Nuclear Fission - Review

Fission The Uranium 236 is very unstable and splits into two smaller nuclei (this is called nuclear fission)

Chain Reaction If there is enough uranium (critical mass) a chain reaction occurs. Huge amounts of energy are released very quickly.                                                                                                                                    

Bang! This can result in a nuclear explosion!

Controlled fission The chain reaction can be controlled using control rods and a moderator. The energy can then be used (normally to generate electricity).

Moderator This slows the free neutrons down, making them easier to absorb by the uranium 235 nuclei. Graphite or water is normally used. 1 eV neutrons are ideal)

Control rods These absorb excess neutrons,making sure that the reaction does not get out of control. Boron is normally used.

Heat The moderator gets hot from the energy it absorbs from the neutrons.

Heat This heat is used to heat water, to make steam, which turns a turbine, which turns a generator, which makes electricity.

Useful by-products Uranium 238 can also absorb neutrons to produce plutonium 239 which is itself is highly useful as a nuclear fuel.

Nuclear Power That’s how a nuclear power station works!

Nuclear power - Advantages High power output Large reserves of nuclear fuels No greenhouse gases

Nuclear power - disadvantages Waste products dangerous and difficult to dispose of Major health hazard if there is an accident Problems associated with uranium mining Nuclear weapons

Solar power

The solar constant

The solar constant The sun’s total power output is 3.9 x 1026 W!

I is the power per unit area The solar constant The sun’s total power output is 3.9 x 1026 W! Only a fraction of this power actually reaches the earth, given by the formula I = P/4πr2 I is the power per unit area For the Earth this is 1400 W.m-2 and is called the solar constant This varies according to the power output of the sun (± 1.5%), distance from sun (± 4%), and angle of earth’s surface (tilt)

Solar power - advantages “Free” Renewable Clean

Solar power - disadvantages Only works during the day Affected by cloudy weather Requires large areas Initial costs are high Difficult to scale

Hydroelectric power

Water storage in lakes “High” water has GPE. As it falls this turns to KE, turns a turbine etc.

Pumped storage Excess electricity can be used to pump water up into a reservoir. It acts like a giant battery.

Tidal water storage Tide trapped behind a tidal barrage. Water turns turbine etc.

Hydroelectric - Advantages “Free” Renewable Clean

Hydroelectric - disadvantages Very dependent on location Drastic changes to environment (flooding) Initial costs very high

Wind power

Wind power Calculating power

Wind moving at speed v, cross sectional area of turbines = A A Cylinder of Air A

Wind moving at speed v, cross sectional area of turbines = A Volume of air going through per second = Av Mass of air per second = Density x volume Mass of air per second = ρAv V A

Wind moving at speed v, cross sectional area of turbines = A Mass of air per second = ρAv If all kinetic energy of air is transformed by the turbine, the amount of energy produced per second = ½mv2 = ½ρAv3 V A

Wind power - advantages “Free” Renewable Clean Ideal for remote locations

Wind power - disadvantages Works only if there is wind! Low power output Unsightly (?) and noisy Best located far from cities High maintainance costs

Wave power

OWC Oscillating water column

Power per unit length A water wave of amplitude A carries an amount of power per unit length of its wavefront equal to P/L = (ρgA2v)/2 where ρ is the density of water and v stands for the speed of energy transfer of the wave

Wave power - Advantages “Free” Reasonable energy density Renewable Clean

Wave power - disadvantages Only in areas with large waves Waves are irregular Low frequency waves with high frequency turbine motion Maintainance and installation costs high Transporting power Must withstand storms/hurricanes