Refrigerator Energy Use in the Laboratory

ELSEVIER Energy and Buildings 22 (1995) 233-243

P I~m

AND BUILDING-

Refrigerator energy use in the laboratory and in

Alan Meier Energy and Environment Division, Lawrence Berkeley Laboratory, Berkeley, CA, USA

the field

Abstract

Residential refrigerators are the largest domestic use of electricity in the US and most developed countries and, as such, have become a target for efficiency improvements. Laboratory tests of energy use are typically used to measure a refrigerator's energy consumption. Three different test procedures are widely used: the US Department of Energy, the International Standards Organization test, and the Japanese Industrial Standard. The features of the tests are compared and conversion factors reviewed. The DOE test is likely to yield the highest consumption of the three procedures. A critical factor is the relationship between a refrigerator's laboratory-based energy use and its consumption in a kitchen. The DOE test is the most carefully validated with field data. The DOE test, on average, over-predicts actual consumption in US homes by about 10%, but there is wide variation in field use for identical units. The factors affecting variation in energy use are reviewed. The ambient temperature in the kitchen is by far the most significant factor, while door openings and humidity are relatively minor. Field measurements suggest that maintenance measures, such as coil cleaning and gasket replacement, save little energy. Replacement of old refrigerators with new, efficient models often cuts refrigerator energy use by 60%. Significant reductions in refrigerator electricity use can be expected during the next decade in the US and Europe.

Keywords: Refrigerators; Energy use; Laboratory tests; Field tests

1. Introduction

Residential refrigerators are the largest end use of electricity in US homes and, altogether, consume about 7% of the nation's electricity. For comparison, the combined electricity demand of the refrigerators roughly equals the combined output of the nation's hydroelectric power plants (see Fig. 1). From a technical perspective, refrigerators are remarkable devices. If one treats a refrigerator as if it were a house, then the kitchen imposes a severe climate on the refrigerator - about

Supply Demand 8% 7%

Fig. 1. Electricity consumption of US residential refrigerators is roughly equivalent to the electricity generated by the nation's hydroelectric facilities.

0378-7788/94/$07.00 1994 Elsevier Science S.A. All rights reserved SSD1 0378-7788(95)00925-N

7000 C-days/year - and the refrigerator's insulation would be considered woefully inadequate. Fortunately, a refrigerator has only a fraction of the surface area of a house, so the refrigeration bill is acceptable.

The energy use of refrigerators has attracted attention recently due to the establishment of federal minimum efficiency standards, utility incentives to encourage consumers to buy more efficient units, programs to accelerate adoption of new technologies, and the shift away from CFC-based insulation and refrigerants. All of these activities involve certain assumptions regarding the energy performance of refrigerators which, in many cases, are based on little or old research. The goal of this paper is to review the current research related to energy use of refrigerators in the laboratory and the field with special emphasis on the US.

Refrigerators are an important consumer durable. There are about 110 million refrigerators in the US, or about 1.15 per home [1]. The average refrigerator lasts 19 years [2]. The size of domestic refrigerators has grown slowly; the typical new 1975 US refrigerator was 4801 and only about 60% had the automatic defrost feature [3]. In 1992, the most popular refrigerator size was 510 1 (18 cubic feet) and about 95% of all new units have automatic defrost. For comparison, the average new refrigerator in Sweden and Japan has a

234 A. Meier / Energy and Buildings 22 (1995) 233-243

capacity of about 400 I. The US market is now dominated by one refrigerator design: the top-freezer with automatic defrost. About 70% of all units sold in the country are top-freezers, 20% side-by-sides, and 10% manual/partial defrost units. Essentially all US refrigerators have just two doors. In contrast, Japanese refrigerators come in a variety of configurations and often have as many as six separate doors. In Scandinavia, separate refrigerators and freezers are very popular. These design differences complicate international comparisons of refrigeration technologies.

2. Laboratory tests of energy use

Electricity consumption of refrigerators has always been recognized as a significant factor. It was recognized early on that field measurements of energy use were inadequate indicators of actual efficiency because of the fluctuating conditions found in typical kitchens. Basic sanitation tests also needed to be performed under standard conditions. As a result laboratory energy test procedures were developed in the 1950s and for- malized in the 1960s. The three most important laboratory energy test procedures are the Department of Energy (DOE), the International Standards Organi- zation (ISO), and the Japanese Industrial Standards (JIS) ~. Each was developed to suit the peculiar conditions found in that region. For example, Japanese homes traditionally lacked central heating, so the JIS test included a test condition with a cool ambient temperature, 15 C. The key aspects of each test procedure are reviewed below and summarized in Table 1.

What makes a 'good' test procedure? The ideal energy test procedure accurately predicts energy use in actual conditions, is easily repeatable, and can be performed at a low cost. In fact, the goals conflict. A test procedure that attempts to closely mimic actual kitchen conditions and human behavior will be very complicated and susceptible to a high degree of measurement uncertainty. It will also be expensive. (An independent testing laboratory charges about $2000 to perform the current DOE test on a single refrigerator.) A 'simple' test procedure may be inexpensive but is liable to ignore features that affect a refrigerator's energy use, thus causing an inaccurate or unfair ranking of models. As a result, every test procedure is a compromise.

3. The Department of Energy test

The DOE test (also referred to as ANSI/AHAM HRF-l-1988) was originally developed over fifteen years

i Canada (through the Canadian Standards Association) and Brazil establ ished test procedures similar to the DOE test.

ago by refrigerator manufacturers. It has undergone various modifications, including adoption as the official government test procedure. The key points of the DOE test are described below, but readers should consult the official test documents because the details can be critical in determining energy consumption.

In the DOE test, the refrigerator is placed in a test chamber maintained at 32 C (90 F). The electricity use is measured for 24 h. During the test period, the doors are kept closed and the refrigerator is empty. Most US refrigerators have two temperature adjustments: one is a thermostat and the other adjusts the proportion of cold air directed to the freezer and fresh- food compartments. As a result, it is difficult to achieve exactly the standard -15 C in the freezer. To avoid continual adjustments, two 24 h measurements are performed but at settings that yield freezer temperatures that straddle the standard temperature. Then the energy use at -15 C is interpolated from the two measurements. The interpolated value is the DOE test consumption for that refrigerator. The test can be performed with a reasonably high degree of consistency. Farley et al. [4] estimated that the measurement uncertainty in the DOE test would be less than 1.7%.

Advances in refrigerator technology have required modifications to the test procedure. The most important modification involves the automatic defrost. This is an important factor because the automatic defrost uses a 400-600 W heater for about 10 rain after about 10 h of compressor run-time. The test procedure was modified so that it captures the whole cycle. (The procedure was further modified after the introduction of 'adaptive defrost', that is, a means to operate the defrost heater only when conditions required it.)

Anti-condensation (or 'anti-sweat') heaters also required a modification. These heaters can add 10% to the refrigerator's energy use, so their setting during the test is significant. The rules regarding anti-sweat heaters are complex (and not entirely logical); the reader should consult the DOE test procedures for an explanation.

Some features are not operated during the test, notably the automatic icemaker and through-the-door drink dispensers. These features have measurable energy penalties in normal operation but no procedures to include them have been considered by DOE or other relevant organizations. A relatively simple test procedure to include the energy use of automatic ice makers has been proposed [5].

Every domestic refrigerator sold in the US must have an 'Energy Guide' label. In addition, AHAM publishes the labeled consumption of every refrigerator sold in the US [6]. This label shows its electricity consumption based on tests of several units. Manufacturers perform the DOE test on a random sample of their refrigerators. The precise number is determined by the statistical

A. Meier / Energy and Buildings 22 (1995) 233-243

Table 1 Comparison of energy consumption test methods

235

Parameter Department of Energy ANSI Japan Industrial Standard JIS C- International Standards HRF-l-1988 9607-1986 Organization ISO/DIS 8187

Test chamber ambient 32+0.6 15+ 1 and 30+ 1 255:0.5 tropical 32 temperature (C)

Weighting of annual results by 365 days @ 32 C 365 days @ 25 *C chamber ambient temperature

Test chamber ambient humidity no specification 45-75%

Door openings none none

Fresh food and frozen compartment loads

Freezer standard temperatures (oc)

Fresh food standard temperature period

No. of thermocouples for each compartment

Anti-condensate heater switch setting

Method of determining consumption at a standard temperature

none (except for low temperature compartment of basic refrigerator)

-15 (freezer of refrigerator/ freezer) -9 (freezer of basic refrigerator)

defrost-to-defrost for auto defrost; at least 3 h with at least 2 compressor cycles for manual defrost

freezer: 3 (except basic) fresh food: 3

test both 'on' and 'off', average results

interpolation of two tests bracketing standard temperature conditions

265 days @ 15 C 100 days @ 30 C

75+5%

fresh food: every 12 min freezer: every 40 rain (during first 10 h of test, duration of opening 15 s)

none for energy consumption test

- 18 (three-star rated unit) - 12 (two-star rated unit) -6 (one-star rated unit)

24 h (48 h or longer if defrost cycle every 2 days or longer)

freezer: 3 flesh food: 1

on for both conditions (for units tested)

run tests at standard temperature, within +0.5 C tolerance

1.6 kg load in freezer with thermal characteristics of lean beef

-18 freezer (three-star unit) 5 fresh food

at least 24 h and whole number of defrost cycles

freezer: 3 fresh food: 3

on if needed to pass condensation test

with all interior temperatures below limits or interpolation of two tests

requirements to establ ish a 95% confidence level, assuming a one-s ided distr ibut ion and random selection. The detai ls are descr ibed in the Federa l Register [8]. Manufacturers typically test four to six units in order to achieve this conf idence level. Mass product ion and improved quality control have resulted in a high degree of consistency among units [9]. Some manufacturers increase the labeled use by an arbitrary amount to allow for unant ic ipated changes in components or minor design changes during product ion that might raise energy use. As a consequence, there is some uncertainty regarding the consistency of the ' label ' consumption and the laboratory test value for a specific unit.

4. The Japanese Industr ial Standard test

The Japanese Industr ia l S tandard (J IS) test procedure is descr ibed in J IS 9607 [10]. The test is used exclusively in Japan and reflects some of the unusual condit ions found in Japan, such as low ambient temperatures and high humidity. The test p rocedure includes door openings and measurements at two ambient temperatures. It also specifies an ambient humidi ty (75%) and specific

inside temperatures . The key features of the J IS test are l isted in Table 1.

Two 24 h measurements are needed to derive a test value, one test at 15 C and another at 30 C. Dur ing these measurement periods, the fresh food and freezer doors are opened and closed 50 and 15 times, respectively, according to a defined schedule. The annual energy consumption, E . . . . . j, is calculated using the formula below:

E15 X 265 + E3o X 100 Eannual

365

where E15 and E3o refer to 24 h electricity consumptions at 15 and 30 C, respectively. This weighted average is the J IS test value and is reported in the specif ications for that refr igerator.

5. The International Standards Organization test

The Internat iona l Standards Organizat ion ( ISO) establ ished an energy test procedure [11]. The test was largely deve loped for European condit ions and refrigerators. It has recently been modif ied to accommodate


Japanese-style, multi-door and multi-compartment units. Three ratings of refrigeration quality were es- tablished, from a 1-star to a 3-star, depending on the unit's ability to maintain a low temperature in the freezer compartment. (A 3-star rating - the highest - can maintain the freezer compartment at -18 C.)

Like the DOE test, the ISO test also relies on a closed-door measurement and interpolation to standard temperatures. However, there are significant differences: ISO uses a lower ambient temperature than DOE (25 versus 32 C) ISO requires a colder freezer compartment standard temperature ( - 18 versus - 15 C) ISO is silent with respect to automatic defrost ISO requires use of anti-condensation heaters only if condensation occurs below the standard temperature A separate 'tropical' ISO energy test procedure has been created. The principal difference is the ambient temperature, 32 C, that is, less than 0.3 C cooler than the DOE test. Most energy data for European refrigerators are based on the 25 C ambient temperature.

6. Comparing the energy test procedures

Each test emphasizes different aspects and makes implicit assumptions about performance. The DOE test assumes that the additional heat gain caused by door opening and food loading can be compensated by an elevated ambient temperature. Furthermore, the energy penalties associated with automatic defrost and anti- condensation features should also be included. The Japanese test assumes that the ambient temperature around a refrigerator will be low, 15 C, that humidity is a significant factor, and that the energy impacts of door openings are important. The ISO test has the mildest conditions. Three of the four significant differences with the DOE test will lead to a lower energy rating than in the DOE test. The ISO test is also easier to perform because it is silent with respect to automatic defrost and is more likely to not require use of anti- sweat heaters.

It is useful to compare the estimates of energy use for identical refrigerators from different test procedures. The impact of advanced technologies in foreign refrigerators can be more rapidly assessed if the energy savings can be easily expressed in terms of the domestic test procedure. In this way, innovations can more easily cross international boundaries. Refrigerator manufacturers periodically compare the energy use of a unit with different procedures but do not publish their findings. As a result, the public record is scanty.

One public comparison of the DOE and JIS tests was performed by Meier [12] in 1987. Nine Japanese refrigerator models (12 units) were tested according

to the DOE test and then compared to the manufacturers' JIS values listed in the specifications (see Fig. 2). The Japanese models used roughly 40% more energy in the DOE test than in the JIS test. The difference was greatest for larger units and those with through- the-door features. In 1988, Alissi et al. [13] tested one 540 1 American refrigerator and reported that the DOE rating was 27% higher than the JIS rating.

In 1993, Meier et al. [9] performed both the DOE and JIS tests on 24 US refrigerators manufactured in 1991. The JIS tests yielded energy ratings on average about 15% less than the DOE tests (see Fig. 2). The difference was negligible for smaller units but the DOE test was roughly 20% higher for side-by-side units.

There are no public comparisons of the ISO test with either the JIS or DOE tests. Since the ISO test has milder conditions, there is good reason to believe that the ISO test will produce energy ratings much lower than the DOE test, possibly by as much as 30%. The magnitude of the difference will depend on the type of model tested. For example, a US model with extensive anti-sweat features (which are allowed to be switched off during the ISO test) and automatic defrost will use much less in the ISO test.

The Japanese will abandon the JIS as early as 1995 and adopt the ISO energy test. The principal reasons for the conversion are the high cost of performing the JIS test and the need to re-test if the unit is destined for export. The trial-and-error needed to achieve the required temperatures without extrapolation is also seen as tedious. In addition, the JIS test is open to considerable subjective interpretation. For example, the test calls for door openings but does not specify which doors. Modern Japanese refrigerators often contain six doors, so the choice of doors is left to the manufacturer.

1600 --

~- 12O0-

g

800- >

400-

0 o ~o

[3 Top Freezer / "1 o Side-by-Side /

J A Manual / / Older Studies / / I

+ OtherData* / J 1 - Equafty Line / / ~ l -- Regressin J + I

/ / ~+ JIS- 0.64xDOE + 250 / / + R 2 = 0.92

/ /

~Other Data" exclude "Manual" and "Older Studies

8~o 12'oo 1~ DOE Value (kWh/year)

Fig. 2. Comparisons of the DOE and JIS energy test procedures.

A. Meier / Energy and Buildings 22 (1995) 233-243 237

A new test procedure is being developed in Canada [14]. This procedure seeks to establish a test procedure that more accurately reflects the way in which a refrigerator is used in a home. Many of the factors, such as door openings, ambient temperature, food loading, etc., are being varied, so as to determine which factors most influence energy use. After these factors are identified, the laboratory test will be designed to include them at average values or levels.

There is periodic discussion about revising the DOE test. Numerous technological advances have under- mined the reasonableness of some procedures. For example, the adaptive defrost mechanism limits the defrost operation to conditions where frost actually exists (rather than dictated by a clock). As a result, the adaptive defrost does not even turn on during the DOE test procedure. Assumptions regarding anti-sweat heaters, automatic ice making, and other features are similarly outdated. To date, only minor, optional, re- visions have been permitted. Changing the DOE test procedure is doubly difficult because the federal refrigerator efficiency standards are based on refrigerator energy use as measured with the DOE test.

7. Energy use of refrigerators in the field

Compared to the laboratory data, field measurements of refrigerator energy use are sparse and uncoordinated. In the US, electric utilities periodically monitor residential appliances. Their objective is to better under- stand residential energy use patterns so that they can more accurately forecast electrical demand. The activities started in earnest in the 1970s. Naturally, the refrigerator was a major target because it is the largest single end use of electricity. Manufacturers also monitor refrigerators. Energy consumption is less important to manufacturers than the compressor duty cycle, ap- pearance of condensation, adequate pull-down and defrost capacity, and so on. Utilities in Europe and Japan have also monitored refrigerators but the studies are smaller, less frequent, and often proprietary. Finally, refrigerators have been monitored by individuals or as part of larger projects.

There are no national surveys of refrigerator energy use comparable in statistical rigor to, say, the US Residential Consumption Survey (RECS) [1]. As a result, it is impossible to accurately estimate the unit energy consumption (UEC) for refrigerators or the national consumption. Stewart [15] estimated that a statistically valid study for the climatically diverse US would require detailed monitoring of at least 36 refrigerators located in three locations in the country. The magnitude of this effort has deterred researchers; however, there have been several attempts to compile, in a non-representative way, monitored energy use.

Spolek [16] assembled data from various studies in- volving 137 refrigerators in the Pacific Northwest and elsewhere. Many of the measurement periods were very short. Nevertheless, Spolek concluded that the DOE label was an excellent predictor of actual energy use in Pacific Northwest homes.

The first national compilation of measured energy use was undertaken by Meier and Heinemeier in 1988 [17]. Metered energy use for 259 refrigerators was collected from any source willing to contribute data. However they rejected data from short-term measurements (which might introduce seasonal bias). The refrigerators were located throughout the US, but prin- cipally in cooler, northern climates. For these units, the average energy use was 1009 kWh/year. This study was updated and enlarged by Meier and Jansky in 1991 [18]. The distribution of energy consumption is shown in Fig. 3. This study found that the 150 top freezers consumed an average of 882 kWh/year and confirmed that side-by-sides consumed more, about 1366 kWh/ year. These estimates are for the stock of refrigerators, whose average age was 10 years but included units ranging from 1 to 30 years old. Again, the data came mostly from cooler, northern climates, so the national averages should be somewhat higher - about 10% - to reflect higher consumptions among refrigerators in warmer, southern regions. Making adjustments for the non-representativeness of the compilation and the introduction of new, efficient units, the 1993 UEC for refrigerators presently in American homes is about 1100 kWh/year.

The compilation by Meier and Jansky also permitted a comparison between the labeled and field energy use. The labeled energy use of the 209 units was obtained

40

~j) 3o

4)

n , -2o

"6

.Q E Zlo

(bat CO0 is r l - 20g

2oo 4oo 6oo

Mumured - Mean: 1009 ~-~ Labeled - Mean: 1160

800 1000 1200 1400 1600 1800 2000 2200 2400

kWh/year

Fig. 3. Distribution of labeled and measured refrigerator energy use (taken from Ref. [18]).


from the AHAM Directories, other reports, and personal communications from the manufacturers. The results are summarized in Fig. 4. There is considerable scatter, but increasing field energy use clearly correlates with higher label energy use. A linear regression yielded the following relationship between label and field energy use"

Annual field use (kWh/year)

= 0.94 (annual label use) - 85

The linear regression is shown as a dark line in Fig. 4. The regression suggests that the label over-predicts field consumption for this group of refrigerators. The field use of an average refrigerator in this compilation (with a labeled consumption of 1160 kWh/year) is about 15% below the label.

A separate relationship for top-freezer design alone was investigated because it is the most popular type of refrigerator. In the field, top-freezers used about 18% less than the label. Again, there was considerable variation among individual units.

In a study confined to one city, Meier et al. [9] compared field energy use of individual refrigerators to their laboratory test values. Fig. 5 summarizes the results. All of the refrigerators were placed in homes in Rochester, NY. As usual for cooler climates, the field use is consistently below the DOE value but the scatter is less than that found in nationwide data. Since the outdoor climate was the same for all units, the remaining variation is due to environmental differences in the kitchens and the behavior of the users.

The studies suggest that the DOE test is a reasonably good predictor of field consumption of a group of units where individual variations in conditions offset each other. Thus, utilities can rely on the labeled consumptions when forecasting electrical demand (after making adjustments for local climate variations).

r . .

O top freezers sJOe-by.sMes """

2000 V Ioottom-freezers ~ . -" . . . . equality ~ , '6~"

'~" - - regression ~3


8.1. Ambient temperature

Most of the thermal load on a refrigerator is con- duction through the walls. For this reason, the temperature of the air around a refrigerator is a significant determinant of energy consumption. Since the compressor efficiency also declines as the ambient temperature rises, a refrigerator's electricity use is very sensitive to the ambient temperature. An example of one refrigerator's temperature sensitivity is shown in Fig. 6. This particular unit was located in New York, but a similar relationship was observed in a Florida home [19], Modest changes in kitchen temperature will have surprisingly large impacts on refrigerator energy use. Electricity consumption varied from 1.25 to 2.6 kWh/day even though the temperature increased only 11 C (from 17 to 28 C). The correlation is so good that it suggests that variations in ambient temperature cause virtually all of the variations in energy consumption. Some points lie off the regression line; these may be due to unusual food loading and periods with more (or fewer) than average defrost cycles. (Note that the regression line's slope and intercept are determined by the refrigerator's efficiency and other environmental factors.)

The strong influence of ambient air temperature is demonstrated in a comparison of kitchen temperature and energy use for 20 refrigerators (Fig. 7). Here the weekly average temperature of the 20 units was plotted against their average energy use. The two lines track almost perfectly, confirming that temperature variations are responsible for almost all the variation in refrigerator energy use.

These results suggest that an important refrigerator energy conservation measure is to keep the kitchen (or wherever the unit is located) as cool as possible. The

Temperature (C) 2O 25 30

5.0 J I I

- - - Labek~d Usage

- - - - C rossover Temperature

4.0 - - Regress ion

3.0

o o . . . . . . . . . . . . . . . . . . . . ; . . . . . ~ _v. -~ _ ~ r ~ -

zo ~ ~

1.0-

E = -0.481 + 0.106T(*C)

0.0 60 ;o 8'0 90

Temperature (F)

Fig. 6. Dai ly electricity consumption as a function of average daily kitchen temperature (taken from Ref. [9]).

4.0

3.0

0 t 0,0

May

~ 2.0

F ie ld Consumpt ion

- - 7 -DayRunn ingAverage N=20

K/tchen Temperature

- - - 7 -DayRunnk lgAverege

1991

95

-55

Fig. 7. Temperature and energy use for 22 1991-vintage refrigerators located in Rochester, NY (taken from Ref. [9]).

simplest strategy is improved natural ventilation. Even a few degree reduction in average temperature can cut electricity consumption by 5-20%.

8.2. Refrigerator compartment temperatures

Just like a house, a refrigerator will use less electricity if its thermostat is re-set to a higher (warmer) temperature. Owing to the single-evaporator design of most refrigerators, a change of temperature in the freezer compartment generally results in a temperature change in the fresh-food compartment. Grimes et al. [20] examined the impact of compartment temperature on energy use on a 1977-vintage automatic defrost refrigerator. The energy consumption rose 26% from the warmest acceptable to the coldest possible settings (when all other test conditions were maintained at the DOE values).

It is also possible to infer the impact of small temperature changes in compartment temperatures on energy use from DOE test results. (Recall that the DOE test involves two tests at two compartment temperatures straddling the standard temperatures.) In a group of 1991 refrigerators, electricity consumption increased about 3.5% for each 1 C decrease in freezer temperature. A more recent study of nine large, 1993- vintage US refrigerators [21] found nearly double the sensitivity: 6.5% for each 1 C reduction in freezer temperature. The higher sensitivity may be a consequence of the new refrigerators' increased efficiency.

Refrigerators in real kitchens should be less sensitive to thermostat settings because typical kitchen temperatures are much lower than those used during the DOE test (32 C). The impact of refrigerator temperatures on field electricity use was examined by Meier et al. [9]. They compared freezer compartment temperatures to field energy use in 20 units. No correlation was observed. The impact of freezer compartment temperatures may be obscured by the more significant


differences in kitchen temperatures. Furthermore, the temperature measurements were made every three months, and subject to considerable error.

Higher temperatures in refrigerator compartments is not a recommended conservation measure. Warmer conditions in the freezer and fresh food compartments accelerate spoilage and increase the likelihood of health hazards. Indeed, the survey by Meier et al. and other studies in the UK [22] and Ireland [23] suggest that many refrigerators are maintained at dangerously un- healthy temperatures (see Fig. 8). Higher storage temperatures are an increasing health hazard because more foods are purchased without preservatives and are therefore vulnerable to bacterial infestation. Health issues notwithstanding, the electricity savings are likely to be offset by the increased costs of accelerated food spoilage.

8.3. Door openings

Many consumers link the number of door openings with a refrigerator's energy consumption. Yet door opening is a relatively modest factor affecting energy use. Grimes et al. [20] measured the change in energy use due to opening the freezer door 24 times and the fresh-food door 24 times at an ambient temperature of 32 C (that is, the DOE test). Together, these openings raised the energy consumption by 6-8%. Alissi et al. [13] repeated the experiment except with 100 door openings. Under extreme conditions, they found a max- imum of 32% increase in energy use. (Typical households will open the refrigerator 40--60 times per day.) In the ten years between these two experiments, refrigerator efficiency had greatly improved. Even though the per- centage increase differed, the absolute impact of 50 openings/day seems to be almost the same, about 0.34 kWh/day. With new, efficient refrigerators, one can reasonably expect that 50 door openings are responsible for about 0.25 kWh/day.

15-

10-

0 -

-5-

~ -10 - -15-

-20 -

-25

- Fresh Food

x Frozen

t ,I t,l,, t ttt,ttt

- 50

-40

-30

-20 3

3 -10

-0

- -10

Fig. 8. Temperatures of fresh food and freezer compartments in 1991-vintage refrigerators (taken from Ref. [9]). Each vertical series of points represents one refrigerator.

In the field, Parker and Stedman [24] estimated that door openings were responsible for 7% of an old refrigerator's energy use. By the same regression tech- nique, Parker and Stedman estimated that a new (1991) unit suffered 19% losses from door opening. The new unit consumed 60% less electricity than the old one, so the door opening losses were constant. Parker and Stedman estimated that each door opening increased energy consumption 9 Wh.

8.4. Humidity

The JIS test requires that the refrigerator be tested at 75% humidity, suggesting that humidity is considered an important factor in Japan. (Indeed, Japan is very humid for several months each year.) The impact of high humidity is relatively modest, however. Grimes et al. [20] found that tests at 60% relative humidity raised energy use only 5% over 40% relative humidity. A modified test that included door openings and a higher humidity caused no observable change in energy use when switching from 40% to 60% relative humidity. This was more recently confirmed by Wong [14] who found that the impact of the humidity was not statistically discernible above the noise inherent in testing. The greatest energy impact of humidity is probably the operation of the electric resistance anti-condensation heaters. These heaters must be set to higher output (or just turned on), leading to increased energy use.

8.5. Air circulation

The major test procedures all assume relatively un- restricted air movement across the condenser coils. But refrigerators in real kitchens are often placed in enclosures. These enclosures may surround the unit on four sides, with only a few centimeters clearance z.

The impact of restricted air circulation will depend on the type of condenser (passive with coils on the rear wall, forced-air underneath, coils embedded in the walls), but it will nearly always increase energy use. In one refrigerator with coils on the rear, improved air circulation cut electricity use 10%, that is, about 100 kWh/year [25]. Cabinet systems (which usually surround the refrigerators) could be easily re-designed to improve air circulation. Natural ventilation might be a new component of building standards (indeed, Swedish building codes have a provision for ventilating the refrigerator). Such modifications may be easier (and cheaper) to implement than achieving comparable savings through design improvements in the refrigerator itself.

2 Part of the enclosure may be heat-generating surfaces. One informal survey found that a stove adjoined the refrigerator in about 20% of the homes.


8.6. Automatic ice makers and through-the-door features

A significant fraction of new refrigerators are equipped with automatic ice makers and cold beverage and ice dispensing. These features are switched off during the DOE test, so it is difficult to estimate their impact on energy consumption. Through-the-door features create paths of low thermal resistance in the refrigerator doors so they use more energy even when those features are switched off. Most manufacturers offer refrigerators that are identical except that one is equipped with through-the-door features. Parker and Stedman compared the energy use of these matched- pairs [19]. The through-the-door features uniformly added 10%, or about 120 kWh/year, to the refrigerators' energy use.

The automatic ice maker consumes more electricity than manual ice making because a 185 W heater is used to release the cubes from the mold. (The energy penalty is compounded because the heat from the resistance heater takes additional compressor energy to remove.) B.R. Laboratories [26] found that automatic ice makers increased the DOE test energy use by up to 26%. These measurements involved letting the ice makers operate until they had filled their storage trays - about 2000 g - and switched themselves off. Meier [5] also modified the DOE test procedure to include the energy use of ice making. The modification involved production of only 500 g of ice (about 1.5 typical trays) during the standard DOE test. When performed on a typical refrigerator, energy consumption increased by 9-12%. The energy consumed by the mold heater was a significant factor.

There are no direct measurements of automatic ice maker energy use in the actual kitchens. Anecdotal evidence suggests that they substantially increase refrigerator energy use. During field monitoring, Proctor [27,28] observed that refrigerators with unusually high consumption generally had automatic ice makers (and were often broken). Consumer Reports [29] found that refrigerators with automatic ice makers experienced a significantly higher frequency of repair. Many of these breakdowns would presumably be connected to increased electricity consumption.

9. Impact of maintenance on energy use

The domestic refrigerator is a remarkable appliance: it is expected to operate for 20 years without any special maintenance. Nevertheless, a refrigerator is a conven- tional mechanical device, with parts that wear out and performance that should deteriorate over time. Will specific repairs or maintenance, restore original performance (or at least reduce energy use)? Few formal

investigations of the impact of maintenance and repairs have been undertaken. This is surprising because utilities have constantly urged their customers to clean their refrigerators' coils and replace torn gaskets.

In California, Bos [30] performed the DOE test on 50 older refrigerators collected in a utility program. The tested values were 40--60% higher than the labeled values for those units. (Some were over 200% higher than the labeled value.) Among these units, roughly one third were improperly charged and 18% had some sort of gasket or cabinet damage. The age of a unit was a poorer predictor of high energy use than its condition and overall level of maintenance.

Cleaning coils is the most common conservation measure. Special brushes are sold to assist consumers in this task. The difficulty in performing this measure depends on the location of the coils (exposed on the back or underneath) and nature of the fouling materials; it can range from a trivial dusting to a major scrubbing. Numerous informal studies of savings from coil cleaning have been undertaken, with measurements ranging in duration from a few days to a year. None of these studies obtained more than a 6% reduction in energy use. The highest savings (6% or 150 kWh/year) were obtained in laboratory tests (with dirty and clean coils) by Bos. The elevated temperatures of the DOE test probably overstated the savings that would have occurred at kitchen temperatures. Persistence of savings is also dubious because the only long-term study observed no savings from standard maintenance operations.

One study of electricity savings from maintenance measures occurred in New York [9]. Twenty seven refrigerators in typical homes were monitored for one year. These units - averaging about 16 years old - were visited by a professional repairman. He cleaned the coils and replaced gaskets where necessary. Mon- itoring continued for.a second year. No savings were observed (see Fig. 9). The contractor encountered

o o o

o

8 o

%,

1000-

o

o

o o %.

Original Condition After Maintenance After Replacement

Fig. 9. Impact of maintenance and replacement on refrigerator energy use. The refrigerators were monitored for one year (first vertical series) and then maintained. They were then monitored for another year (second series). Finally, they were replaced with new units having comparable or better features (third series).


numerous problems replacing gaskets on the old refrigerators and performing other maintenance. Re- placement gaskets were rarely available, so new ones had to be custom-made. Also, many of the refrigerators were in very poor condition so replacing the gaskets often created new problems. For example, sometimes the door no longer closed. The coils proved to be less fouled than expected; some required no cleaning.

In theory, coil cleaning should save energy because of heat transfer between the refrigerant and the environment. Why then are the savings so small? Without formal investigations, it is only possible to speculate. For example, the dirt on the coils may add trivial thermal resistance, so little change in performance should be expected. Alternatively the coils may be oversized so as to perform adequately during peak conditions. As a result, any increased thermal resistance from the dirty coils may become significant only during extreme conditions. In summary, the energy savings from coil cleaning are at best going to be small and of uncertain duration.

10. New refrigerators

The US federal appliance efficiency standards have caused enormous reductions in energy use of new refrigerators (see Fig. 10). Many utilities offer rebates and other incentives to encourage consumers to replace their old refrigerators with new, efficient units [30-32]. These programs have resulted in electricity savings that were observable on the customers' bills. For example, a Wisconsin study [31] analyzed billing data from thou- sands of participants in a utility appliance rebate program. It found savings ranging from 280 to 916 kWh/ year depending on the form of participation.

Metered data confirm the savings observed in utility bills and predicted by labels. In New York, Meier et al. [9] found that new (1991-vintage) refrigerators used about 60% less than the units they replaced. The results

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E

~4oo- =>

AHAM data

~ ~ estimafes

zd o~

" L I l l 1972 1976 1980 1984 1988 1992 1996 2000

Fig. 10. Uni t energy consumption of new refrigerators over time. Data f rom 1972 to 1988 are f rom AHAM. Data for later years are est imates.

are shown in Fig. 9. This reduction caused a 15% drop in the participants' total utility bills. In Florida, Parker and Stedman observed a 62% reduction when they replaced an 18 year old model with a comparable 1991 model [24]. In Southern California, Martinez [33] replaced a 450 1 (16 cubic feet) refrigerator built around 1980 with a 1991 590 1 (21 cubic feet) unit equipped with an automatic ice maker. Even though the new refrigerator was larger and had an ice maker, it used about 30% less energy. Martinez also monitored the performance of 23 post-1991 units and estimated that they averaged only 600 kWh/year.

New European refrigerators are also becoming significantly more efficient although the absence of standards has probably led to a slower rate of improvement. The results are nevertheless observable. In Sweden, Mansson et al. [34] replaced the 1972-vintage refrigerators and freezers in twelve homes with the most efficient units available in 1991. They observed a 50% (210 kWh/year) reduction in refrigerator and a 75% (850 kWh/year) reduction in freezer electricity use.

In Japan, electricity use by refrigerators appears to be still increasing due to the purchase of larger units with more features. There are no immediate plans for efficiency standards in Japan because the government felt that large reductions had already been achieved and further savings were likely to be small.

The 1993 US federal efficiency standards will lead to even greater energy savings through replacement. Monitored data are not yet available but, assuming that the labels accurately predict field consumption, the 60% savings per replacement will probably continue for at least five years and possibly for as long as a decade.

11. Conclusions

Refrigerator energy use in the US will decline steadily during the next decade, but it still represents a significant residential end use. Monitoring refrigerator energy use in the laboratory and the field represents a critical element in achieving the anticipated reductions.

The DOE test procedure yields an energy rating that will be anywhere from 0 to 30% higher than the same unit tested with the JIS or ISO test. More accurate estimates are impossible because there have been few public comparisons of the test procedures, especially between the DOE and ISO tests. Without such information, it is difficult to assess the relative effieiencies of American and European refrigerators.

Despite its lack of realism, the DOE energy test procedure appears to predict field consumption of a group of refrigerators fairly accurately. Its ability to predict individual consumption is poor and is likely to


worsen as high-efficiency units become more sensitive to behavioral factors.

Once a refrigerator has been purchased, there are relatively few opportunities to reduce its energy use. The most important action is to ensure that it faces the lowest possible ambient temperature. Ventilation may also be important. Other measures, such as en- hanced maintenance or altered consumer behavior, will have little impact.

The most reliable way to reduce refrigerator energy use is to replace old units with new ones. This action often cuts electricity use 60%. Programs to collect old refrigerators and encourage consumers to buy new, high-efficiency ones are likely to be highly effective for many years.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewables, Office of Building Technology, of the US Department of Energy under Contract No. DE-AC03-76SF00098.

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Documents

Refrigerator Energy Use in the Laboratory