Drive Systems: Considerations with Engine Applications - Waste Heat Recovery
in the fuel supplied to an engine is released during combustion and is converted
to shaft work and heat. The shaft work drives the compressor, while the heat is
liberated from the engine through the coolant, exhaust gas and surface radiation.
Approximately 70% of the total energy input is converted to heat. Much of this
heat can be recovered from the engine exhaust and jacket coolant, while smaller
amounts are also available from the lube oil cooler and the turbocharger's intercooler/aftercooler.
recovery of this heat is an important consideration in fully utilizing the fuel-input
energy to the engine. The heat can be used to generate steam or hot water, or
it can be used directly in certain industrial applications. Recovered heat is
typically used for space heating, domestic or process water heating, absorption
chilling, or desiccant regeneration.
recovered from the engine jacket accounts for up to 30% of the energy input and
is capable of producing 200°F hot water. Some engines, such as those with
ebullient cooling systems, can operate with water jacket temperatures up to 265°F.
Almost all of the heat transferred to the engine coolant can be recovered, limited
only by the efficiency of the heat exchanger, assuming there is a demand for the
other major source of heat is the engine exhaust. Exhaust temperatures of 850°-1200°F
are typical. Only a portion of the exhaust heat can be recovered, since exhaust
gas temperatures are generally kept above condensation thresholds. Most heat recovery
units are designed for a 300°-350°F exhaust outlet temperature, to avoid
the corrosive effects of condensation in the exhaust piping. Exhaust gas heat
recovery is typically used to generate hot water up to about 230°F or low-pressure
steam (15 psig).
recovering heat in the jacket water and exhaust, approximately 75-80% of the fuel's
energy can be effectively utilized.