Units of energy measurement and
conversion factors
Units of energy measurement
1 calorie 4.186 joules
1 kWh 3.66106
joules
1 thermie 4.1866106
joules
1 BTU (British Thermal Unit) 1.0556103
joules
1 quad 1015
BTU
1 barrel of oil 159 litres
1 US gallon 3.785 litres
Conversion factors
1 tonne of oil equivalent (LHV) 426109
joules
1 tonne of coal equivalent (LHV) 29.36109
joules
1000 m3
natural gas (LHV) 366109
joules
1 tonne liquid natural gas 466109
joules
1000 kWh (primary) 0.0857 TOE (hydro)
1000 kWh (primary) 0.26 TOE (nuclear)
1 tonne natural uranium (PWR) 4.261014
joules
Chapter 1
Basic concepts
Energy is necessary for all life and all economic activity. Like food and water,
energy is indispensable. It has played a fundamental role in the development of
civilisations. It has been the cause of war between peoples who have sought
throughout history to control access to energy resources. During the twentieth
century, easy access to abundant, cheap and concentrated sources of energy
enabled faster economic development. The discovery of electricity, a highly
convenient carrier of energy, revolutionised the use of energy, and it is almost
impossible to imagine a modern house without electricity. However, despite this
progress, a considerable proportion of humankind still cannot satisfy its energy
needs. Since earliest times, human’s energy needs have continually increased.
Today we live in a world where we are always needing more, although it is clear
that we are reaching the limits of this headlong rush for growth. We need to
resolve the thorny problem of how to make further progress without consuming
more energy.
Although everyone has a basic idea of what energy is, in reality it is not a
notion which is that simple to define. The concept has many faces, and the aspect
which interests us here is a particular one that we will now examine more closely.
1.1 What is energy?
Scientists maintain that the fundamental processes (physical phenomena, chemical
reactions, biological processes, etc.) that govern our macroscopic world
(of objects and living creatures) are ruled by a law in which a physical quantity,
which we call energy, is always conserved in an isolated system. This is a fundamental
law and is associated with the fact that the laws of physics do not
change with time. In simple terms, this property translates into the fact that the
results of an experiment do not depend on the time it was carried out provided
that it was done under exactly the same conditions.1
1
Space is also homogenous and isotropic, which leads, respectively, to two laws of conservation: the
law of impulsion and the law of kinetic momentum. The homogeneity (isotropy) of space means that
an experiment yields the same results if one makes a translation (or a rotation) of the experimental
system.
If energy is a physical quantity perfectly defined for the physicist, its dictionary
definition is much less clear. But what interests us here pragmatically is
that a system or a body possesses energy if it can produce work or heat.
According to this definition, petrol contains energy since we can use it to
propel a vehicle. But this same petrol, when burnt, can provide heat.
At the microscopic level, energy can be seen to exist in either an organised
or disorganised form. In the first case we call it work, in the second heat.
Heat represents the lowest form of energy, because it is distributed over all the
degrees of freedom of the system in question, and these are very numerous.2
The particularity of energy is to exist in different forms: mechanical, heat,
nuclear, etc., and it is very often necessary to convert one form of energy into
another. This is done with a certain yield and also loss. When energy passes
from a disorganised form (heat) to an organised form (work) the efficiency is
rarely good. It is through these transformations from one form to another that
humans recover a part of the energy that is exploited for their own needs. It is
therefore not the energy contained in a body that is interesting, but that which
is acquired through its transformation.
All forms of energy are therefore not of the same quality and cannot be
used efficiently to produce work. Thus, a heat source of 300 8C is much more
efficient in producing work than a heat source of 50 8C.
The use of energy enables humans to improve their welfare by helping
them to feed themselves, keep warm and so on. Primary energies are distinguished
from final energies. Primary energy has not undergone any conversion
between production and consumption.3
This is the case with oil, coal, natural
gas, hydropower, wood, solar and wind energy. The final energy delivered to
consumers can be used to meet energy or non-energy needs.
This distinction between primary and secondary energy can have consequences
for the evaluation and comparison of different energy sources as can
be seen in Figure 1.1. Thus nuclear energy and hydropower produce, on a global
scale, much the same quantity of electricity for the consumer. These two sources
have the same output, but statistics show us that nuclear energy actually makes
three times more primary energy than hydropower (Figure 1.1). The reason is
that hydropower is a primary energy and the electricity is produced with an
efficiency close to 100%. The electricity produced by nuclear power, which is the
energy liberated from uranium fission, on the other hand is not accounted for as
a primary energy. Since the efficiency of today’s power-generating stations is
around 33%, nuclear energy creates three times more primary energy than
hydropower, though the energy used by the consumer is the same.
2
In a small glass of water weighing 20 g, the number of degrees of liberty of the water molecules is
a multiple of the Avogadro constant (N = 6.02 6 1023
). There are therefore more than 1024
, that
is more than a million billion billion molecules. The heat contained in this water is spread between
all these degrees of freedom, and therefore each one possesses very little.
3
Crude oil is a primary energy, whereas the petrol or diesel obtained by refining are secondary
energies. Electricity produced by hydropower or photovoltaic panels is primary, whereas energy of
nuclear origin is secondary. Charcoal (secondary energy) is produced from wood (primary energy).
2 Energy
1.1.1 Units
The internationally accepted unit of energy is the joule (J). For macroscopic
transformations this unit is too small so one uses kilojoules (kJ) or megajoules
(MJ): 1 kJ = 1000 J and 1 MJ = 106
J. Table 1.1 lists the twenty SI prefixes
used to form decimal multiples and submultiples of units.
For energy released at the atomic, molecular or nuclear level, the unit used
is the electronvolt (eV) and its multiples. Its definition is: 1 eV = 1.6 6 10719
J.
Energy released in elementary chemical reactions is of the order of a few eV,
Coal
24.8%
Other renewables
0.5%
Biomass & waste
10.5%
Hydropower
2.2%
Nuclear
6.4%
Gas
20.5%
Oil
20.5%
Figure 1.1 World consumption of primary energy in 2004. Totals do not exactly
equal 100% because of rounding errors. [Source: IEA World Energy
Outlook 2006]
Table 1.1 SI prefixes
Prefix 6 by Symbol Prefix 6 by Symbol
yocto 10724
y yotta 1024
Y
zepto 10721
z zetta 1021
Z
atto 10718
a exa 1018
E
femto 10715
f pecta 1015
P
pico 10712
p tera 1012
T
nano 1079
n giga 109
G
micro 1076
m mega 106
M
milli 1073
m kilo 103
k
centi 1072
c hecto 102
h
deci 1071
d deca 101
da
Basic concepts 3
while that released in nuclear reactions is more than a MeV – more than a
million times greater. These elementary values may seem small, but there are a
large number of elementary reactions in the processes carried out at that scale.
For instance, 16 g of methane (1 mol), the basic ingredient of natural gas,
contains N = 6.02 6 1023
molecules.
Power is a quantity of energy by unit of time. The basic unit is the watt
(1 W = 1 J/s). In the energy field, the megawatt (1 MW = 106
W), gigawatt
(1 GW = 109
W) and terawatt (1 TW = 1012
W) are often used.4
In the field of electricity, the usual energy unit is the watt-hour (Wh) and
its multiples. The watt-hour represents an energy of 1 J/s during 1 h. Thus,
1 Wh = 3600 J and 1 kWh = 3.6 6 106
J. The kWh which is a measure of
energy must not be confused with the kW which is a unit of power. Also used
are the MWh (1 MWh = 106
Wh), the GWh (1 GWh = 109
Wh) and the TWh
(1 TWh = 1012
Wh).
1.1.2 Equivalences
To compare different sources of energy it is usual to use as a yardstick the
energy provided by crude oil. The conventional unit is the tonne of oil
equivalent (TOE), the value of which is fixed at 1010
cal (1 cal = 4.18 J) &
42 GJ ( 11,700 kWh) [3]. In fact the equivalence 42 GJ ^ 11,700 kWh corresponds
to a direct conversion between the joule and the kWh, with an efficiency
of 100%. In practice, for the conversions made the efficiency of the
generating station is factored in, which leads to 1 MWh = 0.26 TOE when
electricity is produced by a nuclear power station (33% efficiency), 1 MWh =
0.86 TOE for a geothermal power station (10% efficiency) and 1 MWh = 0.086
TOE for electricity produced by a thermal power station or the photovoltaic
process, etc. (100% efficiency). This explains why 1000 kWh of electricity
represents 0.0857 TOE when produced by hydropower and 0.26 TOE when
produced by nuclear reactors, and hence the factor of 3 as mentioned earlier.
When dealing with combustion, one may refer to lower heating value (LHV)
or higher heating value (HHV). HHV includes the latent heat of the water
vapour produced during combustion whereas the LHV excludes it. As this latent
heat is not usually recovered in current processes, LHV is usually used and the
TOE is defined according to this convention. The calorific value of crude oil
varies slightly between one source and another; it also differs for the various
refined petroleum products (1 tonne of petrol = 1.048 TOE, 1 tonne of liquefied
petroleum gas (LPG) = 1.095 TOE, 1 tonne of heavy fuel oil = 0.952 TOE [3]).
Coal has a lower calorific value than oil, typically between 0.6 and
0.75 TOE depending on its quality (steam coal, coke, anthracite, etc.). Sometimes
the measure of TCE (tonne of coal equivalent) is conventionally set at
4
The horse power (1 HP ¼ 736 W) is an old unit introduced by Watt and now out of use. It was
based on the work that a vigorous horse could carry out, since the power of one horse power is
roughly equivalent to the work of three normal horses [2]. At the end of the nineteenth century,
16,500 horses were needed to work the 38 tramlines of Paris.
4 Energy
1 TCE = 0.697 TOE [3]. Natural gas has a calorific value slightly higher than oil,
1 tonne of liquefied natural gas (LNG) being equivalent to 1.096 TOE (1000 m3
is equivalent to 0.857 TOE) but 1000 m3
of natural gas equivalent to 0.857 TOE.
1.2 Energy and development
Energy has always played a major role in human development. Energy consumption
increased rapidly following the industrial revolution, particularly
during the twentieth century, but this is a much faster process than that which
our ancestors experienced in the past. For thousands of years humans were
happy with a power of a few hundred watts, at first with their own physical
strength then exploiting the strength of their domestic animals. Let us briefly
recall the first stages [4].
Human’s first energy consumption was naturally food. It enabled them to
survive and reproduce. This basic need was extended by other forms of energy
that played an increasingly important role in the development of humankind.
Around 500,000 years ago, humans discovered fire and learned to control
it. This provided them with light to see at night and scare off wild animals, heat
to protect against the cold and to cook their food. About 7000 years ago,
humankind invented charcoal that enabled them to make hotter and more
efficient fires, and so developed new techniques such as pottery, lead and
copper metallurgy, and the manufacture of plaster and lime. Then, about 3000
years ago, they discovered how to smelt and work iron.
As long as humans were hunter-gatherers, their own strength, combined
with their intelligence and skills, and fire, were sufficient. But once they started
to practise settled agriculture they needed new sources of energy to work the
land and mill grain more efficiently. They found this energy in the strength of
domestic animals and slaves. The need for slave labour explains the expansion
of the Roman Empire, for example. Later, renewable forms of energy largely
replaced slaves.
Another form of energy our ancestors needed was for transportation,
which is at the heart of our civilisations today. Early means of transport were
the backs of human beings and animals. Then sea transport was made possible
by using sails to harness wind energy. Coal combustion powered steam engines
and petroleum brought the motor car.
While in the developed countries, energy demand, after growing massively,
is stabilising and is likely to diminish, thanks to improved energy efficiency,5
in
the developing countries it is still growing strongly, as these countries attempt
to attain the economic level of the rich countries.
Ten thousand years before the birth of Christ, the world population is
believed to have been 5 million, rising to 250 million in 1 AD. The first billion
was reached in 1820, and it took 105 years for the population to double to
5
Energy efficiency increases as the same or more work is done with less energy.
Basic concepts 5
2 billion in 1925. Population then increased more rapidly to 3 billion in 1961 and
4 billion in 1976. Six billion was reached in 1999, and in 2009 the population has
already reached 6.8 billion. Demographers forecast that, barring catastrophe,
world population will reach 8 billion by 2020–25. Projections for the end of the
twenty-first century are more uncertain, and population is likely to be smaller than
was projected 10 years ago. The most likely scenario is in the range 9–10 billion.6
Population growth inevitably results in increased demand for energy.
In 2004, world consumption of electricity was 17,400 TWh for a population
of 6.4 billion. Average annual consumption is 2700 kWh/person, but this
figure does not reflect the real situation in which more than 4 billion people
consume less than this value. The domestic electricity consumption of France
in 2004 was 477 TWh, of which 32 TWh were lost [5], representing for France’s
population of some 61 million, an average annual consumption of around
7800 kWh/person.
Life expectancy seems partly correlated to electrical energy consumption
because of its impact on countries’ standard of living. Life expectancy falls
sharply (sometimes as low as 36.5 years) when annual energy consumed is
below 1600 kWh/person. At the end of the twentieth century, 3.5 billion people
consumed less than 875 kWh/person/year, 2.2 billion of those less than
440 kWh/person, and 1 billion less than 260 kWh/person [6]. The rate of infant
mortality also increases sharply when total energy consumption is below
*4400 kWh/person/year [6].
Let us consider some further examples showing the consequences for life
expectancy of inequalities in access to energy. Eighty per cent of world energy
is consumed by 20% of world population and these people have a life expectancy
over 75 years. Sixty per cent consume 19% of energy and their life
expectancy is over 50 years. The remaining 20% of world population consume
1% of global energy and their life expectancy is below 40 years.
In 1796, with a population of 28 million, France consumed an average of
0.3 TOE/person/year. Two hundred years later in 1996, consumption had risen
to 4.15 TOE/person/year. This was an increase by a factor of 14 per person and
28 for France, because the French population doubled over the period. This
corresponds to an average growth in consumption of 1.3% per year, and for
France of 1.75% per year. Currently, the global growth in energy consumption
is forecast to be 2–2.5% per year. In 200 years, life expectancy in France
increased from 27.5 years for men in 1780–89 to 73.5 years in 1994, and for
women from 28.1 to 81.8. In 2006, average French life expectancy was 80 years.
The development of the GDP (gross domestic product) per person gives an
idea of the prosperity of individuals. In France, between 1400 and 1820, it
6
A world population of 8 billion in 2020 represents an average growth of 1.4% from 2000. If this
growth rate was applied over 1000 years, for example, it would result in a total population of 6.5
million billion, which of course is completely unrealistic. An annual growth rate of only 0.2%
would result in a total of 50 billion in 1000 years. Conversely, a rate of decline of 0.2% would
result in a decline in world population from 6 billion to 810 million in 1000 years.
6 Energy
increased by 0.2% per year that is equivalent to a multiplication of prosperity,
over 420 years, of 2.3 times. Since 1950, the increase has been 2.8% per year, a
fourfold multiplication of prosperity in 50 years.
Energy should not be wasted, because while it is cheap today,7
it is likely
that this will change in the future. We must prepare for tomorrow by developing
every possible source of energy, taking into account the economic,
political, security of supply and environmental aspects. In particular, we must
apply the true costs of energy that include what economists call the externalities
(pollution, greenhouse effect, decommissioning, etc.). These are generally
not taken into account, except in rare cases like nuclear energy.
1.3 The Sun
The Sun is a spherical star which is our source of life, for it provides most of the
energy that we use today. In fact, apart from geothermal and nuclear, all
energy comes from the Sun.
The Sun has a radius of 696,000 km and a mass of the order of 1.99 6
1030
kg. Its surface temperature is 5780 K.8
According to the Standard Solar
Model, accepted by the whole scientific community, the Sun’s temperature
increases considerably beneath the surface, and reaches 15.6 million degrees in
the centre. The Sun consists (by mass) of 71% of hydrogen,9
27% of helium
and 2% of heavy elements such as carbon, oxygen and iron. The density and
pressure at the centre are, respectively, 148,000 kg/m3
and 2.29 6 1011
times
atmospheric pressure.
The Sun was formed 4.55 billion years ago by the gravitational contraction
of a cloud of hydrogen, helium and traces of other elements [7]. This process
was rapid until the atoms of the cloud were ionised. The energy could no longer
escape from this cloud and it slowly contracted. Half of the gravitational
energy liberated was converted into radiation and the other half served to heat
the cloud. The contraction continued and the cloud heated up. When the
temperature was close to 1 million Kelvin, thermonuclear fusion reactions
between hydrogen and the light elements such as deuterium, lithium, beryllium
and boron began. As the light elements were present in small quantities, the
liberation of energy was limited, but was enough to form a gas of very high
temperature and set off the fusion reactions between the large numbers of
hydrogen atoms, or, to be more precise, the protons.
Most of the thermonuclear reactions took place in the centre of the Sun in a
volume corresponding to a sphere whose radius was only about 20% of the
Sun’s [8]. They provided the Sun with energy and led to the formation of nuclei
of helium, 4
He, which is a particularly stable element. In the Sun, hydrogen is
7
A ‘barrel’ of some mineral waters costs around $140, or twice as much as crude oil at $70 a barrel.
8
This temperature means that a large part of the Sun’s radiation is in the visible spectrum. The
energy radiated by the Sun is around 4 6 1026
W.
9
Or more than 90% in terms of the number of atoms.
Basic concepts 7
consumed in fusion reactions, the first stage of which is the interaction between
two protons.10
Altogether four protons are needed to generate one nucleus of
helium. These fusion reactions are classified into three families. The first11
occurs
in 85% of cases and liberates 26.2 MeV. The second family (15% of cases)
liberates 25.7 MeV and the third (in only 0.2% of cases) liberates 19.1 MeV.
The average energy liberated by a proton in a fusion reaction is 15 MeV. The
first reaction of each family corresponds to a reaction between two protons. The
probability that the two will react is very small; among the 3 6 1031
m73
protons
present, only 5 6 1013
m73
/s lead to fusion. The energy liberated is 120 W/m3
,
ten times less than the energy the human body needs to survive (^1400 W/m3
) [7].
During fusion, it is the first reaction between two protons which is the
slowest and which governs the process. Some 5 billion years are needed for one
proton (1
H) to fuse with another proton [7] to form a deuteron (2
H). But only
about one second is needed for the deuteron formed during this reaction to
react with a proton to create a 3
He. Around 300,000 years are needed for two
3
He to meet and form a nucleus of helium, 4
He. This low probability of reaction
means that the density of the energy emitted in space by the Sun is very
weak: 200 nW/g, or 7000 times less than the energy released by a human
being’s metabolism (^1.4 mW/g).12
For the Sun, hydrogen is not a renewable energy. This fuel will gradually
run out, and in about 5 billion years, the Sun will become a ‘red giant’. The
central part of the core will contract and heat up until the temperature and
density of matter reaches a point at which thermonuclear fusion of the helium
is triggered. At the same time, the external part of the Sun will expand to form
a ‘red giant’. The Earth will be destroyed during this expansion [9].
1.4 Energy consumption
The various sources of primary energy that we can use are fossil and mineral
resources (coal, oil, gas, uranium) and renewable energies (hydro, solar, wind,
biomass, geothermal). The problem with many of these sources is their availability
and cost.
Until 200 years ago, humans only used renewable energies: wood for
heating and cooking, animal traction for transport, water and wind power for
10
When fusion reactions between neutrinos (ne) and photons (g) are created, the probability of a
neutron formed in the centre of the Sun escaping from the Sun without interaction is 1079
, which
means that only a single neutrino out of a billion interacts before escaping. It is much more
difficult on the other hand for a photon to escape from the Sun on account of the successive
interactions which it undergoes. A photon created within the Sun will take some 50,000 years to
escape from it.
11 1
H + 1
H ! 2
H + e+
+ ne
1
H + 2
H ! 3
He + g
3
He + 3
He ! 4
He + 2 1
H
12
The metabolism of a child, which needs more energy than adult metabolism, is around 3 mW/g.
The metabolism of a bacterium can reach 100 W/g [1].
8 Energy
mechanical energy. During the nineteenth century, coal mining led to the
development of steam engines. In the twentieth century, oil, gas and nuclear
energy were exploited.
Total energy consumption worldwide (commercial and non-commercial) in
2004 was 11.2 GTOE. Fossil fuels (oil, coal, gas) covered more than 80% of
requirements (see Table 1.2). The predominance of fossil fuels can also be seen
in Figure 1.1.
The growth of primary energy consumption in France between 1960 and
2000 is shown in Figure 1.2. During this period, consumption tripled, representing
an annual growth 2.8%. In 2005, consumption reached 276.3 MTOE. However,
huge losses take place between the primary and the final energy used by the
consumer. Thus, in France, the final energy consumed in 2005 was 160.6 MTOE,
Table 1.2 Consumption of primary commercial
energy worldwide in 2004
Energy GTOE %
Oil 3.940 35.2
Gas 2.302 20.5
Coal 2.773 24.8
Nuclear 0.714 6.4
Hydro 0.242 2.2
Biomass & waste 1.176 10.5
Other renewables 0.057 0.5
Total 11.204 100
Totals do not exactly equal 100% because of rounding errors
(Source: IEA World Energy Outlook 2006).
1960 1970 1980 1990 2000
Year
100
150
200
250
MTOE
Consumption of primary
energy in France
Figure 1.2 Development of primary energy consumption in France [5]
Basic concepts 9
only 68% of primary energy. The distribution of energy consumption in France
in 2008 across different energy sources is shown in Table 1.3 [11].
Electricity is used the most widely. Figure 1.3 shows the development of
domestic electricity consumption in France up to 2000 [5]. It increased by a
factor of 6.125 between 1960 and 2000, an annual growth rate of 4.64%.
Between 1970 and 2000, consumption rose from 140 to 441 TWh (482 TWh in
2005). During the 1970s, it was therefore necessary to find new means of generating
electricity. In 1960, large hydropower stations produced 56% of French
electricity but virtually all sites were already exploited. Coal and oil-fired power
stations were developed until the 1973 oil price shock which led to the all-out
development of nuclear energy. That improved the French balance of payments
because oil would have to be paid for in foreign currency (some €1000 per head
of the population would have been needed to buy the necessary oil at $80 a barrel
to produce the necessary electricity in oil-fired power stations).
Table 1.3 Consumption of primary energy in France in 2008
Energy source MTOE %
Coal 12 4.4
Oil 89 32.5
Gas 41 15.0
Electricity (nuclear and renewables) 117 42.7
Thermal renewable energy 15 5.5
Total 274 100
Totals do not exactly equal 100% because of rounding errors (Source:
DGEMP).
1930 1950 1970 1990
Year
0
100
200
300
400
TWh
Domestic consumption of
electricity in France
Figure 1.3 Development of domestic consumption of electricity in France [5]
10 Energy
Table 1.4 shows the distribution of total final energy and electricity consumption
in 2005. The total final energy consumption is 160.7 MTOE as
against 276.2 MTOE of total primary energy consumed in 2005.
In the next century, the world will be confronted with two fundamental
problems. The first relates to cheap fossil fuel reserves and the second to the
greenhouse effect.
1.5 The greenhouse effect
Without the greenhouse effect the average temperature of our planet would
be 718 8C. With this phenomenon it is 15 8C. This represents an average
energy difference of 150 W/m2
.13
Since the beginning of the pre-industrial era
Table 1.4 Distribution of the consumption of final energy and electricity by
economic sector
Total final energy Electricity
Sector MTOE % TWh %
Industry 37.7 23.4 135.8 32.0
Agriculture 2.9 1.8 3.4 0.8
Residential, tertiary 69.8 43.4 246.4 64.3
Transport 50.4 31.3 10.4 2.8
Total 160.7 100 423.7 100
Totals do not exactly equal 100% because of rounding errors (Source: [5, 11]).
13
At the entry to the Earth’s atmosphere, perpendicular to the Earth–Sun axis, the energy received
from the Sun actually averages 1367 W/m2
, a value which is called the solar constant. The
average intensity received on Earth is calculated bearing in mind that while the surface of the
Earth, a sphere of radius R, equals 4pR2
, the Sun only sees a disk with a surface of pR2
. The
average energy received on Earth therefore equals a quarter (pR2
/4pR2
=1/4) of the solar constant,
about 340 W/m2
.
The radiation balance of our planet is in equilibrium with the Sun because the energy received
from the Sun is equal to that emitted by Earth. To calculate this, we use Stephan’s Law which
says that the energy e emitted by surface unit of a body brought to temperature T is e = sT 4
, or
roughly e = T
64:5
À Á4
W/m2
, where s is a constant (s = 5.674 6 1078
Wm72
K74
). Of the average
340 W/m2
that arrive from the Sun, nearly 30% are reflected (100 W/m2
) back into space and
70% (240 W/m2
) are absorbed by our planet. Of these 240 W/m2
, 70 W/m2
(around 20%) are
absorbed by the atmosphere which is warmed and the rest (170 W/m2
, or 50%) heats the continents
and oceans.
If the temperature of the Earth was –18 8C, or 255.16 K, it would emit 240 W/m2
. Thanks
to the greenhouse effect, the average temperature is +15 8C which results in an emission of
390 W/m2
. As 240 W/m2
must be emitted into space to achieve the Earth–Sun balance, 150 W/m2
must be absorbed by the greenhouse effect of the atmosphere.
Basic concepts 11
the greenhouse effect has increased by 2.45 W/m2
, or almost 1% of the energy
emitted by our planet. The effect of this has been to raise the average temperature,
between 1850 and 1995, by around half a degree. This increase is
worrying.
Water vapour is the gas with the biggest greenhouse effect (60–70% of the
total). But the amount generated by humans do not have a significant effect on
its concentration in the atmosphere, and the water cycle is very rapid [10]. This
is not the case with other gases like carbon dioxide (CO2), methane (CH4) and
nitrous oxide (N2O). The halogen gases (CFC, halons, etc.14
) are emitted in
smaller quantities and their impact is minor, but their persistence is much
greater. These halogen gases also play an important role in the destruction of
the ozone layer that protects us from the harmful ultraviolet radiation. Measures
were taken at an international level to limit their use (Vienna Convention
1985, Montreal Protocol 1987), but it will still take several decades to restore
the ozone layer to its 1970s level.
The gases have different levels of greenhouse effect, which can be quantified
as follows: 1.56 Wm72
for CO2, 0.5 Wm72
for CH4, 0.14 Wm72
for N2O
and 0.25 Wm72
for the CFCs. All fossil fuels emit CO2 when burned, as they
contain carbon. Better management of fuel combustion and the choice of fossil
fuel (e.g. for the same quantity of energy generated, the burning of natural gas
emits around half the CO2 of coal) can optimise greenhouse gas emission, but it
is impossible to reduce it to zero because the combustion of carbon compounds
always yields carbon dioxide. By contrast, the production of renewable and
nuclear energy does not contribute to an increase in the greenhouse effect.
The increase in man-made greenhouse gas emissions could have serious
consequences on the environment according to a number of predictive models
[10]. A number of scenarios have been developed to estimate the average
temperature in 2100. They point to an average warming of between 2 8C and
6 8C. The higher values would have dramatic effects on the environment,
including a notable rise in sea level, and the appearance of tropical diseases in
some countries that do not have them today [10]. The Intergovernmental Panel
on Climate Change (IPCC) also predicts on the basis of models that sea levels
could rise by between 15 and 95 cm in 100 years, 95% of European glaciers
will disappear, precipitation patterns will change (with heavier rainfall in
Europe) and major climatic disturbances (cyclones, hurricanes, tornadoes, etc.)
will become more frequent. These predictions are sufficiently worrying for
attempts to be made at a global level for an international agreement on limiting
greenhouse gas emissions. The Kyoto Conference in December 1997 made
a start in this direction, although many experts consider that it did not go far
enough.
14
The CFCs (chlorofluorocarbons) are carbon compounds in which hydrogen atoms are replaced
by atoms of chlorine or fluorine. In halons, some atoms of hydrogen are replaced by atoms of
bromine and/or fluorine.
12 Energy
1.6 Conclusion
Two factors will lead to an increased demand for energy in the future: the
growth of world population and the fact that the developing countries aspire to
increase their standard of living. If we assume an annual global increase of
2–2.5% in energy demand, the world’s energy consumption will double within
30 years. To meet these additional needs, without adding too much to the
greenhouse effect, it will be necessary to develop nuclear and renewable energies,
which currently only account for 20% of the world’s energy consumption.
All energy sources have their own advantages and disadvantages, in terms
of cost, security of supply, impact on the environment, etc. There is no universal
solution, and the best mix of energy sources will vary from country to country.
The consumption of primary energy will remain for several decades largely
dominated by fossil fuel combustion, especially oil. Fossil fuels represent nearly
90% of commercial energy (80% if one includes non-commercial energy) and
nothing else can replace them quantitatively or economically. Between the
beginning and the end of the twentieth century, the world consumption of primary
energy rose from around 1 GTOE to tens of GTOE. This is what enabled
humanity to make such major strides in economic development over that period.
Basic concepts 13