Environmental Engineering Reference
In-Depth Information
has the highest monthly demand, significant savings are possible in rate
reductions. In periods other than the highest demand period, energy is
still saved. Prior to the era of high energy costs, load shedding was used
mainly to avoid demand cost penalties. Now, it is used to limit energy
consumption, by cycling loads on and off for brief periods, as well as to
reduce demand. Other techniques used to limit energy use include the
computer optimization of start times, setpoints, and other operating pa-
rameters based on the weather, temperatures, or occupancy.
Electronic demand limiting includes devices that monitor and mea-
sure the actual demand and provide control actions to limit the operation
of attached devices when the measured demand reaches a specified value.
These devices require two signals, the kilowatt hour (kWh) or demand
pulse, which indicates the units of electrical energy consumed and a tim-
ing pulse, which indicates the end of one demand pulse and the start of
the next one.
Some load shedders use a demand target that is not fixed but increases
at a steady rate. Other devices allow the off-on setpoints to be adjusted in-
dependently for individual loads. Loads can be cycled based on the maxi-
mum demand target, time of day and day or week, rate of demand increase,
heating and cooling temperatures, pressures, fuel flow and rates, occupancy
schedules, inside and outside temperatures, humidity, wind direction and
velocity and combinations of the above factors. Durations can be variable
and changed automatically according to these parameters.
In air conditioning systems, intake and exhaust dampers can be con-
trolled on the basis of air temperatures, so that the mix of air requiring the
least energy is obtained at all times. The start-up and shut-down of air
conditioning, heating, and lighting systems can be regulated according to
inside and outside temperatures as well as occupancy to produce the con-
ditions which consume the least energy.
Utility programs for energy conservation have involved demand-side
management (DSM). These programs try to impact how customers will
use electricity. One technique is to even out the demand for electricity so
that existing power generating stations are operating at efficient capacities
throughout any 24 hour day rather than peaking up during business hours
and late afternoon and then dropping down later in the evening. The other
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