Matter and solar radiation are never in equilibrium in solar cells and quasi-static conversion of the solar energy is not achieved. For this reason, the irreversible thermodynamic engines are described using the method of endoreversible thermodynamics of solar energy conversion (Novikov, 1958; Rubin, 1979; De Vos, 1992). Remember that endoreversible engines are irreversible engines where all irreversibilities are restricted to the coupling of engine to the external world. It is assumed that the inner reversible part of an endoreversible engine is a Carnot cycle. We have also considered the non-Carnot cycles and found out that photon absorption in solar cells can be considered as the external reversible part of an endoreversible engine.
The photon absorption in solar cells is separated into processes with and without work production. These processes are sequencies of transitions from one energy state of the system to another. The energy transitions between particle states are called "photon reemission" if they do not take part in performing work and heat dissipation.
The photon reemission is divided into reversible and irreversible processes. These nonworking processes are regarded as a continuous series of equilibrium states outside the irreversible or endoreversible engine, are isolate into separate processes, and are used to obtain a higher efficiency of the generally non-quasistatic solar energy conversion. We can consider those equilibrium processes as a base of the „exoreversible"additional device for the irreversible or endoreversible engine.
So, we have called the processes without work production photon reemission. Let us now shown that they play a special role in the conversion of solar heat into work.