Faculty of EngineeringFaculty of Engineering
ENERGYENERGY
MEC002J1 The Professional EngineerMEC002J1 The Professional Engineer
Dr S BurgessDr S Burgess
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Content
Energy (definition, conversion of and unit)
Power (definition, unit and examples)
Alternate (renewable) energy
Energy conservation
Task for next week
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Energy
• Capacity for doing work (exists in various forms)
Energy conversion – the process of converting energy from one form to another!
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)
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• Generation of electricity is a good example of energy conversion
In the power station, a fossil fuel (coal/oil) is burned to produce heat
Energy conversion
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 is the rate of doing work, such that, one joule of work is done in one second
Power
• 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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• Domestic user example –
A consumer has the following electrical demand:
Lighting 0.5 kW for 6 hours Water heating 3.0 kW for 2 hoursSockets 3.5 kW for 3 hours Cooking 5.0 kW for 2 hours
Determine the daily energy usage in (a) Joules and (b) kWH
Power
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(a) W = P*t
W = [(500 * 6) + (3000 * 2) + (3500 * 3) + (5000 * 2)] * 3600106.2 MJ
Solution
(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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• 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
Power
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?Questions?