Page 83 - Piston Engine-Based Power Plants

74 Piston Engine-Based Power Plants

Customer heat
exchanger Exhaust

Engine
Heat
recovery Gear
Excess heat box
exchanger
T
Oil cooler

Jacket water

Figure 7.1 Heat sources for a reciprocating gas engine CHP system.

The exhaust gas contains up to one-third of the fuel energy and
30% 50% of the total waste heat from the engine. Exhaust heat is
not normally captured in conventional engines but it is straightfor-
ward to fit a heat recovery system to the exhaust of an engine if the
heat is required. The exhaust temperature is typically between 370 C

and 540 C. This is sufficiently high that it can be used to generate

medium-pressure heat if required, with a maximum pressure of
around 28 bar. Otherwise it can be used to generate hot water.
Engine exhaust gases have also been used directly for drying in some
applications.

The main engine case cooling system can capture up to 30% of the
total energy input. Cooling water exits the cooling system at up to
4
95 C. In a cogeneration system this will be passed through a heat

exchanger to provide a source of hot water. Engine oil and turbo-
charger cooling systems will provide additional energy that can also be
used to supply hot water. The engine cooling jacket and oil cooling
system will typically provide 45% 55% of the total waste heat recov-
ery from an engine system.

If all the heat from the exhaust and the cooling systems of an engine
is exploited, around 70% 80% of the fuel energy can be used.
However this can generally only be fully exploited when there is a
need for hot water. Overall efficiency will also depend on the duty

4 It may be hotter if the cooling system is pressurised.