1. Converting thermal energy to mechanical
(gravitational potential or kinetic) energy
So far we have discussed energy
transfers
Mechanical Energy -> Thermal Energy
Thermal Energy -> Thermal Energy
(conduction, convection, radiation)
A very important process in the energy
Economy is the conversion
Thermal Energy -> Mechanical energy
This is what happens in a car, and in
a coal fired electrical generating plant.

3. In cars,busses, trains, airplanes and trucks the
complete sequence is
Chemical energy -> Thermal Energy ->
Mechanical Energy
whereas in fossil fuel fired generating plants
it is
Mechanical Energy-> Electrical Energy
We only discuss the second step in each
sequence this week.

4. A very important fact about conversions
of Thermal to Mechanical energy is that it
CAN NEVER TAKE PLACE COMPLETELY.
SOME OF THE OUTPUT FROM AN ENGINE
WHICH CONVERTS THERMAL ENERGY INTO
MECHANICAL ENERGY BY DOING WORK
ALWAYS OUTPUTS SOME THERMAL ENERGY
AS WELL

5. How the conversion is done.
Usually with a working fluid, usually a
high pressure gas. The thermal energy
is used to heat the gas. When the gas
is heated, its temperature, and it pressure
go up. (Pressure is the force that a gas
exerts on each square meter of the walls
of its container.)
Then the high pressure gas is used to push
a piston (as in the lab and in your car). The
rod on the rising piston does the mechanical work.
Or the high pressure gas is directed at the
blades of a turbine, which then turns, doing
the mechanical work (as in an electrical generating
plant. )

6. The laboratory this week illustrates the first way. This is sometimes
called a reciprocating engine. h

7. Thermal energy in, lifting weight
out,
piston and
weight
compressing
air
Thermal energy in
, piston rising
Thermal energy out, piston compressing
air

8. The working fluid can be gasoline
vapor (in a car), air (as in the lab)
or water vapor (steam) as in a coal
fired generating plant.

9. If, in the lab experiment, the piston plus
the weight had mass 0.113kg and the
piston rose 1.5cm how much work did
the heated gas do?
A. (0.113x1.5) joules
B x1.5x9.8 joules
C x.015/9.8 joules
D x.015x9.8 joules

10. Answer D joules

11. If .19 joules/degree K are required
to heat the gas from the low temperature
of 23 C to the high temperature of 60 C, then
how much thermal energy is minimally
required as input during the lifting of
the weight?
A x (60-23) joules
B. 0.19x (60-273)joules
C x 60x(1-296/333)
D. (60-23)/.19 joule

12. Answer A. =.
7.03 joules

13. The other common way to convert the
thermal energy to mechanical energy
is a turbine.
It is basically just a propeller: The
gas (for example steam) is expelled at
high pressure from a nozzle after being heated
the stream of gas is directed at the
turbine, where the gas molecules of bounce
off the blades of the turbine, causing them
to turn. We will discuss the physics of such
processes more later when we study wind power.
This kind of engine is used in electrical generating
plants (whether fired by coal, natural gas or
nuclear energy).

15. The turbine generator consists of a series of steam turbines
interconnected to
each other and a generator on a common shaft. There is a
high pressure turbine at one end, followed by an intermediate
pressure turbine, two low pressure turbines, and the generator.
As steam moves through the system and loses pressure and
temperature it expands in volume, requiring increasing
diameter and longer blades at each succeeding stage to extract
the remaining energy. The entire rotating mass may be over
200 tons and 100 ft (30 m) long.

16. Schematic
view of
an engine
QC is never
zero

17. The ratio of the output mechanical
energy to the input thermal energy
is called the EFFICIENCY of the
engine
Efficiency =
(Output Mechanical Energy)/(Input Thermal Energy)
The Maximum Possible Efficiency of such an engine Is
(1-Tc/TH)
Where the temperatures are in K.

18. Use of fossil fuels to generate thermal energy
from chemical energy, often followed by
partial conversion to mechanical energy.
We will discuss
Coal: mainly used in the US to drive turbines
for the generation of electricity
Petroleum: for which the biggest applications
are in transportation and
Natural gas: for which the direct use of the
thermal energy for heating buildings is the
biggest application.

19. In a coal fired electrical generating plant
steam enters the turbines at about 540C
(1000 F) and condenses at the cold end
after driving the turbines which run the electrical
generator at about 35C (95F). What is the
maximum possible efficiency of such a system?
A.1-35/540 B.1-308/813 C. 1-95/1000 D. 308/813

20. Answer:
B. 1-(308/813)= .62 or 62%
In fact coal fired generating plants do not
achieve efficiencies this high. 30% is
more typical.

21. Mojave Generating Station, a 1,580 MW coal
power plant near Laughlin, Nevada

22. Consider the power plant illustrated earlier. It
outputs 1580 MegaWatts (millions of watts).
If it operates at an efficiency of 35%, and one gets
(from the table at the back of your book) 25 million
btu from a ton of coal, how much coal does it
use per day?(1btu=1055joules)
A.3600x24x1580x106/(.35x1055x25x106)tons
B. 25x106x3600x24/(1055x1580x106x.35) tons
C. 3600x24x1580x106/(.65x1055x25x106)tons
D. 25x106x3600x24x1055/(1580x106x.65) tons

23. A. 14,800 tons/day (!)
= about 150 railroad cars full

25. If the US economy uses 23 quads of thermal
energy from coal per year and the efficiency
of coal generating plants is 35%, then
approximately how many such 1500 Megawatt plants
would be required?
1btu=1055 joules, 1 year =3x107seconds,
1 quad = 1015 btu
A. (23x1015 )(1055)/(0.35(1.5x109 )(3x107))=1540
B.(23x1015 )(1055)/((1.5x109 )(3x107))=530
C.1055(1.5x109 )(3x107)/(23x1015 )(0.35)=5900
D.( 23x1015 )(0.35)(1055)/((1.5x109 )(3x107))=190

26. Answer: D.

27. Environmental problems associated with coal use:
* Generation of hundreds of millions of tons of waste products, including fly ash, bottom ash, flue gas desulfurization sludge, that contain mercury, uranium, thorium, arsenic, and other heavy metals
* Acid rain from high sulfur coal
* Interference with groundwater and water table levels
* Contamination of land and waterways and destruction of homes from fly ash spills such as Kingston Fossil Plant coal fly ash slurry spill
* Impact of water use on flows of rivers and consequential impact on other land-uses
* Dust nuisance
* Subsidence above tunnels, sometimes damaging infrastructure
* Coal-fired power plants without effective fly ash capture are one of the largest sources of human-caused background radiation exposure
* Coal-fired power plants shorten nearly 24,000 lives a year in the United States, including 2,800 from lung cancer[44]
* Coal-fired power plants emit mercury, selenium, and arsenic which are harmful to human health and the environment[45]
* Release of carbon dioxide, a greenhouse gas, which causes climate change and global warming according to the IPCC and the EPA. Coal is the largest contributor to the human-made increase of CO2 in the air[46]

30. Mountain top removal coal mining in Boone County,
West Virginia

31. Reserves of Coal
Figure 7.14: World recoverable reserves of coal, 2003.
Fig. 7-14, p. 224

32. Figure 7.15: U.S. coal consumption by sector: 1950–2003.
Fig. 7-15, p. 225

33. US coal production 2005
/acr/acr_sum.html#fes1

34. Minnesota coal generating plants (2005)
5676 MW of generating capacity
43.8% of total electrical generating capacity
34.9 million tons of CO2 emissions/yr
(about 20 tons/person)
US Department of Energy, Energy Information
Agency (EIA) EIA-906/EIA-920

36. In July of 2016, the EIA reports that
2186 thousand megawatt-hours of electrical
energy were produced and consumed in
Minnesota by burning coal. If the plants
had efficiency of 35%, how many tons of
coal were burned?
1 ton of coal =25 x 106 btu
1btu = 1055 joules
A x 106 x 3600/(1055 x
25 x 106 )
B x 106 x 3600/(1055 x .35 )
C x 1012 x 3600/(1055 x.35x25 x 106 )
D x 1012 x 3600/(1055 x 25 x 106 )

38. Petroleum, mostly transportation
In automobiles are rated for FUEL efficiency
which is a related, but different, concept.
The FUEL EFFICIENCY of a car is the average
number of miles you can drive it per gallon.

39. If the fuel efficiency of your car is 35 miles per
gallon how much energy is being provided to
your engine for each mile you drive. (1.25 x 105 btu are
available per gallon of gasoline.)
A x 105 btu
B 1.25 x 105/35 btu
C. 1/ 1.25 x 105 btu
D. 35/ 1.25 x 105 btu

40. Answer B: =3571 btu/mile

41. If the thermal efficiency of your car is
30% how much energy is being used
to push the car forward per mile
(if the gasoline is providing 1.25 x 105/35 btu
per mile)
A..30x 1.25 x 105/35 btu
B x 105/35 btu/.30
C. 30/ 1.25 x 105/35 btu
D. 1/ 1.25 x 105/(35 x.30)btu

42. Answer A.
.30x 1.25 x 105/35 btu=
1071 btu

43. What force is the engine providing to
drive the car forward in the last example
(in Newtons). The energy provided in
a mile was .30x 1.25 x 105/35 btu=
1071 btu
1 btu=1055 joules. 1 mile=1600 meters
A /1055x1600) Newtons
B. 1071x1600/1055 Newtons
C. 1071x1055/1600 Newtons
D. 1071/1600 Newtons

44. Answer C:
706 newtons

45. What power is the engine producing at
35mph in this car? (The force was 708
Newtons). 1 pound(lb)=4.4 newtons,
1 mile = 5280ft, 1 horsepower=550ft-lb/s
A. (708/35)x(3600/5280)x4.4/550 hp
B. (708x35)x(5280/3600)x(1/(4.4x550))hp
C. (708x35)(3600/5280)x4.4x550hp
D. (708/35)x(5280/3600)x4.4/550hp

46. Answer:
B. = 15hp

49. In the energy economy, thermal energy
is obtained for conversion to
mechanical energy Is supplied
mainly by
Fossil fuels (about 80% of total
energy use)
and
Nuclear energy (about 8%)

50. US oil consumption 2003 Fig. 7-7, p. 213
Figure 7.7: U.S. oil consumption by end use, 2003.
US oil consumption 2003
Fig. 7-7, p. 213

51. ----- Hubert's
prediction
___ actual
US oil
production

52. MILLION

53. Gas consumption US 2003

54. Natural Gas
US production

59. In summary,
Most of the natural gas use (about a quarter
fossil fuel use, is for direct heating)
Oil and coal are used almost entirely for
thermal to mechanical energy conversion
with the attendant efficiency limitations.
(The second law limitation on thermal efficiency for
engines does not apply to direct heating
uses of thermal energy. )

60. Table 7-1, p. 209

61. Figure 7.5: World oil reserves and oil use by country, 2003.
Fig. 7-5a, p. 211

62. Figure 7.5: World oil reserves and oil use by country, 2003.
Fig. 7-5b, p. 211