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National Renewable Energy Laboratory
USA Trough Initiative
Thermal Storage for Rankine Cycle
and Combined Cycle Power Plants
Bruce Kelly
Nexant Inc., A Bechtel Technology & Consulting Company
San Francisco, California
Ulf Herrmann
FLABEG Solar International GmbH
Köln, Germany
Thermal Storage for
Parabolic Trough Power Plants
Study objectives:
• Investigate thermal storage options
• Identify, evaluate, and recommend concepts for
Rankine cycle and integrated solar combined
cycle plants
• Develop a conceptual design for an integrated
plant with solar fractions up to 20 percent
Recommendations of
Thermal Storage Evaluation
Near 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
Preferred Near Term Approach
• Hot and cold nitrate salt tanks, charged by an oil-
to-salt heat exchanger
• Tertiary nitrate salts offer favorable combination
of low cost, low vapor pressure, high density,
reasonable specific heat, reasonable melting
temperature, and low chemical reactivity
Salt Steam Generator
Oil Steam Generator
Equipment Arrangement
• With salt steam generator, charge/discharge
cycle imposes two heat exchanger approach
temperatures; with oil steam generator, three
• Salt steam generator requires smaller heat
exchangers; however, auxiliary energy demand
for nitrate salt pumps is higher because all
thermal energy must pass through oil-to-salt
heat exchanger
• Project economics favor oil steam generator
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 tank
ullage volume
• Nitrate salt and gasoline reaction tests
(at 1,100 ºF) by Sandia showed hydrocarbon
vaporization and combustion in air, but no
oxidation reactions with nitrate salt
Oil-to-Salt Heat Exchanger Design
• Likelihood of combustion or explosion is small:
Therminol VP-1 is at least 350 ºF below auto-
ignition temperature, and Dow engineers do not
expect 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
Thermal Storage for Rankine Plants
• Two tank nitrate salt storage systems are viable for
Rankine cycle plants
• Final feedwater temperature of 455 ºF, and main and
reheat steam temperatures of 700 ºF, limits range of salt
temperatures to about 130 ºF; however, nitrate salt and
the oil-to-salt heat exchangers are inexpensive, and the
technical risk should be low
• With a 3 hour system, the capacity factor increases from
25 to 31 percent, and the turbine operates for more hours
at full load
• Unit storage costs of $27 to $32/kWht
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
100
200
300
400
500
600
700
800
900
without Storage, Solar Field size 340000m²
with Storage (450 MWh thermal), Solar Field size: 410000 m²
Direct Normal Insolation
Thermal Storage for Rankine Plants
• System economics may be improved by:
– Allowing the main and reheat steam temperatures
to decay to 690 ºF during storage discharge
– Using a thermocline storage system, which
substitutes low cost limestone for nitrate salt
– Using concrete as the storage medium. Investment
costs under $25/kWht are likely; however, long-
term stability of concrete still has to be proven
Integrated Plant with Thermal Storage
Thermal Storage for Integrated Plants
• Highest solar conversion efficiencies occur with
feedwater temperatures as high as possible
• Integrated plants with annual solar contributions
of 2 to 4 percent are poor candidates for thermal
storage
– Final feedwater temperatures are as high as 620 ºF
– Small difference between hot and cold salt tank
temperatures leads to unit storage costs of $35 to
$40/kWht
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 than
SEGS plants; however, final feedwater temperatures are
comparable, and superheated steam is not required
– Unit storage costs are $24 to $30/kWht
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 Electricity
Solar Share
Thermal Storage for Rankine Cycle Plants
• Hot and cold tank nitrate salt concept is viable
for Rankine cycle plants
• With unit storage costs of $25 to $32/kWht,
adding thermal storage increases levelized
energy cost slightly
• Technical risks are moderate, and thermal
storage should be useful for sites which assign a
value for energy dispatch
Conclusions
Thermal Storage for Integrated Plants
• With a 16 hour storage system, annual solar
contributions up to 17 percent are feasible; net
conversion 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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