exergy analysis of renewable energy cooking...
TRANSCRIPT
156
CHAPTER - 5
Exergy Analysis of Renewable Energy Cooking
Devices
5.1 Introduction
Cooking is one of the most important and old activities for people all over the
world. The problem arises when fuel is either scarce or highly expensive. The
problems are encountered more pronounced in most of the developing countries,
particularly in the remote and rural areas. Cooking accounts for a major share of
energy consumption in developing countries including India. Cooking in a rural area
mainly depends upon the conventional energy sources such as cow dung, straw,
wood, coal and hence, solar cooking can play an important role in the rural areas for
cooking applications. Solar cookers are the most promising devices since firewood
used for coking causes deforestation, health hazard and other social and economical
issues. On the other hand, the commercial fuels such as LPG and electricity are not
available to majority of population besides, the cow dung and agricultural wastes
used for cooking are good fertilizers. In this way, solar cooking may be helpful to
bring prosperity besides, saving the conventional energy resources.
There are basically two types of solar cooker one is box type solar cooker and
other is paraboloid concentrating type solar cooker. The paraboloid type solar cooker
can cook food in lesser time than that of box type solar cooker due to high
concentration ratio and hence higher temperature. Solar cooking saves not only
fossil fuels but also keeps the environment free from pollution without hampering the
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nutritional value of the food [1]. Solar cookers use solar energy as a fuel which is
intermittent in nature and available only in day time, so it is important to explore other
renewable energies such as biomass for cooking applications during the late evening
hours and/or night time.
Majority of households in rural India still use biomass as fuel for cooking and
heating purposes because the percentage of households using LPG is mere 9 % [2].
Coal or biomass like wood, crop residues, cattle dung and charcoal for their cooking
and heating needs is used by almost half of the world population and about 90% of
rural population in the developing nation like India. However, biomass is replaced by
the gaseous fuels like LPG and natural gas in developed countries and most urban
homes in the developing world [3]. Census data 2001 [4] indicated that the
traditional energies is being used by approximately 72% of the population of India for
their cooking needs, of which over 89% of this population lives in rural areas [5].
Total biomass power potential of entire country is 19,500 MW [6] and a very
small percentage of it is being tapped till date. Therefore, a huge potential of this
energy source is untapped. Large amount of energy is used for the household
applications in which energy consumption for cooking plays a major part. Traditional
cook stoves or chulhas having thermal efficiencies below 10% emit large amount of
pollutants, are used by most rural households in the developing countries including
India for cooking applications. Due to the poor thermal efficiencies, traditional
chulhas consume large quantity of fuel which ultimately results in a large amount of
time spent for collecting fuels by covering a large distance. Also due to these
constraints in rural households, women and children are often exposed to high levels
of pollutants, for 3 to 7 hours daily over many years [7]. The concentration of SPM
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rises to 3-6 mg/m3 and that of CO is 5-50 ppm [7] according to studies on indoor air
pollution. Indoor air pollution is responsible for 2.7% of diseases globally [8].
In the present chapter the performance evaluation of renewable energy cooking
devices viz. solar cookers and biomass cook stoves has been carried out. Two
different types of solar cookers and four different types of biomass cook-stoves have
been investigated and evaluated based on energy and exergy analysis. In the case
of solar cookers, the experiments were carried out on clear sky days of different
months of a year with both the cookers filled with different volume of water and rice.
It is found that the exergy efficiency of paraboloid type solar cooker is higher than
that of the box type cooker. On the other hand, the comparative performance of four
different biomass improved cook-stoves was carried out using energy and exergy
analyses. Both the efficiencies of Model-1 are found to be higher than those of other
models studied here while it was found to least for Model-2. Energy analysis of
improved cook stoves was done earlier but the work on exergy analysis of improved
cook stoves is scant. Therefore, this study is expected to fill this gap of exergy
analysis of the improved cook-stoves. This chapter has been divided into two
sections, the first section (section 5.2) deals with the solar cookers while the second
section (section 5.3) deals with the improved biomass cook-stoves and the detail
analysis of the two is given as below:
5.2 Analysis of Solar Cookers
In the present section exergy analysis of solar cooker has been carried out for
which following experimental set-up and procedure has been used.
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5.2.1 Experimental Set-up and Procedure
The present experimental study deals with the comparative performance based
on exergy analysis of two different types of solar cookers viz. box type and
paraboloid type and the photographic view of the system along with the experimental
set-up is shown in Fig. 5.1 and Fig. 5.2 (a-b). The experiment has been carried out
during the clear sky day for different months of a year using water and different food
stuff as the cooling items and the exergetic efficiency of the two types of solar
cookers has also been compared in the study.
Fig.5.1. Photographic view of experimental set-up
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(a) Paraboloid type solar cooker
(b) Box type solar cooker
Fig.5.2 (a-b). Photographic view of solar cookers
The experiments have been done with one and two liters of water along with
250gms of rice in both types of cookers and before starting the experiment, some
measurements like dimensions and mass of water has been measured. On the basis
of measured dimensions, the area aperture for both types of solar cookers has been
calculated. Firstly, the cover plate and reflector of both types of solar cookers have
been properly cleaned and then and then they were kept in the sunlight to carry out
the experiments. In the case of box type cooker, the cooking pot was kept inside the
solar cooker and in the case of paraboloid type cooker, the cooking pot was placed
on the stand at the focal point of the parabolic concentrator so that the reflected rays
fall at its base and both the cookers were placed towards south. After completing the
necessary arrangements, the predefined amount of water in cooking pot has been
taken. A thermocouple wire for the measurement of water temperature inside the
cooking pot at regular periodic intervals has been inserted through a hole in the lid of
pressure cooker. The thermocouple is inserted in such a way its end remains
immersed in the water without touching the walls or the bottom of the pot and the
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other end is connected to a digital temperature sensor in both the cases.
Furthermore, to measure the ambient temperature more precisely, a T- type
thermocouple (copper – constantan) having temperature range of -200 ºC to 350 ºC
and uncertainty of 0.05% has been connected to a digital temperature indicator.
A pyranometer for measuring intensity of solar radiation is mounted on the outer
frame of the paraboloid type in such a manner that no shadow cast on the exposed
area of the dish, normal to the direction of the plane of aperture and is connected to
a data logger. Tracking of the dish has been carried out manually at every 5 minute
to ensure that the dish remains normal to the sun with the motion of the sun. In the
case of paraboloid type cooker, cooking pot in the tray is adjusted in such a way that
the bright spot of the sunrays is positioned at the bottom of the cooking pot.
Measurements of ambient temperature (Ta), water temperature in the cooking pot
(TW), intensity of direct solar radiation on the aperture plane of the parabolic
concentrating solar cooker (Ib) have been recorded at an interval of 5 minutes during
cooking of food stuff. As the water reaches its stagnation temperature during heating,
the parabolic reflector and box type reflectors is shaded by turning it in the opposite
direction and covering it by a black cloth to ensure blockage of solar radiation.
5.2.2 Energy and Exergy Analyses
Energy Analysis
Energy input is given by
SCsi AIE * (5.1)
where sI is the solar radiation and SCA is area of aperture of solar cookers
Energy output is given by
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tTTCmE wiwfpwwi )(. (5.2)
where wm is mass of water,
pwC is specific heat of water, wfT is final temperature of
water, wiT is initial temperature of water, t is time difference. An energy efficiency of
the solar cooker can be defined as the ratio of the energy gained by the solar cooker
(energy output) to the exergy of the solar radiation (energy input).
i
o
E
E
inputEnergy
outputEnergy (5.3)
Exergy Analysis
For the steady-state flow process during a finite time interval, the overall exergy
balance of the solar cooker can be written as follows:
(Exergy)in = (Exergy)out +(Exergy)loss + Irreversibility (5.4)
The availability of the terrestrial solar radiation obtained by superposition of the
availabilities of two lumped sources, a direct beam source and diffuse source. The
availability (exergy) of a solar flux with both beam and diffuse components can be
represented by [10]:
*3
41
34
1s
ad
s
abi T
TI
TT
IEx
(5.5)
where: i is the exergy of solar radiation (W/m2); bI is the intensity of beam radiation
(W/m2); dI is the intensity of direct radiation (W/m2); aT is the ambient temperature
(K); sT is the sun temperature (K); and *
sT is the effective diffuse radiation
temperature (K).
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The Petela [9] expression for the available energy flux, which has the widest
acceptability, can be used to calculate the exergy of solar radiation as the exergy
input to the solar cooker, i.e.
SC
s
a
s
asi A
T
T
T
TIEx
3
4
3
11
4
(5.6)
where; aT is the ambient temperature (K). The sun’s black body temperature of 5762
K results in a solar spectrum concentrated primarily in the 0.3-3.0 μm wavelength
band [11]. Although the surface temperature of the sun ( sT ) can be varied on the
earth’ surface due to the spectral distribution, the value of 5800 K has been
considered for sT . The thermal exergy at temperature T is given as:
dQT
TCm O
T
T
p
1.
0
(5.7)
Equation (5.7) can be applied for non-isothermal processes. Thus, the thermal
exergy content of water at temperature iT can be calculated by the following
equation:
o
wioowipwwi
T
TTTTCmT ln)(.)(
(5.8)
When the temperature of water is increased to temperature Tf, the exergy is
defined as:
oEx tT
TTTTCm
wi
wf
awiwfpww
ln)(.
(5.9)
The exergy efficiency is formed as the ratio of the exergy transfer rate
associated with the output to the exergy transfer rate associated with the necessary
input [12-13]. An exergy efficiency of the solar cooker can be defined as the ratio of
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the exergy gained by the solar cooker (exergy output) to the exergy of the solar
radiation (exergy input).
i
o
Ex
Ex
inputExergy
outputExergy
SC
s
a
s
a
wi
wf
awiwfpww
AT
T
T
Ti
tT
TTTTCm
3
4
3
11
ln)(.
4
(5.10)
5.2.3 Results and Discussion
The comparative study on two different types of solar cookers viz. paraboloid
type and box type has been carried out based on the exergy analysis. Figures 5.3
and 5.4 illustrate the variation of efficiency and solar radiation with respect to time for
one and two liters of water in the paraboloid solar cooker. From Figs. 5.3 and 5.4, it
is found that initially the efficiency is high and then decreases as the time increases
and reaches the stagnant level as the water temperature approaches the optimum
level i.e. near to boiling point. It is seen from these figures that both the efficiencies
are high initially due to the fact that the change in temperature of water inside the
cooker is quick and so as the temperature difference is more. From Eqs. (5.2) and
(5.9), it is clear that the output energy and exergy is directly proportional to
temperature difference, therefore initially the output energy and exergy are high while
at the same time the input energy and exergy are low which is due the fact that the
input energy and exergy are directly proportional to incident radiation (Eqs-5.1 and
5.6) which is low initially. Therefore, the output to input ratio is high as a result the
instantaneous efficiency is found to be higher initially as can be seen from these
figures.
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Fig. 5.3: Exergy and energy efficiency and solar radiation against time of paraboloid solar cooker with 1 liter of water
Fig.5.4: Exergy and energy efficiency and solar radiation against time of paraboloid
solar cooker with 2 liter of water
On the other hand, the solar (energy) radiation has a different trend with time
which can be seen from Figs. 5.3-5.4. The solar intensity has an increasing trend in
0
150
300
450
600
0
7.5
15
22.5
30
10:00 AM 10:15 AM 10:30 AM 10:45 AM 11:00 AM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%)
η (%)
Is(W/m2)
0
123
246
369
492
0
9
18
27
36
10:35 AM 10:55 AM 11:15 AM 11:35 AM 11:55 AM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%)
η (%)
Is(W/m2)
166
the first half, while it is having reverse trend in the second half. Therefore the output
to input ratio i.e. the efficiency of the solar cooker decreases in the afternoon. Also
due to the fact that the temperature difference near boiling point increases slowly
while solar radiation goes down, as a result, we find the results shown in Figs. 5.3-
5.4.
Figures 5.5 and 5.6 illustrate the variation of exergy and energy efficiency and
solar radiation with respect to time for one and two liters of water in box type solar
cooker. From these figures it is found that initially both the efficiencies are high and
then decreases as the time increases and reaches to approximately stagnant level
as the temperature reaches near to boiling point, this variation in exergy and energy
efficiency with respect to time is explained above, however there is fluctuation in
efficiencies which is due to fluctuating nature of solar radiation which can be seen
from Figs. 5.5 and 5.6.
Fig.5.5: Exergy and energy efficiency and solar radiation against time of box type solar cooker with 1 liter of water
0
225
450
675
900
0
7
14
21
28
10:00 AM 10:50 AM 11:40 AM 12:30 PM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%) η (%) Is(W/m2)
167
Fig.5.6: Exergy and energy efficiency and solar radiation against time of box
type solar cooker with 2 liter of water
Mean exergy efficiency of box type and paraboloid type solar cooker in case
of one liter of water is 4.9 and 7.1 % respectively and in case of two liters of water it
is 7.9 and 10.4 % respectively. Therefore it is found that exergy efficiency of
paraboloid solar cooker is higher than the box type solar cooker.
Figures 5.7 and 5.8 illustrate the variation of exergy and energy efficiencies
and solar radiation with respect to time for 250 grams (gms) of rice in both types of
solar cookers as mentioned above. From the figure 5.7 it is found that, initially as the
time increases both the efficiencies increases takes the peak and then decreases
and reaches to stagnant level, however from figure 5.8 it is found that initially
efficiencies are high then decreases and reaches to stagnant level as the time
increases. As far as solar radiation concern it increases with respect to time takes
0
200
400
600
800
0
5
10
15
20
10:30 AM 11:20 AM 12:10 PM 1:00 PM 1:50 PM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%)
η (%)
Is(W/m2)
168
the peak and then decreases which can be seen from Figs. 5.7 and 5.8.
Fig.5.7: Exergy and energy efficiency and solar radiation against time of
paraboloid solar cooker with 250 gms rice
Fig.5.8: Exergy and energy efficiency and solar radiation against time of box type
solar cooker with 250 gms rice
0
150
300
450
600
0
5
10
15
20
11:15: AM 11:25 AM 11:35 AM 11:45 AM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%)
η (%)
Is(W/m2)
0
220
440
660
880
0
5.5
11
16.5
22
11:20 AM 11:40 AM 12:00 PM 12:20 PM 12:40 PM
Eff
icie
ncie
s (
%)
Time (hr)
ψ (%)
η (%)
Is(W/m2)
169
Due to this nature of solar radiation initially temperature increases slowly and
then increases sharply takes the peak and then decreases, therefore initially
temperature difference is small then large and as temperature reaches to stagnant
level again it is small. From Eqs. (5.2) and (5.9), it is clear that output energy and
exergy are directly proportional to temperature difference which changes with respect
to time so as the energy and exergy efficiencies changes. Exergy efficiency with 250
grams of rice of box type and paraboloid type solar cooker is 6.3 and 8.2%
respectively, therefore in this case also exergy efficiency of paraboloid type cooker is
higher than that of box type cooker.
The physical significance of the results is that exergy i.e. the quality of energy
and/or availability of energy which can be calculated by second law of
thermodynamics is always less than the quantity of energy based on first law of
thermodynamics and/or simply the principle of conservation of energy. The results
obtained in this study as given in Figs. 5.3-5.8 clearly shows that the both the
efficiencies of solar cooker are quite low which is the obvious case and can be
explained in terms of irreversibilities associated with these systems. However, the
results obtained and shown in this study exhibits better result than those obtained by
earlier authors [14-16]. This is basically due to the fact that our system is modified
and all the precautions have been taken into consideration to reduce heat/energy
losses. Besides, due to the technological advancement especially in the reflector and
absorber plates/materials we get better performance for both types of cookers than
those obtained by earlier authors given in Refs. [14-16]. Thus the present work gives
better result and hence, these modified solar cookers can perform better to fulfill the
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requirements of these renewable energy based systems for real life applications for
both rural and remote areas around the world in general and for India in particular.
As can be seen from literature studies on solar cooker has been carried out
using energy and exergy analysis [7-8, 14-16]. For example, Buddhi and Sahoo [17]
designed a box-type solar cooker having latent heat storage and showed its
importance in evening hours or when there is no availability of sun. Petela [9]
analysed the simple solar parabolic cooker (SPC), of the cylindrical trough shape,
using exergy analysis and found that the exergy efficiency of the SPC is relatively
very low as compared to energy efficiency. Mawire et al [18] worked on the charging
of oil–pebble bed thermal energy storage (TES) system for a solar cooker and
analyzed the simulated energy and exergy. On the other hand, few studies have
been carried out on design and development of different cookers [19, 20]. But none
of the authors as mentioned above compared the solar cookers viz. paraboloid type
and box type performance based on the exergy analysis as in the present case
therefore, the present study is new in this direction.
5.3 Biomass Cook Stoves
In this section of the chapter, the comparative energetic and exergetic
analyses of four different types of improved cook stoves has been carried out based
on the experimental observations for which following materials and methods were
used.
5.3.1 Material and Methods
Four improved cook stove models selected in this study are given as below:
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1. Model-1: Place a smaller demand on forest and emit less green house gases
[21].
2. Model-2: Having two opening to keep two cooking pots at a time and helpful
for cooking two different meals at a time.
3. Model-3: This model dose not requires specialized fuel and it is affordable for
rural households, abundant oxygen supply ensures smoke free heating [22].
4. Model-4: A variety of solid fuels, wood, twigs, leaf, dung cake, agricultural
waste, raw coal, briquettes etc. can be burnt in the stove at high thermal
efficiency [23]
As per the Bureau of Indian Standards [24], water boiling test of all above
mentioned four cook-stoves was performed and the following instruments/
equipments have been used:
a) Glass cylinders for measuring water
b) Platform Balance
c) Aluminium vessels with lids of proper volumes as per BIS standard
d) Kerosene oil to ignite the process
e) Match stick
f) Selected models of improved cook-stove
g) Stopwatch
h) Thermometer/Thermo-couple
i) Bomb calorimeter
j) Wood fuel in proper size
Thermal efficiency of a cook stove is defined as the ratio of heat actually
utilized to the heat theoretically produced by complete combustion of a given quantity
172
of fuel wood, which is based on the calorific value of a particular fuel wood. To carry
out thermal efficiency of a cook stove few measurements were carried out as below:
5.3.2 Determination of Burning Capacity Rate
If the fuel burning rate per hour is not given by the manufacturer, it need to be
determined in order to chose the capacity of cooking pot and the amount of water to
be taken at a time as per BIS standard [24]. To do so, the combustion chamber was
filled with test fuel in the honey comb fashion up to third-forth of the height of the
combustion chamber in the pattern recommended. To ignite the fire 10 to 15 ml of
kerosene oil has been sprinkled on the fuel wood from the top of the cook stove and
fire was lighted using match stick box. The weight of the cook stoves along with
wood fuel was measured before igniting the fire and again it was done after half an
hour of burning the wood fuel as per the procedure given in BIS standard [24]. To
calculate the burning capacity of the cook stove, the following equation was used
[24]:
Heat input per hour = fXCVMM )(2 21 kcal/h (5.8)
where 1M is the initial mass of the cook-stove along with test fuel before igniting the
fire in kg, 2M is the final mass of the cook stove, after burning for half an hour in kg
and fCV is the calorific value of the fuel wood in kcal/kg, and this weighing applies
only to portable metal stoves, like the present case.
5.3.3 Determination of size of vessel and quantity of water
The size and dimensions of the vessel and the quantity of water to be taken in
each vessel at a time for the thermal efficiency test shall be selected from the table
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given in BIS standard [24], depending upon the burning capacity rate of the cook
stove. After doing preliminary calculations for each cook-stove model, it is found that
the quantity of water to be taken in each pot for water boiling test for the present
cases is to be 8 litres for the Model-3 model and 10 litres for others cook stoves viz.
Models-1,Model-2 and Model-4.
5.3.4 Calculation for the Thermal efficiency
As mentioned above, the thermal efficiency of a cook stove is defined as the
ratio of heat actually utilized to that of the heat theoretically produced by complete
combustion of a given quantity of fuel wood, which is based on the calorific value of a
particular fuel wood and is given as below:
100*
producedHeat
heatUtilizedefficiencyThermal (5.9)
where the utilized heat is the amount of total heat gained by the water in
individual experiment and the heat produced is the amount of heat produced after the
complete combustion of the fuel wood consumed in a particular experiment and
given as below:
utilizedHeat kJffwxWxffwxWxn ))(1868.4896.0())(1868.4896.0)(1( 1312
(5.10)
producedHeat kJxdcXxc )]1000/()[(1868.4 21 (5.11)
efficiencyHThermal 100)/( XproducedHeatutilizedHeat
)]1000/()[(1868.4
100)])(1868.4896.0())(1868.4896.0)(1[(
21
1312
xxdcXxc
xffwxWxffwxWxn
(5.12)
174
The specific heat of aluminium is taken to be 0.896 kJ/kg-0C and the other
parameters used in the above equations are defined as below:
w = mass of water in vessel, in kg;
W = mass of vessel complete with lid and stirrer, in kg;
X = mass of fuel consumed, in kg;
1c = calorific value of wood, in kcal/kg;
x = volume of kerosene consumed, in ml;
2c = calorific value of kerosene, in kcal/kg;
d = density of kerosene, g/cc;
1f = initial temperature of water in 0C
2f = final temperature of water in last vessel at the completion of test 0C; and
n = total number of vessels used.
Before starting the experiment, some measurements like volume of water,
weight of wood, volume of kerosene for the ignition of wood, weight of empty
aluminium vessel along with cover were measured. Then the fuel wood available
from the saw mill was prepared in proper dimension as per BIS standard and the
wood logs (1-2 kg) of proper dimensions were arranged in honey-comb manner
inside the improved cook-stove models to be analyzed. A known minimum
volume of kerosene (which varies for different models) was poured over the wood
logs to initiate the ignition process and fire was lighted with a match stick. As
soon as the fire ignited, the aluminium vessel having known volume of water was
placed on the cook-stove and the time and ambient temperature was recorded at
the beginning of the experiment. The readings of water temperature, pot
temperature, pot cover temperature, outer surface temperature of the cook stove
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etc., were taken periodically after an interval of 5-minutes. As the first pot reaches
to the test temperature i.e. 5 ºC below the boiling point, the second pot placed on
the cook stove and the data was measured in the similar way as for the first pot.
The experiment was continued and the pot being replaced until the complete fuel
wood was consumed properly for water boiling/heating process while the calorific
value of wood and kerosene were determined with the help of Bomb Calorimeter.
5.3.5 Energy and Exergy Analyses
The performance evaluation of four different types of cook-stoves has been
carried out using energy and exergy analyses following earlier authors [25-26] as
below:
Energy analysis
An energy balance for the overall process can be written as:
onaccumulatiEnergylossEnergyeredreEnergyinputEnergy ]cov[ (5.13)
Energy input is given by:
21 cdxcmE wdin (5.14)
where wdm is mass of wood, 1c is calorific value of wood and 2c is calorific value of
kerosene, x is volume of kerosene, d is density of kerosene
Energy output is given by
(5.15)
where pC is specific heat of water, fwT is final temperature of water, iwT is initial
temperature of water, AlCp is specific heat of aluminium , potm is mass of pot, fpT is
final temperature of pot, ipT is initial temperature of pot. In general energy efficiency is
defined as the ratio of output energy to input energy and given by:
)()( ipfpAlpotiwfwpwo TTCpmTTCmE
176
in
o
E
E
inputEnergy
outputEnergy
(5.16)
Exergy analysis
An overall exergy balance can be written as:
onaccumulatiExergynconsumptioExergylossExergyeredreExergyinputExergy ]cov[
(5.17)
Exergy input is given by:
21 )/1( cdxTTcmEx cfuelawdin (5.18)
where aT is ambient temperature, fuelT is temperature of burning fuel. However,
exergy input can also be by chemical exergy of solid industrial fossil fuel, which can
be expressed as follows:
dryfgwhNCV 00 )( (5.19)
where fgh is enthalpy of evaporation of H2O at standard temperature, for water
substance at T = 298.15K, fgh = 2442 kJ/kg, w is mass fraction of moisture in fuel,
0NCV is net calorific value of moist fuel. For dry organic substances contained in
solid fossil fuel consisting of C, H, N, O with mass ratio to carbon 2.67>o/c>0.667,
which in particular includes wood.
c
oc
n
c
h
c
h
dry
3035.01
0383.0)7256.01(2509.01882.00438.1
(5.20)
where c, h, n and o are the mass fractions of C, H, N and O respectively
Exergy input can be given by both the equations 5.15 and 5.16, whichever is more
appropriate is yet to be ascertain and further work in this direction going on in this
research group. Energy output is given by
177
)/1)(()/1)(( fpaipfpAlpotfwaiwfwpwo TTTTCpmTTTTCmEx (5.21)
In general exergy efficiency is defined as the ratio of output exergy to the input
exergy and given by:
in
o
Ex
Ex
inputExergy
outputExergy
(5.22)
5.3.6 Results and Discussion
The performance evaluation and experimental study of four different cook
stove models such as Model-1, Model-2, Model-3 and Model-4 have been carried out
for water boiling test following BIS standard [24]. All these cook stove models were
evaluated for water boiling test with specific quantity of water in the aluminium
container using the same type of fuel wood prepared as per the BIS standard. The
photographic view of various cook stove models during study is given in Fig.5.9. The
quantity of water in the container was calculated using heating capacity rate of the
individual cook stove given in equation-5.8, while the fuel wood was taken arbitrarily
available at the time of each experiment. For example, Model-1 was tested and
evaluated using 1.98 kgs of wood fuel prepared as per the manual having 10 litres of
water in each aluminium container and total 40 litres of water has been used as per
the test procedure. Model-2 was studied with 10 litres of water in each pot and 1.34
kgs of wood fuel, while the Model-3 was evaluated using 8 litres of water in each pot
and 2.0 kgs of wood fuel. On the other hand, Model-4 was evaluated with 10 litres of
water in each pot and a total wood fuel of 1.48 kgs.
178
Fig.5.9. Photographic view of various cook stove models during the experimental
study
As the quantity of water in each pot and the quantity of wood is taken
according to the burning rate as given in Eq. 5.8 for each experiment and power
output of the individual cook-stove was calculated as per the BIS standard. So more
Fig.5.9 (a): Model-1
Fig.5.9 (b): Model-2
Fig.5.9 (c): Model-3
Fig. 5.9 (d): Model-4
179
than one pot was used for each cook stove and the data for all pots for each cook
stove was collected separately. The instantaneous energy and exergy efficiencies
have been evaluated and plotted against the heating time in the graphs as shown in
Figs. 5.10-5.19 and the discussion of results for each cook stove is given individually
along with the comparison among them, as below:
Model-1
As mentioned above, the Model-1 has been evaluated experimentally with 10
litres of water in each pot and total four pots were used consuming 1.98 kgs of wood
fuel prepared as per BIS standard [24]. The temperatures of the pot, pot cover,
ambient air, water and the outer surface of the cook stove were measured at an
interval of five minutes. The energy and exergy efficiencies were evaluated based on
the water temperature using respective analysis and plotted against the heating time
in Figs. 5.10 and 5.11 for each pot and for the individual model as shown in Fig. 5.12.
It is seen from Fig. 5.10 that the energy efficiency increases with time for the first pot,
while it decreases first sharply and then slightly and again sharply for the last pot (pot
4). On the other hand, for second pot (pot 2) the energy efficiency first sharply
decreases then gradually increases, attains its peak and then slightly decreases as
the heating/boiling time increases. Whereas, for the third pot (pot 3) the energy
efficiency first slightly decreases and then increases, attains its peak and again
decreases with the heating time.
180
Fig.5.10. Energy Efficiency vs Time of different pots for " Model-1"
Fig.5.11. Exergy Efficiency vs Time of different pots for "Model-1"
0.00
14.00
28.00
42.00
56.00
5 10 15 20 25 30 35
En
erg
y E
ffic
ien
cy (
%)
Time (minutes)
ηP1
ηP2
ηP3
ηP4
0.00
2.00
4.00
6.00
8.00
5 10 15 20 25 30 35
Exerg
y E
ffic
ien
cy (
%)
Time (minutes)
ψP1
ψP2
ψP3
ψP4
181
Fig.5.12. Variation of efficiencies and time for different pots used in the "Model-1"
Similar results were observed for the exergy efficiency with a slight difference
than that of the energy efficiency for the third pot (pot 3). In other words, the exergy
efficiency for the third pot first increases sharply, attains its peak and then decreases
sharply as can be seen from Fig. 5.11. Also the peaks in the energy and exergy
efficiencies for second and third pots are found to be at almost the same time as can
be seen from Figs. 5.10 and 5.11, however, the peak for the third pot (pot 3) is found
to be higher than that of the second pot (pot 2) for both the cases. Again, the energy
efficiency for all the cases is found to be much higher than that of exergy efficiency,
which can be explained in terms of the quality of energy gained by the hot water
during the testing procedure of our experimental study. Boiling time for second pot is
found to be least and it is found almost same for first and second pot as can be seen
from Fig. 5.12.
0
9
18
27
36
0.00
7.00
14.00
21.00
28.00
P1 P2 P3 P4
Effi
cie
nci
es/
%
Pots
η
ψ
T
182
Model-2
This model was studied with 10 litres of water in each pot and 1.34 kgs of
prepared fuel wood and as mentioned earlier, two pots can be used at a time in this
particular model. The instantaneous energetic and exergetic performance for Model-
2 are shown in Figs.5.13 and 5.14 respectively, while the overall performance for
different pot is shown in Fig.5.15. It is seen from the Fig. 5.13 that energetic
efficiency for pot 1 and pot 2 first increases sharply, attains its peak and then
decreases sharply and again increases sharply, attains its second peak and finally
decreases slowly. Similarly, the exergetic efficiency for pot 1 and pot 2 first increases
sharply, attains its peak and then decreases sharply and again increases first sharply
and then slowly as can be seen from Fig. 5.14. On the other hand, for pot 3 and pot 4
the energy efficiency first decreases and then increases, attains its peak and then
fluctuates and finally goes down, while the exergy efficiency for the third and forth
pots (pot 3 and pot 4) first fluctuates and then increases, attains its peak and then
fluctuates and finally goes down as the heating time increases.
Fig.5.13. Energy Efficiency Vs Time of different pots for "Model-2"
0.00
10.50
21.00
31.50
42.00
5 10 15 20 25 30 35 40 45 50
En
erg
y E
ffic
ien
cy (
%)
Time (minutes)
ηP1
ηP2
ηP3
ηP4
183
Fig.5.14. Exergy Efficiency Vs Time of different pots for "Model-2"
Also the peaks for the second and fourth pots (pot 2 and pot 4) for both the
efficiencies are found to be higher than those of first and third pots (pot 1 and pot 3)
as can be seen from Figs. 5.13 and 5.14. This is due to the fact that the maximum
energy gain is observed in the second opening (mouth), as there is no option for the
passage of the exhaust in the first opening (mouth) of this dual pot cook stove and
the flue gas passes through the second opening (mouth) and has less time and
contact with the first pot. As a result, we obtained the better performance for the pots
kept on the second opening (mouth) of the cook stove viz. for pot 2 and pot 4 as can
be seen from Fig.5.15.
0.00
1.00
2.00
3.00
4.00
5 10 15 20 25 30 35 40 45 50
Exerg
y E
ffic
ien
cy (
%)
Time (minutes)
ψP1
ψP2
ψP3
ψP4
184
Fig.5.15. Variation of efficiencies for different pots used in the "Model-2"
Model-3
In this particular model 10 litres of water in each pot with total four numbers of
pots along with a total quantity of 1.48 kgs of prepared fuel wood were used in the
experimental study. The energetic and exergetic performances of Model-3 are shown
in Figs.5.16 and 5.17 respectively. For the first pot the energy efficiency first
decreases slightly and then increases sharply, while the exergy efficiency first slightly
increases, then decreases slightly and finally increases sharply with time as can be
seen from Figs. 5.16 and 5.17. On the other hand, the exergy efficiency for the forth
pot (pot 4) first increases sharply and then slightly with time. Except the two cases
mentioned above, it is clear from Figs. 5.16 and 5.17 that both the efficiencies first
sharply increase, attain their peaks and then sharply decrease with respect to time.
The peak for the energy efficiency is almost at the same time for different pots,
0
13
26
39
52
0.00
5.00
10.00
15.00
20.00
P1 P2 P3 P4
Eff
icie
ncie
s/%
Pots
η
ψ
T
185
while it is slightly at different time for the exergy efficiency in the case of the forth pot
(pot 4) as can be seen from Figs. 5.16 and 5.17.
Fig.5.16. Energy Efficiency vs Time of different pots for "Model-3"
Fig.5.17: Exergy Efficiency vs Time of different pots for "Model-3"
0.00
7.00
14.00
21.00
28.00
5 10 15 20 25 30
En
erg
y E
ffic
ien
cy (
%)
Time (minutes)
ηP1
ηP2
ηP3
ηP4
-1.25
0.00
1.25
2.50
3.75
5.00
5 10 15 20 25 30
Exerg
y E
ffic
iecy (
%)
Time (minutes)
ψP1
ψP2
ψP3
ψP4
186
Fig.5.18: Variation of efficiencies for different pots used in the "Model-3"
It is also found from Fig. 5.18 that the heating time for the last three pots is
much lesser than that of the first pot, which is due to the fact that the Model-3 took
more time in the ignition at the starting of the experiment. However, due to openings
all around for inlet air the combustion occurs at a faster pace and hence, the heating
time for other pots decreases which can also be clearly seen from Fig. 5.18 for the
first pot in the last phase.
Model-4
For Model-4, 10 litres of water in each pot with total three numbers of pots
along with a total quantity of 1.48 kgs of prepared fuel wood were used in the
0
8
16
24
32
0.00
5.50
11.00
16.50
22.00
P1 P2 P3 P4
Effi
cie
nci
es/
%
Pots
η
ψ
T
187
experimental study and the observations are plotted graphically in Figs.5.19 and
5.20.
Fig.5.19: Energy efficiency vs Time of different pots for "Model-4"
Fig.5.20: Exergy efficiency vs Time of different pots for "Model-4"
0.00
12.00
24.00
36.00
48.00
5 10 15 20 25 30 35 40 45 50 55
En
erg
y E
ffic
ien
cy (
%)
Time (minutes)
ηP1
ηP2
ηP3
0.00
2.00
4.00
6.00
8.00
5 10 15 20 25 30 35 40 45 50 55
Exerg
y E
ffic
ien
cy (
%)
Time (minutes)
ψP1
ψP2
ψP3
188
Figures 5.19 and 5.20 show the variation of the energetic and exergetic
performance of the Model-4 against the heating time for 10 litres of water in each pot
with three numbers of pots as mentioned above. For the first pot (pot 1) it is seen
from these graphs that the energy efficiency first slightly decreases and then sharply
increases, attains its peak and then goes down sharply, while the exergy efficiency
first slightly and then sharply increases attains its peak and then goes down sharply.
On the other hand, both the efficiencies first increase sharply, attain their peaks and
then goes down sharply towards the last phase of the experiment for the second pot
(pot 2). Whereas for third pot (pot 3) both the efficiencies first decrease and then
increase, attain their peaks and then goes down in a fluctuating manner as can be
seen from Figs.5.19 and 5.20.
Also for the first and second pots, the peaks for both energy and exergy
efficiencies are at larger interval unlike other cook stove models mentioned above,
however, the peak for the exergetic efficiency is found to be in the similar range
qualitatively unlike the energy efficiency peak as can be seen from Figs.5.19 and
5.20. Again the exergetic performance is found to be much lower than that of the
energetic performance for all the pots used in this particular model and hence, in the
similar trend those of other cook stoves. It is also observed from figure 5.21 that the
heating/boiling time for the first pot in this particular model is found to be the shortest
than those of other model, which shows the fast ignition process and merit of this
particular model over other type of cook stove models.
The results obtained for this particular model are mixed but slightly different
from other models. For example, for the first pot (pot 1) there is a similarity in the
energy and exergy efficiencies with other models as can be seen from Figs. 5.10-
189
5.21, while towards the end of the experiment these results are different from other
models.
Fig.5.21: Variation of efficiencies and time for different pots used in the "Model-4"
In other words, the energy and exergy efficiencies for all models increases
slowly for longer period of time, while it is not the case for Model-4 as can be seen
from these graphs mentioned above. This is may be due to the fact that in the Model-
4 the ignition process is faster than other types of cook stove models. As a result, we
get the shorter curve for the first two pots (pot 1 and pot 2), while for the third pot (pot
3) the heating power rate of the cook stove decreases slowly and hence, the heating
time prolongs as can be seen from Fig.5.21.
Comparison among different cook stoves
The comparison for total energy and exergy efficiencies among all four cook
stove models is shown in Fig.5.22 for a typical set of operating parameters
mentioned above.
0
13
26
39
52
0.00
7.50
15.00
22.50
30.00
P1 P2 P3
Eff
icie
ncie
s/%
Pots
η
ψ
T
190
Fig.5.22: Variation in efficiencies of different models
It is seen from this figure that the energetic performance of all cook stove models is
much higher than that of exergetic performance, which is due to the fact that the
quantity of energy gained in the hot water for each model is much lesser than the
quality of energy gained due to temperature constrained and hence, it is an obvious
fact in all thermal energy systems.
Also both the energetic and exergetic performances of the Model-1 are found
to be much better than other types of cook stove models followed by the Model-4 as
can be seen from Fig.5.22. Again the energetic performance of the Model-3 is found
to be the lowest whereas the exergetic performance of the Model-2 is found to be the
lowest among all cook stove models studies here. In other words, the Model-2 is
found to be better than that of the Model-3 as far as energetic performance is
concerned, while it is found to be reverse in the case of exergetic performance.
0
6
12
18
24
Model-1 Model-2 Model-3 Model-4
Eff
icie
ncie
s (
%)
Models
ψ
η
191
Since exergetic performance is the true measure of any thermal energy system and
hence, the Model-3 is found to be better over the Model-2, whereas the Model-1 is
found to be the best and the Model-4 falls somewhere in the middle from the point of
view of thermodynamics as well from the point of view of economics.
Also for the Model-1 the heating/boiling time for the first pot (pot 1) is found to
be the longest followed by the Model-2, while it is found to be the least in the case of
Model-3, whereas, it is found to be the second lowest in the case of Model-4 cook
stove. This shows that the ignition time for Model-3 is the shortest followed by Model-
2 and Model-4, while it is found to be the longest in the case of Model -1.
5.4 Conclusions
The present experimental study deals with the performance study of
renewable energy cooking devices viz. solar cookers and biomass cook stoves.
Comparative performance evaluation of two different types solar cookers viz.
paraboloid type and box type based on exergy analysis has been carried out. From
the study following conclusions have been drawn:
I. Initially exergy efficiency is high then decreases sharply and then slowly
decreases and reaches to stagnant level which is due to the fact that, initially
temperature difference is high and decreases with respect to time and
reaches to constant level when it is near to boiling point. On the other hand
solar radiation has different trend with time which is initially low then increases
sharply takes peak and then decreases.
II. Mean exergy efficiency with one and two liters of water of paraboloid type
cooker is higher than that of box type cooker, however, efficiency with two liter
192
is higher than that of with one liter of volume of water. This is due to the fact
that output exergy is directly proportional to the volume of the water.
III. Mean exergy efficiency with 250gms of rice for paraboloid type solar cooker is
found to be higher than that of box type cooker. Therefore, in general it is
found that exergy efficiency of paraboloid type is greater than box type.
Further comparative exergetic and energetic analyses of four different types of
cook-stove models based on the experimental observation using different size of
pots described in the BIS standard revealed that:
I. Both the efficiencies of Model-1 is found to be the higher than that of rest of
the three models i.e. performance of Model-1 is found to be the best among
the all four models. The average exergetic and energetic efficiencies of
second and third pot is always higher than that of first and fourth pot in case of
Model-1 and Model-3 of cook stoves. However, In case of Model-1 both the
efficiencies for second pot are higher than that of first pot and that of fourth pot
is higher than that of third pot.
II. For all cook stove models, the energy efficiency is found to be much higher
than that of exergy efficiency, which can be explained in terms of the quality of
energy gained in the hot water. In other words, the energy gained by the hot
water is much higher that the exergy gained by the hot water at that particular
temperature and hence, we gets the results obtained in these figures.
III. The energetic performance of the Model-3 cook stove is found to be the
lowest whereas the exergetic performance of the Model-2 is found to be the
lowest among all cook stove models studies here. Also both the energetic and
exergetic performances of the Model-1 are found to be much better than other
types of cook stove models followed by the Model-4.
193
IV. The Model-2 is found to be better than that of the Model-3 as far as energetic
performance is concerned, while it is found to be reverse in the case of
exergetic performance. Since exergetic performance is the true measure of
any thermal energy system and hence, the Model-3 is found to be better over
the Model-2, whereas the Model-1 is found to be the best and the Model-4
falls somewhere in the middle from the point of view of thermodynamics as
well from the point of view of economics.
V. In the case of Model-1 the heating/boiling time for the first pot (pot 1) is found
to be the longest followed by the Model-2, while it is found to be the least in
the case of Model-3, whereas, it is found to be the second lowest in the case
of Model-4. This shows that the ignition time for Model-3 is the shortest
followed by Model-2 and Model-4, while it is found to be the longest in the
case of Model-1.
VI. For Model-4 cook stove, both the efficiencies for second pot are higher than
that of first and third pot which may be due to the fact that in this particular
case only three pots have been used. Also for the first and second pots, the
peaks for both energy and exergy efficiencies are at larger interval unlike
other cook stove models mentioned above, however, the peak for the
exergetic efficiency is found to be in the similar range qualitatively unlike the
energy efficiency peak for this particular model.
Thus the concept of exergy analysis has been applied to renewable energy
cooking devices viz. solar cookers and biomass cook-stoves. Since, the exergetic
analysis is more realistic approach than the energetic analysis therefore, the
above conclusions are very useful of such systems. It is also important to note
that the exergy analysis was applied for the first time to biomass cookstoves and
194
few important conclusion were drawn which may be helpful for engineers,
scientists, and decision/policy makers in designing, fabrication and dissemination
of cook-stove to the end users in different part of the world.
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