Energy and Efficiency

Systems

• Open: matter and energy can be exchanged with the surroundings

• Closed: matter is not exchanged with the surroundings but energy is

• Isolated: matter and energy cannot be exchanged with the surroundings

1st Law of Thermodynamics

• Energy cannot be created or destroyed

• Energy can be changed from 1 form to another

• Supported by Joule’s experiments (pages 169, 170)

1st Law

• Whenever energy is changed from 1 type to another, heat is always produced

• Sometimes this not a good thing, sometimes it’s needed

• Examples?

2nd Law

• Heat naturally flows from hot objects to cold objects

• Work must be done to remove heat from objects that are cooler than the surroundings.

• Examples?

Hydroelectric dam

• What energy conversions take place in the power station?

Energy Conversions in a nuclear power plant

Coolant:

Heavy water,

H2O with 12𝐻

Efficiency

• 𝑒𝑓𝑓. =𝑢𝑠𝑒𝑓𝑢𝑙 𝑜𝑢𝑡𝑝𝑢𝑡 𝑤𝑜𝑟𝑘

𝑖𝑛𝑝𝑢𝑡 𝑤𝑜𝑟𝑘× 100%

• Automobile engine 15%-20%(internal combustion)

• Diesel engines 20%-45%(internal combustion)• Steam engine 35%• Steam turbine 40%• Electric motor 95%

WHY do we use

internal combustion

engines and not steam or

electric for cars?

Example

• A certain design of light bulb produces 25.9 J of light. If the bulb is 38.2% efficient, how much input energy is required to operate the bulb?

• 67.8 J

Heat Engines

• Use heat to produce mechanical energy

• Heat pumps uses mechanical energy to transfer heat

• Examples?

Heat Engines

• Newcomen Engine was the first practical device to harness steam to produce mechanical work

• The engine operated by condensing steam drawn into the cylinder, thereby creating a partial vacuum

• this allowed the atmospheric pressure to push the piston into the cylinder.

Watt Engine

• Improved efficiency over Newcomen engine

• Steam engines are large, noisy and inefficient (heat lost to surroundings)

Internal Combustion Engine

• Heat produced by burning fuel

• Smaller, more power because of the energy stored in the liquid fuel

Energy Demand

• Fossil fuel and nuclear still supply most of our energy

World Demand

• Why is the trend generally rising for energy sources?

Energy Alternative Sources

• Solar (photovoltaics)

• Wind

• Tidal/wave

• Biomass (burning fuels produced from waste animal & plant material)

• geothermal

Solar cells (photovoltaic cells)

• Approximately 1000 J/second of solar energy per square metre of Earth’s surface

• Problems with solar energy?

Solar Energy

• Store the energy in the form of molten salt

Biomass

• Converting waste plant/animal material to fuel

• Fuel could be solid, liquid (biodiesel, ethanol) or gas (CH4)

Geothermal

Wind Energy

• Wind is caused by the uneven heating of Earth’s surface

Available Energy During a Four-Day Period

Which of the following electricity-generating technologies most likely produced the graph shown?

A. Photovoltaic panel

B. Hydroelectric dam

C. wind power plant

D. Natural-gas power plant

E. Geothermal

• Read p 192 to 195