Power plant receives heat from a heat source
(the hot spring), output net work, and rejects heat to a heat sink
(the ambient air). Hence, it is a heat
engine. All heat engines in practice are irreversible.
The Carnot principles state that the efficiency of an irreversible
heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs. The thermal
efficiency of a Carnot heat engine working between 140 oC and 30 oC is 26.6%.