Energy:Concepts and Examples

SHORT VERSION – may look weirdSteve ColtEnvi 201

University of Alaska Anchorageafsgc@uaa.alaska.edu

Version 1: 16 Feb 2004

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Energy: What is It?

• Ability to do Work– (formal physics definition)

• Food We can move, think, live• Warmth or Heat, Coolness• Light• Sound (speaker)• Motion (transportation)• Mechanical Power (skilsaw)

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What Forms does it Take?

• Chemical – Bagel, gasoline

• Heat– Warm room, hot water

• Mass (E=Mc2) – Sun, nuclear bomb, nuclear reactor

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Forms of Energy

• Kinetic – energy of motion– Speeding bicycle

• Potential – energy of position– Bicycle at top of hill

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Forms of Energy

• Electric Energy (Charge)– Charged Battery

• Electromagnetic Radiation– Visible Light, microwaves

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Measuring Energy – Common Units

• Joule – metric unit (not very useful)– 1 bagel has 1.2 million joules of food

energy

• Btu – British thermal unit– 1 lb water up by 1 degree f

• kWh – kilowatt hour– Used for electricity – 1 kWh costs 10

cents in Anchorage

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Power: the Rate of Energy Use– Just as speed = distance per time,

• Power = Energy conversion per unit of time

• Measured as Watts (W)– 60-watt light bulb converts electricity

to light (and heat!)…– While it is on!– Technically, 1W = one joule per

second

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Power: Examples• Human (at rest)

– About 100 W (we are always on!)• Small candle

– About 1 W (when it is lit!)• Hair dryer

– 1,000 W = 1 kilowatt = 1 kW– If it is on for one hour it uses:– 1 kW x 1 hour = 1 kilowatt-hour (1

kWh)– (costs 10 cents)

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Power: Horse vs. Hairdryer

• 1 Horsepower = .67 kW = 670 watts– Invented by James Watt so he could

charge for steam power in a way people could understand

– Few horses can actually produce 1 hp– Hairdryer, using 1 kilowatt, = 1.3

horsepower

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Conversion Example: Jet EngineForms:

Chemical -> Heat -> Motion

Power output:

15 Megawatts during take-off,

Much less during cruise

Picture of jet engine

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Energy Flow in Natural Systems

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Solar Radiation Received at Earth’s Surface: average is about 150 watts per

square meter

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Conversion of Solar to Plant Biomass

Only about 1-5% of solar radiation is converted to chemical energy of plant matter

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Conversion of Plants to Animals

• Trophic Level = feeding level• Plants (autotrophs = self-feeders)• Herbivores

– Convert 1-10% of plant energy to biomass

• Carnivores– Convert 1-10% of herbivore energy to

their own biomass

• 16 lbs of corn = 1 pound of beef

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Two Laws of Thermodynamics• First Law:

– Energy is converted from one form to another

• Second Law:– Energy is degraded with every

conversion, and ends up as low-grade heat

• Example: Your Toaster– …Electricity->Heat->(toast + warmth)

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Implication:

• Energy conversions are never 100% efficient

• There is waste heat lost at every step

• Efficiency Varies Dramatically…

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Efficiency of Energy Conversion

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Pre-Industrial Energy Sources

Source: Smil 1994

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Energy Use per Person

Source: Smil 1994

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Energy Use per Person

• The U.S. converts energy at the constant rate of about 10 kW per person

• This is like having 100 “energy slaves” working full-time for every american child and adult person

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Energy Use vs. Human Development

The Human Development Index combines life expectancy, education, and income into an index that ranges from zero to 1

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Impacts: Sulfur and Nitrogen

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Impacts: Land Use from Production

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Changing Energy Efficiency

Light Bulbs Jet Engines

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Overall, we seem to be getting more efficient…..

This is the “T” factor – how much energy per dollar of economic output

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But World energy use still grows by 1-2%/yr

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References• Smil, Vaclav. 1994 Energy in World History. Westview.• Smil, Vaclav. 2003. Energy at the Crossroads. MIT Press.• Scott Brennan and Jay Withgott. 2003. Environment:

The Science Behind the Stories. Pearson/Cummings.• BP Statistical Review of World Energy. 2003.