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 Heat Thermoelectric Generator This talk Current Thermoelectricity • Thermoelectricity: Electricity produced from heat • First observed by Seebeck in 1822 Hot End Wire Cold End Replica of the apparatus TH Tc V = S.DT i Thomas Seebeck Traditional Uses Seiko “Thermic” watches 5°C body heat, 60mW Doped Poly Si, .3% efficiency Cassini space probe 32.8Kg radioactive plutonium fuel, InGaAs thermocouple, 628 Watts, 3-4% efficiency Cat Mutator Docile Cat Radioactive Plutonium Pellet The Physics e Electrons: current flow e e e e e p p p p p p p Phonons: heat flow e e e e Hot end Cold end When a wire is heated electrons and phonons diffuse • Electrons – Higher electron diffusion Þ 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) Traditional Materials Ideally for large thermoelectric current • Low phonon flow – Const temperature difference Þ Low thermal conductivity • Many high energy electrons – Small resistance Þ High electrical conductivity • Many phonon electron collisions – Large voltage per unit temperature difference Þ High Seebeck constant Constant Metals Insulators Semiconductors Seebeck Small High Acceptable Electrical High Very Low Variable Thermal High X Medium®High Nanotech allows constants be controlled independently & precisely New Thin-film Wires e e e e e p p p p p p e e e e Hot end Cold end Thin film (few nanometers) • 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 Maximum theoretical efficiency of any generator Chip temperatures • Cold end (Tc) Temperature Difference Max. efficiency of a Bismuth Telluride – 27°C Generator • Hot end (TH) 50 7.1% – 77° C, 52 ° C 25 3.7% • M for Bismuth Telluride – 6x better Horizontal Generator Horizontal Generator Hot end (nanowire bundles) Cold end Wiring Layers Die • Run a bundle of Bismuth Telluride nanowires from processor hot spot to cold spot • Temperature difference: ~50 degrees Vertical Generator Wiring Layers Hot surface Cold surface Die Vertical Generator • Run a bundle of Bismuth Telluride nanowires from logic level to the heat spreader • Temperature difference: ~20 degrees Multiple Generators Vertical Generator Purr Cold surface Hot surface Die Rough Estimates Parameters Horizontal Vertical Length 1mm .25mm Area 300nm x 300nm 1cm x 1cm Resistance 13MW .3 mW Temp Diff 50 25 (50) Real Power .13mW .15W (.6W) Theoretical 7.1W 3.7W For Bismuth Telluride: • Seebeck coefficienct 243mV/K • Resistivity: 1.2 x 10-5 ohm/meter 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!
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