5. Energy Efficiency

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Energy Efficiency:

Energy Efficiency

Efficiency of Energy Conversion:

EGEE 102 - S. Pisupati 2 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.

Energy Efficiency:

EGEE 102 - S. Pisupati 3 Energy Efficiency Energy Conversion Device Energy Input Useful Energy Output Energy Dissipated to the Surroundings

Illustration:

EGEE 102 - S. Pisupati 4 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

Efficiency of Some Common Devices:

EGEE 102 - S. Pisupati 5 Device Electric Motor Home Oil Furnace Home Coal Furnace Steam Boiler (power plant) Power Plant (thermal) Automobile Engine Light Bulb-Fluorescent Light Bulb -Incandescent 90 65 55 89 36 25 20 5 Efficiency Efficiency of Some Common Devices

Vehicle Efficiency – Gasoline Engine:

EGEE 102 - S. Pisupati 6 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

Heat Engine:

EGEE 102 - S. Pisupati 7 Heat Engine A heat engine is any device which converts heat energy into mechanical energy. Accounts for 50% of our energy conversion devices

Carnot Efficiency:

EGEE 102 - S. Pisupati 8 Temperature is in ° Kelvin !!!!!!! Carnot Efficiency Maximum efficiency that can be obtained for a heat engine

Illustration:

EGEE 102 - S. Pisupati 9 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 ?

Slide10:

EGEE 102 - S. Pisupati 10 540 ° C = 540 +273 ° K = 813 °K 20 °C = 20 + 273 = 293 °K

Inference:

EGEE 102 - S. Pisupati 11 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.

Schematic Diagram of a Power Plant:

EGEE 102 - S. Pisupati 12 Schematic Diagram of a Power Plant

Boiler Components:

EGEE 102 - S. Pisupati 13 Boiler Components

Overall Efficiency:

EGEE 102 - S. Pisupati 14 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

Overall Efficiency Cont..:

EGEE 102 - S. Pisupati 15 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%

System Efficiency:

EGEE 102 - S. Pisupati 16 System Efficiency The efficiency of a system is equal to the product of efficiencies of the individual devices (sub-systems)

Efficiency of a Light Bulb:

EGEE 102 - S. Pisupati 17 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

System Efficiency of an Automobile:

EGEE 102 - S. Pisupati 18 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

Efficiency of a Space Heater:

EGEE 102 - S. Pisupati 19 Efficiency of a Space Heater Electricity = 24% Fuel Oil = 53% Natural Gas = 70%

Heat Mover:

EGEE 102 - S. Pisupati 20 Heat Mover Any device that moves heat "uphill", from a lower temperature to a higher temperature reservoir. Examples. Heat pump. Refrigerator.

Heat Pump Heating Cycle:

EGEE 102 - S. Pisupati 21 Heat Pump Heating Cycle Source: http://energyoutlet.com/res/heatpump/pumping.html

Heat Pump Cooling Cycle:

EGEE 102 - S. Pisupati 22 Heat Pump Cooling Cycle Source: http://energyoutlet.com/res/heatpump/pumping.html

Slide23:

EGEE 102 - S. Pisupati 23 Coefficient of Performance (C.O.P) Effectiveness of a heat pump is expressed as coefficient of performance (C.O.P)

Example:

EGEE 102 - S. Pisupati 24 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.

Solution Cont.:

EGEE 102 - S. Pisupati 25 Solution Cont. T hot = 70 °F = 21°C =21 + 273 =294K T cold = 30 °F = -1°C =-1 + 273 =272K C.O.P = 294 294 - 272 = 294 22 = 13.3

Consequences:

EGEE 102 - S. Pisupati 26 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.

More C.O.P.’s:

EGEE 102 - S. Pisupati 27 More C.O.P.’s Compare the ideal coefficients of performance of the of the same heat pump installed in Miami and Buffalo. Miami: T hot = 70°F, T cold = 40°F Buffalo: T hot = 70°F, T cold = 15°F Miami: T hot = 294°K, T cold = 277°K Bufalo: T hot = 294°K, T cold = 263°K

Slide28:

EGEE 102 - S. Pisupati 28 = 294 (294-277) = 17.3 = 294 (294-263) = 9.5 Miami Buffalo

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