Solar energy is heat from the sun (temperature TS = 5800 K) transported to the earth (temperature T0 = 300 K) in the form of radiation.

When heat is involved, energy conversion is limited by the second principle of thermodynamics which states that entropy must not be destroyed. If an entropyfree form of energy (mechanical, chemical, electrical energy) is to be produced, the entropy which accompanies the solar heat energy has to be given away to a reservoir on earth and with it some energy. This process of energy conversion from heat is realized in a heat engine. Since entropy may also be generated in the conversion process, the highest efficiency for the conversion of heat requiring the smallest amount of energy to remove the entropy is found for reversible processes in which no entropy is produced. A state in which no entropy is produced is a state of equilibrium. However, in equilibrium, no net energy current flows from one body to another. A net energy current requires a deviation from equilibrium.

Although widely used, a thermodynamic equilibrium between two systems with everything in equilibrium and in which all intensive quantities have the same values in both systems fortunately does not exist because it would be the death of the universe. Instead, there is a possible and independent equilibrium for each intensive quantity, which then has the same value in different systems. There can be pressure equilibrium by the free exchange of volume, but the temperatures will differ. There can be temperature (thermal) equilibrium between two systems by a free exchange of entropy, while the chemical potentials of the particles in each

system differ, because their exchange is inhibited. It is important to call equilibria by their proper names to distinguish between them.