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Environmental Physics
Chapter 3:
Energy Conservation
Copyright © 2008 by DBS
Introduction
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‘Energy Intensity’ (Btu/GDP ratio) – shows conservation trend since 1980’s
– High EI indicate high price of conversion of energy into GDP
– Low EI indicates a lower price of conversion
Shows that economy can grow without a commensurate increase in energy consumption
Figure 1.16: United States energy use (Btu) compared to
GDP over time, and their ratio.
Law of Conservation of Energy
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First law of thermodynamics - The total energy of a system can be increased by doing work on
it or by adding heat
W + Q = Δ(KE + PE + TE)
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When no work or heat is added (W + Q) = 0, we have a ‘closed system’
Δ(KE + PE + TE) = 0
The total amount of energy in an isolated system will always remain constant
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If our isolated system is the universe, then, law of conservation of energy follows:
Total energy in the universe is a constant and will remain so
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In short, the law of conservation of energy states that energy can not be created or destroyed, it
can only be changed from one form to another or transferred from one body to another, but the
total amount of energy remains constant (the same).
Law of Conservation of Energy
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Note that the “law of conservation of energy”, a scientific law is very different to “energy conservation” – reducing
energy use through the use of reduced activity or increased efficiency
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As an example of this law consider the “nosecracker”
Total mechanical energy is conserved, no energy is transferred to the system by work or heat,
Δ(KE + PE) = 0
Initial mechanical energy = final mechanical energy
Max
PEG
If the ball is released
from ‘A’ to swing A-B-CB-A, it will not go higher
than A on its return
PE →KE
In reality frictional forces at the pivot and air
resistance result in thermal energy loss –
stops the ball swinging
Max KE
PE = 0
Law of Conservation of Energy
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Since energy in an isolated system is not destroyed or created or generated, one might wonder
why we need to be so concerned since energy is a conserved quantity
The final result of most energy transformations is waste heat
Waste heat is not useful for doing work - Energy is said to be degraded
Energy Conversion Examples
Passive solar technologies convert sunlight into usable heat,
cause air-movement for ventilation or cooling, or store heat for
future use, without the assistance of other energy sources
Passive solar home, Oswego, New York. Thirty-five percent of the heating needs
are provided by solar energy.
Energy Conversion Examples
Change in energy of the system = Net energy added (energy in – energy out)
ΔE = Ein - Eout
e.g. passive solar house,
ΔE = 0,
Ein = Eout
Solar input =
(energy loss through walls +
energy stored in the materials of the house)
Figure 3.2: A passive solar energy house.
Energy in = energy out + energy stored.
Energy Conversion Examples
e.g. fossil-fueled steam power plant
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7.
Combustion of fuel in the boiler unit using air
Combustion generates heat that boils water into steam
(water gains TE) along with combustion gases
High temperature, high pressure steam drives a turbine
Turbine drives a generator for producing electricity
Steam leaves the turbine and passes through condenser
Condenser takes cold water and passes it through a
heat exchanger to condense the steam
Cold water becomes warmer water producing thermal
pollution
Efuel + Eair + Ewater in = Eelectricity + Ewater out + Ecombustion gases
Figure 3.3: Block diagram of a fossil-fueled electric generating station.
Energy in = energy out, because no energy is stored.
Energy Conversion Efficiencies
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Although energy is conserved processes are not 100% efficient
The efficiency (η) of an energy conversion process is defined as:
η = Eout/Ein x 100 %
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η = eta
The energy input that does not go into useful work goes into unusable energy forms
(such as waste heat)
e.g. Power plant:
Only a fraction of the chemical energy in fuel is transformed into electricity…
Efficiency = η = Eelectricity/Efuel x 100 % = 35 %
…while essentially all of the fuel’s chemical energy is converted into heat during
combustion, 65% of this heat is transferred to the water leaving the condenser and the
gases leaving the stack
Energy Conversion Efficiencies
e.g. light bulb is only 10% (η = 10/100 = 0.1) efficient
Eelec = Elight + Eheat
and
Eheat = Eelec – Elight
Where η = Elight/Eelec
 Elight = η Eelec
Eheat = Eelec – Elight = Eelec – η Eelec = (1 - η)Eelec
If Eelec = 60W,
Eheat = (1 – 0.1) Eelec = 0.9 x 60W = 54W
and Elight = 60W – 54W = 6W
Energy Conversion Efficiencies
Efficiencies range
from 5 to 95 %
The laws of
thermodynamics place a
limit on η
Energy Conversion Efficiencies
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Efficiencies can be multiplied for processes with several stages:
e.g.
coal → electricity
Transmsssion generator → home
electricity → light
= 35 % efficiency
= 90% (loss 10 %)
= 5 % (loss 95%)
Overall efficiency = ηgeneration x ηtrans. x ηconver.
= 0.35 x 0.90 x 0.05 = 0.016
 0.016 x 100% = 1.6 %
Figure 3.4: Calculation of the overall efficiency for a multistep process involves
multiplying the efficiencies of the individual steps.
Question 7
Table 3.5 examines the conversion
processes in an internal combustion
engine and an electric vehicle. For both
systems, calculate the overall
efficiencies.
Gasoline ICE = 10 %, EV = 18 %
Energy use in Developing Countries
Developed Countries
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Depends on muscle power!
¾ of the world’s population use ¼ of the energy
Major disparity in energy use per capita
US = 10 x France, 70 x Kenya
Figure 3.5: An interesting method of
pumping water in Burkina Faso, West
Africa. The boy is doing work using
his PE, gained by jumping in the air.
Developing Countries
Energy use in Developing Countries
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Since 1960, developing countries have quadrupled
their energy use and doubled their per capita use
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Caused massive environmental problems
Figure 3.6: Growth in energy use, GDP,
and population in developing countries:
1960–2000.
Energy use in Developing Countries
A rapidly growing economy in China has led to increased use of coal
combustion. About 75% of its electricity is generated with coal.
Pollution in
China and India
Huai River Basin
http://www.nytimes.com/packages/khtml/2004/09/11/int
ernational/20040912_CHINA_FEATURE.html
Energy use in Developing Countries
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Developing countries’ energy plans follow the example set by industrialized countries
Using technologies fueled by fossil fuels
China: coal (dirtiest fossil fuel) provides 70% of electricity (growing by 7% per yr)
Energy use in Developing Countries
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Industrialized countries are beginning to increase efficiency and move to renewable fuels,
developing countries are burning more and more fossil fuels
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In the next 30 years global population is expect to grow 33% from 6 to 8 billion primarily in
developing countries
Sustainable Development
Building a house from handmade mud and straw bricks.
A Barrel, A Calorie, A Btu? Energy
Equivalencies
British Thermal Units:
1 Btu = energy required to raise the
temperature of 1 lb H2O by 1 °F
Food Calorie:
1 food calorie = 1kcal = 1000
calories = energy required to raise
the temp. of 1 kg H2O by 1 °C
1 Btu = 252 calories = 1055 J
Where 1 cal = energy required to raise
the temperature of 1 g H2O by 1 °C
A Barrel, A Calorie, A Btu? Energy
Equivalencies
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e.g. Barrel of crude oil (42 gallons)
– Heat average home for 4 days to 20 °C when outside is 0 °C
– Refined into gasoline and run a car for 1300 km (780 miles)
Since different engines have different efficiencies far better to use ‘heating values’
Heating value is defined as
the amount of heat the fuel
could provide if completely
burned
A Barrel, A Calorie, A Btu? Energy
Equivalencies
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Heating value of coal, uranium, natural gas etc. can be equated to the heating value of so many
barrels of oil
MBPD = million barrels of oil per day
e.g. burning 500 x 106 tons of coal provides same energy as burning 6 MBPD of oil for a year
US consumption is 7 billion barrels per year or 20 MBPD
A Barrel, A Calorie, A Btu? Energy
Equivalencies
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G
Question
If one ton of bituminous coal is burned to generate electricity, how many kWh could be
produced if the efficiency of this conversion is 35 %?
One ton bituminous coal = 25 x 106 Btu (from Table 3.4)
Process is 35 % efficient:
η = Eout / Ein
Eout = 0.35 x 25 x 106 Btu = 8.9 x 106 Btu
= 8.9 x 106 Btu x 1 kWh
3414 Btu
= 2560 kWh
Summary
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Doing work on, or adding heat to an object increases an object’s total energy
First law thermodynamics:
W + Q = Δ(KE + PE + TE)
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Total energy of an isolated system is conserved
Energy in = Energy out + Energy stored in the system (for an isolated system)
Energy may be transformed from one form to another
Efficiency is the ratio of useful energy output to the energy input
Problems
2.
A ball of mass 0.5 kg is dropped from a height of 5 m. What is the greatest velocity it will have just
before hitting the ground?
PE = mgh = 0.5 kg x 9.8 m/s2 x 5 m = 24.525 J = 25 J
At ground PE = KE = ½ mv2
½ x 0.5 kg x v2 = 25 J
v2 = (2 x 25 J)/0.5 kg = 100 m/s
v = 10 m/s
Problems
3. How many pounds of coal has the same heating value as 20 gal gasoline?
From Table 3.4: 1 gal gasoline = 1.24 x 105 Btu
20 gal gasoline = 20 x 1.24 x 105 Btu = 2.48 x 106 Btu
From Table 3.4: 1 ton coal = 25 x 106 Btu
1 ton = 2000lb
2000 lb coal = 25 x 106 Btu
1 lb coal = 1.25 x 104 Btu
2.48 x 106 Btu
1.25 x 104 Btu / 1lb coal
= 198.4 lb coal
Problems
4.
A household furnace has an output of 100,000 Btu/hr. What size electrical heating unit (in kW)
would be needed to replace this?
1 watt = 3.41 Btu/hr
100,000 Btu/hr x 1 W
3.41 Btu/hr
x 1 kW
= 29.3 kW
1000 W
Problems
7.
If 60 % of US petroleum use goes for transportation, and 1 % of this goes for buses, how much oil
could be saved per year if all buses were electrified (assuming the electrical energy was
generated from non-petroleum resources)? What is this in MBPD?
1 % of 60% = (60/100) x 100% = 0.006 x 100 = 0.6 %
0.6 % of all oil consumed per year is for buses
From Appendix G (for 2003):
US oil = 39.07 x 1015 Btu x
1 bbl
5.8 x 106 Btu
= 6.74 x 109 bbl / yr
6.74 x 109 bbl / yr x (0.6/100) = 0.040 x 109 bbl / yr
0.040 x 109 bbl / yr = 1.1 x 105 bbl / day
1.1 x 105 bbl / day x
MBPD
= 0.11 MBPD
106 bbl / day