POWER September 2012 - 82

PLANT DESIGN
Water and Power: Will Your Next
Power Plant Make Both?
In much of the developing world, two essentials are often in short supply: potable
water and reliable electricity. Some countries have invested heavily
in desalination and combined cycle technologies to simultaneously solve
both problems.
By David G. Daniels, M&M Engineering Associates Inc.
I
n the U.S., people have come to assume
that a cheap and abundant supply of
clean, potable water will always be available
at the turn of a valve. However, water
supplies in some regions have developed
serious problems, lately caused by drought.
Meanwhile, coastal regions, such as Florida,
are stressing underground aquifers to meet
water demand while risking contamination
from saltwater intrusion. Perhaps it is time
to consider a new and virtually limitless
source of freshwater-the ocean.
Evaluate the Options
Two primary technologies are available to
desalinate seawater. Most desalination plants
(86%) in the U.S. are based on membrane
technology, called seawater reverse osmosis
(SWRO). The second technology approach
is thermal distillation, which produces more
than 40% of the world's desalinated water.
SWRO certainly has some capital and deployment
time advantages over thermal distillation.
SWRO also seems to better fit the
compartmentalized nature of U.S. business
and government, where, generally speaking,
municipalities retain responsibility for providing
potable water and investor-owned utilities
provide electrical power. Some SWRO
projects have been colocated at existing
power plants to take advantage of the warmer
seawater available from the plant's cooling
water discharge; the warmer ocean water increases
the SWRO process efficiency.
Development of desalination in many parts
of the developing world has taken a different
route. Usually, the markets for electrical
power and freshwater supplies are developed
simultaneously, often by the country's sovereign
government or by a group of large
international businesses based on long-term
power and water supply agreements made
with the host country. Thermal distillation is
normally the technology selected for these
very large projects.
There are synergies that can be exploited
when thermal distillation and power generation
are developed in concert. Thermal distil82
lation
of seawater requires a large supply of
cheap energy-and generally large amounts
of steam. However, the newest technologies
only require very low-pressure, low-grade
steam as an energy source. On the other
hand, power plants are often challenged
to develop the means of disposing of large
amounts of low-temperature waste heat produced
in the condenser, usually with oncethrough
cooling, cooling towers, or massive
air-cooled condensers.
In the U.S., once-through cooling has
been the historical cooling method of choice,
but pending Clean Water Act Section 316(b)
changes (final release is expected in 2013)
will eliminate this option for plants constructed
in the future.
Option 1: Reverse Osmosis
The typical SWRO process consists of
pretreatment, reverse osmosis (RO), and
post-treatment steps. Pretreatment includes
1. Tampa Bay Water produces 25 million gallons per day of drinking water
from seawater. The desalination plant is located next to Tampa Electric's Big Bend
Power Station and " catches " the plant's warm cooling water discharge for desalination. Starting
from the upper left, screens filter out shells, wood, and other debris greater than ¼ inch from
ocean cooling water for landfill disposal. The cooling water next is given time for the heavier
solids to settle and be removed. The sand filters remove smaller solids from the water, diatomaceous
earth filters remove microscopic materials, and cartridge filters protect the reverse
osmosis membranes. Next, ocean water, under high pressure, is pumped through reverse osmosis
membranes to remove the salt. The concentrated salt water (only 1% to 1.5% more salty)
is mixed with ocean water and returned to the discharge canal that leads to the bay. The clean
water undergoes several additional treatment steps before arriving at the Regional Blending
Station 14 miles away, where the desalinated water is blended with treated surface water and
then delivered through the municipal water system to customers. Source: Tampa Bay Water
Diatomaceous earth filters
Intake canal
Initial
Tampa
Bay
Discharge canal
Power plant
chemical
treatment
44 mgd
Recycled water
(water removed
from solids is
recycled)
Residual
treatment
(RO) 1st pass
Pump
Cartridge filters
Concentrate return
Concentrated
salt water
Energy recovery
turbine
www.powermag.com
POWER | September 2012
(RO) 2nd pass
Pump
Posttreatment
Holding
tank
5
mg
Pump
Solids processing
Regional blending
and high service
pumping
Traveling
screens
settlement Sand filters
Particle
Meter
19 million gallons per day
25 mgd of clean water

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POWER September 2012

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