Powering Your Car with Waste Heat

For sake of illustration, let's imagine that an internal combustion engine sends 1/3 of its output to propulsion, 1/3 to engine cooling, and 1/3 to exhaust.

Further imagine that the combustion temperature is 1000 K, the exhaust temperature 750 K , the coolant temperature 360 K, and the ambient temperature 300 K. ( I suspect these are fairly realistic, but am least sure about the combustion temperature.)

Under these crude assumptions, the combustion process has a Th of 1000, half dissipating to a Tc of 500, and half to a Tc of 360; average of 430. Theoretical Carnot efficiency is (1-Tc/Th ) = (1-430/1000) = 0.570.

If the exhaust cools to about 400 K (average exhaust temp 575), the theoretical Carnot efficiency would be (1-400/575) ≈ 0.304. Practical efficiency will be considerably less, because of the large area of gas-to-liquid or gas-to-gas heat exchangers.

The engine coolant might range from 360 K down to 350 K in a heat recovery system (average Th = 355). Here, (1-Tc/Th) = (1-300/355) ≈ 0.155. This Carnot efficiency isn't very high, but it would improve if heating colder air.

One could quibble over these numbers, but they do illustrate the hurdles of recovering energy from low-intensity sources.