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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-60559-1
Cover image: Top right: GaAs photovoltaic array integrated on a microsystem, placed on top of a US penny. Photo by Inhee Lee and Eun Seong Moon, University of Michigan
Other images provide by the Editor
Cover design by Russell Richardson
Preface
Indoor photovoltaics (IPV) has grown in importance over recent years. This can in part be attributed to the creation of the Internet of Things (IoT) and Artificial Intelligence (AI) along with the vast amounts of data being processed in the field, which have been a massive accelerator for this development. Moreover, since energy conservation is being imposed as the national strategy of many countries and is being set as a top priority throughout the world, understanding and promoting IPV as the most promising indoor energy harvesting source is considered by many to be essential these days.
There are many contributing factors as to why this book is so special. First of all, I would like to deeply thank all the authors from around the world who found the time to contribute chapters on top of their world-class research, industrial work, and the launch of companies, projects and actual rockets. The book has exceeded my expectations in many ways. Not only did we manage to cover all topics that are essential for building an IPV device, but this book actually contributes to the research in the field. It has yielded collaborations among the chapter authors and the research community, many of which will hopefully be a seed for long-lasting and growing collaborations. Many chapters contain original research that was performed solely for this book, and some chapters contain work that is being published for the first time for a larger audience. On top of all that, the book was already in steady demand even during its writing.
Finally, I would like thank my publisher, Martin Scrivener, for his great and always helpful support, and Jean Markovich, who did outstanding work in copyediting the chapters. I am most grateful for this opportunity and the joy this work brings.
I hope, you, the reader, will find this book helpful in designing and understanding your IPV system.
Monika Freunek Müller
Bern, Switzerland, August 2020
1 Will Photovoltaics Stay Out of the Shadows?
Joseph A. Paradiso
MIT Media Lab, Cambridge, Massachusetts, USA
Abstract Advances in ultra-low-power electronics capable of useful applications have caused interest in energy harvesting to balloon in recent years. When needed power drops to the point where harvesting is feasible, however, an embedded battery presents serious competition for many use cases, and the particulars tend to rapidly become niche-driven, as they depend on the ambient energy environment that the system experiences. Photovoltaics have long been a popular energy harvesting option, and recent developments expand their opportunities for low-light indoor applications.
Keywords: Indoor photovoltaics, energy scavenging, low-power electronics, human-powered systems, Internet of Things
1.1 Introduction
I’m going to start this book introduction with a confession. Although I’m known for developing a variety of energy harvesting approaches and implementations dating back a couple of decades now, I’ve never actually really used any of them. Yes, we and others back then and since [1–4] siphoned parasitic power from various modes of human motion and activity – for example, from footfalls alone, my students and I made compact shoe-mounted piezo harvesters in the late 1990s that sent radio transmissions and shoe-integrated magnetic generators that could power a music system with loudspeakers [5, 6]. Others since have done more evolved work in siphoning power from the foot [e.g., 7, 8], and even products or online DIY projects have appeared [e.g., 9–11], but nothing has hit even close to the mainstream or for that matter even established a strong niche market. The dream of capturing energy from human motion is an opium to the public that hearkens to the appealing magic of perpetual motion machines since there is an eagerness to see this kind of invention. But when the rubber meets the road, even though ingenious implementations have been designed, nothing has gained much traction.
This is for a variety of reasons. One is physical; living bodies are evolved to conserve their energy—our tendons act as springs while walking to store and release ambulant power, and our vascular system constricts to avoid excessive heat loss. Hence, pulling significant power from moving or walking can become tiring, and large-area thermoelectrics can feel cold. Another is expense and complication. Building harvesters into shoes, where most tappable human energy resides, can get pricey and can interfere with footwear designs that are already finely optimized for their purpose (walking, running, climbing, etc.). The complication involved in large mechanical harvesters also introduces fragility; again, going back to the shoe, such devices always break there well before the shoe is worn out. Then there is the issue of power distribution; without wiring the body or clothing, the energy needs to be used where it is harvested. Although you can perhaps glean a good fraction of a Watt from a shoe, it’s hard to use it there; the power needs to be transported to a smart AR headset, for example, where that level of energy would be welcomed (ditto for pretty much any kind of mechanical harvester—you need to tap limb motion or the inertial reaction of a heavy mass in a backpack [12] to get to this power level). Sure, you could charge a battery in your shoe and move it—people have implemented this several times. But even with multiple wearable devices, the convenience of powerline charging them, at least in the wired world, has well outpaced this strategy.
OK, so let me backpedal a little from my leading sentence. There is one energy harvesting technology that I have used in my work routinely, and that’s photovoltaics.
