Table 1. Chiller and TES Capacity and Design Day Cooling Loads.
Primary Chiller Capacity
(New Packaged Chiller Plant)
2,900 tons (operates off peak and,
often, as needed, on peak)
Secondary Chiller Capacity
(Rehabilitated Chiller Plant)
Approximately 1,800 tons (operates off peak
and, at times, as needed, on peak)
Peaking & Reserve Chiller Capacity
Administration Building
Albemarle Building
New Revenue Building
Highway Building
(operates only rarely and always off peak)
485 tons
450 tons
750 tons
375 tons
modate the incorporation of two additional large existing state buildings and
three large future buildings. Second, the
new TES tank was oversized beyond that
necessary for leveling present peak day
loads – not an idle investment, as it can
be used now to shift larger cooling loads
and achieve larger electric demand management. Third, the new packaged chiller
plant was designed to allow for easy
future capacity expansion.
TES Capacities
New Chilled-Water TES Tank
Old Chilled-Water TES Tank
(discharges on peak, recharges off peak)
3,700 tons of discharge ( 26,270 ton-hr total)
800 tons of discharge ( 7,100 ton-hr total)
Total Available Capacity
Chillers Only
Chillers Plus TES Discharge
6,760 tons
11,260 tons
Design Day Peak Load
Design Day 24-hr Average Load
With Initial
Buildings Only
4,872 tons
3,500 tons
Also With Legislature and
Legislative Office Building
5,881 tons
4,162 tons
The existing in-building chillers, which
can provide peaking and reserve cooling
capacity, are not designed for the low
( 39 F) chilled-water supply temperature.
However, they are used on a limited basis,
and are not planned to feed the district
cooling network. In those instances when
in-building chillers are dispatched, they
are used solely to serve load within their
own individual buildings, effectively shedding that load from the district cooling
system.
Reliability and Redundancy
The project now provides a minimum
of “N+ 1” redundancy in critical mechanical
items such as chillers and pumps. The use
of an integrated district cooling system
permits this to be economically achieved,
whereas it would not have been economically feasible if serving each building
only with its own in-building chiller plant.
This added redundancy and reliability is
a benefit that the state did not previously
enjoy, and would not now be enjoying
without the use of district cooling.
The upgrade of Raleigh’s state government energy infrastructure, under the
direction of Pepco Energy Services, achieved
an innovative and extremely beneficial
result. The magnitude of the project
benefits, and even the economic viability
of having a project at all, was dependent
on the integration of the key technical
elements: district cooling, TES, packaged
chiller plants, pressure-independent flow
control valves ¬and the tactical use of
various existing assets.
Results and Benefits
Management of Energy and Energy Costs
Each state building is now independently metered and billed in accordance
with that building’s applicable standard
electric utility tariff. Because buildings
have been connected to the district cooling network, the Energy Conservation
Center can be used to adapt to and control
the load profiles of the individual buildings. This is made possible primarily
though the use of the TES. These strategies
allow the state to select the most advantageous electric rate tariff for each building, further increasing the energy cost
savings. Through the avoidance of in-building chiller operation in many of the
large buildings, the state can benefit from
the use of the more economical Small
Time-of-Use tariff.
The old chiller plant and the new
Energy Conservation Center, each with a
nearby TES tank, are billed on the attractive TES tariff. Excess storage and chiller
capacity also allow both chiller plants to
avoid operation during the day-time peak
periods of high demand and energy charges
during the cooler months. Additionally,
water flow within the network is managed through the use of variable speed
pumps to further enhance system efficiency and control.
A new custom-applied Siemens Energy
Management Control system monitors,
controls and coordinates the two chiller
plants, the TES systems, the individual
building chilled-water needs and the
district cooling flow. Also, each facility
served by the system is metered to determine the actual flow rates and temperature differentials.
The project is a showcase
of what district energy can
deliver, especially when it is
leveraged with complementary
technologies that are not as
readily deployed on the scale
of individual building systems.
Expandability for Future Load Growth
Several steps were taken in designing
the project to accommodate future district
cooling system expansion. First, the piping
headers were sized large enough to accom-
The result for the performance contractor is an attractive rate-of-return
applied to a large investment, thus creating a large net present value. The result
for the state is a much better-than-antici-pated solution to their cooling issues, with
much more new infrastructure, much
