Table Of ContentSpringer Series in Materials Science 268
Dario Narducci · Peter Bermel
Bruno Lorenzi · Ning Wang
Kazuaki Yazawa
Hybrid and Fully
Thermoelectric
Solar Harvesting
Springer Series in Materials Science
Volume 268
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Dario Narducci Peter Bermel
(cid:129)
Bruno Lorenzi Ning Wang
(cid:129)
Kazuaki Yazawa
Hybrid and Fully
Thermoelectric Solar
Harvesting
123
DarioNarducci NingWang
Department ofMaterials Science ChineseAcademy of Sciences
University of Milano-Bicocca Institute of Soil andWater Conservation
Milan Yangling
Italy China
PeterBermel Kazuaki Yazawa
BirckNanotechnology Center BirckNanotechnology Center
PurdueUniversity PurdueUniversity
West Lafayette, IN West Lafayette, IN
USA USA
BrunoLorenzi
University of Milano-Bicocca
Milan
Italy
ISSN 0933-033X ISSN 2196-2812 (electronic)
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ISBN978-3-319-76426-9 ISBN978-3-319-76427-6 (eBook)
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Preface
Thermoelectricity is, on many counts, a solution in search of a problem.
Thermoelectricity as a physical phenomenon was discovered in the nineteenth
century, and its very first application was provided by Peltier who built a portable
cooler just a few years after the discovery of the thermoelectric effect that was
named after him. Applications of the Seebeck effect to convert heat into electric
power began instead over the first part of the twentieth century, especially in the
SovietUnion,butfoundamajoropportunitywithdeep-spaceexploration,wherever
sunlight is too weak for solar panels to power spacecrafts. Plutonium oxide
radioactive decay was used in Radioisotope Thermoelectric Generators (RTGs) by
NASA in many missions including Apollo (along with photovoltaic panels),
Pioneer, Viking, Voyager, Galileo, and Cassini—and, quite recently, to power the
Curiosity rover on Mars. Although inefficient, RTGs were preferred over alternate
technological solutions because of their remarkable reliability and lifetime due to
the absence of moving parts.
For terrestrial applications, instead, thermoelectric converters still struggle to
find their path. As known, efficiency progress of thermoelectric materials and
generators have been delayed by the need of separately controlling electron and
phonon transport in solids, keeping phonon mean free paths as small as possible
while increasing the corresponding quantity for charge carriers—a result partly
achieved just over the last decade by the introduction of nanotechnology. Despite
their limited efficiency, also for non-space applications, thermoelectric generators
have been considered to harvest heat when lack of moving parts and small sizes
were at a premium. Deployment of thermoelectric generators to recover heat from
car mufflers prompted quite a bit of basic and applied research, stimulating the
searchfornovellow-cost,geo-abundantmaterials.Theautomotiveindustry,which
waslookingforwaystocontainCO emissionsinthewakeoftheParisAgreement,
2
reported, however, severe problems concerning the economic sustainability
of thermoelectric converters. Although paying back their cost over the average car
lifetime, they required an extra capital cost for customers that made thermoelectric
harvesters a viable solution only for the high-end automotive segment.
Furthermore,thevolatilityoftheenvironmentalpolicies(bothinEuropeandinthe
v
vi Preface
USA) along with the advancements (achieved and/or announced) in the making of
electric cars turned out to cool down the initial enthusiasm toward thermoelectric
harvesters.
Thermoelectric generators (either miniaturized or fully integrated) are also
consideredaninterestingsupportingtechnologyfortheso-calledInternetofThings
(IoT), namely networks of interconnected, distributed devices embedding elec-
tronics, software, sensors, and actuators. Although IoT devices are not necessarily
wireless, physical autonomy is a key aspect of a large subset of such networks.
Thus, unplugged devices would ideally pair with their being wireless as of data
exchanges. Batteries may be once again a viable and ready solution. However,
especially when device maintenance isinconvenient orimpossible, harvesters may
either complement or replace batteries, targeting the making of deploy-and-forget
network nodes. Development of suitable thermoelectric harvesters has advanced
overthelastyears.Inthiscase,economicalaspectsarearathermarginallimitation,
thehurdletobeovercomebeingessentiallythatofthepowerdensityavailableover
small (sometimes very small) temperature differences. Body heat harvesters are a
good example of this issue.
If the IoT might provide a good driver to move thermoelectric generators from
niche to bulk application for microharvesting (i.e., power outputs in the order of
somemilliwatts),useofthermoelectricitytocomplementthedominatingrenewable
energysource,i.e.,photovoltaics,mightplayasimilarroleformacroharvesting.In
1954, when Mária Telkes published her pioneering work on thermoelectric sun
power converters, the efficiency of thermoelectric generators (3–4%) was compa-
rable to that of the first photovoltaic cells (4–6%). As known, in the following
years, efficiency of photovoltaics skyrocketed to two-digit figures, while thermo-
electrics could not break the 10–% efficiency threshold till relatively recent times,
thereforeleavingtheraceforsolarharvesting.Itcouldre-enteritnow,however,as
a partner more than as a competitor. Currently, the photovoltaic market is domi-
nated by silicon panels, with efficiencies now leveled off to about 20%. Use of
multiple junctions, capable of efficiencies already exceeding 40% is limited by
capitalcosts.Also,powercosts,albeitheavilydopedbytaxationandincentivesby
publicauthorities,havealsoreachedaplateausincemorethan5years.Thisimplies
that,unlessradicallynewfabricationtechnologiesareconceived,capitalandpower
costsfor silicon-based photovoltaics maynotbeexpectedtodecreasesubstantially
over the next decades. Novel photovoltaic materials have surfaced, instead, cur-
rently with lower efficiencies but also with promising lower costs, should they be
fully promoted to production. Perovskites are only the most recent example.
Differently from silicon, such materials would take significant advantages from
being paired to a thermoelectric stage, remarkably increasing the total electric
power output of the solar harvester. Furthermore, preliminary economic estimates
show that their power costs, payback periods, and capital investments per electric
watt would be sustainable.
It is therefore not surprising that both fundamental and applied research on
hybrid (photovoltaic–thermoelectric) solar harvesters have lately gained a great
momentum.Thenumberofpaperscoveringthistopichasincreased,exceeding500
Preface vii
articles published in the year 2017 (Data source: Scopus). With a publication rate
still increasing linearly at the noteworthy rate of 46 papers/year2 since 2006, the
fieldisclearlystillunderacceleratedexpansion.Thisisoneofthemainmotivations
behind this book—namely providing a self-consistent yet thorough and in-depth
introductiontohybridharvesterstothenewcomers.Thisbookisprimarilythought
asaprimertoexperiencedscientistswillingtoenterthislivelyfieldofresearch.Itis
meant both for scientistsworking onthermoelectrics andwilling tocatch thebasic
physicsofphotovoltaics;andforprofessionalsworkingonphotovoltaicscienceand
technologywhoarecurioustoconsidertheapplicationofthermoelectricharvesters
to enhance the total efficiency of solar converters. Therefore, further to core
chaptersonfullandhybridthermoelectricsolarharvesters,introductorychapterson
thermoelectricity and photovoltaics are provided. This turns out to make the book
also suitable for Ph.D. students and post-docs, who will use the first chapters to
haveanintroductiontobothharvestersbeforemovingtotheirjoinedapplicationsto
solar converters. But we also hope that this book may be of some interest to
entrepreneurs and public decision-makers looking for an insight into the opportu-
nities (and the limits, too) of hybrid solar harvesters. Although all authors of this
book are actively engaged in this research field, we did our very best to uncover
also what hybridization cannot afford to do. However, as will be shown, premises
look as good as promises for this technology, and it seems fair to suggest more
attention to the opportunities it might deliver in the short- and mid-terms.
Taking a picture of a quickly moving subject is not simple at all, and some
blurring is unavoidable. This book is no exception, and it inescapably reflects the
viewsandthesentimentsofitsauthors.Wearefullyawarethatwecouldnotcatch
all possible approaches to modern hybrid solar harvesters. We then apologize for
any promising approach we did not mention or that we insufficiently covered. We
did not aim at being exhaustive in a field showing such a rapid growth simply
because a catalog of ideas, hints, and strategies would have suffered a too rapid
obsolescence, becoming outdated over the time needed just to publish it. Our aim
wasinsteadtowriteaself-contained,agileintroductiontothetopic,whichwasyet
missing—as an invitation to senior and junior colleagues to join the effort. This
said, any suggestion, criticism, and comment from readers on how to update and
completeourpicturewillbehighlyappreciated,andmightbeusedforforthcoming
editions of this book.
Milan, Italy Dario Narducci
West Lafayette, USA Peter Bermel
Cambridge, USA Bruno Lorenzi
Yangling, China Ning Wang
West Lafayette, USA Kazuaki Yazawa
Acknowledgements
Bruno Lorenzi has received funding from the European Union’s Horizon 2020
research and innovation programme under the Marie Skłodowska-Curie grant
agreement No. 745304.
ix
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Solar Harvesting: Photovoltaics and Beyond . . . . . . . . . . . . . . . . 1
1.1.1 The Emergence of Renewable Energy Sources . . . . . . . . . 1
1.1.2 Photovoltaics: A Technological Success History . . . . . . . . 3
1.2 Aims of This Book. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2 A Primer on Thermoelectric Generators. . . . . . . . . . . . . . . . . . . . . . 11
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Fundamentals of Thermodynamics of Thermoelectricity . . . . . . . . 12
2.2.1 Thermoelectricity in Linear Thermodynamics . . . . . . . . . . 12
2.2.2 Thomson Effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Thermoelectric Efficiency in the Constant-Property Limit . . . . . . . 17
2.3.1 Dirichlet Boundary Conditions . . . . . . . . . . . . . . . . . . . . . 18
2.3.2 Neumann Boundary Conditions . . . . . . . . . . . . . . . . . . . . 25
2.4 Thermoelectric Efficiency in the Presence of Large
Temperature Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.1 Thermoelectric Potential. . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.4.2 Comparison to CPL Efficiency . . . . . . . . . . . . . . . . . . . . . 33
2.4.3 Compatibility and Efficiency . . . . . . . . . . . . . . . . . . . . . . 33
2.4.4 Engineering Figure of Merit. . . . . . . . . . . . . . . . . . . . . . . 34
2.5 Finite–Rate Thermoelectric Efficiency . . . . . . . . . . . . . . . . . . . . . 36
2.5.1 Efficiency of Finite–Rate Thermal Engines . . . . . . . . . . . . 36
2.5.2 Application to Thermoelectric Generators . . . . . . . . . . . . . 39
2.6 Thermoelectric Efficiency Under Non-steady State
Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.7 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
xi
