that, depending on the energy requirements, can enable wireless applications that work independent of cables and batteries.
To date, energy harvesting technology provides three primary sources of energy to power wireless modules: motion, light, and temperature differences. But before further researching platform approaches for these various modalities, it is necessary to understand the basic principles of a wireless energy harvesting system.
Basic principles of energy harvesting
Since most energy harvesters deliver only very small amounts of power, it is a pre-requisite that they accumulate energy over time while losing only a small fraction of it in the process. Therefore, a fundamental requirement of such systems is an extremely low idle current so that only a tiny amount of energy is consumed while in sleep mode.
Standard consumer electronics devices today have a standby current in the range of a few milliamps (mA), whereas power-optimized embedded designs typically achieve standby currents in the range of a few microamps (µA), an improvement factor of 1,000. In comparison, the latest generation of energy harvesting wireless sensors requires standby currents of 100 nanoamps (nA) or less, an improvement by a factor of 10,000 or more.
When in active mode, the available energy has to be used as efficiently as possible. This requires an optimized communication protocol used for transmitting data wirelessly. There are protocols in sub-1 GHz and 2.4 GHz spectrum ranges that are optimized for ultra-low-power communication, such as the ISO/IEC 14543-3-1X standard. Transmissions are just 0.7 milliseconds in duration and transmitted at a data rate of 125 kilobits per second (kbps). Although the transmission power can be up to 10 milliwatts (mW), only 50 microwatt seconds of energy are required for a single transmission. The range is up to 100 feet in buildings and 1,000 feet in the free field.
Selecting the appropriate energy harvesting technique is dependent upon the application, with factors for selecting motion, light, or temperature as a system’s primary power source contingent upon environment, output power, and other variables. Once identified, however, leveraging a complete platform that provides room for component optimization can help ease product integration and improve overall performance of the energy harvesting system.
Energy of motion
A core element of many wireless energy harvesting solutions is an electro-mechanical energy converter, which, as its name suggests, converts mechanical energy into electrical energy that becomes immediately available.
In electro-mechanical energy converters a magnetic flux is passed through two magnetically conductive laminations by a small but very strong magnet, and enclosed in a U-shaped core wrapped in induction coil (Figure 1). The magnetic parts are held in position by a plastic frame and spring-loaded clamp, but the U-shaped core leading through the coil is movable and can be positioned in either of two ways. In each position the core touches the opposite magnetic pole, resulting in a reversal of the magnetic flux in the U-core. This design ensures maximum magnetic flux alteration through the coil with minimal movement of the core, and therefore high efficiency with the smallest amount of energy waste.
Energy of light
Light is one of the most popular sources of renewable energy, and solar modules are often used to power different kinds of sensors, including temperature, window and door contacts, humidity, light level, occupancy, or CO2 sensors. However, using miniaturized solar modules, indoor light can also be used to supply electricity for ultra-low-power wireless radio modules.
For example, sensor modules can be outfitted with modern solar cells that generate an operating voltage of 3 V at 200 lux – enough power to transmit a measured value every 15 minutes in uninterrupted operation mode after just 3.6 hours of daytime charging. An additional charge capacitor can ensure an adequate power reserve to bridge intervals when little or no light energy can be harvested, such as in complete darkness where a fully charged energy storage device can ensure reliable operation for up to four days.
Furthermore, radio modules that execute sensor and actuator operations rapidly and turn off promptly when not in use can help save energy. For this purpose, sensor modules must incorporate special timers that only draw about 100 nA of current, fully deactivating all other components during sleep phases and waking them again when they are required to operate.
Energy of temperature
Temperature differentials contain a lot of power and are ideally suited as a source for energy harvesting. The yield from temperature variances can be enough to operate not only wireless sensors, but wireless actuators as well.
Energy delivered by thermo generators (so-called Peltier elements) often have a pronounced drawback – namely that they only produce very small voltages of about 10 milliVolts (mV) per 1 ºC. Electronic circuitry connected to this, such as a sensor module, requires a typical supply voltage of 3 V. A DC/DC converter closes this gap, with such optimized oscillators beginning to resonate upwards of 10 mV input voltage (Figure 3). At 20 mV or more (about a 2 °C temperature differential), a useful output voltage of more than 3 V is generated.
When using a heatsink, approximately 100 μW of energy is produced from a temperature difference of only 7 °C. A typical battery-less wireless module that is waked every two minutes to send a transmission only requires about 5 μW, and the remaining 95 μW is enough to power a number of actuators, such as heating valves, air flaps, or other mechanical devices.
Advances in energy harvesting: More efficient, lower consumption, higher capacity storage
The next generation of radio technology will enable up to ten times longer radio ranges for wireless transmissions of more than three kilometers for outdoor applications with high range requirements. The increased energy demands for such long distances will need to be met through progression of energy harvesting technology and the associated building blocks.
Lowering device energy consumption is an important adjustment for improved performance, especially for critical functions such as the sleep and receiver current of sensor nodes. Tests have already shown that a 10x reduction in timer currents is technically achievable for the next generation of sensor modules.
Advances are also being made that will improve the efficiency of motion, light, and temperature-based energy harvesting technology in the coming years, with research revealing efficacy of new designs and innovative deployment models:
- Motion – New types of mechanical energy harvesters using rotational motion, for example, can make use of energy of flowing gases and liquids.
- Light – Light will remain one of the most frequently used energy sources, as next-generation products will combine higher efficiency solar cells with improved performance under low light conditions. While today’s limit of operation is light intensities of about 100 lux at 5 percent efficiency, tomorrow’s solar cells based on organic material or dye-sensitized technology will operate down to 10 lux light intensity with more than 10 percent efficiency.
- Temperature – Harvesting temperature differentials is only beginning, with one new option being to harvest day and nighttime temperature differences in outdoor applications. These harvesters, which already work in the laboratory, will enable very robust sensor nodes that operate independently of light and are not sensitive to dirt. For example, they may be deployed underground, which is often the case in industrial environments.
Besides the energy need, research is also evaluating improved storage components. The target is to store harvested energy for weeks to several months without new ambient energy impulses being required. This could enable battery-less sensors that can “sleep” for much longer periods of time, retaining energy stores until an incident wakes them to measure and send signals. This is particularly interesting for alarm systems in dark environments, for example in a forest or other poorly illuminated areas.
Future applications for energy harvesting wireless
Current wireless energy harvesting architectures and emerging technologies are enabling new applications in almost every field of industry based on communications powered by the surrounding environment. The specific benefits of energy harvesting are currently contributing to the improvement of existing systems and the development of new IoT use cases in applications that include structural health monitoring, environmental monitoring, healthcare applications, and automated logistic processes, though the potential applications are almost as unlimited as the available energy sources themselves.