Abstract: An apparatus for generating or recovering energy on board an aircraft effectively utilizes the cabin exhaust air stream, which has a high thermal energy and a differential pressure, before it is discharged into the environment outside the aircraft. In this manner, the total efficiency of the aircraft is improved. The apparatus essentially includes an energy conversion unit (8) connected to the cabin exhaust air duct (2) to generate or recover energy from the exhaust air stream (5). Bleed or tap air (11) from the engine (10) is provided from a bleed air port (10A) through a cabin supply air duct or tap air duct (3) to an air conditioning plant (12) for final preparation of cabin supply air. A heat exchanger (4) is arranged between the cabin exhaust air duct (2) and the tap air duct (3), upstream of the energy conversion unit (8) in the cabin exhaust air duct (2).