Cooling Options for Geothermal and Concentrating Solar Power Plants

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1 Cooling Options for Geothermal and Concentrating Solar Power Plants EPRI Workshop on Advanced Cooling Technologies July 9, 2008 Chuck Kutscher National Renewable Energy Laboratory

2 Geothermal

3 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 Net Output - kw 3,000 2,500 2,000 1,500 1,000 Unit 200 Performance Data Ambient Temperature F (@weather station)

4 Water-Saving Options Approach Pros Cons ACC + WCC in Series ACC + WCC in Parallel ACC w/ Evap Media ACC w/ Spray Nozzles - ACC can handle desuperheating load - Simple design - Improves approach to dry bulb - Can achieve good approach to wet bulb on inlet air - Simple, low cost of nozzles - Low pressure drop - Cost of dual equipment - Condensate temp. very limited - Condensate temp. limited by dry bulb - Cost of media - Pressure drop lowers flow rate and LMTD - Overspray and water waste - Cost of water treatment or mist eliminator - Nozzle maintenance - Potential damage to finned tubes Deluge of ACC - Highest enhancement - Water treatment or protective coating needed

5 Spreadsheet Model of Evaporative Enhancements to Existing Air-Cooled Plants

6 System 1 - Spray Cooling Low cost, low air pressure drop High water pressure Over-spray and carryover or cost of mist eliminator Nozzle clogging

7 System 2 - Munters Cooling High efficiency, minimum carryover High air pressure drop (reduces air flow rate and decreases LMTD) High cost

8 System 3 Hybrid Cooling Inexpensive and simple, used in poultry industry Over-spray, carryover, and nozzle cleaning

9 System 4 Deluge Cooling Excellent performance Danger of scaling and deposition without pure water

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11 Example Analysis: Net Power Produced 900,000 Total Kilowatt-hours Produced Kilowatt-hours 800, ,000 No Enhancement Spray Cooling 600,000 Munters Cooling Deluge Cooling Hybrid Cooling 500,000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

12 Example Cost Results Cents/kWh Incremental Cost of Added Electricity Discount Rate = 10%, Plant Life = 25 years $0/kgal $0.5/kgal $1/kgal System 1 - Spray Cooling System 2 - Munters Cooling System 3 - Hybrid Cooling System 4 - Deluge Cooling Note: Value of of electricity will will be be affected by by time-of-day rates and and capacity payments.

13 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

14 Coated Fin Test Results OMP-coated fin unaffected by salt spray Plain fin pitted

15 Measurements at Mammoth

16 Measurements at Mammoth Binary-Cycle Geothermal Power Plant Munters system Hybrid spray/munters system

17 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

18 Munters Performance at Mammoth 3,000 2,500 Unit 200 Performance Data Net Output - kw 2,000 1,500 1, before Munters after Munters Ambient Temperature F (@weather station)

19 Mammoth Measurement Results: 2002 Munters system modified, brine used for cooling water. Munters efficiency dropped from 79% to 66%

20 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

21 Tabbed Fin Concept Tabbed Plate Fin Tabbed Plate Fin Heat Exchanger

22 Individual Fins GEA fins w/spacers NREL tabbed circular fin fin

23 Detailed CFD Model Isometric Views: Heat Flux and Total Pressure Surface Heat Flux Total Pressure

24 CSP: The Other Solar Energy Linear Fresnel Parabolic trough Power tower Dish-Stirling

25 354 MW Luz Solar Electric Generating Systems (SEGS)

26 New 64 MW Acciona Solar Parabolic Trough Plant

27 CSP Power Plant with Thermal Storage Hot Tank HX Cold Tank

28 Study of Evaporative Pre-Cooling for Trough Plants Air-Cooled Water-Cooled Air-Cooled with Spray Enhancement

29 Electricity Produced [MWe-hr] Water Cooled Evaporatively Pre-cooled Air Cooled Month Number

30 6.0% Effect of Purchase Price of Electricity on Yearly Revenue (Water Cost = $2/kgal) Water Cooled Evaporatively Pre-cooled Percent Increase in Yearly Revenue (Compared to Air-Cooled) 4.0% 2.0% 0.0% -2.0% % Price of Electricity [$/kwh]

31 Report on Reducing CSP Water Usage Hybrid air/water cooling systems can reduce water use 80% with modest performance and cost penalties

32 1.00 Wet Fraction of wet cooling tower net plant output in. HgA 8 in. HgA 4 in. HgA 2.5 in. HgA Dry Fraction of wet cooling tower water consumption

33 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