Environmental Engineering Reference
In-Depth Information
EXAMPLE 2.4
An electrical generating plant uses steam at 700°C (973K), and on the downside the steam is
cooled by water to 300°C (573K). The maximum efficiency possible is around 0.41 or 41%.
Since efficiency is always less than 1, for a system or device to continue to operate, energy
must be obtained from outside the system. For a series of energy transformations there is a total
efficiency, which is the product (multiply) of the individual efficiencies.
EXAMPLE 2.5
Efficiency of incandescent lights in your home from a coal-fired plant:
Transformation
Efficiency, %
Mining of coal
96
Transportation of coal
97
Generation of electricity
38
Transmission of electricity
93
Incandescent bulb (electricity to light)
5
Overall efficiency (coal to light)
1.6
You can see why fluorescence lights, efficiency 15-25%, for commercial buildings and compact
fluorescence lights for your home are so important. Now light-emitting diodes (LEDs), efficiency
25-50%, are available. Countries, states, and even cities are setting regulations to phase out incan-
descent lighting. This also says that day lighting can save money, especially during the summer, as
you do not need air conditioning to reduce the heat given off by the lights.
In the physical world, subsidies or economics (dollars) do not change the final outcome, all they
do is tilt consumption or use in favor of different energy resources. For example, at some point in the
future it will take more energy to drill for oil than the amount of energy in the oil produced. At that
point, it is foolish to subsidize the drilling for oil as an energy source. It might be that the product
is so useful as a liquid fuel or as a feedstock for other products that it could be subsidized by other
energy sources. Another example is that a glass of orange juice is a net energy loser in temperate
climates. What is the energetics of producing ethanol from corn?
Prior to the oil crisis of 1973, industry and business maintained that efficiency was not cost-
effective and that the gross domestic product (GDP) was tied directly to the amount of energy
used. However, industry changed and the United States saved millions (10 9 ) of dollars since 1973
by increased efficiency in industry and higher efficiency for transportation. However, much more
conservation and efficiency has to be done in the coming decades.
Every U.S. president since 1973 has called for energy independence, primarily in reaction to the
importation of foreign oil. In 2006, President G. W. Bush's energy policy maintained that we have to
drill for more oil and gas, and as in the past, the automobile industry was fighting against increasing
fuel efficiency. The automobile industry's argument is couched in terms of economics—we cannot
compete with foreign manufacturers of small cars, consumers will not buy fuel-efficient cars (adver-
tising pushes large motors, acceleration and power, and SUVs)—and safety. In past discussions with
students, they stated that gasoline in the United States would have to be around $1/L ($4/gallon)
before they would buy a fuel-efficient vehicle. Of course, Europeans have been paying those prices
for quite a long period, and it is not surprising that with oil over $100/bbl in 2008, the sale of fuel-
efficient vehicles has increased. The safety issue means everybody should drive a truck or an M1
tank—to heck with fuel efficiency—or at least we all deserve big Cadillacs.
Another example of efficiency is cogeneration, which is today referred to as combined heat. In the
production of electricity, the low-grade (lower-temperature) energy can be used for other processes.
In most cases, 60% of the heat from electricity generation by steam (coal, oil, gas, and even nuclear)
is not used. In Europe, some electric power plants have heating districts associated with them.
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