Cooling Options for Geothermal and Concentrating Solar Power Plants EPRI Workshop on Advanced Cooling Technologies July 9, 2008 Chuck Kutscher National Renewable Energy Laboratory
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 Unit 200 Performance Data • Output of air-cooled plant 3,000 can drop > 50% in summer, 2,500 when electricity Net Output - kW 2,000 is highly valued 1,500 1,000 500 - 40 45 50 55 60 65 70 75 80 85 Ambient Temperature °F (@weather station)
Water-Saving Options Approach Pros Cons ACC + WCC in Series - ACC can handle - Cost of dual equipment desuperheating load - Condensate temp. very limited ACC + WCC in Parallel - Simple design - Condensate temp. - Improves approach to limited by dry bulb dry bulb ACC w/ Evap Media - Can achieve good - Cost of media approach to wet bulb - Pressure drop lowers on inlet air flow rate and LMTD ACC w/ Spray Nozzles - Simple, low cost of - Overspray and water nozzles waste - Low pressure drop - 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
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 900,000 800,000 Kilowatt-hours 700,000 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
Example Cost Results Incremental Cost of Added Electricity Discount Rate = 10%, Plant Life = 25 years 9.9 10.0 $0/kgal $0.5/kgal 8.7 9.0 8.5 $1/kgal 8.0 7.6 7.2 7.2 7.0 6.4 6.0 5.7 Cents/kWh 5.2 4.9 5.0 4.2 4.0 3.5 3.0 2.0 1.0 0.0 System 1 - Spray System 2 - System 3 - System 4 - Cooling Munters Cooling Hybrid Cooling Deluge Cooling Note: Value of electricity will be affected by time-of-day rates Note: Value of electricity will be affected by time-of-day rates and capacity payments. and 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 Results Coated Fin Test Results OMP-coated fin Plain fin pitted unaffected by salt spray
Measurements at Mammoth
Measurements at Mammoth Binary-Cycle Geothermal Power Plant Munters 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 3,000 2,500 Net Output - kW 2,000 after Munters 1,500 1,000 before Munters 500 - 40 45 50 55 60 65 70 75 80 85 Ambient Temperature °F (@weather station)
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 Fin Tabbed Plate Fin Heat Exchanger Tabbed Plate Fin Tabbed Plate Fin Heat Exchanger
Individual Fins NREL tabbed circular fin NREL tabbed circular fin GEA fins w/spacers GEA fins w/spacers
Detailed CFD Model Isometric Views: Heat Flux and Total Pressure Surface Heat Flux Total Pressure Surface Heat Flux Total Pressure
CSP: The Other Solar Energy Linear Fresnel Parabolic trough Power tower Dish-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 Hot Tank HX Cold Tank
Study of Evaporative Pre-Cooling for Trough Plants • Air-Cooled • Water-Cooled • Air-Cooled with Spray Enhancement
12000 10000 Electricity Produced [MWe-hr] 8000 6000 4000 Water Cooled Evaporatively Pre-cooled Air Cooled 2000 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Month Number
Effect of Purchase Price of Electricity on Yearly Revenue (Water Cost = $2/kgal) 6.0% Water Cooled Evaporatively Pre-cooled Percent Increase in Yearly Revenue 4.0% (Compared to Air-Cooled) 2.0% 0.0% 0.00 0.04 0.08 0.12 0.16 0.20 0.24 -2.0% -4.0% Price of Electricity [$/kWh]
Report on Reducing CSP Water Usage • Hybrid air/water cooling systems can reduce water use 80% with modest performance and cost penalties
1.00 Wet 0.99 2.5 in. HgA Fraction of wet cooling tower net plant output 0.98 4 in. HgA 0.97 6 in. HgA 0.96 8 in. HgA 0.95 Dry 0.94 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
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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