thermoelectrics: impacts on the environment and sustainability

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Thermoelectrics: Impacts on the Environment and Sustainability ANDREAS PATYK 1,2 1.—Karlsruhe Institute of Technology, Institute for Technology Assessment and Systems Analysis, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. 2.—e-mail: [email protected] Energy saving in power generation, industry, transport, and residential applications by using waste heat with thermoelectrics (TE) may be important for an environmentally sound and sustainable energy system. It is probable that operable TE generators (TEG) will be developed for numerous applica- tions and will save energy and reduce CO 2 emissions from plants. However, the environmental profile of a technology is not sufficiently described by just the energy and CO 2 inputs and outputs of the core process. Necessary pre- ceding and subsequent processes, other environmental impacts, and compet- ing technologies have to be considered as well. Furthermore, sustainability covers aspects beyond environmental soundness. So far, comprehensive studies on TE and the environment/sustainability have not been available. In this paper, the following selected aspects are discussed: resource availability, specific energy consumption of TEG production, specific energy and CO 2 savings in different application fields by TE and competing technologies, and the global potential of TE. Key words: Thermoelectric, energy saving, sustainability, resources, environment INTRODUCTION One of the great challenges is the establishment of a sustainable energy system. Research and develop- ment (R&D) into thermoelectrics (TE) is at least partly motivated by the assumption that TE—espe- cially when using waste heat—can contribute sig- nificantly to achieving this goal. However, according to a recent analysis, 1 their contribution seems to be relatively small, even if strong improvements in the figure of merit, ZT, etc. are anticipated; efficiency advantages of mechanical engines, for instance, will persist. On the contrary, another study states a much more optimistic perspective, 2 albeit without details. In the present paper, some aspects of sus- tainability regarding TE use for conversion of waste heat in transport, combined heat and power gener- ation, and residential applications are analyzed. Additionally, the overall potential of TE is assessed for thermal condensation power plants and indus- trial processes. The underlying analyses are preliminary. There- fore, the main messages provide hints at tendencies and issues to be considered when developing and implementing TE materials, generators, and sys- tems, but no final statements about pros or cons of TE, certain materials, etc. SUSTAINABILITY There is no complete agreement on how to assess the sustainability of policies, technologies or prod- ucts. However, there is agreement on the basic principles of sustainability, given by the definition of ‘‘sustainable development’’ of the UN World Commission on Environment and Development (WCED; the so-called Brundtland Commission): Sustainable development is development that meets the needs of the present without compromis- ing the ability of future generations to meet their own needs. 3 From this definition it is clear that sustainability is not completely described by environmental soundness, resource usage, and eco-efficiency only; it includes social and also cultural and institutional (Received July 8, 2009; accepted November 9, 2009; published online December 11, 2009) Journal of ELECTRONIC MATERIALS, Vol. 39, No. 9, 2010 DOI: 10.1007/s11664-009-1013-y Ó 2009 TMS 2023

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Page 1: Thermoelectrics: Impacts on the Environment and Sustainability

Thermoelectrics: Impacts on the Environment and Sustainability

ANDREAS PATYK1,2

1.—Karlsruhe Institute of Technology, Institute for Technology Assessment and Systems Analysis,Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. 2.—e-mail:[email protected]

Energy saving in power generation, industry, transport, and residentialapplications by using waste heat with thermoelectrics (TE) may be importantfor an environmentally sound and sustainable energy system. It is probablethat operable TE generators (TEG) will be developed for numerous applica-tions and will save energy and reduce CO2 emissions from plants. However,the environmental profile of a technology is not sufficiently described by justthe energy and CO2 inputs and outputs of the core process. Necessary pre-ceding and subsequent processes, other environmental impacts, and compet-ing technologies have to be considered as well. Furthermore, sustainabilitycovers aspects beyond environmental soundness. So far, comprehensivestudies on TE and the environment/sustainability have not been available. Inthis paper, the following selected aspects are discussed: resource availability,specific energy consumption of TEG production, specific energy and CO2

savings in different application fields by TE and competing technologies, andthe global potential of TE.

Key words: Thermoelectric, energy saving, sustainability, resources,environment

INTRODUCTION

One of the great challenges is the establishment ofa sustainable energy system. Research and develop-ment (R&D) into thermoelectrics (TE) is at leastpartly motivated by the assumption that TE—espe-cially when using waste heat—can contribute sig-nificantly to achieving this goal. However, accordingto a recent analysis,1 their contribution seems to berelatively small, even if strong improvements in thefigure of merit, ZT, etc. are anticipated; efficiencyadvantages of mechanical engines, for instance, willpersist. On the contrary, another study states amuch more optimistic perspective,2 albeit withoutdetails. In the present paper, some aspects of sus-tainability regarding TE use for conversion of wasteheat in transport, combined heat and power gener-ation, and residential applications are analyzed.Additionally, the overall potential of TE is assessedfor thermal condensation power plants and indus-trial processes.

The underlying analyses are preliminary. There-fore, the main messages provide hints at tendenciesand issues to be considered when developing andimplementing TE materials, generators, and sys-tems, but no final statements about pros or cons ofTE, certain materials, etc.

SUSTAINABILITY

There is no complete agreement on how to assessthe sustainability of policies, technologies or prod-ucts. However, there is agreement on the basicprinciples of sustainability, given by the definitionof ‘‘sustainable development’’ of the UN WorldCommission on Environment and Development(WCED; the so-called Brundtland Commission):

Sustainable development is development thatmeets the needs of the present without compromis-ing the ability of future generations to meet theirown needs.3

From this definition it is clear that sustainabilityis not completely described by environmentalsoundness, resource usage, and eco-efficiency only;it includes social and also cultural and institutional

(Received July 8, 2009; accepted November 9, 2009;published online December 11, 2009)

Journal of ELECTRONIC MATERIALS, Vol. 39, No. 9, 2010

DOI: 10.1007/s11664-009-1013-y� 2009 TMS

2023

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aspects. However, the environment and resourcesplay a special role because they are the materialbasis for all human activities, and resource con-sumption directly addresses intergenerational jus-tice. Therefore, environment- and resource-relatedissues are quite often in the foreground of simplifiedor early-stage sustainability assessments of tech-nologies, etc., which also applies to this paper.

Environmental and economic sustainabilityassessments of technologies require lifecycleapproaches, which are typically applied to theproducts of technologies, describing their intendedfunctions. A lifecycle approach means considerationof the production, use, and disposal/recycling ofproducts, including supply of the required equip-ment, auxiliary materials, energy, and material re-sources. This perspective is needed because weakpoints of technologies can be hidden in preceding orsubsequent processes which are unavoidably linkedto the core process of interest. In the case of TE it isprobable that, for numerous applications, operableTE generators (TEG) will be developed and will saveenergy and reduce CO2 emissions from plants.However, the lifecycle-related balance may be dif-ferent because of hidden weak points of, e.g.,material production.

The lifecycle results of the technology under studyhave to be compared with those of competing tech-nologies with the same function and, if scarceresources are used, with technologies with differentfunctions that use the same resources.

RESOURCE AVAILABILITY AND ACCESSJUSTICE

Three aspects of resource availability and accessjustice are discussed briefly (Table I).

Resource Prices

It is trivial that prices limit the availability of allgoods to people who are able to pay for them. Ifresource prices for TE materials contribute signifi-cantly to the costs of TE systems, they willdetermine the fields of application and users of TE.On the other hand, prices are very volatile. Devel-opment in recent years has been driven by real eco-nomics as well as by speculation. Nevertheless,whether and which companies decide to produce orimplement TEG in their products and for whichapplications depends on their expected cost, includ-ing resource prices. Examples of applications are:

� Cars for the global upper and middle class,enabling additional power production for comfortfeatures without additional fuel consumption.

� Combined heat and power production (CHP) forall people with own micro-CHP plants or gridconnection, enabling cheaper and more environ-mentally sound power supply.

� Woodstoves for the underclass in the third world,enabling reduced toxic emissions when cooking,and saving biomass, including nonrenewableresources from primary forests.

Locations and Geographical Distributionof Deposits

A technology cannot be sustainable without a soundresource base. Concentration of deposits in a fewcountries will limit the security of supply if thesecountries are politically unstable or uncooperative,or if they have less well-developed institutions, etc.On the other hand, especially in the case of exploi-tation of resources in poor countries, fair pricesfor resources, local re-investment, and acceptable

Table I. Production, reserves, ratios of reserves to production, and main deposits of some TE materials

ProductionReserves

Reserve

Static Ratio of Reserves toProduction

Prices (2004 = 1)2008 Base Reserves Reserve Base05:06:07:08:09kt kt kt Year Year

Antimony165 2100 4300 13 26 1.2:1.8:2.0:2.2:1.5

Reserve base [%] China 56, Thailand 10, Russia 9, Bolivia 7, RSA 5, Tajikistan 3, USA 2, other 8Bismuth

5.8 320 680 55 117 1.2:1.5:4.2:3.6:2.3Reserve base [%] China 69, Peru 6, CND 4, Bolivia 3, Mexico 3, USA 2, Kazakhstan 1, other 11Tellurium

0.135 22 48 163 356 7.4:6.8:6.3:17:13Reserve base [%] USA 12, Peru 8, Canada 3, other 77Reserve base: part of an identified resource that meets specified minimum criteria related to current mining and pro-duction practicesProduction tellurium without USAReferences [4–6]

Italic value denotes production tellurium except USA.

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working conditions and social standards are pre-conditions for the sustainability of technologiesbased on these resources. Only a minor part of TEmaterial resources are exclusively available fromdeposits in ‘‘high-standard’’ countries (e.g., OECDstates). Although this is true for nearly all materialresources, it is no reason to disregard the corre-sponding considerations.

Ratio of Reserves to Production

This aspect refers directly to intergenerationaljustice. Two indicators are used: the static (or sta-tionary) ratio of reserves to production based oncurrent consumption, and the dynamic ratio ofreserves to production, which refers to future con-sumption. The dynamic ratio takes into accountthat consumption is subject to future changes due todecreases or increases of established and theappearance of new consumers (evidently, this con-cept is more realistic but suffers from specificuncertainties). For instance, the static ratio ofantimony is only 15 years to 30 years (reserve andreserve base), i.e., one generation or less. Especiallyin the case of a currently very small annual pro-duction, the dynamic ratio has to be assessed. Fortellurium, e.g., the static ratio is about 160 years to360 years. However, based on the use of half of thecurrent tellurium input per kW generation, and itsuse in 1-kW TEGs for cars, the overall resourceswould be consumed by only 15 to 32 million cars (aquarter to a half of world car production in 20077).On the other hand, up to now the ratios of reservesto production of ores have been maintained in mostinstances because of new technologies for explora-tion, mining, etc.

Summarizing the Three Aspects and TheirUncertainties and Imponderability

Resource limitations do not demand that weabandon TE technology, but they do mean that weshould diversify its material and resource base andincrease material efficiency and recycling. Sustain-ability of resource consumption and technology

application requires significantly more than tech-nical efficiency; it also includes fair participation ofall stakeholders in the supply chain and access tothe product by groups with the biggest relativeadvantage. Concrete decisions require specific casestudies.

ENERGY CONSUMPTION OF TEGPRODUCTION

This analysis gives an impression of the optimi-zation potential of TEG production and generatesbase data for comparison of TE applications (see thenext section). Because of the state of systems anal-ysis of TE as well as of TE development itself,the analysis is done for a generic TEG (Table II).Generic means that the TEGs are defined by con-version efficiencies, materials, specific surfaces, andmasses, and by flat-rate values for the energy con-sumption of different manufacturing techniqueswhich all are aimed at current, or are plausible forfuture, TEGs and their production processes, but notidentical to certain existing TEGs and processes.

Results (Fig. 1)

The sequence of primary energy consumption(PEC; energy consumption including all conversion

Table II. Specifications of generic thermoelectric generators

TEG Specification

TEG 0 Based on existing Bi2Te3 TEG (e.g., Ref [8]); rounded-off material masses;specific energy consumption of TE material supply = tellurium; system efficiency 5%

Opt. 1 Efficiency and TE material qualitative and quantitative = TEG 0; other components optimized:material masses halved

Opt. 2 Double efficiency of TEG 0/Opt. 1; TE material Bi2Te3 based (specific energy consumption doubled);material masses (TEG module and other components) half of TEG 0

Opt. 3a Efficiency = Opt. 2; film technology I: high energy consumption, TE material:specific energy consumption guess based on different electronic materials (low); mass 5% of TEG 0

Opt. 3b Efficiency = Opt. 2; film technology II: low energy consumption, TE material:specific energy consumption guess based on different electronic materials (high); mass 1% of TEG 0

Fig. 1. Primary energy consumption of TEG production.

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losses up to the well/mine; base data: ecoinvent9) isas follows: Opt. 3b< Opt. 2< Opt. 3a< Opt. 1<TEG 0. In all variants, the largest share corre-sponds to the supply of aluminum for the heat sink.Only a medium to small shares corresponds to theTE materials themselves and module manufactur-ing. The optimization of the auxiliary componentsmay be a promising approach for very different TEconcepts to improve the energy balance. For thispurpose, the production of TE materials and mod-ules has to be analyzed with specific data for realTEG.

ENERGY AND CO2 SAVINGS: DIFFERENT TEAPPLICATIONS AND PHOTOVOLTAICS

The most sustainable application of a technologyor resource is that with the greatest advantages andleast disadvantages (and, of course, a favorableoverall assessment) as compared with the referencesituation. TEG can be implemented in differentsystems to utilize waste heat. Three completelydifferent TE applications with different beneficia-ries are compared. Some currently important orpromising TE materials can also be used in otherenergy technologies; this, for instance, applies totellurium, a component of Bi2Te3 for TE and CdTefor photovoltaics (PV). Therefore, the TE applica-tions and PV are compared. The systems aredescribed briefly in Table III.

The common reference or functional unit forcomparison of energy technologies is the productionor consumption of 1 MJ (or kWh) of a certain form ofenergy. For the comparison of production and use ofthe TE applications among one another and withPV, such a reference is not meaningful because ofthe different uses of the generated electricity. Insuch cases, indicators should refer to input or out-put flows related to environmental goals, e.g., sav-ing of primary energy, CO2 or scarce materialsresources. Here, the comparisons are done on thebasis of energy and CO2 savings related to telluriuminput, indicating the efficiency of the use of scarcematerial resources for the reduction of energy con-sumption and CO2 emissions.

Results: Production

As an intermediate result, Fig. 2 shows the PECshares of the base systems (without TEG) and of theTEG for the three applications. For the simplestsystem, the woodstove, the very small TEG(including batteries) accounts for a quarter of theoverall production expenditure. For the most com-plex system, the car, the share is only about 5%. Inthe case of the CHP plant, the share is quite highbecause of the performance of the TEG.

Results—Production and Use (Fig. 3)

By far the most efficient application is the wood-stove. Evidently, this is not due to the efficiency of

Table III. Specifications of thermoelectric and photovoltaic applications

Application Base System TE Generator

Passenger car Typical car; production9; diesel fuel consumption10;lifetime 150,000 km

Opt. 2; g 10%

CHP 100 kWel; diesel; gel 42%; weight, material shares estimatedfrom company information and literature; lifetime 20 a

Opt. 2; g 10%

Woodstove Material (mainly): 3 kg stainless steel; fuel consumptionw/o TEG 6 kg wood/(day 9 family),11

20% primary forest12; lifetime 5 a

TEG 0; g 5%

CdTe-PV South D 2009 Current g (8%); site in Germany; power generation13; production9; lifetime 20 a –CdTe-PV Sahara 20xx Optimized g (12%), site in Sahara; power generation13; production9; lifetime 20 a –

Fig. 2. Shares of base systems (without TEG) and TEG in the pri-mary energy consumption of the overall system’s production.

Fig. 3. Primary energy consumption and saving, and CO2 saving byproduction and use of thermoelectric and photovoltaic systemsrelated to the required tellurium quantities.

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electricity generation but to the improvement ofwood combustion. A precondition for this result isthat nonregenerative biomass, e.g., wood from pri-mary forests, is burned and that, therefore, thereleased CO2 is assessed like fossil CO2. Actually,the efficiency is higher than that of CdTe PV withimproved performance and very high solar radia-tion. The resource efficiency of TE in CHP is com-parable to that of current CdTe PV operating in,e.g., the South of Germany. TE in cars is much lessresource-efficient because of the short operationtime compared with stationary applications (disre-garding recycling).

GLOBAL ENERGY SAVING POTENTIAL

The potential of TE for making individual sectorsand the overall energy system more efficient andsustainable can be assessed from the specificreduction rates of TE applications and the energyproduction or consumption of the correspond-ing sectors. Figure 4 summarizes the specific(plant-related) and overall (related to the globalPEC) energy saving potential of TE applications(global energy data taken from or generated basedon Teske et al.14; reference year 2005; primarysources: IEA statistics). Besides the applicationsanalyzed above, power production from thermalcondensation plants and industrial energy use andrecovery are roughly assessed.

The use in CHP shows only small overall poten-tials. There is no significant change of this outcomereferring to future years in the scenarios describedby Teske et al.14 (reference according to IEA and[r]evolution). Even in the very ambitious ‘‘[r]evolu-tion’’ scenario, the overall share of CHP in globalpower production in 2030 is small, as is the impactof improvements of CHP by TE. This is remarkablebecause small- and medium-scale CHP (consumingliquid or gaseous fuels) is considered to be animportant component for a sustainable low CO2

energy system. For transport (with internal com-

bustion engines) and the woodstove, considerablereductions can be achieved. However, the largestsaving seems to be possible through improved tra-ditional use of biomass.

Large waste-heat potentials mostly have quitelow temperature levels and low energy densities.Examples are exhaust gases and condensed steamof large thermal condensation power plants. Forcondensers based on Kyono et al.,15 a specific fuelsaving in the range of 1% to 2% can be estimated.Related to the world PEC, this results in only smallsavings. On the other hand, current improvementsin power-plant efficiencies are bundles of 1% to 2%measures.

In industrial processes, numerous different situ-ations regarding temperature levels, heat flowrates, etc. appear. Generalized statements are veryuncertain. According to Hendricks and Choate,16 inthe US manufacturing industry about 6% of con-sumed energy is converted into recoverable wasteheat, but only certain production processes areassessed to be suitable for TE application. Adoptingthe figure mentioned for all industries in the worldand assuming a TE system efficiency of 10%, thePEC saving by TE is assessed to be of the sameorder of magnitude as for power generation bythermal power plants.

The overall potential of TE to reduce CO2 emis-sions can be estimated to be roughly of the sameorder of magnitude as the energy saving potential.

CONCLUSIONS

Primary energy consumption and CO2 emissionsassociated with the production and use of TEGsshow reasonable specific savings for three applica-tions: car, CHP, and woodstove. These results areprobably robust against more detailed investiga-tions. So, for two important impacts with respect tosustainability and environment—energy resourceconsumption and climate change—there areadvantages compared with no measures (for appli-cations in thermal condensation power plants andindustrial processes, rough estimations show smal-ler advantages). However:

� Other technologies could be comparably resource-efficient or even better (in the case of telluriumCdTe photovoltaics).

� The maximal overall PEC saving potential of TEapplication

� In cars, CHP, thermal condensation powerplants, and industry is about 3% of the globalPEC only—based on very optimistic assump-tions, e.g., TEG efficiencies of about 10%, and100% implementation.� In woodstoves alone, is about 3% but not from

large amounts of produced power but due to theuse of power for combustion improvement inthis simple and vital application.

Fig. 4. Specific (plant-related) and overall (related to global primaryenergy consumption) energy saving potential of thermoelectricapplications.

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� High specific savings, high resource efficiencies,and large overall potentials typically do notcoincide in the individual applications or sectors.Therefore, different stakeholders—TE develop-ers, producers, users, and the public (givingsupport to the implementation)—may have dif-ferent views on the preferred application of TE,use of resources, etc.

When developing and implementing TE, thesesummarized results have to be considered. Asmentioned above, these analyses are preliminary.Therefore, the listed tendencies and issues providea framework for more detailed analyses ratherthan direct instructions. Sound recommendationson how to optimize TE materials, modules, andsystems and on identifying optimal applicationswith respect to the environment and sustainabilityrequire information about additional environmen-tal impacts and different aspects of sustainability(costs, employment, working conditions, andtechnology accessibility) and about competingtechnologies. The most efficient way to get thisinformation is by integrating sustainabilityassessments into TE R&D.

The present paper seems to confirm the assess-ment by Cronin Vining.1 However, the great chal-lenge of developing a sustainable energy supply stilllacks a correspondingly great solution. Even ifthis is no really fascinating vision, TE will contrib-ute to sustainable energy supply because nearlyall promising future energy technologies stillneed more time for development, will remain moreexpensive than expected, have limiting resourcedemands, etc. Therefore, contributions to energysaving on the order of 1% to 5% are needed aswell.

REFERENCES

1. C.B. Vining, Nat. Mater. 8, 83 (2009).2. G. Angerer, L. Erdmann, F. Marscheider-Weidemann, M.

Scharp, A. Lullmann, V. Handke, and M. Marwede,Rohstoffe fur Zukunftstechnologien (Stuttgart: FraunhoferIRB Verlag, 2009).

3. World Commission on Environment and Development(WCED), Our Common Future (Oxford: Oxford UniversityPress, 1987).

4. US Geological Survey (USGS); US Department of the Inte-rior (USDOI), Mineral Commodity Summaries 2009(Washington, 2009).

5. Website of metalsplace.com (http://metalsplace.com). June 2009.6. Website of metalprices.com (http://www.metalprices.com).

June 2009.7. Verband der deutschen Automobilindustrie (VDA), Annual

Report. Frankfurt, 2008.8. Website of Hi-Z (http://www.hi-z.com). August 2008.9. R. Frischknecht, ed., Ecoinvent data v2.0 (Dubendorf:

Coinvent Centre, EMPA, 2007).10. H. Hopfner, W. Knorr, A. Patyk, U. Fritsche, C. Hochfeld,

and W. Zimmer, Potenziale zur Minderung von Treibhaus-gas- und Schadstoffemissionen: Integrierte Betrachtung vonKraftstoffen und Antrieben. Report in order of the GermanBundestag, Berlin/Darmstadt/Heidelberg 2006.

11. P. van der Sluis, (Philips Research): personal communica-tion. 2009.

12. Wissenschaftlicher Beirat der Bundesregierung GlobaleUmweltveranderungen (WBGU), Welt im Wandel: Energie-wende zur Nachhaltigkeit (Berlin, Heidelberg: Springer-Verlag, 2003).

13. European Commission 2007, PVGIS—PV Estimation Utility(http://re.jrc.ec.europa.eu/pvgis/apps/pvest.php?lang=de). 2007.

14. S. Teske, J. Beranek, E. Blomen, W. Graus, W. Krewitt,T. Pregger, O. Schafer, S. Schmid, S. Simon, S. Tunmore,and A. Zervos, Energy [r]evolution—A Sustainable GlobalEnergy Outlook. Scientific Research: DLR Stuttgart, EcofysUtrecht, 2008.

15. T. Kyono, R.O. Suzuki, and K. Ono, IEEE Trans. EnergyConvers. 18, 330 (2003).

16. T. Hendricks and W.T. Choate, Engineering Scoping Studyof Thermoelectric Generator Systems for Industrial WasteHeat Recovery (Washington: USDOE, 2006).

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