Environmental Engineering Reference
In-Depth Information
consumption. To provide context to water use, at 3% consumption, >3 billion gallons per
day were consumed. The above discussion clearly illustrates that water is an essential
source for energy use and generation. In the next several sections, a discussion for water
use for different energy areas is presented.
28.2 Water Use in Power Generation
In a 2009 report by the US Government Accountability Ofice (USGAO) [13], it was shown
that as of 2007, nearly 75% of electric power generation in the United States was through
thermal (e.g., coal- or gas-ired power plants, nuclear, and solar-thermal, among others)
methods. Water withdrawal and consumption are largely related to cooling water to oper-
ate the power plants. Speciically, as all thermal methods inherently rely on some form of
a thermodynamic cycle, the heat generated needs to be dissipated at as low a temperature
as possible to obtain maximum work achievable,
W
, which based on a simple thermody-
namic analysis can be expressed as
T
T
C
W
≤−
1
Q
(28.1)
H
where
T
C
is the temperature of the heat sink,
T
H
is the temperature of the hot reservoir,
and
Q
is the heat generated. The use of water for cooling requirements is possible owing
to the high speciic heat of water, which at room temperature is 4.181 kJ/kg-K. A glance
at thermodynamic tables makes this clear: the heat of vaporization of water is 2 J/mm
3
or 2 GJ/ m
3
; therefore, for a 1 GW power plant, 0.5 m
3
(500 liters) of water will need to be
evaporated per second. It is generally known that depending on speciic operating and
source water conditions, including those of the surrounding environment, approximately
1%-3% of water in the cooling towers is lost to evaporation as a function of circulation rate.
Of this evaporated water, 5% capture and reuse could potentially lead to savings in excess
of 200 million gallons per day (MGD) across the United States.
It is commonly assumed that use of renewable sources of energy will mitigate many
crises, including impending water shortages. However, the need for water withdrawal
and consumption for energy use is highly dependent on the method to produce energy.
For example, renewable sources such as hydroelectricity use 1.55-6.05 m
3
of water for
every gigajoule of energy produced [14]. Furthermore, methods such as solar-thermal
(distinct from solar-based photovoltaic approaches, see Section 28.7) are comparable to
other thermal methods in water consumption. In the sections to follow, several energy
generation and use methods are discussed with a brief review of water consumption for
each method.
Water is used in thermoelectric power plants for a variety of activities, depending on
the type of plant, including lue gas desulfurization, boiler feedwater makeup, gasiica-
tion process makeup water, processing and washing of cooling systems, irrigation for wet
recirculating pond systems, scrubber dilution water, and ash control, yet cooling remains
the dominant use of water in thermoelectric plants [15]. Currently, there are four types of
cooling schemes in these plants: once-through, wet recirculating, dry, and hybrid cooling
as shown in Figure 28.7 [16]. Once-through cooling is used in many older power plants and
