the coils has improved chilled-water usage
efficiency (increased Delta T).
With coils typically oversized by design
(to be conservative), standard valves have a
tendency to hunt, even under design conditions; peak load may only require 50 percent
to 75 percent of design flow as a result of
cooling-coil performance characteristics (fig.
1). When the load is at 90 percent, the flow
required is only 50 percent. As such, the
selected valve must be able to offer tight
Princeton University.
control at flows well below design.
Only a part of the planned coil and
valve replacements have been implemented
so far. These replacements have already
increased campus Delta T by more than 1 F
(fig. 2). This demonstrates that valve replacements can have a positive impact on Delta T,
as the projected impact of coil replacement
was only 0.4 F, and that project is not yet
complete. The Delta T increase represents
approximately 1,000 tons of cooling now
available on design days and reduced pumping energy requirements the rest of the year.
to 2003. HEATMAP® software was used to
locate ‘choke’ points and cost-effective
locations to reinforce the distribution system piping. All existing loads and a 10-year
construction forecast were included in the
model. Where available, historical temperature, pressure and flow operating data were
used to verify the model.
The study identified two major piping
additions as ‘required’ to meet specific construction deadlines and recommended several smaller projects
be done ‘when
possible.’ The first
project added
2,500 linear ft of
30-inch supply
pipe and converted
the existing supply
line into a parallel
return. Completed
during the winter
of 2003, it resulted
in a noticeable
reduction in total
differential head
and pumping energy. A second part of this
project will include installation of 1,000 linear
ft of
24-inch supply and return piping in winter
2004-2005.
Figure 2. Chilled-Water Temperature Differential, Princeton University,
2004 vs. 2002.
Distribution Upgrades
Kattner/FVB (now FVB Energy) conducted a thermal-hydraulic study of
Princeton’s distribution system from 2002
Real-Time Economic
Dispatch
In August 2003, commercial electric
purchases in New Jersey were deregulated.
Prior to that, the electric tariff provided a
simple price structure but little opportunity
to reduce operating costs. With deregulation, Princeton now purchases power at the
wholesale market rate. At night prices are
often as low as $20/MWh – far below
Princeton’s marginal cost to generate power.
With a traditional tariff system, cogeneration runs in a load-following manner.
Any campus load not met by cogeneration
is imported from the grid. Demand structure can penalize cogeneration for reduced
capacity. In a dynamic deregulated energy
market, cogeneration output can be
reduced when the cost to generate is higher than the market.
During the day, prices can rise into the
hundreds of dollars per MWh. With a
cogeneration system and boilers that can
burn either diesel fuel or natural gas, the
choice of steam- or electric-driven cooling,
and the opportunity to purchase and/or
generate power, the most cost-effective
means to deliver energy at any given time is
far from obvious. Historically, the plant was
operated for high reliability with simple rules
based on conventional wisdom, such as
“steam chillers are cheaper than electric.”
In response to the wholesale market, a
real-time economic dispatch system (see fig.
3) was developed by Princeton and Icetec
Inc. that predicts campus energy demands
and recommends the most cost-effective
combination of equipment for operators to
use to meet those requirements. The model
inputs include real-time data for weather,
NYMEX gas and oil prices and futures, campus energy demands, equipment efficiencies
and availability. Using this system allowed
the plant operator’s focus to shift from simply meeting demand to delivering energy in
the most cost-effective manner.
While this system could be fully automated, Princeton chooses to have plant
operators use it as an expert reference
source while they manually take into
account reliability issues such as local storms
and campus events. The addition of thermal
storage to Princeton’s energy plant will provide considerable operating flexibility and
cost-saving opportunities.
