rankine combined cycle power plants
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National Renewable Energy LaboratoryUSA Trough Initiative
Thermal Storage for Rankine Cycle
and Combined Cycle Power Plants
Bruce KellyNexant Inc., A Bechtel Technology & Consulting Company
San Francisco, California
Ulf Herrmann
FLABEG Solar International GmbH
Kln, Germany
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Thermal Storage for
Parabolic Trough Power Plants
Study objectives:
Investigate thermal storage options
Identify, evaluate, and recommend concepts forRankine cycle and integrated solar combinedcycle plants
Develop a conceptual design for an integratedplant with solar fractions up to 20 percent
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Recommendations of
Thermal Storage EvaluationNear term
Hot and cold tank nitrate salt
Mid term
- Nitrate salt thermocline storage with taconite or
limestone fill material- Concrete
Long term
Cascaded phase change material(R&D required, not investigated here)
Cost goal of $25/kWht
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Preferred Near Term Approach
Hot and cold nitrate salt tanks, charged by an oil-to-salt heat exchanger
Tertiary nitrate salts offer favorable combinationof low cost, low vapor pressure, high density,
reasonable specific heat, reasonable meltingtemperature, and low chemical reactivity
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Salt Steam Generator
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Oil Steam Generator
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Equipment Arrangement
With salt steam generator, charge/discharge
cycle imposes two heat exchanger approachtemperatures; with oil steam generator, three
Salt steam generator requires smaller heatexchangers; however, auxiliary energy demandfor nitrate salt pumps is higher because allthermal energy must pass through oil-to-saltheat exchanger
Project economics favor oil steam generator
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Oil-to-Salt Heat Exchanger Design
Tube rupture will expose organic heat transfer
fluid to nitrate salt oxidant. Fluid will vaporize,and vapor will accumulate in thermal storage tankullage volume
Nitrate salt and gasoline reaction tests(at 1,100 F) by Sandia showed hydrocarbon
vaporization and combustion in air, but nooxidation reactions with nitrate salt
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Oil-to-Salt Heat Exchanger Design
Likelihood of combustion or explosion is small:
Therminol VP-1 is at least 350 F below auto-
ignition temperature, and Dow engineers do notexpect nitrate salt decomposition/oxidation
reaction
Nitrate salts are stable in presence of nitrogen;
permanent nitrogen ullage gas could be used in
storage tanks
Standard TEMA heat exchanger
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Thermal Storage for Rankine Plants
Two tank nitrate salt storage systems are viable forRankine cycle plants
Final feedwater temperature of 455 F, and main andreheat steam temperatures of 700 F, limits range of salttemperatures to about 130 F; however, nitrate salt and
the oil-to-salt heat exchangers are inexpensive, and thetechnical risk should be low
With a 3 hour system, the capacity factor increases from
25 to 31 percent, and the turbine operates for more hoursat full load
Unit storage costs of $27 to $32/kWht
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Summer Output of a
50 MWe Rankine Cycle Plant
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12 14 16 18 20 22 24
Hour
0
100200
300
400
500
600
700
800900
without Storage, Solar Field size 340000m
with Storage (450 MWh thermal), Solar Field size: 410000 m
Direct Normal Insolation
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Thermal Storage for Rankine Plants
System economics may be improved by:
Allowing the main and reheat steam temperaturesto decay to 690 F during storage discharge
Using a thermocline storage system, whichsubstitutes low cost limestone for nitrate salt
Using concrete as the storage medium. Investmentcosts under $25/kWht are likely; however, long-
term stability of concrete still has to be proven
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Integrated Plant with Thermal Storage
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Thermal Storage for Integrated Plants
Integrated plants with annual solar contributions
of 8 to 10 percent are (thermodynamically)potential candidates for thermal storage
Main steam saturation temperatures are higher thanSEGS plants; however, final feedwater temperatures arecomparable, and superheated steam is not required
Unit storage costs are $24 to $30/kWht
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Annual Performance of
Integrated Plant with Storage
0
200
400
600
800
1,000
1,200
1,400
CC ISCCS: 0 h 1.5 h 3 h 8 h 12 h 16 h
Thermal Storage Size [h]
0.0%
2.0%
4.0%
6.0%
8.0%
10.0%
12.0%
14.0%
16.0%
18.0%
20.0%
Solar Thermal Energy
Steam Turbine Gross Electricity
Gas Turbine Gross ElectricitySolar Share
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Thermal Storage for Rankine Cycle Plants
Hot and cold tank nitrate salt concept is viablefor Rankine cycle plants
With unit storage costs of $25 to $32/kWht,adding thermal storage increases levelizedenergy cost slightly
Technical risks are moderate, and thermalstorage should be useful for sites which assign avalue for energy dispatch
Conclusions
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Thermal Storage for Integrated Plants
With a 16 hour storage system, annual solarcontributions up to 17 percent are feasible; netconversion efficiencies are in the range of 29 to
32 percent
With storage unit costs about $25/kWht,levelized energy costs are not affected markedly
Conclusions
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