had an average annual total plant efficiency between 0.90 k W/ton and 1. 2
k W/ton; the California plant had an
average annual total plant efficiency
between 0.80 k W/ton and 1.0 k W/ton.
Even in very humid climates such
as the UAE (with design wet bulbs
greater than 89 F), district cooling is
still a part-load and part-lift industry
with a large portion of the hours occurring at less than 74 F wet bulb and 60
percent capacity. One mistake many
engineers make is confusing “load” and
“lift” in centrifugal chiller plants. “Lift”
can be simply defined as the difference
between the exiting condenser water
temperature (out of the chiller) minus
the chilled-water supply temperature.
When talking about large district cooling plants, the use of variable-speed and
high-performance control algorithms,
one must talk first in terms of lift optimization and, secondly, load optimization. Load optimization directs the plant
control sequences to run everything as
close to full load as possible, while lift
optimization lends itself to the use of
variable-speed chillers and components.
One mistake many engineers
make is confusing “load”
and “lift.”
In contrast to conventional control
methodologies, relational controls take
advantage of the ability to optimize the
operation of all variable-speed HVAC-system components in relation to one
another and in response to real-time
building loads. Additionally, software
solutions available today are delivering
ultra-high performance across a wide
range of plants without compromising
reliability.
A paradigm shift is leading
to significantly improved
kilowatts per ton.
These solutions can be quickly
installed, and they provide persistent,
reliable optimization and real-time
measurement and verification capabilities
that are unprecedented in the industry.
Along with M&V reporting, new Web-based software dashboards provide
real-time monitoring of plant performance metrics and operator alerts if the
system is not performing as expected,
thus helping to maintain savings over
time. This represents a paradigm shift
that is leading to significantly improved
kilowatts per ton (COP), providing
enhanced automatic plant control and
meeting the goals of long-term reduced
energy consumption and operating costs.
As can be seen in figure 2, the
sharp drop in plant efficiency occurs at
very low loads ( 10 percent and below) in
the California example vs. the UAE
example. This is a result of a two-chiller
plant serving the California campus vs.
eight chillers in the UAE plant. When
one chiller is operating at capacities
below 20 percent load, chiller performance falls, and auxiliary equipment
must maintain the minimum flow
requirements of the chiller condenser
and evaporator bundles. Low load is
often a problem in newer systems
because the plants are built for future
growth as the campus builds out. But,
without variable speed mitigating these
low-load conditions, performance with a
standard constant-speed plant at capacities below 20 percent can be anywhere
between 2 and 5 k W/ton.
Ben Erpelding, PE, CEM, is
director of engineering for
Optimum Energy LLC. He has
more than 11 years’ experience in
energy efficiency, distributed gen-
eration, renewable energy and
demand response. Prior to joining the firm, he
measured and verified actual performance and
cost savings for energy efficiency retrofits, pho-
tovoltaic installations, demand response audits,
and combined heat and power projects while
at the nonprofit San Diego Regional Energy
Office. Erpelding received a master of science
degree, with an emphasis in combined heat
and power, and a bachelor of science degree in
mechanical engineering from San Diego State
University. His email address is ben.erpelding@
optimumenergyco.com.
