TECHNICAL AND ECONOMIC FEASIBILITY OF DRY AIR COOLING...

TECHNICAL AND ECONOMIC FEASIBILITY OF DRY AIR

COOLING FOR THE SUPERCRITICAL CO2 BRAYTON

CYCLE USING EXISTING TECHNOLOGY

Sandeep R Pidaparti, Patrick J. Hruska, Anton Moisseytsev, James J. Sienicki, Devesh Ranjan

The 5th International Symposium – Supercritical CO2 power cycles San Antonio, Texas March 28-31, 2016

Background

• Dry air cooling is an attractive option for the S-CO2 Brayton cycle • Technically feasible but is it economical? • If feasible completely eliminates the need for water, increasing the range of applicability

• Previous investigation of Dry air cooling option1 • Used Heatric Hybrid HX technology for Air-to-CO2 cooler – Significant increase in plant capital cost

per unit electrical output ($/kWe)

• Goal of the current investigation • Identify more economical Air-to-CO2 cooler to reduce the plant capital cost • Perform cycle optimization to identify optimal cycle operating conditions using the modified Air-to-

CO2 cooler

1) "Investigation of a Dry Air Cooling Option for an S-CO2 Cycle" by A. Moisseytsev, 4th S-CO2 Symposium, Pittsburgh, PA, September 9-10, 2014

Reference conditions and Assumptions

1

ABR S-CO2 CYCLE TEMPERATURES, PRESSURES, HEAT BALANCE, AND EFFICIENCIES

516.6 164.4 104.8

19.79

CO2 367.0 403.9

1360.5 kg/s 403.9 19.96 7.722

92.8% 7.751

528.0 516.6

0.100 19.80

174.6 185.0

89.7 182.0 19.97 7.696

Na 7.643 19.98

1267.0 96.3% 88.8 171.3 184.9

kg/s 7.559 90.1% 19.99 7.682

373.0

367.0

19.95

32.8 84.3 84.3 89.8

7.621 20.00 20.00 7.666

31.25

7.400 89.1%

Efficiency = 42.27 % 5%

32.7 89.6

7.628 7.635

32%

Q,MW T,oC 6,000 30.0 35.5

Input P,MPa 0.84 kg/s 0.226 0.101

250

68%

Cycle

94.6%

26.4

95.5%

137.2

156.4

338.2

95.5%

28.5

TURBINE

HTR

RHXCOMP. #2

COOLER

COMP. #1

Eff. =

LTR

Eff. =

LTR

Eff. =

Eff. =

e (TS) =

e (TS) =

e (TS) =

• 250 MWt (~105 MWe) S-CO2 cycle for a Advanced Fast Reactor (AFR) • 42.27% cycle efficiency

• 31.25oC minimum cycle temperature

• 7.4 MPa minimum and 20 MPa maximum

cycle pressures

• 30oC inlet water temperature • Assumed same for air • Atmospheric pressure

Air-to-CO2 cooler design

• Cross flow between CO2 and air • CO2 flowing through the SS316 tubes with external aluminum fins (3 passes) • 3 fans per each unit blow air (assumed uniformly) over the finned tubes

15’ wide

1” tube diameter 2.5” fin diameter 10 fins per inch

61’ long

CO2 inlet

CO2 outlet

Exploded view

Fins closer view

CO2 – solid lines Air – dashed lines

2

10.5’ tall

Air inlet

Air outlet

Outputs

Cooler modeling

Set Model Inputs

Number of cooler units CO2 mass flow rate

CO2, air inlet temperatures CO2 inlet pressure

CO2 outlet temperature

Determine air side properties Air side heat transfer coefficient Air side pressure drop

Discrete Sub-section modeling

Energy Balance

Resistance Network Effectiveness/NTU

Air, CO2 temperatures CO2 pressure drop

CO2 heat transfer coefficient

Conductance

Fan power CO2 outlet pressure

5

Cooler model verification Variable Harsco Industrial

Air-X-Changers Calculated

(EES model) Calculated

(EES model)

Inputs

Number of HEX units 86 86 86

CO2 flow rate per unit [Kg/s] 10.22 10.22 10.22

CO2 inlet temperature [oC] 89.61 89.61 89.61

CO2 inlet pressure (MPa) 7.736* 7.736* 7.635

Air flow rate per unit [kg/s] 317.2 317.2 317.2

Air inlet temperature [oC] 30 30 30

Outputs

Heat transfer capacity [MWth] 1.691 1.696 1.61

CO2 outlet temperature [oC] 32.7 33.12 32.64

CO2 pressure drop [KPa] 6.895 6.645 6.802

Air outlet temperature [oC] 52 51.2 51.11

Air pressure drop [KPa] Not provided - 0.1112

*The CO2 inlet pressure provided in the Harsco quotation didn’t match the proposed design parameter.

Fans efficiency is calculated by using the estimated pressure drop from EES (built in finned tube geometry)

6

η𝒇𝒂𝒏𝒔𝑾 𝒇𝒂𝒏 =𝒎 𝒂𝒊𝒓∆𝑷𝒂𝒊𝒓

ρ𝒂𝒊𝒓

Using, 𝑾 𝒇𝒂𝒏=32.95hp per fan → η𝒇𝒂𝒏𝒔=41%

S-CO2 air cooled cycle optimization

• Parameters for optimization Cycle minimum pressure Cycle minimum temperature

•

•

• Cycle maximum pressure (Limited by pressure containment capability of heat exchangers)

• Cycle conditions demand mechanical design changes •

•

•

•

•

Reactor heat exchanger (RHX) High temperature recuperator (HTR) Low temperature recuperator (LTR) Piping Turbomachinery components (Not implemented in this study)

Restricted to along the pseudo-critical line to take benefit of high fluid density

Minimum pressure (MPa)

Minimum Temperature (oC)

Maximum pressure (MPa)

7.4 31.25 18-30

7.628 32.5 18-30

8 35 18-30

8.864 40 19-30

9.688 45 20-30

Selected conditions for optimization study

Pressure containment capabilities of Heatric PCHEs (From their website)

3

PCHE design methodology

• Heatric design methodology was used to estimate the plate thickness (t2), ridge thickness (t3) as maximum pressure changes

• Semicircular channels are approximated as rectangular channels for design purposes • ASME 13-9

Parameters RHX HTR LTR

HEX type Z/I PCHE Platelet PCHE Platelet PCHE

Unit length (m) 1.5 0.6 0.6

Unit width (m) 0.6 1.5 1.5

Unit height (m) 0.6 0.6 0.6

Design temperature (oC) 550 450 300

Design pressure (MPa) 16-30 16-30 16-30

Hot side (Na) Cold side (CO2) Hot side (CO2) Cold side (CO2) Hot side (CO2) Cold side (CO2)

Channel diameter (mm) 6.0 2.0 1.3 1.3 1.3 1.3

Channel depth (mm) 4.0 1.0 0.65 0.65 0.65 0.65

Pitch to diameter ratio 1.083 Variable Variable Variable Variable Variable

Plate thickness (mm) Variable Variable Variable Variable Variable Variable

PCHE design methodology

1.6

1.5

1.4

1.3

1.2

1.1

1

0.9

0.8

Pit

ch/D

iam

ete

r

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Design pressure (MPa)

Channel Pitch to diameter ratios vs Design pressure

RHX

HTR

LTR

1.8

1.7

1.6

1.5

1.4

1.3

1.2

1.1

1

0.9

0.8

Pla

te t

hic

kne

ss (

mm

)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Design pressure (MPa)

Plate thickness vs Design pressure

RHX

HTR

LTR

9

• Estimation of heat exchangers cost

𝑼𝒏𝒊𝒕 𝒄𝒐𝒔𝒕 = 𝑼𝒏𝒊𝒕 𝒗𝒐𝒍𝒖𝒎𝒆. 𝝆𝒔𝒔𝟑𝟏𝟔.𝑴𝒂𝒕𝒆𝒓𝒊𝒂𝒍 𝒄𝒐𝒔𝒕 + 𝑭𝒂𝒃𝒓𝒊𝒄𝒂𝒕𝒊𝒐𝒏 𝒄𝒐𝒔𝒕

𝑯𝑬𝑿 𝒄𝒐𝒔𝒕 = 𝑼𝒏𝒊𝒕 𝒄𝒐𝒔𝒕. 𝑵𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒖𝒏𝒊𝒕𝒔

Material cost – 7.64 $/kg 𝝆𝒔𝒔𝟑𝟏𝟔 – 8,000 kg/m3 Unit volume – 0.54 m3

10

Piping design methodology

• Minimum wall thickness is estimated using ASME B31.1 design procedure

𝑴𝒊𝒏 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 =𝟎.𝟓𝑷𝒅𝒆𝒔𝒊𝒈𝒏.𝑰𝑫

𝑷𝒅𝒆𝒔𝒊𝒈𝒏.𝒀+𝑺𝒂𝒍𝒍𝒐𝒘𝒂𝒃𝒍𝒆 .𝑾 . 𝟏−𝑼𝑻𝑷−𝑪𝑨 −𝑷𝒅𝒆𝒔𝒊𝒈𝒏

𝑵𝒐𝒎𝒊𝒏𝒂𝒍 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔 = 𝑴𝒊𝒏 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔. (𝟏 − 𝑼𝑻𝑷 − 𝑪𝑨)

𝑺𝒂𝒍𝒍𝒐𝒘𝒂𝒃𝒍𝒆 - Allowable stress for SS316 (Table 1A of ASME B&PV code section II, Part D) 𝒀 – Coefficient from ASME B31.1 Table 104.1.2(A) 𝑾 – 0.975 (Weld joint strength reduction factor) UTP – Under tolerance allowance (12.5%) CA – Corrosion allowance (12.5%)

• As per the recent 3-D plant layout, there are 25 pipe sections connecting different components • Lengths • Inner diameters • Design pressure • Design temperature

• Estimation of piping cost 𝑶𝑫 = 𝑰𝑫 + 𝟐.𝑴𝒊𝒏 𝒕𝒉𝒊𝒄𝒌𝒏𝒆𝒔𝒔

𝑴𝒂𝒕𝒆𝒓𝒊𝒂𝒍 𝒗𝒐𝒍𝒖𝒎𝒆 =𝝅

𝟒. 𝑶𝑫𝟐 − 𝑰𝑫𝟐 . 𝑳

𝑴𝒂𝒕𝒆𝒓𝒊𝒂𝒍 𝒄𝒐𝒔𝒕 = 𝑼𝒏𝒊𝒕 𝒗𝒐𝒍𝒖𝒎𝒆. 𝝆𝒔𝒔𝟑𝟏𝟔. 𝑴𝒂𝒕𝒆𝒓𝒊𝒂𝒍 𝒗𝒐𝒍𝒖𝒎𝒆

Fabrication cost is neglected assuming that it is small compared to the material cost

Cost based optimization

• Plant capital cost based optimization technique is employed

• Plant capital cost per unit net electrical output is calculated as,

• Rest of plant capital cost is calculated for the reference conditions • Minimum pressure – 7.4MPa • Minimum temperature – 31.25oC • Maximum pressure – 20MPa • Water cooling • 4480.432 $/KWe is the reference value calculated • 104.6 MWe is the reference grid power calculated

• A code was developed in Matlab to perform the optimization

Rest of plant capital cost

31

$

𝑲𝑾𝒆=

𝑹𝒆𝒔𝒕𝒐𝒇𝒑𝒍𝒂𝒏𝒕𝒄𝒂𝒑𝒊𝒕𝒂𝒍𝒄𝒐𝒔𝒕+𝑹𝑯𝑿 𝒄𝒐𝒔𝒕+𝑯𝑻𝑹 𝒄𝒐𝒔𝒕+𝑳𝑻𝑹 𝒄𝒐𝒔𝒕+𝑪𝒐𝒐𝒍𝒆𝒓 𝒄𝒐𝒔𝒕+𝑷𝒊𝒑𝒊𝒏𝒈 𝒄𝒐𝒔𝒕

𝑷𝒆𝒍𝒆𝒄−𝑷𝒇𝒂𝒏

Optimization code flow chart

START

READ THE INPUT VARIABLES (MIN, MAX PRESSURE and MIN TEMPERATURE)

SET THESE VARIABLES IN INPUT FILES FOR PDC DEPENDING ON MAX PRESSURE CHANGE RHX, HTR and LTR parameters (Pitch

to diameter ratio, plate thickness)

BEGIN THE OPTIMIZATION WITH REFERENCE NUMBER OF HEX UNITS (RHX – 96 units, HTR – 48 units, LTR – 48 units)

VARY SPLIT FRACTION BETWEEN COMPRESSORS FROM 0.6 to 0.8 in steps of 0.01

FIND OPTIMUM SPLIT FRACTION WHICH YIELDS MAXIMUM CYCLE EFFICIENCY

AND SET THE VALUE IN CYCLE INPUT FILE

VARY NUMBER OF RHX units from 32 to 272 in steps of 8 FIND OPTIMUM NUMBER OF RHX UNITS WHICH MINIMIZES PLANT $/KWe AND

SET THE VALUE IN RHX INPUT FILE

VARY NUMBER OF HTR units from 18 to 198 in steps of 6 FIND OPTIMUM NUMBER OF HTR UNITS WHICH MINIMIZES PLANT $/KWe AND

SET THE VALUE IN HTR INPUT FILE

VARY NUMBER OF LTR units from 18 to 198 in steps of 6 FIND OPTIMUM NUMBER OF LTR UNITS WHICH MINIMIZES PLANT $/KWe AND

SET THE VALUE IN LTR INPUT FILE

DID OPTIMUM SPLIT FRACTION CHANGE?

NO

STOP

YES

12

• The cycle efficiency decreases after a certain maximum pressure due to drop in turbine and compressors efficiency

• It is required to change design of turbomachinery equipment to maintain constant efficiencies

• These values might change if the efficiencies of compressors and turbine are maintained constant

Results – Fixed turbomachinery design

42.5

42

41.5

41

40.5

40

39.5

39

38.5

38

37.5

Pla

nt

eff

icie

ncy

(%

)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Cycle maximum pressure (MPa)

Effect of cycle maximum pressure on plant efficiency

7.4MPa, 31.25C - Water cooling 7.4MPa, 31.25C 7.628MPa, 32.5C 8MPa, 35C 8.864MPa, 40C 9.688MPa, 45C

5400

5350

5300

5250

5200

5150

5100

5050

5000

4950

4900

4850

4800

4750

4700

Pla

nt

Ca

pit

al

cost

($

/KW

e)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30


Effect of cycle maximum pressure on plant capital cost

7.4MPa, 31.25C - Water cooling

7.4MPa, 31.25C 7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

13

• Optimum conditions are • Minimum pressure – 8MPa • Minimum temperature – 35oC • Maximum pressure – 24 MPa

90.5

91

91.5

92

92.5

93

93.5

Turb

ine

eff

icie

ncy

(%

)

18 19 20 21 22 23 24 25 26 27 28 29 30

Maximum cycle pressure (MPa)

Turbine efficiency as a function of max cycle pressure

7.4MPa, 31.25C

7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

90

89.5

89

88.5

88

87.5

87

86.5

86

85.5

Co

mp

ress

or

#1 e

ffic

ien

cy (

%)

18 19 20 21 22 23 24 25 26 27 28 29 30


Compressor #1 efficiency as a function of max cycle pressure

7.4MPa, 31.25C

7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

90

89

88

87

86

85

92

91

Co

mp

ress

or

#2 e

ffic

ien

cy (

%)

18 19 20 21 22 23 24 25 26 27 28 29 30


Compressor #2 efficiency as a function of max cycle pressure

7.4MPa, 31.25C

7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

Results – Fixed turbomachinery design

14

Results – Fixed turbine efficiency

• Optimum conditions didn’t significantly change compared to previous case • Minimum pressure – 8MPa • Minimum temperature – 35oC • Maximum pressure – 25 MPa

• $/kWe of air cooled plant is still about 5% higher compared to water cooled plant

37

38

39

40

41

42

43

Pla

nt

eff

icie

ncy

(%

)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30



7.4MPa, 31.25C - Water cooling 7.4MPa, 31.25C 7.628MPa, 32.5C 8MPa, 35C 8.864MPa, 40C 9.688MPa, 45C

5400

5350

5300

5250

5200

5150

5100

5050

5000

4950

4900

4850

4800

4750

4700

Pla

nt

Ca

pit

al

cost

($

/KW

e)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30


Effect of cycle maximum pressure on plant capital cost

7.4MPa, 31.25C - Water cooling 7.4MPa, 31.25C 7.628MPa, 32.5C 8MPa, 35C 8.864MPa, 40C

15

Effect of cycle minimum pressure

43

42.5

42

41.5

41

40.5

40

39.5

39

38.5

38

Pla

nt

eff

icie

ncy

(%

)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30



7.7MPa, 32.5C

8.2MPa, 35C

9.2MPa, 40C

5200

5100

5000

4900

4800

5400

5300

Pla

nt

ca

pti

al

cost

($

/kW

e)

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30


Effect of cycle maximum pressure on plant captial cost

7.7MPa, 32.5C

8.2MPa, 35C

9.2MPa, 40C

16

39.5

40

40.5

41

41.5

42

42.5

43

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Pla

nt

effi

cien

cy (

%)


Effect of cycle maximum pressure on the plant efficiency

7.8MPa

7.9MPa

8MPa

8.1MPa

8.2MPa

4800

4850

4900

4950

5000

5050

5100

5150

5200

5250

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Pla

nt

cap

tia

l co

st (

$/k

We)


Effect of cycle maximum pressure on the plant captial cost

7.8MPa7.9MPa8MPa8.1MPa8.2MPa

Water vs Air cooled optimal conditions

17

ABR S-CO2 CYCLE TEMPERATURES, PRESSURES, HEAT BALANCE, AND EFFICIENCIES

516.6 164.4 104.8

19.79

CO2 367.0 403.9

1360.5 kg/s 403.9 19.96 7.722

92.8% 7.751

528.0 516.6

0.100 19.80

174.6 185.0

89.7 182.0 19.97 7.696

Na 7.643 19.98

1267.0 96.3% 88.8 171.3 184.9

kg/s 7.559 90.1% 19.99 7.682

373.0

367.0

19.95

32.8 84.3 84.3 89.8

7.621 20.00 20.00 7.666

31.25

7.400 89.1%

Efficiency = 42.27 % 5%

32.7 89.6

7.628 7.635

32%

Q,MW T,oC 6,000 30.0 35.5

Input P,MPa 0.84 kg/s 0.226 0.101

250

68%

Cycle

94.6%

26.4

95.5%

137.2

156.4

338.2

95.5%

28.5

TURBINE

HTR

RHXCOMP. #2

COOLER

COMP. #1

Eff. =

LTR

Eff. =

LTR

Eff. =

Eff. =

e (TS) =

e (TS) =

e (TS) =

About 1-2% increase in the plant $/kWe compared to the water cooled cycle

Summary and Conclusions

• More economical Air-to-CO2 cooler was identified and commercially available

• The cooler was modeled and the calculations were verified with the vendor specifications

• Using this cooler design, air cooled cycle optimization was performed in reference to the water cooled cycle

• New optimal conditions were calculated which results in only about 1-2% increase in the plant $/kWe compared to water cooled cycle

• Shows that dry air cooling is both technically and economically feasible

Questions?

Thank you for your time!

Piping design methodology

Section Pipe ID (m) Pipe length (m)

1 0.68302 29

2 0.68302 2

3 0.68302 12

4 0.68302 30

5 0.5 9.25

6 0.5 5.5

7 0.5 13

8 0.5 9.25

9 0.5 5

10 0.5 55

11 0.5 21

12 0.5 5

13 0.5 10

14 0.5 10

15 0.68302 10

16 0.68302 15

17 0.68302 2

18 0.5 12

19 0.45 38

20 0.25 17

21 0.25 21

22 0.25 11

23 0.25 11

24 0.25 12

25 0.25 25

0.19

0.18

0.17

0.16

0.15

0.14

0.13

0.12

0.11

0.1

0.09

0.08

0.2

Pip

e t

hic

kne

ss (

m)

RHX to turbine pipe thickness vs cycle maximum pressure

7.4MPa, 31.25C

7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

0.05

0.048

0.046

0.044

0.042

0.04

0.038

0.036

0.034

0.032

0.03

Pip

e t

hic

kne

ss (

m)

18 19 20 21 22 23 24 25 26 27 28 29 30


18 19 20 21 22 23 24 25 26 27 28 29 30


turbine to HTR hot side pipe thickness vs cycle maximum pressure

7.4MPa, 31.25C

7.628MPa, 32.5C

8MPa, 35C

8.864MPa, 40C

9.688MPa, 45C

10

TECHNICAL AND ECONOMIC FEASIBILITY OF DRY AIR COOLING...

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