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Lecture 10 Introduction to Environmental Eng. Energy Flows and Balances

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Units of Measure –BTU – British Thermal Unit Amount of energy required to heat 1 pound of water 1 degree Fahrenheit –Calories Amount of energy required to heat 1 gram of water 1 degree Celcius –Joules –Kilowatt-Hours –All are measures of energy and are interchangeable

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Energy Balances and Conversion Many types of energy –Heat, kinetic, potential, electrical –Engineers utilize one form of energy and place in another that can be used for work –This energy conversion is always less than 100% efficient

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Energy Balances and Conversion A black box “energy balance” Rate of energy accum. = rate in – rate out + rate produced – rate of energy consumed Energy in Energy out Prod/cons. Wasted

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Energy Balances and Conversion Energy can not be produced or consumed, just transformed And energy systems can be at steady-state Rate in = Rate out + Rate Wasted

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Energy Balances and Conversion Example –What is the efficiency of a gas powered car if gas has an energy content of 20,000 BTU/gal, a car uses 20 gallons per week and in that time produces 10 x 10 6 joules? Energy In = 20 gal x 20,000 BTU/gal = 40,000 BTU x 1054 Joules/BTU = x10 6 Joules Energy Out = 10 x10 6 Joules Efficiency = 10 x10 6 Joules/42.16 x10 6 Joules X 100 = 23.72%

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Energy Balances and Conversion How is the Energy content of a source measured? –A bomb calorimeter Source is placed in a steel ball, placed in a water bath Pure Oxygen is added The source is ignited and combusted Change in the temperature of the water is the result of the energy content of the source

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Energy Balances and Conversion Heat Energy Balance –Heat energy = mass of material x absolute temperature Analogous to mass flows 0 = heat energy IN – heat energy OUT 0 = [T 1 Q 1 + T 2 Q 2 ] – [T 3 Q 3 ] –T = absolute temperature –1 & 2 input streams, 3 output stream

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Energy Sources and Availability Renewable Energy –Hydropower, Wood and other biomass, Solar power, Refuse and other waste materials Non-renewable Energy –Nuclear Power, Coal, peat and other similar materials, Natural gas, Oil How long will they last? 50, years?

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Energy Balances and Conversion Energy Equivalence –Arithmetic or Conversion Energy Equivalence Energy production isn’t just based on the caloric content of the substance, In considering energy production all other costs of production must be taken into account How much more does it cost? What will the different infrastructure costs be? Example methane gas from Wastewater treatment?

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