System Cooling of Outdoor Wi-Fi Antenna

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1 System Cooling of Outdoor Wi-Fi Antenna Robert Raos, Solectron Corporation MEPTEC Thermal Symposium February 16, 2005

2 Topics Requirements and Constraints Cooling Methods and Trade off Analysis System Cooling Simulation Thermal Testing Results and Comparison to Simulation

3 Requirements and Constraints Thermal Requirements: Max. Allowable Temp at Inlet of Electronics Compartment: 60 deg C Max. Allowable Temp in Electronics Compartment: 66 deg C Max. External Ambient Temp: 46 deg C Internally Generated Heat Load: 200 W distributed as follows: Solar Heat Loading: 753 W/sq. m, Evenly distributed on Front, One Side, and Top Surfaces (Total 3 Surfaces) ~ 200 W Other Constraints Maximum Weight Budget for Thermal Management Components: 12 lbs Maximum Cost for Thermal Management Components: $120

4 Thermal Model of a Sealed Box Exposed to Sun i i Shadow Side r t F4 F3 t Sunny Side r to m ti ts F7 Wr F6 Wi F5 F2 Ws F1 Legend : F = Heat Flux, Wr = Reflected Solar Radiation Shadow Side Wi = Internal Heat Load Ws = Incident Solar Radiation to m = Surface Temp., Shadow Side ti = Avg. Internal Air Temperature ts = Surface Temperature Sunny Side

5 Solar Radiation Heat Loading A3 Area in 2 Area m 2 32 A2 A1 A A Total A Use Method-1 : GR-487-CORE Sec 3.25 Incident Solar Load W i = 753 W/m 2 x m 2 = 803 W 3 α = Absorptance of Surface Wα = Absorbed Solar Load = W i X α For Painted white surface, α.20 Wα = 803 x.20 = 160 W

6 Cooling Methods Investigated Thermo-Electric Cooling Rankin Cycle A/C or Refrigeration Unit Phase-Change Materials (PCM) Heat Storage Air-to-Air Heat Exchanger

7 Thermo-Electric Cooling High Cost High Weight

8 AC / Refrigeration High Cost High Weight

9 Phase-Change Materials (PCM) Heat Storage PCM Temperature Phase change of PCM with temperature Latent Heat of Fusion (or Vaporization) Time Phase Change Material: TH29 Inorganic Salt Melting Temperature : 29C Latent Heat : 180 J/gm Density : 1500 kg/m 3 Weight Required: 100 lbs. Enclosure Int Fins Ext Fins High Cost High Weight PCM

10 Air-to-Air Heat Exchanger Internal Fins Internal Fans External Fans Ambient Air External Fins Internal fans provide air circulation over internal fins External fans ensure airflow of ambient air over external fins. Heat transfer is by a combination of convection to and from the fins to the air and conduction between internal and external fins The internal and ambient air do not mix Enclosure

11 Cooling Methods Selection Criteria, Ranking & Recommendation Criteria Thermal Performance Cost TEC *** * A/C or Refrigerati on Unit **** ** PCM ** ** Air-to-Air Heat Exchanger * **** Weight / Volume * * ** **** Reliability *** ** **** **** Ease of assembly & field service *** * ** **** Air-to-Air Heat Exchanger Wins For This Application

12 Air-to-Air Heat Exchanger Air Flow Schematic External Blowers Front (2) External Blowers Rear (2) Separator Internal Blowers (2) Antenna Electronics Double Sided Folded Fin Heat sink De-icing Fabric Outer Enclosure Vertical Section Outside Air In Heater Outside Air In

13 Simulation with Macroflow

14 Summary of Results: Macroflow Location T in deg C T out deg C Flow Rate CFM Electronics Compartment Internal Blowers Heat Exchanger Max Delta T to Ambient: = 37 deg C at outlet of electronics area

15 Flotherm CFD Model

16 Air Z=.7, External Heat Sink

17 Air Z=1, Internal Heat Sink

18 Air Z=3, Electronics Compartment

19 Air Z=3.7, Antenna/Fabric Gap

20 Air X=20.5, Through Heat Sinks

21 Air Z=3.5 Isometric View

22 Air X=21.3 Vertical Section View

23 Air Y=35 Horizontal Section View

24 Air X=35.7, CPU Vertical Section View

25 Fan Curve vs. System Performance Fan Curve for ebm #RG90-18/06 Pressure (in H2O).6 x x x (17.3,.38) HS, External x (21.3,.28) Antenna-Fabric Gap x x (22.75,.17) HS Internal x x x x Flow Rate (cfm)

26 Electronics Compartment Air Flow and Temperatures Summary 70% 9.02cfm 49 deg C Total Top: 45.51cfm, Avg Temp. 58 deg C 50% 8.73cfm 63 deg C 60% 9.72cfm 51 deg C 60% 10.11cfm 60 deg C 70% 7.93cfm 67 deg C Antenna, Left 20.5W Butler 0W Antenna, Right 20.5W 70% Radio 70% 59.52W CPU 26.4W PSU 70W 49 70% 5.47cfm 49 deg C 50% 12.89cfm 49 deg C 30% 9.75cfm 57 deg C 50% 12.68cfm 54 deg C 70% 4.69cfm 60 deg C Total Bottom: 45.48cfm, Avg Temp 54 deg C

27 Max. Air Temperature Violation Summary Maximum Allowable Air Temperature: 66 deg C Max. Air Temp. deg C Location Top of Radio PCB Top of PSU Module Right Corner of Right Antenna PCB Mitigation Plan Open Up Vent from 30% to 50 % Add an internal fan for PSU Exhaust None (within margin of error)

28 Electronics Compartment Predicted vs. Measured Temperatures Summary 49 (56) 51 (60) 58 (55) 63 (60) 60 (61) 67 (61) Antenna, Left 20.5W Butler 0W Antenna, Right 20.5W (55) 68 (64) 56 (57) 49 (50) 54 (52) deg C Radio 59.52W CPU 26.4W PSU 70W (X) = Measured

29 Air Temperature Simulation vs. Measurement Comparison Location Simulation Temp. deg C Measured Temp deg C Delta deg C Error % Inlet, Left Inlet, Left Inlet, Center Top of Radio PCB Top of CPU PCB Top of PSU Outlet, Left Outlet, Left Outlet, Center Outlet, Right Outlet, Right Outlet, Right

30 Conclusion High power dissipation sealed outdoor enclosures can be effectively cooled with low cost air-to-air heat exchangers. CFD simulation tools like Flotherm are very effective in predicting the internal air temperatures and in helping to optimize the thermal design for this type of application.