IB DP Physics: Topic 8. Energy production: 8.1 Energy sources: Study Notes

8.1 Energy sources

Essential Idea:
The constant need for new energy sources implies decisions that may have a serious effect on the environment. The finite quantity of fossil fuels and their
implication in global warming has led to the development of alternative sources of energy. This continues to be an area of rapidly changing technological innovation.

Understandings:

• Specific energy and energy density of fuel sources
• Sankey diagrams
• Primary energy sources
• Electricity as a secondary and versatile form of energy
• Renewable and non-renewable energy sources

Applications and Skills:

• Solving specific energy and energy density problems
• Sketching and interpreting Sankey diagrams
• Describing the basic features of fossil fuel power stations, nuclear power stations, wind generators, pumped storage hydroelectric systems and solar power cells
• Solving problems relevant to energy transformations in the context of these generating systems
• Discussing safety issues and risks associated with the production of  nuclear power
• Describing the differences between photovoltaic cells and solar heating panels

Data booklet reference:

• $$Power=\frac{energy}{time}$$
• $$Power=\frac{1}{2}A\rho v^3$$

Big Ideas

Most energy sources can be traced back the sun, our ultimate primary source

Energy sources must be compared based on many factors including energy density, cost, availability, politics, safety, and environmental impact

No energy source can be converted to electricity with 100% efficiency

All energy sources have advantages and drawbacks and it important to understand the complete picture

Every object with a temperature above 0 K emits thermal radiation

Radiation intensity is related to separation distance by the inverse square law (similar to force fields)

The Earth’s climate relies on a delicate thermal energy balance where total energy in equals total energy out

Global Energy Usage

 Rank Energy Source % 1 Oil 32% 2 Coal 28% 3 Natural Gas 22% 4 Biomass 10% 5 Nuclear 5% 6 Hydropower 2.5%

Efficiency

 Sankey Diagram Rules:Width of the arrow proportional to the amount of energy

Energy Density

 Definition Units Specific Energy Energy transferred per unit mass J kg-1 Energy Density Energy transferred per unit volume J m-3

Primary and Secondary Sources

 Primary Energy Sources Secondary Energy Sources Energy sources found in the natural environment(fossil fuels, solar, wind, nuclear, hydro, etc.) Useful transformations of the primary sources(electricity, pumped storage for hydro, etc.)

Fossil Fuels

 Number of years left in global reserves Coal ~100-150 years Oil ~50 years Natural Gas ~50 years
 Describe the process of Fracking: 1.      Drill hole into shale rock2.      Inject fracking fluid at high pressure to create cracks 3.      Extract newly released natural gas4.      Seal fracking fluid in the hole

Nuclear Power

 Why is the concentration of U-235 important?Only U-235 can undergo a fission chain reaction What is done with the nuclear waste?Stored on-site in spent fuel pools and/or concrete dry cask storage

 Moderator Control Rods Slows down neutrons to be absorbed by U-235Made from Water or Graphite (carbon) Absorbs neutrons to limit number of chain reactions Made from Boron

Renewable Energy

 Variable Symbol Unit Power P W Cross-Sectional Area A m2 Air Density ρ kg m-3 Air Speed v m s-1
 Data Booklet Equations:
 Photovoltaic Cells Solar Concentrator Solar Heating Panel Converts solar energy directly into electricity. Useful in solar panels on top of building or solar farms connected to the energy grid Mirrors focus sunlight onto a central tower. The high thermal energy is converted to steam and runs turbines to produce electricity Sun’s radiation is absorbed by black pipes that transfer thermal energy to the water flowing through them. Replaces hot water heater.
 Biomass Coal Geothermal Hydropower Natural Gas Nuclear Petroleum Solar Wind Renewable ✓ ✓ ✓ ✓ ✓ Produces CO2 ✓ ✓ ✓ ✓

Thermal Energy Transfer

 Conduction Convection Radiation Energy is transferred through molecular collisions Energy circulates through the expansion and rising of hot fluids Energy is transferred through electromagnetic radiation. Can travel through a vacuum
 Emissivity Black Body Radiation Sun ~1 An idealized object that absorbs all the electromagnetic radiation the falls on it Earth ~0.6 Black-Body 1

Power Emissivity

Variable Symbol

Unit

Power

P

W

Emissivity

e

Surface Area

A

m2

Temperature

T

K

Max Wavelength

λmax

m

 Data Booklet Equations:

Intensity

Variable Symbol

Unit

Data Booklet Equations:

Intensity

I

W m-2

Power

P

W

Area

A

m2

 Greenhouse Gases Positive Feedback Loop Negative Feedback Loop Water Vapor (H2O) Melting ice (decreases albedo) Cloud formation (increases albedo) Carbon Dioxide (CO2) Melting permafrost (releases methane) Increased photosynthesis (uses CO2) Methane (CH4) Rising ocean temp releases methane Climate Change leads to renewables

Power Delivered by Wind Generator:

The volume of air that moves through  the blades in a time t  is given by

V = Ad = Avt, where v is the speed of  the air and A = πr2.

The mass m is thus m = ρV = ρAvt.

EK = (1/2)mv2 = (1/2)ρAvtv2 = (1/2)ρAv3t.

Power is $$\frac{E_K}{t}$$  so that

$$\frac{E_K}{t}=\frac{1}{2} A\rho v^3$$
Where  $$A = \pi r^2$$

Sankey diagrams

Energy degradation in systems can be shown   with an energy flow diagram called a Sankey diagram.

Sankey diagrams show the efficiency of each energy conversion.

Suppose the actual energy values are as shown:

The efficiency of a conversion is given by

$$Efficiency =\frac{output}{Input}$$

For example, the efficiency of the first energy conversion (chemical to potential) is

efficiency = 80 MJ / 100 MJ = 0.80 or 80%.