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60 TRG-TRC016-EN
notes
period three
System Variations
doubled, and even tripled, during periods of high demand in the first summer
of deregulation. Utility deregulation will occur differently in various locations,
but the possibility of high electricity costs during peak periods has building
owners and operators looking for ways to use different fuels during those
periods.
Another reason for using different fuel types is that it provides the building
owner with leverage to negotiate for reduced utility prices from competing
utilities. If a building uses both electricity and natural gas for cooling, and
can switch between the two, the owner can often negotiate better rates for
both cooling and heating.
A chilled-water system that uses more than one type of fuel is referred to as a
hybrid system. The most obvious option for using an alternate fuel is an
absorption chiller. This type of chiller can be powered by natural gas, fuel oil,
or even waste heat in the form of steam or hot water.
Another option is to use natural gas to operate an engine-and-generator set that
produces electricity, and then use that electricity to run a standard electric
chiller. This indirect coupling of the gas engine to the chiller allows the flexibility
of operating the chiller using the gas engine during times of high electricity
costs, and operating the chiller on utility (line) electricity during times of low
electricity costs. A second benefit of indirect coupling is that the engine can
be sized to provide enough power for the chiller, the pumps, and, in a water-
cooled system, the cooling tower. If the engine is also to be used for emergency
backup, the pumps and cooling tower would not need a second generator to
provide them with power.
An alternative to this approach is to directly couple the engine and chiller.
A significant drawback of this approach is that the building owner does not
have the flexibility to switch between natural gas and electricity—the chiller
must always operate on natural gas. Also, only the chiller is connected to the
engine. If emergency backup is necessary, a second generator is required to
operate the pumps and cooling tower.
Fuel Choice Options
absorption
thermal storage
indirectly-coupled,
gas-engine
chillers
control interface
control interface
power
power
Figure 67