Handbook of air conditioning and refrigeration / Shan K

Gas-Engine Chiller
Gas Engines
HEAT PUMPS, HEAT RECOVERY, GAS COOLING, AND COGENERATION SYSTEMS 12.27
A gas-engine chiller is often a combination of a gas cooling system and a cogeneration system.
Heat released from the exhaust gas and the engine jacket cooling water are all recovered to increase
system efficiency. Figure 12.11a shows the energy flow of a gas-engine chiller system.
If the efficiency of the gas engine is 35 percent, the mechanical efficiency including the transmission
gear train is 95 percent, and the chiller’s COP � 4.5, then for every 10,000 Btu (10,550 kJ)
of fuel energy input to the gas-engine chiller, there is a cooling output Q rf of
Q rf � 10,000 � 0.35 � 0.95 � 4.5 � 15,000 Btu (15,825 kJ)
In addition, there is a heating output of 4000 Btu (4220 kJ) from the exhaust gas and engine
jacket to supply hot water or low-pressure steam for an absoption, space heating, or domestic hot
water unit.
A gas-engine chiller has a prominent advantage over a motor-driven chiller because the former
can vary its speeds at various operating conditions: high speed at overloads and low speeds at partload
operation. Engine reliability is the key to user acceptance for gas-engine chillers. Several
hundred engine-driven chillers were installed in the 1960s and early 1970s. According to reliability
records of these systems reported in Davidson and Brattin (1986), the reliability of gas-engine
chillers matches the requirements of HVAC&R systems.
Gas engines can be used to drive screw, reciprocating, or centrifugal compressors. Gas-enginedriven
screw chillers are becoming more and more popular. Many manufacturers offer packaged
units for easier field installation. The capacity of gas-engine chiller packaged units varies from 30
to 500 tons (105 to 1760 kW). Gas-engine cooling systems can also be coupled to direct-expansion
(DX) refrigeration systems and rooftop packaged units. Gas-fired internal combustion engines for
cooling in buildings follow the developments of gasoline and diesel engine technology. There are
two kinds of gas engines: heavy-duty industrial applications and light-duty automotive engines. Industrial
heavy-duty gas engines run a minimum of 30,000 h of full-load service between major
overhauls, and cost about 5 times as much as automotive engines. The service life of an automobile
engine is only 2000 to 5000 h.
Recently, manufacturers have produced packaged automotive gas-engine chillers of 150-ton
(525-kW) capacity. One manufacturer also offers one 150-ton (525-kW) gas-engine chiller and
integrated hot water absorption chiller to give a total maximum output of 180 tons (630 kW).
Automotive gas engines are suitable for compressors that require speeds far above 1800 rpm. This
packaged gas-engine chiller is also equipped with microprocessor-based controls to coordinate and
monitor the operation of the engine and chiller. An operating cost as low as one-half that of similarsize
electricity-driven units is claimed, depending on the local utility rate structure.
Engine-driven chillers are maintenance-intensive. The maintenance cost of a gas-engine chiller
may be between 10 and 20 percent of the energy cost, and it should be added to the operating cost
during economic analysis. D’zurko and Epstein (1996) made a cost analysis between gas-engine
chiller and electric chillers for offices, schools, restaurants, retail stores, and hospitals in New York
State. Most have a simple payback of 2.1 to 11.9 years.
For a gas-engine chiller with a cooling output of 15,000 Btu (15,825 kJ), if its by-product, 4000-
Btu (4220-kJ) heat output from the exhaust gas and engine jacket is also counted, then the simple
payback period is usually between 2 and 5 years.
Two types of automotive engines are used to drive the chillers: gas engines operated on diesel cycle
and operated on an Otto cycle. Because a diesel engine has a higher thermal efficiency than an Otto
engine, more diesel engines are used in gas-engine chillers. Most diesel engines are operated on a
four-stroke cycle, i.e., an intake stroke, a compression stroke, a power stroke, and an exhaust stroke,
to produce power. A diesel engine can also operate on a two-stroke cycle, an intake stroke and an