EES 119/219

Lecture 16

Energy sources

• Current energy sources

– Predominantly fossil fuels: 80à90 %

– Currently in use:

• Coal

• Oil

• Natural gas

– Future use: unconventional hydrocarbons

• Shale oil

• Tar sands

• Coalbed methane

• Gas hydrates

• Alternative Energy sources

– Nuclear Energy

• Fission – in use

• Fusion – under research

– Solar Energy

• Direct

• Photovoltaics

• Solar heating etc.

• Indirect

• Hydropower

• Wind

• OTEC etc.

• Biomass

– Geothermal

– Tidal

Nuclear Power

• Energy associated with nuclear bonds

• Nuclear Fission

– Thermal reactors – in use; limited potential

– Breeder reactors – not available, large potential

– Thorium reactors – not available, large potential

• Nuclear Fusion – very large potential, not ready

– Magnetic confinement

– Inertial confinement

Principles of nuclei (Fig. 1)

• Building blocks

– Proton

– Neutron

– Electron

• Z Proton number

• N Neutron number

• M Mass number

• E Electron number

Definitions

• Isotopes: nuclei which have the same number of protons, but differ in number of neutrons à different mass, same element àidentical chemical behavior

• Isobars: nuclei which have the same mass, but differ in composition of nucleus à same mass, different elements

• Isotones: nuclei which have same number of neutrons but differ in number of protons à different mass, different element

The atom

• Atom is made up of nucleus and surrounding electron shell

• Nucleus contains almost all of the mass

• Chemical reactions result by interaction of electrons

• Nuclear reactions involve the nucleus, which is positively charged

Binding energy changes with configuration of nuclides (Fig. 2)

Mass defect and binding energy

Example: ¹H + ¹n à ²H (=D)

m_H + m_n – m_D = Dm

1.007825 + 1.008665 – 2.014102 = 2.388x10^-3 amu

Observation: The mass of the building blocks is greater than the resulting nucleus

à Mass defect à Energy à E = mc²

The challenge in fusion

• Bring two nuclei together which have positive charges

• Steps:

– Remove electrons à Plasma

– Plasma is state of matter where electrons and nuclei are separated à needs very high temperatures (>10,000^oC)

– Inject kinetic energy to overcome Coulomb repulsion (Fig. 3)

Energy Barrier

• Energy Barrier ~ 5 keV

• E_i = 3/2 kT

• k Boltzmann constant: 8.125x10^-5 eV/deg

• à 40x10^{6 o}K needed

Current approach

• D-T reaction and breeding of tritium

• D+T à ⁴He + n + 17.6 MeV

• n + ⁶Li à ⁴He + T + 4.8 MeV

• n + ⁷Li à n + T + ⁴He – 2.7 MeV

• D in seawater: D/H = 1/6500

• Li: ⁶Li 7.4% ⁷Li = 92.6 %

Principal approaches in fusion

• Magnetic confinement – Tokamak Reactor (Fig. 4)

– Charged particles are deflected while moving through a magnetic field

– Build very strong, torroidal magnets which force charged particles to come together

• Inertial confinement – Laser approach (Fig. 5)

– Insert target gas into small container, surrounded by material which evaporates easily

– Bombard capsule with laser from all sides simultaneously

– Evaporation produces short pulses of extreme pressure on inside to cause fusion

Fusion - assessment

• Very large energy potential

• Plentiful source materials (D; Li) available

• Very demanding engineering challenges

– Magnetic confinement (Tokamak; ITER)

– Inertial confinement (Laser; NIF)

• Large facilities needed

• No radioactive end-product, but radioactive T used and high n flux à some radioactive by-product likely

• System under investigation, but not very close to being available à > 30 yrs away