POWER September 2012 - 54

WATER MANAGEMENT
Water Conservation Options for
Power Generation Facilities
The electric power industry is a large water user and is dependent upon reliable
water supplies. Adopting new water-conserving technologies for
power production can help alleviate the impact of future water shortages.
Several water use reduction technologies are available, each with different
benefits and costs.
By Kent D. Zammit, Electric Power Research Institute
B
y far the largest use of water in power
generation is for condenser cooling.
Thermal power plants require a large
amount of cooling water to condense the
steam turbine exhaust steam. The lower the
condensing temperature, the lower the backpressure
on the steam turbine, which increases
plant thermal efficiency. The most effective
method of rejecting this heat is through
the use of cooling water. Traditionally, power
plants have used three methods for condenser
cooling: once-through, evaporative, and dry
cooling. Each has unique advantages and disadvantages.
With
once-through cooling, water is withdrawn
(typically from a lake, river, or ocean),
pumped through a condenser, and returned to
the source at the same rate but at an increased
temperature (Figure 1). Once-through cooling
provides the best power plant efficiency
of all the alternatives (such as cooling towers)
because the source water tends to be the
lowest temperature heat sink available for
most of the year.
Evaporative cooling is often used when larger
water volumes are unavailable, as in the arid,
western U.S. The recirculating water is cooled
either in a cooling tower or cooling pond and
then recycled back to the condenser (Figure
2). Evaporative cooling typically provides
the second-best plant efficiency, because the
condensing temperature approaches the ambient
wet-bulb temperature. Plants with cooling
towers typically withdraw less than 5% of the
cooling water of a comparable plant with oncethrough
cooling, but the actual water consumption
(evaporative water loss) is higher.
Figure 3 shows a schematic of water use
in a typical 500-MW thermal plant with a
wet cooling tower. The cooling tower in this
example requires 6,253 gpm of freshwater
when running at full load. Makeup is required
to replace the water lost to evaporation, drift,
and cooling tower blowdown.
With dry cooling, heat is rejected from the
condenser to the atmosphere by passing air
54
1. Once-through cooling. Steam exhausted
from the steam turbine typically enters
the condenser from above and is condensed
by the relatively cold cooling water flowing
through the condenser tubes. Source: EPRI
Exhaust steam from turbine
Condenser
2. Wet cooling tower and recirculating
cooling. A cooling tower cools
the warm water from the condenser through
evaporation, rather than heating large volumes
of ocean or river water for once-through
cooling. The disadvantage is that freshwater
makeup is required to replace evaporation,
drift, and blowdown from the cooling tower.
Source: EPRI
Hot water from
condenser
Hot water
returned to source
Cold water from source
Condensed steam to boiler
Cold water to
condenser
Ambient
air
Makeup
Blowdown
3. Balanced water use. This schematic shows the water makeup requirement for a typical
500-MW thermal plant using evaporative cooling towers. Source: EPRI
Steam
1000F
Condense
vapor
Fan
Drift
eliminator
Use less water
for cooling
TWB
Air
=75F
15-30 Macfm
Makeup water
6,253 gpm
Treat blowdown
water for reuse
Blowdown water
1,250 gpm
Cold water
82F
Air
Warm water
102F
250,000 gpm
Boiler ash
Condenser
Expand water
resource
www.powermag.com
Water 109F
(2.5 " HgA)
Improve cycle
efficiency
Evaporation
5,000 gpm
and drift loss
2.5 gpm
Cooling
tower
500 MW
generator
Steam
2,500,00 lb/hr
Turbine
Utilize waste heat
(60% wasted)
Boiler
Flue gas
340F
4,060,000 lb/hr
POWER | September 2012
http://www.powermag.com
POWER September 2012

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