School of Electronics and Computer Science, University of Southampton, Southampton, United Kingdom
This book has presented an overview of the design and implementation of autonomous systems that are powered using ambient forms of energy within the operating environment. The availability, over recent years, of low-cost and low-power radio frequency devices has led to the development of many commercial wireless sensor nodes. Examples have been given of how such systems can be used in specific application areas. A variety of energy harvesting mechanisms has been described within this text; these include techniques based on kinetic, solar, and thermal energy. Examples have also been given that demonstrate how multiple harvesting sources can be coupled together to increase the available power within a given environment. Owing to the variability in electrical output characteristics between different types of harvester, a wide range of interfacing circuits are required to ensure efficient connectivity with the autonomous system. In broad terms, the amount of electrical energy generated in small (typically milliwatts at best) and careful design of the electronic systems is required in order to maximize the useable quantities of power for the overall system. The ability to store electrical energy is desirable for many applications, and devices such as supercapacitors and planar rechargeable batteries have been described.
The reader should at this point appreciate that the decision to employ energyharvesting techniques for a given scenario should not be taken lightly. There are many design challenges and technical issues that need to be thoroughly assessed before committing to the energy harvesting route. We hope that many of these issues are addressed within this book and that any reader new to this subject area will not have unrealistic expectations regarding the harvesting approach, as is often the case. Indeed, the whole issue of powering small-scale electronic devices is being addressed from a variety of angles and we must not ignore the fact that almost all of today’s portables electronic systems use only a small fraction of the electrical power that their predecessors used and they are more computationally intensive. If the fuel cell technology matures to a point where low-cost, miniaturized, safe, reliable, and high-energy density devices are readily available, then the opportunities for energy harvesting will almost certainly be limited. At this point in time, however, such scenarios appear to be a long way off.
Another key aspect that emerges from this study is that self-powered autonomous systems are rather complex entities comprising many different types of subsystems that can interact with each other. The nature of, say, a solar harvesting system is vastly different from that of a vibration harvesting system, not simply because of the different form of ambient energy, but also because of the requirements for the electronic interfacing between the respective harvester and autonomous system. Traditionally, the design issues for the generator, storage, RF module, sensor, and microprocessor have been addressed in isolation, but the need for a holistic process becomes evident. This will require a standardized approach to the design process, and we have already seen examples of this in several of the main building blocks. Perhaps the most advanced standards to date are in the areas of low-power wireless communications, in particular the IEEE 802.15.4 standard and the high-layer protocols such as ZigBee, 6LoWPAN, and Bluetooth Low Energy. Standards for energy harvesting will undoubtedly emerge since bodies such as the International Society for Automation (ISA) have already formed a working group to this end.
Advances in enabling technologies will continue to have an impact on future harvesting systems. A majority of energy harvesters are, by their nature, currently rather large and bulky devices. In many instances, the power output is a direct function of the physical size and scaling down is not necessarily desirable in all cases. There are, however, many benefits that arise from systems that are inexpensive, lightweight, and flexible, which can be easily integrated into a wide variety of application areas. Advances in microelectromechanical systems and nanoelectromechanical systems (MEMS/NEMS) materials and processes will play a key role in the future of energy harvesting systems, especially as power consumption levels in end user systems continue to fall.