Download 3a Energy forms, units efficiency etc

The Bases of Energy: forms, units
and efficiency
Energy Efficiency
Energy Input
Energy
Conversion
Device
Useful Energy
Output
Energy Dissipated
to the Surroundings
Useful Energy Output
Efficiency 
Total Energy Input
2
Illustration
• An electric motor consumes 100 watts (a joule per second
(J/s)) of power to obtain 90 watts of mechanical power.
Determine its efficiency ?
Useful Energy Output
Efficiency 
Total Energy Input
3
Efficiency of Some Common Devices
Device
Electric Motor
Home Oil Furnace
Home Coal Furnace
Steam Boiler (power
plant)
Power Plant (thermal)
Automobile Engine
Light Bulb-Fluorescent
Light Bulb -Incandescent
Efficiency
90
65
55
89
36
25
20
5
4
Vehicle Efficiency – Gasoline Engine
25% Of the gasoline is used to propel a car, the rest is
“lost” as heat. i.e an efficiency of 0.25
5
Heat Engine
• A heat engine is any
device which converts
heat energy into
mechanical energy.
• Accounts for 50% of
our energy conversion
devices
6
Carnot Efficiency
• Maximum efficiency that can be obtained for
a heat engine
Temperature is in ° Kelvin
7
540 ° C = 540 +273 ° K = 813 °K
20 °C = 20 + 273 = 293 °K
 293
CarnotEfficiency()  1 
x100  64%
 813
8
Inference
A maximum of 64% of the fuel energy can go to
generation. To make the Carnot efficiency as
high as possible, either T hot should be
increased or T cold should be decreased.
9
Schematic Diagram of a Power Plant
10
Boiler Components
Chemical Energy Input (100 BTU
Boiler
)
Thermal Energy (88 BTU
Turbine
)
Mech. Energy (36 BTU
Generator
Elec. Energy Output (10.26 Wh
)
)
11
Overall Efficiency
Overall Eff = Electric Energy Output (BTU) x 100
Chemical Energy Input (BTU)
= 35 BTU x 100
100 BTU
= 35%
Overall Efficiency of a series of devices =
(Thermal Energy)x(Mechanical Energy) x(Electrical Energy)
Chemical Energy
Thermal Energy
Mechanical Energy
12
Overall Efficiency Cont..
(Thermal Energy)x(Mechanical Energy) x(Electrical Energy)
Chemical Energy
=
Thermal Energy
Mechanical Energy
E boiler E turbine E generator
x
x
= 0.88
x 0.41 x 0.97
= 0.35 or 35%
13
System Efficiency
The efficiency of a system is equal to the product
of efficiencies of the individual devices (subsystems)
14
Efficiency of a Space Heater
Electricity = 24%
Fuel Oil = 53%
Natural Gas = 70%
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