Answer
0 percent. 100 percent.
Work Step by Step
Equation 20.14 gives the efficiency of a Carnot engine: $e_{Carnot}=\frac{T_H-T_C}{T_H}$.
If the hot and cold temperatures are the same, the efficiency is zero, and the engine does no work. A heat engine requires a finite temperature difference between the reservoirs.
If the cold reservoir were at 0 K, the efficiency would be perfect, at 100 percent. No heat would be expelled into the cold reservoir. However, one statement of the second law of thermodynamics, found on page 656, states that heat cannot be converted completely into mechanical work. Some heat must remain, which is expelled to a low-temperature reservoir. Therefore, a heat engine cannot have a cold reservoir at absolute zero.
