First & Second Law of Thermodynamics – Energy Basics

Energy Basics is a series covering fundamental energy concepts.

Posts in this series include kW vs kWh and Average vs Marginal Carbon Emissions.

Most people associate thermodynamics with textbooks full of strange symbols and complex equations.  It is no surprise that thermodynamics is rarely a topic that gets people excited.

This post aims to highlight what is exciting about thermodynamics.  The practical insights that stem from thermodynamics are the foundation of energy engineering.

The First Law

The First Law is about conservation.  All that we can do is convert one form of energy to another (most often converting between fuel, power and heat).  We can never generate energy out of nothing.

This is all great – but how does it help us?

The First Law allows engineers to use mass & energy balances. Engineers use balances to model energy systems.  Unknowns such as flow rates of water or temperatures of gases can be calculated using balances.

A simple example of power generation – we generate 4 units of power from 10 units of fuel.  The First Law allows us to calculate the heat generation at 6 units.

What the First Law doesn’t do is place any limit on the amount of power we can generate (except limiting it at 10).  This is where the Second Law steps in.

The Second Law

The Second Law has multiple definitions.  One form of the Second Law limits how much power we can generate from fuel. No matter how advanced our technology becomes we will never be able to convert 100% of our fuel into power.

There are fundamental thermodynamic limits on the amount of high quality energy we can convert from one form to another.  We will never be able to convert 100% of our fuel into power.  Even with more advanced technology, we will always end up with some of our fuel converted into low quality energy (i.e. heat).

The Second Law tells us that heat is a necessary by-product of power generation. To maximize the value we get from our fuel requires finding a place for this heat. Combined Heat and Power is a range of technologies that aim to do this.

Essentially 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 above are the two I find the most practical.