simulation of adsorption generators for optimal … files/presentations... · •beware...

SIMULATION OF ADSORPTION GENERATORS FOR OPTIMAL

PERFORMANCE

Bob Critoph, Steven Metcalf, Ángeles Rivero Pacho University of Warwick

Heat Powered Cycles, Nottingham, 27-29 June 2016

Contents

• Introduction

• Plate designs

• Shell and tube designs to date

• Adsorbent material

• Simulation of shell and tube adsorbers

• Simulation of finned tubes

• Shell and tube results

• Finned tube results

• Conclusions

Plate designs Topmacs project for car air conditioning: Nominal bed conductivity was

0.4 W/mK but thermal mass of steel too high.

Past shell and tube designs

Carbon packed between 1.2 mm tubes

• Recommended by consultants

• Beware consultants!

• Output power low due to lower than expected heat transfer in shell

and tube generators.

• Measured conductivity approx 0.2 W/mK.


Shell and tube heat exchanger

150 mm

40

0 m

m

0.8 mm

3 mm 1.2 mm


Carbon packed between 1.2 mm tubes

What is the optimum tube diameter and pitch? Could it be made better??


What is the optimum tube diameter and pitch? Could it be made better??

What is the optimum tube diameter and pitch? Could it be made better?? Simulate heat pump cycles in Matlab, using a range of carbon adsorbents.

Adsorbent material ADSORBENT

Density

(kg m-3)

Specific heat

(J kg-1 K-1)

Conductivity

(W m-1 K-1) x0 n K

Granular 208C 650 175+2.245*T(K) 0.1 0.2775 5.445 1.46

208C + lignin

binder 791 175+2.245*T(K) 0.32 0.2775 5.445 1.46

208C + silane

binder 704 175+2.245*T(K) 0.26 0.2344 4.453 1.318

208C

Grain monolith 750 175+2.245*T(K) 0.6 0.3629 3.6571 0.9

75 % 208C +

25% ENG 770 175+2.245*T(K) 1.03 0.2775*0.75 5.445 1.46

50 % 208C +

50% ENG 1025 175+2.245*T(K) 1.65 0.2775*0.5 5.445 1.46

n

satT

TKxx 1exp0

Simulation of shell and tube adsorbers

Water flow

Steel

Carbon

Radial direction

Axial direction

Adiabatic surface

nl nodes

nr nodes

• 2-bed cycle with heat recovery

• 3 cycles for periodicity

• Typical time step 0.02 s

Simulation of shell finned tube adsorbers

• 2-bed cycle with heat recovery

• 3 cycles for periodicity

• Time step in metal ≈ 1/20 x carbon

Water flow

Steel

Carbon

Radial direction

Axial direction

Adiabatic surface

nl nodes

nr nodes

Aluminium

Shell and tube results

1.15

1.17

1.19

1.21

1.23

1.25

1.27

1.29

1.31

1.33

1.35

0 1000 2000 3000 4000 5000 6000 7000

CO

Ph

Output power per unit volume (kW/m3)

time 20

time 30

time 40

time 50

time 60

time 70

time 90

time 110

time 130

time 150

time 170

time 200

Envelope

Range of heat recovery times Heating, cooling times

• Existing 1.2 mm tube diameter

• Existing 3 mm tube pitch

• 208C carbon with lignin binder

• Return water from load 50C

• High temperature water 170C

• TEVAP 5C

Shell and tube results – materials comparison

1.15

1.17

1.19

1.21

1.23

1.25

1.27

1.29

1.31

1.33

1.35

0 1000 2000 3000 4000 5000 6000 7000 8000

ENG + carbon (50%)ENG + carbon (75%)Lignin + carbonMonolithic grainsSilane blockVibrated grainsEnvelope - ENG + carbon (50%)Envelope - ENG + carbon (75%)Envelope - Lignin + carbonEnvelope - Monolithic grainsEnvelope - Silane + carbonEnvelope - Vibrated grains


CO

Ph

Conclusion: restrict further analysis to 208C + lignin binder Examine different tube diameters and pitches

1.15

1.17

1.19

1.21

1.23

1.25

1.27

1.29

1.31

1.33

1.35

0 2000 4000 6000 8000 10000 12000 14000

Pitch = 2.5 mm

Pitch = 3 mm

Pitch = 4 mm

Pitch = 5 mm

Envelope - Pitch = 2.5 mm

Envelope - Pitch = 3 mm



Shell and tube results – pitch comparison with 1.2 mm tube

Conclusion: Present pitch not far from optimum 2.5 to 4 mm Could finned tubes offer an improvement?

CO

Ph


Finned tube results (3 mm diameter) O

utp

ut

he

atin

g p

ow

er

pe

r u

nit

vo

lum

e

(kW

/m3

)

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10 30 50 70 90 110

CO

Ph

Half heating/cooling time (s)

Power - Time rec. = 0s - Low hw Power - Time rec. = 50s - Low hwPower - Time rec. = 0s - High hw Power - Time rec. = 50s - High hwCOPh - Time rec. = 0s - Low hw COPh - Time rec. = 50s - Low hwCOPh - Time rec. = 0s - High hw COPh - Time rec. = 50s - High hw

Finned tube results • Higher pitches are feasible, desirable • Higher pitches lead to water side heat transfer

becoming dominant. • Graph shows effect of increasing water side heat

transfer x 10

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10 30 50 70 90 110

CO

Ph

Ou

tpu

t h

eati

ng

po

wer

per

un

it v

olu

me

(kW

/m3)

Half heating/cooling time (s)

Power - Time rec. = 0s - Low hw Power - Time rec. = 50s - Low hwPower - Time rec. = 0s - High hw Power - Time rec. = 50s - High hwCOPh - Time rec. = 0s - Low hw COPh - Time rec. = 50s - Low hwCOPh - Time rec. = 0s - High hw COPh - Time rec. = 50s - High hw

Finned tube, 3 mm dia. , high water H.T.

• 3 mm diameter is not necessarily optimal • Higher pitches mean easier manufacturing • Changing cycle times allow good modulation

1.15

1.17

1.19

1.21

1.23

1.25

1.27

1.29

1.31

1.33

1.35

0 2000 4000 6000 8000

CO

Ph


Pitch = 6 mm

Pitch = 8 mm

Pitch = 10 mm

Pitch = 12 mm

Pitch = 16 mm






Conclusions

• Control strategies provide a useful degree of modulation with acceptable COP variation

• Existing shell and tube design is not far from optimal

• Finned tube designs should be easier to manufacture and have better performance

• Water side heat transfer will be limiting

• Future work will identify desired tube and fin dimensions

Thanks for your attention!

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