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Internet of Things (IoT) power design is always a challenge. Since IoT devices are mostly deployed in remote locations, frequent battery replacement is not always practical. Designers have to find alternate ways to power them portably. Minimizing power consumption is usually the Holy Grail; however, comes by trading off features, connectivity, range, and even built-in security capabilities (such as cryptography).
In spite of low power connectivity options (such as LoRa, LPWAN), energy-efficient circuits, and improvements in battery design, devices mostly exceed battery life. What if instead of solely relying on a static power source, IoT devices can dynamically generate power? Energy harvesting is a promising option on the horizon to enable just that.
The number of IoT sensors and devices deployed in remote locations can easily scale to thousands. Frequent truck rolls to replace batteries not only affect profit margins and the return-on-investment curve, but it also runs the risk of losing data.
The capability to siphon energy from surroundings is highly valued in IoT use cases. Harvesting energy from environmental sources such as sunlight, motion, ambient RF, heat, wind, vibration, etc., is not an entirely new concept. But the growing demand for power-efficient, safe, and durable systems that require minimum to no maintenance is driving its demand.
According to IDTechEx research, the energy harvesting market size is set to grow from $400 million in 2017 to $2.6 billion (USD) in 2024. The growing adoption of IoT and wireless sensor networks ties to these forecasts.
Energy harvesting involves transducers to convert energy from ambient sources into electricity to power the electronics. Depending on energy sources, transducer technology could be piezoelectric, thermoelectric, electromagnetic, photovoltaic, radiofrequency, etc.
Currently, photovoltaic cells are commonly used to power toys, gadgets, and even home appliances by converting solar energy into electricity. In the case of RFID, strong local signals aimed directly at the sensor are rectified. Powercast’s P2110 RF Powerharvester is another example that converts low-frequency RF signals into DC electricity (5.25V, up to 50mA). P2110 can be used to design battery-free wireless sensor nodes that can operate with very low RF input (-11.5dBm) in various industrial applications such as smart grids, building automation, military, agriculture, etc.
Energy-harvesting’s biggest promise is to prolong IoT battery life if not completely dispense it. Technologies to harvest energy have come a long way over the last decade. Yet, cost and complexity barriers to install and integrate are still there and must be carefully considered. It should be reasonable when compared to the overall IoT solution.
Another consideration is the differential of power usage in machine-to-machine (M2M) communications. An agricultural sensor, for example, might be sending/receiving data in bursts at various periods during the day. Other than that, it’s mostly idle drawing very little power. Power consumption spikes up when bursts of data are sent. The energy harvester needs to handle M2M energy burst requirements in terms of peak-load current (amps) and operating voltage requirements (volts).
Although IoT energy harvesting technologies are still being incubated in labs worldwide, there are already some encouraging success stories.
Rectenna is a device that uses a flexible antenna to capture AC electromagnetic waves (including the ubiquitous Wi-Fi signals) and converts those into DC electricity. The antenna connects to a two-dimensional semiconductor with the thickness of a few atoms. As the AC signal goes into the semiconductor, DC voltage is generated, which can be used to power electronic circuits, PMICs, or to recharge batteries.
When exposed to typical Wi-Fi power-levels (approximately 150µW [microwatts]), during lab trials, the rectenna produced about 40µW. The conversion rate is fairly good and the power is enough to drive a silicon chipset or to light up an LED.
Scientists at the National Center for Nanoscience and Technology (NCNT) in Beijing were able to design a single platform to harvest both solar and wind energy (Figure 1). It integrates a triboelectric nanogenerator (to convert wind energy into electricity) with a highly efficient solar cell. The component was tested to produce 8mW of power on the solar side, and up to 26mW by the wind harvester, which translates to high power density for a 120mm × 22mm × 2mm platform.
This hybrid nanogenerator was designed mainly as a source of renewable energy for smart cities. However, it can also power embedded and IoT devices.
Figure 1: Illustration of hybrid solar and wind harvesting cells. (Source: NCNT)
E-peas’s AEM40904 is a tiny-footprint (5-mm × 5-mm) PMIC that extracts AC power from ambient RF sources. The boost converter has 94 percent efficiency and supports a very low power startup (380mV/3 µW) and low RF input power levels (−18.5dBm up to 10dBm).
The harvested energy can simultaneously power a wide variety of IoT and embedded systems, and store excess energy in rechargeable batteries and capacitors. System designers can explore this capability in wireless IoT applications such as wearables, home automation, industrial monitoring, etc., to extend battery lifetime.
Energy harvesting systems (EHS) are driving a fusion between harvesting energy and battery storage, not necessarily a replacement. When considering EHS for system design, the cost versus value is a major consideration. As EHS modules mature, the cost-prohibitive component can be expected to slide down. It is also important to weigh in use-case imperatives, for example, power management for factory equipment is quite different from agricultural sensors.
Some EHS experts consider photovoltaic, ambient RF, and vibration to emerge as the “big three” harvesting options, because of their relatively lower cost and ease of installation.
The electronic design should continue to focus on low power consumption. Designers can choose energy harvesting to address the needs of specific IoT use cases. When power isn’t a constraint it is possible to offer many attractive features and security capabilities.
Sravani Bhattacharjee has been a Data Communications technologist for over 20 years. She is the author of “Practical Industrial IoT Security,” the first released book on Industrial IoT security. As a technology leader at Cisco till 2014, Sravani led the architectural planning and product roadmap of several Enterprise Cloud/Datacenter solutions. As the principal of Irecamedia.com, Sravani currently collaborates with Industrial IoT innovators to drive awareness and business decisions by producing a variety of editorial and technical marketing content. Sravani has a Master's degree in Electronics Engineering. She is a member of the IEEE IoT Chapter, a writer, and a speaker.