The rules for energy conversions

The last chapter described how different Earth system processes are related to different forms of energy and entropy. This formulation in terms of energy sets a basis for making them comparable. The rules for converting one form of energy into another are described by the laws of thermodynamics. They ensure the conservation of energy during the conversion process, and set the direction into which these conversions occur.

Historically, these laws have grown out of the need to understand and improve the work output of steam engines in the mid-nineteenth century. Since then, their basis has been extended much beyond steam engines to all forms of energy transfer. The purpose of this chapter is to show those aspects of the laws of thermodynamics that have the most direct relevance to understand energy conversions by Earth system processes. The foundations set by the laws then allow us to make quantitative predictions of the direction in which the dynamics take place in Earth systems and set upper limits on energy conversion rates, as described in the following chapter. In total, there are four laws of thermodynamics that are numbered from zero to three. They are summarized in Table 3.1.

The zeroth law sets the basis for comparing thermodynamic systems. It establishes the state of thermodynamic equilibrium as a reference state, which is the state of a system in which there is no net transformation or exchange of any physical quantity. The zeroth law formulates that if two systems are in thermodynamic equilibrium with a third system, then the two systems are also in thermodynamic equilibrium. As we will see in the following, the state of thermodynamic equilibrium serves as an important reference point, as it sets the “target” state for the dynamics that take place within a system and the exchanges with other systems. The zeroth law also comes into play when the equilibrium between different forms of energy is needed to describe energy conversion processes. This is, for instance, the case for radiative processes, in which conversions between radiative and thermal energy are involved, or for phase transitions, in which conversions between liquid and gaseous phases are involved. However, as this law represents a more formal aspect, we will not deal with the zeroth law in greater detail here.