cooling options for geothermal and concentrating solar...
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Chuck KutscherNational Renewable Energy Laboratory
Cooling Options for Geothermal and Concentrating Solar Power Plants
EPRI Workshop on Advanced Cooling Technologies
July 9, 2008
Geothermal
Relevance• Air-cooled geothermal plants especially susceptible to
high ambient temperature
• Plant power decreases ~1% of rated power for every 1ºF rise in condenser temperature
• Output of air-cooled plant can drop > 50% in summer, when electricity is highly valued
Unit 200 Performance Data
-
500
1,000
1,500
2,000
2,500
3,000
40 45 50 55 60 65 70 75 80 85Ambient Temperature °F (@weather station)
Net
Out
put -
kW
Water-Saving OptionsApproach Pros Cons
ACC + WCC in Series - ACC can handle desuperheating load
- Cost of dual equipment- Condensate temp. very
limited ACC + WCC in Parallel - Simple design
- Improves approach to dry bulb
- Condensate temp. limited by dry bulb
ACC w/ Evap Media - Can achieve good approach to wet bulb on inlet air
- Cost of media- Pressure drop lowers
flow rate and LMTDACC w/ Spray Nozzles - Simple, low cost of
nozzles- Low pressure drop
- Overspray and water waste
- Cost of water treatment or mist eliminator
- Nozzle maintenance- Potential damage to
finned tubesDeluge of ACC - Highest enhancement - Water treatment or
protective coating needed
Spreadsheet Model of Evaporative Enhancements to Existing Air-Cooled Plants
System 1 - Spray Cooling
• Low cost, low air pressure drop• High water pressure• Over-spray and carryover or cost of mist
eliminator• Nozzle clogging
System 2 - Munters Cooling
• High efficiency, minimum carryover• High air pressure drop (reduces air flow
rate and decreases LMTD)• High cost
System 3 – Hybrid Cooling
• Inexpensive and simple, used in poultry industry
• Over-spray, carryover, and nozzle cleaning
System 4 – Deluge Cooling
• Excellent performance• Danger of scaling and deposition without
pure water
Example Analysis: Net Power Produced
Total Kilowatt-hours Produced
500,000
600,000
700,000
800,000
900,000
Kilo
wat
t-hou
rs
No EnhancementSpray CoolingMunters CoolingDeluge CoolingHybrid Cooling
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Example Cost Results
5.2
6.4
7.67.2
8.5
9.9
5.7
7.2
8.7
3.54.2
4.9
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0C
ents
/kW
h
System 1 - SprayCooling
System 2 -Munters Cooling
System 3 -Hybrid Cooling
System 4 -Deluge Cooling
Incremental Cost of Added ElectricityDiscount Rate = 10%, Plant Life = 25 years
$0/kgal$0.5/kgal$1/kgal
Note: Value of electricity will be affected by time-of-day ratesand capacity payments.
Note: Value of electricity will be affected by time-of-day ratesand capacity payments.
Geothermal Analysis Conclusions
• Deluge most attractive if scaling/corrosion issues can be addressed
• Systems 1 to 3 obtain ~40 kWh/kgal of water; deluge can produce an average of ~60 kWh/kgal
• Results very sensitive to water costs, electric rate structure, installation costs
Coated Fin Test ResultsCoated Fin Test Results
OMP-coated fin unaffected by salt spray
Plain fin pitted
Measurements at Mammoth
Measurements at Mammoth Binary-Cycle Geothermal Power
PlantMunters system
Hybrid spray/Munters system
Mammoth Measurement Results: 2001
• Field instrumentation: Type T thermocouples, optical dew point (chilled mirror) hygrometer, handheld anemometer
• Munters had 79% saturation efficiency; hybrid was 50%
• Flow rate with Munters dropped 22-28%
• Munters increased net power 62% (800 kW to 1,300 kW) at 78ºF ambient
Munters Performance at Mammoth
Unit 200 Performance Data
-
500
1,000
1,500
2,000
2,500
3,000
40 45 50 55 60 65 70 75 80 85Ambient Temperature °F (@weather station)
Net
Out
put -
kW
after Munters
before Munters
Mammoth Measurement Results: 2002
• Munters system modified, brine used for cooling water. Munters efficiency dropped from 79% to 66%
Geothermal Conclusions• All operators of air-cooled plants interested in evaporative
enhancement
• Costs at existing plants are site-specific and negotiable; $0.50 to $2.00 per thousand gallons
• Reclaimed water becoming more widely available
• Two-Phase Engineering showed successful use of nozzles with brine
• Can reduce average cost of electricity by about 0.3¢/kWh, depending on cost of water
• Capacity payments can be as high as 30 ¢/kWh and lower average cost of electricity by 2–3 ¢/kWh
Tabbed Fin Concept
Tabbed Plate FinTabbed Plate Fin Tabbed Plate Fin Heat ExchangerTabbed Plate Fin Heat Exchanger
Individual Fins
GEA fins w/spacersGEA fins w/spacers NREL tabbed circular finNREL tabbed circular fin
Detailed CFD Model Isometric Views:
Heat Flux and Total Pressure
Surface Heat FluxSurface Heat Flux Total PressureTotal Pressure
CSP: The Other Solar Energy
Parabolic trough
Linear Fresnel
Power towerDish-Stirling
354 MW Luz Solar Electric Generating Systems (SEGS)1984 - 1991
New 64 MW Acciona Solar Parabolic Trough Plant
CSP Power Plant with Thermal Storage
HX
HotTank
ColdTank
Study of Evaporative Pre-Cooling for Trough Plants
• Air-Cooled• Water-Cooled• Air-Cooled with Spray Enhancement
0
2000
4000
6000
8000
10000
12000
0 1 2 3 4 5 6 7 8 9 10 11 12 13Month Number
Ele
ctric
ity P
rodu
ced
[MW
e-hr
]
Water CooledEvaporatively Pre-cooledAir Cooled
Effect of Purchase Price of Electricity on Yearly Revenue(Water Cost = $2/kgal)
-4.0%
-2.0%
0.0%
2.0%
4.0%
6.0%
0.00 0.04 0.08 0.12 0.16 0.20 0.24
Price of Electricity [$/kWh]
Perc
ent I
ncre
ase
in Y
early
Rev
enue
(C
ompa
red
to A
ir-C
oole
d)
Water CooledEvaporatively Pre-cooled
Report on Reducing CSP Water Usage
• Hybrid air/water cooling systems can reduce water use 80% with modest performance and cost penalties
0.94
0.95
0.96
0.97
0.98
0.99
1.00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fraction of wet cooling tower water consumption
Frac
tion
of w
et c
oolin
g to
wer
net
pla
nt o
utpu
t
Dry
8 in. HgA
6 in. HgA
4 in. HgA
2.5 in. HgA
Wet
CSP Cooling Conclusions
• Water cooling most economic • Water-cooled trough plant uses about 800
gal/MWh of which 20 is for mirror washing; power towers use less, linear Fresnel uses more; dish/engine air-cooled
• Air cooling eliminates 90% of water use but increases LEC by 2 to 10%
• Hybrid (parallel air/water) reduces cost penalty while still saving about 80% of the water
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