Kelvin-Planck statement

No process is possible whose sole result is the absorption of heat from a reservoir and the conversion of all of this heat into work.

It is impossible to convert heat completely into work in a cyclic process.

That is, it is impossible to extract energy by heat from a high-temperature energy source and then convert all of the energy into work. At least some of the energy must be passed on to heat a low-temperature energy sink. Thus, a heat engine with 100% efficiency is thermodynamically impossible.