The Internet of Things (IoT) is big business; IDC is forecasting annual growth of 13.6%, leading to annual spending of $1.2 trillion by 2022. In terms of nodes and gateways deployed, MarketsandMarkets expects the number to be around 17 billion by 2023.
The rapid growth of the IoT is being fueled by better communications networks, particularly advances in cellular to 4G/5G, and the greater availability of energy-efficient smart devices.
One of the greatest challenges, and potentially the largest costs, of the IoT is the fact that many nodes are deployed remotely or in hard-to-access places. This has an impact on carrying out repairs as well as regular maintenance, which includes battery replacement.
The cost of sending out a person to replace a coin battery will exceed the cost of the battery by hundreds of times. Until this is resolved, the growth of the IoT will not reach its full potential.
Energy from Thin Air
As IoT nodes become ever more efficient, so it becomes possible to look at different ways of powering them. Potentially one of the most viable methods currently being considered is energy harvesting, which will use limitless energy sources located close to where the nodes are deployed.
Probably the most obvious source for nodes deployed outdoors is solar energy, where electrical power is generated from the sun’s rays using photovoltaic (PV) cells. Alongside solar, other potential sources being considered include piezoelectricity, thermoelectricity and radio-frequency (RF) emissions. All of these potential sources are dependent on environmental factors such as weather and, as such, tend to be fairly irregular.
PV technology has improved significantly, and solutions are now relatively cost-effective and energy-efficient. Developments allow flexible PV cells to be printed onto irregular surfaces, such as the surface of the IoT device, enabling easier deployment. The big challenge with solar/PV is that the sun only shines for half a day, on average (less in winter, more in summer).
As more portable devices are deployed there is an ever-increasing amount of ambient RF energy available, which could be used to charge low-power IoT nodes wirelessly. If there is warmth present, such as near a motor, combustion engine, halogen light bulb or even a warm human or animal body, this can be scavenged via a thermoelectric harvester and turned into electrical energy. Even vibrations from machinery, doors being closed or other mechanical devices can be harvested, in this case by using a piezoelectric device.
The decision as to which energy source to select for a given application will depend on the location of the IoT device and the environment in which it is deployed. In some cases, a hybrid approach may be used, combining two energy sources, such as solar and RF or thermal and solar. Given the fluctuating nature of these energy sources, some form of local storage will be needed – this could be small, thin-film or printed flexible rechargeable batteries or supercapacitors, or a combination of the two technologies, depending on the needs of the application.
Battery and Supercapacitor Technology
One example of a 3V thin-film battery is the Renata CP042350. This 25mAh capacity mercury-free device is so thin it can be bent for more than 1000 cycles without damage. The CP042350 weighs just 0.86g and has a shelf life of up to 10 years at temperatures of 23°C. Importantly for IoT applications, the self-discharge is extremely low, at an annual rate of 1%. Constructed using lithium manganese-dioxide (Li/MnO2) chemistry, the device can operate at temperatures between -40°C and 60°C.
An alternative to a battery is a supercapacitor such as Murata’s DMH series of ultra-thin supercapacitors that provide 35mF of capacitance in a 20mm x 20mm x 0.4mm package. With a 300mΩ equivalent series resistance (ESR) and a 4.5V DC rating, the devices have an operating temperature range of -40°C to 85°C. Their small size and ultra-thin form factor allow the DMH supercapacitors to be fitted almost anywhere, including being placed under a typical coin cell battery.
Intelligent and Efficient Power Management
It is important that the energy is harvested as efficiently as possible, and then used at as low a rate as possible. Fortunately, a number of new power management ICs (PMICs) are available to assist in this task.
Analog Devices’ LTC3588-2 combines an efficient full-wave bridge rectifier with a high-performance step-down buck converter, together providing a full energy-harvesting solution. The device is compatible with all types of energy source including solar PV, magnetic transducers, piezoelectric sources and others. A miserly quiescent input current of only 830nA flows when in under-voltage lock-out (UVLO) mode, with a 16V rising threshold that enables efficient energy extraction. The UVLO threshold also facilitates input-to-output current multiplication through the high-efficiency synchronous buck regulator. In order to maximize efficiency, the buck converter reduces both input and output quiescent currents to a minimum while in regulation. An input protective shunt provides over-voltage protection (OVP).
The device can deliver up to 100mA of output current continuously, and the output voltage can be selected as either 3.45V, 4.1V, 4.5V or 5.0V using a dedicated pin. The LTC3588-2 is ideally suited to many types of supercapacitors, as well as Li-ion and LiFePO4 batteries.
Another PMIC intended for energy harvesting is the S6AE101A from Cypress. This device covers an input range of 2V to 5.5V and is particularly suited to use with small PV cells as small as 1cm2. With an output that can be configured for voltages between 1.1V and 5.2V, the PMIC uses an on-board switch to store energy from the PV cell in a dedicated output capacitor. On startup, the PMIC consumes just 1.2µW and draws only 250nA while operating. The device is housed in a small 3mm x 3mm SON-10, making it ideal for cramped PCB layouts.
The S6AE101A is often used to augment the power available for functions such as sensing, processing and communication within BLE beacons and battery-free wireless sensors for building/factory automation and smart agriculture.
Maxim Integrated provides energy-harvesting charger and protector devices under the part number MAX17710. These devices are versatile power managers that provide complete functionality across a range of applications ranging from 1µW to 100mW. The devices incorporate a lithium (Li) charger with 1nA of standby IQBATT, 625nA for linear charging and a 1µW boost-charging capability.
The MAX17710 devices have low-dropout (LDO) voltages (1.8V, 2.3V or 3.3V), and can charge from a source voltage as low as 0.75V. They are supplied in 3mm x 3mm x 0.5mm UTDFN packages.