Subsystem environmental/thermal control is a vital process in the overall integration of processes leading to quality operational aircraft. We do a good job of providing control of the required environments and subsystem component temperatures. However, current aircraft environmental control/thermal control system design does not attempt to recover or utilize waste thermal energy. Creative methods to control and use the waste thermal energy will enhance aircraft performance. Unless efficient energy recovery and conversion systems are developed and integrated with the other aircraft subsystems we will find it difficult to achieve full performance potential in new aircraft.Identification of the most efficient utilization of available waste energy sources is a difficult proposition requiring numerous trade-offs. A comprehensive methodology that systematically conducts feasibility and trade studies and provides visibility into the ramification of design options is needed to objectively evaluate recovery methods.