AlthoughSecond law of thermodynamics an entropy balance has been referred to as the Second Law of Thermodynamics in the previous chapters, its classical formulation goes hand in hand with thermodynamic cycles. Therefore, the focus is now on these cycles. There are basically two different types of thermodynamic cycles: Clockwise cycles, on the one hand, convert heat into mechanical energy and are denoted as thermal engines. Counterclockwise cycles, on the other hand, are refrigerators and heat pumps that lift thermal energy from a lower to a higher temperature level. Initially, however, the cycles are represented in a so-called black-box notation, i.e. the energy fluxes at their boundary are taken into account without focussing on the detailed physical processes that take place inside the machine. Finally, a detailed description of which components these machines consist of is given in Part II.