The laws of thermodynamics are empirical rules, meaning they are determined through experimentation, that indicate the natural mechanisms governing energy exchanges. Being empirical rules, the laws of thermodynamics should be called thermodynamic principles. However, the use of both terms is common.
The first law of thermodynamics states:
The total amount of energy in any isolated system (without interaction with any other system) remains constant over time, although that energy can transform into another form of energy.
This is the well-known law of conservation of energy, which has become popularized as “energy cannot be created or destroyed, only transformed.” However, despite being an expression that has entered colloquial language, it is worth pausing for a moment to consider the implications of this principle. The most obvious is that we cannot generate energy from nothing and, therefore, we must find “stores” of energy, which we call energy sources. On the other hand, any system that receives energy must, to be in equilibrium, release the same amount. For example, when heating a body, its temperature increases, and its response is to emit a greater amount of heat until the body emits the same amount of energy it receives, reaching equilibrium.
The second law of thermodynamics states:
The amount of entropy in the universe tends to increase over time.
This principle tells us that in every energy exchange, part of the energy becomes unusable and cannot be harnessed again. This increase in entropy is what we know as process losses. There are multiple formulations of this principle, which essentially boil down to the fact that in any real process, we will have losses.
Despite their apparent simplicity, these two principles are fundamental and necessary when analyzing the correct use of energy in engineering and the viability of energy sources, such as hydrogen or solar energy.
