Technology

Installation

Review

A case study on

energy-efficient

technologies

prepared by the

New Technology

Demonstration Program

Energy Savings in Refrigerated Walk-in Boxes

Results of an assessment of a technique for saving energy in refrigerated

walk-in coolers

Introduction and

Background

The purpose of this Technology

Installation Review 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 manufacturer’s 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.

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, 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 semihermetic 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 mediumtemperature, medium-to-large cooler

with a dedicated (or rack) refrigeration system that operates with singlephase 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. 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 autotransformer is installed in the evaporator 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 cooling; 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 thermistors. Control, therefore, is not

directly tied to compressor operation, 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 (approximately 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 models 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, mediumtemperature (28-40° F) walk-in coolers with shaded pole evaporator fan

motors and where the refrigeration

system has been oversized with respect

to actual loads. Although this technology 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 magnitude of savings must be carefully

calculated for each specific site with

the major parameters fully accounted

for until better estimating methodologies 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 facilities. There are an estimated 129,000

walk-ins (coolers and freezers) combined in Federal, state, and local government buildings. Since Federal

facilities make up only 13% of the

total number of government buildings,

there are an estimated 12,000 walkin-coolers in Federal facilities that

are potential candidates for this technology.[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 practical 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 implementation, 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

manufacturer’s perspective. Although

implementation costs are well established at $400 to $500 per controller

plus $100 for installation, the magnitude of savings is highly site specific.

In addition, for the reasons stated

below, the methodology used to estimate 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 walkin box installation, existing energy

use must be disaggregatated to separate 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 performance, each situation has to be

considered on its own merits. These

calculations should include consideration of all of the major factors identified in this report.

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 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 lowefficiency shaded pole 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 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 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 walkins (coolers and freezers combined) in Federal facilities to

which this technology is applicable is about 12,000.

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 shaded pole motors (~35%)

is significantly less than that of

permanent split capacitor 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 60% 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[3]; thus

the market size claimed by the

manufacturer could be significantly overstated. Based on

recent CBECS data there are a

total of approximately 700,000

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 New Technology Demonstration Program appears to be

underestimated.

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.
Appendix A: Performance

Assessment

This section contains amplifying

details that may be important to the

interested reader.

Test Results

As shown in Table 1, the results

from these tests are highly variable

in both percentage and magnitude of

savings. The results are skewed by

the fact that power factor was not

included in the calculated savings

for the evaporator fans for all but one

test (test E). These results underscore

the difficulty in predicting savings

and generalizing results from one

site to another. There is no clear

correlation between tests other than

the fact that some savings are shown

for all sites. The variation and lack

of correlation in these results is not

surprising given that the tests were

conducted in installations and under

operating conditions significantly

different from one another. The tests

span a wide range of box sizes, load

ratios, evaporator motor types, and

weather conditions.

Motor performance

There are two types of electric

motors that are used for walk-in

coolers: shaded pole and permanent

split capacitor. These are four-pole,

direct-drive motors typically rated at

1/10 to 1/20 hp. According to manufacturers, about 75% of unit cooler

production is shipped with shaded

pole motors.

The lack of true power measurements on the evaporator motors has

resulted in considerable confusion

in interpretation of the results. In

the field tests, power input and savings

were computed from measurements

of voltage and current to calculate

VA—(Volt-ampere—apparent power)

as opposed to measuring wattage

(active power) with a watt meter or

Table 1. Summary Results of Vendor-Sponsored Tests (vendor data [4])

Ambient Compressor

Test Box Temp Range Cooler size, duty cycle Avg. Daily Energy Savings, Energy cost

Test Purpose Location Date Located (Daily average) cu ft before/after, % Motor type Energy Savings, % Avg. daily KWh savings, $/yr(a)

A(b) Food Stockton, Feb-Mar Inside 40-548° F 865 21/13 permanent 44 6.0 175

service CA 1997 split capacitor

B Food Fremont, Feb-Mar Inside 55-67° F 5695 31/31 permanent 15 7.4 229

sales CA 1997 split capacitor

C Food Walnut Mar Outside 58-68° F 2739 16/12 shaded pole 59 8.8 569

service Creek, CA 1997

D(c) Food Sacramento, Jul-Aug Inside 64-107° F 2287 69/65 shaded pole 13 19.2 657

service CA 1997

E(d) Food Sacramento, Aug-Sep Outside 70-83° F 2500 38/33 shaded pole 24 9 400

sales CA 1993

(a) These are energy savings only, equipment and maintenance savings are not included.(b) This test did not include condenser fan energy in compressor energy measurements since condensers were common with other refrigeration units.(c) Compressor was not dedicated; a display case was supported. Therefore, the magnitude of compressor savings that should be credited to Nesci control is not known.

No outside temperatures measured.

(c) A watt meter was used for this test.
profile, actual loads, and head pressure control scheme. Annual savings

should be computed using techniques

that account for any performance

change with weather as well as oversizing of the condensing unit. If

testing is done it should include tests

conducted both in winter and summer conditions to establish the effect

of weather on performance. Better

yet, a methodology could be developed, and validated through testing,

that would consider all of the relevant

factors.

Appendix B: References

1. “Methods and Devices for Energy

Conservation in Refrigerated

Chambers,” ERIP Recommendation

No. 670, NIST/OTI, September

1995.

2. “Energy Savings Potential for

Commercial Refrigeration Equipment,” Arthur D. Little, Inc. Ref.

No. 46230/NTIS PB97-128730,

June 1996.

3. “Commercial Buildings Characteristics 1995, Preliminary Data”

Energy Information Administration, DOE/EIA Precbecs; private

communication with Joelle Davis.

4. “Refrigeration Monitoring Test

Results,” Nevada Energy Control

Systems, Inc., various test reports,

1993-1997.

5. Fasco Industries, Private communication with John Uhrig,

November 1997.

6. Specification Sheet, Necsi.

watt transducer (except for test E).

Power savings will only be accurate

using this method if the power factor

is close to unity and is constant

between full and low speed. Computer simulated performance curves

for shaded pole and permanent split

capacitor motors operating at full

and reduced voltage were obtained

from Fasco.[5] A performance curve

for a typical 10-inch fan was overlaid on these curves to identify the

operating point for these motors.

These curves accurately reflect field

experience and testing conducted by

Nesci; i.e., motor speed at 40 Volt

AC input is about 400 rpm. These

tests do not, however, show the effect

of using an auto-transformer to operate the motor at low speed. Likewise,

they do not indicate the range of

power factors that occur across various manufacturers models.

The Fasco data does show that for

permanent split capacitor motors,

power factor is relatively high and

does not vary much with speed. However, for shaded pole motors, power

factors are ~0.60 and decrease at

low speeds. This situation is complicated by the addition of the autotransformer. Theory (and some

additional testing not included elsewhere in this report) indicates that

the added inductance may actually

increase (i.e., improve) power factor

for permanent split capacitor motors

but decrease (i.e., worsen) it for

shaded pole motors. This issue is

further complicated by the lack of

data from motor manufacturers,

especially with auto-transformer

speed control.

Ignoring the change in power factor

with speed for shaded pole motors

underestimates savings by about 5%.

However, for motors with power

factors ~0.60, savings magnitudes

are overestimated by about 40%

(without including the effect of autotransformer added inductive reactance) when VA is uncorrected for

power factor. Overestimates of total

(compressor plus fan) savings will

vary with the proportion of the total

savings that are derived from evaporator fan energy. For example, for

test D actual overall savings are about

30% lower than reported (again, ignoring any effect that the auto-transformer

has on power factor).

Annual Energy Savings Estimates

Claims made in the manufacturer’s

literature suggest that total operating

cost savings can be as high as 50%.[6]

Necsi test reports show savings of

combined compressor and fan savings in the range of 10 to 60%.[4]

Other claims include operating life

improvements for fan motors and

compressors due to reduced run times,

extended product life due to better

humidity control, and reduced dessication due to lower air velocities.

Although tests results show significant percentage savings under

certain operating conditions, it is

clear that annual savings magnitudes

are highly dependent on a number

of site- and system-specific factors.

Savings magnitudes are driven by

several factors, the primary ones

being: type of evaporator motor, sizing of refrigeration system, weather
Produced for the U.S. Department

of Energy by the Lawrence Berkeley

National Laboratory

DOE/EE-0170

June 1998

For More Information

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General Contacts

Ted Collins

New Technology Demonstration Program

Program Manager

Federal Energy Management Program

U.S. Department of Energy

1000 Independence Avenue, SW, EE-92

Washington, DC 20585

(202) 586-8017

Fax: (202) 586-3000

theodore.collins@hq.doe.gov

Steven A. Parker

Pacific Northwest National Laboratory

P.O. Box 999, MSIN: K5-08

Richland, Washington 99352

(509) 375-6366

Fax: (509) 375-3614

steven.parker@pnl.gov

Technical Contact

William L. Carroll

Lawrence Berkeley National Laboratory

1 Cyclotron Road, MSIN: 90-3026

Berkeley, California 94720

(510) 486-4890

wlcarroll@lbl.gov

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