Conservation of Energy

• If only conservative forces act (e.g. only gravity), then Mechanical Energy is constant:

KEi + PEi = KEf + PEf

• Today we will generalize this to include all forms of energy!

ITAIPU (Brazil/Paraguay)

http://www.solar.coppe.ufrj.br/itaipu.html 12.6 GW capacity

ITAIPU (Brazil/Paraguay)

18 turbines, each producing roughly 715 MW12.9 GW total output!

(700 m3/s, effective height ~110m for each turbine)

http://www.solar.coppe.ufrj.br/itaipu.html

Residential Electric Power Bill

$145.6/1948 kWh = $0.0747/kWh

Avg. Power: 1948kWh/31day.24h/day = 2.6 kW.

Commercial Power Costs

Energy cost estimates for a project out at IUCF: use (i.e. energy) charge: $3105/(360hr*575kW)=$0.0147/kWh peak use (i.e. maximum power demand) charge: $13.11/kW Why is this different from the residential calculation?

DVB:Energy can be neither created nor destroyed;It can only be changed from one form to another

First Law of Thermodynamics(generalized energy conservation)

Quantify:

W + Q = PE +KE + TE

Work+Heat exchanged= Change in (PE, KE and Thermal E)

The rub, as we shall see, is that not all forms ofenergy are equally useful. In particular, thermal Energy is very easy to produce, but difficult to use.

Energy Conversions (Table 2.2)

AutomaticallyHappens!

digestion

thermal

A key question to consider is with what EFFICIENCY can each of theseConversions be accomplished (Useful energy output/ total energy input)

EfficiencyEfficiency = (useful E transferred)/(total E input)

Device Transfer effected Efficiency

Generators Kinetic-Electrical 70-99%

Elec. Motors Elect.-Kinetic 50-90%

Transformer Electrical-Electrical 95-99%

Gas Furnace Chem.-Thermal 70-95%

Wind Turbine Kinetic-Electrical 35-50%

Power Plant Nucl/Chem-Elect. 30-40%

Fuel Cell Chem-Electrical 50-70%

Solar Cell Radiant-Electrical 5-28%

Fluoresc. Light Electrical-Radiant 20%

Incandescent light Electrical-Radiant 5%

Energy Losses in a Carhttp://www.fueleconomy.gov/feg/atv.shtml

NOTE: only 13% gets to the wheelsand ALL of that goes to thermalenergy (eventually)

http://www.fueleconomy.gov/feg/hybrid_sbs_cars.shtml

See also:

Efficiency example (H&K p75)

Not all hybrids are created equal!

http://www.fueleconomy.gov/feg/hybrid_sbs.shtml

BMW hydrogen sports car

“Our long-term EfficientDynamics strategy can be summed up with one vehicle: the BMW Hydrogen 7. As the world's first luxury performance sedan with hydrogen drive, it runs on the most plentiful element in the world and emits virtually nothing but water vapor. And because the infrastructure for refueling a hydrogen internal combustion engine is not yet complete, the V-12 engine also runs on gasoline at the push of a button, though emissions will result. “

http://www.bmwusa.com/uniquelybmw/EfficientDynamics?PANELID=4

Coal-fired Electric Power Plant

Lighting: Overall efficiency

Incandescent light!

The Electromagnetic Spectrum

DVB:Energy can be neither created nor destroyed;It can only be changed from one form to another

First Law of Thermodynamics(generalized energy conservation)

Quantify:

W + Q = PE +KE + TE

Work+Heat exchanged= Change in (PE, KE and Thermal E)

The rub, as we shall see, is that not all forms ofenergy are equally useful. In particular, thermal Energy is very easy to produce, but difficult to use.

EfficiencyEfficiency = (useful E transferred)/(total E input)

Device Transfer effected Efficiency

Generators Kinetic-Electrical 70-99%

Elec. Motors Elect.-Kinetic 50-90%

Transformer Electrical-Electrical 95-99%

Gas Furnace Chem.-Thermal 70-95%

Wind Turbine Kinetic-Electrical 35-50%

Power Plant Nucl/Chem-Elect. 30-40%

Fuel Cell Chem-Electrical 50-70%

Solar Cell Radiant-Electrical 5-28%

Fluoresc. Light Electrical-Radiant 20%

Incandescent light Electrical-Radiant 5%

Lighting: Overall efficiency

Incandescent light!

Roughly 1/3 of the residential/commercial energy sector goes to providing lighting; there are gains to be made here!

Black Body Spectra

0.00E+00

5.00E-12

1.00E-11

1.50E-11

2.00E-11

2.50E-11

0 500 1000 1500 2000 2500

Wavelength (nm)

Inte

nsi

ty

Series1

Series2

Light from hot objects

VisibleUV IR

6000K

2500K (x10)

Lighting efficiency comparison

http://www.mge.com/home/appliances/lighting/comparison.htm

LED’s(white)

Light Emitting Diodes (LED)

http://alt-e.blogspot.com/2005/04/energy-efficiency-led-lights-to.html

$58/bulb

Consumes 6 W; “replaces” 50 W incandescent bulb

http://store.starrynightlights.com/ledlightbulbs.html

LED’s are best in cases where you want directional lighting (e.g. “task lighting”, or stop lights), since they emit a beam (unlike most other lights that emit in all directions)! They are not yet ready for high-luminosity applications. (e.g. street lights) due to power limitations.

What is Temperature?

• It is the quantity that determines the direction of heat flow (from higher Temperature to lower temperature) when two objects are brought into thermal contact. (“zeroth law of thermodynamics”)

• For gases under normal circumstances this is related to the average energy per molecule, BUT this relationship is not valid in all circumstances nor for all materials.

Example (H&K p95)

You can heat up a beaker of water (or anything else)by either doing work (friction converts work to thermal energy)or by “heat transfer”

Generalized Energy conservation (H&K p79)

Energy OUTEnergy IN

Energy Stored

Specific Heats• Ice (near 0oC) 2090 J/kg.K=2090 J/kg.Co

• Water (near 0oC) 4186 J/kg.K=1000 cal/kg.Co

• Aluminum 900 J/kg.K• Copper 387 J/kg.K

This is how thermal energy and temperature are related!!

Temperature Scales:• 0oC = 273.15 K ice melts at this temp• 100oC = 373.15 K water boils at this temp• 0 K: absolute zero, the coldest temperature possible.• Note that 1Co = 1 K. The Celsius degree and the Kelvin

are the same size! The only difference in the scales is the position of the origin, hence the two scales are equivalent for talking about temperature DIFFERENCES, but not for talking about absolute temperatures.

Phase transitions in water