The First & Second Law of Thermodynamics

Simple enough to be summarized in two sentences.

1 minute read

The First and Second Law are foundational in energy engineering. They are also simple enough to be summarized in only two sentences.

The First Law means we can only ever break even. The Second Law means we can never break even, and will always lose.

the first law of thermodynamics

The First Law is the law of conservation. Energy cannot be created or destroyed. We can only convert energy from one form to another.

The practical implication of the First Law is the ability to draw mass & energy balances. Engineers use balances to understand and model energy systems. 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. A simple energy balance allows us to calculate the heat generation at 6 units.

The First Law limits the amount of power generation at 10. The Second Law further limits the fraction of fuel we can convert to power.s

the second law of thermodynamics

The Second Law limits the amount of useful work we can do. It restricts us even more than conservation - we always end up losing exergy and generating entropy.

Conversion of energy results in a loss of energy quality. We can never convert all of the energy in fuel into useful work. We will always generate lower quality forms of energy - commonly heat.

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 attempts to make use of this heat in industry or for heating.

summary & further reading

The First Law tells us we can only ever break even when converting energy. The Second Law then tells us we 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.

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!