concentrated solar thermoelectric power

CONCENTRATING SOLAR POWER PROGRAM REVIEW 2013

Concentrated Solar Thermoelectric Power Principal Investigator: Prof. Gang Chen Massachusetts Institute of Technology

Cambridge, MA 02139 [email protected]

http://web.mit.edu/nanoengineering/

Subcontractor: Prof. Zhifeng Ren University of Houston

DOE EERE CSP Program: Funding Category: Seed Concept


Solar to Electricity Conversion

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Solar Electricity: PV

http://www.treehugger.com/Solar-Thermal-Plant-photo.jpg

Solar Electricity: Thermal-Mechanical

http://www.homesolarpvpanels.com


Thermoelectric Devices and Effects

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COLD SIDE

HOT SIDE

Nondimensional Figure of Merit

kTSZT

2σ=

Reverse Heat Leakage Through Heat Conduction

Joule Heating

Seebeck Coefficient Electron Cooling

-

I

+

N P

COLD SIDE

HOT SIDE


Solar Thermoelectric Energy Conversion

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• US Patent No. 389124: E. Weston in 1888 • M. Telkes, JAP, 765, 1954

Efficiency: 0.63%


Heat Flux Considerations

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LLTkq K 100

KmW1∗

≈∆

=

q=1000 W/m2 (1 Sun); L=100 mm q=100,000 W/m2 (100 Sun); L=1 mm


Optical vs. Thermal Concentration

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Optical Concentration

Optical Concentrator

p n

Selective Surface Area As

Enclosure

Solar Radiation


Flat Panel Solar Thermoelectric Generators (STEGs)

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Kraemer et al., Nature Materials, May, 2011


Two-Stage Concentrated STEGs

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p n

p n

Skutterudite stage

Bi2Te3 stage

Selective surface

Heat sink

I1

I2

concentrator

sunlight


Program Goals

• Demonstrate 10% solar-to-electric energy conversion by using optical concentration and a multi-stage TEG

• Limit required optical concentration to less than 10X, ideally less than 4X

• Demonstrate the potential for 24-hour operation by incorporating phase change materials into design

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Core Team

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Professor Gang Chen, Principal investigator

Professor Zhifeng Ren Co-Principal Investigator

Ken McEnaney Device Engineer

Daniel Kraemer System Engineer

Dr. Sveta Boriskina Optical Engineer

Dr. Weishu Liu Materials Scientist

Dr. Feng Cao Materials Scientist

Dr. Qing Jie Materials Scientist

Lee Weinstein System Engineer


Major Tasks and Challenges • Devices

– Electrodes – Device joining

• Emittance – Selective surfaces – Optical design

• Systems – Modeling – Integration – Testing

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Effect of electrodes and contacts: • Parasitic losses have a linear negative effect on

system performance Effect of absorptance/emittance:


TEG Optimization: Critical Parameters

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Th

Tc

• Geometry must be optimized to • Reduce parasitic effects of electrode and

contact resistance • Reduce effect of radiative losses from

sidewalls

• For each thermoelectric leg: p-type skutterudite

p-type Bi2Te3

n-type skutterudite

n-type Bi2Te3

n hot electrode

n mid electrode

n cold electrode

p hot electrode

p mid electrode

p cold electrode

Ideal leg efficiency

Decreasing leg area


Electrodes: Material Properties • Bi2Te3 electrodes: <1% of system resistance • New p-type skutterudite electrode alloy:

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Electrical resistivity reduced by a factor of 3+

Thermal conductivity increased 50% – 200%


Electrodes: Contact Resistance • Bi2Te3/copper contact resistance < 1 μΩ•cm2

• Skutterudite/electrode contact resistance:

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n-type: <1 μΩ•cm2

p-type: 6 μΩ•cm2

electrode skutter.

electrode skutter.


TEG Device Testing • Currently testing single-stage devices; 9% efficiency

recorded. • Multi-stage devices should boost efficiency to ~13%

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Selective Surfaces • Optimized for operating temperature (500 – 600 °C) • Manufactured via sputtering • Surfaces with copper substrates not stable at high T

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400 500 600 700 800 900 1000 11000

20

40

60

80

100

Refle

ctan

ce (%

)

Wavelength (nm)

Cu SS Cu_500_7d_in vacuum SS_500_7d_in vacuum Cu_550_7d_in vacuum SS_550_7d_in vacuum Cu_500_7d_in air


Emittance Testing • Direct measurement of total hemispherical emittance at

high T:

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Sample surface

Heater

Thermocouple

Leads


Receiver Cavity • Receiver cavity can reduce heat loss from black surface

or selective surface

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With blackbody absorber: With 20% selective surface:

Selective Surface Only

SS + Cavity

Black Surface + Cavity


Additional Leverage

• Experience: – Materials – Devices – Heat Transfer – Multiple Start-ups

• Other Programs: – EFRC S3TEC: thermoelectric materials – ARPA-E HEATS: Storage

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Transformative Potential

• Third way: sunlight into electricity • Low cost due to:

– Solid state power block – Low optical concentration – Small amount of materials – Thermal storage

• Large market – Rooftops – Small communities – Power plants

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concentrated solar thermoelectric power

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