Environmental Engineering Reference
In-Depth Information
TABLE 28.2
Summary of Water Consumption for Thermoelectric Power Plants with Once-Through
and Wet Recirculating Cooling
Cooling Type
Once-Through
Wet Recirculating with Cooling Tower
Type of Power
Generation
Gallons
Consumed/MWh
% Withdrawal
Consumed
Gallons
Consumed/MWh
% Withdrawal
Consumed
Fossil fuels
200-300
0.45%-1.5%
300-700
70%-100%
Natural gas
100
0.5%-1.3%
180
78%
Nuclear
400
0.67%-1.6%
400-800
31%-90%
Source:
Badr, L., G. Boardman, and J. Bigger,
Journal of Energy Engineering
, 138, 246, 2012.
water withdrawal is very low. The speciic heat capacity of air ranges from 1.006 kJ/kg-K
for dry air to approximately 1.06 kJ/kg-K for air at 100% humidity and 30°C, and is approx-
imately four times lower than the speciic heat capacity of water, a major drawback for the
system as it causes lower eficiency cooling for similar low rates of air. This lower cooling
eficiency causes decreased eficiency of net electricity production, which can be further
exacerbated because of the energy needed to power fans, the higher backpressures for dry
cooling, and the dependence on ambient conditions, which for a plant in a hot, dry loca-
tion can reduce annual electrical production by about 2% compared with a plant in a cool,
humid climate [15]. Spray cooling can be used to help alleviate the capacity issues of dry
cooling during hot periods. In the spray cooling method, a spray of water is injected to the
cooling stream where it evaporates and aids cooling. Although scaling and corrosion of the
heat exchanger tubes can be an issue with this method, careful correlation of ambient con-
ditions can ensure minimal corrosion and water use [20]. The spray water requirements
are stringent, and therefore the water must be treated to remove contaminants likely to
cause scaling or precipitation fouling. Most water treatment involves using membrane-
based ilters with nanoscale openings to remove most foulants. A more complete descrip-
tion of the membrane technologies for water treatment is available in Chapter 27.
Several approaches are being considered to reduce the water footprint for thermoelectric
power generation. Prohibiting the construction of new once-through cooling for thermo-
electric plants might seem counterintuitive since plants with advanced cooling actually
consume more water; however, because of shortage risks discussed above, this policy
could bear fruit in the long term [18] by reducing overall withdrawal. Furthermore, it has
been estimated that the United States has the potential energy capacity of 3 × 10
6
MW of
wind and solar energy, which is approximately three times the 2008 installed electricity
capacity. Greater implementation of solar photovoltaics and wind turbines are of beneit
due to their low water consumption as discussed in Section 28.7 [18]. Additionally, mini-
mization of water for thermoelectric cooling cycles is under investigation through proj-
ects between the National Energy Technology Laboratory (NETL) and Sandia National
Laboratory (SNL), including using reclaimed water to cool power plants, which is used
in 57 power plants in the United States. Currently, most of these plants utilize water from
wastewater treatment plants and 46 of them use the water for cooling towers. The volume
of reclaimed water used and the decade reclaimed water began being used by number
of facilities is summarized in Tables 28.3 and 28.4 [21], respectively. Production of water
by trapping lue gas vapor or using waste heat to desalinate water has also been sug-
gested as a means of water procurement for these plants [18]. Desalination at present is
