2010 On Line Technocracy Study Course project
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In the previous lessons we have found that while energy may be converted from one to another of its forms it is never destroyed...
Lesson 5
...We also found that there is a fundamental tendency for all other forms of energy to change into heat, and for all heat to come to the same temperature. When a difference of temperature exists it is possible to convert heat into work, but if no temperature difference exists no heat can be converted into work even if, literally, oceans of heat exist.
Definition of an
Engine.
An engine may be defined as any type of machine which takes energy in any form
and converts it into work.
The initial form of the energy converted may be mechanical, as in the case of wind and falling water; it may be chemical, as in the case of coal, oil and wood; it may be electrical, as in driving an electric motor from a power line; or it may be radiant energy, as in the case of using the sun's heat to drive an engine.
TABLE 2
Examples
of Engines Converting Mechanical Energy into Work:
Engine Energy
Used
Windmill . . . . . . . . . . . . . . . . . . . . . Kinetic energy of the wind
Sailing
vessel . . . . . . . . . . . . . . . . . . Kinetic energy of the wind
Water
wheel . . . . . . . . . . . . . . . . . . . Potential energy of water
Examples
of Engines Converting Chemical Energy into Work:
Engine Energy
Used
Steam
Engine . . . . . . . . . . . . . Fuel---Coal, oil or wood
(a) Reciprocating type (piston)
(b) Steam
turbines
Internal combustion
engines:
(a) Gas . . . . . . . .
. . . . . . . . engine Gas
(b) Gasoline engine . . . . . . Gasoline
(c) Diesel . . . . . . . . . . . . . . Fuel oil
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Examples of Engines Converting Electrical Energy into Work:
Engine Energy
Used
Various forms of
electric
motors
Electrical energy from power lines or from electric batteries
NOTE: An engine which makes the initial conversion of energy into work is called a Prime mover. In electric power systems mechanical or heat energy is converted first into work which is used to drive the electric generators. These convert work into electrical energy. The engine which drives the generator in this case is the prime mover. Electric motors converting this electrical energy back into work are not prime movers, but 'secondary movers,' instead.
Efficiency of
Engines.
The efficiency of an engine is defined as
the ratio of energy converted into work, to the total energy initially
supplied.
efficiency = work output / energy input
Therefore, in order to measure the efficiency of an engine it is necessary to know both the total energy taken during a given time and the work done in that time by the engine.
In the case of a waterfall, the available energy per unit of time is determined by the amount of water passing through the water wheel in that time, and by the height of the fall. Suppose the fall is 100 feet high, and that 990 pounds of water per minute fall through the water wheel. In this case the energy input would be 990 x 100 equals 99,000 foot-pounds per minute. Since 33,000 foot-pounds per minute is one horsepower, then the input into this wheel would be three horsepower.
Suppose the output of the wheel were only two horsepower due to frictional losses or to poor design of the wheel. Then the efficiency of this wheel would be:
efficiency= 2 h.p. / 3 h.p. =66.2/3%
The maximum efficiency possible in this case would be 100 percent, with an output of 3 horsepower.
Modern hydro-turbine installations such as the 70,000 horsepower units at Niagara Falls have an efficiency of approximately 92 percent. That is, they convert into electrical energy 92 percent of the energy supplied by the water.
Efficiency of
Heat Engines.
In order to measure the efficiency of a
heat engine we have to measure the heat supplied to the engine as well as the
engine's output of work. We cannot measure the heat directly, but we can measure
the fuel that is used; then we can determine the heat input if we know the
amount of heat that is produced by a given amount of fuel.
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Heat Value of
Fuel.
It was pointed out in Lesson 3 that when
certain chemical reactions take place heat is evolved. Also, for the same
amount of substances taking part in a given reaction, the same amount of heat
is always produced.
Now, the production of heat by the burning of a fuel results from the chemical reaction due to the chemical combination of that fuel with oxygen. Fuel plus oxygen equals waste products plus heat. If the fuel be of a particular grade, then the number of calories of heat produced by burning one gram is the same for all the fuel of that grade. The number of gram calories produced by burning one gram of the fuel, or the number of British thermal units produced per pound, is called the heat value of that substance.
Heat values are obtained by placing a measured amount of fuel in a gas-tight container surrounded by compressed oxygen. This is placed in a heat insulated vessel of water and the fuel ignited by an electric spark. When the spark occurs the fuel burns and the heat which is released is taken up by the water. The amount of water is 'known, and the rise of temperature is measured. From this the number of calories or British thermal units is obtained.
TABLE 3
Heat Value
Fuel Gram
Calories British Thermal
Coal per
Gram Units per lb.
Bituminous, low grade . . . . . . . . . 6,000 11,000
Bituminous, high grade . . . . . . . . . 8,000 14,000
Anthracite, low grade . . . . . . . . . 7,000 12,500
Anthracite, high grade . . . . . . . . . 7,500 13,500
Liquid fuel
Gasoline . . . . . . . . . 11,000 20,000
Fuel oils . . . . . . . . . 10,500 18,500
Wood
Oak . . . . . . . . . 4,500 8,500
Pine . . . . . . . . . 5,000 9,000
The average consumption by central power stations in the United States in 1932 was 1.5 pounds of coal per kw.-hr., figuring the heat value of coal to be 13,100 B.t.u. per pound of coal.
Then 1.5
lbs. coal produces 19,650 B.t.u.
3,411 B.t.u. = 1.0 kw.-hr.
Therefore:
efficiency = work done / heat used = 1 kw.-hr. / 19,650 B.t.u. = 3,411 B.t.u. / 19,650 B.t.u.= 17.3%
Thus the average efficiency of all the central power stations in the United States in 1932 was only 17.3 percent.
TABLE 4
Engine Efficiency
Water wheels . . . 70 to 92%
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Steam engines
(a) Locomotives . . . 5
to 10%
(b) Stationary reciprocating engines . . . 10 to 17%
(c) Steam turbines . . . 15 to 30%
(d) Hartford mercury vapor station . . . 33.1%
(e) Average of all central power stations in U.S. in 1932 . . . 17.3%
Internal combustion
engines
(a) Gasoline engine (automobile type) . . . 15 to 28%
(b) Gas engines . . . 25%
(c) Diesel engines . . . 29 to 35%
The above discussion of engines has been presented in some detail not because we are interested in having the reader become an engineer, but because this, it is hoped, will help to clarify the relationship between matter and energy. It was stated at the outset that all the matter on the earth is composed of 92 chemical elements, and that, whether this matter is in the form of living organisms or rocks, its movement involves a degradation of energy.
Engines do not create work or energy; they are instead converters of energy---they convert energy from one form to another.
In our next lesson we shall show that the human body is itself an engine that converts energy into heat and work in strict and exact accordance with the laws of thermodynamics.
April 6, 2007 After 10 years of research, the Massey scientists claim to have "the most efficient porphyrin dye in the world." Benefits of the dyes over traditional silicon-based solar panels include the ability to operate in low light, 10x cheaper production, and flexible application -- starting with canvassing roofs, walls and windows, but eventually moving on to wearable items that can charge your electronics stash. A working prototype for "real applications" should b e ready in a couple years.
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