Powering IoT Devices: Technologies and Opportunities
Internet of Things (IoT) devices are supposed to be deployed 'everywhere' and to be accessed 'any time' from 'anywhere'. A high number of these devices perform monitoring and control tasks in the smart-x applications and difficult-to-access areas. For successful realization of these applications an IoT device should be tiny and autonomous de-facto while including sensing/actuating, processing and wireless communications capabilities.
These simple pre-requisites imply strict requirements for the energy storage and power management of IoT devices to ensure their 'perpetual' operation, given neither cable-power nor battery replacement are viable options in those conditions, or simply because of convenience (quick "cable-free" and "no maintenance" installations are very appealing indeed).
It appears that the best alternative which might be suggested for large-scale self-contained IoT is ambient energy sources. In this article we give a brief overview of what's happening in energy harvesting, storage and power management solutions.
There are four main ambient energy sources available in the environment: mechanical, thermal, radiant and biochemical. These energy sources are characterized by different power densities (Figure 1).
Figure 1: Ambient sources power densities before conversion (Source: CEA-Leti)
Energy harvesting of ambient sources is a clear target for 'perpetual' powering or recharging of IoT devices. Although a number of energy harvesting technologies have been proposed and presented in the context of IoT recently, radiant and thermoelectric sources are the most engineering-viable options to power or charge a small electronic system.
The main challenge currently for thermoelectric solutions is to increase the thermoelectric materials' intrinsic efficiency, in order to convert a higher part of the few mW of thermal energy which is possible to harvest while keeping the device size small. For this purpose micro and nanotechnologies, e.g. superlattices, are being considered.
As for solar radiation, perovskite technology  has the potential to increase solar conversion efficiency to more than 30% in the very near future instead of 20% for state-of-the-art cells. Other advantages over existing photovoltaic technologies include material properties that simplify the manufacture of high-performance perovskite cells. At the moment, perovskite technology is in the investigation and prototyping phase where researchers, apart from getting the highest conversion rates, try to figure out how toxic the technology is and whether it is possible to substitute some non-toxic elements for lead in the cells.
Energy harvesting alone is obviously not good enough in many application scenarios. Low power devices are expected to require 50 mW in transmission mode and less in standby or sleep modes. Energy harvesting devices cannot generate this amount of energy in a continuous active mode; therefore an intermittent operation mode has to be incorporated when using energy harvesting-powered devices. Moreover, ambient sources performances are also influenced by weather conditions. That is why energy storage is still required to ensure smooth operation of a device.
Batteries and super capacitors
The technical and practical challenges facing energy storage in emerging IoT electronics cannot be met by any one incumbent technology. Most 'things' are powered by non-rechargeable batteries for reasons of cost, availability and convenience. However, the need for replacement, limited energy resources and ecological implications, which will become a severe problem when powering billions of IoT devices, may prevent non-rechargeable batteries from being used as primary energy storage.
In rechargeable applications, Nickel Metal Hydride (NiMH) batteries have, for the most part, replaced Nickel Cadmium (NiCd) batteries. As compared to their NiCd relative, NiMH batteries are not potentially harmful to the environment. Despite being easily accessible, NiMH batteries are known for their extremely high discharge rate, which makes their application for storing such relatively small amounts of harvested energy rather questionable. In general, a nickel-based battery (1.2 V) will discharge 10% to 15% of its capacity within the first 24 hours after charging following which the discharge rate is an additional 10% to 15% per month. Lithium (Li) cells have the highest energy density, the lowest leakage current per month (<10%), but require an additional circuit for the charging. A single Li cell provides a system with high voltage (3.7 V). Li-ion and NiMH technologies guarantee around 500 charge-discharge cycles per cell which makes them practically useless/questionable for long-term IoT deployments.
The recent trend has been towards replacing rechargeable batteries with super capacitors characterized by 'unlimited' charge-discharge cycles (> 100,000). However, high self-discharge (up to 25%/day) motivates developers to keep searching for powering solutions for IoT devices.
One of the options for powering IoT devices is a 3D printed zinc rechargeable battery  developed by Imprint Energy. These zinc-based batteries are created using a process similar to screen printing and do not require heavy insulation. It allows for forming them into any desired shape, which allows for customized application. A significant advantage of this technology is that the batteries are slim and flexible. From a technical point of view, this customization ensures the required capacity and voltage of the cell which helps to avoid extra power conditioning.
Another promising battery technology for IoT devices is a solid-state thin-film one that uses solid electrolytes. This technology has low power density but high energy density, making it a good candidate for long-term deployment of IoT devices. Such a thin, bendable battery can also be easily integrated into compact IoT devices. A solid-state battery can be manufactured in typical IC packages. To enable a significant reduction in size and system integration cost, the battery can be integrated with an IC in a single package . Table 1 summarizes the pros and cons of technologies available for powering IoT devices.
|Rechargeable batteries||Rechargeable||Limited charge-discharge cycles
Feasible with energy harvesting
|Printable batteries||Easy fabrication process
Customizable cell (voltage, capacity, size)
Thin and flexible
|May damage at 40-50 °C
Not mature enough
|Solid-state batteries||Easy integration with IC
Easy to miniaturize
Thin and flexible
|Low power density
Not mature enough
|Super capacitors||'Unlimited' charge-discharge cycles||Self-discharge|
|Energy harvesting||Continuously replenishes energy resources||Depends on ambient conditions (in some cases)
Requires extra hardware and power conditioning
Not mature enough
Table 1: Technologies for powering IoT devices: their pros and cons.
The enhancement of battery technologies is of pivotal importance for developing advanced functionality and design of thin, flexible, lightweight, and low cost electronics for IoT applications.
While high-efficiency energy harvesting and proper power storage technology materials are a must, in most low-power systems, power management is also worth consideration given it allows switching certain parts of a system or putting them in a low-power state when they are not required, while enabling battery recharge. In fact, apart from simply managing a battery and storing the ambient energy, low-power electronics powered by energy harvesters is highly dependent on the following fundamental aspects: impedance match between the energy source and transducer and the electrical system, voltage regulation, maximum power point tracking (MPPT).
Due to the stringent form factor constraint in most IoT devices, key IC manufacturers address the design and fabrication of complete energy harvesting solutions. These complete solutions are feasible not only in terms of space reduction, but are of great help for engineers and researchers to address the above mentioned aspects in terms of circuit tuning and setup. Indeed, a number of complete power management solutions have evolved from research prototypes to commercial products. Solutions include piezoelectric transducers with rectifier and regulation functions  as well as solar and thermoelectric harvesters which become much more efficient when employing a MPPT mechanism on board  to replenish a rechargeable battery.
Future outlook and conclusions
In this article we have presented a brief overview on recent trends in energy harvesting, storage and power management solutions. The biggest growth opportunities over the next several years are expected in wireless power transfer, thermoelectric and solar harvesting technologies adopted for powering IoT devices. This growth is justified by significant progress in materials science, engineering and extensive prototyping. Innovative energy storage solutions for IoT have already appeared on the market. This tremendous progress in energy harvesting and storage technologies is largely due to adoption in RFID tags, a segment that will increase up to a $100 million market by 2019 and $583 million by 2021 . In terms of power management, a number of complete solutions are available on the market for powering IoT devices spanning from home appliances to wearable and flexible electronics.
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Andrey Somov (IEEE Member) is a Senior Researcher in the Area of Smart Internet of Things at CREATE-NET Research Center, Italy. He graduated at "MATI"-Russian State Technological University, Russia (2004) and holds the diploma of Electronics Engineer from the same institution (2006). Andrey received his PhD (2009) from the University of Trento, Italy, for work in the field of power management in sensor networks. Before starting his PhD, Andrey worked as an electronics engineer at VNIIEM corporation, Russia. In the fall of 2008 he was a visiting scholar at the University of California, Berkeley, USA, where he conducted research in energy efficient sensor networks. Andrey has been General Chair of the 6th International Conference on Sensor Systems and Software (S-Cube'15) and the 'IoT360' Summer School on the Internet of Things (IoT) in 2014 and in 2015. His current research interests include power management for IoT devices, cognitive IoT and associated proof-of-concept implementation.
Raffaele Giaffreda is the Head of Smart IoT research group at CREATE-NET and Coordinator of EU FP7 13m€ collaborative project iCore (www.iot-icore.eu), merging IoT and Cognitive Computing. He graduated at Politecnico Torino (Italy) and also holds an MSc in Telecom Engineering from UCL (UK). He worked for Telecom Italia (1994-1995) and British Telecom (1998-2008). He is a recognised IoT expert with a strong background in network technologies. He has published more than 30 articles in journals and international conferences and given tutorials and talks on IoT at IEEE conferences and recently at RE-WORK Technology Summit. Besides research his interests also cover the innovation aspects of relating Internet of Things, cognitive technologies and systems of smart objects. He also has a leadership role in the innovation and pilots activities of the European Research Cluster on IoT and serves as the Editor-in-Chief of the IEEE IoT eNewsletter.
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