Indoor Photovoltaics E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 422

Veröffentlichungsjahr: 2020

Beschreibung

This is the first and most comprehensive guide on the modeling, engineering and reliable design of indoor photovoltaics which currently is the most promising and energy efficient power supply for edge nodes for the Internet of Things and other indoor devices.

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 has 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.

The book provides the engineer and researcher with guidelines, and presents a comprehensive overview of theoretical models, efficiencies, and application design. This unique and groundbreaking book has chapters by leading researchers on:

• Introduction to micro energy harvesting
• Introduction to indoor photovoltaics
• Modeling indoor irradiance
• Characterization and power measurement of IPV cells
• Luminescent solar concentrators
• Organic photovoltaic cells and modules for applications under indoor lighting conditions
• High-efficiency indoor photovoltaic energy harvesting
• Indoor photovoltaics based on ALGAAs alloys

Leseprobe

Scrivener Publishing100 Cummings Center, Suite 541jJBeverly, MA 01915-6106

Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])

Indoor Photovoltaics

Materials, Modeling and Applications

Edited by

Monika Freunek Müller

This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2020 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.

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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 MichiganOther images provide by the EditorCover 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

1Will Photovoltaics Stay Out of the Shadows?

Joseph A. Paradiso

MIT Media Lab, Cambridge, Massachusetts, USA

AbstractAdvances 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. Energy harvesting, by its nature, is a nichey opportunity. Different kinds of harvesting suit environments with different energy reservoirs subject to the constraints of the application and attached device (e.g., energy levels required/available, the conversion efficiency at that energy, and allowable area/volume/mass for the harvesting technology). If your device is buried inside a wall, in a closed shipping box, or embedded in the human body, for example, there is no light available, and then perhaps, if the expense is warranted, thermal conversion (on a warm/hot steampipe, for example) is used [13], or mechanical harvesting (if there’s a lot of vibration, ideally at a higher frequency as in an engine) [14], and RF harvesting of beamed or ambient radio with a backscatter tag can work for very low power applications [15]. Within the body, power is generally proximately inductively coupled from transmitting coils on the skin, but ongoing work looks to harvest small amounts through adaptive RF beamforming [16], vibrational scavenging via inertial reaction and induced strain (including on the heart—an idea with a long legacy [17] but only now nearing implementation [18]) and using the body’s own energy transport mechanisms via implantable biofuel cells [19]. But in most places where people spend time, there is light by default, as we need it to see, and here photovoltaics are often a top choice if energy harvesting is mandated.

Solar cells offer a harvesting mechanism that has been applied across many orders of magnitude of scale and plant capacity, from urban power systems to spacecraft to mm-scale “smart dust” [20] stacked IC dies gestating in research labs that glean power from a top silicon layer with a micro embedded solar array that also doubles as a coarse video sensor [21]. Photoelectric conversion technology has developed enormously since Einstein won the Nobel Prize in part for explaining it a century ago, thus playing his role (to his purported later chagrin) in ushering in the dawn of quantum mechanics. They have gone through decades of laborious development, enabled through (and likewise inspiring) advances in materials, power conversion and energy storage. Indeed, they will play a major role in renewable energy, together with safe, economical nuclear power and within a few decades fusion reactors designed either by humans or the AIs that will augment us by then. The market and societal need for large-scale solar power is now clear, and has driven the technology and manufacturing into a variety of performance achievements with different form factors while lowering the cost. Thanks to Elon Musk, one notable player out of many in this field, I can now tile my roof with aesthetically-appealing solar shingles and power my Tesla electric car significantly off-grid, even with the available solar illumination up here in New England. The world is now lacking only in political will to buck economic inertia and transition to an emission-free/renewable energy basis in which the capture and harnessing of incident solar photons will be a major part.

That, of course, is the topic of many other books. But the broad, recent expansion of photovoltaics and associated research has also driven the lower end, making light conversion increasingly feasible for low-power embedded applications. Already as a child growing up in the 1960s, I delighted in buying the cheap selenium solar cells they used to sell at the Radio Shack (the dominant electronics component chain store in the US that enabled generations of fellow DIY-ers before it recently went out of business) and hooking them up to small motors, etc., to see them work with a little bit of daylight. With decades of subsequent improvement, solar cells became cheaper and better—certainly at the microwatt level, if you could afford the surface area for some cm’s worth of cell area, they have powered simple LCD calculators with indoor light since the 1980s, and watches have even hidden tiny solar arrays in their face—we were proud back then to show off these versions of common gadgets that never needed batteries replaced. But they stayed nichey, as in those days there wasn’t much you could do when constrained to microwatts.

Moving to recent times, the long-anticipated explosion of wireless devices in our living environments has taken hold, and simultaneously the amount of function you can achieve at lower power has expanded. Processing, sensing, and even wireless communication have exploited advances in microelectronics fabrication/design, smart power management, and frameworks/protocols to do much more with fewer ergs, while energy conversion and dynamic optimal load matching has become much more efficient, even at the microwatt level and for a variety of different source impedances. This increased function per microwatt has boosted what can be done with a little bit of exposed PV cell, and applications have followed. Already in the mid-1990s we designed and deployed scores of smart IR beacons for indoor location at the MIT Media Lab that ran off light from proximate florescent bulbs [22]. During the last decade, we have run several of our embedded wireless sensing systems from small PV cells (e.g., our Tidmarsh environmental sensing node powers from a tiny solar cell atop its package; a few hours of diurnal daylight or a bit longer in overcast skies will serve to keep it powered [23]), and for low duty-cycle applications, we have built similar embedded light-powered sensors that work indoors.

This is no longer unusual, and various PV-powered wireless sensors have appeared for years now, some by commercial vendors, like the Schneider HOMES temperature/humidity/CO2 sensor developed by our former collaborators at Schneider Research in Grenoble a decade ago. But nonetheless, although the outdoor market for low-power wireless PV devices is robust (witness, for example, commonplace solar-powered outdoor lighting), few indoor PV-powered sensors have achieved widespread commercial viability. This is mainly due to the fact that as power decreases to the point where energy harvesting becomes an option, an embedded battery can last potentially many years, often approaching either the anticipated device lifetime, the battery’s shelf life, or just lasting long enough to make replacement a limited irritation (witness how we tolerated batteries in smoke alarms over the past decades), now often announcing over a wireless link when batteries need to be replaced. Batteries pose the ultimate dilemma to energy harvesting, as they are just so cheap and convenient. Plus, they don’t run the risk of failing in darkness, and can deliver brief pulses of high current (e.g., a commodity wireless IoT sensor bought today can draw a couple of amperes from a pair of AA cells for a few seconds when first joining an 802.11 network).

Things are changing now on many fronts, and this situation will evolve. For one, indoor photovoltaics are themselves improving. Although the efficiency of real high-end silicon solar modules in bright sunlight can significantly exceed 20%, recombination and spectral mismatch effects in the low light conditions we find indoors can cut efficiencies at least in half [24]. Indeed, the efficient world of LED and fluorescent lighting that we live in now emits at wavelengths less friendly to canonical silicon, and partly in response, ongoing research in photovoltaics has developed many approaches to converting light efficiently across broader spectral ranges. Exploiting these techniques to exhibit better performance at spectra now commonly exhibited indoors, as well as shifting away from straight silicon to new materials like perovskite, GaAs, and Cd compounds, hint at raising this efficiency by a factor of 2 to 3, albeit introducing some complexity in toxicity, manufacturing, etc.; the following chapters in this book explore this in detail. Similarly, needed power levels are still decreasing, not only for processing and sensing, but also for joining “heavier” mainstream networks like 801.11 or 5G, not to mention continued evolution in their low-power brethren like the now ubiquitous BLE and the multitude of other short-range contenders (e.g., ZigBee, Z-Wave, RF-backscatter, etc.).

The sheer number of wireless devices in the environments we live and work in will also continue increasing; at some point, this will cross a level at which battery replacement is less tolerable, together with a profusion of sensors in hard-to-access areas that still have low but usable levels of light (e.g., for environmental control, material integrity, leakage and structural assessment, safety, etc.). Also, as lighting has become more efficient and networked, and cheap renewable energy drops basic power costs, one possibility would be for areas of dwellings to illuminate for an interval when people aren’t present just to charge the sensors; they will tell the smart home or IoT servers when they are running low and need an illumination hit, or just raise the shades to let enough daylight in. Long-range inductive charging is a potential competitor here [25], but efficiency loss with range, orientation, and still unacceptably large EMI are show stoppers here, together with the lack of infrastructure; our homes and offices are amply lit, but don’t yet (and probably won’t) have large induction coils in the floors and walls (on the other hand, I see Helmholtz-coil-laden closets, draws, and shelves as potential ways to conveniently charge wireless electronics embedded in clothing).

Hence, we’re at a crossroads here—the dominance of batteries in low-power sensors and systems may well be close to ending as IPV technology advances and smart environments can coordinate their assets to conserve the energy of distributed wireless embedded systems and even recharge them when needed. Both batteries and photovoltaics are differently weighted in terms of manufacturing energy, toxicity, etc., and it’s not clear how much photovoltaics will come out ahead over a device’s lifetime, but the race is certainly on, and the incentives will continue.

I’m delighted to see this book arrive now—this is certainly the right time for it, and many of the things I’ve barely hinted at above are explored much more fully in the chapters ahead. But let’s check back in after another decade passes to see what will be energizing the low-power devices scattered throughout our environments—and, in what I currently find most exciting and profoundly important, how the data they produce is used and leveraged. In the context-driven world of the future, every bit will mean something—here’s hoping that they all will serve us well!

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