ent of a technique for saving energy
in refrigerated walk-in coolers, and to
evaluate the potential for this technol-
ogy in Federal facilities. The focus
of this study was on a single manu-
facturer of the technology, Nevada
Energy Control Systems, Inc. (Necsi);
no other vendors for this technology
could be found.
Previous studies were inconclusive
about the overall efficacy of this
technique due to uncertainties in a
number of areas.[1] Previous evalu-
ations also lacked the benefit of the
results from recent manufacturer
sponsored tests and did not address
some fundamental issues about the
overall efficacy of this technology
that are critical to understanding its
potential. The primary objective of
this assessment was to determine if
the previous studies combined with
recent vendor sponsored test results
substantiate the manufacturers claims
that this is a cost effective energy
saving technique with significant
potential in Federal facilities. Sec-
ondary objectives included evalua-
tion of intangible benefits such as
equipment life and reliability issues,
and humidity and airflow effects on
product.
Technology Description
Walk-in boxes are used in a wide
variety of applications but their use
in food sales and service facilities
dominates all other uses (see market
size discussion below). There are
two major classes of walk-ins: low
temperature (-10 to -20
°
F) and
medium temperature (-10 to 30
°
F).
In most cases the panels, refrigera-
tion components and controls are
ordered separately and assembled
on site by local refrigeration con-
tractors. Some smaller units are
supplied fully assembled. Sizes are
typically 80 to 750 sf and 8 to 10 ft
high; or about 640 cu ft to 7500 cu
ft; the average size appears to be in
the range of 2000 cu ft. Condensing
units are split systems with semi-
hermetic compressors rated at 1.5
to 5 hp and operate with HCFC-22
or HFC-404A refrigerants.[2]
The typical target application
of the Necsi control is a medium-
temperature, medium-to-large cooler
with a dedicated (or rack) refrigera-
tion system that operates with single-
phase powered evaporator fans. The
control is not intended for freezers
since most freezers use electric defrost
and cycle evaporator fans off during
the compressor off-cycle. These sys-
tems also tend to have longer run
times and less load variation than
coolers, which limits their energy
savings potential.
The U.S. Department of Energy
requests that no alterations be
made without permission in any
reproduction of this document.
DOE/EE-0170 Principles of Operation
The design and operation of the
control are very simple. An auto-
transformer is installed in the evapo-
rator motor electric power circuit.
This auto-transformer supplies low
voltage to the motors (typically one
or more banks of 1/20-hp motors)
whenever there is no call for cool-
ing; i.e., when the compressor is off
in dedicated compressor systems
or when the liquid line solenoid is
closed in a rack system. The switch
to low voltage is caused by a logic
circuit that senses the decrease in
temperature difference across the
expansion device using two ther-
mistors. Control, therefore, is not
directly tied to compressor opera-
tion, only to sensing of flow via
temperature sensors. This allows
the technique to be also used on rack
systems where a solenoid valve in
the liquid line cycles each unit cooler
on and off for space temperature
control. At low voltage (approxi-
mately 35% of primary voltage) the
motor operates at reduced speed
(typically ~400 rpm) and airflow.
Power input (including the effect of
auto-transformer losses) is reduced
by about 75-85% when operated at
low speed.
This speed control technique is
limited to single-phase shaded pole
and permanent split capacitor motors.
The controller is supplied in two mod-
els rated at 10 amp, 110/115 vac and
5 amp, 208/220 vac, respectively.
Potential of Applications
Limitations
The preferred application for these
controls are medium-sized, medium-
temperature (28-40
°
F) walk-in cool-
ers with shaded pole evaporator fan
motors and where the refrigeration
system has been oversized with respect
to actual loads. Although this technol-
ogy is applicable to installations with
permanent split capacitor motors, the
savings magnitudes will be greater
with shaded pole motors due to their
lower overall efficiency. The magni-
tude of savings must be carefully
calculated for each specific site with
the major parameters fully accounted
for until better estimating method-
ologies are developed. Assumptions
based on previous test results from
other sites should not be generalized
to a candidate site.
Federal Sector Potential
Applications of this technology
in Federal facilities are subject to
the same limitations and caveats
that apply to private sector facili-
ties. There are an estimated 129,000
walk-ins (coolers and freezers) com-
bined in Federal, state, and local gov-
ernment buildings. Since Federal
facilities make up only 13% of the
total number of government buildings,
there are an estimated 12,000 walk-
in-coolers in Federal facilities that
are potential candidates for this tech-
nology.[1,2] The estimated maximum
savings (if all potential applications
were retrofitted) using conservative
estimates of percentage savings is
about 155,000 Mbtu/yr. The actual
potential is somewhat less than this
since many walk-ins will not be prac-
tical candidates for this technology,
and it is not known how typical
Federal facilities are compared to
all other government facilities.
Cost Effectiveness
Since cost effectiveness is driven
by the magnitude of savings and its
cost relative to cost of implementa-
tion, savings percentages per se are
not good indicators of the efficacy of
this technique. A detailed estimate
of cost effectiveness was beyond the
scope of this study, but it is clear
that this computation is much more
complex than it appears from the
manufacturers perspective. Although
implementation costs are well estab-
lished at $400 to $500 per controller
plus $100 for installation, the magni-
tude of savings is highly site specific.
In addition, for the reasons stated
below, the methodology used to esti-
mate annual energy savings may be
inaccurate. Based on results of a
single test, these savings could be
understated or overstated depending
on the specific operational and system
design aspects of a particular site.
Using percentages demonstrated by
the test results can be misleading if
they are applied to historical energy
use based on billing meter data.
Because these results reflect the sum
of condensing unit and evaporator
energy only and not the entire walk-
in box installation, existing energy
use must be disaggregatated to sepa-
rate out compressor and fan energy
before percentages are applied.
Overall, the cost effectiveness
based on test results and analyses to
date cannot be generalized. Lacking
a well-developed energy savings
estimation methodology that accounts
for all major factors that affect per-
formance, each situation has to be
considered on its own merits. These
calculations should include consider-
ation of all of the major factors iden-
tified in this report.
Summary and Conclusions
This review produced mixed results
in terms of verifying the expected
savings magnitudes and cost effec-
tiveness of using this technique in
Federal facilities. On the positive
side are these issues:
1. The technology is simple, direct,
and relatively inexpensive to apply.
2. The technology does save energy
both directly by reducing evapo-
rator fan energy and indirectly via
reduction of compressor energy
due to reduced heat gain from
the fan motors. The overall
potential of this technology is
enhanced by the fact that about 75% of all walk-in boxes use low-
efficiency shaded pole motors.
3. The energy savings measure-
ment methodology, although
not ideal because power factors
are assumed to be unity, appears
to be substantially correct for
permanent split capacitor evapo-
rator and compressor motors
and reasonably reflects savings
in active power under the test
conditions encountered. This
is not the case, however, for
shaded pole motors (see below).
In addition, by ignoring the effect
of change in power factor from
full speed to low speed, savings
for shaded pole motors are
underestimated by about 5%.
4. Stratification due to low speed
evaporator fan operation does
not appear to be a significant
problem.
5. The total savings potential for
Federal facilities appears to be
greater than previous estimates
even for low estimates of savings.
The estimated number of walk-
ins (coolers and freezers com-
bined) in Federal facilities to
which this technology is appli-
cable is about 12,000.
On the other hand, the following
issues should be taken into account
when this technology is being con-
sidered for use:
1. The percentage and magnitude
of savings (and therefore cost
effectiveness) are highly variable
and dependent on specific site
conditions. The energy savings
potential for a particular site
cannot be generalized to other
sites from any of the information
available without further analy-
sis and/or testing. Three major
factors affect the magnitude of
savings at a particular site:
Temperature dependence
extra