INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. 2012; 36:724 736
Published online 28 February 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1827
Comparative study based on exergy analysis of solar air
heater collector using thermal energy storage
V. V. Tyagi1, A. K. Pandey2, G. Giridhar3, B. Bandyopadhyay3, S. R. Park4 and S. K. Tyagi2, ,y
1
Centre for Energy Studies, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India
2
School of Infrastructure Technology & Resource Management, Shri Mata Vaishno Devi University, Katra 182320, Jammu and
Kashmir, India
3
Solar Energy Centre, Ministry of New and Renewable Energy (MNRE) Gwal Pahari, Gurgaon 122002, India
4
Renewable Energy Research Centre, Korea Institute of Energy Research, PO Box 103, Yuseong, Daejeon 305 343, South Korea
SUMMARY
This communication presents the comparative experimental study based on energy and exergy analyses of a typical
solar air heater collector with and without temporary heat energy storage (THES) material, viz. paraffin wax and
hytherm oil. Based on the experimental observations, the first law and the second law efficiencies have been
calculated with respect to the available solar radiation for three different arrangements, viz. one arrangement
without heat storage material and two arrangements with THES, viz. hytherm oil and paraffin wax, respectively. It
is found that both the efficiencies in case of heat storage material/fluid are significantly higher than that of without
THES, besides both the efficiencies in case of paraffin wax are slightly higher than that of hytherm oil case.
Copyright r 2011 John Wiley & Sons, Ltd.
KEY WORDS
phase change material; temporary heat energy storage; hytherm oil; solar air heater collector; exergy analysis
Correspondence
*S. K. Tyagi, School of Infrastructure Technology & Resource Management, Shri Mata Vaishno Devi University, Katra 182320, Jammu
and Kashmir, India.
y
E-mail: [e-mail]
Received 3 September 2010; Revised 6 January 2011; Accepted 6 January 2011
1. INTRODUCTION electric systems such as room heater, heat pipes, heat
pump systems are employed to produce heating.
Thermal comfort plays a very important role on the In some countries, where the atmospheric temperature
health, growth, working efficiency and feeling of is very low, natural heating like solar energy is not
human beings. All living beings including humans are sufficient. In such a case, refrigeration and fuel-fired
very much concerned about the suitable climate and systems are proven to be suitable heating devices [1].
thermal comfort, especially, temperature and humid- Continuous efforts have been made by numerous
ity. Owing to the increasing pressures of energy researchers on different types of heat pumps in order
demand, the degradation of environment, global to improve their performance and to make them cost
warming and depletion of ozone layer, etc., there is a effective. Some of the heat pumps developed so far still
need for efficient energy utilization and waste heat have not gained much importance. This may be due to
recovery. In the excessively hot climates it is necessary various factors, such as low coefficient of performance,
to reduce the temperature and humidity, whereas in the high investment and operational costs and/or their
excessively cold climate there is a need to increase the limited heat producing capacity. Owing to the limited
temperature within the presence of suitable moisture resources of energy and the increasing demand, there is
content. If the temperature drops below thermal a concern in the scientific community to rethink and to
comfort level, especially, in the winter season, the develop the energy efficient system which is not only
heating devices such as burning of wood, coal, etc. are economical but also environment friendly. The energy
found to be traditional systems for heating and being consumption in buildings, commercial installations
used for decades, in the undeveloped and poor and space air conditioning constitutes a huge share of
countries. With the advancement of technologies, the total energy consumption not only in the developed
Copyright r 2011 John Wiley & Sons, Ltd.
724
Exergy analysis of solar air heater collector V. V. Tyagi et al.
world but also in the developing countries. Facing ever exergy analysis of a 35-kW parabolic trough-based
the increasing pressure of energy demand, environ- solar thermal power plant situated near the capital of
mental degradation, global warming and depletion of India. They [10,11] observed that most of the exergy
ozone layer due to various reasons most commonly the is lost in solar collector system followed by high-
industrialization [2]. temperature heat exchanger, whereas the exergy loss in
The efficient use of energy is a hot topic of research, the low-temperature heat exchanger is found to be very
especially, after the Kyoto and the Montreal Protocols. less unlike the energy losses. Because exergy is the
It is well known that there is a huge potential of low- quality of energy, once it is lost, is lost forever and can
grade energy usage such as solar energy, and waste heat not be recovered unlike energy. They also mentioned
from the industry for hot water, space heating and crop some techniques to decrease the loss in different com-
& grain drying. This will not only helpful in the saving ponents of a solar thermal power plant and how to
of high-grade energy resources but also can decrease the increase the efficiency of solar thermal devices. Fath
ozone depletion and release of greenhouse gases. This in [12] studied the performance of the simple design solar
turn can help in the long run for solving the huge air heater; the conventional flat plate absorber is pre-
environmental degradation and global warming pro- sented by a set of tubes filled with a thermal energy
blems. Owing to high pressure from the scientific com- storage material [13] predicted in the thermal perfor-
munity, some governments have decided to emphasize mance of four common types of single pass solar air
the use of solar and other renewable energy, besides, to heater.
restrict the wastage of energy in different form by means For commercial applications, ability of the drier
of penalty on the industries for the release of waste heat to process continuously is very important to dry the
into the environment [1 3]. products for its safe storage level and to maintain the
In such a case, the heat exchanger, solar air heater quality of the product. Normally thermal storage sys-
collector and heat pump systems can be used to extract tems are employed to store thermal energy, which
the heat from low-grade energy sources and the waste includes sensible heat storage, chemical energy storage
from industries to utilize it for other low- and medium- and latent heat storage. The solar drier is an energy
temperature industrial applications, such as in dye efficient option in the drying processes [13]. The use
industry, heating, cleaning and so on. This not only of forced convection solar driers seems to be an
can utilize the freely and abundantly available renew- advantage compared with traditional methods and im-
able energy and waste heat for higher temperature proves the quality of the product considerably [14 16].
applications but also can reduce a huge potential of the Normally thermal storage systems are employed to
environment degradation. Solar energy is freely avail- store heat for both short and long periods [17].
able, clean and can be utilized for different heating/ Common sensible heat storage materials used to store
cooling and space conditioning applications such as sensible heat are water, gravel bed, sand, clay, concrete,
domestic, agricultural and industrial sectors with suit- etc. [15 17]. Mohanraj and Chandrasekar [18] analyzed
able design and modifications as per the requirements. heat storage material for copra drying of a flat plate
But it is fluctuating in nature and also available only in solar air heater.
the daytime and hence, there is a need for thermal heat In recent years, few authors [19 25] have studied
storage so that the heat collected from the collector can different features of solar collector system using
be stored and used when there is no availability of sun various approaches. For example, Mohanraj and
light [2,3]. Chandrasekar [18] and Kurtbas and Durmus [19] have
The thermal energy storage is defined as the tem- studied the solar air heater for different heating
porary storage of the thermal energy at high or low purposes, whereas Luminosu and Fara [20] and
temperatures. Energy storage can reduce the gap Torres-Reyes et al., [21] have studied the optimal
between energy supply and energy demand, and it thermal energy conversion and design of a flat plate
plays an important role in energy conservation, and solar collector using exergy analysis. On the other
hence, maximizes the use of available energy. Kovarik hand, Bakos et al. [22], Kaushik et al. [23] and Tyagi
and Lesse [4] studied the optimal flow for low- et al. [24] have studied the optimum design of a para-
temperature solar heat collector, whereas Farries bolic trough collector (PTC) and have given some
et al. [5] studied the energy conservation by adaptive fruitful results, especially, the mass flow rate of the
control for a solar-heated building. The optimal and moving fluid and the concentration ratio of the PTC
semioptimal control strategies and sensitivity for the collector.
mass flow rate and other parameters for liquid solar Ozturk and Demirel [25] experimentaly investigated
collector system were carried out by Orbach et al. [6] the thermal performance of a solar air heater having its
and Winn et al. [7]. Bejan et al. [8,9] studied the second flow channel packed with Raschig rings based on the
law analysis and exergy extraction from solar collector energy and exergy analyses. It was found that the
under time varying conditions. average daily net energy and exergy efficiencies were
Keeping this aspect in mind, Singh and Kaushik [10] 17.51 and 0.91%, respectively. In addition, the energy
and Misra [11] carried out a thorough study about and exergy efficiencies of the packed-bed solar air
Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd. 725
DOI: 10.1002/er
V. V. Tyagi et al. Exergy analysis of solar air heater collector
heater increased as the outlet temperature of the heat tubes are filled with hytherm oil, in a way that the THES
transfer fluid increased. Potdukhe and Thombre [26] material is coated around the copper tube and heat is
designed, fabricated, simulated and also tested a solar stored in the material and transferred to the copper tube
dryer fitted with a novel design of absorber having and finally to the blowing air inside it. It also overcomes
inbuilt thermal storage capabilities. The length of the sudden drop in the outlet temperature to the hot air,
operation of the solar air heater and the efficiency of due to fluctuation in the solar radiation arises due to
the dryer were increased, and better quality of agri- cloud and/or other reasons. On the other hand, there
cultural products in terms of colour value was obtained is no temporary heat storage material in the third
compared with open sun drying. MacPhee and arrangement.
Dincer [27] worked on thermodynamic analyses of the Asbestos cloth was used for covering the copper
process of charging of an encapsulated ice thermal tubes exposed into the open air, viz. outside the ETC
energy storage device through heat transfer. The en- tubes for insulation purpose to reduce/minimize the
ergy efficiencies are found to be more than 99%, heat loss to the ambient air. Calibrated J-type thermo-
whereas the thermal exergy efficiencies are found to couples made of copper-constantine with a tempera-
vary between 40 and 93% for viable charging times. ture range of 200 13501C were used to measure air
The results confirm the fact that energy analyses, temperature at different state points. A Total of 13
and even thermal exergy analyses, may lead to some sensors have been used in the experimental set-up, the
unrealistic efficiency values. cross-sectional view of a tube along with THES in
In the present study, evacuated tube collector Figure 1(a) and the schematic of the experimental set-
(ETC)-based solar air heater collector with and with- up in Figure 1(b) can be seen. In this arrangement, one
out thermal energy storage has been studied using the thermocouple has been used for measuring the input
experimental data measured for typical days and time air temperature and other four were used for measur-
at Solar Energy Centre, Gurgaon, India. The measured ing the outlet temperature of air at different state
data include solar radiation, temperature of the air/ points. The outlet air temperature of the first tube is
working fluid at different state points for different the inlet of the second tube and the outlet of the second
mass flow rates of air. Based on the measured data, tube is the inlet of the third tube and so on. In this
different properties of air such as density, specific heat arrangement where THES is used, one thermocouple
etc. have been calculated using online air calculator has been inserted inside the collector tube for mea-
and with the help of above-mentioned parameters suring the temperature of storage material, besides,
other properties such as enthalpy and entropy were the same number of thermocouples has been used at
calculated. Finally, energy, exergy, first and second different state points mentioned above. To measure air
law efficiencies were calculated. Based on findings, flow rate, a Rotameter of 200 LPM capacity is used,
conclusions were made about the most probable time which has been placed between the compressor and
in a day at which the first law and second law effi- inlet of ETC (Figure 1(b)). Air was forced circulated
ciencies are found to be the maximum for all the three through the system using half HP air compressor.
cases. In this analysis, it is observed that both the For measuring solar radiation Pyranometer with
efficiencies are significantly higher in the case where multiplication factor 8.52 10 6 VW 1 m2 has been
thermal energy storage materials, viz. hytherm oil and used in the experiment. The Pyranometer is kept on the
paraffin wax have been used than that of without horizontal surface nearby the experimental set-up in
storage. However, both the efficiencies are slightly the open air, so that no shadow and/or reflection of
better in case of paraffin wax than that of hytherm oil, solar radiation from any other surface/object falls on
filled within the tubes of solar collector. it. For collection of data, the HP data acquisition unit
attached with a computer has been used in this study.
The specifications of the ETC are given in Table I.
As shown in Figure 1(a), solar collector consists of a
2. EXPERIMENTAL SET-UP AND double-walled evacuated glass tubes. Forced air flow
DESCRIPTION is used as a working fluid in the system and PCM/
hytherm oil as a heat storage material/fluid so that this
In the present experimental study, the solar air heater stored heat can be used for drying when solar radiation
collector with and without temporary heat energy is not available and/or suddenly fluctuates due to any
storage (THES) has been made of an ETC. A total of reason in the daytime and/or late evening hours. There
12 ETC collector tubes (four for PCM, four for hytherm are four vacuum tubes in each arrangement and a
oil and four for without THES individually) have been black-absorbing coating is done on the outer surface of
arranged in the series. The copper tube of 12 mm the inner tube. The tubes are made of glass and the
diameter has been inserted inside the evacuated tubes for specification is given in Table I, while the length
air circulation. Out of three arrangements, mentioned exposed to sunlight is 172 cm and inclined at 451. The
above, one is to fill Paraffin wax inside ETC tubes and volume flow rate of the circulating fluid is measured by
outside the copper tube; in second arrangement ETC volume flow meter before it enters the first tube. There
726 Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy analysis of solar air heater collector V. V. Tyagi et al.
Figure 1. (a) Cross-sectional view of ETC tube with THES and (b) schematic of the experimental set-up.
is a vacuum between the annular spaces of double- thermodynamics viz. the law of conservation of energy,
walled glass tubes to reduce the heat loss by conduc- while the exergy analysis is based on the second law
tion and convection. Whenever fluid enter in the first of thermodynamics, i.e. using the concept of entropy
tube its temperature rises, which can be identified by generation and/or the law of degradation of the quality
measuring temperature with the help of thermocouple of energy, as given in the next section.
provided at inlet and outlet of each tube. Data were
collected from the system for few days in different
months by varying the volume flow rate of air.
Energy and exergy analyses have been carried out to 3. ENERGY ANALYSIS
evaluate the first and second law efficiencies of solar
air heater collector system with and without THES. The energy analysis is based on the first law of
Volume flow rate of the fluid in the system is specified thermodynamics and the corresponding first law
and the schematic description is shown in Figure 1(b). efficiency has been calculated. The energy analysis is
The energy analysis is based on the first law of based on the fact that it is an upper limit of efficiency
Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd. 727
DOI: 10.1002/er
V. V. Tyagi et al. Exergy analysis of solar air heater collector
is the mass flow rate of air, the first law efficiency of the
Table I. Details about the solar air heater collector and storage
materials. collector system is given by
_
ZźQf=QcźmCpDT=AIs ð4Þ
Specification of collector tubes Values
where Z is the abbreviation used for first law efficiency
Total length 179.5 cm
of the system.
Inner length 176 cm
Coating length 172 cm
Inner diameter 44 mm
Outer diameter 57.5 mm 4. EXERGY ANALYSIS
Properties of the Paraffin wax
The rate at which exergy is collected by the solar
as a PCM
collector can be increased by increasing the mass flow
Melting point 53.041C
rate of the working fluid. Since the collector tubes are
Specific heat 2.05 kJ kg1C 1
the most expensive component of any solar thermal
Latent heat of fusion 183.1 kJ kg 1
system which needs advanced material and associated
Thermal conductivity 0.21 (solid) (W m K 1)
technology to build, therefore, it requires large invest-
Density at 701C 0.769 kg m 3
ment. In order to reduce the capital cost, we need to
Properties of HP Hytherm 500 Oil
optimize the dryer area, as the fuel (sunlight) is free.
Kinematic viscosity @ 40 c, cst 27 35
Again, for large mass flow rates, the fluid outlet
Flash point coc, c, min 194
temperature is very low and requires more power to
Viscosity index 95
pump/blow air/fluid through it. On the other hand, low
Power point c max 0.0
flow rate results in high outlet temperature of the
Copper strip corrosion 3 h @ 100 c 1.0
working fluid with high specific work potential. But due
(astm), max
to the nature of entropy generation, exergy losses
Neutralization number mg koh gm 1, 0.15
increase due to the temperature differences and hence,
max
the optimum mass flow rate is required. The exergy
260c 0.731
analysis has been performed based on the configuration
280c 0.751
of solar air heater collector shown in Figure 1(b). The
300c 0.772
exergy received by collector is given by [8 11,23,24,28]
260c 0.097
280c 0.096
Exc ź Qcð1 Ta=TSÞð5Þ
300c 0.095
where Ta is the ambient temperature, and TS is the
Measured by Differential Scanning Calorimeter (DSC).
temperature of the source while, the exergy received by
fluid is written as [8 11,23,24,28]:
_
ExfźmðEo EiÞźm_½ðho hiÞ Taðso siÞŠ ð6Þ
with which the solar radiation can be converted into
heat and the heat can be transferred for useful
where ho is the output specific enthalpy, hi is the input
applications at a given frequency spectrum and
specific enthalpy, so is the output entropy, si is the input
intensity. Energy incident on the evacuated tube is
entropy, and m_ is the mass flow rate of air blowing
given by
through the collector tubes. The output specific enthalpy
of the fluid is given by [23,24,28]
QcźAIs ð1Þ
hoźCPo TO ð7Þ
where Qc is the energy incident on the collector tube,
A is the projected area of collector tube exposed to the where TO is outlet temperature, and CPo is the specific
sun light, and Is is the intensity of solar radiation at heat of air at outlet. The specific enthalpy of inlet air is
any particular site. Useful energy gained from the given by [23,24,28]
collector can be written as
hiźCPi Ti ð8Þ
Quźat IsA ð2Þ
where CPi is the input specific heat, Ti is the inlet
temperature. While the entropy difference has been
where a is the absorptance of inner surface of ETC, t is
calculated using the following set of equations [23,24,28]:
the transmittance of outer surface of the collector.
Useful energy transmitted into the evacuated tubes is CPiźa1k Ti ð9Þ
absorbed by fluid, and can be calculated using the first
CPoźa1k TO ð10Þ
law of thermodynamics, viz. the law of conservation of
energy:
dsźdq=TźCP dT=Tźða1bTÞðdT=TÞ
_
Qu ź QfźmCp DT ð3Þ
źa dT=T1kdT ð11Þ
where Qf is the energy absorbed by air, Cp the specific Using Equations (9 10), the values of constants
heat of air and DT is the temperature difference and m_ a and k can be calculated and hence, the entropy
728 Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy analysis of solar air heater collector V. V. Tyagi et al.
difference thereafter using Equation (11). The second
law efficiency, i.e. exergy efficiency of the system can be
written as [8 11,23,24,28]:
cźExf=Exc
_
źm½ðho hiÞ TOðso siÞŠ=Qcð1 TO=TSÞ ð12Þ
and first law efficiency can be written as [28]
_
Zźmðho hiÞ=Qc ð13Þ
HoðKJ=secÞźm_ ho ð14Þ
HiðKJ=secÞźm_ hi ð15Þ
Qcź4AIs cos 45 t a ð16Þ
where Exc is the exergy received by collector, Exf is the
exergy received by fluid, a and k are constants, Z is the
first law efficiency and c is the second law efficiency of
the solar collector-dryer system. Based on the above-
mentioned equations, a sample calculation has been
made and the results are shown in Table II, for a
typical set of operating conditions.
5. ERROR ANALYSIS
The data given on solar radiation, thermal energy
storage materials, solar collector tubes, digital tem-
perature displayer and sensors, air flow meter, air
compressor, rotameter have been measured/calculated
using different instruments. For example, solar radia-
tion data have been compared with the weather
monitoring systems available with the Ministry of
New and Renewable Energy, Government of India and
found to be in good agreement with those taken by the
Pyranometer with an error of 0.1 0.2%. Properties of
the thermal energy storage material such as latent heat,
melting temperature, etc. have been measured with the
differential scanning calorimeter (DSC) and were
found to be almost similar as given by the supplier
with an error of 0.2 0.5%. The error in the efficiency
of the compressor is found to be 0.5 1% depending on
the ambient air conditions, whereas the error in the
rotameter is found to be between 2.0 and 3.3%. The
error in the properties of air was found to be around
0.5 1% which is mainly due to the presence of
moisture in the circulating air over the period. The
temperature sensors were calibrated before and after
use with the standard scale available in the laboratory
with an error of about 0.05%.
As it is well known wind speed also affects heat
transfer rates to and from the solar collector and hence
affects the energy and exergy efficiencies of the system.
Besides, the deposited dust particles on the collector,
moisture content in the ambient air, location, or-
ientation and so on, only slightly affect the overall
performance of the experimental system.
There is an error of around 0.5 0.8% in the exergy
and energy efficiencies due to various parameters and
Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd. 729
DOI: 10.1002/er
c
c
f
II
I
1 1
0.071
984.07
137.37
129.74
29.23
2.52
8.63
20.74
0.075
982.63
159.54
155.39
36.42
3.36
9.23
20.51
0.081
981.21
181.12
159.49
40.54
4.31
10.63
23.13
0.085
979.68
204.81
178.18
48.33
5.50
11.38
23.90
0.089
978.31
225.23
187.64
51.09
6.67
13.06
25.43
0.091
977.99
224.61
178.81
49.18
6.70
13.62
26.78
0.091
977.55
226.57
188.71
50.82
6.87
13.53
25.80
0.091
977.86
227.21
208.66
57.04
6.85
12.03
23.25
0.089
978.18
227.83
191.94
50.61
6.82
13.48
25.18
0.088
978.49
228.46
165.15
43.36
6.80
15.67
29.16
0.088
978.75
223.25
143.94
36.25
6.50
17.92
32.59
0.086
979.15
215.38
140.02
33.71
6.06
17.98
32.18
0.084
979.99
205.38
109.32
24.72
5.48
22.16
38.80
0.081
980.71
197.92
55.34
11.67
5.06
43.33
73.04
1 1
1 2
1 1
1008.99
1009.85
1010.77
1011.88
1012.95
1013.07
1013.31
1013.19
1013.07
1012.95
1012.70
1012.35
1011.77
1011.32
P
;
o
1 1
P
;
i
1
Table II.
Sample calculation for different parameters for a typical set of operating conditions.
1,in
S
O
Time (h)
T
(K)
T
(K)
T
(K)
m (gm s
)
C
(J kg
K
)
C
(J kg
K
)
k (J kg
K
)
a (J kg
K
)
D
s (J kg
K
)
Q (W)
E (W)
E (W)
Z
(%)
Z (%)
10:00
308
395
353
0.580
1005.8
10:30
309
401
362
0.582
1005.9
11:00
310
413
371
0.584
1005.9
11:30
311
424
381
0.586
1006.1
12:00
312
426
390
0.587
1006.2
12:30
313
429
391
0.590
1006.1
13:00
314
427
393
0.592
1006.1
13:30
313
428
392
0.590
1006.1
14:00
312
421
391
0.588
1006.0
14:30
311
419
390
0.586
1006.2
15:00
311
413
388
0.586
1006.1
15:30
311
407
385
0.586
1006.2
16:00
310
398
380
0.586
1005.9
16:30
309
389
376
0.584
1005.9
V. V. Tyagi et al. Exergy analysis of solar air heater collector
the performance accuracy of the instruments. But as seen from these graphs, which is obvious because of the
mentioned above, wind speed and deposition of dust fluctuation in solar radiation throughout the day.
particle were not taken into account in the present In case of temporary storage material, both the
experimental study. Therefore, the overall influences of efficiencies have peaks at different times than those
these input errors on the total results can also be very obtained without THES. We note that graphs with
small and hence, can be neglected in the present study. phase change material and hytherm oil, have their
However, to make an error-free system, all the para- peaks at about 16:30, while it is in the first half, in
meters mentioned above must be taken into account general, for those without THES, besides, they are
for the better accuracy and performance of such sys- fluctuating in nature as can be seen from Figures 2 6.
tems for real-life applications. The peak with THES occurs at around 16:30 h, which
is because solar radiation goes down sharply, while the
circulating air gets heated by temporary storage
material at almost constant temperature for some
time. As a result, the output to energy input ratio with
6. RESULTS AND DISCUSSION
temporary storage increases sharply, and hence, the
first and second law efficiencies attain their peaks
A comparative study on first and second law analyses
during the late afternoon with some shifting due to
of a typical solar air heater collector system with and
different mass flow rates.
without thermal heat energy storage (viz. hytherm oil
However, due to finite heat storage capacity and
and paraffin wax) has been carried out at different
mass of the storing material, the stored energy of
mass flow rates using hourly solar radiation. The solar
THES decreases afterwards and hence, both the effi-
radiation first and second law efficiencies against time
ciencies in most cases decrease in the same pattern,
are shown in Figures 2 6. From the graphs it is found
as can be seen from the graphs (Figures 2 6). As
that both the efficiencies increase as the time increases
mentioned above, the temporary storage material has
in all the three cases (with and without THES). But
finite heat capacity and limited mass due to the space
there are some fluctuations in the efficiencies as can be
Figure 2. Solar radiation and efficiencies versus time for 10 LPM flow rate (a) with PCM; (b) with hytherm oil; and (c) without THES.
730 Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy analysis of solar air heater collector V. V. Tyagi et al.
Figure 3. Solar radiation and efficiencies versus time for 20 LPM (a) with PCM; (b) with hytherm oil; and (c) without THES.
available in the evacuated tubes; the instantaneous than that of the second law efficiency, because exergy
fluctuation in solar radiation is compensated by the represents the quality of energy which is obviously
storage material that supplies heat at almost constant enhanced with the increase in the temperature unlike
temperature. However, if the solar radiation fluctuates the quantity of energy. In addition, as explained by
more often and/or for a longer time due to weather several authors [20 24,28], exergy once lost is lost
constraints, the fluctuation is also found in the effi- forever and cannot be recovered, unlike energy.
ciencies of the collector with THES. But in the case Moreover, the exergy loss is more in the collector
where there is no temporary storage, the fluctuation in receiver-assembly and not in the low-temperature uti-
both the efficiencies is found to be more frequent and lity unlike energy. This results in more losses in exergy
significant, as can be seen from Figures 2 6. It is also than in energy and hence, we find that the second law
important to note that the solar radiation is given in efficiency is much less than that of the first law effi-
kW m 2 in Figures 2 6, which is done to fit the effi- ciency. The curves for second law efficiency are found
ciency and radiation curve on the same axis. to be smoother than that of first law efficiency, which
It is also noted that the fluctuation in the efficiencies may be due to the fact that exergy losses are less
is in a phase where there is no temporary storage sensitive to the input energy, viz. solar radiation, while
material, whereas the fluctuation in both the effi- it is the reverse in the case of energy losses.
ciencies is in the opposite phase, where temporary However, as the mass flow rate of the working fluid
storage is being used, as can be seen from Figures 2 6. increases, the efficiencies in all the three cases increase
It can be explained with the same physical significance due to the heat gain by the moving fluid (viz. by the
that the THES material behaves as a temperature air) in the receiver tube increase, as a result, we get
regulator by supplying additional heat during the higher output, resulting in higher efficiency for all
fluctuation in solar radiation. From the observations, the cases. In addition, whatever the mass flow rate,
it is found that the first law efficiency is much higher first law efficiency is always greater than second law
Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd. 731
DOI: 10.1002/er
V. V. Tyagi et al. Exergy analysis of solar air heater collector
Figure 4. Solar radiation and efficiencies versus time for 30 LPM (a) with PCM; (b) with hytherm oil; and (c) without THES.
efficiency. In the case where temporary storage decrease and again increase ; this is because the heat
material has not been used, only one peak is observed stored in the PCM/hytherm is supplied by THES
and it shifts towards the origin for both the efficiencies which does not happen in the case of without THES.
as the mass flow rate of working fluid is increased. In case of empty collector tubes the efficiencies attain
This is due to the fact that the role of temporary their peaks in the first half once, and then go down
storage material is significant in this way. In other further as time increases as solar radiation also
words, the peak observed is around 13:00 h (Figure 2c). decreases;this is found to be different in the case with
In case of 10 LPM, it is found to be 11:05 and 12:40 h THES.
in case of medium mass flow rates. However, in cases As can be seen from the literature [17 24,28 30],
where temporary storage has been used, both the some studies have been carried out in solar collector/
efficiencies attain their highest peaks at approxi- dryer/heaters systems using energy and exergy analyses
mately 4:30 h (Figures 2 6) for all mass flow rates with and without phase change materials and/or ther-
because at this time solar radiation goes down sharply mal energy storages. For example, Ahmad et al. [17]
and heat is supplied by THES up to some reasonable studied the thermo-hydraulic (effective) efficiency of
duration. packed bed solar air heater without phase change
Both the efficiencies of solar air heater collector material using energy analysis. Mohanraj and
using PCM are slightly better than in the case of hy- Chandrasekar [18] studied the energetic performance
therm oil. But in general, both the efficiencies with with and without thermal energy storage for copra
THES have been observed to be better than those drying. Kurtbas and Durmus [19] carried out energy
without THES. Besides, as the temporary storage and exergy analysis of a solar air heater without
material regulates the supply of heat at almost con- phase change material and gave some new results
stant temperature, both the efficiencies are found to be for the performance enhancement of such systems.
smoother than that without THES as can be seen from The performance study of the flat plate collector has
Figures 2 6. However, both the efficiencies increase been carried out by some authors [20 21] using
slowly and attain peaks at nearly 16:30 h, and then exergy analysis without thermal energy storage. The
732 Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy analysis of solar air heater collector V. V. Tyagi et al.
Figure 5. Solar radiation and efficiencies versus time for 40 LPM (a) with PCM; (b) with hytherm oil; and (c) without THES.
performance analysis and parametric study of para- 7. CONCLUSIONS
bolic trough concentrating collector have been carried
out by different authors [22 24] using energy and
The comparative study based on the first and second
exergy analyses without thermal energy storage and
law analyses of a typical solar air heater collector
some useful results were also obtained. For example,
system with and without temporary thermal energy
in Reference [23 24] the authors studied the effects of
storage has been carried out at different mass flow
concentrating ratio and mass flow rate of the working
rates using hourly solar radiation. From the present
fluid on the first and second law analyses of con- experimental study, some interesting results are found
centrating collector and observed that the mass flow
and can be summarized as follows:
rate is a critical parameter and should be chosen very
It is found that there is fluctuation in both the
carefully to obtain the best performance of these solar
efficiencies which is mainly due to the fact that
collectors. Tyagi et al. [28] carried out the performance
solar radiation also fluctuates throughout the day
of an evacuated tube solar collector without any phase
as can be seen clearly from the figures given in this
change material-based thermal energy storage. Similar
paper. In addition, as time increases, both the
studies have been carried out by other authors on flat
efficiencies first increase and then decrease in case
plate collectors and/or solar air heaters, such as Koca
without temporary storage material and the
et al. [29], and Akbulut and Durmus [30] using energy
similar trend is found for solar radiation.
and exergy analyses and some useful results were
given. But none of the studies mentioned above is In case of without THES material, the efficiency
concentrated on the evacuated tube solar air heater increases with time, attains its peak in the first
collector using different thermal energy storage mate- half in general (Figures 2 6) and then decreases
rials and hence, the work presented in this paper is new after that. However, in cases where temporary
and unique of this kind. heat storage material is used, both the efficiencies
Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd. 733
DOI: 10.1002/er
V. V. Tyagi et al. Exergy analysis of solar air heater collector
Figure 6. Solar radiation and efficiencies versus time for 50 LPM (a) with PCM; (b) with hytherm oil; and (c) without THES.
increase with time, attain their peaks at approxi- can be seen from the graphs. It can be explained
mately 16:30 h with a small fluctuation with flow with the same physical significance that the THES
rate and then decrease smoothly. This is due to material behaves as a temperature regulator by
the fact that the solar radiation sharply decreases supplying additional heat during the fluctuation
in the late afternoon and heat is supplied only by of solar radiation.
THES for some time. But due to the limited mass From the observations, it is also found that the
and capacity of the storage material, this supply first law efficiency is much higher than the second
also decreases slowly after a certain time. As a law efficiency, because exergy represents the
result, the stored heat energy decreases smoothly, quality of energy, which is obviously enhanced
and hence, both the efficiencies again decrease with the increase in the temperature unlike the
towards the late evening hours. quantity of energy, and also as explained by
several authors mentioned in this paper. Thus
As the mass flow rate increases, peaks of both
efficiencies slightly shift towards the origin in case exergy, which is the quality of energy, once lost is
without storage material/fluid. However, in case lost forever and cannot be recovered, unlike
with THES, there is a small shift in the peak due energy. Also exergy loss is more in the collector
to different mass flow rates of the working fluid receiver-assembly and not in the low-temperature
except in case of 40 LPM which is because of a utility unlike energy. This results in more losses in
shift in the peak of solar radiation during that exergy than that of the energy and hence, we
particular day. found the second law efficiency much less than
that of the first law efficiency.
It is also noted that the fluctuation in the
efficiencies is in a phase where there is no The curves of the second law efficiency are found
temporary storage material, whereas the fluctua- to be smoother than that of the first law efficiency,
tion in both the efficiencies is in the opposite which may be due to the fact that exergy losses
phase where temporary storage is being used, as are less sensitive to the input energy, viz. solar
734 Int. J. Energy Res. 2012; 36:724 736 r 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Exergy analysis of solar air heater collector V. V. Tyagi et al.
University, Katra is duly acknowledged. This work
radiation, while it is the reverse in the case of
was financially supported by the Ministry of New &
energy losses.
Renewable Energy, Government of India.
Thus, the results obtained in this study will be very
useful and informative for real-life applications using
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