The Kelvin–Planck statement of the second law limits a heat engine.
It says that no cyclic heat engine can take heat from a single reservoir and convert all of it into an equivalent amount of work with no other effect. A practical heat engine must reject some waste heat to a cooler reservoir. This restriction explains why no engine can have 100% thermal efficiency when operating cyclically between finite-temperature reservoirs.
The statement is named for William Thomson, known as Lord Kelvin, and Max Planck. It is also called the heat-engine statement of the second law. The limitation is not that engines cannot produce work; rather, it rules out a particular impossible arrangement—complete conversion of heat from one reservoir into work during a cycle.
A steam turbine may be part of a heat-engine system, but “steam turbine” describes a device, not the general thermodynamic category being constrained. Fuel cells and electric motors operate through different energy-conversion mechanisms. The Kelvin–Planck statement is equivalent in consequence to the Clausius statement, which concerns transferring heat from cold to hot without external work.