Energy Consumption of Domestic Refrigerators

8/2/2019 Energy Consumption of Domestic Refrigerators

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Energy consumption of domestic refrigeratorsusing electronic energy saving devices

J. Van den Keybus12 ([email protected]),

R. Belmans2 ([email protected])

Katholieke Universiteit Leuven, Dept. ESAT, Div. ELECTA

Introduction

Peoples awareness of the need for reduction of their energy consumption is in-creasing, their motivations being both environmental as well as economic. There-

fore, energy saving is gaining importance, even in the common household. The

most effective way to reduce domestic energy consumption is to tackle the largest

consumers first. In an average household this would be the refrigerator, which is

continuously connected to the supply. For this purpose, electronic energy savers

are sold, which have to be simply plugged between the refrigerator and the mains

outlet. They are supposed to reduce the power cons umption of the refrigerator, al-

legedly leading to a substantial reduction of the energy costs.

This paper discusses the effectiveness of such devices, based on measurements

on a refrigerator. Two energy saving devices are evaluated in 24 hour tests.

Operating principle

Both refrigerators and deep freezers use a single-phase induction motor to drive

the compressor. Energy saving is achievable by reducing the motor voltage after

startup, effectively reducing the magnetic flux in the machine as well as the iron

losses. On the other hand, an insufficient magnetization of the machine causes ex-

cessive rotor and stator winding heating so an optimal working voltage must be

found. This optimum depends on the load condition of the motor.

Energy saving devices (further in this text abbreviated as ESD) employ phase

chopping similar to traditional lamp dimmers using triacs. This produces a more

distorted current, leading to extra losses both in the motor and the distribution

grid.

When the motor voltage is reduced, t he torque-speed curve lowers as the torqueis proportional to the squared rms voltage. If the load does not change, the ma-

chine will operate at a slightly reduced speed. However, in the case of a refrigera-

1 The author currently holds a research scolarship of the Belgian Instituut voor de aanmoe-

diging van Innovatie door Wetenschap en Technologie in Vlaanderen (IWT).2 The research is part of the research program Generisch Basisonderzoek aan de Univer-

siteiten (GBOU) of the IWT.


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tor, the mechanical load of the motor the compressor does change (it is also

proportional to the speed squared) and requires a lower torque. After a while, a

new torque-speed equilibrium is reached. A slight decrease of the speed already

reduces the cooling performance of the refrigerator considerably.

The thermostat within the refrigerator compensates for the loss of cooling

power by extending the motor on time. Due to the operation of the thermostat, a

simple power measurement is not sufficient to assess the energy efficiency of a re-

frigerator using an ESD. It is necessary to measure the total energy consumption

over an extended period of time.

Measurements

The uncertainty of the energy measurement is one on/off-cycle. The cycle time

of the refrigerator used in the tests is about 15 min. To have a tolerance of about

1 % on the energy measurements (assuming the power measurements have an er-

ror which is at least one order of magnitude lower), the measurement must span

about 100 cycles. Therefore, each test lasts for 24 hours. In the next sections, addi-

tional error sources and the measurement results are discussed.

Also a 2 months lasting duration test is performed, to test the reliability of the

ESD and to detect possible damage to the refrigerator motor, caused by the used

voltage control technique.

The refrigerator is an old type of the SIEMENS brand. During the energy

measurements (last test) also a second, older electronic energy saving device is in-

troduced (ESD B). The other test are all performed with the same, recent and

available ESD.

Measurement equipment

A Voltech PM3000A Power Analyzer is used to measure the energy consumption.

The precision of this equipment is much better (< 0.1 %) than any other uncer-

tainty in the test setup and is therefore neglected.

Environmental changes

The room in which the refrigerator is placed, is air-conditioned with a HVAC in-

stallation. The room temperature is kept constant at 20 C, because the energyconsumption of a refrigerator, although theoretically constant when a constant Co-

efficient of Performance (COP) is assumed, may vary a little with the difference

between the inside and outside temperature. Due to practical reasons, the room

temperature is not varied.


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Refrigerator content

The content of a refrigerator plays an important role. The objects inside the refrig-

erator have a heat capacity, to which the rate of temperature change in the refriga-

tor is inversely proportional. This is due to the fact that the heat flow is constant

and that the inside and outside temperature are also kept constant.

A first series of tests is done with an empty refrigerator. The second series is

performed with 48 small glass bottles of water inside. After insertion of the bot-

tles, there is a 24-hour waiting period.

The rate of temperature change obviously has a direct influence on the duty cy-

cle of the thermostat, which switches the cooling source on and off on two prede-

termined temperatures. When switched on, the cooling source produces a constant

heat flow. When it is off, the heat leakage through the wall isolation is also con-

stant but opposite in direction and usually much smaller. When the refrigerator is

empty, the on- and off-cycles are short because only a small amount of heat must

be extracted. When it is full, the on- and off-cycles are longer. The graph in figure

1 shows the thermostat on- and off behaviour in different tests.

The small xs in the graph are set whenever the refrigerator is in the on-state.

There are 8 horizontal series, each covering 3 hours. 4 sets of data are shown in

this figure, the corresponding measurement setup of which is given in table 1.

Table 1. Different setups used in the tests.

Sequence Load condition ESD used

1

2

3

4

Empty yes

Empty - no

Full yes

Full no

The on- and off-cycle times are a bit shorter (both using an empty and a full

refrigerator) when an ESD is used. The switching frequency is much lower when

objects are present in the fridge (full).

The duty cycle is important because it takes a few minutes for the refrigerant to

reach an equilibrium between evaporation and condensation. In this time, up to

10 % more power is needed. To facilitate this startup, ESDs do not cut back the

supply voltage during the first seconds. Figure 3 shows a typical current

consumption with and without the use of an ESD.

When on- or off-times are short, the compressor wastes energy more often in

reaching the steady state. In other words, an empty refrigerator uses more energy

than a filled one.Figure 3 also clearly shows that at any instant, the energy saving by the ESD is

never more than 5 W. Additionally, when using the ESD, the refrigerator runs

longer to compensate for this power loss. An ESD consumes also a little energy

when the compressor is not operating.


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0 2000 4000 6000 8000 10000-9

-8

-7

-6

-5

-4

-3

-2

-1

Time [s]

Series

Fig. 1. ON states of the refrigerator in the 24-hour test.

Surprisingly, using an ESD increases both the duty cycle and the switching fre-

quency. This effect is probably due to a reduced overall performance of the cool-

ing cycle.

80

85

90

95

100

105

110

0 100 200 300 400 500Time [s]

Power

[W]

Fig. 2. Power consumption during the first minutes after startup.


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Power measurement

Table 2 lists the rms mains voltage, current, active and apparent power consump-

tion of the refrigerator with and without ESD. The Total Harmonic Distortion

(THD) of the current is also listed.

Table 2. Power consumption

ESD used ? Ther-

mostat

U

[V]

I

[A]

P

[W]

S

[VA]

PF

[-]

ITHD

[%]

On 219,7 742,3m 92,18 163,11 0,565 6,70No

Off 0,0 0,0m 0,00 0,00 - 0,00

On 219,1 646,9m 87,50 142,01 0,616 31,71Yes

Off 219,6 67,2m 0,62 14,76 0,041 20,61

The 4.5 W power reduction does not mean that the compressor gets 4.5 W less

power when using an ESD. The strong harmonic distortion of the motor current

causes magnetic fields in the rotor rotating at speeds different from the synchro-

nous speed. The result is rotor heating and reduced output torque, which is diffi-

cult to measure because the motor and compressor are tightly integrated in a her-

metic package.

Energy consumption measurements

Table 3 summarizes the total energy measurements. In this test, also a second,

older ESD is evaluated (ESD B). In this table, the ESD used in the previous sec-tions is called ESD A. The working principle of the second type is unknown.

Table 3. Energy consumption

Test Load ESD Time

span

[h]

Etot

[Wh]

Eeco

[Wh]

Ein

[Wh]

PF

Measd.

[-]

Saving

(*)

[%]

1 empty none 24 835,5 0,0 835,5 0,560

2 empty none 24 815,8 0,0 815,8 0,570

3 empty none 24 853,5 0,0 853,5 0,568

-

4 empty A 24 795,0 17,5 777,6 0,529

5 empty A 24 832,0 18,8 813,2 0,536

+ 2,56

6 empty B 24 852,5 30,0 822,5 0,554 - 2,107 full none 24 762,1 - - 0,578 -

8 full A 24 770,1 - - 0,526 - 1,05

(*): Compared to the identical case without energy saver.

Clearly, the ESD only provided marginal energy savings only in the rather unu-

sual case of an empty refrigerator. This is contrary to the alledged 10 % energy

saving mentioned by the ESD manufacturer.


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Electrotechnical issues

The electrical schematic of a refrigerator, along with its supply voltage and current

waveform using an ESD is shown in figure 3.

M~

Main winding

Aux. winding

Startup relayMains

Motor

Thermostat

Fig. 3. Schematic and voltage and current waveform of a refrigerator using an ESD.

A short-circuited split pole winding (not on the schematic) provides the neces-

sary phase shift to obtain a rotating field. An auxiliary winding, powered by a re-

lay increases the field amplitude briefly to facilitate startup. No capacitor is used

in the schematic.

The influence of the steep voltage transients using an ESD, clearly visible in fi-

gure 3, on the life of the motor is still a subject of discussion in drive technology.

The two month duration test showed no signs of deterioration, but it is expectedthat a refrigerator motor is not designed with such repeating voltage transients in

mind.

Conclusions

Generally, energy savings by flux reduction is a questionable practice when the

mechanical motor load is above 40 % of its rated value. Refrigerator motors, by

design, match their compressor load well and flux reduction is expected to yield

only marginal energy savings, if any.

Also, although the power consumption is lower, the reduction of speed associ-

ated with flux reduction is compensated for by the thermostat. leading to a comp a-rable overall energy consumption. Experiments using an ordinary refrigerator con-

firm these statements.

In specific situations, the use of an ESD may reduce the energy consumption,

whereas in others, it may even increase it. The high current distortion and the un-

known influence on the refrigerators lifetime are factors discouraging the wide-

spread use of these devices.

Energy Consumption of Domestic Refrigerators

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