parabolic trough solar plants

8/2/2019 Parabolic Trough Solar Plants

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.

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