How High Can You
New approaches for cooling
system efficiency
Feature
Story
Go?
Ben Erpelding, PE, Director of Engineering, Optimum Energy LLC
The volatility of energy costs and emphasis on reducing carbon footprints have increased the
urgency of improving district cooling
plant energy performance. At the same
time, facility managers and owners face
increased pressure to reduce operating
costs, take on new loads and continue
to provide 100 percent reliability and
zero downtime for their chilled-water
services. This poses a dilemma for plant
operators, as energy efficiency usually
ranks lower as a priority than reliability,
availability and even construction time
schedules.
It is possible, however, for chilled-water plants to achieve new levels of
efficiency – what Optimum Energy calls
“ultra-high performance” – without compromising reliability. It’s already being
done in plants from California to the
United Arab Emirates using approaches
that represent a new paradigm in system optimization.
What Is Ultra-High
Performance?
In district cooling plants, ultra-high
performance (a term first introduced in
a 2001 ASHRAE article by Tom Hartman)
is defined as the delivery of chilled
water at an average annual efficiency of
0.45 to 0.70 k W/ton (annual kilowatt-hours divided by annual ton-hours) [ 7. 8
to 5.0 coefficient of performance (COP)],
including all chiller, chilled-water distribution pumping, condenser water pumping and cooling tower fan energy. [This
annual average efficiency is based on an
all-electric plant and excludes hybrid
(gas- or steam-driven) plants and deep
lake/ocean water cooling plants; these
can achieve electrical efficiencies of less
than 0.30 k W/ton.]
While 0.45 k W/ton is achievable,
plant design, equipment, load profile
and climate (wet-bulb) push these levels
higher. The transitional climates, such
as New York, allow free cooling (
plate-and-frame heat exchanger) strategies
throughout the winter; temperate climates
like Los Angeles have year-round cooling
loads at relatively low wet bulbs; the
arid subtropical climate of the Middle
East (e.g., Dubai) is known for extreme
wet-bulb designs (89 degrees F); and
tropical climates, such as Singapore, have
a nearly constant outdoor wet-bulb temperature year-round. Though the efficiency variance in ultra-high performance
plants is minimal (0.45 to 0.7 k W/ton),
engineers need to understand how to
mitigate each climate’s effect on potential
cooling plant efficiency.
A Winning Combination
Achieving ultra-high performance in
district cooling plants requires a combination of best-of-class plant design and
sophisticated optimization algorithms,
as well as a fully automated plant and
extensive control system. Only this combination makes it possible to sustain
consistent operation of a chiller plant at
levels less than 0.6 k W/ton for extended
periods across different facility types
and global locations.
In the past, district cooling plants
held strong to the control theory that
because their plants have multiple
chillers, equipment can be sequenced
accordingly to keep them as fully
loaded as possible. Control strategies
were written into the plant design and
specification by the engineer of record,
and custom programming was required
by the control contractor. Plant efficiency,
therefore, was limited by the equipment
full-load efficiency and the technical
expertise of the on-site programmer.
This old paradigm has resulted in average
annual plant operating efficiencies of
0.8 to 1. 1 k W/ton.
Today higher levels of efficiency are
attainable by employing ( 1) variable-frequency drives throughout the plant – all
chillers, chilled-water and condenser-
