# Is it possible to fully utilise excess heat from an aircraft to power other systems required for the aircraft to function?

Is it possible to fully utilize excess heat from an aircraft engines to power other systems required for the aircraft to function, using heat exchangers and other clever engineering tools and techniques? This would seem to me to amount to massive energy efficiency and fuel savings.

• Is the pilot included in "other systems needed for the aircraft to function" because the cabin heat is exactly like that in my little plane :) – Jamiec Jan 11 '18 at 13:32
• Harvesting heat for electricity is not very efficient. – Ron Beyer Jan 11 '18 at 14:55
• In a sense, bleed air from jet engines is using this technique. – kevin Jan 11 '18 at 15:37
• And massive weight and cost increase. – user3528438 Jan 11 '18 at 15:54
• "This would amount to massive energy efficiency and fuel savings.", No it really would not be. The Avionics, lights, hydraulics, environmental controls really do not draw very much energy compared to the power needed to keep the plane flying. The additional weight of a heat-capture/convert system would be a far bigger factor. – abelenky Jan 11 '18 at 16:01

So yes some of the waste heat of the aircraft engines can be used. But thermodynamics says that never all heat can be transformed - the theoretical maximum efficiency is a function of the maximum and minimum temperatures. Only a heat engine with a perfect Carnot cycle, a heatsink of 0 K or an operating temperature of $\infty$ K and no heat loss other than through the engine process will transform all heat. And we cannot make that in an environment that is neither 0K nor $\infty$ K.