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
