KELVIN PLANCK STATEMENT

It is impossible to construct a cyclically operating device such that it produces no other effect than the absorption of energy as heat from a single thermal reservoir and performs an equivalent amount of work.

The only option then is that the engine converts part of the energy it receives as heat into work and rejects the rest to another thermal reservoir the temperature of which is less than the temperature of the source. Two thermal reservoirs, one of high temperature (source), from which the working fluid receives energy as heat, and the other of low temperature (sink), to which the working fluid rejects energy as heat, are needed for a heat engine. Once the heat engine rejects a part of the energy it receives, its efficiency becomes less than one. Thus the Kelvin Planck statement further implies that no heat engine can have a thermal efficiency of one (hundred percent). This does not violate the first law of thermodynamics either.