WSP11937

• File Number: 8040.200
• Description: Section D General Studies-Energy
• Date: 1/1/1974
• Author: Davis-Wood-USGS
• Title: Energy-Oil Shale-Geological Survey Circular 703-Water Demands for Expanding Energy Development
• Water Supply Pro - Doc Type: Publication

is used where the plant is near an abundant source of
water, such as the sea, <11arge.lake, or a large river. As
the name suggests, water from an infinite (for practical
purposes) suurce is circulated through the condenser and
carries the waste heat away to a point of discharge else-
where on the water body. The heat is dissipated through
increased evaporation from the slightly warmer water
body and by conduction to the atmosphere_
Where no large water body is available, a natural or
artificial pond may be used for storage and as a heat
sink. In this mode, heat is dissipated mainly through
surface evaporation from the warmed pond. Where the
cooling capacity of the pond is inadequate, sprayers may
be used to increase evaporation. Sprayers may also be
used together with canals in once-through systems to
,educe the impact of heated discharge on fish and other
aquatic biota.
Where water is in short supply or discharge of heated
water is unacceptable, and ponds are not practicable,
cooling towers generally are employed. In wet cooling
towers some of the warm water evaporates through con-
tact with an air draft, either naturally induced or driven
by fans, thus cooling the remaining water. Dry cooling
towers dissipate heat directly to an air draft in a fashion
similar to an automobile radiator. Although dry cooling
towers are effective in reducing water consumption,
their capital cost greatly exceeds that of wet cooling
processes, and their use results in a loss of thermal effi-
ciency as well. They find their greatest use in cold cli-
mates and to date have seen little use in the United
States in steam-electric power generation.
Various combinations of these cooling techniques are
applied to achieve maximum economy in combination
with acceptable environmental effects. The cooling sys-
tem is quite independent of the type of fuel; rather, it
depends mainly on local factors such as availability of
water, terrain features, and potential environmcntal
impacts.
The cooling demand, regardless of how the waste heat
is dissipated, is governed by the thermal efficiency of the
plant, which is expressed as electrical output as a per-
centage of energy input. Maximum thermal eHiciency is
achieved by use of very high steam temperatures and
inlet pressures. In the newer modern fossil-fueled plants,
for example, thermal efficiency of 40 percent is achieved
with inlet temperatures as lugh as I,OOO'F (538'C) and
pressures of 3,500 psi (246 kg per cm').
The evaporative demand of a fossil-fueled steam-
electric generator may be expressed as (Cootner and
Liif, 1965, p. 74):
Gallons evapurated/kwhr=0.39 (aH - 1),
where H is overall thermal efficiency, and a is boiler-
furnace efficiency (usL"dly about 0.9). The boiler.

furnace efficiency. normally about 90 percent, repre-
sents the fuel energy that is not lost in Oue gases. The
heat energy lost in flue gases is about 10 percent. An
additional 5 percent of the input is dissipated to the
atmosphere through in-plant losses and uses. Thus about
85 percent of the input energy is used in driving the
turbines or is disposed of as thermal waste in the form of
warmed water.
Present nuclear plants are less efficient than fossil-
fueled plants because of safety restrictions on maximum
steam temperatures, and nuclear plants dissipate waste
heat almost entirely to cooling water because nu flue
gases are emitted. A rypical nuclear plant of 31.percent
thernlal efficiency releases about 50 percent more heat
to cooling water than a fossil-fueled plant of comparable
power output.
With respect to consumption of water, geothermal
plants are the least efficient form of steam-electric gener-
ation. Because of inherent low temperature and pressure
of natural steam used, the geothermal plants at the Gey-
sers Field. Calif., for example, have an overall thermal
efficiency of only about 14 percent, the remaining
energy being dissipated by evaporative cooling with com-
parably greater water consumption. The source of
cooling water is the condensed geothermal '!>team, about
80 percent being consumed in the cooling process. The
remaining 20 percent, which is of poor quality, is
injected into the producing formation (Finney, 1972).
The rapid growth of electric consumption in the
United States in recent decades is reflected in increased
water withdrawals for thermal-electric power. Surveys of
water use compUed at 5-year intervals by the U.S. Geo-
logical Survey (fig. 2) show that by 1965 withdrawals by
thermal.electric plants exceeded irrigation withdrawals,
to become the leading class of withdrawal use in the
Nation. This reOects not only the rapid growth of elec-
tric demand but also the fact that most plants employed
once-through systems for condenser cooling. Concern
over thermal pollution of water bodies. however, has
resulted in a trend to greater use of closed evaporative
systems employing cooling lowers, ponds, or sprayers.
Thus the rapid growth of thermal-electric withdrawal
should level off considerably in coming decades. TillS
effect, coupled with the influence of improvements in
thermal efficiency, can be observed in figure 3. Nonethe-
less, consumptive use by thermal-electric generation will
continue to grow and will increase relative to withdraw-
als. This seeming paradox is due to the fact that closed
cooling systems have a greater evaporation loss relative
to once-through systcms as well as additional water con-
sumption not applicable to once-through systems. The
principal water economies of once-through systems are
attributable to a greater proportion of conductive heat

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