TOPIC 8
ENERGY PRODUCTION
NEW
SL and HL CORE
1
Read Tsokos pp. 415 – 429 ( also scanned on Moodle)
and do the following questions.
May need to use internet sources outside the book to
answer some of the questions.
Units : Power, J ,MJ, W, MW kWh, eV = 1.6 x 10-19 J
Power = Work = energy per second
t
P = Fd
t
P=Fv
v =d
t
Watt ( W) = Js-1
1 MJ = 1 x 106 J ( 1 million J)
1 eV = 1.6 x 10-19 J
1MeV = 1 million eV
1 MW = 1 x 106 W ( 1 million W)
1GW = 1000 MW
The Kilowatt hour:
Defined as the energy used in kilowatts in one hour.
1kW = 1000 W = 1000 Js-1
1kWh = 1000 Js-1 x 3600 s ( in 1 hr) = 3.6 x 106 J
1kWh = 3.6 x 106 J
IN Data Booklet
2
Example 1
A power plant produces 500 MW of power.
a) How much energy is produced in one second? Express your answer in (i)
j (ii) kWh
b) How much energy in joules is produced in one year?
Check answers:
a) 5 x 108 J , 140 kWh
b) 1.6 x 1016 J
1. Energy degradation and Power generation Tsokos
pp. 415-433
Energy degradation, production and sources pp. 415 – 418
Read and do the following. May need to use internet sources outside the
book to answer some of the questions.
1. Definitions :
a) Sankey diagram – diagram that represents energy flows
b) Energy degradation – excess energy lost and is “ less useful” and can not
be used to perform mechanical work
c) Non- renewable energy source ( give 2 examples)– finite sources that will
run out e.g : fossil fuels, nuclear
d) Renewable energy source ( give 3 examples ) - energy that can renew
itself eg. : solar, wind, wave - tidal, geothermal, hydroelectricity
e) Energy density ( state unit) – energy , in joules (J), that can be obtained
from one cubic meter m3 : Jm-3 . High energy density = high power output
f) Specific Energy – energy , in joules (J), that can be obtained from one Kg
: Jkg-1 .
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2. Answer these questions
Example 2:
Degraded energy is energy that is
A. stored in the Earth’s atmosphere.
B. available from non-renewable energy sources.
C. converted into work in a cyclical process.
D. no longer available for the performance of useful work
Example 3:
Power plants
Efficiency = output energy
input energy
Power plant problems basically deal with how much useful energy that they can
produce ( output) to be used as electricity based on how much energy they are
using ( input) to produce that energy. Power plants input energy sources are
usually coal ( fossil fuels) , nuclear or hydro. ( water dams) and the challenge is
to get away from these and use renewable sources : solar, wind, geothermal and
more hydro.
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Example 4:
Explain fig. 1.1 below and why you only get 25% efficiency rating using the
formula for efficiency . Also, explain how much is considered degraded energy:
Efficiency = output energy
input energy
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Efficiency, output, input , rate and specific energy:

Need input energy in order to produce output energy and you never get
100 %. For example , nuclear reactors take huge amounts of energy from
nuclear reactions and take what’s left over ( output) to send to cities and
homes
Efficiency = output energy
Input energy
Rate = input energy
Specific energy


Rate = rate of input energy that is being used up to produce output
energy. For example the rate that a power station is using up its fuel,
natural gas, to produce electricity – its output energy.
Specific Energy – the amount of energy that can be obtained from 1 kg
of substance.
Example 5:
a) A power station has an output power of 500 MW and overall efficiency of
27%. It uses natural gas as a fuel. How much input energy is being consumed by
the natural gas?
b) Natural gas has an specific energy of 56 MJ kg-1. 1 MW = 1 MJs-1 .
Calculate the rate of consumption of natural gas in the power station in
kgs-1 by using the specific energy and the input energy ( in MW)
calculated in part a above.
Rate = input energy
Specific energy
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Rate
vs.
input
Both indicate how much energy is used over a certain period of time. The only
difference is the units : rate is in kgs-1 and input is in W = Js-1.
Example 6:
A power plant produces electricity by burning coal, using the thermal energy
produced as input to a steam engine, which makes a turbine turn, producing
electricity. The plant has a power output of 400 MW and operates at an
overall efficiency of 35%.
a) Calculate the rate at which thermal energy is provided by the burning
coal. i.e calculate the energy input that the burning coal provides for the
power plant.
b) Calculate rate at which coal is being burned in kgs-1. Specific Energy of
coal is 30MJkg-1.
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Example 7:
A coal burning power plant produces 1.0 GW of electricity. The overall efficiency
of the power plant is 40%. Taking the specific energy of coal to be 30 MJkg-1 ,
calculate the amount of coal that must be burned in one day.
Ans. : 7.2 x 106 kg day-1
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KNOW BASICS of STEAM and COMBUSTION ENGINES
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2. Fossil Fuels, Nuclear Power, Solar Power, Wind Power, Wave
Power pp. 418 – 433.
1. Fossil Fuels
Example 8:
This question is about power generation.
(a)
Describe the origin of fossil fuels.
(b)
An electrical power generating station using fossil fuels as its source of
energy has an output of 2 GW. It has been suggested that this station
should be replaced by wind turbines, each providing 0.8 MW of
electrical power.
(i)
State two advantages of the use of wind power ( Can not simply
say pollution free).
(ii)
State and explain two disadvantages of using wind turbines to
replace the fossil-fuel generating station.
(c ) List 4 advantages of fossil fuels
(d) List 3 disadvantages of fossil fuels
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2. Nuclear Power pp. 420 – 423
Fuel is typically U – 235
n1 + 235 U  236 U  140 Xe + 94 Sr + 2 1 n
0
92
92
54
38
0
Neutron must bombard U 235 to initiate rxn. The rxn is self sustaining and is called a
chain rxn. For rxn to keep going a certain minimum number of U 235 must be present
otherwise the neutrons escape without causing further rxns. – this is called critical mass.
U 235 will only catch the neutrons if they are not moving too fast. The neutrons that are
produced during the rxn are too fast so they have to be slowed down.
The Moderator, Control Rods and Coolant:
Slowing down the neutrons is achieved by collisions of neutrons with a moderator which
is a material surrounding the fuel rods. The moderator is usually water or graphite. The
fuel rods are the tubes that contain 235. NOTE: if water is the moderator, do not confuse
with the other water that is used as a coolant. The moderator does not cool down the
reaction AND the coolant does not slow down the reaction
If the rxn gets out of hand, i.e too many neutrons flying around, control rods are used. These
absorb excess neutrons when the rxn begins to get out of hand and are introduced by the
engineer- tech. when needed. An atomic bomb has no control rods.
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Example 9 : This question is about nuclear power.
(a)
A fuel often used in nuclear reactors is uranium. Explain why uranium is a
non-renewable energy source.
(b)
One nuclear reaction that takes place in a reactor is
1 n + 235 U  236 U  140 Xe + 94 Sr + x 1 n
0
92
92
54
38
0
(i)
State the number x of neutrons produced in this reaction.
(ii)
Using the equation explain what is meant by a chain reaction.
(iii)
Explain how, in a reactor, the production of energy in a chain reaction
is controlled.
(c)
Outline how the energy produced in fission reactions is transferred to thermal
energy.
(d)
State one advantage of nuclear power production compared to fossil fuel
power production.
(e)
When a uranium nucleus fissions, approximately 180 MeV of energy is
released. The overall efficiency of a nuclear reactor is 23 and its output
power is 450 MW. Calculate the number of fissions required per second:
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Example 10:
Example 11:
Example 12:
Critical mass refers to the amount of fissile material that
A. will allow fission to be sustained.
B. is equivalent to 235 g of uranium.
C. will produce a growing chain reaction.
D. is the minimum mass necessary for fission to take place
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Example 13:
The commercial production of energy by nuclear fusion is not yet possible mainly due to
difficulties with
A.
obtaining plentiful supplies of a suitable fuel.
B.
reaching the high temperatures required.
C.
confining the hot plasma.
D.
disposing of the radioactive waste products.
Example 14 :
List 3 advantages and 4 disadvantages of nuclear energy :
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3. Solar Power pp. 423 – 425
1. Define and explain :
a) Active solar devices :
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b) Photovoltaic cell :
List 3 advantages and 4 disadvantages of solar energy :
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Example 15:
This question is about solar power.
(a)
Describe, in terms of energy transformations, the difference between a
photovoltaic cell and an active solar heater.
(b)
A photovoltaic cell of area 6.5  10–4 m2 is situated on the roof of a house.
The cell has an efficiency of 8%. At a time when the power of the solar
radiation incident on the photovoltaic cell is a maximum, the energy of a cell
delivers an input power of 47 W .
(c)
(i)
Calculate the amount of power output per square meter of 1
photovoltaic cell delivered after loss due to the efficiency rating.
(ii)
State one reason why the power of solar radiation at any particular
region does not have a constant value.
A power of 30 kW is required to produce adequate hot water for the house.
(i)
Use the data from (b) to determine the minimum number of
photovoltaic cells required to generate this power.
(ii)
State and explain whether it is more practical to use photovoltaic cells
or an active solar heater to provide hot water for the house.
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4. Hydroelectric, Wind and Wave Power pp. 425-429
Hydroelectric
fig. 1.14
Power generated by hydroelectricity depends on how much water can drop
from a certain height over time. It requires large volume flow rates ( Q)
and large heights ( h) and basically depends on how fast and from how
high water flows and drops. Gravity and density are constant.
Power formula for Hydroelectricity
P = pQgh  units in W
p = density of water = 1000kgm-3
Q = volume rate in Ls-1 ( 1 L = 1kg)
g = gravity = 9.8 ms-2
h = height in m
Example 16:
Find the power developed when water in a stream with flow rate 50Ls-1 falls from
a height of 15m.
Check Ans. 7.4 MW = 7.4 x 106W
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Example 17 :
Example 18 :
List 3advantages and 3 disadvantages of hydroelectricity.
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Wind
Example 19 :
This question is about wind power.
The maximum theoretical wind power P for air of speed v moving normally
through area A where  is the density of air is given by
P
(a)
ρ Av 3
.
2
Units = W
(i)
Air of density 1.3 kg m–3 and speed of 9.0 m s–1 is incident on a
wind turbine having blades of diameter 15 m. Calculate the
maximum wind power incident on the turbine.
(ii)
State why it is impossible in practice to extract all of the power P
in (i) from the air.
(iii) State two reasons why wind turbines are not placed close to one
another.
Wave
List 3 advantages and 6 disadvantages of wave power.
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3.The Greenhouse Effect and Global Warming
pp. 433– 453 . Focus on these notes only.
1. Black body law / Stefan – Boltzmann Law: emitted
radiation
Any body in the universe will radiate energy in the form of electromagnetic
radiation.
The Stefan – Boltzmann law states that the amount of energy per second
or power (P) radiated by a body depends on the surface area (A),
absolute temperature (T), and the properties of the surface called
emissivity ( e) :
P = e σ AT4
T emp. Must be in K . If given in C convert to K :
K = C + 273
Where σ is the Stefan-Boltzmann constant = 5.67 x 10-8 Wm-2 K-4
Emissivity ( e) and Black body – do not confuse emit with reflect
Emissivity describes the ability of a body’s SURFACE to emit radiation.
Dark and dull surfaces emit better than light and shiny surfaces. The
black body is a theoretical body that is a “ perfect” emitter of radiation.
The values of e range from 1 to 0 with 1 being the perfect emitter. There
are no units ( similar to an index for example the index of refraction) Some
examples:
Black body
e=1
Ice
e = 0.1
( good emitter and absorber)
( not a good emitter but a good reflector)
Surfaces that are black and dull , as opposed to light and shiny, are also
good absorbers of radiation. Thus we wear dark clothes in the winter to
absorb the radiation. White T shirts are good reflectors of radiation which
is why people wear them during hot summer days (Arabians use white
clothing in desserts).
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Temperature and Wavelength
As temperature increases, energy increases, frequency increases and
wavelength decreases.
Questions
Example 20 :
By what factor does the power emitted by a body increase when the
temperature is increased from 1000 C to 2000 C ?
Check Ans. 2.5- 2.6 acceptable range correct
Example 21 :
By what factor does the rate of radiation from a body increase when the
temperature is increased from 500 C to 1000 C ?
Check Ans 1.8
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Intensity vs. Wavelength Graphs – Black Body Spectra
With increasing temperature the peak of the curve occurs at higher
intensities and slightly shorter wavelengths. This makes sense because
higher temperatures = higher frequencies and shorter wavelengths.
If you reverse this graph and put intensity on the x –axis and wavelength on
the y- axis. Black bodies with higher temperatures should peak at a shorter
wavelengths AT EVERY POINT ON THE GRAPH. SEE example 22 below:
Example 22
Two black bodies X and Y are at different temperatures. The temperature of body Y is
higher than that of body X. Which of the following shows the black body spectra for
the two bodies?
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2. The Albedo – α : radiation reflected or scattered
The albedo of a body is defined as the ratio of power of radiation
reflected or scattered from the body to the total power incident on the
body:
α = total scattered OR reflected power
total incident power
total reflected radiation = total incident – total absorbed
Bodies with high reflective surfaces have a high albedo. Snow, for example , has
an albedo of 0.85 ( no units = type of index) indicating that snow reflects most of
the radiation that hits it ( incident) whereas charcoal has a low albedo of only
0.04 , meaning that it reflects very little light incident on it. Albedo are always
lees than 1.0. Also remember that snow and ice are poor emitters of radiation
emissivity = 0.1
Example 23 :
Calculate the average albedo of earth if the average incident solar
radiation is 350 Wm-2 and the average absorbed radiation is 250 Wm -2.
Global Warming
A HIGH albedo, mainly due to the polar ice caps, is important to avoid
global warming. This means that the earth will reflect much of the
incoming radiation back out , If the polar ice caps melt the earth’s albedo
decreases.
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Example 24 :
The diagram below shows a simplified model of the energy balance for Earth.
The albedo of the Earth according to this model is equal to
A.
2
.
340
B.
100
.
340
C.
238
.
340
D.
240
.
340
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3. Greenhouse effect – Basics
The greenhouse effect is the warming of the earth. It is caused when short
wave UV radiation from the sun is transmitted in through the
atmosphere and other green house gases. However , the radiation is
reemitted as infrared radiation by the earth’s surface, which is absorbed
by various gases in the earth’s atmosphere. The infrared radiation is
then partly radiated back towards the surface of the earth. In other words
the UV radiation from the sun can get in to the earth’s atmosphere but the
infrared radiation can not get out.
The gases primarily responsible for this effect are : water vapor, carbon
dioxide , methane and nitrous oxide.
Global warming
Due to the greenhouse effect and human activities which have been increasing the
concentration of greenhouse gases the earth is undergoing global warming.
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The change in sea level is always varying but the present value is about 100m more
than the last ice age 18 000 years ago. Changes in sea level affect the amount of
water that can evaporate and the amount of thermal energy that can be exchanged
with the atmosphere. In addition, changes in sea level affect ocean currents. The
presence of these currents is vital in transferring thermal energy from the warm
tropics to colder regions.
Example 25 :
It is hypothesized that global warming may lead to significant changes in the average sealevel. This hypothesis assumes that
A.
average rainfall will increase.
B.
icebergs will melt.
C.
glaciers will melt.
D.
the rate of evaporation of seawater will increase.
OK
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