(1) It is not necessary that a heat engine's hot environment...

(1) It is not necessary that a heat engine's hot environment be hotter than ambient temperature. Liquid nitrogen $(77 \mathrm{K})$ is about as cheap as bottled water. What would be the efficiency of an engine that made use of heat transferred from air at room temperature $(293 \mathrm{K})$ to the liquid nitrogen "fuel" (Fig. 19)?

(1) It is not necessary that a heat engine's hot environment be
hotter than ambient temperature. Liquid nitrogen $(77 \mathrm{K})$ is
about as cheap as bottled water. What would be the efficiency of
an engine that made use of heat transferred from air at room
temperature $(293 \mathrm{K})$ to the liquid nitrogen "fuel" (Fig. 19)?

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Key Concepts

Heat Engine Efficiency

Heat engine efficiency is defined as the ratio of the work output to the heat input. It represents how effectively an engine converts supplied heat into useful work. The maximum efficiency achievable, as dictated by the Carnot efficiency, underscores the importance of the temperature difference between the heat source and the sink.

Carnot Efficiency

This is the theoretical maximum efficiency that a heat engine operating between two thermal reservoirs can achieve. It is calculated using the formula ? = 1 - (T_c/T_h), where T_c is the absolute temperature of the cold reservoir and T_h is that of the hot reservoir. The concept illustrates how the efficiency of any engine is fundamentally limited by the temperatures between which it operates.

Thermal Reservoirs

In thermodynamics, a thermal reservoir is a body with a large thermal capacity that can provide or absorb heat without undergoing a significant change in temperature. For a heat engine, the hot reservoir supplies the heat, and the cold reservoir receives the rejected heat. The efficiency of an engine depends critically on the temperatures of these reservoirs.

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