Energy, Power, and Climate Change

Chapter 7.1 Energy degradation and power generation

Energy Degradation

Sankey Diagram of energy flow in a flash light.Degradation of Energy – As energy is transformed from on type to another there is energy lost in the form of heat. This energy becomes less useful and is therefore degraded.

The width of the arrow is proportional to energy flow.

Efficiency, e = work/input energy

If the input is 100 J and the useful work that came from the transfer is 5 J, then e, is 5%.

Energy Degradation

Can you think of three forms of degraded energy?

1. Heat2. Sound3. Deformation

Does this violate conservation of energy?

US Annual Energy Flow

History of Energy

Civilization has increased the demand for energy per person from 8 MJ per day to 300 MJ per day. This is an increase of 3750 %.

As population increases this equates to an enormous demand for energy.

Annual world production of energy is at 1.5 x 10^21 J

Where do we get that energy from?

Energy Sources

Renewables Non-renewables

Infinite sources of energy.

Solar, Wind, Wave, Tidal, Geothermal

Finite sources which are being depleted and will run out.

Fossil Fuels, Nuclear Fuels,

Facts About Fuels

Type of fuel Renewable? CO2 emissions?

Fossil fuels No Yes

Nuclear No No

Hydroelectric Yes No

Wind Yes No

Solar Yes No

Wave Yes No

Energy Density

Fuel Energy Density (MJ/kg)Fusion fuel 300,000,000

Uranium-235 90,000,000

Gasoline (Petrol) 46.9

Diesel 45.8

Biodiesel 42.2

Crude oil 41.9

Coal 32.5

Sugar 17.0

Wood 17.0

Cow dung 15.5

Household waste 10

Energy Density – The energy that can be obtained per unit mass. The unit is in Joule/Kg.

E=mc2 is used for nuclear fuels.

Potential Energy is used to determine energy density of hydroelectric plants.

Fossil Fuels

Created over millions of years through natural decomposition of plant and animal matter.

By product is CO2 and H2O.Used for transportation and electricity

production.Coal burning power plants are 30%

efficient.Natural Gas is typically 42% efficient.

Fossil Fuel

1. Fossil Fuel is burned in a furnace.

2. This heats water which turns to steam.

3. Steam has pressure to spin a turbine.

4. Turbine converts rotational energy in to electrical energy.

Nuclear Fission

U 238 is most abundant isotope of Uranium found in nature.

U-235 is the isotope (.7% found naturally) used as fission fuel source.

Uranium is enriched until 3% of U-235 is present.

This creates critical mass for a sustainable reaction.

Nuclear Fission

Fission: splitting a heavy nucleus into lighter nuclei only

plutonium & some isotopes of uranium

Nuclear Fission

Fission - example◦235

92U + 10n → 236

92U → 14054Xe + 94

38Sr +210n +

energy

◦Fissioning 1.0 g of uranium-235 yields 1.2×1012 J, enough energy to keep 375 100-watt light bulbs lit for a year.

◦Super High Energy Density

Nuclear Fission

chain reaction: neutrons in products can cause more fission critical mass:

mass, density needed for self-sustaining chain reaction

Nuclear Fission

Fuel Rods and Control Rods are used to moderate the reaction taking place. (more fuel rods more reactions, and more control rods less reactions).

During the Uranium reactions Plutonium 239 is created. This does not occur naturally and is a fuel that can be used in nuclear weapons. (Hence the national security concerns for nuclear power).

Nuclear Fusion

Fusion – small atoms form a larger atom. 2

1H + 31H → 4

2He + 10n + energy

Both Deuterium and Tritium at these high temperatures are ionized and therefore positively charged.

Conditions must be met to overcome this awesome electric force.

Engineering monstrosities such as NIF and Tokamak have been built to do this.

Nuclear Fusion - Tokamak

Magnetic Confinement Fusion

Fusion Fuel is heated to a plasma and then accelerated with electric and magnetic fields to initiate fusion.

Fusion has occurred, but not enough to outweigh the energy put in to create it.

Nuclear Fusion NIF

National Ignition Facility at LLNL

Inertial Confinement Fusion

192 Lasers impact target simultaneously to produce environment consistent with the core of our sun.

Solar Power

Solar power harnesses the brightness of the sun… 1400 W/m2.

What assumptions go into this number?Luminosity of the sun is 3.9 x 1026 Watts.Active solar devices – Low reflecting

materials absorb heat in order to convert solar energy into hot water.

Photovoltaic cells – converts sunlight into direct current.

Solar Power

Solar energy is variable by about 5%

What could cause this?

Daily insolation- Total amount of energy received by one square meter in a day.

Hydroelectric Power

Power derived from moving water masses.

P = mgh/Δt = ρΔVgh/ Δt

Wind Power

P = Cp½ ρ A v3

ρ = air densityA = area spanned by bladesv = velocity of windCp = efficiency factor or power coefficient.

Wave Power

P/L = ρgA2v/2

P /L = power per unit lengthρ = density of waterg = gravityA = amplitude of wavev = waves velocity