Thermodynamics for Energy Engineers

Let’s start at the end - here are two simple definitions of the First & Second Law of Thermodynamics:

• The First Law limits us to only ever breaking even,
• The Second Law says we can never break even, and will always lose.

Both of these Laws have alternative definitions and interpretations. The insights I have chosen to highlight are how they relate to practical energy engineering.

The First Law of Thermodynamics

The First Law is a law of conservation - energy cannot be created or destroyed, only only converted from one form to another. The First Law applies to both mass & energy, and allows for conversion between the two.

The practical implication of the First Law for energy engineers is the ability to draw mass & energy balances. Unknowns such as flow rates of water or temperatures of gases can be solved for using balances.

A common energy transformation is converting fuel into power and heat. For example:

• we generate 4 units of power,
• from 10 units of fuel,
• an energy balance allows us to calculate the heat generation at 6 units.

In this example, our efficiency of power generation is 40%.

The First Law puts no limit on the efficiency of power generation - the First Law limits the amount of power generation at 10.

In reality, it’s not possible to generate power at 100 % efficiency. The Second Law further limits the fraction of fuel we can convert to power.

The Second Law of Thermodynamics

The Second Law limits the amount of useful work we can do with energy.

It restricts us even more than the First Law - the Second Law makes it explicit that we can never convert all of the energy in fuel into useful work. We always end up losing exergy and generating entropy when we convert energy.

Converting heat (i.e. fuel) into work (i.e. electricity) will always generate lower quality forms of energy, such as heat or sound.

The Second Law means that heat is a necessary by-product of electricity generation. To maximize the value we get from our fuel requires finding a place for this heat. Combined heat and power (CHP) attempts to make use of this heat in industry for for heating.

Summary

We end where we started, with our simple definitions of the First & Second Law:

• The First Law limits us to only ever breaking even,
• The Second Law says we can never break even, and will always lose.

Further Reading

Carnot’s theorem limits the maximum efficiency of heat engines using the ratio of the heat supply and rejection temperatures.

Exergy is a measurement of maximum useful work.

Entropy is similar to exergy, and measures the irreversibility of a process.

Thanks for reading!