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Energy Solutions October 23, 2009
Rankine (steam) Cycle Cooling
Options
Babul Patel
Nexant, Inc.San Francisco, CA
1
Rankine Cycle Cooling Considerations
n Steam turbine output and Rankine cycle efficiency is dependent on turbine exhaust conditions (Temperature & Pressure)
n Exhaust conditions are established by – Water temperature in once through cooling system– Wet bulb temperature for the wet cooling tower– Ambient dry bulb temperature for dry cooling system
n For concentrated solar thermal plant (CSP) exhaust cooling options are wet tower, dry cooling (air cooled condenser) or wet-dry hybrid system
2
Turbine Exhaust Cooling Options
Wet Cooling Tower
3
Turbine Exhaust Cooling Options
Air Cooled Condenser
4
Turbine Exhaust Cooling Options
Hybrid Cooling System
5
CSP Plant Cooling Options
n For CSP plant water is a scare resource
n For most CSP plant, once through cooling is not an option
n CSP plant output is dependent on ambient conditions - maximum output during high ambient temperatures
6
CSP vs. Fossil Plant Cooling
n Major difference between CSP and fossil fueled power plants
– Fossil plant cooling options can be evaluated using annual mean dry bulb and wet bulb temperatures
– Water use and plant output determined with average conditions are reasonable for economical analysis
– For CSP plant it is necessary to evaluate hourly performance and determine plant output and water use for wet tower or loss of capacity for dry cooling
7
CSP vs. Combined Cycle Cooling
n For a 100 MW combined cycle plant with 60% capacity factor– Plant output at 32C and higher temperature is 17% of
annual GWh output– Difference between dry cooling and wet cooling is
<0.5% of the annual GWh output
n For a 100 MW CSP plant– Plant output at 32C and higher temperature is >34%
of annual GWh output– Difference between dry cooling and wet cooling can
be ~6% of the annual GWh output
8
Design Basis –CSP Plant
n 100 MWe Nominal Solar Trough Plant n Location Barstow/Daggett, CAn Solar Field Output Excelenergy/SAMn Rankine Cycle Analysis GateCycle 6.0n Turbine Exhaust Pressure
Design 85 mbar (2.5 in HgA) Maximum 271 mbar (8 in HgA)
n Design Turbine Gross Efficiency 37.7%
9
Dry, Wet and Hybrid Cooling Design Basis
Design Parameter Units Value
Wet Cooling Tower
Ambient Dry Bulb Temperature °C (°F) 40 (104)
Wet Bulb Temperature °C (°F) 20 (68)
Dry Cooling
Ambient Dry Bulb Temperature °C (°F) 20 (68)
Hybrid Cooling
Ambient Dry Bulb Temperature °C (°F) 40 (104)
Wet Bulb Temperature °C (°F) 20 (68)
Condenser Pressure @ Design Conditions
mbar (in HgA)
85 (2.5)
10
160 MW Cooling Tower Duty5 C Approach Temperature
20 C Inlet Wet Bulb Temperature0.98 Exit Relative Humidity
0.028 Evaporation Loss Fraction8 Number of Fans (Bays)
508 kW Total Fan Power4 Cycles of Concentration
Design Parameters for Wet Cooling Tower
Condenser Pressure 85 mbar @ 40 C Dry Bulb, 20 C Wet Bulb Temperature
11
Design Parameters for Air Cooled Condenser
20 C inlet air temperature39 C outlet air temperature
43 C condensing steam temperature165 MW condenser duty
10 C log mean temperature difference (LMTD)162.5 kJ/h-m2 C overall heat transfer coefficient, U
366,127 m2 heat transfer area
Condenser Pressure 85 mbar @ 20 C Ambient Temperature
12
Dry Bulb Temperature Distribution* – Barstow, CA
0
50
100
150
200
250
300
350
400
450
500
27 32 37 42 47 52 57 62 67 72 77 82 87 92 97 102 107 112
Dry Bulb Temperature, oF
Ann
ual P
erio
d, h
ours
.
* Expected distribution when solar plant will be on line (3421 hrs on line)
13
Relative Humidity vs. Dry Bulb Temperature*–Barstow, CA
y = 0.00010001x2 - 0.02455648x + 1.61571832
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
30 40 50 60 70 80 90 100 110 120
Dry Bulb Temperature, oF
Rel
ativ
e H
umid
ity
.
* Expected distribution when solar plant will be on line (3421 hrs on line)
14
Turbine Output – Wet Cooling Tower Case
0
20
40
60
80
100
120
30 40 50 60 70 80 90 100 110 120
Ambient temperature, F
Net
pla
nt o
utpu
t, M
We
15
Turbine Output – Air Cooled Condenser
0
20
40
60
80
100
120
30 40 50 60 70 80 90 100 110 120
Ambient temperature, F
Net
pla
nt o
utpu
t, M
We
16
Wet vs. Dry Cooling – Lost MW
0
5
10
15
20
25
30
35
40
45
30 40 50 60 70 80 90 100 110 120
Ambient temperature, F
Wet
tow
er -
Dry
tow
er, M
We
17
Water Use kg/MWh vs. Ambient Temperature
y = -0.0353858638x2 + 20.3388269465x + 333.8363287753
1,000
1,500
2,000
2,500
3,000
3,500
30 40 50 60 70 80 90 100 110 120
Ambient temperature, F
Tow
er m
akeu
p, k
g/M
Whe
18
Plant Output – Wet to Dry
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 ca
se p
lant
out
put
Dry 271 mbar
250 mbar
203 mbar
135 mbar
85 mbar
Wet 85 mbar
Design Ambient Temperature 40CWet Bulb Temperature 20C
19
Annual GWh vs. Annual Water Consumption
260
270
280
290
0 200 400 600 800 1000
Makeup water consumption, MTx1000/year
Net
pla
nt o
utpu
t, G
Whe
Dry 271 mbar
250 mbar
203 mbar
135 mbar 85 mbar
Wet 85 mbar
20
Conclusions
n Designing cooling system for CSP plant require detailed technical and economical analysis
n Water resources are scares at most solar site
n Dry cooling has high capital cost, high power consumption, and significant loss of capacity
21
Conclusions (contd.)
n Wet cooling has lower capital cost, no loss of output, but significant loss of water and waste disposal problems
n Hybrid cooling can save significant water use, maintain high capacity, but with much higher capital costs and higher maintenance costs
