tors. The RFP specified that numerous
and varied types of energy conservation
measures could be included within the
scope of the project. The RFP also request-
ed that several concerns be addressed
regarding specific inefficient and poorly
operating cooling systems. These included
( 1) an aging and relatively small district
cooling plant that serves a core of
10 of the state’s oldest buildings;
( 2) an old, unused 740,000-gal chilled-
water TES tank associated with the
old cooling network; and
( 3) aging, inefficient chillers and
degraded cooling towers in four
large buildings remote from the
existing district cooling network.
(Their capacity was to be replaced,
presumably within those buildings.)
(Note: The state complex is also served
by an existing steam district heating network that was not included in the RFP.)
The state selected the proposal offered
by Pepco Energy Services. The company
then performed the detailed audit phase
of the project throughout the first half
of 2005. Project implementation began
in November 2005, with completion of
the energy conservation measures in late
2006 and 2007. The expanded district
cooling system was commissioned and
placed in service during spring 2007 and
commissioned during the summer.
Courtesy Natgun Corporation.
Raleigh’s upgraded district energy system includes a new 26,270 ton-hr thermal energy storage tank.
Designed to blend architecturally with an adjacent parking structure, the tank was partially buried, which
minimize its height and aesthetic impact.
The Solution
The successful solution involved an
approach integrating existing assets that
were modernized and improved as well as
new equipment and infrastructure. These
improvements involved four groups of
energy conservation measures: lighting
upgrades in various buildings, HVAC and
control improvements within the buildings,
water conservation initiatives, and a major
expansion and modernization of the district cooling system. The latter included a
new TES system, a new packaged chiller
plant, upgrades to existing chillers and
the existing TES system, system control
modifications to ensure a high delta T
throughout the chilled-water system, and
expansion of the existing cooling loop
to accommodate additional and future
facilities.
The project benefits are as follows:
● More than $2 million per year in
energy savings
● Approximately $18.9 million in new
infrastructure
● Reductions in energy usage of more
than 20,000,000 k Wh/year
● Reductions in water consumption of
more than 10,000,000 gal/year
● Improved building comfort
● More than $7 million in future capital
cost avoidance
● Expanded cooling capacity and system
redundancy
The district energy solution was
dependent on the integrated use of four
important elements: district cooling,
thermal energy storage, packaged chiller
plant and improved chilled-water delta T.
District Cooling Is Backbone
The energy services contractor based
its unique and successful proposal not
on replacing the four large in-building
chiller plants with new in-building plants,
but rather on a major expansion of the
existing district cooling network to include
those four buildings as well as other new
additions.
The old central chiller plant, which
had served 10 state buildings totaling 1. 3
million sq ft of space, was modernized;
the existing unused chilled-water TES
tank was repaired and re-commissioned.
However, the primary investment in new
chiller capacity was focused on an all-new,
larger-scale, highly energy-efficient packaged chiller plant located at a more centralized site within the expanded district
cooling network. The expanded system
now serves 20 buildings that total 3. 5
million sq ft. No new chillers were purchased for any individual buildings.
The new packaged chiller plant became
the primary source of chilled-water generation. The old, rehabilitated chilled-water plant became a secondary source at
a satellite location. Additionally, the most
reliable and efficient of the remaining in-building chiller plants became available
as distributed, local, tertiary sources of
chilled-water generation, used only for
peaking capacity or emergency backup
capacity in extreme cooling conditions.
The majority of the new piping used
in the district cooling system is direct-buried, high-density polyethylene (HDPE)
pipe. HDPE was chosen due to its ease of
installation, longevity, and life-cycle-cost
considerations. Insulated steel piping is
used for all above-ground and in-building applications.
TES Enhances Solution
Critical to the successful economics
of the project, in addition to rehabilitation of the old, relatively small TES tank,
was installation of a new 2. 7 million gallon
TES tank, located at the site of the new
packaged district cooling plant. This large,
new stratified chilled-water TES tank provides two key economic benefits: It minimizes the use of chillers during high-cost,
on-peak utility periods, greatly reducing
demand costs and also benefiting from
