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Thermal analysis of the defrost cycle in a domestic freezer

Pradeep Bansal*, David Fothergill, Ryan Fernandes

Department of Mechanical Engineering, The University of Auckland, Private Bag – 92019, Auckland, New Zealand

a r t i c l e i n f o

Article history:

Received 7 January 2009

Received in revised form18 November 2009

Accepted 30 November 2009

Available online 13 January 2010

Keywords:

Refrigerator-freezer

Calculation

Heat transfer

Defrosting 

Experiment

Efficiency

Energy consumption

a b s t r a c t

This paper presents a thermal analysis of a defrost cycle in order to design more efficient

defrosting mechanisms in household refrigerators and freezers. A simple heat transfer

model has been developed to determine energy flows from a defrost heater across variouscomponents of a refrigerator/freezer. The study measures power consumption and

temperatures of a single temperature vertical empty freezer (in normal operation with and

without defrosts) to determine the heat distribution from the radiant type electric defrost

heater and its effect on power consumption. The surface temperature of the defrost heater

was measured to be 520 C for minimal frost and 560  C for heavy frost. The efficiency of 

a defrost heater was measured to be 30.3%, while power consumption of the freezer was

found to increase by 17.7% due to automatic defrost.

ª 2009 Elsevier Ltd and IIR. All rights reserved.

Analyse thermique du cycle de dé givrage d’un congé lateurdomestique

Mots clé s : Réfrigérateur-congélateur ; Calcul ; Transfert de chaleur ; Dégivrage ; Expérimentation ; Efficacité  ; Consommation d’énergie

1. Introduction

The energy efficiency of household refrigerators and freezers

is receiving considerable attention since these can account for

up to 11% of household energy costs (GSA, 2008). Today’s

refrigerators come with advanced electronics and ’intelligent’

features, and hence are more sophisticated and energy effi-

cient. New technologies are continually being discovered and

developed to improve the performance of the system (e.g.

door alarms, temperature sensors).  Young (2008) found that

over the last two decades the efficiencies of 620 L fridge

freezers have increasedby 150% in theUSA. There has been an

interest and drive by government bodies internationally to

increase the efficiency of modern appliances and electronics,

and a promotion of the use of energy efficient products in

recent years. Manufacturers of household products are

* Corresponding author. Tel.: þ64 9 373 7599x88176; fax:  þ64 9 373 7479.E-mail address: p.bansal@auckland.ac.nz (P. Bansal).

w w w . i i fi i r . o r g

a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m/ l o c a t e / i j r e f r i g

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 3 ( 2 0 1 0 ) 5 8 9 – 5 9 9

0140-7007/$ – see front matter  ª 2009 Elsevier Ltd and IIR. All rights reserved.

doi:10.1016/j.ijrefrig.2009.11.012

mailto:p.bansal@auckland.ac.nzhttp://www.iifiir.org/http://www.elsevier.com/locate/ijrefrighttp://www.elsevier.com/locate/ijrefrighttp://www.iifiir.org/mailto:p.bansal@auckland.ac.nz

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gearing up to improve the efficiencies of their products so that

they can be more competitive in the market. Energy ratings

stickers are put on appliances displaying their power

consumption. This is a key decision making factor thatconsumers look for when purchasing new appliances. The

energy consumption figures for energy ratings are determined

by testing the appliance to the relevant standard (Bansal, 2003)

of that country.

Frost free household refrigerators and freezers have auto-

matic mechanism (Bansal and Xie, 1999) to remove frost

before it noticeably degrades their performance. This mech-

anism is usually an electric radiation-type heater that is

controlled by a timer and a thermostat to periodically melt the

frost. Theheat required to melt the frost can be determined by

the amount of frost on the evaporator and its temperature.

Zakrzewski (1984) defined the efficiency of a defrost process in

terms of the energy required to melt the frost  Q melt, and thetotal energy input, including heat losses during defrost

(Q losses), by the following correlation:

h ¼

  Q melt

Q defrosth¼ Q melt þ Q losses

i!

  (1)

Although these systems have been used and researched in

the industry (Radcenco et al., 1995;  Inan et al., 2002;  Tudor

et al., 2005; Ozkan and Ozil, 2006; Tso et al., 2006) for a number

of years, there is still not enough information available in the

open literature about heat transfer effectiveness, energy flows

and the effect of defrost on the power consumption of 

domestic refrigerator/freezers. This paper, therefore,uncovers the physics of the defrost process in domestic

refrigerators by developing a simple heat transfer model to

quantify the resultant energy consumption as a result of 

defrosting, and investigate how this energy gets dissipated in

various parts of the freezer beyond the evaporator. The theory

was supplemented by in-depth experiments performed on

a single temperature empty vertical freezer with an electric

radiation-type defrost heater (in normal operation with and

without defrosts). The specific objectives of the study were to

have better fundamental understanding of the defrost cycles,

to analyse the heat distribution from a defrost heater in the

freezer space, and to quantify the effects of the defrost cycle

on the power consumption of the freezer.

2. Heat transfer modelling of the freezersystem

Modelling the freezer system will determine the heat loadswithin the system, identify the sources and the distribution of 

these loads within the system, and compare the effects of 

different defrost conditions on the freezer. Heat entering into

the system is absorbed by the components in the freezer, such

as the shelves, metal lining, internal air and the evaporator. In

steady state, this heat gain must be equal to the heat removed

by the refrigeration system during the compressor ‘on’ cycle.

A simple heat transfer model of the system has been devel-

oped in Engineering Equation Solver (EES, 2008)  to calculate

various heat transfer components of the process (Fernandes,

2008). The system was a vertical single temperature upright

freezer with a gross volume of 308 L and was charged with

105 g of refrigerant 134a. The radiant defrost electric heater

was rated to 450 W. The fin and tube evaporator was a two

tube and eight pass, sizing 500 mm, 255 mm and 60 mm

width, height and depth respectively. The fin and tube surface

area was 1.8 m2. The evaporator was covered from the main

cabinet with an opening below the evaporator and heater. The

re-circulated air hit the heater first, before the evaporator.

2.1. Assumptions

The following assumptions were made in modelling the

freezer system to simplify the problem and minimise the

number of measurements to be taken:

  The freezer system is symmetrical, i.e. the temperature on

one wall is equal to the temperature on the opposite wall.

This assumption is also made for the evaporator where

temperatures on the left and right are assumed equal.

 The growth of frost is uniform and constant on all parts of 

the evaporator.

  The rate of power consumed is the average power of the

cycle.

 The defrost heater temperature is its average temperature

in steady state.

  The heat loads of thesystem are determined only during the

‘off’ cycle.

Nomenclature

A   area (m2)

Cp   specific heat (J kg 1 K1)

F   view factor

H   enthalpy (J kg 1)

M   mass (kg)

Nu Nusselt number

Pr Prandtl number_Q    rate of heat transfer (W)

Q melt   energy required to melt the frost (J)

Q losses   losses in the defrost process (J)

Q defrost   total energy input for defrosting (J)

Ra Raleigh number

R   thermal resistance (K W1)

T   temperature (K)

DT   temperature difference (K)

Dt   time interval (s)

t   time (s)U   overall heat transfer coefficient (W m2 K1)

3   emissivity

h   Defrost efficiency

s   Stefan Boltmann constant

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  During defrost, natural convection as a result of heater

surface temperature only affects the evaporator and frost on

it, and not other components.

 The emissivity of the radiant heater and the evaporator are

respectively 0.85 and 0.07 (The Engineering Toolbox, 2009).

The emissivity of the evaporator does not change with the

amount of frost. The emissivity of the evaporator is

considered due to its close proximity to the heater and anyfrost at the lower part of the evaporator would melt quickly.

2.2. Heat transfer from ambient to the freezer

Heat transfer from ambient into the freezer occurs through

walls, door and seals of the freezer due to the temperature

difference of about 40 C between freezer inside (about20  C)

and the outside environment (about 20  C). For convective

heat transfer, the Nusselt (Nu) number of a vertical plate has

been evaluated, following   Churchill and Chu (1975a), as

a function of Raleigh and Prandtl numbers in Eq.  (2):

Nu ¼

26640:825 þ   0:387,Ra1=6n

1 þ

0:492Pr

9=16o8=273775

2

(2)

2.3. Heat gain within the freezer

The rate of heat transfer entering the freezer control volume

from the outside will be distributed among the various

components within the freezer, such as the freezer shelves,

the sheet-metal lining and the evaporator surface, and can be

given by:

_Q  ¼

m,Cp,DT

Dt

  (3)

2.4. Defrost heater modelling

The majority of the defrost electric energy (at high heater

surface temperatures of around 550 C) is converted to

radiation, and the remaining to convection. With the heater

being placed at the bottom ofthe evaporator (Figs. 1 and 2), theamount of radiation emitted towards the evaporator enables

the determination of heater effectiveness. The heat directed

towards the side walls is waste heat. The heat directed

downwards towards the drain is not all waste as this prevents

the melted water, after defrost, from freezing and blocking the

drain. The natural convection from the heater is calculated

similarly to the heat transfer through the wall, except that the

Nusselt number,   Churchill and Chu (1975b), correlation for

a horizontal cylinder, and the length, l, in the Grashof number

are replaced by the diameter of the heater   d. The Nusselt

correlation is only valid for Ra 1012.

Nu ¼

26640:6 þ   0:387,Ra1=6n1 þ

0:559

Pr

9=16o8=273775

2

(4)

Fig. 3 shows the radiation network for the defrost heater,

where radiation exchange between components2, 3, 4 and 5 is

assumed to be negligible. The sum of the heat energy received

Fig. 1 – Defrost heater and evaporator cross-sectional view inside the evaporator enclosure. Left, model; right, actual (mm).

Fig. 2 – Evaporator without cover.

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by the surfaces should be equal to the energy entering the

heater. In order to determine the direction of the heat transfer

between two surfaces i  and  j, thermal resistances in the radi-

ation network can be calculated as the surface resistance by

Eq. (5), andthe resistancedue to radiationview factorby Eq.(6):

Ri  ¼ 1 3iAi,3i   (5)

Ri/ j ¼

  1

Ai,Fi/ j

  (6)

where 3i and Ai are respectively the emissivity and area of the

surface. This determines whether the surface is emitting or

absorbing radiation. The view factors for radiation from the

circular heater to the surroundings are calculated following 

Bejan and Kraus (2003), as:

F1/2;3  ¼

 12p

tan1ðb1Þ tan

1ðb2Þ

F1/3;4  ¼1 2,F1/2;3

2

  (7)

where b1 and b2 relate how the surfaces are oriented. Using Eq.

(7), the values for view factors are shown in   Table 1. A

significant amount (39.8%) of the radiation emitted by the

heater is directed towards the evaporator. The drain receives

25%, while the remaining radiation is directed at the evapo-

rator cover and the back wall.

The net rate of heat transfer from radiation, between the

heater surface (at temperature Ti¼550  C) and each individual

surrounding surface (at temperature  T j¼40  C) can be calcu-

lated following  Oppenheim (1956) as:

_Q i/ j  ¼s,

T4i   T4 j

Ri þ Ri/ j þ R j

(8)

The total radiation heat transfer from the heater can be

found by the summation of Eq.  (8). The temperature of the

surrounding surfaces was measured during a normal defrost.

For simplicity the average value of 40  C was used in thermal

modelling.

It can be observed from the resulting rate of heat transfer

(as given in Table 2) that only 32% of the total heat from the

heater is utilised in heating the evaporator and frost, while the

remaining heat is wasted to heat up the surrounding components increasing the cooling load of the freezer. Inter-

estingly the amount of heat received by the evaporator is less

than the view factor towards the evaporator. Table 2 shows

the calculated values from the model to each component in

the evaporator enclosure.

Knowing the rate of heat transfer to the evaporator and the

amount of frost on the evaporator, the time to melt the frost

(in minutes) can be calculated from:

tdefrost ¼

(mfrost,

Cp;frost,DT þ hfusion;frost

60, _Q heater to evaporator

)  (9)

For the evaporator temperature of  20  C, the defrost time

would be 22 min.

3. Experimental procedure

For experimental work, a vertical freezer was chosen with

fixed operating temperature and a simple operation cycle,

where a fan blows the air over theevaporator. The freezer was

modified by adding a switch to the control box allowing the

freezer defrost heater to run internally or be controlled

externally. The external switch prevents the freezer control

box from initiatingthe defrost cycle, thus allowing the manual

start of the defrost heater. T-type thermocouples were placed

evenly around the freezer, as shown in Fig. 4, to measure the

Fig. 3 – Radiation network diagram.

Table 1 – View factors of radiation emitted from theheater.

Direction of view factors View factor

F1/2 (heater to evaporator) 0.3976

F1/3 (heater to drain) 0.25

F1/4 (heater to cover) 0.1762

F1/

5 (heater to back) 0.1762

Table 2 – Heat transfer from defrost heater to freezercomponents.

Freezer components Heat transfer rate(W)

Heater to drain (radiation) 161.8

Heater to back wall and cover (radiation) 147.7

Natural convection 109.9

Heater to evaporator/frost (radiation) 33.6

Total heat from heater to freezer internals 453

Total heat from heater to the evaporator 143.5

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temperatures of the internal and external walls and the

internal air. On each wall two thermocouples were placed in

the centre of the walls and a third from the top and bottom.

These measurements were used for calculating the heat

transfer rates. Five T-type thermocouple pairs (exposed and

insulated) measured the evaporator surface temperatures

during defrost. The exposed thermocouples were very sensi-

tive to the changes in the defrost heater temperature, while

the insulated thermocouples showed gradual temperature

increases. The thermocouples were placed on the left handside of the evaporator, as it was assumed that the heat

distribution from the heater is symmetrical. A separate K-type

thermocouple was used to measure the heater surface

temperature. The thermocouples were calibrated during the

temperature range from   30  C to 40  C to within   0.1  C

accuracy.

Freezer air temperatures were measured using thermo-

couples inserted into a brass mass to collect an even

temperature change with minimal fluctuation. Thermocou-

ples in the evaporator were inserted into an aluminium

tube with spacers used to stop it from touching the

aluminium. This shielded the thermocouple from the radi-

ation from the defrost heater, allowing a proper tempera-ture measurement to be taken. Wall temperatures were

measured by sticking the thermocouples to the wall with

foil tape.

3.1. Humidification of the freezer

3.1.1. Methodology and equipment

To measure the effectiveness of the defrost heater as well, as

to show the effects of a frosted evaporator on the power

consumption of the freezer, a humidification procedure had to

be developed. The device used for humidification consisted of 

two sponges wrapped over element heaters, which had

a specific amount of water poured over them. This device was

then put inside the freezer and the freezer allowed to run as

normal. This device was assisted by three auxiliary heaters

placed inside the freezer. These heaters regulated the

temperature inside the freezer compartment so that the

evaporated water from the humidification device did not

condense on the freezer walls and could safely be assumed to

only cause frost to form on the evaporator.

3.2. Power consumption tests

The purpose of the tests was to measure the power

consumption of the freezer for a prolonged period of time and

for different operating parameters, e.g. for an empty freezer in

normal operation with no defrost, normal operation with

automatic defrost and humidification with and without

defrost. The humidification tests consisted of frosting the

evaporating 500 g of water into the freezer. The conditions for

the test included the following:

 Ambient temperature at 20  C, and relative humidity of 65%

  Empty freezer – no mass packs, but one wire shelf for

placing thermocouples.

3.3. Defrost effectiveness test

This test is used to quantify the effectiveness of the defrost

heater in a given freezer. The test involves humidifying the

freezer with 500 g of water, letting the freezer reach a steady

state of operation, initiating a defrost for 22 min after which

returning the freezer back to normal operation. This simu-

lates a typical defrost function. The conditions for the test

are similar to the power consumption test, but there are

more components in the freezer like the auxiliary heaters

and the humidifier. The test will be initiated in the following 

order:

Fig. 4 – Thermocouple placement on the evaporator (left) and the freezer (right).

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1. Set air inside the environmental chamberat 20 C and 65%

relative humidity

2. Open the freezer and let it normalise to ambient

conditions

3. Add 500 g of water to the humidifier

4. Once normalised (i.e. freezer temperature is 20  C), close

the freezer doors, and wait for 2 min to start the freezer

5. Set the auxiliary heater power6. Start freezer, auxiliary heaters and humidifier

7. After the humidifier switches off, switch off auxiliary

heaters

8. Run the freezer for 12 h period

9. Start defrost, turn off freezer for 22 min, and collect the

melted water

10. Wait for 2 min and turn the freezer back on

4. Results and discussion

4.1. Refrigeration cycles

The power consumption test for normal operation with

defrost was conducted first to gather information regarding 

the refrigeration system, e.g. freezer cycles, system tempera-

tures, cycle times and power consumption during the opera-

tion. The average time and temperatures measured over the

24 h period are given in  Table 3. The internal freezer temper-

atures are higher than the freezer air (20.3  C), while the

freezer external wall temperatures are higher than the

ambient 20  C due to a hotwall condenser (Bansal and Chin,

2003)   being placed inside the freezer walls. The average

evaporator temperature is only 3.3  C lower than the freezer

air temperature. For a freezer, this indicates that within14 min and 26 s the freezer temperature increases to the

maximum allowable temperature of   17  C for normal

operation.

As shown in Table 4, the freezer air temperature reaches

maximum (4.1  C) soon after the defrost cycle, while the

corresponding drain and wall temperatures reach 47.5  C and

2  C respectively. The power consumption values in  Table 4

are the average values during a cycle, while the power value

during a defrost cycle is the maximum power consumed by

the freezer.

4.2. Heat transfer without defrost

Heat infiltration into the freezer, as determined by the heat

transfer model (as explained in Section 2), is given in Table 5.

These are based on the measured freezer temperatures during 

the ‘off’ cycle as inputs. It is interesting to note from Table 5

that the heat infiltration rate through the door seals is the

highest among all components.

Thedatafor heat gain by thefreezer components, based on

the heat transfer model (Table 5), agrees with the calculated

heat gain from measured system temperatures over time

(Table 6) to within 9.5% (i.e. a discrepancy of 3.8 W). This is an

excellent agreement, where the discrepancy maybe due to the

heat absorbed by the internal plastics.

4.3. Power consumption tests

4.3.1. No defrost

Power consumption tests were conducted for three empty

freezer operations: normal operation, normal operation

without defrost and 500 g of frost on evaporator with and

without defrost, and power consumption was measured for

steady state operation over a 24 h period.  Figs. 5 and 6 are the

photos taken of the evaporator during tests. The frost growth

is greater at the bottom rows of the evaporator for the 500 g of 

frost, while for normal operation with minimal frost (Fig. 5) is

evenly distributed across the evaporator. For normal opera-

tion without defrost the yearly power consumption is

412 kWh, while the annual power consumption of a frosted

evaporator is 532 kWh. As expected, the power consumptionin   Fig. 7   shows that the normal operation with no defrost

consumes the least amount of power. This is due to shorter

Table 3 – Measured average temperatures of the freezerover a 24 h period.

Compressor case

temperature

38.4  C Evaporator

temperature

23.6  C

Ambient temperature 20.0 C Freezer air

temperature

20.3  C

External side wall

temperature

21.8  C Internal side

wall temperature

17.6  C

External door

temperature

19.2  C Internal door

temperature

18.2  C

External back wall

temperature

21.9  C Drain

temperature

18.7  C

On time 13 min 17 s Off time 14 min 26 s

Pow er consumed 51.8 W Defrost time 11 min 47 s

Table 4 – Minimum and maximum measured values.

Minimum Maximum

Freezer air

temperature

20.3  C Freezer air

temperature

4.1  C

External side wall

temperature

18.3  C External side

wall temperature

29.9  C

External back walltemperature 17.0 

C External backwall temperature 30.2 

C

Internal side wall

temperature

20.5  C Internal side

wall temperature

2  C

Drain temperature   22.4  C Drain temperature 47.5 C

On time 2 min 00 s On time 59 min 00 s

Off time 1 min 30 s Off time 17 min 00 s

On cycle power

consumed

100 W Defrost power

consumed

480.2 W

Table 5 – Calculated heat infiltration into the freezerduring compressor ‘off’ cycle.

Heat transfer through walls Heat transfer rate (W)

Side walls 12.1

Door seals 16.4

Door 6.2

Back wall including compressor step 5.3

Total 40

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‘on’ cycle times, while still maintaining ‘off’ cycle times (of 

around 15 min) comparable to other tests (Fig. 8). Normal

operation with the defrost data set does not contain a defrost

cycle, thus providing good comparison against other data sets

which are for ‘on’ and ‘off’ cycles only. The power consump-

tion for defrosting normal operations is (13 W) 17.7% higher

than normal operation with no defrost due to the heavy frost

build up on the evaporator (Figs. 6 and 7).

4.3.2. Effect of defrost on power consumption

Frost growth on the evaporator degrades its thermal perfor-

mance, and hence defrosting is necessary. However,defrosting the evaporator adds heat into the system, thereby

resulting in a longer compressor ‘on’ cycle as can be seen in

Fig. 8.   Fig. 8   displays a comparison between the normal

operation of the freezer with and without a defrost cycle. The

average ‘on’ cycle time (Fig. 8, top graph) for no defrost is

about 15 min, while after the defrost, the cycle has a much

longer ‘on’ time of about 1 h (Fig. 8, bottom graph). The power

consumed during a normal operation including a defrost

cycle for a 24 h period is 1.26 kWh, which is higher than the

normal operations without a defrost. An automatic defrost of 

an evaporator with 500 g of frost is 25 min 30 s and the

following ‘on’ cycle of 80 min consumes a total of 0.32 kWh of 

power. The power consumption before defrost was 1.46 kWhper day (532.9 kWh per annum). The power consumption

after defrost for the test was 1.28 kWh (467.2 kWh per

annum).

Fig. 9  illustrates the ‘on’ cycle times following a defrost

cycle for the normal operation test without addition of 

water. The average ‘on’ cycle time after defrost was

54.6 min, while for the normal freezer operation it was

11.8 min, thus suggesting that the freezer needs 66.4 min to

defrost the evaporator before returning to its regular ‘on/off’

cycle.

4.4. Defrost effectiveness

4.4.1. Heater temperature

Fig. 10 illustrates the temperature of the defrost heater rela-

tive to the freezer air for a defrost cycle. When the heater is

turned on, the defrost heater temperature reaches its steady

state value of 560  C within 5 min. Over the entire defrost

cycle, the temperature of the freezer air increases by only 6  C

despite the temperature around the heater being higher than

40  C. The heater temperature is higher with heavy frost

(560  C) on the evaporator than minimal frost (520  C) due to

partial blockage of the air flow, resulting in the lower heat

transfer coefficient (U ).

Eq.  (10)   shows that to maintain the same heat flow rate

with a lower heat transfer coefficient, the temperature

difference must increase since the change in surface area of 

the evaporator due to the frost is negligible.

_Q  ¼ U,A,DT   (10)

4.4.2. Evaporator temperatures

The evaporator temperatures during defrosting with

minimal frost and with 500 g of frost are shown in Fig. 11.The

respective defrost times for the 500 g frost and minimal frost

test are 22 min and 10 min. It can be seen that the tempera-ture of the bottom row of the evaporator (with minimal

defrost) which is closest to the heater, increases very quickly

from below 20  C to under 60  C in 5 min. The temperature

rise for the initial 5 min is the same for both tests, and then

the bottom row of the evaporator with minimal frost gradu-

ally increases to its maximum temperature of 66  C just

when the heater was switched off. The frosted evaporator

reaches maximum temperatures 15 min into defrost, reach-

ing 100  C and steadily declining after that. The drops in

temperature during defrost are due to streams of melting 

water from above cooling local zones where the thermo-

couples were placed.   Fig. 12   displays the temperatures

measured at the top row of the evaporator, where the

Table 6 – Calculated heat gain within the system asa result of infiltration.

Heat gain within the system Heat transfer rate (W)

Evaporator 9.5

Freezer air 0.3

Metal linings including shelf 26.4

Total heat infiltrated into freezer 37.2

Fig. 5 – Frost build up during normal operation.

Fig. 6 – Heavy frost (500 g frost test).

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Fig. 7 – Power consumption from different operations during a steady state on/off cycle.

Fig. 8 – Normal operation and defrost power cycle.

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temperature rise is significantly slower than that for thebottom row. This is due to the distance from the heater as

well as frost in between the heater and the top row. The top

row temperature peaks after defrost has turned off, similar to

the freezer air temperature.

4.4.3. Heat gain during a defrost cycle

Table 6   compares the total heat absorbed by the freezer

components to the total heat added into the freezer from the

defrost heater as well as through heat infiltration for 20 min.

The heat input component is calculated from the power

readings of the freezer, while heat absorbed is calculated

from the temperature change in the freezer components.

Data reveals that (i) only 28% of the total heat goes intomelting the frost and 10% towards heating the melted water

to 30  C exiting the drain, (ii) an insignificant amount of 

energy goes to heating up the freezer air, (iii) the evaporator

absorbs 11% of the total energy in the system with the

majority coming from the heater, (iv) 43% of the heat during 

defrost is absorbed by various freezer components, while (v)

only 8% of the heat input within the system is unaccounted

for. This is a very satisfactory energy balance across the

system, which could be further improved by adding scope to

the model for temperature measurement in the ducts above

the evaporator and fan where this heat is most likely

absorbed.

4.4.4. Defrost efficiency

The efficiency of the defrost heater for the upright freezer was

calculated to be 30.3% by Eq. (11):

h ¼

  Q melt

Q defrost h ¼ Q melt þ Q lossesi

!¼

171:4 kJ566:2 kJ

¼ 30:3% (11)

where  Q melt  is the theoretical heat required to melt 500 g of 

frost and  Q losses is the calculated energy absorbed by freezer

components not critical to defrosting.

This value is similar to the theoretical percentage of heat

from the heater directed towards the evaporator.   Table 7

shows the calculated heat gains in the system from measured

temperatures during defrost.

Fig. 9 – Following ‘on’ cycle after defrost.

Fig. 10 – Freezer air temperature relative to heater

temperature.

Fig. 11 – Temperatures from the bottom row of evaporator

during defrost.

Fig. 12 – Temperatures from the top row of evaporator

during defrost.

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4.4.5. Defrost melting comparisonThe theoretical time to defrost 500 g of frost, using Eq.   (9),

was calculated to be 22 min. This time was compared with

the experimental time to defrost 500 g of frost by measuring 

the actual amount of melted water. After defrosting, an

average 425 g of water was collected as against the expected

amount of 500 g. It was observed after the defrost tests that

there was still frost on the upper sections of the evaporator

and water droplets on the fins and tube of the evaporator. A

key observation made during the defrost tests was that

some of the melted water dropping from the evaporator

contacts the heater and evaporates. The water evaporating 

absorbs heat energy and reintroduces moisture back into

the system.

5. Conclusions and recommendations

The study analyses the energy flowdistributionof defrost heat

into the freezer space and quantifies the effects of the defrost

cycle on the power consumption of the freezer. From the

results of this research, the following conclusions can be

drawn:

  The comparison between minimal frost and 500 g frost

shows that under normal usage conditions – i.e. introduc-

tion of moisture into the system via door opening andregularly taking items in and out of the freezer – less energy

will be consumed when an automatic defrosting mecha-

nism is integrated into the freezer operation.

  The defrost heater testing shows that a radiant defrost

heater is perhaps not the best option to use. The tempera-

tures that the radiant heater reaches are unnecessarily high

for the purpose of melting the frost on the evaporator. Also,

the majority of people would not expect to find an element

that reaches temperatures in excess of 500 C in their

freezer. This could be considered as a safety issue.

The defrosting process should be investigated further in

order to improve its efficiency. This study has shown that

a freezer with 500 g frost and normal defrost cycle will

consume respectively about 30% and 17.7% more energy to

melt the frost than with minimal frost. Because the energy

saving is only 12.3%, there is still room for improvement in

this area. Two options to consider are:

 Embedding a heater into the evaporator itself which uses

less energy and operates at lower temperatures. This wouldhave more direct contact with the frost and will have the

effect of less radiated energy to the other freezer

components.

  Reversecycledefrostisoftenusedinheatpumps,whichcould

also be investigated in domestic freezers. This may result in

higher energy efficiency of the overall defrost process.

r e f e r e n c e s

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Table 7 – Calculated heat gains by thefreezer componentsduring defrost.

Components Total heat transferred (kJ)

Heat input to the system

Heater 566.2

Heat transfer through walls 48

Total heat input to the freezer 614.2Heat absorbed by various components

Evaporator 67.2

Evaporator cover 26.4

Freezer air 1.7

Heat to melt frost 171.4

Heating water 57.6

Auxiliary heaters 23.3

Freezer shelves 20

Defrost heater 51.6

Metal linings 51.4

Drain plastic 93.8

Total heat absorbed 564.4

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