Comparison of Energy and Exergy Efficiency of Community and Domestic Type Parabolic Solar Cookers

Abstract

The work presented in this article essentially consists of evaluating the energy and exergy efficiency of two types of parabolic solar cookers. The experiments on both the cookers were performed in summer and winter, both in the climatic conditions in India. The measurements were done by using a microprocessor-based on-line data acquisition system using class I solar pyranometer and Pt. 100 temperature sensors. The energy end exergy efficiencies of both the cookers were experimentally evaluated. The energy output of the community solar cooker varied between 2.73 to 43.3 W and 7.77 W to 33.4 W for the domestic solar cooker. The exergy output for community solar cooker was in the range of 1.92 to 2.58 W, whereas for the domestic solar cooker, it varied from 0.65 to 1.45 W. The energy efficiency of the community solar cooker varied from 8.3% to 10.5% and for the domestic solar cooker, it varied from 7.1% to 14.0%.

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Comparison of Energy and Exergy
Efficiency of Community and Domestic
Type Parabolic Solar Cookers
S. K. Shukla a
a Department of Mechanical Engineering, Institute of Technology,
Banaras Hindu University, India
Version of record first published: 07 Oct 2009
To cite this article: S. K. Shukla (2009): Comparison of Energy and Exergy Efficiency of Community
and Domestic Type Parabolic Solar Cookers, International Journal of Green Energy, 6:5, 437-449
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COMPARISON OF ENERGY AND EXERGY EFFICIENCY
OF COMMUNITY AND DOMESTIC TYPE PARABOLIC
SOLAR COOKERS
S. K. Shukla
Department of Mechanical Engineering, Institute of Technology, Banaras Hindu
University, India
The work presented in this article essentially consists of evaluating the energy and exergy
efficiency of two types of parabolic solar cookers. The experiments on both the cookers were
performed in summer and winter, both in the climatic conditions in India. The measurements
were done by using a microprocessor-based on-line data acquisition system using class I solar
pyranometer and Pt. 100 temperature sensors. The energy end exergy efficiencies of both the
cookers were experimentally evaluated. The energy output of the community solar cooker
varied between 2.73 to 43.3 W and 7.77 W to 33.4 W for the domestic solar cooker. The exergy
output for community solar cooker was in the range of 1.92 to 2.58 W, whereas for the domestic
solar cooker, it varied from 0.65to 1.45 W. The energy efficiency of the community solar cooker
varied from 8.3% to 10.5% and for the domestic solar cooker, it varied from 7.1% to 14.0%.
Keywords: Comparison of parabolic solar cookers; Energy efficiency; Exergy efficiency
INTRODUCTION
Use of a solar cooker is highly desirable option particularly in countries like India
where deforestation and depletion of resources is a serious problem. The use of solar energy
for food preparation appears in the work of Nicolas Saussure (1740–1799). He used an
insulated box, a so-called oven, which was comprised of glass blocks on the top of a
blackened surface. The sunlight entered the box through the glass cover and was absorbed
by the black surface. This raised temperature inside box the up to 88C. Another work by
Herschel, cited by Ghai (1953), used a solar oven, which was simply a black box buried in
sand for insulation and was provided with a double glass cover through which solar energy
entered the box. A temperature of 116C was recorded and vegetables and meat were
cooked. Mouchot, cited by Ghosh (1976), used a parabolic concentrator to intensify the
solar radiation onto a cooking pot. Ghosh (1976) was the first to develop simple box cooker
in India. His design consisted of two wooden boxes, one placed within the other, and the top
was covered with the glass sheet fitted in a wooden frame. He could achieve a temperature
of 120C inside his oven. Later on a simple plane reflector was added to enhance the
incident solar energy, which considerably improved the box cooker’s performance. A
International Journal of Green Energy, 6: 437–449, 2009
Copyright Taylor & Francis Group, LLC
ISSN: 1543-5075 print / 1543-5083 online
DOI: 10.1080/15435070903227912
Address correspondence to S. K. Shukla, Department of Mechanical Engineering, Institute of Technology,
Banaras Hindu University, Varanasi, 221005, India. E-mail: skshukla.mec@itbhu.ac.in
437
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major advancement took place in the 1950s when a number of solar cookers that involved
reflecting concentrations were designed, developed, and tested (Ghosh 1976; Ghai 1953;
Duffie et al. 1961). A comprehensive study of the focusing-solar cookers that had truncated
parabolic-reflecting concentrators was reported by Ghai (1953) and his associates (Ghai
1953; Lof and Fester 1961). Various geometries of reflectors and mounting configurations,
such as vertical and the equatorial, were studied using parabolic reflectors. The perfor-
mance data for the cookers having a truncated parabolic concentrator (made up of spun
aluminum) was also reported under various operating conditions. Further, the efforts made
to popularize this cooker amongst the people were not successful at that time.
The simplest concentrator uses a flat receiver. In this case, direct as well as diffuse
radiation are directly received and absorbed by the flat receiver, which is further augmented
by the reflected radiations. The concentration factor for such types of reflectors is generally
not more than four. The analysis of such reflectors was carried out by Telkes (1959),
whereas analysis of Vee-trough collector by Hollands (1971). This can further be improved
by using more plane reflectors in a series known as a compound wedge collector. A
parabolic reflector focuses the parallel rays of the sun on a small area and gives a very
high concentration ratio, and thus high temperature can be attained. The fundamental
operation problem with solar collectors is the collection and delivery of solar energy to
users with minimum losses. The optimum operating conditions for solar collectors can be
investigated using different modes of performance. The common aim is to optimize the
thermal efficiency of any collector, which is defined as the ratio of useful energy output to
that of incident solar energy during the same time period.
The performance of solar cookers can be evaluated by the traditional way, which is
based on the energy efficiency defined as the ratio of output energy to the input energy
supplied. In this article, an analytic framework is provide based on quality of energy, which
is based on the second law of thermodynamics. The exergy is very good tool for measuring
the performance of a thermal system (Kaushik et al. 2000). The amount of useful energy
(exergy) delivered by solar collectors is found to be affected by heat transfer irreversibil-
ities between the sun and the collector, between the collector and the ambient air, and inside
the collector. The present work is based on the sensible heating of a known quantity of
water up to its boiling point.
DESCRIPTION OF THE SYSTEMS
Community-Type Solar Cooker (SPDC)
To achieve higher temperature from solar energy, it is necessary to focus solar energy
on the absorber. A parabolic mirror reflects solar radiation, which is parallel to its axis. This
property of the parabolic mirror is used in the construction of concentrating solar collectors
in the form of parabola with the absorber placed at its focal point. The Scheffler solar
cooker was installed at Holistic Health Centre of IIT, Delhi in the month of May, 1998 by
keeping in mind that the position of the solar cooker should be such that it could face the sun
without any obstacle in the surroundings through out the day. To locate the actual direc-
tions, a flat, leveled platform of about 60 cm diameter was made in the early morning and a
set of concentric circles were marked on it. A flat iron strip of length 53 cm with several
holes is placed above the center of the concentric circles and turned such that the sun light is
passed through the holes and created small light spots on the leveled surface. The spot
position was marked when the spot crossed a circle, and this was done for about two hours.
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In the afternoon, we also marked the spot position when the same light point passed the
same circle, and then the exact east-west direction was drawn by joining the two corre-
sponding marks (one from the morning and the other from the afternoon) of the same circle.
The solar radiation reflected by the primary reflector was able to enter the cooking place
and get further reflected by the secondary reflector to reach the bottom of the pot, which is
placed just adjacent to the opening window inside the kitchen. A movable iron cover was
also provided on the opening window so as to control the amount of solar radiation reaching
the pot and hence controlling the heating process as desired by the cook. The complete
installation of the cooker was completed by IMEC (Valsad) in the month of May, 1998. By
following the design given by the manufacturer (IMEC, Valsad, India), the various
dimensions and distances of the solar cooker installed are evaluated as given in the structure
view diagram shown in Figure 1.
Reflector daily tracking of 15/h can be accomplished by using a mechanical clock as
is done in an equatorial telescope mount. The mechanical drive is translated using gears and
chains from a bicycle. The seasonal adjustment of 23.50 must only be done once a week.
Domestic-Type Parabolic Solar Cooker (SDC)
The solar parabolic domestic cooker (SDC) is the most powerful type of collector,
which concentrates sunlight at a single, focal point (Figure 2). This solar cooker has an
aperture diameter 1,400 mm (lengthwise) and 1,400 mm (widthwise) and focal length (b) of
270 mm. The dimensions of the SPC are shown in Figure 2. The reflector needs to be
reoriented after every 25–30 minutes, which can be done quite easily. The cooker has
Figure 1 Structure view of solar cooker.
EFFICIENCY OF PARABOLIC SOLAR COOKERS 439
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concentration ratio 31.36. The SPC is constructed of a steel profile and an acrylic mirror
sheet with an aluminum polished surface, which has reflectivity of about 0.8. The structural
frame of the SDC, made of steel and acrylic mirror sheet, is screwed on the frame (a). The
thickness of acrylic mirror sheet is 0.5 mm. In the center of the concentrating reflector, a
cooking pot (c) of aluminum of diameter 16 cm is placed. The cooking pot is made of
galvanized steel sheet of 0.5 cm thickness. All outer surfaces of the cooking pot are painted
with matte black. Thus, the acrylic mirror sheet acts as a reflector to concentrate the sun’s
rays on the cooking pot. The reflector could be adjusted, according to Funk (2000). The
emissivity of the cooking pot and shield surfaces are 0.87 and 0.45, respectively.
EXPERIMENTATION
Experiments were performed and data for ambient air temperature, T
a
, water tem-
peratures in the cooking pot, T
w
, total solar radiation energy, I, on a reflector surface, and
wind speed are recorded. The average temperature of the water in the pot was determined
by averaging the measurements of the thermocouples. The water temperature in the pot was
sampled at each time period. Every 30 minutes the recorder averaged and stored the value
of the water temperature. Solar radiation at the reflector surface and ambient air tempera-
ture were also recorded during the tests. Solar radiation was measured by the solar-energy
sensor. The system tests were conducted between 10:30 and 16:00 solar time. The tests
were conducted at the ambient temperature, T
a,
between 293 K and 313 K. Experimental
data was recorded for water temperatures, T
w,
between 313 K and 363 K. The solar
radiation energy, I, varied in the range of 450–1100 W/m
2
. The amount of 40 kg of water
per square meter of intercept area of the system was used into the cooking. The system was
adjusted every 30 minutes to keep the solar radiation focused on the cooking pot. The
temperatures of the cooking pot and reflector surfaces were not determined in the experi-
ment. A data logger was used for taking and storing readings from the sensors. The recorded
Figure 2 Structural view of solar domestic cooker.
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data were stored in memory for printing or on disk for using in the computer. Readings
could be taken at regular intervals, different for each channel.
The Measurements
The tests for the solar cookers were conducted during the months of March to June
1998. During the experimental period, the following quantities were measured: ambient air
temperature, T
a
, water temperatures in the cooking pot, T
w
, total solar radiation energy, I,
on a horizontal surface, and wind speed. The temperature of the water in the pot was
measured by using calibrated copper–constantan thermocouples, which have a low cost, an
acceptable accuracy, and a rapid response. The location of the thermocouples in the
cooking pot allowed for determining the temperature within the water. Thermocouple
beads were immersed in the water 10 mm above the pot bottom, at the center. The average
temperature of the water in the pot was determined by averaging the measurements of the
thermocouples. The water temperature in the pot was sampled every 30 min.
Solar radiation at a horizontal surface and ambient air temperature were also recorded
during the tests. Solar radiation was measured by the solarimeter. A solarimeter is a
pyranometer, a type of measuring device used to measure combined direct and diffuse
solar radiation. An integrating solarimeter measures energy developed from solar radiation
based on the absorption of heat by a black body. Resolution is 1 W/m
2
. This meter provide
an accurate reading from 50–1,200 W/m
2
. The sensor is a silicon photovoltaic (PV) cell
mounted on the front of the meter. Meters are calibrated on clear days in natural sunlight
and adjusted to a reference cell measured against pyranometers at Sandia National
Laboratories. Meters are pointed directly at the sun for calibration. Off-angle calibration
is not done, but performance seems consistent up to ,40off direct normal .Wind speed
was measured using the anemometer with the range of 0.3–75 m/s. The accuracy of the
anemometer is 2%+0.2 m/s. The wind speed was always below 1 m/s and was recognized
as small thus the effect of wind and its direction was neglected.
The ambient temperature is measured by digital thermometer, which has resolution
0.1 K. The accuracy of the digital thermometer is 1 K (from 273 to 353 K) and 5–10 K
(other temperature range).
The tests were conducted between 09:00 and 14:00 solar time and at the ambient
temperature, T
a,
between 298 K and 303 K. Experimental data was recorded for water
temperatures, T
w,
between 301.1 K to 343 K for SBC and 301.1 K to 351 K for SPDC. The
solar radiation energy, I, varied in the range of 450–700 W/m
2
. The SPDC was adjusted
every 30 minutes to keep the solar radiation focused on the cooking pot. The temperatures
of the reflector surfaces and absorber of solar cookers were also determined in the
experiment. Readings could be taken at regular interval of 30 min, different for each
cooker.
ANALYSIS FOR THE ENERGY AND EXERGY EFFICIENCY
In this experimental study, both of the cookers were tested based on the test condi-
tions outlined by Funk (2000) who presented and evaluated the test standard of the solar
cookers proposed at the Third World Conference on Solar Cooking (Avinashilingam
University, Coimbatore, India, January 6–10, 1997).
EFFICIENCY OF PARABOLIC SOLAR COOKERS 441
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Energy Efficiency of Solar Cooker
Energy efficiency of a solar cooker can be defined as the ratio of energy-output rate
(only the increase of the water energy due to temperature increase) to the energy-input rate
(the energy of solar radiation). The energy-input rate to the SC can be calculated as follows:
Ei¼IAa(1)
And energy-output rate of the solar cookers were obtained in watts as follows:
E0¼mwcpwðTwf Twi Þ
t(2)
Thus the instantaneous energy efficiency of the SPDC was calculated as follows:
¼E0
Ei¼
mwcpwðTwf Twi Þ
t
IAa
(3)
The calculated values for energy efficiency based on experimental data recorded on
different days for community and domestic solar cookers are given in Tables 1 and 2.
Exergy Efficiency of Solar Cooker
Exergy efficiency of any process is a ratio of the exergy-transfer rate associated with
the output to the exergy-transfer rate associated with the driving input (Kotas 1990).
The following expression for the available energy flux, which has the widest accept-
ability, was used to calculate the exergy of solar radiation (exergy-input rate to the solar
cookers):
Exi ¼I1þ1
3
Ta
Ts

4
4
3
Ta
T

"#
A(4)
And the exergy-output rate from the SCs was calculated by using:
Exo ¼
mwcpw ðTwf TwiÞTaln Twf
Twi
hi
t(5)
Table 1 Useful energy and efficiencies of community solar cooker on different days.
S No. Date Total incident
energy (kWh)
Initial
temperature
Final
temperature
Useful
energy (kWh)
Efficiency (%)
1. 19.06.98 9.4 26
0
c94
0
c 0.75 8%
2. 20.06.98 9.3 27
0
c98
0
c 1.32 14%
3. 24.06.98 8.7 27
0
c98
0
c 0.7 7.8%
4. 03.12.98 4.2 24
0
c80
0
c 0.3 7.1%
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The instantaneous exergy efficiency of the SC can be defined as the ratio of the increased
water exergy to the exergy of the solar radiation:
¼Exo
Exi ¼
mwcpw ðTwf TwiÞTaln Twf
Twi
hi
t
I1þ1
3
Ta
Ts

44
3
Ta
T


A
(6)
In the present work, the Petela expression (Petela 2003) was used to calculate the exergy of
solar radiation as the exergy input to the SPDC and SDC. Assumed are the following data:
the time interval 1,800 s; the constant specific heat for water, c
p,
is 4,186 J/kg/ K; and the
solar radiation temperature, T
s,
is 6,000 K.
UNCERTAINTY ANALYSIS
An important argument is the accuracy of measured data as well as the results
obtained by experimental studies. Therefore, uncertainty analysis is necessary to validate
the experimental results (Holman 1994; Hepbaslı and Akdemir 2004). Errors and uncer-
tainties in the experiments can arise from instrument selection, instrument condition,
instrument calibration, environment, observation and reading, and test planning (Asan
and Namli 1997). Uncertainty analysis is needed to prove the accuracy of the experiments.
An uncertainty analysis was performed using the method described by Holman (1994). This
calculation method concerning uncertainty analysis was also used in many studies (Asan
and Namli 1997). In the present study, the temperatures were measured with appropriate
instruments explained previously.
Type B uncertainty, being dependent upon the measuring instrument, cannot be
reducedbyaugmentingthenumberofmeasurements. Generally, sensor specifications
furnish the accuracy, a, of an instrument, which is the maximum deviation from the true
value. It is assumed that all values inside the interval 2aare equally probable. In this case,
it is possible to demonstrate that the uncertainty can be obtained from the following
equation:
B¼a
ffiffiffi
3
p(7)
Type B uncertainties of these parameters are presented in Table 3.
Table 2 Useful energy and efficiencies of domestic solar cooker on different days.
S No. Date Total incident
energy (kWh)
Final temperature
of water
Useful energy
(kWh)
Efficiency (%)
1. 12.10.98 3.4 80
0
c 0.35 10.5%
2. 20.10.98 5.3 96
0
c 0.418 8.8%
3. 21.10.98 4.4 96
0
c 0.41 9.5%
4. 22.10.98 4.7 93
0
c 0.39 8.3%
EFFICIENCY OF PARABOLIC SOLAR COOKERS 443
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RESULTS AND DISCUSSION
The hourly variation of water temperature, T
w,
in the pot and solar intensities falling
on the solar cookers has been shown in Figures 3 and 4. This is according to the atmospheric
conditions of the experimental place. The water temperature in the pot increases during the
day and rises from 297.1 K to 357 K for SPDC and 297.1 K to 345.2 K for SDC. The
maximum value of water temperature was recorded during 12:30 hours for both the
cookers, after about 3 hours of heating. During this period, the ambient air temperature
varied from 297.1 K to 304.1 K, whereas the solar radiation ranged from 450 to 750 W/m
2
.
In the present experiment, while the water temperature in the SPDC at 9:00 was
only 297.1 K, it reached 357 K at 12:30 since the solar radiation reached the maximum
value (735 W/m
2
) at this time. When the solar radiation increase after 9:30, the water
temperature of SDC is increased from 297.1 to 342.5 at 12:30, which is the maximum
value of the measured temperature. The water temperature of the SDC did not increase
much during the experimental period. It is important to mention that the SDC is domestic
solar cooker with a smaller aperture area then the SPDC. When the average daily and
maximum temperatures of water in both SCs were taken into account, the temperature
difference of water was 10.4 and 14.6 K, respectively. For the same time interval, the
temperature rise of water in SPDC was about 10.4 K higher than that of the water
temperature in SDC. The study showed that the water temperature in the SPDC was
always greater than the water temperature in the SDC.
Initially the temperature rise above the ambient (the temperature difference) in both
SCs was small; it increased with time. The highest temperature difference was obtained in
the SPDC. While the temperature difference in the SPDC was only 3.1 K at 9:00 in the
Table 3 Type ‘‘b’’ uncertainties of measured variables.
S.No. Variables Accuracy Uncertainties
1. Solar radiation 4023
2. Ambient temperature 1K 0.58K
3. Glass temperature 2% 1.15
4. Air velocity 2% 1.15
9:30 10:30 11:30 12:30 13:30 14:30
290
300
310
320
330
340
350
360
Water Temperature (K)
Time of Da
y
(
h
)
SPDC
SDC
Figure 3 Variation of water temperature with the time of the day.
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morning, it reached 60.6 K at 12:300 in the afternoon. On the other hand, the temperature
difference in the SDC varied between 3.1 and 47.1 K.
For the period of time covered by Figure 3, the average daily temperature difference
in the SPDC and SDC was 40.14 and 28.1 K, respectively. It was found that the average
daily temperature difference in the SPDC was about 30% higher than that of the SDC. The
results showed that the SPDC was able to keep the maximum water temperature in the pot
60.6 K higher than the ambient temperature. Under these conditions, the SPDC provided
the water heating effect of 60.6 K. This good performance was due to the thermal properties
of the absorber surface of the SPDC.
It is clear that the maximum heating effect of the SPDC was only 60.6 K for the period
of time covered by Figure 3. Thus, the heating effect of the SPC was lower than that of the
SPDC. The SDC did not substantially increase the temperature of the water in the cooking
pot. This result is due to the small size of the cooker. The difference of the heating effect
between the SPDC and SDC relate essentially to the absorbing and reflecting properties for
the solar radiation. In other words, the difference of heating effect between the SBC and
SPC could be attributed to the constructional features of the SBC and SPC.
The energy-output rate is calculated using Equation 2, and its variation during the day
is shown in Figure 5. It is seen that the value of energy-output rate varied from 4.65 to 39.3
W for SPDC and 7.44 to 33.49 W for SDC. It is also clear from the figure that the energy-
output rate increases at a faster rate in the first 1 hour and then its value increases at slower
rate during 11:00–14:00. The energy output per hour dropped from 39.3 W (at 9:30 in the
morning) to 20.9 W (at 12:30 in the afternoon). During the experimentation, the average
daily energy-output rate of the SPDC and SDC is found to be 22 W and 20.5 W.
The exergy-output rate is calculated using Equation 5 and its variation as a function
of time is shown in Figure 6. It may be noted from the figure that the trend of variation of the
exergy-output rate is significantly different from those of the energy-output rate. It is also
seen that during the first one hour the rate of increase of exergy output was faster than
energy-output rate. It is due to the fact that temperature of the water rises rapidly during this
period due to instant rise in solar intensity and heat addition to the water is at higher
temperatures minimizing the other heat losses. However the values of the energy output for
the SPDC is found greater than SDC during the day.
9:30 10:30 11:30 12:30 13:30 14:30
400
450
500
550
600
650
700
750
Solar Radiation (W/m2)
Time of Day (h)
Solar Radiation
Figure 4 Variation of solar radiation with the time of day.
EFFICIENCY OF PARABOLIC SOLAR COOKERS 445
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The variation of the instantaneous energy efficiency as a function of time for the
SPDC and SDC is presented in Figure 7. It is seen that the energy efficiency of the SPDC is
lower than SDC and varied from 0.42% to 5.27%. It may be attributed to the geometrical
error in design of SPDC. In case of the SDC, it varies from 4.7% to 29.81%. However the
daily average energy efficiency of the SPDC and SDC was found to be 3.54% and 16.55%.
Thus, the difference in energy efficiency between the SPDC and SDC is 79%. The above-
presented results indicate that the present SPDC is well designed and its energy perfor-
mance is quite efficient. This is expected, since the temperature difference in the SBC is
about 30% higher than that of the SPC. This is consistent with results of Todd and Miller
(2001), who found that the average efficiency for the basic cooker was between 8% and
22% for the best cooker design tested.
Similarly the variation in exergy efficiency of the SPDC and SDC are shown in
Figure 8. The exergy efficiency of the SPDC varies from 0.07% to 0.3% and the average
daily exergy efficiency was found to be 0.2%. For SDC, its variation is from 0.58% to
1.11%, whereas the average daily exergy efficiency was found to be 0.9%. When the
average daily energy and exergy efficiencies of the SCs are considered, the energy
9:00 9:30 10:00 10:30 11:00 11:30 12:00
0
5
10
15
20
25
30
35
40
Energy Output (W)
Time of Day (h)
SPDC
SDC
Figure 5 Variation of energy output with the time of the day.
9:00 9:30 10:00 10:30 11:00 11:30 12:00
0.5
1.0
1.5
2.0
2.5
Exergy Output (W)
Time of Da
y
(
h
)
SPDC
SDC
Figure 6 Variation of exergy output with the time of the day.
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efficiency of the SPDC was higher by 79% than that of the SPC, whereas the exergy
efficiency of the SPC was higher by 76.6% than that of the SPDC.
Figure 9 shows the sensible heating and cooling curve for water for domestic type
parabolic solar cookers. The optical parameter of the solar cooker is obtained from the
sensible heating curve. It is observed that the optical efficiency increases in turn the
difference of water and ambient temperature increases as the bright spot moves toward
the center of the pot bottom and then decreases as the bright spot moves away from the
center. Hence it is clear that the optical parameter for the parabolic cookers does not remain
constant as in case of box-type solar cooker.
CONCLUSION
In order to evaluate the performance of the two solar cookers, energy and exergy
efficiencies of the solar cookers are calculated. Also the energy efficiency of the solar
cookers is compared with their exergy efficiency and the following conclusions are drawn:
9:00 9:30 10:00 10:30 11:00 11:30 12:00
0
5
10
15
20
25
30
Energy Efficiency (%)
Time of Da
y
(
h
)
SPDC
SDC
Figure 7 Variation of energy efficiency with the time of the day.
9:00 9:30 10:00 10:30 11:00 11:30 12:00
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Exergy Efficiency (%)
Time of Day (h)
SPDC
SDC
Figure 8 Variation of exergy efficiency with the time of the day.
EFFICIENCY OF PARABOLIC SOLAR COOKERS 447
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1. The energy efficiency is found to be higher than the exergy efficiency. This is due to the
reason that the energy efficiency incorporates the amount of energy transferred during
the cooking period.
2. It is inferred that the exergy efficiency of both the solar cookers is always lower than
the energy efficiency at all temperatures, which is in accordance with the previous
results.
3. It was also found that the average daily exergy efficiency for the SPDC was only 2.15%
during the experimental period. It indicates that both the solar cookers investigated in
this study are inefficient in terms of the exergy efficiency. This confirms that the
sensible heat-energy storage systems are inherently inefficient devices in terms of the
exergy efficiency.
The time variations of these efficiencies were calculated based on the applied
formulae and measurement data. The results show that the community-size solar cooker
has high energy efficiency, high exergy efficiency, and low characteristic boiling time as
compared to domestic size solar cooker. Though the efficiency seems to be low, in terms of
the wood saved per household, it comes out to be around 9,200 gms for the cookers. The
annual savings in the amount of the wood is about 75 million tons. This will release pressure
on the forest and help in saving the environment.
NOMENCLATURE
A= intercept area, m
2
c
p
= specific heat, Jkg
-1
K
-1
E= energy, W
I= instantaneous solar radiation, Wm
-2
M= mass, kg
Q= heat energy, J
T= temperature, K
t= time, s
= energy efficiency,%
E
x
= exergy, W
= exergy efficiency, %
9:30 10:30 11:30 12:30 13:30 14:30
10
20
30
40
50
60
Cooling CurveHeating Curve
(Tw-Ta) (Ο C)
Time of da
y
(
h
)
Figure 9 Sensible heating and cooling curve for SDC.
448 SHUKLA
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Subscripts
A= ambient
b= beam
d= direct
e= effective diffuse radiation
f= final
i= input, initial
o= output, outside
s= sun
sc = solar cooker
t= total
w= water
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