Kelvin–Planck statement
No cyclic engine can turn heat from a single reservoir entirely into work with no other effect.
Definition
The Kelvin–Planck statement of the second law asserts that no process is possible whose sole result is the complete conversion of heat drawn from a single reservoir into work. Every heat engine must reject some heat to a colder reservoir; a perfectly efficient engine is impossible.
This is why a heat engine needs two reservoirs, not one. An engine that ran by cooling the ocean alone would conserve energy perfectly yet still be impossible — it would be a perpetual-motion machine of the second kind. The cold sink is mandatory.
The statement is equivalent to the Clausius statement and to the Clausius inequality; the three are interchangeable expressions of the second law, each easiest to apply in a different setting.
History
William Thomson (Lord Kelvin) gave the original form in 1851; Max Planck sharpened it into its modern wording in the 1880s and 1890s, which is why the combined name has stuck.