Energy, Power, and Climate Change Topic 8

Energy conversions

!   Energy transferred from one form to another (conservation of energy)

!   Can an engine work endlessly?

Energy conversions

!   Energy transferred from one form to another (conservation of energy)

!   Can an engine work endlessly? – No

!   This is because some energy has to be trasnfered to the surrounding from the system as thermal energy

!   This energy can’t be harvested. It can’t do work.

!   Let’s look at this from a microscopic view. Open to page 170 in purple book.

Energy conversions

!   This unavailable energy is known as degraded energy.

!   Definition: thermal energy transferred to the surroundings that is no longer available to produce useful work.

Energy conversions

Energy conversions !   All energy eventually turns to heat

!   Eventually everything will be the same temperature… end of the universe???

Sankey diagrams

!   Used to represent energy conversions

!   Arrow represents conversions

!   Width power or energy consumed at the stage

!   Degraded energy is shown with an arrow up or down

Sankey diagrams

!   Examples on page 171 in purple book

Question

!   Draw a Sankey diagram for: !   A) an electric light bulb

!   B) a bicycle dynamo producing electricity to illuminate a light from mechanical energy

Question !   Draw a Sankey diagram for:

!   A) an electric light bulb

!   B) a bicycle dynamo producing electricity to illuminate a light from mechanical energy

Electrical generators

!   Converts mechanical energy to electrical

!   When charged particles move in an magnetic field, they produce a force

!   Explain what happens when the wire moves upward.

Electrical generators !   Electrons going up experience a force

!   Force will make them move to one side of the wire

!   Causing a potential difference and a current will flow.

!   In generators, coils are used instead of straight wires.

Generating electricity

!   Coal – fired power station !   Fuel is used to release thermal energy

!   This thermal energy is used to boil water to make steam

!   Steam used to turn turbines

!   Motion of turbine used to generate electricity

!   Transformers alter the potential difference.

!   Open to page 66 in study guide

Coal – fired power station

Generating electricity

!   Look at the Sankey diagrams of electricity generation on P.180 and 181 in purple book and 66 in study guide

Sources of energy

!   Renewable VS non-renewable !   Renewable: energy sources that can’t be used up

!   Non-renewable: energy sources that can be used up and eventually run out

!   Open to page 67 in study guide

!   Homework: read and summarize pages 173 – 177 in ur purple book.

Energy density !   Useful comparison between fuels

!   Definition: energy liberated per kg

!   Fuel choice is influenced by energy density.

!   A fuel with high energy density costs less to transport

!   The greater the mass of the fuel that needs to be transported, the greater the cost

!   Table of energy densities on page 67 in study guide.

Nuclear power

!   What happens in a fission reaction?

Nuclear fission !   What happens in a fission reaction?

!   Large nuclei split into smaller nuclei

!   173 MeV is released in this reaction. How did we calculate that?

!   How much Joules is that?

!   This amount of energy is just for 1 atom.. Do you usually only have 1 atom?

Nuclear chain reaction

!   To split nucleus, energy is required to overcome strong nuclear force

!   This energy can be supplied by adding a neutron to the uranium nucleus.

!   Binding energy is now increased and nucleus can’t get rid of energy, so it splits into two

!   New neutrons are now released and can be captured by other uranium nuclei.

!   This is what we call a chain reaction

Nuclear chain reaction

Factors affected chain reaction !   Speed of neutron

!   KE about 1eV !   If too fast it will pass by nuclei without binding !   It is slowed down by moderator nuclei (small nuclei)

!   Mass of radioactive nuclei !   If too small neutrons will have to be slowed down a lot..

And by then the neutron would have left the reacting uranium

!   The minimum amount of mass is called the critical mass

Nuclear fuel

!   99.3% of natural uranium is U – 238 .

!   It’s a weak alpha emitter

!   It absorbs the neutron but doesn’t split (prevents chain reaction).

!   U – 235 makes the other 0.7% and is much useful as it produces a chain reaction and energy can be harvested.

!   Therefore U – 235 needs to be increased by process called enrichment.

Nuclear fuel

!   3% of U – 235 used to fuel a nuclear reactor

!   Removed U – 238 (so that to increase the proportion of U – 235) is called depleted uranium.

!   This depleted uranium is very tough and heavy and used for making radiation shielding and weapons (armour piercing ammo).

Nuclear fuel

! Pu – 239 can be also used as a fuel for energy or in the manufacturing of weapons

Nuclear fuel !   fuel made from small cylinders stalked together to

make fuel rods

!   Fuel rods are bundled together

!   A reactor will contain many of these bundles

Nuclear reactor !   Three important components:

!   Moderator !   To slow down neutrons

!   Control rods !   Easily absorbed by neutron !   Can be added or removed to control chain reaction

!   Heat exchanger !   Reactor is in a sealed area !   The temperature increase because of reaction !   This thermal energy is transferred to water. !   The steam produced will turn the turbines and produce

electricity

Nuclear reactor !   Description

is on page 185 in purple book

Nuclear reactor

!   Look at Sankey diagram on page 69 in study guide

Controlling production

!   Controlling the number of neutrons

!   U -238 will absorb some of the neutrons therefore lower the reaction rate (not easily changed so hard to control)

!   Introduce rods (Boron) that are neutron absorbing.

!   Loss of control: mix of U – 235 with moderator (ATOMIC BOMB)

Advantages VS disadvantages

Advantages Disadvantages

Extremely high energy density

Process produces nuclear waste that is just stored

Reserves of uranium large compared to oil

Large risk if anything should go wrong

Nonrenewable (nonrenewable but lasts a long time)

Problems with nuclear energy

!   Extraction of uranium !   Radiation exposure to mine workers

!   Another method: leachingà solvent used to dissolve uranium and then pumped out à this though leads to groundwater contamination

!   Meltdown !   Overheat of fuel rods causing them to melt

!   Fuel then can’t be removed and pressure vessel will burst (Chernobyl)

!   Meltdown can be caused by malfunction in cooling system or leak in pressure vessel.

Waste

!   Low level waste !   Extraction, fuel enrichment, heat transfer from fuel rods

all leave some traces of radioactive material !   Material disposed should be in places away from human

contact for 100 – 500 years !   Old reactors – can’t be touched for many years then it is

demolished or kept in concrete.

!   High level waste !   Disposal of fuel rods (half life for thousands of years) !   Currently they are disposed of in:

!   water: to cool off then they are sealed in steel cylinders !   Reprocessed to remove uranium and plutonium.

homework

!   Solve questions 6 – 9 on page 187

!   Read about fusion on pages 187 – 189

!   Watch videos on website

Solar power !   Two ways to harvest radiated energy from the sun:

!   Photovoltaic cell

!   Active solar heater

Photovoltaic cell !   Solar energy à electrical energy

!   Uses semiconductors to do this

!   Produces small potential difference and not much current (used for low energy devices)

!   Use in series: more voltage / use in parallel: high current

Active solar heater

!   Also know as solar panel

!   Solar energy à thermal energy

!   hot water is typically produced and would cut down on use of electricity

Advantages VS disadvantages

Advantages Disadvantages

Very ‘clean’ production (no harmful chemical by-products)

Can only be used during the day

Renewable source Unreliable (bad weather)

Free Low energy density

Solar constant !   Amount of energy that

falls per second on an area of 1 m2 above the Earth’s atmosphere that is at right angles to the sun’s rays.

!   About 1400 Wm-2

!   Look at diagram on page 71 in study guide

!   Different parts of the earth (different latitudes) will receive different solar radiation

Inverse square law of radiation

!   The longer the distance from radiation point source à the more area it will cover à less power is received.

!   What happened when you doubled the distance?

Inverse square law of radiation

!   Surface area of sphere is 4πr2

!   Intensity, power received per unit area, is:

!   I inversely proportional to r2

!

I =P4"r2

Wave power

!   Uses kinetic energy of the wave to generate electricity

!   Two main techniques: !   Oscillating water column (OWC)

! Pelamis

!   (let’s watch the videos)

!   Open to page 72 in study guide

Hydroelectric power

!   GPE water à KE water à KE turbines à electrical energy

!   Water can gain its PE from: !   Water cycle: rain stored in reservoirs as high up as possible

!   Tidal power: trap water at high tides and release at low tides

!   Water pumped up from low reservoir to high reservoir. They can get this energy from coal power station when coal power station is shut and excess heat can be used to pump water up.

Advantages VS disadvantages

Advantages Disadvantages

Very ‘clean’ production (no harmful chemical by-products)

Only in certain areas

Renewable source Construction of dam will need land being buried under water

Free

Same as solar and wind (next)