EGEE 102 – Energy Conservation And Environmental Protection

Energy Efficiency

Definition and Scope of Energy Efficiency

• Energy efficiency improvements refer to a reduction in the energy used for a given service (heating, lighting, etc.) or level of activity.

• The reduction in the energy consumption is usually associated with technological changes, but not always since it can also result from better organization and management or improved economic conditions in the sector ("non-technical factors").

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Efficiency of Energy Conversion

• If we are more efficient with the energy we already have there will be less pollution, less reliance on foreign oil and increased domestic security.

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Energy Efficiency

Energy Conversion

Device

Energy InputUseful Energy Output

Energy Dissipated to the Surroundings

InputEnergyTotal

OutputEnergyUsefulEfficiency

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InputEnergyTotal

OutputEnergyUsefulEfficiency

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 ?

= 90 W x 100 = 90 %100 W

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Device Electric Motor Home Oil Furnace Home Coal Furnace Steam Boiler (power plant) Power Plant (thermal) Automobile Engine Light Bulb-Fluorescent

90

65

55

36

2520

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Efficiency

Efficiency of Some Common Devices

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25% Of the gasoline is used to propel a car, the rest is “lost” as heat. i.e an efficiency of 0.25

Source: Energy Sources/Applications/Alternatives

Vehicle Efficiency – Gasoline Engine

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Heat Engine

• A heat engine is any device which converts heat energy into mechanical energy.

• Accounts for 50% of our energy conversion devices

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Temperature is in ° Kelvin !!!!!!!

Carnot Efficiency

• Maximum efficiency that can be obtained for a heat engine

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Illustration

For a coal-fired utility boiler, The temperature of high pressure steam would be about 540°C and T cold, the cooling tower water temperature would be about 20°C.Calculate the Carnot efficiency of the power plant ?

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540 ° C = 540 +273 ° K = 813 °K20 °C = 20 + 273 = 293 °K

CarnotEfficiency () 1 293

813

x100 64%

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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.

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Schematic Diagram of a Power Plant

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Boiler Components

Boiler

TurbineGenerator

Chemical Energy Input (100 BTU)

Thermal Energy (88 BTU)

Mech. Energy (36 BTU)

Elec. Energy Output (10.26 Wh)

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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

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Overall Efficiency Cont..

(Thermal Energy)x(Mechanical Energy) x(Electrical Energy) Chemical Energy Thermal Energy Mechanical Energy

= E boiler x E turbine x E generator= 0.88 x 0.41 x 0.97= 0.35 or 35%

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System Efficiency

The efficiency of a system is equal to the product of efficiencies of the individual devices (sub-systems)

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Extraction of Coal 96% 96%Transportation 98% 94% (0.96x0.98)

Electricity Generation 38% 36 % (0.96x0.98x0.38)

Transportation Elec 91% 33%Lighting

Incandescent 5% 1.7%Fluorescent 20% 6.6%

Step Efficiency Cumulative Efficiency

Efficiency of a Light Bulb

Step

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System Efficiency of an Automobile

Production of Crude 96% 96%Refining 87% 84%Transportation 97% 81%Thermal to Mech E 25% 20%Mechanical Efficiency-Transmission 50% 10%Rolling Efficiency 20% 6.6%

Step Step Efficiency Cumulative Efficiency

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Efficiency of a Space Heater

Electricity = 24%Fuel Oil = 53%Natural Gas = 70%

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Heat Mover

• Any device that moves heat "uphill", from a lower temperature to a higher temperature reservoir.

• Examples.• Heat pump.• Refrigerator.

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Heat Pump Heating Cycle

Source: http://energyoutlet.com/res/heatpump/pumping.html

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Heat Pump Cooling Cycle

Source: http://energyoutlet.com/res/heatpump/pumping.html

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Coefficient of Performance (C.O.P)

Effectiveness of a heat pump is expressed as coefficient of performance (C.O.P)

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Example

Calculate the ideal coefficient of performance (C.O.P.) For an air-to-air heat pump used to maintain the temperature of a house at 70 °F when the outside temperature is 30 °F.

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Solution Cont.

Thot = 70 °F = 21°C =21 + 273 =294K

Tcold = 30 °F = -1°C =-1 + 273 =272KC.O.P = 294

294 - 272 = 294

22 = 13.3

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Consequences

• For every watt of power used to drive this ideal heat pump, 13.3 W is delivered from the interior of the house and 12.3 from the outside.

• Theoretical maximum is never achieved in practice This example is not realistic. In practice, a C.O.P in the range of 2 - 6 is typical.

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