1. Micro-refrigeration Gareth F. Davies SIRAC Meeting, SELEX Galileo, Luton 20 October 2011

2. 2 Overview Need for microprocessor cooling Need for microprocessor cooling Conventional microprocessor cooling Conventional microprocessor cooling Alternative technologies Alternative technologies Development of micro-refrigeration system Development of micro-refrigeration system Results from modelling work Results from modelling work Benefits and future applications Benefits and future applications Next steps Next steps

3. 3 Need for microprocessor cooling Moore’s Law: approx. doubling of transistors every 2 years Moore’s Law: approx. doubling of transistors every 2 years Concentration of chip components results in increased heat output Concentration of chip components results in increased heat output To keep chip to acceptable temperature (< 85°C) need more efficient cooling than heat sinks/fans To keep chip to acceptable temperature (< 85°C) need more efficient cooling than heat sinks/fans

4. Conventional microprocessor cooling 4 (a)Typical microprocessor package attached to heat sink (b) Expanded section showing heat spreader and heat sink

5. Heat distribution across microprocessor 5 The microprocessor die generally has a surface area of 2 to 3 cm 2 Heat generation is unevenly distributed across the surface of the microprocessor die - overall heat flux from current high performance chips typically 150 W cm -2 - however, within “hot spots” heat fluxes are approaching 1kW cm -2

6. Alternative IC cooling technologies 6 TechnologyMaximum heat flux achieved (W cm -2 ) Comments Fan + heat sink50About the limit for this technology Water jet impingement 450High jet velocities needed Heat pipes 10-300Average 100 W cm -2 Micro-channels 450Single and 2-phase systems. Flow 10x less with 2-phase. Porous media 500Pressure drop 1.2 bar with water. Reported by Thermacore. Immersion cooling100Achievable with surface enhancement of heat source

7. 7 Development of a micro-refrigeration system Evaporator Microprocessor Compressor Expansion Orifice Condenser Suction Line Hot Gas Line Fan High Pressure Liquid Line Low Pressure Liquid/Gas Line LSBU in collaboration with the University of Oxford and Newcastle University has developed an electronic cooling device which uses a porous media heat exchanger in a micro-refrigeration system together with a small oil-free compressor LSBU in collaboration with the University of Oxford and Newcastle University has developed an electronic cooling device which uses a porous media heat exchanger in a micro-refrigeration system together with a small oil-free compressor

8. 8 Porous media evaporator A porous media heat exchanger has been developed by Newcastle University in conjunction with Thermacore A porous media heat exchanger has been developed by Newcastle University in conjunction with Thermacore Current device is designed to lift 350 W from 2.75 cm 2, with 2 g s -1 refrigerant (R134a) and a pressure drop of < 20 kPa Current device is designed to lift 350 W from 2.75 cm 2, with 2 g s -1 refrigerant (R134a) and a pressure drop of < 20 kPa

9. 9 Design of Oil-Free Compressor (Image supplied by University of Oxford) Static part of motor consists of a series of wire wound laminated cores Cores are slotted to leave a rectangular gap Moving assembly comprises a number of rectangular magnets arranged in a line and occupying the gap Low cost components High efficiency Simple, robust construction Compressor is designed to provide 100W shaft power Expected to provide 250W cooling at COP 2.5

10. 10 Development of model for micro- refrigeration system Output screen shows many parameter values, updated each time step A 1-D transient, lumped parameter model of the micro-refrigeration system has been developed at London South Bank University (LSBU). Model used to analyse system performance and investigate control of system “transients” A detailed model of the oil-free compressor has also been developed

11. 11 Results for Compressor Model Piston displacement oscillates similar to motor force Displacement lags motor force by approx. 90  (phase angle) Steady sinusoidal pattern seen when transient forces have decayed Piston Displacement and Motor ForcePiston Velocity and Damping Force Piston velocity is mirror image of damping force Damping  -(velocity) i.e. deceleration effect Piston velocity has sharper peak in compression phase

12. 12 Benefits and Applications Benefits of micro-refrigeration: efficient removal of heat from specific areas/surfaces; control of temperatures; reduced thermal stress; increased lifetimes for high power systems In the future, micro-refrigeration systems are likely to provide cooling for a wide range of applications Examples include: i. i.use of liquid microchannel heat exchangers in 3D chip architecture i.e. interspersed between layers of vertically stacked chips ii. ii.cooling of personal protective clothing for use in hot, hazardous environments iii. iii.cooling of defence electronics; high power electronics; laser diode cooling iv. iv.cooling of server racks in data centres

13. 13 Next steps Main focus for developing micro-refrigeration systems during recent years has been on single phase pumped refrigerant e.g. water Now turning to two-phase cooling systems – a range of small scale micro- refrigeration systems of different capacities have been reported recently One problem that has been highlighted for a number of the systems is the need for oil-free compressors It is planned to continue with development of the micro-refrigeration system described here and to adapt it to work with ammonia

14. 14 Further Information See website: http://www.minicool.co.uk or Email: info@minicool.co.uk