Introduction | Table
of Contents | Chiller Models
Operating Environment for the Chiller System
Several conditions and assumptions relating to the environment in which
the chiller system would be operating had to be specified in order to determine
energy consumption and energy cost. The building cooling model, the weather
data, and the electric and gas rates are discussed below.
Building Cooling Load Model
As in the studies of Weber (1988), Joyce (1990), and Liu (1997), the building
cooling model used is a linear function of the outdoor dry bulb temperature,
Tdb. This study considers the cooling needs of a 400,000
square foot building in operation twenty-four hours per day. Figure 2.1
plots the cooling load profile. The design load of 1000 tons is reached
at 98oF or greater and decreases linearly to 200 tons at a temperature
of 60 oF. Below 60 oF, the load is taken to be constant
at 200 tons for internal heat gain due to lights, people, and equipment.
The building cooling load profile is described by the following equations:
Load = 200 tons (Tdb<60oF)
Load = (21.053*Tdb) - 1063.156 tons
(60oF< Tdb<98 oF)
Load = 1000 tons (Tdb>98oF)
Figure 2.1: Building Cooling
Load Profile (Joyce,1990)
With this cooling load profile and the weather data, discussed below,
it was possible to plot the ton-hours for the chiller systems which describe
the cooling load and the run time for the chillers. In the case of the
multiple chiller systems, the ton-hours were divided into two parts: the
base ton-hours (the portion of the cooling load met by the baseloaded chiller)
and the peaking ton-hours (the portion of the cooling load met by the peaking
chiller). Figure 2.2 below shows how the ton-hours are distributed among
months. The dark portion of bar represents the base ton-hours, and the
lighter portion of the bar represents the peaking ton-hours. The total
length of the bar represents the total ton-hours of the chiller system.
Figure 2.2: Ton-Hours for a
1000 Ton Chiller System
Weather Data
The building location was chosen to be Atlanta, Georgia, and weather data
was obtained from the National Climatic Data Center in Local Climatological
Data Monthly Summary reports (1994). This data is taken at Hartsfield Atlanta
International Airport. A wet bulb and dry bulb temperature is listed in
these reports and used in this study for every third hour. Therefore, the
building cooling load is calculated every three hours for an entire year.
This type of weather data is different than that used in other studies
which employed the bin method for sorting annual weather information. The
bins represent five degree temperature ranges. Weather data sorted using
the bin method describes the weather by listing the number of hours that
the temperature lies within each bin for each month of the year. The new
form of weather data used in the current study was necessary in order to
accurately reflect the weather on an hourly basis to correspond with the
hourly electric rates.
Electric Rates
The electric rates for the city of Atlanta were used for the calculation
of energy costs. Specifically Georgia Power's real time pricing (RTP) rates
were considered for this study. RTP rates consist of two parts: a base
charge and an incremental charge. The base charge covers electrical energy
used up to some base level. Electrical energy usage above or below this
base level is at an incremental RTP rate which varies hour to hour. The
incremental RTP charge may be either positive or negative, depending on
whether energy consumption falls above or below this base level. The RTP
rates used in this study were for the Westin Peachtree Plaza Hotel in Atlanta
for 1994. These rates vary hour to hour over a wide range. In 1994, the
rates ranged from as low as 1.5 cents/kWh to as high as 79.5 cents/kWh.
Since the weather data was listed every three hours, the RTP rates for
the hour of, the hour preceding, and the hour following that specific third
hour were averaged to obtain the mean RTP rate for each three hour period.
The incremental RTP charge is representative of the current deregulated
electric market rate (Power Value, 1998). The energy use differences between
the different chiller systems will be charged at this RTP rate.
Gas Rates
Natural gas rates for the Atlanta area were obtained from Atlanta Gas Light
Company for 1995. These rates were monthly rates for interruptible, seasonally
nominated natural gas. The rates also include transportation charges and
ranged from a low of 16.5 cents/therm for the month of August to a high
of 26 cents/therm for the month of December.