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