The document discusses energy and exergy using an analogy of representing energy as "little men". It states that the 1st law of thermodynamics means the total number of men remains the same during energy conversion. Exergy is represented by the height of the men and refers to the quality of energy. The 2nd law means men lose height and courage when moving to lower platforms. Heating a room with electricity versus hot water is compared in terms of exergy levels. Energy losses occur throughout the processes of generating and transferring energy.

1. Energy and exergy explained
Minute Lectures
A representation of energy
A good analogy often helps to understand the concepts of physics
So let’s represent energy by little men
The number of men is equivalent to
the amount of energy
The 1st law of thermodynamics says:
during energy conversion, the
total amount of energy remains the same.
Or: no men can ever disappear (or appear)

2. Energy and exergy explained
Minute Lectures
A representation of exergy
Another basic concept is “exergy”, or the “quality of energy”.
This can be represented by the height at which the men are
standing.
If the energy is heat, exergy is
linear with temperature.
Just like height, exergy is relative to a reference level. This
reference is the outside atmosphere, which we assume to be at
0° Celsius or 273.15° Kelvin.

3. Energy and exergy explained
Minute Lectures
The 2nd law of thermodynamics: the more men fall
down, the fewer of them will be able to climb up
again
Or: when men move to a lower platform, some
become discouraged. They are still capable of
walking on a flat surface or falling down to a lower
platform, but they lack the courage to climb up to a
higher level again without a helping hand. At
ground zero, all men are discouraged
Height = exergy = “courage”

4. Energy and exergy explained
Minute Lectures
The total amount of courage of the men on a certain
platform is the exergy level of that platform. This is
also called the Carnot factor (C) calculated by:
Cplatform = 1 – (Tatmosphere / Tplatform)
Electricity, natural gas, oil and all other fuels, have a
Carnot factor ‘1’. Indeed, their energy can
(theoretically) be converted to heat at any desired
temperature. They are on the highest exergy level
The exergy level C

5. Energy and exergy explained
Minute Lectures
Heating a room
Example: the heating of a room
E(C,J) is electrical energy with a Carnot factor C and an
amount of energy J (from Joule).
QT(C,J) is heat at a temperature T, with a Carnot factor C
and an amount of energy J
Now let’s compare heating a room with an electric radiator,
to heating a room with a heating element containing hot
water at 65°C

6. Energy and exergy explained
Minute Lectures
By an electric radiator
C1 = 1
C2 = 1 – (273.15/293.15) = 0.07

7. Energy and exergy explained
Minute Lectures
By a heating element with water at 65°C
C1 = 1 – (273.15/338.15) = 0.19
C2 = 1 – (273.15/293.15) = 0.07

8. Energy and exergy explained
Minute Lectures
Energy Losses
Not all the men that landed at the exergy level of the room will stay
there, unfortunately. Through cracks and chinks, some of them will
leave the room and fall down into the atmosphere.

9. Energy and exergy explained
Minute Lectures
When the room is heated by a hot water radiator,
the hot water comes from a boiler.
Boiler (1/2)

10. Energy and exergy explained
Minute Lectures
The boiler converts the fuel energy into combustion heat
(1200°C), reducing the exergy level.
The combustion heat is transferred to hot water – a
process by which exergy goes further down
In the conversion, some energy is lost.
Boiler (2/2)

11. Energy and exergy explained
Minute Lectures
The electrical power
for a radiator comes
from a power station
Power station (1/2)

12. Energy and exergy explained
Minute Lectures
In a power station
 Fuel creates heat in a boiler, which is then transferred to
steam. The exergy level goes down (some men become
discouraged)
 This process causes some loss of energy (some men fall into
the atmosphere)
 Electricity is generated with the steam. The exergy level goes
up. But not all the men will crawl up again; some of them are
discouraged and will stay were they are. It is impossible to
convert 100% of the heat energy into electrical energy
Power station (2/2)

13. Energy and exergy explained
Minute Lectures
Is it possible to take energy out of the atmosphere? Yes,
but remember: at atmosphere level, all men are
discouraged, so they will need some help.
Example: a heat pump.
The heat of the atmosphere is transferred to a medium at
low pressure = a life buoy. This buoy is taken up to a
higher pressure and higher temperature by a
compressor, driven by electrical energy. On average,
one unit of electrical energy can draw three units of
heat out of the atmosphere
Heat pump (1/2)

14. Energy and exergy explained
Minute Lectures
Heat pump (2/2)

15. Energy and exergy explained
Minute Lectures
Round-up
During an energy conversion process, no energy can be
lost.
But it is not only the amount of energy that counts, the
“quality level” or “exergy” is just as important.
Exergy is a measure that indicates to what degree energy
is convertible to other forms of energy.
It is impossible to take energy from a lower exergy level to
a higher exergy level without a price.

16. Energy and exergy explained
Minute Lectures
Links and references
• Heat pumps and their contribution to energy
conservation, Ed E Camatini and T Kester,
Noordhoff, Leyden, 1976, ISBN 9028600566
• Why thermal power plants have a relatively low
efficiency
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