there's more to energy than joules

Energy and exergy explained

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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)


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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.


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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”


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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


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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


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By an electric radiatorC1 = 1

C2 = 1 – (273.15/293.15) = 0.07


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By a heating element with water at 65°C

C1 = 1 – (273.15/338.15) = 0.19C2 = 1 – (273.15/293.15) = 0.07


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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.


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When the room is heated by a hot water radiator, the hot water comes from a boiler.

Boiler (1/2)


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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)


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The electrical power for a radiator comes from a power station

Power station (1/2)


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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)


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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)


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Heat pump (2/2)


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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.


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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>>

there's more to energy than joules

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