Waste Heat Recovery with a Reciprocating Engine
Reciprocating engines convert 20% to 40% of the fuel energy they consume into shaft horsepower. The remainder is removed with the cooling system and rejected in the exhaust. A waste heat recovery system on the engine’s exhaust can increase fuel thermal efficiency to around 55%. If the cooling system energy is recovered as well, the efficiency may be increased to about 75%.
The exhaust temperature of reciprocating engines varies from 800°F to 1350°F depending on the size, efficiency and whether it is supercharged. Because reciprocating engines do not use large amounts of excess air, the combustion products constitute a larger percentage of the exhaust. The specific gravity of reciprocating engine exhaust gases is shown, along with that of air, on Figure 3200-13.
For reciprocating engines, a variety of systems and combinations of systems can be used, depending on the heating medium, and the temperature required by the heat user.
Figure 3200-14 shows a typical hot water system in which the engine jacket cooling water has been replaced by the hot water heating fluid. Additional heat energy is then recovered from the hot exhaust gases.