two tank indirect thermal storage system: parametric ... · two tank indirect thermal storage...

Two Tank IndirectThermal Storage System

Parametric Studies forFLABEG Solar International GmbH

and Solar Millennium AG

Bruce Kelly

Nexant, Inc.A Bechtel-Affiliated Company

San Francisco, California

Indirect Nitrate Salt Thermal Storage

Parametric Studies• Select oil-to-salt heat exchanger approach

temperature which provides lowest levelized energy cost

• Analysis of oil-to-salt heat exchanger (exchange area, pressure losses, and cost), storage inventory temperatures (nitrate salt volume, tank dimensions, and costs), part load steam generator performance (Therminol temperature distribution, and live/reheat steam temperatures), part load Rankine cycle performance (live/reheat steam pressures, and cycle efficiency), and annual plant electric output

Rankine Cycle

• Generator output of 55 MWe

• Single reheat cycle with 6 feedwater heaters

• 100 bar / 373 ºC live steam

• 16.5 bar / 373 ºC reheat steam

• Final feedwater temperature of 235 ºC

• Condenser pressure of 90 millibar

• Gross cycle efficiency of 0.375

GateCycle Rankine Cycle Model

MODEL:

CASE:

POWER:

HR:

EFF:

SOLMIL

SOLMIL

55.03

0.00

0.00ST1

0.08

41.01

41.55

2348.7

P T

W H

CND1

CNDPMP

FWH4

FDWPMP

ST2

FWH5

33.48

6.31

242.77

2811.9

P T

W H

13.99

2.78

351.61

3154.2

P T

W H

DA1

MU1

FWH0

FWH1

0.37

2.28

74.03

2515.2

P T

W H

CT1

CIRCPM

IPST

18.50

53.97

208.48

2712.3

P T

W H

16.50

53.97

373.00

3195.8

P T

W H

6.18

2.74

254.63

2966.7

P T

W H

5.17

48.45

235.08

2929.1

P T

W H

FWH2

3.04

2.64

183.35

2830.7

P T

W H

1.17

2.51

104.02

2675.3

P T

W H

1.72

381.02

815.56

859.76

P T

W H 1.72

381.02

575.82

587.65

P T

W H

VRHTR

VSUHTR

STMGEN

100.01

60.29

373.00

3011.7

P T

W H M1

SP1

V2

6.18

2.74

254.63

2966.7

P T

W H

6.18

61.53

160.01

675.49

P T

W H

5.03

0.01

152.11

2726.6

P T

W H

V3100.67

0.01

311.46

2726.6

P T

W H 18.750

Exh loss

100.67

61.53

235.01

1015.0

P T

W H

9.31

49.75

70.03

293.80

P T

W H

101.36

61.53

190.02

811.75

P T

W H

TMX1

SP2

100.01

60.29

373.00

3011.7

P T

W H

102.05

0.00

161.58

687.92

P T

W H

TMX2 16.50

53.97

373.00

3195.8

P T

W H

102.05

0.00

161.58

687.92

P T

W H

S2

S4

S5S7

S8

S9

S11

S12

S15

S16

S17

S26

S27

S28S10

S32

S33S34

S35

S36

S1

S18

S19

S22

S24

S30

S6

S13

S14

S3

S31

S20

S29

S23

S39

S40

S41 S21

S25

S37

S38

S42

Steam Generator Model

Inputs from GateCycle analysis

• Feedwater flow rate

• Live steam flow rate

• Cold reheat steam flow rate and temperature

Steam Generator Model (Continued)

- Nitrate salt duty equals water/steam duty

- Reheater Therminol outlet temperature equals 225.2 ºC

- Adjust evaporator pinch point temperature to give preheater Therminol outlet temperature of 301.4 ºC

Iterate on Therminol flow rate and inlet/outlet temperatures for each heat exchanger, subject to following constraints:

Steam Generator Model (Continued)

• Select trial water/steam and Therminol velocities

• Calculate inside and outside heat transfer coefficients, overall heat transfer coefficient, log mean temperature difference, and heat exchange area

• Calculate number of tubes, length of tubes, and baffle spacing

• Iterate on tube side velocities and baffle spacing to meet pressure loss constraints

Steam Generator Model (Continued)

Oil-to-Salt Heat Exchanger Model

• Select trial nitrate salt (shell side) and Therminol (tube side) fluid velocities

• Calculate inside and outside heat transfer coefficients, overall heat transfer coefficient, log mean temperature difference, and heat exchange area

• Calculate number of tubes, length of tubes, and Therminol pressure losses, and nitrate salt pressure losses

Oil-to-Salt Heat Exchanger (Continued)

• Iterate on number of heat exchangers and tube length to meet pressure loss constraints

• Design philosophies

– Thermal storage discharging duty: 124.1 MWt to 135.2 MWt part load steam generator duty

– Thermal storage charging duty: 146.8 MWt full load steam generator duty, or 124.1 MWt to 135.2 MWt part load steam generator duty

Oil-to-Salt Heat Exchanger (Continued)

Approach Heat transfer Number ofTemperature, ºC area, m2 exchangers

5 36,390 86 28,566 87 22,827 68 19,187 69 16,529 510 16,197 411 12,761 4

Thermal Storage Model• Two approach temperatures on oil-to-salt heat

exchanger cause Therminol supply temperature to steam generator to be 12 ºC to 24 ºC below design value of 373 ºC

• With Therminol flow rate to the steam generator fixed at the design value, thermal input is 84 to 92 percent of design

• Oil-to-salt heat exchanger, steam generator, and Rankine cycle all operate at part load during thermal storage discharge

Thermal Storage Model (Continued)

Thermal Storage Model (Continued)

• Trial Therminol, water, and steam temperatures for superheater - evaporator - preheater combination are selected, from which the log mean temperature difference for each shell is calculated

• Trial Therminol outlet temperature for reheater is selected

• Thermal duties on the water and steam sides of four heat exchangers are calculated

Thermal Storage Model (Continued)

• Therminol flow rates through the reheater and the superheater - evaporator - preheater combination are calculated based on trial temperature distribution and thermal duties

• Therminol flow rate through the reheater is fixed at the design flow rate

• Tube side, shell side, and overall heat transfer coefficients were calculated for four heat exchangers

Thermal Storage Model (Continued)• Required log mean temperature difference for

each shell is calculated, using the thermal duties on the water and steam sides and the overall heat transfer coefficients:

• Calculated log mean temperature difference is developed using standard formula:

)2m C)(Area,o-2kJ/m ,overall(U

kJ/sec Q,Co ,difference etemperatur mean log Required =

Co ,difference etemperatur Lesser

Co ,difference etemperatur Greaterln

Co ,difference etemperatur Lesser - Co ,difference etemperatur Greater

Thermal Storage Model (Continued)

• Required and calculated values are compared; if the values differed, a new trial Therminol temperature distribution is selected, and the process repeated until the log mean temperature differences converged

• Resulting values for live steam temperature, live steam flow rate, reheat steam temperature, and reheat steam flow rate are exported to the GateCycle program

Thermal Storage Model (Continued)• In GateCycle, live steam pressure is determined

by the steam flow rate through the turbine. GateCycle calculates live steam pressure, condenser pressure, final feedwater temperature, and gross generator output, and exports these values back to the part load steam generator model

• Iterations are repeated until constant values for live and reheat steam pressure and temperature, and final feedwater temperature are obtained

System Fluid Temperatures

280

290

300

310

320

330

340

350

360

370

380

390

4 5 6 7 8 9 10 11 12

Oil-to-Salt Heat Exchanger LMTD, oC

Syst

em F

luid

Tem

pera

ture

s, o C

Hot nitrate salt tank

Cold nitrate salt tank

Therminol supply to collector field

Rankine Cycle Output

44

45

46

47

48

49

50

51

52

4 5 6 7 8 9 10 11 12

Oil-to-Salt Heat Exchanger LMTD, oC

Gro

ss R

anki

ne O

utpu

t, M

We

Unit Thermal Storage System Costs

$29

$30

$31

$32

4 5 6 7 8 9 10 11 12

Oil-to-Salt Heat Exchanger LMTD, oC

Uni

t Sto

rage

Cos

t, $/

kWht

Optimum Design Parameters

• Oil-to-salt heat exchanger approach temperature of 7 ºC

• Oil-to-salt heat exchanger design duty during thermal storage discharging equal to part load steam generator duty of 131.3 MWt (90 percent of design steam generator duty)

Equipment Arrangement

Cold Salt Tank Hot Salt Tank

Cold saltpump

Hot saltpump

Immersion heater(typical of 4)

Immersion heater(typical of 4)

Oil-to-salt heatexchanger (typical of 6)

Distributionring header

Distributionring header

Cooing air pipes(typical of 24)

Cooing air pipes(typical of 24)

1.0 m

Isolation valve(typical of 4)

12.5 m

Equipment Arrangement

Distributionring header

Immersionheater

(typical of 4)

Cooling air pipes(typical of 24)

Isolationvalves

(typical of 6)

1.0 m

Oil-to-saltheat exchangers

(typical of 6)

Cold Salt Tank

37.58 m

Hot Salt Tank

38.16 m

Hot saltpump

A and B

Elevatedplatform

40 m

Immersionheater

(typical of 4)

Cold saltpump

A and B