Moran M.J., Shapiro H.N. Fundamentals of Engineering Thermodynamics
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(a) For each storage method, identify its principal sources of
exergy destruction and develop an exergetic efficiency for it.
Use principles of this chapter with assistance from the
technical literature as required.
(b) From these storage methods, identify a subset of them
well suited for storage duty associated with a 300-MW wind
farm. On the basis of cost and other pertinent factors for
such duty, place the subset in rank order.
7.8D The objective of this project is to design a low-cost,
electric-powered, portable or wearable consumer product
that meets a need you have identified. In performing every
function, the electricity required must come fully from
human motion. No electricity from batteries and/or wall
sockets is allowed. Additionally, the product must not be
intrusive or interfere with the normal activities of the user,
alter his/her gait or range of motion, lead to possible physical
disability, or induce accidents leading to injury. The product
cannot resemble any existing product unless it has a valuable
new feature, significantly reduces cost, or provides some
other meaningful advantage. The final report will include
schematics, circuit diagrams, a parts list, and a suggested
retail cost based on comprehensive costing.
7.9D In the 1840s, British engineers developed atmospheric
railways that featured a large-diameter tube located between
the tracks and stretching the entire length of the railroad.
Pistons attached by struts to the rail cars moved inside the
tube. As shown in Fig. P7.9D, piston motion was achieved by
maintaining a vacuum ahead of the piston while the
Atmosphere acting Vacuum
Piston
Fig. P7.9D
Design & Open-Ended Problems: Exploring Engineering Practice 423
atmosphere was allowed to act behind it. Although several
such railways came into use, limitations of the technology
then available eventually ended this mode of transportation.
Investigate the feasibility of combining the atmospheric
railway concept with today’s technology to develop rail
service for commuting within urban areas. Write a report,
including at least three references.
7.10D Pinch analysis (or pinch technology) is a popular
methodology for optimizing the design of heat exchanger
networks in complex thermal systems. Pinch analysis uses a
primarily graphical approach to implement second-law
reasoning. Write a paper, including at least three references,
in which the role of pinch analysis within thermoeconomics
is discussed.
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424
ENGINEERING CONTEXT
In the twenty-first century we will be challenged to responsibly provide
for our growing power needs. The scope of the challenge and how we will address it are discussed in the
introduction to power generation beginning on p. 426. You are encouraged to study this introduction before
considering the several types of power-generating systems discussed in the present chapter and the next.
In these chapters, we describe some of the practical arrangements employed for power production and
illustrate how such power plants can be modeled thermodynamically. The discussion is organized into three
main areas of application: vapor power plants, gas turbine power plants, and internal combustion engines.
These power systems produce much of the electrical and mechanical power used worldwide. The objective
of this chapter is to study vapor power plants in which the working fluid is alternately vaporized and con-
densed. Chapter 9 is concerned with gas turbines and internal combustion engines in which the working
fluid remains a gas.
Major ways for generating electricity are considered in the chapter introduction.
George Doyle/Getty Images, Inc.
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Vapor Power
Systems
8
When you complete your study of this chapter, you will be able to…
c
Demonstrate an understanding of the basic principles of vapor power plants.
c
Develop and analyze thermodynamic models of vapor power plants based on the Rankine
cycle and its modifications, including:
c
sketching schematic and accompanying T–s diagrams.
c
evaluating property data at principal states in the cycle.
c
applying mass, energy, and entropy balances for the basic processes.
c
determining power cycle performance, thermal efficiency, net power output, and mass
flow rates.
c
Explain the effects on Rankine cycle performance of varying key parameters.
c
Discuss the principal sources of exergy destruction and loss in vapor power plants.
LEARNING OUTCOMES
425
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Introducing Power Generation
An exciting and urgent engineering challenge in the decades immediately ahead is to
responsibly meet our national power needs. The challenge has its roots in the declining
economically recoverable supplies of nonrenewable energy resources, effects of global
climate change, and burgeoning population. In this introduction, we consider both con-
ventional and emerging means for generating power. The present discussion also serves
to introduce Chaps 8 and 9, which detail vapor and gas power systems, respectively.
Today
An important feature of a prudent national energy posture is a wide range of sources
for the power generation mix, thus avoiding vulnerabilities that can accompany over-
reliance on too few energy sources. This feature is seen in Table 8.1, which gives a
snapshot of the sources for nearly all U.S. electricity today. The table shows heavy
dependance on coal for electricity generation. Natural gas and nuclear are also sig-
nificant sources. All three are nonrenewable.
The United States has abundant coal reserves and a rail system allowing for smooth
distribution of coal to electricity producers. This good news is tempered by significant
human-health and environmental-impact issues associated with coal (see Energy &
Environment, Sec. 8.2.1). Coal use in power generation is discussed further in Secs.
8.3 and 8.5.3.
Use of natural gas has been growing in the United States because it is competitive
cost-wise with coal and has fewer adverse environmental effects related to combustion.
Natural gas not only provides for home heating needs but also supports a broad
deployment of natural gas–fueled power plants. Natural gas proponents stress its value
as a transitional fuel as we move away from coal and toward greater reliance on
renewables. Some advocate greater use of natural gas in transportation. North Amer-
ican natural gas supplies seem ample for years to come. This includes natural gas from
deep-water ocean sites and shale deposits, each of which has environmental-impact
issues associated with gas extraction. For instance, the hydraulic drilling technique
known as fracking used to obtain gas from shale deposits produces huge amounts of
briny, chemically laden wastewater that can affect human health and the environment
if not properly managed. Despite increasing domestic natural gas supplies, natural gas
in liquid form (LNG) is imported by ship to the United States (see Energy & Envi-
ronment, Sec. 9.5).
The share of nuclear power in U.S. electricity generation is currently about the
same as that of natural gas. In the 1950s, nuclear power was widely expected to be a
dominant source of electricity by the year 2000. However, persistent concerns over
reactor safety, an unresolved radioactive waste–disposal issue, and construction costs
in the billions of dollars have resulted in a much smaller deployment of nuclear power
than many had anticipated.
In some regions of the United States, hydroelectric power plants contribute sig-
nificantly to meeting electricity needs. Although hydropower is a renewable source,
it is not free of environmental impacts—for example, adverse effects on aquatic life
in dammed rivers. The current share of wind, solar, geothermal, and other renewable
sources in electricity generation is small but growing. Oil currently contributes only
modestly.
Oil, natural gas, coal, and fissionable material are all in danger of reaching global
production peaks in the foreseeable future and then entering periods of decline.
Diminishing supplies will make these nonrenewable energy resources ever more
costly. Increasing global demand for oil and fissionable material also pose national
security concerns owing to the need for their importation to the United States.
Table 8.1 shows the United States currently has a range of sources for electricity
generation and does not err by relying on too few. But in years ahead a gradual shift
to a mix more reliant on renewable resources will be necessary.
Current U.S. Electricity
Generation by Source
Coal 48.5%
Natural gas 21.6%
Nuclear 19.4%
Hydroelectric 5.8%
Other renewables
a
2.5%
Oil 1.6%
Others 0.6%
a
Wind, solar, geothermal, others
Source: Summary Statistics for
the United States, Energy Infor-
mation Administration, 2009,
http://www.eia.doe.gov/cneaf/
electricity/epa/epates.html.
TABLE 8.1
426 Chapter 8 Vapor Power Systems
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Tomorrow
Looming scarcities of nonrenewable energy resources and their adverse effects on
human health and the environment have sparked interest in broadening the ways in
which we provide for our electricity needs—especially increasing use of renewable
resources. Yet power production in the first half of the twenty-first century will rely
primarily on means already available. Analysts say there are no technologies just over
the horizon that will make much impact. Moreover, new technologies typically require
decades and vast expenditures to establish.
Table 8.2 summarizes the types of power plants that will provide the electricity
needed by our population up to mid-century, when electric power is expected to
play an even larger role than now and new patterns of behavior affecting energy are
likely (see Table 1.2).
There are several noteworthy features of Table 8.2. Seven of the twelve power
plant types listed use renewable sources of energy. The five using nonrenewable
sources include the three contributing most significantly to our current power mix
(coal, natural gas, and nuclear). Four power plant types involve combustion—coal,
natural gas, oil, and biomass—and thus require effective means for controlling gas-
eous emissions and power plant waste.
The twelve power plant types of Table 8.2 are unlikely to share equally in meeting
national needs. In the immediate future coal, natural gas, and nuclear will continue
to be major contributors while renewables will continue to lag. Gradually, the respec-
tive contributions are expected to shift to greater deployment of plants using renew-
ables. This will be driven by state and national mandates requiring as much as 20%
of electricity from renewables by 2020.
Of the emerging large-scale power plant types using a renewable source, wind is
currently the most promising. Excellent wind resources exist in several places in the
United States, both on land and offshore. The cost of wind-generated electricity is
becoming competitive with coal-generated power. Other nations with active wind-
power programs have goals of supplying up to 30% of their total electricity needs by
wind within a few years, providing models for what might be possible in the United
States. Still, wind turbines are not without environmental concerns. They are consid-
ered noisy by some and unsightly by others. Another concern is fatalities of birds and
bats at wind-turbine sites (see Energy & Environment, Sec. 6.13.2).
Large-Scale Electric Power Generation through 2050 from Renewable and
Nonrenewable Sources
a
Power Plant Type Nonrenewable Source Renewable Source Thermodynamic Cycle
Coal-fueled Yes Rankine
Natural gas–fueled Yes Brayton
b
Nuclear-fueled Yes Rankine
Oil-fueled Yes Rankine
c
Biomass-fueled Yes Rankine
Geothermal Yes Rankine
Solar-concentrating Yes Rankine
Hydroelectric Yes None
Wind Yes None
Solar-photovoltaic Yes None
Fuel cells Yes None
Currents, tides, and waves Yes None
a
For current information about these power plant types, visit www.energy.gov/energysources. The Rankine cycle
is the subject of the current chapter.
b
Brayton cycle applications are considered in Chap. 9. For electricity generation, natural gas is primarily used with
gas turbine power plants based on the Brayton cycle.
c
Petroleum-fueled reciprocating internal combustion engines, discussed in Chap. 9, also generate electricity.
TABLE 8.2
Introducing Power Generation 427
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428 Chapter 8
Vapor Power Systems
Owing to higher costs, solar power is currently lagging behind wind power. Yet
promising sites for solar power plants exist in many locations, especially in the South-
west. Active research and development efforts are focused on ways to reduce costs.
Geothermal plants use steam and hot water from deep hydrothermal reservoirs
to generate electricity. Geothermal power plants exist in several states, including
California, Nevada, Utah, and Hawaii. While geothermal power has considerable
potential, its deployment has been inhibited by exploration, drilling, and extraction
costs. The relatively low temperature of geothermal water also limits the extent to
which electricity can be generated economically.
Although fuel cells are the subject of active research and development programs
worldwide for stationary power generation and transportation, they are not yet widely
deployed owing to costs. For more on fuel cells, see Sec. 13.4.
Power plants using current, tides, and waves are included in Table 8.2 because their
potential for power generation is so vast. But thus far engineering embodiments are
few, and this technology is not expected to mature in time to help much in the decades
immediately ahead.
The discussion of Table 8.2 concludes with a guide for navigating the parts of this
book devoted to power generation. In Table 8.2, seven of the power plant types are
identified with thermodynamic cycles. Those based on the Rankine cycle are consid-
ered in this chapter. Natural gas–fueled gas turbines based on the Brayton cycle are
considered in Chap. 9, together with power generation by reciprocating internal com-
bustion engines. Fuel cells are discussed in Sec. 13.4. Hydroelectric, wind, solar-
photovoltaic, and currents, tides, and waves are also included in several end-of-chapter
Design and Open-Ended Problems.
Power Plant Policy Making
Power plants not only require huge investments but also have useful lives measured
in decades. Accordingly, decisions about constructing power plants must consider the
present and look to the future.
Thinking about power plants is best done on a life-cycle basis, not with a narrow
focus on the plant operation phase alone. The life cycle begins with extracting
resources required by the plant from the earth and ends with the plant’s eventual
retirement from service. See Table 8.3.
To account accurately for total power plant cost, costs incurred in all phases should
be considered, including costs of acquiring natural resources, plant construction,
power plant furnishing, remediation of effects on the environment and human health,
and eventual retirement. The extent of governmental subsidies should be carefully
weighed in making an equitable assessment of cost.
Power Plant Life-Cycle Snapshot
1. Mining, pumping, processing, and transporting
(a) energy resources: coal, natural gas, oil, fissionable material, as appropriate.
(b) commodities required for fabricating plant components and plant construction.
2. Remediation of environmental impacts related to the above.
3. Fabrication of plant components: boilers, pumps, reactors, solar arrays, steam and wind turbines,
interconnections between components, and others.
4. Plant construction and connection to the power grid.
5. Plant operation: power production over several decades.
6. Capture, treatment, and disposal of effluents and waste products, including long-term storage
when needed.
7. Retirement from service and site restoration when the useful life is over.
TABLE 8.3
TAKE NOTE...
Here we provide a guide for
navigating the parts of the
book devoted to power
generation.
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Capture, treatment, and proper disposal of effluents and waste products, including
long-term storage where needed, must be a focus in power plant planning. None of
the power plants listed in Table 8.2 are exempt from such scrutiny. While carbon
dioxide production is particularly significant for power plants involving combustion,
every plant type listed has carbon dioxide production in at least some of its life-cycle
phases. The same can be said for other environmental and human-health impacts
ranging from adverse land use to contamination of drinking water.
Public policy makers today have to consider not only the best ways to provide a
reliable power supply but also how to do so judiciously. They should revisit entrenched
regulations and practices suited to power generation and use in twentieth-century
power generation but that now may stifle innovation. They also should be prepared
to innovate when opportunities arise. See the Horizons feature that follows.
Policy makers must think critically about how to promote increased efficiency.
Yet they must be watchful of a rebound effect sometimes observed when a resource,
coal for instance, is used more efficiently to develop a product, electricity for
instance. Efficiency-induced cost reductions can spur such demand for the product
that little or no reduction in consumption of the resource occurs. With exceptional
product demand, resource consumption can even rebound to a greater level than
before.
Decision making in such a constrained social and technical environment is clearly
a balancing act. Still, wise planning, including rationally decreasing waste and increas-
ing efficiency, will allow us to stretch diminishing stores of nonrenewable energy
resources, gain time to deploy renewable energy technologies, avoid construction of
many new power plants, and reduce our contribution to global climate change, all
while maintaining the lifestyle we enjoy.
Power Transmission and Distribution
Our society must not only generate the electricity required for myriad uses but also
provide it to consumers. The interface between these closely linked activities has not
always been smooth. The U.S. power grid transmitting and distributing electricity to
consumers has changed little for several decades, while the number of consumers and
their power needs have changed greatly. This has induced significant systemic issues.
The current grid is increasingly a twentieth-century relic, susceptible to power outages
threatening safety and security and costing the economy billions annually.
Introducing Power Generation 429
Policy makers in 10 northeastern states (Con-
necticut, Delaware, Maine, Maryland, Massachusetts,
New Hampshire, New Jersey, New York, Rhode Island,
and Vermont) with a total population approaching 50 million
have boldly established the nation’s first cap-and-trade pro-
gram to harness the economic forces of the marketplace to
reduce carbon dioxide emitted from power plants. The aim of
these states is to spur a shift in the region’s electricity supply
toward more efficient generation and greater use of renewable
energy technology.
The group of 10 states agreed to cap the total level of CO
2
emitted annually from power plants in the region starting in
2009 and continuing through 2014. To encourage innovation, the
total CO
2
level then will be reduced 2.5% annually over the next
four years to achieve a 10% decrease by 2019. Power plant oper-
ators have agreed to purchase allowances (or credits), which
represent a permit to emit a specific amount of CO
2
, to cover
their expected CO
2
emissions. Proceeds of the sale of allowances
are intended to support efforts in the region to foster energy
efficiency and renewable energy technology. A utility that emits
less than its projected allotment can sell unneeded allowances
to utilities unable to meet their obligations. This is called a trade.
In effect, the buyer pays a charge for polluting while the seller
is rewarded for polluting less.
Costing carbon dioxide creates an economic incentive for
decreasing such emissions. Accordingly, cap-and-trade provides
a pathway for utilities to reduce carbon dioxide emitted from
their power plants cost effectively. If the cap-and-trade program
of this group of states is as successful as many expect, it could
be a model for other regions and the nation as a whole.
Reducing Carbon Dioxide Through Emissions Trading
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430 Chapter 8
Vapor Power Systems
The principal difference between the current grid and the grid of the future is a
change from an electricity transmission and distribution focus to an electricity man-
agement focus that accommodates multiple power generation technologies and fos-
ters more efficient electricity use. A twenty-first-century grid will be equipped for
real-time information, conducive to lightning decision making and response, and
capable of providing consumers with quality, reliable, and affordable electricity any-
where and anytime.
This is a tall order, yet it has driven utilities and government to think deeply about
how to bring electricity generation, transmission, and distribution into the twenty-first
century and the digital age. The result is an electricity superhighway called the smart
grid. See the Horizons feature that follows.
Considering Vapor Power Systems
8.1 Introducing Vapor Power Plants
Referring again to Table 8.2, seven of the power plant types listed require a thermo-
dynamic cycle, and six of these are identified with the Rankine cycle. The Rankine
cycle is the basic building block of vapor power plants, which are the focus of this
chapter.
The components of four alternative vapor power plant configurations are shown
schematically in Fig. 8.1. In order, these plants are (a) fossil-fueled, (b) nuclear-fueled,
(c) solar thermal, and (d) geothermal. In Fig. 8.1a, the overall plant is broken into
four major subsystems identified by the letters A through D. These letters have been
omitted in the other three configurations for simplicity. The discussions of this chap-
ter focus on subsystem B, where the energy conversion from heat to work occurs. The
function of subsystem A is to supply the energy needed to vaporize the power plant
working fluid into the vapor required by the turbine of subsystem B. The principal
The smart grid is envisioned as an intelligent system that
accepts electricity from any source—renewable and non-
renewable, centralized and distributed (decentralized)—
and delivers it locally, regionally, or across the nation. A robust
and dynamic communications network will be at its core, enabling
high-speed two-way data flow between power provider and end
user. The grid will provide consumers at every level—industry,
business, and the home—with information needed for decisions
on when, where, and how to use electricity. Using smart meters
and programmable controls, consumers will manage power use in
keeping with individual requirements and lifestyle choices, yet in
harmony with community, regional, and national priorities.
Other smart grid features include the ability to
c Respond to and manage peak loads responsibly
c Identify outages and their causes promptly
c Reroute power to meet changing demand automatically
c Use a mix of available power sources, including distributed
generation, amicably and cost-effectively
And all the while it will foster exceptional performance in electric-
ity generation, transmission, distribution, and end use.
The smart grid will accommodate emerging power technolo-
gies such as wind and solar, emerging large-scale power consum-
ers such as plug-in and all-electric vehicles, and technologies yet
to be invented. A more efficient and better-managed grid will
meet the increase in electricity demand anticipated by 2050
without the need to build as many new fossil-fueled or nuclear
power plants along the way. Fewer plants mean less carbon diox-
ide, other emissions, and solid waste.
Our Electricity Superhighway
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difference in the four power plant configurations shown in Fig. 8.1 is the way working
fluid vaporization is accomplished by action of subsystem A:
c Vaporization is accomplished in fossil-fueled plants by heat transfer to water pass-
ing through the boiler tubes from hot gases produced in the combustion of the
fuel, as shown in Fig. 8.1a. This is also seen in plants fueled by biomass, municipal
waste (trash), and mixtures of coal and biomass.
c In nuclear plants, energy required for vaporizing the cycle working fluid originates
in a controlled nuclear reaction occurring in a reactor-containment structure. The
pressurized-water reactor shown in Fig. 8.1b has two water loops: One loop circulates
(a) Fossil-fueled vapor power plant.
Boiler
Cooling tower
Warm water
Pump
Fossil fuel
Air
Cooled water
Feedwater pump
Combustion gases
to stack
Turbine
Stack
Condenser
Makeup water
+
–
Electric
generator
B
A
C
D
(b) Pressurized-water reactor nuclear vapor power plant.
Cooling tower
Warm water
Pump Cooled water
Feedwater pump
Turbine
Condenser
Boiler
Pressurizer
Containment structure
Pump
Reactor
vessel
Makeup water
+
–
Electric
generator
Control
rods
Fig. 8.1 Components of alternative vapor power plants (not to scale).
8.1 Introducing Vapor Power Plants 431
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432 Chapter 8
Vapor Power Systems
water through the reactor core and a boiler within the containment structure; this
water is kept under pressure so it heats but does not boil. A separate loop carries
steam from the boiler to the turbine. Boiling-water reactors (not shown in Fig. 8.1)
have a single loop that boils water flowing through the core and carries steam
directly to the turbine.
c Solar power plants have receivers for collecting and concentrating solar radiation.
As shown in Fig. 8.1c, a suitable substance, molten salt or oil, flows through the
receiver, where it is heated, directed to an interconnecting heat exchanger that
replaces the boiler of the fossil- and nuclear-fueled plants, and finally returned to
(c) Concentrating solar thermal vapor power plant.
Cooling tower
Warm water
Molten salt
or oil
Pump Cooled water
Feedwater pump
Turbine
Condenser
Receiver
Heat
exchanger
Makeup water
+
–
Electric
generator
Pump
(d) Geothermal vapor power plant.
Cooling tower
Warm water
Pump Cooled water
Pump
Turbine
Condenser
Heat
exchanger
Organic substance
Makeup water
Return water
injected
underground
Hot water
and steam
from underground
+
–
Electric
generator
Pump
Fig. 8.1 (Continued)
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