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

There are heat engines that convert heat into mechanical energy, which can drive useful things around the aircraft. Examples that actually drive systems that we use are

• The steam turbine used for electrical power generation in remote area's, driven by the waste heat from gas turbines.
• The stirling engine, used for driving submarines due to it being whisper quiet.
• The classical steam engine, also very quiet.

All these engines come at a weight penalty of course. It makes a lot of sense to add a steam turbine to a gas turbine in a power station; adding a steam turbine next to the jet engine is less clearly beneficial. It has not been done.

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.

• Nitpick: You need a cold side of 0K or an infinite operating temperature, not both. (1-Tc/Th). More practically, the temperature differential is dictated by the environment (outside air, waste heat) and not an engineering choice. – MSalters Jan 16 '18 at 10:42
• @MSalters right-o, have incorporated. – Koyovis Jan 16 '18 at 10:52