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Aperture Array LNA Cooling

(How best not to cool an LNA!)

Is it economically viable (or even physically possible) to cool the tens of thousands of front-end LNAs used in an SKA aperture array station?

Presentation Overview:

- 1 – Aperture Array Review
- 2-PAD

- 2 – LNA Cooling Costing Model
- physics and features
- results

- 3 – LNA Cooling Measurement
- description
- results

Presentation Overview

1000

SKA Reference Design

100

SKADS Benchmark

10

Field of View (deg2)

1

0.1

0.01

0.001

0.1

1

10

100

Frequency (GHz)

- Overall SKA concept
- Low Frequency(0.1-0.3GHz)Sparse Apertura Array
- Mid Frequency(0.3-1.0GHz)Dense Aperture Array
- High Frequency(1.0-20GHz)Small Dishes

- Aperture arrays are the only technology that provide survey speeds great enough to allow deep HI surveys
- FoV = 250deg2

- Benchmark document available to download online at:
- http://www.skads-eu.org/p/memos.php

1 – Aperture Array Review

1 – Aperture Array Review

Look out for talk by:Georgina Harris

1 – Aperture Array Review

Front-end PCB

- Look out for talks by:
- Chris Shenton (digital), Tim Ikin (analogue)

1 – Aperture Array Review

- SKADS benchmark scenario document:
- predicts the cost of an SKA aperture array station to be 3484k€
- assumes a Tsys of 50K for mid-frequency aperture array
- a saving of 200k€ can be made if Tsys is reduced to 40K (§8.4)

- Reducing Tsys
- “Vital to get below 50K” Peter Wilkinson
- Tsys might even be greater than 50K
- future developments will see noise LNA decrease (14K from previous talk)
- however cooling may still be required especially at high frequencies
- cooling will also deliver temperature stabilisation

1 – Aperture Array Review

front-end PCB

coax to antenna

twisted pair to receiver

cooling block

cooling lines

plastic casing

plastic casing

o-ring track

cooling block

hose fittings

milled fluid channel

- Possible concept for cooling the front-end module using a metallic cooling block

front-end PCB

warmfluid out

coldfluid in

1 – Aperture Array Review

- The costing model / simulation code:
- includes physics dealing with thermodynamics and hydrodynamics
- costing includes: non-recurring expenses, replacement, electrical power
- does not include: labour costs, no uncertainty analysis
- written as a simple Matlab script (should be easy to convert, eg. Python)
- might be able to become a ‘design block’ in the general SKA costing model

- Assumptions / principle limitations
- best estimates for input parameters used, some more inaccurate than others
- chiller cost is assumed to be linearly proportional with power consumption, more costing ‘data points’ required to make a more accurate relationship
- chiller cooling capacity efficiencies assumed to be equal for small and large chillers, more ‘real’ chiller specifications data are required

- The Matlab script is currently available to download online at:
- http://www.physics.ox.ac.uk/users/schediwy/cooling/

2 – Cooling Costing Model

- For the results in this presentation the code is configured to:
- compare cost of a cooling system with the total cost SKA aperture array as specified in the SKADS Benchmark Scenario document (3500k€/station)
- compare the power consumption with total station use (1000kW/station)

- Three scenarios are compared:
- 1 chiller located at the centre of the aperture array – “Model A”
- 16 chillers distributed throughout the aperture array – “Model B”
- 256 chiller distributed throughout the aperture array – “Model C”

- The Matlab script is currently available to download online at:
- http://www.physics.ox.ac.uk/users/schediwy/cooling/

2 – Cooling Costing Model

- Key:
- chiller
- pipe ‘D’
- pipe ‘C’
- pipe ‘B’

- SKA aperture array station
- Model A
- chillers = 1
- pipe ‘D’ = 16
- pipe ‘C’ = 256
- pipe ‘B’ = 4096
- pipe ‘A’ = 65536

~60m

2 – Cooling Costing Model

- Key:
- chiller
- pipe ‘D’
- pipe ‘C’
- pipe ‘B’

- SKA aperture array station
- Model B
- chillers = 16
- pipe ‘D’ = 0
- pipe ‘C’ = 256
- pipe ‘B’ = 4096
- pipe ‘A’ = 65536

~60m

2 – Cooling Costing Model

- Key:
- chiller
- pipe ‘D’
- pipe ‘C’
- pipe ‘B’

- SKA aperture array station
- Model C
- chillers = 256
- pipe ‘D’ = 0
- pipe ‘C’ = 0
- pipe ‘B’ = 4096
- pipe ‘A’ = 65536

~60m

2 – Cooling Costing Model

LNA cooling block

Individual Component Heat Power Absorption

Total System Heat Power Absorption

pipe A

pipe A

pipe C

pipe D

pipe B

total cooling capacity available from the chiller

pipe A

pipe B

pipe B

pipe C

pipe C

pipe D

pipe D

LNA cooling block

pipe A

pipe B

pipe C

pipe D

- Assumed ambient temperature 30°C, desirable LNA temperature −20°C
- Cooling much below this temperature is not possible with a glycol/water mixture
- The chiller cooling capacity was adjusted to compensate for the total heat power absorbed by the cooling system
- Insulation thickness was increased until the LNA was the dominant factor

2 – Cooling Costing Model

pipe C

pipe D

pipe B

pipe A

Total System Heat Power Absorption

LNA cooling block

pipe A

pipe C

pipe D

pipe B

total cooling capacity available from the chiller

pipe A

pipe B

pipe C

pipe D

LNA cooling block

pipe A

pipe B

pipe C

pipe D

- Assumed ambient temperature 30°C, desirable LNA temperature −20°C
- Cooling much below this temperature is not possible with a glycol/water mixture
- The chiller cooling capacity was adjusted to compensate for the total heat power absorbed by the cooling system
- Insulation thickness was increased until the LNA was the dominant factor

2 – Cooling Costing Model

pipe ‘C’ext = 100mmint = 20mm

pipe ‘D’external radius = 150mminternal radius = 50mm

pipe ‘B’ext = 58.5mmint = 6.5mm

pipe A0.82m3

pipe ‘A’ext = 34mmint = 2mm

pipe B2.17m3

pipe D5.02m3

pipe C2.57m3

- Pipe and insulation dimension:

- Fluid volumes:

2 – Cooling Costing Model

pipe A

pipe C

pipe D

pipe B

LNA cooling block

pipe A

pipe D

pipe D

pipe C

pipe B

pipe D

chiller

pipe C

pipe C

LNA cooling block

pipe B

pipe B

pipe A

pipe A

0 1 2 3 4 5 6 7 8 9Position in Loop

- Chiller pressure must be great enough to drive fluid through cooling system
- If there is too much pressure resistance the chiller flow rate will decrease
- Flowrate was set so that Reynolds number is above 10,000 for all pipes
- Dominated by inertial forces, viscous forces are minimised, turbulent flow

2 – Cooling Costing Model

- Model A60.0k€ (1.7%)

- Model B 51.1k€ (1.5%)

- Model C44.0k€ (1.3%)

6.6k€

17.5k€

6.6k€

6.6k€

9.9k€

17.0k€

16.1k€

9.9k€

9.9k€

7.4k€

2.7k€

14.1k€

1.6k€

7.4k€

4.0k€

7.4k€

7.4k€

2.7k€

- All cooling models only cost a small fraction of the total aperture array
- Model C results in the lowest price, mainly due to the reduction in coolant used
- limitation: model currently does not take into account the difference in cooling efficiency (coefficient of performance) of different classes of chillers

2 – Cooling Costing Model

- Model A= 42.5kW × 1= 42.5kW

- All models require only a small fraction (~4%) of the electrical power of the total aperture array (~1000kW)
- Because of chiller assumption electrical power consumption of all models is very similar
- Balance could change when chiller efficiencies are considered in detail

- Model B= 2.57kW × 16= 41.1kW

- Model C= 0.152kW × 256= 38.9W

2 – Cooling Costing Model

- Close-up photo of the Avago LNA showing the cold finger in contact with the PCB

thermocouple probe

GaA LNA

cold finger in contact with the PCB

3 – Experimental Cooling Work

- The housing used to trap nitrogen to eliminate water condensation as the PCB warms-up to room temperature

LNA PCB

cold finger

LN2 reservoir

50Ω terminator

3 – Experimental Cooling Work

- Plot of the broad-band noise temperature of the LNA PCB recorded at three different LNA temperatures (−50°C, −10°C and +30°C)

3 – Experimental Cooling Work

- Plot of LNA noise temperature of the LNA PCB at 700MHz measured at 17 different LNA temperatures

3 – Experimental Cooling Work

- Conclusions
- cooling 10,000’s of LNA is not physically ridiculous
- cooling could be economically beneficial
- cost a small fraction of the full aperture array (<2%)
- electrical power use is a small fraction of the full aperture array (~4%)

- Further Work
- only three models were studied in detail; further optimisation of parameter space may result in
- more work required on some cost inputs, particularly chiller assumptions
- presently work on low-loss potting compounds to minimise condensation problems

- The Matlab script is currently available to download online at:
- http://www.physics.ox.ac.uk/users/schediwy/cooling/

Conclusions

- The Matlab script is currently available to download online at:
- http://www.physics.ox.ac.uk/users/schediwy/cooling/

Presentation End

4 – Extra Slides

power supplies

spectrum analyser

50Ω cold load

copper coax

gain chain v02

liquid nitrogen bath

- Photo of the experimental set-up used to measure the noise temperature of the LNA at various LNA temperatures

4 – Extra Slides

- Prandtl number
- coolant specific heat
- coolant dynamic viscosity
- coolant thermal conductivity

- Reynolds number
- coolant density
- coolant dynamic viscosity
- coolant flow velocity
- pipe hydrodynamic diameter

- Hagen-Poiseuille Law
- coolant volumetric flow rate
- coolant dynamic viscosity
- pipe length
- pipe cross-sectional area

- Heat transfer coefficient
- coolant thermal conductivity
- pipe Nusselt number
- pipe hydrodynamic diameter

- Dittus-Boelter correlation
- Reynolds number
- Prandtl number

- Heat power absorbed
- heat transfer coefficient
- ambient temperature
- coolant initial temperature
- insulation thickness
- insulation thermal conduction
- pipe surface area

4 – Extra Slides

- Insulation
- Chiller flowrate large enough so that Reynolds number is above 10,000 for all pipes
- means: flow is dominated by inertial forces, viscous forces are minimised, flow is turbulent

- Cooling agents other than a glycol-water mixture would be too expensive, therefore minimum temperature limited to about −30°C
- Incompressible fluid – very small effect
- Laminar flow -
- Wall friction – Darcy-Weisbach equation – easy to include in the future
- Joint/Corner effects
- Chiller efficiencies

4 – Extra Slides

- Schematic representation of three models investigated using a Matlab cooling and costing simulation

pipe D

pipe B

pipe A

pipe C

x16

x16

x16

x16

antennapairs

Model A1 central chiller

chillers

subtiles

1

16

256

4096

65536

large pipe

small pipe

x16

x16

x16

antennapairs

Model B16 distributed chillers

subtiles

chillers

16

256

4096

65536

- The physical layout of three concepts are shown on the next slide

large pipe

small pipe

x16

x16

antennapairs

subtiles

chillers

Model C256 distributedchillers

256

4096

65536

4 – Extra Slides

3

4

Cascade Element:

1

2

50Ω Terminator

Copper Coax

Gain Chain v02

Spectrum Analyser

Avago LNA

- Factors affecting Tsys:
- sky temperature
- front-end LNA
- rest of system

4 – Extra Slides

4 – Extra Slides

- A photo of the demo-board after warming back up to room temperature

excess condensation collects on cold finger

no condensation visible on PCB

4 – Extra Slides

LN2 evaporated

Cold Finger Removed

4 – Extra Slides

4 – Extra Slides

2.56m

4 – Extra Slides

~13mm

~4mm

~13mm

~13mm

30 Leads

~35mm

Low DensityPoly Pipe 13mm X 100M: A$43.02

CAT 7

37 Leads

37Leads

~13mm

4 – Extra Slides