Transcript MEC002J1 Energy

Faculty of Engineering
ENERGY
MEC002J1 The Professional Engineer
Dr S Burgess
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Content
Energy (definition, conversion of and unit)
Power (definition, unit and examples)
Energy conservation
Alternate (renewable) energy
Task for next week
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Energy
• Capacity for doing work (exists in various forms)
Potential energy – a form of energy a body possesses by
virture of its position (ex. a coiled spring)
Kinetic energy – a form of energy a body possesses by
virture of its motion (ex. releasing a coiled spring)
Energy conversion – the process of converting energy
from one form to another!
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Energy conversion
• Generation of electricity is a good example of energy conversion
In the power station, a fossil fuel (coal/oil) is burned to
produce heat
Heat is used to boil water
Boiling water is turned into high pressure steam
Steam drives a turbine which is connected to a generator
The generator produces electricity as it rotates
Four stages – chemical, heat, mechanical & electrical energy!
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Unit of energy
• Energy is measured in Joules (J)
• In basic electrical theory – an e.m.f. source is measured in
terms of the number of joules of work necessary to move one
coulomb of electricity around the circuit.
• Expressed as: E = W/Q
E is the e.m.f. in volts (V)
W is the energy in joules (J)
Q is the quantity of electricity in coulombs (C)
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Unit of energy
Q [quantity of electricity in coulombs (C)] can also be expressed
as:
Q = I*t
where, I = current in amps (A)
t = time in seconds (s)
Thus, if E = W/Q
then W = E*Q and W = E*I*t
Q
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Power
• Power is the rate of doing work, such that, one joule of work is
done in one second
• It is the ratio of energy and time – its unit is the Watt (W).
After James Watt (1736-1819)
• The ratio is expressed as: P = W/t
Re-arranging we get: W = P*t
But W = E*I*t thus P*t = E*I*t
P = E*I
This is the total power dissipated by an electric circuit
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Power
• In an electric circuit, when current flows through a resistor, heat
is dissipated between the points of connection. This involves the
current (I) and potential difference (V). We now write:
P = V*I
• There are several ways of expressing power:
I = V/R , P = V2/R , P = I2 *R
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Power
• Instrument for measuring power is a ‘Wattmeter’
• Instrument for recording energy is a ‘Integrating meter’
Typical domestic Wattmeter
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Power
• Domestic user example –
A consumer has the following electrical demand:
Lighting
Water heating
Sockets
Cooking
0.5 kW for 6 hours
3.0 kW for 2 hours
3.5 kW for 3 hours
5.0 kW for 2 hours
Determine the daily energy usage in (a) Joules and (b) kWH
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Solution
(a)
W = P*t
W = [(500 * 6) + (3000 * 2) + (3500 * 3) + (5000 * 2)] * 3600
106.2 MJ
(b)
W = P*t
W = [(0.5 * 6) + (3 * 2) + (3.5 * 3) + (5 * 2)]
29.5 kWh
Since: 1 kWh = 3.6 MJ, then 29.5 kWh = 106.2 MJ
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Power
• Electric kettle example –
An electric kettle is rated at 3 kW @ 230V
What is the cost of using the kettle on 28 occasions per week if it
takes two minutes to boil?
Take one unit (a single kW) of electricity to cost 7.8p
Ans –
W = P*t = kWh
W = 3 * 2/60 * 28 = 2.8 kW
cost per week = 2.8 kW * 7.8p = 21.84 pence
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Power
• The last example shows the cost of a single electrical appliance.
• With natural energy resources dwindling the cost of electricity will
rise in the future, this is why we (as engineers) are interested in
energy conservation and alternate energy sources!
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Energy conservation
• Low energy light bulbs
• Reduced thermostat setting
• Better insulation materials (i.e. loft, walls, hot water tank etc)
• Switch off appliances
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Alternate (renewable) energy
• Solar energy
• Wind
• Biomass
• Heat pumps
• Bio Fuels
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Task
Write a small report (2 – 3 pages) on a form of alternate
energy.
The choice of alternate energy source is yours.
Include – text, diagrams & pictures. Show calculations
on usage, energy saving, system efficiency etc….
Submit report to School Office (Rm 5D06)
by 5pm Tuesday 23 Oct 2007.
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Questions?