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
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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
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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
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⎜ ⎜ ⎜
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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!
