a thermoelectric cat warmer from microprocessor waste heat
DESCRIPTION
A Thermoelectric Cat Warmer from Microprocessor Waste Heat. Simha Sethumadhavan Doug Burger Department of Computer Sciences The University of Texas at Austin. Motivation. Hot laptops Cold cats Frozen whiskers Reduced pest control. Solution. Purr. On chip Thermoelectric Generator. - PowerPoint PPT PresentationTRANSCRIPT
A Thermoelectric Cat Warmer from Microprocessor Waste Heat
Simha Sethumadhavan Doug Burger
Department of Computer SciencesThe University of Texas at Austin
Motivation• Hot laptops
• Cold cats– Frozen whiskers– Reduced pest control
Solution
Purr
HeatOn chip
Thermoelectric Generator
CurrentThis talk
Thermoelectricity• Thermoelectricity: Electricity produced from heat• First observed by Seebeck in 1822
ThomasSeebeck
Replica ofthe apparatus
Hot End Cold End
TH Tci
Wire
V = S.T
Traditional Uses
Cassini space probe32.8Kg radioactive plutonium fuel, InGaAs thermocouple, 628 Watts, 3-4% efficiency
Seiko “Thermic” watches
5°C body heat, 60WDoped Poly Si, .3% efficiency
Cat Mutator
Radioactive Plutonium Pellet
DocileCat
The Physics
When a wire is heated electrons and phonons diffuse
• Electrons– Higher electron diffusion more current (good)
• Phonons– Collide with other phonons and increase heat flow (bad) or– Either transfer their momentum to electrons (good) or– Lose their momentum due to boundary collisions (good)
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p pe
ep
ep p
e
eee ep
e Phonons: heat flow Electrons: current flow
Hot end Cold end
Traditional Materials
Constant Metals Insulators SemiconductorsSeebeck Small High AcceptableElectrical High Very Low VariableThermal High X MediumHigh
Ideally for large thermoelectric current• Low phonon flow
– Const temperature difference Low thermal conductivity• Many high energy electrons
– Small resistance High electrical conductivity• Many phonon electron collisions
– Large voltage per unit temperature difference High Seebeck constant
Nanotech allows constants be controlled independently & precisely
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p pe
ep
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Hot end Cold endThin film (few nanometers)
New Thin-film Wires
• Thin film and metal boundary do not align – More phonon boundary collisions – More electron phonon collisions
• Figure of Merit (M = seebeck2. elec/therm)– Traditional Poly Si is 0.4– Thin-film Bismuth Telluride is 2.38 – [Venkatasubramanium et al. Nature 2001]
Generator Efficiency
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Efficiency = Th - Tc
Th• 1+ M −1
1+ M + TcTh
⎛
⎝
⎜ ⎜ ⎜
⎞
⎠
⎟ ⎟ ⎟
Maximum theoretical efficiency of any generator
Temperature Difference
Max. efficiency of a Bismuth TellurideGenerator
50 7.1%25 3.7%
Chip temperatures• Cold end (Tc)
– 27°C• Hot end (TH)
– 77° C, 52 ° C• M for Bismuth Telluride
– 6x better
Horizontal Generator
• Run a bundle of Bismuth Telluride nanowires from processor hot spot to cold spot
• Temperature difference: ~50 degrees
Die
Hot end Cold endHorizontal Generator (nanowire bundles)
Wiring Layers
Vertical Generator
DieVerticalGenerator
Wiring Layers
Cold surface
Hot surface
• Run a bundle of Bismuth Telluride nanowires from logic level to the heat spreader
• Temperature difference: ~20 degrees
Multiple Generators
Die
VerticalGenerator
Cold surface
Hot surface
Purr
Rough Estimates
For Bismuth Telluride:• Seebeck coefficienct 243V/K• Resistivity: 1.2 x 10-5 ohm/meter
Parameters Horizontal VerticalLength 1mm .25mmArea 300nm x 300nm 1cm x 1cmResistance 13M .3 Temp Diff 50 25 (50)Real Power .13W .15W (.6W)Theoretical 7.1W 3.7W
Conclusions• Limitations
– Manufacturing– Engineering: Hinders cooling, peripheral circuitry overheads– Only cats are supported
• Final thoughts– Thermoelectric heat extraction looks interesting– Newer materials can improve power output further– How far can this be pushed? – When does this become interesting to architects?
Thank You!