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FEMP
New Technology
Demonstration Program



Technical Brief


FEMP

Federal Energy Management Program


Tom Webster, P.E.
Center for the Built Environment
University of California, Berkeley CA

Lawrence Berkeley National Laboratory
LBNL 57651

August 1998

This work was supported by the
Assistant Secretary for Energy
Efficiency and Renewable Energy,
Federal Energy Management Program,
of the U.S. Department of Energy under
Contract No. DE-AC03-76SF00098.



Energy Savings in
Refrigerated Walk-in
Boxes


Introduction and Background
The purpose of this technical brief is to
provide an overview of the results of an
assessment of a technique for saving energy
in refrigerated walk-in coolers, and to
evaluate the potential for this technology in
Federal facilities. The focus of this study
was on a single manufacturer 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 evaluations 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. Secondary objectives
included evaluation of intangible benefits
such as equipment life and reliability issues,
and humidity and airflow effects on product.

Summary and Conclusions
This review produced mixed results in terms
of verifying the expected savings
magnitudes and cost effectiveness 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 evaporator fan

1 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
(SP) motors.
3.

The energy savings measurement
methodology, although not ideal
because power factors are assumed to be
unity, appears to be substantially correct
for permanent split capacitor (PSC)
evaporator 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 SP
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 significantly
greater than previous estimates even for
low estimates of savings. The estimated
number of walk-ins (coolers and
freezers combined) in Federal facilities
to which this technology is applicable is
about 12 K; this number is somewhat
greater than previous NTDP estimates.

On the other hand, the following issues
should be taken into account when this
technology is being considered 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 analysis and/or
testing. Three major factors affect the
magnitude of savings at a particular site:

Temperature dependence -
Extrapolation of results from a
single test to annual performance
cannot be assumed to be accurate
without a more complete
understanding of the system
performance variation with ambient
temperature. Performance can be
affected by the head pressure
control strategy used for the
condensing unit, dependence of
cooler load on outside weather, and
the weather patterns at a site.

Oversizing - System design capacity
relative to actual loads (condensing
unit and unit cooler)

has a major
impact on savings since it directly
affects the time that the evaporator
fans operate at low speed.
Oversizing can vary appreciably
between sites.

Motor type - The motor efficiency
for SP motors (~ 35%) is
significantly less than that of PSC
motors (~50%-65%). The
magnitude of savings will vary
depending on the type of motor
used. Efficiency at reduced speed,
although extremely low (~4-5%), is
about the same for both motors.
2.

Power factor was not used in computing
the savings for any of the sites except
for one test where a wattmeter was used.
Since the power factor for shaded pole
motors is about .6 or less, the savings
magnitudes are significantly
overestimated for the sites with these
motors.
3.

Although not necessarily germane to the
technical assessment focus of this
report, there is a large discrepancy
between Bureau of Census data used by
Necsi and that based on Commercial
Buildings Energy Consumption Survey
(CBECS) data [2]; thus the market size
claimed by the manufacturer could be
significantly overstated. Based on
recent CBECS data there are a total of
approximately 700 K walk-in boxes to
which this technology is applicable. As
stated above, however, CBECS data also
show that the potential in the Federal
sector computed by the NTDP appears
to be significantly underestimated.

2 4.

Intangible benefits such as improvement
in relative humidity, reduced space
temperature, and increases in
compressor and fan motor longevity,
although important in some applications,
in general appear to be of limited
impact. The selection of this technique
should not be made based solely on
these intangibles. However, there is no
indication that these effects negatively
impact the use of this technology.
Reduced desiccation due to reduced air
velocities over open organic products
does appear to be of significant benefit,
although the magnitude of this effect has
not been substantiated.
5.

Based on motor manufacturers input, the
bearing life for evaporator motors when
operated below 500 rpm is of some
concern. However, because of the way
that these motors are operated with the
Necsi control, motor problems do not
appear to be significant.
6.

Some care should be exercised when
interpreting percentage savings claims.
The savings shown by Nesci are the
combined compressor and evaporator
fan energy savings only; these typically
account for about 90% of total energy
consumption of a walk-in box but will
depend on which loads are included on
the meter used to measure walk-in box
energy use.

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 and medium temperature. In
most cases the panels, refrigeration
components and controls are ordered
separately and assembled on site by local
refrigeration contractors. 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. [3]

The typical target application of the Necsi
control is a medium temperature, medium to
large cooler with a dedicated
(or rack)

refrigeration systems 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 systems also tend to have
longer run times and less load variation than
coolers which limits their energy savings
potential.

Principles of Operation
The design and operation of the control is
very simple. An autotransformer is installed
in the evaporator fan motor electric power
circuit. This autotransformer supplies low
voltage to the motors (typically one or more
banks of 1/20 hp motors) whenever there is
no call for cooling; i.e., when the
compressor is off in dedicated compressor
systems
or when the liquid line solenoid
is closed in a rack sys