How we can reduce the waste heat from systems such as car exhaust systems or large power plants and/or use it to enhance product performance ?

Edit: let’s focus on one system for example a cars exhaust system loses a lot of heat. So could a thermoelectric generator be used to convert this heat loss into electrical energy ? And if so what can this electrical energy be used for ? What are the benefits of this ?

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    $\begingroup$ That's a rather large topic. We've been striving to decrease waste for decades if not centuries, and spend millions if not billions of dollars every year on it. Can you narrow it down to specific classes of solutions you're interested in, such as "things an average person can do to their 5 year old car" or "things that can be retrofit onto power plants?" Or are you looking for the general rules of thumb like maximizing temperature differentials in a carnot engine or using multiple stages? $\endgroup$ – Cort Ammon - Reinstate Monica Jan 26 '19 at 0:33
  • $\begingroup$ Please see my edit :) $\endgroup$ – Dan Khan Jan 26 '19 at 0:37
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    $\begingroup$ Even thermoelectric generators suffer performance issues from heating. You need a temperature gradient for them to work, so if the entire generator gets heated to uniform temperature then nothing happens. You could use the hot exhaust gas to preheat the fuel, which makes it easier to combust. $\endgroup$ – Drew Jan 26 '19 at 0:58
  • $\begingroup$ @Drew, I don't think the car companies or customers would want to preheat gasoline. You don't want to preheat air either, as this reduces the density of the air in the intake manifold. $\endgroup$ – David White Jan 26 '19 at 1:06
  • $\begingroup$ @DavidWhite I agree, that was a bad idea from a practical standpoint. $\endgroup$ – Drew Jan 26 '19 at 1:18

In theory one can use multiple stages to increase efficiency, such as adding a thermoelectric generator stage to your internal combustion engine. Indeed, if one looks to the extreme, we find structures like Matrioshka Brains which are built like giant onions around a star with many stages, striving to achieve 99% efficiency or more.

However, there are practical considerations. Many approaches for adding a second stage interact with the first stage. In your example, if the thermoelectric generator takes up too much space inside the exhaust system, it decreases air flow, making the engine less efficient by a larger margin than your generator. One also has to consider the losses due to hauling around more mass. One also has to consider the cost of the hardware. There's a reason we don't see a market for "DIY add-on stage for a F-150."

Typically, these balances have been explored. Eeeking 10 or 15 watts out of the exhaust of a 150,000 watt engine is typically not worth the extra mass.

One interesting place where we do in fact make such tradeoffs is in turbochargers. Turbochargers compress the air entering the engine, providing many opportunities such as extra performance. However, compressing air is not easy. It takes a lot of energy to do so. In a turbocharger, the flow of exhaust is used to spin a turbine to compress the air.


From a thermodynamic standpoint, you don't want to reduce the "waste" heat from the engine. You want to increase the temperature difference between the heat source (the combustion gases in your cylinder) and the temperature of the exhaust gases exiting the tailpipe. Any thermo-electric device that can reduce the exhaust gas temperature will generate electricity that will relieve some of the power requirements of the alternator, increasing fuel economy. Likewise, any engine design that can increase cylinder combustion temperatures without melting the engine, such as a ceramic engine, will allow you to either run higher combustion temperatures or eliminate the radiator and water cooling system altogether, again increasing fuel efficiency because you will not be sending a LOT of heat to the environment via the radiator.

Having stated that, as other posters have pointed out, there are disadvantages to the thermo-electric device in the exhaust system, and no one has yet been able to design a ceramic engine that is sturdy enough to last a reasonable amount of time, as thermal stress during startup tends to crack ceramic engines.


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