Environmental Engineering Reference
In-Depth Information
vide some or all of the cooling without using electricity during the high
cost, peak period. The electric rate structure is one of the primary determi-
nants of the choice of cooling medium. Hybrid plants are generally a bet-
ter option because the cost of cooling with electricity during some periods
can be less than the cost of cooling with natural gas.
Absorption chillers, especially the double-effect type, are used for
utility rate structures with high peak period demand. These include usage
charges or rate schedules with ratchet clauses for the demand charges. There
is a significant first cost premium for this equipment. Maintenance costs
are generally comparable with electric chillers, but the absorption chiller
requires more day-to-day maintenance. Engine-driven chillers provide an
alternative to absorption chillers when natural gas cooling is desired. En-
gine-driven chillers utilize the same type of equipment as electric chillers
for cooling, but replace the electric motor with a natural gas fueled engine.
One problem with this equipment is that the maintenance and operation
staff may be unfamiliar with the requirements of the engine. Especially for
truck-derivative engines, maintenance costs are significant and must be ac-
counted for in the operating costs. Noise and vibration are also concerns
that must be addressed. A major benefit of engine-driven chillers is the op-
portunity to capture waste heat from the engine as a mechanical cogenera-
tion systems. Another option is the use of dual-drive chillers with both an
electric motor and a natural gas engine available to drive the chiller.
Thermal storage systems can be used to shift the cooling load from
high cost periods to low cost times of day. The major design concern is al-
lowing for sufficient storage and re-charging capacity to allow for some
load and temperature increase for overnight periods. There is a significant
danger of poor operation and the inability to fully transfer loads if some
spare capacity is not provided.
Water-side economizers used in areas with low wet bulb tempera-
tures, especially when the use of an air-side economizer cycle is not feasi-
ble. One problem that must be considered is the change-over from econo-
mizer operation to chiller operation since the low condenser water tem-
perature can affect the operation of the chiller. The use of a water-side
economizer also affects the cooling tower since there are collateral benefits
in providing a larger cooling tower.
Primary-secondary chilled water distribution systems were devel-
oped to allow a constant flow through chillers, required by the chiller
manufacturers, with variable flow for the load side of the system to im-
prove efficiency. The main applications are in multibuilding systems or
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