parabolic trough solar plants
TRANSCRIPT
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Development of a Molten-salt ThermoclineThermal Storage System for ParabolicTrough Plants
James E. Pacheco
Steven K. Showalter
William J. Kolb
Sandia National Laboratories
Forum 2001: Solar Energy: The Power to Choose
April 24, 2001
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Need for Thermal Storage for
Trough Plants Enables dispatchability without fossil fuel
Provides load shifting Increases revenue from plants Allows higher solar fraction in combined cycle
(CC) plants
Improves economics of solar in CC plants
Decouples collection from electricity production
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
How Thermal Storage Can be
Integrated into a Solar Plant Direct Heat transfer fluid is the same as the storage media
Examples: SEGS 1 (mineral oil), Solar Two (molten salt)
Indirect Heat transfer fluid is transferred to another media
Examples: SEGS plant (synthetic oil transfers heat to molten salt in atwo tank system).
Collector Field
Oil-to-SaltHeat
Exchanger
ThermalStorage
SteamGenerator
Turbine
Condenser
Hot Tank
Cold Tank
OilSaltSteam/Water
OilSaltSteam/Water
Indirect Thermal Storage System
Estimated Capital CostOf a Two-Tank Molten Salt
System: $31/kWht
Long-term Goal: $10/kWht
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Objective of Thermocline
Development Project Reduce the capital costs of storage for
parabolic trough plants relative to the
baseline (indirect two-tank molten-saltsystem).
Address the technical risks associated with
thermocline storage:Compatibility of filler materials with molten-salt
Unproven concept
Safety issues of fuel (Therminol) next to oxidizer(nitrate salt)
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Description of Thermocline System
A thermocline molten salt system:
- Uses a single tank to storageenergy
- Has a thermal gradient thatseparates the hot from coldfluid.
- Uses a low-cost filler materialto displace higher-cost moltennitrate salt.
HTF from Field
HTF Return
Salt-to-OilHeat
Exchanger
Thermocline Tank
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Development Activity Divided into 3 Areas
Model thermocline system: Simulate
performance and economics Evaluate candidate filler materials: Isothermal
and thermal cycling tests
Test a small pilot-scale thermocline: 2.3 MWhcapacity to validate performance and operatingcharacteristics
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NM
National Renewable Energy Laboratory, Golden CO
Modeling a Thermocline System
Transient thermal behavior simulated by the Schumannequations which describe the heat transfer between afluid and a packed bed.
Finite difference representation of these equationsenabled calculations of the thermal gradient and inletand outlet temperatures as a function of time.
Also enabled calculation of extracted energy andcapacity.
)()(
)( fbvffpf
fp TThy
T
A
mCTC +
=
)()1()( bfvb
bp TThT
C =
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S O L A R T H E R M A L E N E R G Y
SunLabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Model Results
0
24
6
8
10
12
14
1618
290 300 310 320 330 340 350 360 370 380 390
Temperature, deg C
0.0 hrs0.5
1.01.5
2.02.5
3.0
275
300
325
350
375
400
425
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Time, hours
In
Out
275
300
325
350
375
400
425
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50Time, hours
In
Out
Charging Discharging
Thermal Gradient During Charging
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Screening Potential Filler Materials
in Nitrate Salt Seventeen commonlymined minerals andcrushed rock were
evaluated for theircompatibility withnitrate salts
The ideal filler materialwould have thefollowing properties: Inexpensive
Widely available
Low void fraction Compatible with nitrate salts
Have a high heat capacitance
Chemical Name Mineral Name Chemical Formula
Aluminum oxide Corundum Al2O3Aluminum oxyhydroxide Bauxite* AlOx(OH)z
Barium carbonate Witherite BaCO3Barium sulfate Barite BaSO4
Calcium carbonate Marble CaCO3Calcium fluoride Fluorite CaF2Calcium sulfate Anhydrite CaSO4
Iron (II,III) oxides Taconite Fe2O3,Fe3O4Iron titanate Ilmenite FeTiO3
Magnesium carbonate Magnesite MgCO3Silicon carbide Carborundum SiC
Tin oxide Cassiterite SnO2Calcium hydroxyphosphate Hydroxylapatite Ca5(PO4)3 OH
Calcium flurophosphate Fluorapatite Ca5(PO4)3FCalcium carbonate Limestone CaCO3H2O
Silicon dioxide Quartzite SiO2
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Isothermal Tests of Filler Materials
Evaluated candidate materials in nitrate salts at 400 C.Samples were removed at 10, 100, 400 and 1000 hours
of exposure.
Samples were washed, weighed, and photographed.The salt was analyzed for contaminants.
Results indicated the most promising materials were:quartzite, taconite, marble, NM limestone,apatite,corrundum, scheelite, and cassiterite.
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Thermal Cycling Tests
Evaluated how well materials held up tothermal cycling condition expected in athermocline system. At least 350 cyclesbetween 290 and 400 deg C were
conducted on each sample. Samples tested: taconite, marble, NM
limestone, quartzite, and silica sand.
Results: NM Limestone fell apart
Marble softened and individual grainsappeared to enlarge
Taconite pellets held together well.
Absorbed some salt in pores Quartzite rock and silica sand held up
remarkable well and were selected asfiller material for pilot-scale test
ColdTank
HotTank
Chamber
Thermal Cycling System
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Thermocline Test
Evaluate on a larger (pilot-scale)a molten-salt thermoclineconcept.
Fabricated a 6 m tall by 3 mdiameter carbon steel tank.
Filled tank with a 2:1 mixture ofquartzite rock and silica sand toa level of 5.2 m.
Sodium nitrate and potassiumnitrate were melted and added to
tank. A propane heater simulated the
heat input from the solar field(via the salt-to-oil heatexchanger).
A forced-air cooler simulatedheat rejection to the steamgenerator (through the salt-to-oilexchanger).
2.9 MW Propane FiredSalt Heater
Thermocline Storage Tank10 diameter x 20 high
2.2 MW Salt toAir Cooler
Flow meter
Flow meter
Drain sump
Carbon steelcold pump
Stainless steelhot pump
TC
TC
TC
TC
TC
25 ft
28 ft
22 ft
17 ft
15 ft14 ft
3ft 3in
-14 in
7 in
FCV 101
FCV 201
HV 101
HV 201
HV 301
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Tests Conducted
Verification of heatcapacity of system
Evaluation of size andshape of thermalgradient
Evaluation of change ofshape of gradient overtime
Measurement of heatloss over timePilot-Scale Thermocline System
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Thermocline Test Results
Capacity measured to be 2.44 MWh (slightly higher than 2.3 MWhdesign estimate.) Height of gradient during charging matches modeled profile. Gradient tended to become more tapered after 41 hours, but can
still yield useful energy at a reasonable temperature potential.
Heat loss was higher than modeled (likely due to heat loss throughpump penetrations at the top (which werent modeled.)
0.0001.000
2.000
3.000
4.000
5.000
6.000
280 300 320 340 360 380 400
Bed Temperature, Deg C
13:30
13:00
12:3012:00
11:30
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
280 300 320 340 360 380 400
Temperature, deg C
0
3121
11
41 hrs
Profile during discharging and stagnant with heat loss
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S O L A R T H E R M A L E N E R G Y
Sun
LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Economic Analysis for a 688 MWht
Two-tank and Thermocline SystemsComponent Two-TankMolten Salt
Thermoclinewith Quartzite
Nitrate Solar Salt, $k 11800 3800
Filler Material, $k 0 2200
Tank(s), $k 3800 2400
Salt-to-oil Heat
Exchanger, $k
5500 5500
Total, $k 21100 13900
Specific Cost, $/kWh 31 20
Thermocline Molten-Salt System is 65% the cost of aTwo-tank Molten-Salt System
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S O L A R T H E R M A L E N E R G Y
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LabSandia National Laboratories, Albuquerque, NMNational Renewable Energy Laboratory, Golden CO
Summary
A molten salt thermocline system has beendeveloped that is lower cost than a two-tankmolten salt system.
Isothermal and thermal cycling tests showedthat silica sand and quartzite rock as well as
taconite were compatible with nitrate salts.
The feasibility of a molten-salt thermoclinesystem was proven on a pilot scale 2.3 MWh
storage system.