It is not necessary that a heat engines hot environment be hotter than ambient temperature. Liquid nitrogen

Question:

It is not necessary that a heat engine€™s hot environment be hotter than ambient temperature. Liquid nitrogen (77K) 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 (293K) to the liquid nitrogen €œfuel€ (Fig. 15-25)?

It is not necessary that a heat engine’s hot environment