Micro Energy Harvesting
Micro Energy Harvesting
Introduction to Micro Energy Harvesting
Danick Briand, Eric Yeatman and Shad Roundy
1.1 Introduction to the Topic
We are living in an increasingly intelligent world where countless numbers of autonomous wireless sensing devices continuously monitor, provide information on, and manipulate the environments in which we live. This trend is growing fast and will undoubtedly continue. The vision of this intelligent world has gone by many names including "wireless sensor networks,""ambient intelligence," and, more recently, "the Internet of Things (IoT)." Regardless of the current buzzwords, this vision will continue to take shape. We are now realistically talking about a trillion or more connected sensors populating the world. Almost all of these wireless connected devices are currently powered by batteries that have to be periodically recharged or replaced. This state of affairs is simply not practical if we are to have many hundreds of sensors per person on the planet. Alternative autonomous power supplies are becoming more and more critical. Furthermore, these power sources must be small, inexpensive, and highly reliable. This need has given rise to a new field of research, study, and engineering practice, usually referred to as Energy Harvesting. This book is intended to cover the engineering fundamentals and current state of the art associated with energy harvesting at the small scale, or Micro Energy Harvesting.
The term "Energy Harvesting" usually refers to devices or systems that capture (or harvest) ambient energy in the environment and convert it into a useful form, which is usually electricity. Large-scale renewable power generation such as solar arrays, wind farms, and ocean wave generators can be considered forms of energy harvesting. However, for the purposes of this book, we define the term somewhat more narrowly. We define energy harvesting as technologies, devices, and systems that capture ambient energy to replace or augment the batteries in wireless devices. The title of this book is " Micro Energy Harvesting ". In the energy harvesting literature, the word "micro" sometimes refers to different aspects of the system. The first is the power level. Micro energy harvesting systems generally produce power best described by microwatts, usually 10-100s of microwatts. Sometimes, "micro" refers to the scale of the energy harvesting device, that is, micrometers. While the overall dimensions of micro energy harvesters are usually in the millimeter or centimeter range, the key features of the transducers are usually microscale. Finally, "micro" sometimes refers more to the fabrication method, using highly parallel fabrication techniques common to the semiconductor and MEMS industries. While many of the techniques covered in this book can be applied across different size scales, we are generally concerned with energy harvesters that can be mass produced at microscales using microfabrication techniques. In practice, some of the devices will be macroscale devices that are moving toward microscale implementations.
Micro energy harvesting covers a broad range of technologies and relies on quite a broad range of fundamental science. A typical engineer or scientist working in the field will most likely be an expert in only a few areas. However, in order to make good engineering decisions, it is important to be well grounded in the entire breadth of the field. This book is intended, in part, to provide a solid foundation in a broad range of technologies that comprise the field of micro energy harvesting.
An energy harvesting system is comprised of four different components as depicted in Figure 1.1 . Some form of environmental energy is available (e.g., thermal, solar, vibration, RF, wind). A device or subsystem captures that energy and presents it to an energy converting transducer (e.g., thermoelectric stack, piezoelec