Energy consumption analysis of domestic oven

Abstract

For cookers with oven to be interesting for the market, they need to exhibit a high energy efficiency. On the basis of an analysis of individual influences on the energy efficiency of ovens, several solutions were proposed for improving oven operation. An analytical solution was made for describing the nonstationary temperature field within a test brick with convection at a constant heat transfer coefficient and a continuously variable temperature of the surrounding fluid using Green's functions. The influences of door sealing, vapor outlet slit, additional insulation, the oven regulation regime, door glazing and irradiation within the oven were experimentally analyzed. Because of the proposed improvements, the analyzed mass produced oven was moved to the A class of energy efficiency according to standard EN 50304:2001.

Full text

Energy consumption analysis of domestic oven
M. Penšek1, N. Holeþek1, H. Gjerkeš2, I. Golobiþ2*
1Gorenje d.d., Partizanska 12, Velenje, Slovenia
2University of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, Ljubljana, Slovenia
* iztok.golobic@uni-lj.sil
Abstract
For cookers with oven to be interesting for the market, they need to exhibit a high energy efficiency. On the basis of
an analysis of individual influences on the energy efficiency of ovens, several solutions were proposed for improving
oven operation. An analytical solution was made for describing the nonstationary temperature field within a test brick
with convection at a constant heat transfer coefficient and a continuously variable temperature of the surrounding fluid
using Green’s functions. The influences of door sealing, vapor outlet slit, additional insulation, the oven regulation
regime, door glazing and irradiation within the oven were experimentally analyzed. Because of the proposed
improvements, the analyzed mass produced oven was moved to the A class of energy efficiency according to standard
EN 50304:2001.
Introduction
Kitchen ovens consume large amounts of energy in
households. By increasing their energy efficiency, it is
possible to reduce greenhouse gas emissions via
reduction of energy consumption. The energy efficiency
of ovens is marked on energy labels. Oven manufacturers
are increasingly forced to adjust their products to market
requirements by developing ever more energy efficient
ovens that fulfill Class A criteria [1-3]. Functionality,
design and energy efficiency all need to be taken into
account [4-6].
Standard EN 50304:2001 [7] prescribes measurement
of energy consumption after the oven is switched on and
once a Hipor test brick saturated with moisture is heated
from 5 to 55 oC. Depending on the oven size, the
electrical energy consumption in the given case should be
below 800 Wh in conditions of natural air convection
without the fan, or forced air convection with the oven
fan switched on.
This paper focuses primarily on an experimental
theoretical analysis of individual influential parameters
that affect the energy efficiency of mass produced ovens.
Modeling of heat transfer within the oven
In order to be able to improve the energy efficiency of
an oven, one must analyze the influence of the greatest
possible number of parameters that affect it. However, a
numerical approach in the form of 3D oven modeling
does not enable the analysis of a greater number of
parameters because of its complexity. Since the standard
for determining the energy efficiency prescribes
measurement of consumed energy for the required
increase in the temperature inside a moist test brick while
it is heated in the oven, we decided to approach the
description of the nonstationary temperature field in the
test brick analytically.
Fourier’s equation for heat convection

t
T
a
1
t,xg
x
T
2
2
w
w
O

w
w (1)
was solved analytically for a plate using Green’s function
(general solution) [1]:
Strojniški vestnik - Journal of Mechanical Engineering 51(2005)7-8, 405-410
UDK - UDC 536.2
Izvirni znanstveni þlanek - Original scientific paper (1.01)
405

For the nonstationary continuous temperature variation of
the surrounding fluid, we assumed that


ff

ffff  ,
tk
3
tk
1,maks,TekekTTT 42

(3)
By using constants k1 through k4 under the following
conditions:
k1> 0, k2> k3, k4< k2< 0, (4)
it is possible to capture the variation of temperature with
time as can be seen in Figure 1.
Figure 1 Example of variation of the surrounding fluid’s
temperature with time.












(2)
n
G
fda,xt,xG
f
da
dxd,xg,xt,xG
a
xF0,xt,xG
cL
axdxF0,xt,xGt,xT
t
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1i xx
i
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§W
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c
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
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¼
º
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¬
ªcc
O
U

c

cc
This can serve as the basis for analyzing processes in the
oven.
The maximum temperature is expressed as
ff
fff
f


,
31
,0,
maks,T
kk
TT
T (5)
Taking into account a constant heat transfer coefficient
for heat transfer to the surrounding fluid and variable
ambient temperatures according to equation (3), the
nonstationary temperature field of a semiendless plate is
in the form:






 
(6) e1
2sin2
cos
L
x
cos
T
L4
ee
2sin2aLk
cos
L
x
cosTTaLk4
ee
2sin2aLk
cos
L
x
cosTTaLk4
L
x
cos
cossin
sin
eT2t,xT
2
2
m
2
2
m
4
2
2
m
2
2
2
m
L
at
1m mm
2
m
mm
,
L
at
tk
1m mm
2
m
2
4
mm,maks,m3
L
at
tk
1m mm
2
m
2
2
mm,maks,m1
1m
m
mmm
m
L
at
0
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In order to determine the nonstationary temperature
variation in a three-dimensional brick, the principle of
superposition is used.
t [s]
T[°C] Tmaks
T0
T
Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 405-410
Penšek M. - Holeþek N. - Gjerkeš H. - Golobiþ I.
406

Experimental set up and test procedure
Figure 2 shows two cooker models that were used to
perform oven measurements, namely Gorenje Pininfarina
and Gorenje E774W. The model Gorenje E774W had an
oven insulated with glass wool with a thickness of 3 cm,
and Pininfarina had glass wool with a thickness of 5 cm.
The oven door was double-glazed. Oven controls were set
so as to allow temperature oscillations within the oven of
about 10 K around the set temperature. When the oven
operated with natural air convection only (switched off
fan), two heaters were used – an upper and a lower one.
During oven operation with forced air convection, the
lower heater and the peripheral heater (which is installed
around the fan) were switched on.
Figure 2 Gorenje E774W and Gorenje Pininfarina.
Figure 3 shows an oven schematic with the positions of
the heaters and fan opening.
Figure 3 Schematic of oven in the Gorenje E774 W
cooker.
The consumed energy was measured using a Siemens
Simeas P power meter, which measures the input voltage
and current, after which an integral processor calculates
the desired values. The measurement accuracy of the
power meter for the measurement of electric voltage and
current is r0.2 %, and for measuring power it is r0.5 %.
The weight of the brick was measured using a Siemens
Siwarex U balance. The balance was placed on top of the
cooker. A bore with a diameter of 1 mm was drilled
through the plate, insulation and the oven, and a thin wire
was led through that connected the balance with a basket
containing the test brick that was placed in the oven.
The temperature was measured using thermocouples
Class 1 according to EN 60548-2 standard in a steel
casing with an external diameter of 1 mm. The
thermocouples were fit snugly inside the bores within the
test brick. For all measurements according to standard EN
50304:2001, measurement of two temperatures is
required. In our case, however, 10 were done to be able to
analyze heat conduction within the test brick. A
schematic of the measurement sites on the test brick and a
picture of the thermocouples installed in the test brick are
shown in Figure 4.
Figure 4 Measurement sites on Hipor test brick.
For the regulation of oven heaters, electronic elements
were added to the oven with output relays for switching
the heaters on, as well as power supply for external
electronics that transformed the software PID controller
data into an electric signal and sent it to the relays, which
then switched oven heaters on or off. The PID controller
was connected with the LABView PC program, via
which the oven operation regime was determined for
individual heaters and data were captured. The
temperature profile was also set and it was determined
which heaters should be switched on. Settings of the PID
controller were also made.
The objective was to achieve Class A of energy
efficiency for the oven according to standard EN
50304:2001. The standard for Class A prescribes that a
Hipor test brick saturated with moisture should be heated
from 5 to 55 oC after the NG 500 oven is switched on,
whereby the total electrical energy consumption needs to
be lower than 800 Wh, for both natural air convection and
for forced air convection. A new test brick has to be dried
in the oven before use for 3 hours with forced air
circulation at a temperature of t175 °C. The weight of
the dried brick without thermocouples has to be measured
within 5 min after its removal from the oven. Then the
brick can be used for up to 20 measurements to measure
7
1
8,7
10
9
10
1
4
6
2
3
9
8
64
11
50
230
50
11
114
22
12
3
4
6
y
z
x
Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 405-410
Energy consumption analysis of domestic oven
407

the time and consumed energy that is necessary for an
empty oven to be heated from the ambient temperature by
180 K in the case of natural convective air circulation, or
by 155 K in the case of forced air circulation with the use
of a fan. Two thermocouples required by the standard are
fitted into the test brick such that the measuring sites are
in the center of the brick, and the distance between the
thermocouples is 50 mm (marks 1 and 10 in Figure 4).
Results and discussion
Figure 5 shows the influence of the rate of heating of the
surrounding air on the increase in brick surface
temperature in the middle of the test brick, as an example
of using equation (6).
Figure 5 An example of using equation (6).
Figure 6 shows a comparison between the measured and
calculated temperature curves during heating of dry brick
at natural convection. The differences primarily resulted
from inaccurate approximation of the temperature
variation of the surrounding air, nonobservance of the
irradiative share of heat transfer from the oven walls to
the test brick, and a nonuniform initial temperature field
in the test oven.
Figure 6 Comparison of the measured and calculated
temperature profiles during heating of dry test brick at
natural air convection, D = 16 W/m2K.
Infrared camera ThermaCam S60 manufactured by Flir
was used for thermographic measurements. Figure 7
shows IR photographs of the test brick and Figure 8 the
time variation of the mass of moist test brick during
drying, while it is heated with forced air convection and
overheated to 155 K.
0
40
80
120
160
200
240
280
320
0 10203040506
t [min]
T [°C]
0
Tamb 1
Tsu rf 1
Tamb 2
Tsu rf 2
Tsu rf 3
Tamb 3
Figure 7 IR photographs of the test brick after 8, 16 and
32 min of heating with forced air convection and
overheating of 155 K.
1800
1850
1900
1950
2000
0 1020304050
t (min)
m (g)
0
40
80
120
160
200
240
0123456789101112131415
t [min]
T [°C]
pt2 calc pt4 calc pt 7 calc
amb meas pt2 meas pt4 meas
pt 7 meas amb calc
Figure 8 Time variation of the mass of moist test brick
during heating with forced air convection with 155 K
overheating.
Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 405-410
Penšek M. - Holeþek N. - Gjerkeš H. - Golobiþ I.
408

The influences of sealing of the door, vapor outlet slit,
additional insulation, the regulation regime, door glazing
and irradiation in the oven were analyzed.
The sealing of door and gaps/slits on the oven was
performed using an aluminum self-adhesive insulating
tape. This measure reduced the energy consumption from
the baseline of 917 Wh to 801 Wh.
The effect of the vapor outlet channel was tested by
plugging the vapor outlet slit in addition to sealing all of
the gaps and slits on the oven. After this, the oven’s
energy consumption decreased to 772 Wh.
When measuring the effect of glazing, the existing
double-glazed door was replaced with a triple-glazed one.
The measurements showed that an additional glass sheet
on the door not only increases safety by lowering the
temperature of the external glass surface, but also
contributes to a higher energy efficiency of the oven.
Figure 9 shows the temperature variations in the brick at
position 1 (T pt1) and consumed energy (E) at a given air
temperature in the oven (T amb) for double (2G) and
triple (3G) door glazing. The energy consumption was
837 Wh.
Figure 9 Effect of double (2G) and triple (3G) door
glazing.
The influence of insulation layer thickness was
analyzed by laying an additional glass wool layer on the
sides and on the upper and lower external oven surface.
On the rear, no insulation was added because of the
vicinity of the electric heaters. A 3 cm layer of glass wool
was added – a total of about 1300 g. Figure 10 shows a
comparison of the basic oven and the additionally
insulated oven (isol). The energy consumption of the
oven with additional insulation was 846 Wh.
.
0
100
200
300
400
500
600
700
800
900
1000
010203040
t (min)
E (Wh)
0
40
80
120
160
200
T(°C)
E
E is ol
T amb
T pt 1
T pt 1 isol
Figure 10 Effect of insulation.
The effect of irradiation inside the oven was tested by
placing an aluminum foil onto the oven’s inside walls,
except for the upper and lower sections, where the heaters
are. The foil was arranged in two layers at a distance of
about 4 mm. The result for energy consumption is
practically the same as the result for measurement with
additional thermal insulation, i.e. 845 Wh.
The oven has an integral ON-OFF PID controller. The
control regime was analyzed by setting different
tolerances for the desired and actual air temperature in the
oven. It turned out that small oscillations have a favorable
effect on oven energy consumption. Figure 11 shows a
comparison of energy consumption (E), temperature
variation in the oven (T amb) and temperature in the test
brick at measurement site 1 (T pt 1) between the standard
control regime and a regime with smaller tolerances
between the desired and actual oven temperature (E reg,
T amb reg, T pt 1 reg). Energy consumption at improved
regulation amounted to 745 Wh.
0
100
200
300
400
500
600
700
800
900
1000
0 10203040
t (min)
E (Wh)
0
40
80
120
160
200
T(°C)
E 3G
E 2G
T pt1 3G
T amb
T pt 1 2G
0
100
200
300
400
500
600
700
800
900
1000
0 1020 3040
t (min)
E (Wh)
0
40
80
120
160
200
T(°C)
E
E r eg
T amb
T pt 1
T pt 1 reg
T amb r e
g
Figure 11 Effect of control.
Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 405-410
Energy consumption analysis of domestic oven 409

On the basis of analysis of the influence of individual
parameters on the energy consumption required for
heating the test brick from 5 to 55oC, the above-
mentioned measures were quantitatively compared.
Figure 12 shows that the control method can contribute at
most.
With a balanced combination of individual measures, it
was possible to achieve a shift to a higher energy
efficiency class for the studied oven. The acquired
knowledge may be a good basis for designing ovens with
a higher energy efficiency.
Conclusions
On the basis of an analysis of individual influences on
the energy efficiency of ovens, several solutions were
proposed for improving oven operation. An analytical
solution was made for describing the nonstationary
temperature field within a test brick with convection at a
constant heat transfer coefficient and a continuously
variable temperature of the surrounding fluid using
Green’s functions. The influences of door sealing, vapor
outlet slit, additional insulation, the oven regulation
regime, door glazing and irradiation within the oven were
experimentally analyzed.
On the basis of the proposed improvements, the
analyzed mass produced oven was moved to the A class
of energy efficiency according to standard EN
50304:2001.
600
650
700
750
800
850
900
950
E (Wh)
1234567
Step
Step Description E (Wh)
1 Standard 917
2 Sealing of the door and gaps 801
3 Sealing of the door, gaps and vapor outlet 772
4 Control with smaller oscillations 745
5 The added 30 mm insulation layer around the oven 846
6 Aluminum foil glued over the interior oven surface 845
7 Triple glazed door 837
Figure 12 Quantitative comparison of measures.
References
[1] B. Flipsen, J. Koot and G. Timmers, Design for
energy efficiency as a basis for innovations in kitchen
appliances, Energy Efficiency in Household Appliances
and Lighting, Springer, pp. 182-191, 2001
[2] A.K. Meier and J.E. Hill, Energy test procedures for
appliances, Energy and Buildings 26 (1), pp. 23-33, 1997
[3] T.M.I. Mahlia, H.H. Masjuki and I.A. Choudhury,
Theory of energy efficiency standards and labels, Energy
Conversion and Management 43 (6), pp. 743-761, 2002
[4] Efficient Domestic Ovens, Final report of the SAVE
II Project 4.1031/D/97-047, Helsinki, 2000
[5] J.P. Clinch and J.D. Healy, Cost-benefit analysis of
domestic energy efficiency, Energy Policy 29 (2), pp.
113-124, 2001
[6] B.M. Shaughnessy and M. Newborough, Energy
performance of a low-emissivity electrically heated oven,
Applied Thermal Engineering 20 (9), pp. 813-830, 2000
[7] Standard EN 50304:2001; CENELEC, Brussels, 2001
Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 405-410
410 Penšek M. - Holeþek N. - Gjerkeš H. - Golobiþ I.