54 The Electrochemical Society Interface • Fall 2008

Thermoelectric generators are all solid-state devices that convert heat into electricity. Unlike

traditional dynamic heat engines, thermoelectric generators contain no moving parts and are completely silent. Such generators have been used reliably for over 30 years of maintenance-free operation in deep space probes such as the Voyager missions of NASA.1 Compared to large, traditional heat engines, thermoelectric generators have lower efficiency. But for small applications, thermoelectrics can become competitive because they are compact, simple (inexpensive) and scaleable. Thermoelectric systems can be easily designed to operate with small heat sources and small temperature differences. Such small generators could be mass produced for use in automotive waste heat recovery or home co-generation of heat and electricity. Thermoelectrics have even been miniaturized to harvest body heat for powering a wristwatch.

Thermoelectric Power

A thermoelectric produces electrical power from heat flow across a temperature gradient.2 As the heat flows from hot to cold, free charge carriers (electrons or holes) in the material are also driven to the cold end (Fig. 1). The resulting voltage (V) is proportional to the temperature difference (∆T) via the Seebeck coefficient, α, (V = α∆T). By connecting an electron conducting (n-type) and hole conducting (p-type) material in series, a net voltage is produced that can be driven through a load. A good thermoelectric material has a Seebeck coefficient between 100 µV/K and 300 µV/K; thus, in order to achieve a few volts at the load, many thermoelectric couples need to be connected in series to make the thermoelectric device (Fig. 1).

A thermoelectric generator converts heat (Q) into electrical power (P) with efficiency η.

(1)

The amount of heat, Q, that can be directed though the thermoelectric materials frequently depends on the size of the heat exchangers used to harvest the heat on the hot side and reject it on the cold side. As the heat exchangers are typically much larger than the thermoelectric generators themselves, when size is a constraint (or high P/V is desired) the design for maximum power

P = ηQ

Small Thermoelectric Generatorsby G. Jeffrey Snyder

may take precedence over maximum efficiency. In this case the temperature difference (and therefore thermoelectric efficiency as described below) may be only half that between the heat source and sink.3

The efficiency of a thermoelectric converter depends heavily on the temperature difference ∆T = Th – Tc across the device. This is because the thermoelectric generator, like all heat engines, cannot have an

efficiency greater than that of a Carnot cycle (∆T/ Th). The efficiency of a thermoelectric generator is typically defined as

(2)

Where the first term is the Carnot efficiency and ZT is the figure of merit for the device. While the calculation of

FiG. 1. Schematic of a thermoelectric generator. Many thermoelectric couples (top) of n-type and p-type thermoelectric semiconductors are connected electrically in series and thermally in parallel to make a thermoelectric generator. The flow of heat drives the free electrons (e-) and holes (h+) producing electrical power from heat. (Copyright Nature Publishing Group,2 reprinted with permission.)

η

The Electrochemical Society Interface • Fall 2008 55

a thermoelectric generator efficiency can be complex,4 use of the average material figure of merit, zT, can provide an approximation for ZT.

(3)

Here, Seebeck coefficient (α), electrical resistivity (ρ), and thermal conductivity (κ) are temperature (T) dependent materials properties.

Recently, the field of thermoelectric materials is rapidly growing with the discovery of complex, high-efficiency materials. A diverse array of new approaches, from complexity within the unit cell to nanostructured bulk, nanowire and thin film materials, have all lead to high efficiency materials.2

Heat to Electricity

For large electrical power generation applications traditional dynamic thermal to electric generators (e.g. Rankine, Brayton, or Stirling engine) have several times the efficiency of a thermoelectric system. However, such dynamic systems are expensive and do not scale easily for small applications. When high quality combustible fuel is available, internal combustion engines are cost effective and reasonably efficient in the 100 W to 100 kW range but tend to be noisy. For applications requiring less than 100W, the scalability of thermoelectrics gives them a clear advantage.

Power from Waste Heat

Efforts are already underway to demonstrate waste heat recovery in automobiles (Fig. 2).5 In this ~1 kW range, even relatively inefficient thermoelectrics can be competitive for use with such waste heat sources (e.g. automobile exhaust) when design, fabrication, and maintenance cost are factored in. The thermoelectric

zT = α2T ρκ

generator will extract waste heat from the exhaust that will deliver DC electrical power to recharge the battery. By reducing or even eliminating the need for the alternator, the load on the engine is reduced thereby improving fuel efficiency by as much as 10%.

Instead of recovering waste heat, Co-generation recovers some of the useful work wasted on heat. Often a high energy content fuel with a high flame temperature (such as natural gas) is used for low ∆T heating (e.g. home heating or hot water). In electricity–heat cogeneration, electricity is produced with nearly 100% efficiency (as opposed to ~40% for power plants) because the remaining energy is used for heating instead of being wasted. In applications such as home co-generation, the desire for silent, vibration, and maintenance free operation will favor thermo-electrics. Residential co-generation and automotive waste heat recovery are two examples where “small” systems could have an impact on the global energy consumption if implemented on a large scale.

Portable Power

For small portable applications, power sources that are smaller and lighter than conventional batteries are of great commercial interest. The energy density of a combustible fuel is 10 to 100 times that of a battery. Thus in principle, even for low thermal-to-electric conversion efficiencies (e.g. a few percent), a small combustor supplying heat to a thermoelectric generator could provide greater energy density than a battery. However, a 10% efficient generator can require at least 500°C. In addition, to ensure that the heat is directed through the thermoelectric and not lost in the exhaust, the heat exchangers must be carefully designed. One such design is that of a “Swiss roll” combustor where catalytic combustion is localized

FiG. 2. Conceptual design of thermoelectric generator producing electricity from waste heat in the engine exhaust. Copyright BMW, reprinted by permission.

to a small region to maintain a high source temperature while the exhaust is directed through a counter flow heat exchanger to preheat the incoming gasses (Fig. 3).6 The thermal efficiency of such combustors can be 80-95%. Perhaps the best way to achieve high efficiency with such a device is to include a fuel cell just before the catalytic combustion. A single chamber fuel cell7 produces electricity when placed in a hot fuel-air mixture. The unreacted fuel could then be combusted by the catalyst for use by the thermoelectric.

Harvesting MicroPower

The ability to fabricate exceedingly small semiconducting thermoele- ments has enabled the possibility of harvesting very small amounts of heat for low power applications such as wireless sensor networks, mobile devices, and even medical applications.8 Various thin film techniques have been utilized to produce small thermoelectric devices including electrochemical MEMS,9 CVD10 and sputtering11,12 with at least three companies (Micropelt, Nextreme, ThermoLife) recently established to produce devices (Fig. 4). Devices utilizing the thermoelectric material in a direction parallel to the deposition plane12 lose some efficiency due to the thermal shorting of the substrate, but gain from the density and length

FiG. 3. Single-chamber solid-oxide fuel cell with Swiss roll combustor.6 The high power density fuel cell produces electricity from a mixture of fuel and air. The unreacted mixture is then catalytically combusted at the center of the Swiss roll heat exchanger that could be placed between two thermoelectric generators for additional electrical power.

56 The Electrochemical Society Interface • Fall 2008

Snyder(continued from previous page)

of the thermoelectric elements such that 5V can be produced with a 10K temperature drop (ThermoLife).

At the same time, manufacturers of bulk thermoelectric devices, which typically have thermoelectric elements of 1-2 mm in length, can now reduce the size of the thermoelectric elements13 even to 100µm.14

A good example of thermoelectric energy harvesting is the thermoelectric wristwatch, which utilizes thin bulk thermoelectric devices (Fig. 5). The watch is driven by body heat converted into the electrical power by the thermoelectric. At least two models have been built, one by Seiko and another by Citizen. The Seiko watch14 (Fig. 5) under normal operation produces 22 µW of electrical power. With only a 1.5K temperature drop across the intricately-machined thermoelectric modules, the open circuit voltage is 300 mV, and thermal to electric efficiency is about 0.1%.

Although no longer in production, the thermoelectric wristwatch demonstrates the viability of utilizing thermoelectric in small power sources. As the cost of producing these devices drops with mass production and the increasing need for remote power sources, viable applications will undoubtedly arise. In addition, with the ever increasing economic and

social cost of energy production, small thermal to electric power sources for cogeneration and waste heat recovery may someday play a significant role, however small.

Acknowledgments

The assistance of Sossina Haile, Jeongmin Ahn, Rama Venkatasubramanian (RTI), Joachim Nurnus (MicroPelt), Eric Toberer, Paul Ronney, BSST, Andreas Eder (BMW), and Matsuo Kishi (Seiko) is gratefully acknowledged.

About the Author

G. JEFFrEY SnYdEr is a faculty associate at the California Institute of Technology (Caltech) in Pasadena, California. His interest is focused on thermoelectric engineering and advanced materials development such as complex Zintl phase and nanostructured bulk thermoelectric materials (http://thermoelectrics.caltech.edu/). He may be reached at jsnyder@caltech.edu.

FiG. 4. Microfabricated thermoelectric elements (Copyright Micropelt). Completed device (RTI) next to a penny (USA 1 cent.) (Copyright RTI.)

References

1. L. A. Fisk, Science, 309, 2016 (2005). 2. G. J. Snyder and E. S. Toberer, Nature

Materials, 7, 105 (2008). 3. G. J. Snyder, “Thermoelectric Energy

Harvesting,” in Energy Harvesting Technologies, edited by S. Priya (Springer, in press).

4. G. J. Snyder, “Thermoelectric Power Generation: Efficiency and Compatibility,” in Thermoelectrics Handbook Macro to Nano, edited by D. M. Rowe (CRC, Boca Raton, 2006), Ch. 9.

5. K. Matsubara, in Twenty-first International Conference on Thermoelectrics, Proceedings, ICT’02, 418 (2002).

6. J. Ahn, Z. Shao, P. D. Ronney, and S. M. Haile, in The 5th International Fuel Cell Science, Engineering & Technology Conference (Fuel Cell 2007), 250832 (2007).

7 Z. P. Shao, S. M. Haile, J. Ahn, P. D. Ronney, Z. L. Zhan, and S. A. Barnett, Nature, 435, 795 (2005).

8. J. A. Paradiso and T. Starner, IEEE Pervasive Computing, 4, 18 (2005).

9. G. J. Snyder, J. R. Lim, C.-K. Huang, and J.-P. Fleurial, Nature Materials, 2, 528 (2003).

10. R. Venkatasubramanian, C. Watkins, D. Stokes, J. Posthill, and C. Caylor, 2007 IEEE International Electron Devices Meeting - IEDM ‘07, 367 (2007).

11. H. Böttner, J. Nurnus, A. Gavrikov, et al., J. Microelectromechanical Systems, 13, 414 (2004).

12. I. Stark and M. Stordeur, in Eighteenth International Conference on Thermoelectrics. Proceedings, ICT’99, 465 (1999).

13. G. J. Snyder, A. Borshchevsky, A. Zoltan, et al., in Twenty-first International Conference on Thermoelectrics, 463 (2002).

14. M. Kishi, H. Nemoto, T. Hamao, M. Yamamoto, S. Sudou, M. Mandai, and S. Yamamoto, in Eighteenth International Conference on Thermoelectrics Proceedings, ICT’99, 301 (1999).

FiG. 5. Seiko Thermic, a wristwatch powered by body heat using a thermoelectric generator; (a) the watch, (b) cross-sectional diagram.14 (Copyright by Seiko Instruments Incorporated, reproduced with permission.)

01/04/2011 1

Overview of Thermoelectric Power Generation

Technologies in Japan

Takenobu Kajikawa,Shonan Institute of Technology,

Fujisawa, Kanagawa, 251-8511 JAPANkajikawa.tk@sand.ocn.ne.jp

01/04/2011 2

OUTLINE• Introduction

Classification of applications of thermoelectric power generation (TEG) Experiences of major demonstrations for thermoelectric power generation system

• Progress of the Development of TEG Applications Waste heat recovery systemsRenewable energy sources

• Future ProspectsCREST project “Development of high-efficiency thermoelectric materials and systems”NEDO project “Development of nano-structured thermoelectric materials using

Clathrates”

• Conclusions

01/04/2011 3

Classification of TEG Applications at present in Japan

I. Large scale TEG systems recovering waste heat in the energy system

II. Applications to the renewable energy sources such as solar, geothermal and ocean

III. Dispersed small TEG systems as energy-harvesting or ubiquitous power sources

01/04/2011 4

Impressive Records of TEG Demonstrations in Japan

• 500kW Diesel engine cogeneration TEG・・・ (I)

• Municipal solid-waste TEG systems ・・・ (I)

• Hot-springs TEG ・・・(II)

• TEG Wristwatch・・・ (III)

・Wall-embedded type,

・Separate type using working medium such as air and organic fluid

・In-line type

Kusatsu Hot springs

150W / 4.5 years run

1kWeTEG

01/04/2011 5

Ongoing Developments of Thermoelectric Applications in Japan

• Waste Heat Recovery SystemsIndustrial furnaces (Komatsu/KELK, Showa Cable Systems)

Solid waste incinerator (Showa Denko)

Motorcycles/Automobile (Atsumitec)

• Renewable Energy SourcesSolar thermal energy (JAXA group &China, TDS group)

Geothermal energy (Hot springs) (TOS/Kusatsu)

01/04/2011 6

Waste Heat Recovery SystemsIndustrial furnaces

(Komatsu/KELK, Showa Cable Systems)

Solid waste incinerator (Showa Denko)

Motorcycles/Automobile (Atsumitec)

01/04/2011 7

InverterLED/fluorescent

Gas

Residual carburizing Gas

Gas Carburizing Furnaces

TEG

TEG application to a gas carburizing furnace by KOMATSU

200 W class TEG recovering the exhaust heat for a gas carburizing furnace installed in the large gear manufacturing process

Power Conditioner with MPPT

Battery

(CO,N2,H2)

20-30 kWt

Sixteen thermoelectric modules were installed in the test facility.

The modules have been commercialized by Komatsu & KELK as outcome of the JATeCS project.

01/04/2011 8

[Specifications of the thermo modules ]Size : 50mm x 50mm x 4.2mm Weight : 47gOutput : Max. 24W(when 280 °C in the hot side and 30 °C in the cold side)Temperature range for use : 280 °C (max.) and 250 °C or lower (normal) in the hot side and 150 °C (max.) in the cold sideConversion efficiency : Max. 7.2%Material : BiTe alloy

HDK2010B0054

Heat cycle test

(TH = 250 -50 °C and TC = 30 °C in air .)

TEG Module using Bi-Te alloys

01/04/2011 9

(a) Generator #1 (b) Generator #2

TEG systems tested using two types heat collector

(a)Thermoelectric generator #1. Plain-type heat collector. (b)Thermoelectric generator #2. Fin-type heat collector.

01/04/2011 10

Test Results of TEG system for a gas carburizing furnace

The overall system efficiency was about 4.4% for #1 generator and 4.2% for #2 one respectively. The conversion efficiency for the TE power conditioner with MPPT was obtained about 85%.

01/04/2011 11

TEG Application to a preheating furnace for copper wire manufacture by SWCC Showa

Cable Systems Co.Ltd.

Temperature is kept about 1123K.

01/04/2011 12

Installation of test unit ,TEG module and Test resultsModule specification

55mmx110mmx7.5mm

24 couples made of layered oxides such as LaNiO3 for n-type, and Ca3Co4O3 for p-type.

Max. ZT is 0.14 at 1123K.

Test unit consists of 32 modules.

Current (A)

Pow

er (W

)

Volta

ge (V

)

Installed TEG unit at the monitor hatch

Long run test proved 3000h in durability.

01/04/2011 13

TEG Application to the recovery of the combustion heat for industrial solid waste

incinerator by PLANTEC and SHOWA DENKO

Overall view of the industrial solid waste incinerator system in Chiba Pref..

TEG unit was installed at the monitor hatch for the durability test.

01/04/2011 14

Thermoelectric performance of the module based on the filled Skutterudites

30mmx30mmx4mm

18 couples

Ce-Co-Sb for n-type

La-Fe-Sb for p-type

Power output was obtained 21.6W at 550K in temperature difference for a module. Power density was 2.4W/cm2, and module efficiency was about 6%. The durability was proved over 3000h.

01/04/2011 15Power Characteristics

Waste heat recovery from Motorcycle

On the streetsOn the test bench

TEG system Load: LED Lighting

823K

Heusler alloy TE module

823K to 670K

Time variation of exhaust gas temperature with driving modes; Mode variations are insensitive to gas temperature.

by ATSUMITEC Co.Ltd.

01/04/2011 16

Goal: 2.5W/g, 1.4W/cm2,0.3W/cm3 at 300 K in ΔT

using Heusler alloy Fe2VAl(Ir,Ti,Ta)

Improvement of TE performance for Heusler alloy

Advanced TEG unit design of 25mmx70mm mounted 12 modules

Fe1.82Ir0.02VAl Fe1.82 V0.8Ti0.11Ta0.11Al

01/04/2011 17

Renewable Energy SourcesSolar thermal energy

(JAXA group & China,TDS group)

Geothermal energy (Hot springs)(TOSHIBA/Kusatsu)

01/04/2011 18

Development of Solar Thermo Hybrid Generation System

One of Strategic international Cooperative Program on Science and Technology : JAXA group and China

Concept of Solar Thermal Power TEG System combined with Hot House System

01/04/2011 19

Demonstration solar thermal powered TEG test facility with solar tracker

Front-view of solar collector with 100 Fresnel lens and 2 PV-units for tracking driving power Back-view of the collector

mounted TEG/Fin units

Inside view of single Fresnel lens collector

Surface temperature was obtained 563K, power output was only 0.65W although the rating value was 4W, because of unexpected thermal resistance in the system and insufficient tracking performance.

01/04/2011 20

Solar Thermal Powered TEG /Desalination System promoted by TDS group

The system can produce Heat, Electricity and Fresh water as Key resources for human beings.

01/04/2011 21

Conceptual Energy Balance & Temperature Allocations

TEG System

ηTE=10%

01/04/2011 22

Cost Competitiveness

Fresh water cost :0.99 US$/m3 by the proposed concept

1.3 US$/m3 by PV/Desalination Plant

Assumptions for the cost estimation:

Capacity:10,000t/d、 Plant availability:0.9, Site: the Middle East, Plant Life: 20years, Inflation rate:2.0%, Construction year:2010, Efficiency of TEG:5%, Efficiency of PV:20%

01/04/2011 23

Continuation in Kusatsu Hot-springs TEG System by TOSHIBA /Kusastu

01/04/2011 24

Future Prospects

• Two major projects aiming the enhancement of thermoelectric performance supported by the government (NEDO,JST)

• Development of TEG Power Management Technology

• Roadmap for TEG Applications

01/04/2011 25

Collaborative Research and Development

Design of novel clathrates by first-principle calculationsYamaguchi Univ.

Synthesis of single crystalstoward higher ZT

Synthesis of bulk materials for modules by sintering techniqueYamaguchi Univ.

Optimization of TE modulesAIST, KELK

Hiroshima Univ.

Design of TE power generation unit for furnaces

DENSO 500 W/unit

NEDO project “Development of Nano-StructuredThermoelectric Materials using Clathrates” FY2009-2011

Project Goal; ZT=1.3 at 200-300 ºC

01/04/2011 26

p-type

BGS

BGS

n-type

Temperature（℃）

BGAS

ZT for type-VIII Ba8Ga16Sn30 and Al substituted samples

). Saiga, Takabatake et al., J. Alloys and Compds, 507, 1 (2010).

S. Deng, Takabatake et al., J. Appl. Phys. 108, 073705 (2010

1.ZT values are 1.0 for n-type, and 1.2 for p-typearound 200 C.

2.A module of p- and n-types can be made from the same compound.

3.Preparation of Module was successful.

Preparation of Modules using Ba8Ga16Sn30 by AIST Team

Power Output: 1.7W (217 mW/cm2)

01/04/2011 27

01/04/2011 28

Nanocube of La-doped SrTiO3 modifying grain boundary with 2DEG Nb-doped SrTiO3

01/04/2011 29

TEG Applications in CREST Project

• Waste Heat Recovery from various sectors of our society, industries, infrastructures, public welfare, transportations

• Renewable Energy such as Solar Energy

Waste heat recovery TEG using Oxide modules

Concept of Solar Thermal TEG

Hybrid solar cell composed of dye-sensitized solar cell and TEG

350W TEG system

01/04/2011 30

LoadPower conditionerTE modules

Virtual load

IN OUT

Controller

Development of TEG power management technology

Thermoelectric Power Output P（W)Pow

er

Co

nd

itio

ner

Co

nve

rsio

n E

ffic

ien

cyη

C（％）

: High voltage type: Low voltage type: TEG Simulator

The DC-DC converter installed MPPT (Maximum Power Point Tracking) control has been successfully developed to achieve 96.7% in conversion efficiency as a 100W class power conditioner.

96.7％！！

01/04/2011 31

Energy harvesting TEG(Bio-heat, ground heat, air、Solar)

Waste heat recovery(Industrial, private, vehicle)

Solar thermal

Large scale waste heat recoveryfrom industry

２０４０２０１０ ２０２０ ２０３０

η=8%

η=15%

η=20%

Quantum effect

Phonon Scattering1mW

1W

1kW

1MW

1μW

η>30%Smart grid cogeneration TEG

TEG car

Solid waste combustion

OTEC,LNG cold, Low grade heat from plants

Robust Nanostructured

Synegy Nano

PGEC

10kW

100kW

Stove Heat

Structure control of complex compoundsStructure Control

Natural SL

Geothermal

Large scale central TEG

Synthesis of Atomic Network/Cluster Materials

Sca

le o

f TE

G s

yste

ms

Year

Mod

ule

Effi

cien

cy (%

) TEG Applications Roadmap10MW

01/04/2011 32

Conclusions• Several ongoing demonstration tests on TEG systems

applying to various fields such as industrial furnace, solid waste incinerator, motorcycle and solar thermal energy system have been actively progressed by private companies mostly. Durability of TEG systems for practical fields is the key at the next stage.

• The establishment of the advanced thermoelectric material technology is the urgent matter on the realization of TEG system in the society. Two ongoing national projects are introduced, in which the goal in ZT is 1.3~1.5 for the medium and wide temperature range applications.

• The roadmap on TEG systems is presented for forthcoming 30 years as well as the prospect on the enhancement of module efficiency with several key-words of materials innovation.

01/04/2011 33

Thank you for your kind attention !

The author would like to express his hearty gratitude to

Dr.H.Kaibe, KOMATSU,Mr.K.Nakajima,Showa Denko Co.Ltd.,Mr. M.Minowa, SWCC SHOWA CABLE SYSTEMS Co.Ltd., Mr.Y.Uchiyama, ATSUMITEC Co.Ltd.,Dr.M.Niino, JAST, Dr.K.Kisara,JAXAProf. Y.Horita, Tokyo Institute of Tech., Dr.Y.Saito,JGC Corp.,Prof. T.Takabatake, Hiroshima University, Prof. K.Koumoto, Nagoya University, andMr.T.Oishi, Mr.T.Shindo,TOSHIBA for their cooperation and sincere support.