MRS Spring 2010-Wood Stove

7/31/2019 MRS Spring 2010-Wood Stove

1/23

One application ofthermoelectrics: wood stoves for

residential cooking

March 9th 2009


2/23

Related work Electricity generation as by-product of space

heating in cold rural area of Lebanon

BiTe TEmodule


3/23

Natural convection heat sink

4.2 W per module, 0.24$/W, no heat wasted

Energy Conversion and Management 46 (2005) 16311643


4/23

Use some electricity to generate

more MIT Rajeev Ram group shows it is worth to use part of

the generated electricity to pump an active heat sink

100

101

102

103

104

10-2

10-1

10 0

101

102

103

Powerdensity(W/cm2)

h=20,Baseline SiGe

h=0.5,Baseline SiGe

h=0.5,

h=0.5,/2

L (m)

2

Forced liquid cooling usingelectric pump

W1W2

Net output W=W1-W2


5/23

Philips wood stoves

Philips wood stoves use part of generated heatto pump a thermoelectric fan to improvecombustion efficiency and generate more heat!

Air preheating:Q2 Q1, W=Q1-Q2

Even smarter: forcedcooling air ispreheated and fed inburning chamber, so

nothing lost!

BiTe module: W=Q1-Q2

Fan: W


6/23

Performance and Data sheet

Adjustable power between 1.5-5.5kW

Low emission figures not changed

Fan 1Watt, 150 liters/minute

Heated air inlet at top of stove, 200-300C

Peltier Thermo power generator (1.5-2.5Watt) starts after 4-7 minutes

About 100 Watt of heat to be dissipatedby heat sink

Performance against traditional stove:

up to 94% carbon monoxide reduction up to 93% particulate matter reduction up to 45% wood saving (CCT)


7/23

Why Philips is developing

advanced wood stoves > 2 Billion people still rely on wood for cooking

1.6 million people die each year from cooking relatedemissions

400 million stoves world wide market, mostly indeveloping countries

Test consumeracceptance

Drawbacks

Te is toxic and expensive

BiTe must work at low temperaturerange (


8/23

BiSbTe: high ZT at low temperature

range

Hest insulator athot side is neededto move the TEelement to lowtemperature range

P-type

Nano Lett., Vol. 8, No. 8, 2008


9/23

BiSbTe electrical conductivity

2

4

6

8

10

12

14

16

0 50 100 150 200 250

bulknano

Electricalconductivity(104 S/m)

Temperature (C)

Y = M0 + M1*x + ... M8*x8

+ M9*x9

11.018M0

-0.065122M1

0.0002117M2

-2.1893e-07M3

0.99984R

Y = M0 + M1*x + ... M8*x8

+ M9*x9

14.31M0

-0.076284M1

0.00019874M2

-1.784e-07M3

0.99995R


10/23


11/23

BiSbTe Seebeck

120

140

160

180

200

220

240

260

0 50 100 150 200 250

bulknano

Seebeck(uV/K)

Temperature (C)

Y = M0 + M1*x + ... M8*x8

+ M9*x9

205.03M0

0.70863M1

-0.0044033M2

1.5726e-06M3

0.99944R

Y = M0 + M1*x + ... M8*x8

+ M9*x9

173.72M0

0.52979M1

-0.0014583M2

-7.77e-07M3

0.99961R


12/23

Some basic principles before

numerical calculations Largest temperature gradient notnecessarily results in largest poweroutput: self compatibility

Electrical impedance match:maximum power, near maximumefficiency

Thermal impedance match: maximumpower

Largest internal thermal impedance:largest efficiency

Smaller heat sink thermal resistanceis always better, if not using power

Using active heat sink may increase

net electrical power due to its lowerthermal resistance (micro channels) orincreased heat source (wood stoves)

Active heat sink thermal conductivitynot necessarily linear to driving power

Tc

Th

RL

Rth-sink

Rth-in, RLin, V0

K

T

ZTs

2

11

Th point is: you justshould not put batterieswith similar emf but quitedifferent internal

resistances in series!


13/23

Optimization of bulk BiTe based

wood stoves

0

0.2

0.4

0.6

0.8

1

0%

1%

2%

3%

4%

0 5 10 15 20

Poweroutput(W/cm

2)

Efficiency

Heat sink at hot side (K/W)

0

0.2

0.4

0.6

0.8

1

0%

1%

2%

3%

4%

0 2 4 6 8 10

Poweroutput(W/cm

2)

Efficiency

TE element thickness (mm)

Cold side heat sink resistance 1K/W, for an TE module area of30mmx30mm with 50% filling factor, hot side 800 degree C, cold side50 degree C.

@2.4mm, 3.6K/W, output power is maximized at 0.787 W/cm2 (materialarea), with efficiency 2.56%


14/23

Optimization of nano BiTe based

wood stoves

Cold side heat sink resistance 1K/W, for an TE module area of30mmx30mm with 50% filling factor, hot side 800 degree C, cold side50 degree C.

@1.4mm, 5.4K/W, output power is maximized at 0.935 W/cm2 (material

area), with efficiency 4.61%

0

0.2

0.4

0.6

0.8

1

0%

1%

2%

3%

4%

5%

6%

0 5 10 15 20

Poweroutput(W/cm

2)

Efficiency

Heat sink at hot side (K/W)

0

0.2

0.4

0.6

0.8

1

0%

1%

2%

3%

4%

5%

0 2 4 6 8 10

Poweroutput(W/cm

2)

Efficiency

TE element thickness (mm)


15/23

Use SiGe: cheap and large work

temperature range

APPLIED PHYSICS LETTERS 93, 193121 2008

Large ZT at T>200C

Nanomaterials (preparedby ball-milling Si and Geand hot-pressing) haveimproved ZT

N-type


16/23

Thermal conductivity

2

3

4

5

6

0 200 400 600 800 1000

bulknano

Thermalconductivity(Wm

-1K-1)

Temperature (C)

Y = M0 + M1*x + ... M8*x8

+ M9*x9

4.6529M0

-0.0012977M1

-1.0797e-06M2

2.0776e-09M3

0.99805R

Y = M0 + M1*x + ... M8*x8

+ M9*x9

2.4819M0

0.0016613M1

-4.6281e-06M2

3.9035e-09M3

0.98016R


17/23

Electrical conductivity

2

4

6

8

10

12

14

0 200 400 600 800 1000

bulknano

Electrical

conductivity(10

4S

/m

)

Temperature (C)

Y = M0 + M1*x + ... M8*x8

+ M9*x9

12.406M0

-0.018646M1

6.9334e-06M2

5.8801e-09M3

0.9997R

Y = M0 + M1*x + ... M8*x8

+ M9*x9

9.827M0

0.0053567M1

-6.8222e-05M2

9.7962e-08M3

-3.7764e-11M4

0.99476R


18/23

Seebeck

-3

-2.5

-2

-1.5

-1

0 200 400 600 800 1000

bulknano

Seebeckcoefficient(10-4 V

/K)

Temperature (C)

Y = M0 + M1*x + ... M8*x8

+ M9*x9

-0.91549M0

-0.0033317M1

2.9075e-07M2

1.71e-09M3

0.99974R

Y = M0 + M1*x + ... M8*x8

+ M9*x9

-1.2286M0

0.0016329M1

-2.0576e-05M2

3.2238e-08M3

-1.4377e-11M4

0.99598R


19/23

Compatibility factor

SiGe is very self-compatible along temperaturegradient direction

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 200 400 600 800 1000

bulknano

Comp

atibilityfactor(/V)

Temperature (C)


20/23

Bulk SiGe TE wood stoves

W increases when sink resistance decreases Optimal TE element thickness (for maximum power) increases with sink resistance


21/23

Optimal TE thickness Heat sink resistance 1K/W, for an TE module area of 30mmx30mm with 50% filling

factor, hot side 800 degree C, cold side 50 degree C. 4.51 W/cm2 power density (using TE material area) and 5.33% efficiency can be

achieved. Optimal TE thickness is 2.5mm.

0

1

2

3

4

5

0%

1%

2%

3%

4%

5%

6%

7%

8%

0 0.2 0.4 0.6 0.8 1

Po

wer(W/cm

2)

Efficiency

TE thickness (cm)


22/23

Nano SiGe 6.17 W/cm2 power density (using TE material area) and 7.25%

efficiency can be achieved. Optimal TE thickness is 2mm.

0

1

2

3

4

5

6

7

0%

2%

4%

6%

8%

10%

12%

0 0.2 0.4 0.6 0.8 1

Po

wer(W/cm

2)

Efficiency

TE thickness (cm)


23/23

Summary

SiGe may be a better choice of TE material for Philipswood stoves for (1) cheaper (2) non-toxic (3) no thermalinsulator (4) less materials more power out

Power density of 4.51 W/cm2 (bulk) and 6.17 W/cm2

(nano alloys) can be achieved, with efficiency above 5%. As a comparison, original BiTe solution offers power

output 2W for 30mmx30mm area module, with efficiency2% in experiment, 0.787 W/cm2 (material area) withefficiency 2.56% for bulk in theory, and 0.935 W/cm2

(material area) with efficiency 4.61% for nano BiSbTe intheory.

Documents

MRS Spring 2010-Wood Stove