An auxiliary power unit system of an aircraft includes an auxiliary power unit (APU), a cooling unit for the APU including a heat exchanger, an air inlet in communication with the APU and/or with the cooling unit, an air inlet door unit located at the air inlet, a first duct configured to draw air into the APU and having an entrance in communication with the air inlet, a second duct having an entrance in communication with the air inlet, the heat exchanger being at least partially located within the second duct, an air turbine located within the second duct downstream of the heat exchanger, and an electrical generator coupled to the air turbine.