Though the balance of entropy has been denoted as Second Law of Thermodynamics in the previous chapters, its classical formulation comes along with thermodynamic cycles. Thus, the focus now is on these cycles. However, there are two different types of thermodynamic cycles: Clockwise cycles on the one hand convert heat into mechanical energy and are named thermal engines. Counterclockwise cycles on the other hand are fridges and heat pumps, that lift thermal energy from a lower to a higher temperature level. However, at that stage cycles are represented in a so-called black-box notation, i.e. the fluxes at its border are balanced without focussing on the detailed physical processes that run inside the machine.