General Dwight D. Eisenhower is reported to have said that “you will not find it difficult to prove that battles, campaigns, and even wars have been won or lost primarily because of logistics.” Without proper supply (e.g., ammunition), catastrophe looms. It has even been argued that “at worst, military disaster may be the price of logistics neglect”.
With his characteristic wit Benjamin Franklin was driving home the same point in his oft-quoted humorous observation: “A little neglect may breed great mischief…for want of a nail the shoe was lost; for want of a shoe the horse was lost; and for want of a horse the rider was lost.”
Likewise for the Internet of Things (IoT), close attention must be given to the fundamental “gluing” components, i.e., the “must-have” elements without which the Internet of Things will be unable to fully materialize its huge potential.
The purpose of this brief article is to highlight among the IoT enablers the vital importance of ambient power sourcing, i.e., extracting energy from external sources (a.k.a. energy harvesting), and to introduce some of the related challenges as well as some of the interesting research that could possibly solve the IoT power conundrum.
The Internet of Things must rest on solid foundations
Many enabling dimensions of the Internet of Things are increasingly debated at all levels of industry, academia and government around the world (see in this regard, in the United States, the hearing on the Internet of Things organized by the U.S. House of Representatives Judiciary Subcommittee on Courts, Intellectual Property and the Internet on July 29, 2015). As it can be expected, security, privacy, interoperability, regulation, legislation, education and spectrum are among the crucial building blocks that are usually addressed.
However, if the expansion of the digital society shaped in great part by the Internet of Things does indeed generate a mind-boggling number of connections – the Wireless World Research Forum (WWRF) is anticipating 7 trillion wireless devices by 2020 – we can fairly assume that powering this plethora of devices is rapidly going to become a (if not the) mission-critical issue.
Energy harvesting as a national strategic priority
Energy harvesting is certainly not new as a research domain. Over the last decade, many academic and commercial groups have been involved around the world in the development and application of cost-effective technologies aimed at capturing energy from ambient sources. The emergence of the Internet of Things provides a timely impetus to that research.
Beyond making business sense, the insertion of energy harvesting capabilities in all aspects of life makes also sound strategic sense. With a multitude of sensors and actuators embedded in a country’s infrastructure (roads, bridges, levees, canals, rail networks, dams, etc.), connecting them to the grid power source is not only very expensive but also exposes them to various possible nefarious attacks. Independent power sourcing makes those nodes less vulnerable to malicious intent.
Beyond the eco-friendliness of self-powered systems and straightforward geopolitical advantage gained from energy independence, security considerations also contribute to making energy harvesting a national priority.
In terms of national importance, in the United States, the role of energy harvesting for the military is not overlooked at the Defense Advanced Research Projects Agency (DARPA). For instance, DARPA’s new (April 2015) Near Zero Power RF and Sensor Operations (N-ZERO) program seeks to overcome the power limitations of persistent sensing by developing wireless, event-driven sensing capabilities that would allow physical, electromagnetic and other sensors to remain dormant—effectively asleep yet aware— until an event of interest awakens them. N-ZERO seeks to exploit the energy in signal signatures to detect and recognize attention-worthy events while rejecting noise and interference. As a result, sensors will remain “asleep yet aware”.
These breakthrough power-saving technologies for national security are bound to spill over into the civil sector and help move the Internet of Things forward: “The value of interconnected sensors is undermined by their constant need for either a local power source or to have their batteries recharged or replaced. Thus, the Internet of Things today is applicable only to devices that can be either plugged into a wall outlet or located where people can actively take care of them. ‘By advancing state-of-the-art sensing capabilities for national security through N-ZERO, DARPA could help make the Internet of Things more efficient and effective across countless scenarios and environments, thus transforming the way people live,’ [DARPA Program Manager Troy] Olsson said.”
Energy harvesting in ambient environment
Among the multiple ambient energy sources, the wireless energy-harvesting technology has dramatically grown recently due to prevalence of wireless signals, such as TV, radio, cellular, satellite, and WiFi signals, especially after the early 1990s.
The concept of wireless energy harvesting has been raised by Nikola Tesla and Heinrich Hertz: radiate wireless power to free space and convert the wireless power to usable direct current (DC) power.
This concept of wireless power transfer requires no motion, pressure, or heat flows to generate power. There are two main types of wireless energy transfer (WPT) and harvesting: near-field or farfield systems. Near-field WPT systems utilize electric/magnetic induction or magnetic resonance to transfer power wirelessly. For the far-field WPT systems, the energy harvesting devices utilize antennas to collect remotely radiated electromagnetic waves and diode/transistor-based circuitry, such as rectifiers and charge pumps, for the RF–DC conversion. Numerous available renewable ambient energy sources exist in nature. Broadly utilized ambient energy sources are presented below.
Solar power is one of the most commonly used sources, featuring high power. A solar panel can also operate in a hybrid mode in conjunction with other types of energy source. The photovoltaic technology has been well developed over the last 60 years after the first siliconbased solar cell had been demonstrated in the 1950s and its physical (flexibility, durability, etc.) and electrical (efficiency, output voltage, etc.) properties keep improving.
Thermal energy of the power source is also widely utilized. Electrical power is directly generated by exploiting the temperature difference in thermoelectric devices taking advantage of thermoelectric effects. Thermoelectric devices can operate continuously as far as there is a temperature difference or a heat flowing across them, while they are usually rigid and heavy compared to other energy-harvesting devices, such as solar cells. The thermoelectric energy-harvesting devices typically require relatively large form factors in terms of volume to generate useful amounts of power.
The piezoelectric effect generates electrical voltages or currents from mechanical strains, such as vibration or deformation. Typical piezoelectric-based energy harvesters keep creating power when there is a continuous mechanical motion, such as acoustic noises and wind, or they sporadically generate power for intermittent strains, such as human motion (walking, clicking a button, etc.).
Ambient RF energy has a relatively low energy density compared to other energy sources. However, a larger amount of total available power can be harvested by utilizing a high gain antenna. The available or existing ambient energy density of ambient RF and wireless sources keeps increasing due to the ever expanding wireless communication and broadcasting infrastructure, such as analog/digital TV, AM/FM radio, WiFi networks, and cellular networks. The ambient RF power density is usually higher in downtown urban areas and in the proximity of the power sources (e.g., TV towers). The RF energy-harvesting technologies could be especially useful in charging a battery or powering up electronics wirelessly in scenarios in which it is hard to replace the batteries of the deployed wireless networks (e.g., bridges, buildings). It is also useful when the wireless networks are deployed in difficult to access areas (e.g., chemical plants and aircrafts), and they can operate at any time of the day and at any topology as far as there exists a minimal ambient power. Ambient RF energy-harvesting systems can be easily integrated with different types of antennas as well as with other harvesting technologies, such as the solar cells.
A more detailed review (on which this section is based) of these various ambient energyharvesting technologies and their applicability in the development of self-sustaining wireless platforms can be found in an October 2014 IEEE paper on “Ambient RF Energy-Harvesting Technologies for Self-Sustainable Standalone Wireless Sensor Platforms”.
As the efficiency of energy harvesting improves, the range of applications for self-powered sensors goes on expanding (examples of these possibilities can be found in this recent [August 2015] White Paper from EnOcean on “Energy Harvesting Wireless Power for the Internet of Things”).
When everything is said and done, the “Internet of Things” expression among similar terms (e.g., “ubiquitous computing”, “connected objects”, “Industry 4.0”, etc.) is akin to an umbrella that embraces a profound societal transformation, i.e., the insertion, thanks to the timely convergence of a multitude of mutually-reinforcing trends, of almost anything and everything into the communications fabric. This is a tall order, which we can barely comprehend and whose power equation cannot be solved only through conventional means.
While battery reliability, safety and storage capacity continue to benefit from impressive technological achievements, and offer undeniable possibilities, they will not be enough to support the anticipated wide expansion of the Internet of Things. There will be places where changing batteries (e.g., when completely drained or defective) will be too risky or costly and where power will be required to be scavenged from ambient sources.
While independently powering “things” with size, weight, and capabilities of the likes of smartphones and tablets is not yet within reach (we are about 2 to 3 orders of magnitude below that), energy harvesting is advancing by leaps and bounds, making it, at least for the time being, well suited for low-power devices.
As summarized elsewhere: “[regarding energy harvesting for smartphones and other wireless portable consumer electronic devices] there are a lot of efforts under way. The efficiency will grow slowly within the next five years, and that will be a solution for low-power devices. There is also work on kinetic and thermal energy harvesting, too. Couple that with ambient wireless energy for wearables—let’s say you have harvesting integrated with that—that could be another 150 milliwatts. So you may have a real solution in the next four to five years. But you have to combine different forms of harvesting. No one alone will be sufficient for the complete powering of practical wearable devices” (see Manos Tentzeris’ May 18, 2015 interview with Semiconductor Engineering).
However, constraints in bandwidth and data storage infrastructure coupled with groundbreaking developments in architecture (see for example the current debate around edge computing vs. cloud computing, e.g., “either or” or “both”), process technologies and materials, could quickly re-define the power requirements for IoT devices and make, much more swiftly than currently expected, energy harvesting a cornerstone of the large-scale deployment of the Internet of Things.
This article expresses the views and opinions of the authors and does not necessarily represent the position of the Georgia Institute of Technology (“Georgia Tech”, Atlanta, GA, United States), the University System of the (U.S.) State of Georgia, or the (U.S.) State of Georgia.
(*) Dr. Manos M. Tentzeris is Professor of Electromagnetics within the School of Electrical and Computer Engineering (ECE) at Georgia Tech. He has published more than 550 papers in refereed Journals and Conference Proceedings, 4 books and 23 book chapters. A recipient of many national and international awards, he is a recognized authority in the field of energy harvesting. Dr. Tentzeris is the Founder and Chair of the newly formed IEEE MTT-S TC-24 Technical Committee, which promotes activities related to RFID technology, science and applications. He is an IEEE Fellow and has served as the IEEE Distinguished Microwave Lecturer. He is currently an IEEE C-RFID (Committee on RFID Technologies) Distinguished Lecturer. He will be chairing the Special Session on “Additive
Manufacturing Techniques for RF Modules” at the European Microwave Conference 2015 in Paris, France, September 6-11, 2015.
(**) Alain Louchez is the Managing Director of the Georgia Tech Center for the Development and Application of Internet of Things Technologies (CDAIT).
Center for the Development and Application of Internet of Things Technologies (CDAIT pronounced sedate) www.cdait.gatech.edu