Page 61 - Sustainable On-Site CHP Systems Design, Construction, and Operations

40 CHP B a s i c s

and heat from the IC engine coolant loop. The IC engine type of system is most often
used where the electrical loads are 2 to 3 MW or less, and where the need for thermal
heat is low in comparison to the need for electrical power (i.e., a relatively low thermal-
electric ratio). Since IC engine prime movers tend to be more fuel efficient than combus-
tion turbines, there is more electrical power and less thermal energy proportionally
derived per unit of fuel.
IC engines are available in sizes from less than 50 kW to more than 15 MW. Com-
bustion turbine generators range from approximately 1 MW to well over 100 MW.
Combustion turbines require high-pressure gas supply or gas compressors to provide
the necessary gas pressure. Combustion turbines typically have higher exhaust
temperatures, higher volume exhaust and, as a result, combustion turbine generators
are better suited for applications where high temperature recovered heat is required,
such as high-pressure steam, or low-pressure high temperature oil. Combustion turbine–
based CHP systems also have higher thermal-electric ratios than IC engine–based
systems. In addition, since combustion turbines tend to be less thermally efficient in
producing electricity, the application favors situations where there are high year-round,
24 hours a day thermal loads and where the cost of piping thermal energy to those loads
is relatively low.
Internal combustion engines are better suited for applications which require lower
recovered waste heat temperatures, such as hot water or low-pressure steam (typically
less than 15 psig). IC engines can be used with lower quality fuels. An example is an IC
engine powered on methane gas from a wastewater treatment plant or a garbage land-
fill. Internal combustion engines require less specialized maintenance, training, and
auxiliary equipments than combustion turbine generators.

IC Reciprocating Engines

Engine Types
IC reciprocating engines are machines that translate the linear movement of pistons into
the rotational movement of a crankshaft through a combustion process. Combusting fuel
heats and expands the fuel-air mixture inside a cylinder which drives the piston. Most
of engines today are multicylinder for smoother power delivery. The two basic types of
IC reciprocating engines are spark ignition and compression ignition. Each of these
engine types is available to operate in either a four- or two-stroke combustion cycle. The
strokes of the four-stroke cycle are intake, compression, power, and exhaust. Engines
are either naturally aspirated or turbocharged. In naturally aspirated engines, air and
fuel are mixed in a carburetor and the intake stroke draws in the fuel-air mixture. A
turbocharged engine has a compressor which compresses air and discharges that air
into the combustion chamber during the intake stroke. In most cases, only the air is
injected by the turbocharger and fuel is directly injected into the combustion chamber
eliminating carburetors or external mixing of fuel and air. In either case, a turbocharged
engine can deliver more power because there is a greater density of fuel and air in
the process.
The two-stroke cycle differs from the four-stroke cycle in that it combines the power
and intake strokes into one stroke while the exhaust and compression strokes are com-
bined into a second common stroke. Start-up time for reciprocating engines can be fast,
around 10 seconds for diesel-fueled engines. Warm-up times take significantly longer
and depend upon the mass of the system. The warm-up time can be reduced if the system