chapter - 3 exergy analysis of solar heating...
TRANSCRIPT
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CHAPTER - 3
Exergy Analysis of Solar Heating Devices
3.1 Introduction
It is well known that thermal comfort plays a very important role on the health,
growth, working efficiency and feeling of human beings. All living beings including
humans are very much concerned about the suitable climate and thermal comfort,
especially, temperature and humidity. Due to the limited resources of energy and the
increasing demand, there is a concern in the scientific community to rethink and to
redevelop the energy efficient system which is not only economical but also
environment friendly. The increasing pressures of energy demand, the degradation
of environment, global warming and depletion of ozone layer etc., has created the
need of efficient energy utilization and waste heat recovery options for commercial
and industrial installations [1].
The thermal energy storage is defined as the temporary storage of the thermal
energy at high or low temperatures. The energy storage can reduce the gap between
energy supply and energy demand, and it plays an important role in energy
conservation, and hence, maximizes the use of available energy. Kovarik and Lesse
[2] studied the optimal flow for low temperature solar heat collector while Farries et
al. [3] studied the energy conservation by adaptive control for a solar heated
building. The optimal and semi optimal control strategies and sensitivity for the mass
flow rate and other parameters for liquid solar collector system were carried out by
Orbach et al. [4] and Winn and Winn [5]. Bejan [6, 7] studied the second law analysis
and exergy extraction from solar collector under time varying conditions.
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Use of solar energy depends on the type of application, one of the most common
application of solar energy at low temperatures is heating of water for useful
purposes. The most common types of collector used in solar water heating systems
(SWH) are flat plate collectors (FPCs), evacuated tube collectors (ETCs) and
compound parabolic collectors (CPCs). Out of these three, FPCs and ETCs are the
most widely used collectors for small-scale water heating applications. Lot of studies
have been carried out on the improvement and advancement of FPCs [8,9] and
ETCs [10,11] . Mason and Davidson [12] had worked on the selective absorbing
surface of vacuum tubes and the use of single or double glazing. The use of
transparent insulating materials over the absorber surface has been suggested by
Goetzberger and Schmidt [13].
The present chapter deals with the exergy analysis of solar heating systems
which has two sections first one is the evacuated tube collector (ETC) based solar
air heater with and without thermal energy storage and the second one is the exergy
analysis of ETC based direct flow solar water heater (SWH). The evacuated tube
collector based solar air heater collector with and without thermal energy storage has
been studied using the experimental data measured at a typical location in North
India. The measured data includes solar radiation, temperature of the air/working
fluid at different state points for different mass flow rate of air. Using the air
properties like enthalpy and entropy etc. energy, exergy, first and second law
efficiencies were calculated. Based on findings, conclusions were made about the
most probable time in a day at which the first law and second law efficiencies were
found to be the maximum for all the three cases. In this analysis it was observed that
both the efficiencies are significantly higher in the case where thermal energy
storage materials viz. hytherm oil and paraffin wax have been used than that of
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without any thermal energy storage. However, both the efficiencies were found to be
slightly better in the case of paraffin wax than that of hytherm oil, filled within the
tubes of solar collector. For solar water heater, the experiments have been carried
out at different volume flow rate such as 10, 15, 20 25 and 30 LPH and has been
found that the present system shows the better performance at 15 LPH flow rate and
also this SWH shows the better efficiency than those of other ETC based SWHs
investigated by earlier authors [20-21].
3.2 Solar Air Heater
This section presents the exergy analysis of ETC based solar air heater with and
without thermal energy storage for which following experimental set up and
procedure has been used.
3.2.1 Experimental Set-up and Description
In the present experimental study, the solar air heater collector with and without
thermal energy storage (TES) have been fabricated using evacuated tubes (ETC).
Total 12 numbers of tubes viz. 4 for PCM, 4 for hytherm oil and 4 without TES
individually, have been arranged in the series. The copper tube of 12mm diameter
has been inserted inside the evacuated tubes for air circulation. Out of three
arrangements, mentioned above, one arrangement was filled with Paraffin wax
inside ETC tubes and outside the copper tube, in second arrangement ETC tubes
were filled with hytherm oil, in a way that the temporary heat energy storage material
is coated around the copper tube and heat is stored in the material and transferred to
the copper tube and finally to the blowing air inside it. It also overcomes the sudden
drop in the outlet temperature to the hot air, due to fluctuation in the solar radiation
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arises due to cloud and/or other reasons. On the other hand, there was no temporary
heat storage material in the third arrangement.
Asbestos cloth was used for covering the copper tubes exposed into the open air
viz. outside the ETC tubes for insulation purpose to reduce/minimize the heat loss to
the ambient air. Calibrated J-type thermocouples made of Copper-Constantine with
temperature range -200oC to 1350oC were used to measure air temperature at
different state points and total 13 sensors have been used in the experimental set-
up. The cross-sectional view of a tube along with the temporary heat energy storage
in Fig.3.1 (a) and the schematic and photographic of the experimental setup are
shown in Figs.3.1 (b-c) respectively.
Fig.3.1(a): Cross-sectional view of ETC tube with TES
In this arrangement, one thermocouple has been used for measuring the input air
temperature and other four were used for measuring the outlet temperature of air at
different state points. The outlet air temperature of the first tube is the inlet of the
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second tube and the outlet of the second tube is inlet to the third tube and so on. In
this arrangement where TES is used, one thermocouple has been inserted inside the
collector tube for measuring the temperature of storage material, besides, the same
number of thermocouples have been used at different state points mentioned above.
To measure air flow rate, a Rotameter of up-to 200 LPM capacity was used, which
has been placed between the compressor and inlet of ETC [Fig.3.1 (b)].
Fig.3.1 (b): Schematic of the experimental set up
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Fig.3.1 (c): The photographic view of the experimental set-up
Air was forced circulated through the system using half HP air compressor. For
measuring solar radiation pyranometer with multiplication factor 8.52x10-6 V/(W/m2)
has been used in the experiment. The pyranometer is kept on the horizontal surface
nearby the experimental set-up in the open air, so that no shadow and/or reflection
of solar radiation from any other surface/object falls on it. For collection of data, HP
data acquisition unit attached with a computer has been used in this study. The
specifications of the evacuated tube collector are given in Table-3.1. As shown in
Fig.3.1 (a) solar collector consists of a double walled evacuated glass tubes. Forced
air flow was used as the working fluid in the system and PCM/hytherm oil as a heat
storage material/fluid so that this stored heat can be used when solar radiation is not
available and/or suddenly fluctuates due to any reason in the day time and/or late
evening hours.
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There were four vacuum tubes in each arrangement having a black absorbing
coating on the outer surface of the inner tube. The tubes were made of glass and the
specification is given in Table-3.1, while the length exposed to sunlight is 172cm and
inclined at 450. The volume flow rate of the circulating fluid is measured by volume
flow meter before it enters the first tube. There is a vacuum between the annular
spaces of double walled glass tubes to reduce the heat loss by conduction and
convection. Whenever fluid enter in the first tube its temperature rises, which can be
identified by measuring temperature with the help of thermocouple provided at inlet
and outlet of each tube.
Table3.1: Details about the solar air heater collector and storage materials
Specification of collector tubes Values
Total length 179.5 cm
Inner length 176 cm
Coating length 172 cm
Inner diameter 44 mm
Outer diameter 57.5 mm
Properties of the Paraffin wax as a PCM
Melting point 53.04C *
Specific heat 2.05 kJ/kg°C
Latent heat of fusion 183.1 kJ/kg *
Thermal conductivity 0.21 (solid) (W/m K)
Density at 70 °C 0.769 kg/m3
Properties of HP Hytherm 500 Oil
Kinematic viscosity @ 40 c, cst 27-35
Flash point coc, c, min 194
Viscosity index 95
Power point c max 0.0
Copper strip corrosion 3hr @ 100 c (astm), max 1.0
Neutralisation number mg koh/gm, max 0.15
260c 0.731
280c 0.751
300c 0.772
260c 0.097
280c 0.096
300c 0.095 *Measured by Differential Scanning Calorimeter (DSC)
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Data was 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
evaluate the first and second law efficiencies of solar air heater collector system with
and without thermal energy storage. Volume flow rate of the fluid in the system is
specified and the schematic description is shown in Fig.3.1 (b). The energy analysis
is based on the first law of thermodynamics viz. the law of conservation of energy,
while the exergy analysis is based on the second law of thermodynamics, i.e. using
the concept of entropy generation and/or the law of degradation of the quality of
energy, as given in the next section.
3.2.2 Energy Analysis
The energy analysis is based on the first law of thermodynamics and the
corresponding first law efficiency has been calculated. The energy analysis is based
on the fact that it is an upper limit of efficiency with which the solar radiation can be
converted into heat and the heat can be transferred for useful applications at a given
frequency spectrum and intensity. Energy incident on the evacuated tube is given by:
sc AIQ (3.1)
where Qc is the energy incident on the collector tube, A is the projected area of
collector tube exposed to the sun light, and Is is the intensity of solar radiation at any
particular site. Useful energy gained from the collector can be written as:
AIQ su (3.2)
where is the absorptance of inner surface of evacuated tube collector, is the
transmittance of outer surface of the collector. Useful energy transmitted into the
evacuated tubes is absorbed by fluid, and can be calculated using the first law of
thermodynamics, viz. the law of conservation of energy:
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TCmQQ pfu (3.3)
where Qf is the energy absorbed by air, Cp the specific heat of air and T is the
temperature difference and m is the mass flow rate of air, the first law efficiency of
the collector system is given by:
spcf AITCmQQ // (3.4)
where is the abbreviation used for first law efficiency of the system
3.2 3 Exergy Analysis
The rate at which exergy is collected by the solar collector can be increased by
increasing the mass flow rate of the working fluid. Since, the collector tubes are the
most expensive component of any solar thermal system which needs advanced
material and associated technology to build and therefore, requires large investment.
In order to reduce the capital cost, we need to optimize the dryer area, as the fuel
(sunlight) is free. Again, for large mass flow rates, the fluid outlet temperature is very
low and requires more power to pump/blow air/fluid through it. On the other hand,
low flow rate results in high outlet temperature of the working fluid with high specific
work potential. But due to the nature of entropy generation, exergy losses increase
due to the temperature differences and hence, the optimum mass flow rate is
required. The exergy analysis has been performed based on the configuration of
solar air heater collector shown in Fig.3.1(b). The exergy received by collector is
given by [6-7, 16-18]:
)/(1( SaCc TTQEx (3.5)
where aT is ambient temperature, and ST is the temperature of the source while,
the exergy received by fluid is written as [6-7, 16-18].
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)]()[()( iOaiOiOf ssThhmEEmEx
(3.6)
where oh is output specific enthalpy, ih is input specific enthalpy, os is output
entropy, is is input entropy, and m is mass flow rate of air blowing through the
collector tubes. The output specific enthalpy of the fluid is given by [16-18]:
oh = oPC OT (3.7)
where OT is outlet temperature, and oPC is the specific heat of air at outlet. The
specific enthalpy of inlet air is given by [16-18]:
ih = iiP TC (3.8)
where iPC is input specific heat, iT is inlet temperature. While the entropy difference
has been calculated using the following set of equations [16-18]:
iP TkaCi
(3.9)
OP TkaCO
(3.10)
kdTTdTaTdTbTaTdTCTdqds p /)/()(// (3.11)
Using Eqs.(3.9-3.10), the values of constants a and k can be calculated and
hence, the entropy difference thereafter using Eq.(3.11). The second law efficiency
i.e. exergy efficiency of the system can be written as [6-7, 16-18]:
)/1(/)]()[(/ SOCiOOiOCf TTQssThhmExEx (3.12)
and first law efficiency can be written as [18]:
CiO Qhhm /)( (3.13)
oO hmKJH sec)/( (3.14)
ii hmKJH sec)/( (3.15)
45cos4 sC AIQ (3.16)
where CEx is the exergy received by collector, fEx is the exergy received by fluid, a
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and k are constants, is the first law efficiency and 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-3.2, for a typical set of
operating conditions.
3.2.4 Uncertainty Analysis
The data given on solar radiation, thermal energy storage materials, solar
collector tubes, digital temperature displayer and sensors, air flow meter, air
compressor, rotameter has been measured/calculated using different instruments.
For example, solar radiation data has been compared with the weather monitoring
systems available at Solar Energy Centre, Gurgaon (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 it is 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 found to be between 2.0-3.3%. 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 that the 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, orientation, and so on, only slightly affect the overall
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performance of the experimental system. Since, there is an error of around 0.5-0.8%
in the exergy and the energy efficiencies due to various parameters and the
performance accuracy of the instruments. But as mentioned above, the wind speed
and deposition of dust particle were not taken into account in the present
experimental study. Therefore, the overall influences of these input errors on the
total results can also be very small and hence, can be neglected in the present
study. However, to make an error free system, all these parameters mentioned
above must be taken into account for the better accuracy and performance of such
systems for real life applications.
3.2.5 Results and Discussion
Comparative study on first and second law analyses of a typical solar air heater
collector system with and without thermal energy (TES) storage (viz. hytherm oil and
paraffin wax) have been carried out at different mass flow rates using hourly solar
radiation. Three different arrangements viz. solar air heater without TES, with PCM
and with hytherm has been taken for performance evaluation of solar air heater. The
inlet, outlet temperatures and solar radiation with respect to time for different mass
flow rates (i.e. 10, 20, 30, 40, 50LPM) with and without TES recorded and the
performance has been illustrated in Figs. (3.2-3.6). It is noted that in case of without
TES the outlet temperature increases as time increases attains its peak at near
about 13:00, 14:20, 12:00, 13:40, and 13:00 hrs for different mass flow rates viz. 10,
20, 30, 40, and 50 LPM respectively and then decreases gradually after 16:30 hrs.
The similar trend has been observed for solar radiation but with fluctuations
throughout the daytime.
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Fig.3.2. Inlet, outlet temperatures and solar radiation vs Time for 10 LPM
Fig.3.3: Inlet, outlet temperatures and solar radiation vs Time for 20 LPM
100.0
325.0
550.0
775.0
1,000.0
20
45
70
95
120
10:00 13:00 16:00 19:00
Tem
pera
ture
s(°
C)
Time (hr)
Tin Thtm Tpcm T Is(W/m2)
0.0
250.0
500.0
750.0
1,000.0
20
45
70
95
120
10:00 13:00 16:00 19:00
Tem
pera
ture
s(°
C)
Time (hr)
Tin Thytherm Tpcm T Is(W/m2)
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Fig.3.4: Inlet, outlet temperatures and solar radiation vs Time for 30 LPM
Fig.3.5: Inlet, outlet temperatures and solar radiation vs Time for 40 LPM
0.0
250.0
500.0
750.0
1,000.0
10
40
70
100
130
10:00 13:00 16:00 19:00
Tem
pera
ture
s(°
C)
Time (hr)
Tin Thtm Tpcm T Is(W/m2)
200.0
400.0
600.0
800.0
1,000.0
10
40
70
100
130
10:00 13:00 16:00 19:00
Tem
pera
ture
s(°
C)
Time (hr)
Tin Thtm Tpcm T Is(W/m2)
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Fig.3.6: Inlet, outlet temperatures and solar radiation vs Time for 50 LPM
In, arrangements with TES, the outlet temperature increases as time
increases attains its peak at 14:50, 15:40, 14:30, 13:30 and 13:30 hrs for different
mass flow rates viz. 10, 20, 30, 40, and 50 LPM respectively and decreases
afterwards slowly. But as heat is stored in the PCM during the day time it is also
supplied to the working fluid during low insolation and/or in the absence of solar
radiation, generally after 16:30 hrs during winter time. The heat released by TES to
the working fluid is nearly up to 20:30 hrs, which gives back up of approximately four
hrs every day. As observed from Figs. (3.2-3.6), outlet temperature is good enough
even after 16:30 hrs, it is very useful for many low and medium temperature heating
applications. This shows the usefulness of thermal energy systems for such devices.
Also, as the mass flow rate increases the optimum temperatures also increases in all
0.0
250.0
500.0
750.0
1,000.0
10
40
70
100
130
10:00 13:00 16:00 19:00
Tem
pera
ture
s(°
C)
Time (hr)
Tin Thtm Tpcm T Is(W/m2)
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the three cases as can be seen from Figs. (3.2-3.6). It is also found that the optimum
outlet temperature of the working fluid at different mass flow rates is slightly higher in
the case of PCM than those of with hytherm oil, but, in general the optimum outlet
temperature with TES is better than those of without TES.
The solar radiation, first and second law efficiencies against time are shown in
Figs. (3.7-3.11). From the graphs, it is found that both the efficiencies increase as
the time increases in all the three cases (with and without TES). But there is some
fluctuations in the efficiencies as can be seen from these graphs, which is obvious
because of the fluctuation in solar radiation throughout the day. In case of temporary
storage material, both the efficiencies have peak at different times, than those
obtained without TES, If we see graphs with phase change material and hytherm oil,
have their peak at about 16:30, while it is in the first half in general, for those without
TES, besides, they are fluctuating in nature as can be seen from Figs. (3.7-3.11).
The peak with TES occurs at around 16:30hrs, which is due to the fact that solar
radiation goes down sharply, while 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 temporary storage increases sharply, and hence, the first and
second law efficiencies attain their peak during the late afternoon with some shifting
due to different mass flow rate.
But due to finite heat storage capacity and mass of the storing material, the
stored energy of TES decreases afterwards and hence, both the efficiencies in most
of the cases decrease at the same pattern, as can be seen from the graphs [Fig(3.7-
3.11)]. As mentioned above that the temporary storage material has finite heat
capacity and limited mass due the space available in the evacuated tubes, the
instantaneous fluctuation in solar radiation is compensated by the storage material
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which supplies heat at almost constant temperature. However, if the solar radiation
fluctuates more often and/or for longer time due to weather constraints, the
fluctuation is also found in the efficiencies of collector with TES. But in the case,
where there is no temporary storage, the fluctuation in both the efficiencies is found
to be more frequent and significant, as can be seen from Figs. (3.7-3.11). It is also
important to note that the solar radiation is given in kW/m2 in Figs. (3.7-3.11), which
is done to fit the efficiency and radiation curve on the same axis.
It is also noted that the fluctuation in the efficiencies is in phase where there is no
temporary storage material, while the fluctuation in both the efficiencies is in the
opposite phase where temporary storage is being used, as can be seen from Figs.
(3.7-3.11).
Fig. 3.7(a): Solar radiation and efficiencies vs time for 10 LPM flow rate with PCM
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Fig. 3.7(b): Solar radiation and efficiencies vs time for 10 LPM with hytherm oil
Fig.3.7(c): Solar radiation and efficiencies vs time for 10 LPM without TES
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Fig. 3.8(a): Solar radiation and efficiencies vs time for 20 LPM with PCM
Fig. 3.8(b): Solar radiation and efficiencies vs time for 20 LPM with hytherm oil
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Fig.3.8(c): Solar radiation and efficiencies vs time for 20 LPM without TES
Fig. 3.9(a): Solar radiation and efficiencies vs time for 30 LPM with PCM
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Fig. 3.9(b): Solar radiation and efficiencies vs time for 30 LPM with hytherm oil
Fig. 3.9(c): Solar radiation and efficiencies vs time for 30 LPM without TES
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Fig. 3.10(a): Solar radiation and efficiencies vs time for 40 LPM with PCM
Fig. 3.10(b): Solar radiation and efficiencies vs time for 40 LPM with hytherm oil
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Fig. 3.10(c): Solar radiation and efficiencies vs time for 40 LPM without TES
Fig. 3.11(a): Solar radiation and efficiencies vs time for 50 LPM with PCM
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Fig .3.11(b): Solar radiation and efficiencies vs time for 50 LPM with hytherm oil
Fig. 3.11(c): Solar radiation and efficiencies vs time for 50 LPM without TES
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It can be explained with the same physical significance that the thermal energy
storage material behaves as temperature regulator by supplying additional heat
during the fluctuation in solar radiation. From the observations, it is found that first
law efficiency is much higher than that of the second law efficiency, because exergy
represents the quality of energy which is obviously enhances with the increase in the
temperature unlike the quantity of energy. Also as explained by several authors [14-
18], exergy once lost is lost forever and cannot be recovered, unlike energy. Also the
exergy loss is more in the collector receiver-assembly and not in the low temperature
utility unlike energy. This results in more losses in exergy than that of energy and
hence, we found that the second law efficiency is much less than that of the first law
efficiency. The curves for second law efficiency are found to be smoother than that of
first law efficiency which may be due to the fact that exergy losses are less sensitive
to the input energy viz. solar radiation, while it is reverse in the case of energy
losses.
However as the mass flow rate of the working fluid increases, the efficiencies in
all the three cases increases which is due to the fact that the heat gain by the
moving fluid (viz. by the air) in the receiver tube increases, as a result, we get higher
output, resulting in the higher efficiency for all the cases. Also whatever the mass
flow rate, first law efficiency is always greater than that of the second law efficiency.
In case where temporary storage material has no been used, only one peak is
observed and it shifts towards origin for both the efficiencies as the mass flow rate of
working fluid is increased. This is due to the fact that role of temporary storage
material is significant in this way.
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Table 3.2: Sample calculation for different parameters for a typical set of operating conditions
Time
(hrs)
T1,in
(K)
Ts
(K)
To
(K)
m
(gm/s)
CP,i
(J/kg-K)
CPo
(J/kg-K)
k
(J/kg-K2)
a
(J/kg-K)
∆ s
(J/kg-K)
Qc
(W)
Ec
(W)
Ef
(W)
ηII
(%)
ηI
(%)
10:00 308 395 353 0.580 1005.8 1008.99 0.071 984.07 137.37 129.74 29.23 2.52 8.63 20.74
10:30 309 401 362 0.582 1005.9 1009.85 0.075 982.63 159.54 155.39 36.42 3.36 9.23 20.51
11:00 310 413 371 0.584 1005.9 1010.77 0.081 981.21 181.12 159.49 40.54 4.31 10.63 23.13
11:30 311 424 381 0.586 1006.1 1011.88 0.085 979.68 204.81 178.18 48.33 5.50 11.38 23.90
12:00 312 426 390 0.587 1006.2 1012.95 0.089 978.31 225.23 187.64 51.09 6.67 13.06 25.43
12:30 313 429 391 0.590 1006.1 1013.07 0.091 977.99 224.61 178.81 49.18 6.70 13.62 26.78
13:00 314 427 393 0.592 1006.1 1013.31 0.091 977.55 226.57 188.71 50.82 6.87 13.53 25.80
13:30 313 428 392 0.590 1006.1 1013.19 0.091 977.86 227.21 208.66 57.04 6.85 12.03 23.25
14:00 312 421 391 0.588 1006.0 1013.07 0.089 978.18 227.83 191.94 50.61 6.82 13.48 25.18
14:30 311 419 390 0.586 1006.2 1012.95 0.088 978.49 228.46 165.15 43.36 6.80 15.67 29.16
15:00 311 413 388 0.586 1006.1 1012.70 0.088 978.75 223.25 143.94 36.25 6.50 17.92 32.59
15:30 311 407 385 0.586 1006.2 1012.35 0.086 979.15 215.38 140.02 33.71 6.06 17.98 32.18
16:00 310 398 380 0.586 1005.9 1011.77 0.084 979.99 205.38 109.32 24.72 5.48 22.16 38.80
16:30 309 389 376 0.584 1005.9 1011.32 0.081 980.71 197.92 55.34 11.67 5.06 43.33 73.04
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In other words the peak observed around 13:00hrs (Fig.3.7c). In case of 10LPM,
while it is found to be 11:05hrs and 12:40hrs in case of medium mass flow rates.
However in cases where temporary storage have been used, both the efficiencies
attains their highest peak at near about 4:30hrs [Figs. (3.7-3.11)] for all mass flow
rates because of the fact that at this time, solar radiation goes down sharply and
heat is supplied by TES up to some reasonable duration.
Both the efficiencies of solar air heater collector using PCM are slightly better
than those in the case of hytherm oil. But in general, both the efficiencies with TES
have been observed to be better than those of without TES. Besides, as the
temporary storage material regulate the supply of heat at almost constant
temperature, both the efficiencies, are found to be smoother than that of without TES
as can be seen from Figs. (3.7-3.11). However both the efficiencies increases slowly
and attains peak at nearly 16:30hrs, and then decreases and again it increases
which is due to the fact that the heat stored in the PCM/hytherm is supplied by TES
which does not happen in the case of without TES. In case of empty collector tubes
the efficiencies once attain their peak in the first half, in general and then goes down
further as time increases because of the fact that solar radiation also decreases,
while it is found to be different in the case with TES.
3.3 Direct Flow Evacuated Tube Collector Solar Water Heater
Present section represents the exergy analysis of direct flow ETC based solar
water heater for which following materials and methods has been used.
3.3.1 Experimental Set-up and Procedure
In the present experimental study the direct flow evacuated tube collector
(ETC) solar water heater using energy and exergy analysis has been investigated.
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Total 9 numbers of ETC tubes have been arranged in the series and the
photographic view of the experimental set-up is shown in Fig.3.12 (a-b). The
evacuated tube collector and copper tubes have been used of the same dimensions
as mentioned in table-1. The glass wool was used to cover the copper tubes
exposed into the open air viz. outside the ETC tubes for insulation purpose to
reduce/minimize the heat loss to the ambient air. Calibrated J-type thermocouples
made of Copper-Constantine with temperature range -200°C to 1350°C were used to
measure air temperature at different state points.
In this arrangement, one thermocouple has been used for measuring the inlet
water temperature and other for measuring the outlet water temperature at the
defined state point. The outlet water temperature of the first tube is the inlet of the
second tube and the outlet of the second tube is inlet to the third tube and so on. To
measure water flow rate, a rotameter of 200 LPM capacity is used, which has been
placed before the inlet of ETC. Water was circulated through
Fig.3.12 (a): ETC based Solar water heater
Fig.3.12 (b): Hot water storage tank
Fig.3.12: Photographic view of experimental set-up
89
the system directly from the water tank situated at the roof of the building. For
measuring solar radiation Suryamapi has been used in the experiment. The
Suryamapi is kept on the horizontal surface nearby the experimental set-up in the
open air, so that no shadow and/or reflection of solar radiation from any other
surface/object falls on it. Data of temperatures at different state points has been
collected by using digital temperature displayer. The volume flow rate of the
circulating fluid was measured by volume flow meter before it enters the first tube.
Whenever fluid enter in the first tube its temperature rises, which can be identified by
measuring temperature with the help of thermocouple provided at inlet and outlet of
each tube. Data was collected from the system for few days in different months by
varying the volume flow rate of water and has been evaluated to find out the
efficiency of solar water heater.
3.3.2 Energy and Exergy Analysis
Same set of equations as mentioned in sections 3.2.2 and 3.2.3 have been used for
energy and exergy analysis of the present solar water heater. Based on the above
mentioned equations, calculations have been made for a typical set of operating
conditions and the sample calculation is given in Table-3.3.
3.3.3 Results and Discussion
Present experimental study is an attempt to investigate the performance
evaluation of direct flow evacuated tube collector (ETC) based solar water heater
using energy and exergy analyses. Flat plate collectors based solar water heaters
(SWH) were very famous initially but in the modern scenario solar water heaters
based on evacuated tube collectors (ETC) are of more interest because of low cost
90
and increased efficiency of the system. In conventional ETC based SWHs, storage
tank is at the top of the collectors and hot and cold water remains in the same tank.
Hot water always remains at the top of the storage tank and cold water at the bottom
level this is because of the density difference between hot and cold water. Solar
water heaters based on ETC used in the present study is different from conventional
ETC based SWH. In this system water at the inlet is taken from the storage tank
which is already situated at the roof of the building and hot water at the outlet is
stored in properly insulated water tank so that to avoid heat losses.
The experiments were performed for typical climatic conditions of Katra, India.
The experiments have been carried out in the winter season i.e. in the month of
January and February at different volume flow rates viz. 10, 15, 20, 25 and 30 litres
per hour (LPH). Data has been collected manually for a clear sky day of the months
January and February. The measured data includes solar radiation and
temperatures at different state of points (inlet, outlet and ambient). Based on the
collected data calculations for energetic and exergetic efficiencies have been made
and analysed. The discussion of results is given as below:
Figure 3.13, illustrates the variation in solar radiation and efficiencies (energy
and exergy) with respect to time for 10 LPH volume flow rate. From the figure it can
be seen that experiment starts at 11:30 AM when irradiation is quite good i.e. about
500 W/m2. From the figure it is also observed that solar radiation first increases with
time then remains almost constant for some time then again decreases which is an
obvious case in practice. It also seen from the figure that initially both the efficiencies
are low which is due to the fact that initially solar radiation is also low and also
temperature difference is less initially because it takes some time to warm the water.
91
Table 3.3- Energetic and exergetic efficiencies at 15 LPH volume flow rate for a typical set of parameters
Time (Hrs) Intensity (W/m2) Qin (W) Qo (W) Exin (W) Exo (W) η (%) ψ (%)
11:30 660 1,841.40 1,027.11 943.69 87.95 55.78 9.32 11:35 645 1,799.55 962.13 922.07 79.44 53.47 8.62 11:40 640 1,785.60 1,059.25 915.09 90.70 59.32 9.91 11:45 645 1,799.55 1,027.11 922.24 87.95 57.08 9.54 11:50 640 1,785.60 1,027.11 915.09 87.95 57.52 9.61 11:55 640 1,785.60 994.73 915.09 85.18 55.71 9.31 00:00 650 1,813.50 995.90 929.05 81.98 54.92 8.82 00:05 660 1,841.40 997.07 943.51 87.89 54.15 9.31 00:10 670 1,869.30 1,063.15 957.63 93.43 56.87 9.76 00:15 670 1,869.30 1,162.24 957.81 108.85 62.18 11.36 00:20 670 1,869.30 1,129.83 957.81 105.82 60.44 11.05 00:25 670 1,869.30 1,196.02 957.99 118.88 63.98 12.41 00:30 670 1,869.30 1,229.81 957.99 125.57 65.79 13.11 00:35 680 1,897.20 1,197.38 971.92 115.06 63.11 11.84 00:40 690 1,925.10 1,263.59 986.40 128.63 65.64 13.04 00:45 695 1,939.05 1,265.21 993.36 128.41 65.25 12.93 00:50 720 2,008.80 1,298.98 1,029.29 139.18 64.66 13.52 00:55 720 2,008.80 1,298.98 1,029.29 139.18 64.66 13.52 13:00 740 2,064.60 1,333.04 1,057.68 142.40 64.57 13.46 13:05 725 2,022.75 1,399.56 1,036.43 157.35 69.19 15.18 13:10 720 2,008.80 1,368.45 1,028.91 149.36 68.12 14.52 13:15 720 2,008.80 1,335.77 1,029.10 149.73 66.50 14.55 13:20 705 1,966.95 1,435.50 1,007.66 164.66 72.98 16.34 13:25 700 1,953.00 1,502.05 1,000.70 180.60 76.91 18.05 13:30 700 1,953.00 1,469.56 1,000.70 176.69 75.25 17.66 13:35 700 1,953.00 1,536.41 1,000.70 188.68 78.67 18.86 13:40 690 1,925.10 1,537.15 986.40 196.61 79.85 19.93 13:45 720 2,008.80 1,571.51 1,029.10 200.42 78.23 19.48 13:50 725 2,022.75 1,572.32 1,036.24 204.49 77.73 19.73 13:55 715 1,994.85 1,642.69 1,021.95 221.86 82.35 21.71 14:00 715 1,994.85 1,543.03 1,022.14 201.26 77.35 19.69 14:05 705 1,966.95 1,612.17 1,007.84 218.36 81.96 21.67 14:10 720 2,008.80 1,649.53 1,029.29 227.52 82.12 22.10 14:15 705 1,966.95 1,686.18 1,007.84 236.74 85.73 23.49 14:20 710 1,980.90 1,724.96 1,015.37 256.28 87.08 25.24 14:25 705 1,966.95 1,755.60 1,007.84 255.09 89.25 25.31 14:30 700 1,953.00 1,786.51 1,000.70 263.92 91.48 26.37 14:35 690 1,925.10 1,410.52 986.40 158.58 73.27 16.08 14:40 570 1,590.30 994.73 814.85 82.13 62.55 10.08 14:45 530 1,478.70 893.68 757.67 66.20 60.44 8.74 14:50 510 1,422.90 793.15 729.08 51.89 55.74 7.12 14:55 470 1,311.30 825.48 671.77 51.43 62.95 7.66 15:00 460 1,283.40 824.04 657.60 51.50 64.21 7.83 15:05 450 1,255.50 756.98 643.42 45.23 60.29 7.03 15:10 430 1,199.70 690.18 614.83 37.13 57.53 6.04 15:15 410 1,143.90 687.51 586.34 35.03 60.10 5.97 15:20 400 1,116.00 588.84 571.83 22.65 52.76 3.96 15:25 390 1,088.10 489.20 557.84 19.00 44.96 3.41 15:30 280 781.20 324.15 400.50 7.46 41.49 1.86
92
Input energy is low initially and the corresponding outlet temperature is also low
and therefore temperature difference (∆T) is low and hence corresponding energy
and exergy output is also low. As solar radiation increases both the efficiencies i.e.
energetic and exergetic increases with time while, solar radiation starts decreasing
after some point of time but both the efficiencies are still increasing in nature. These
efficiencies takes peak at a particular point of time then these also decreases slowly
afterwards. This variation in efficiencies with solar radiation can be explained in
terms of increased outlet temperature due to copper tube inserted inside ETC tubes.
As explained above, in this system copper tubes are inserted inside the ETC tubes
due to which temperature at the outlet doesn’t drop suddenly, it takes time and
hence we got better efficiency even if solar radiation decreases.
Fig.3.13: Solar radiation and efficiencies Vs time for 10 LPH volume flow rate
Variation in solar radiation and efficiencies with time for 15 LPH volume flow
rate has been shown in Fig.3.14. From the figure it is seen that initially irradiance
level is good i.e. approximately 660 W/m2 increases slowly and maintains good level
0
23
46
69
92
11:30 00:30 13:30 14:30 15:30
Eff
icie
ncie
s (
%)
Time (hr)
η ψ Is
93
of irradiance for a long period of time and then decreases suddenly. As explained
above, in this case also both the efficiencies increases as solar radiation increases
but drops suddenly as the corresponding solar radiation drops which is unlike the
case of 10 LPH volume flow rate. At 10 LPH after some time solar radiation
decreases but in this case as the solar radiation decreases both the efficiencies
suddenly decreases.
Fig.3.14: Solar radiation and efficiencies Vs time for 15 LPH volume flow rate
Figure 3.15 shows the variation in energy and exergy efficiencies and solar
radiation with time at 20 LPH volume flow rate. From the figure it is observed that for
this particular day solar radiation is very much fluctuating in nature. A mixed
response in the variation of efficiencies with solar radiation has been found in this
case. Sometimes as the solar radiation drops due to clouds corresponding
efficiencies also decreases suddenly. However, sometimes solar radiation is high
and the corresponding efficiencies are low. This mixed response in the variation of
0
185
370
555
740
0
23
46
69
92
11:30 00:50 14:10 15:30
Eff
icie
ncie
s(%
)
Time(hr)
η
ψ
Is
94
efficiencies is due to fluctuation in solar radiation which is intermittent in nature.
Variation in solar radiation and efficiencies with time at 25 LPH and 30 LPH has
been shown in Figs. 3.16 and 3.17 respectively. Same nature of variation in
efficiencies with solar radiation has been found as that of 10 LPH volume flow rate.
Fig.3.15: Solar radiation and efficiencies vs time 20 LPH volume flow rate
Fig.3.16: Solar radiation and Efficiencies vs time 25 LPH volume flow rate
0
185
370
555
740
0
17
34
51
68
10:50 11:50 12:50 13:50 14:50
Eff
icie
ncie
s(%
)
Time(hr)
η
ψ
Is
0
173
346
519
692
0
14
28
42
56
11:30 00:45 14:00 15:15
Eff
icie
ncie
s(%
)
Time(hr)
η
ψ
Is
95
Fig.3.17. Solar Radiation and efficiencies Vs time for 30 LPH volume flow rate
Comparative variation in energetic and exergetic efficiencies at different flow
rates is shown in Fig.3.18. From the figure it has been found that both the
efficiencies for 15 LPH volume flow rate is highest among all the flow rates at which
the present ETC based SWH has been experimented and analysed. The average
energetic and exergetic efficiencies at 15 LPH flow rate are 66.57 % and 13.38 %
respectively. As can be seen from the figure, as the volume flow rate increases 15
LPH onwards both the efficiencies decreases. Both the efficiencies for 10 LPH flow
rate are second best for the system and average energy and exergy efficiency has
been found to be 62.17% and 11.14 % respectively. Energy and exergy efficiencies
for 30 LPH flow rate has been found to least which are 30.36% and 2.22%
respectively. Energy and exergy efficiencies for 20 LPH were 45.45% and 5.24%
and for 25 LPH flow rate has were 35.49% and 3.30% respectively.
0
180
360
540
720
0.00
13.00
26.00
39.00
52.00
11:00 00:15 13:30 14:45 16:00
Eff
icie
nc
ies(%
)
Time(hr)
η
ψ
Is
96
Fig.3.18: Comparative variation in efficiencies at different volume flow rates
Therefore performance of direct flow ETC based solar water heater with nine
tubes has been found to be best at 15 LPH flow rate i.e. 15 LPH is the optimum flow
rate for nine tube ETC based solar water heating system. This is due to the fact that
at this flow rate output energy/exergy is better comparative to other flow rates
presented in this study therefore losses are low and ultimately SWH works better at
15 LPH volume flow rate. Also performance of this SWH has been found to be better
than that of other SWHs investigated by earlier authors [20-21]. This better
performance of the present system is due to the fact as explained above. Energy
efficiency has been found to be always higher than those of exergy efficiency. This is
due to the fact that energy efficiency is based on first law of thermodynamics and
doesn’t consider losses associated with the system. However, exergy efficiency is
based on second law of thermodynamics and considers the losses/irreversibilities
associated with the system. Therefore exergy represents quality of energy rather
than quantity.
0.00
17.00
34.00
51.00
68.00
10 15 20 25 30
Eff
icie
ncie
s (
%)
Volume flow rate (LPH)
η
ψ
97
The solar air dryer has been designed and fabricated in the central workshop of
the university to study the performance of such system with different modifications
for various applications. The site built was not brought from the market because of
the novelty of the idea and its design features. In general, the flat plate solar collector
are available in the market however, ETC based solar dryer are occasionally
available and also here copper tubes were inserted inside the tubes to study its
quality over those available in the market. The outlet temperature was measure to be
more than 150ºC which not only a uniqueness of this system but also a very useful
for many applications in real life. Further, to check the novelty of idea the same
system was converted into solar water heater with direct flow from the roof top water
tank and direct use/storage of hot water in a separate tank which may be filled with
PCM for later use. However, based on the requirements and site specification, the
optimization of design parameters can be carried out and the technology may be
transferred to the willing party for marketing the product.
3.4 Conclusions
The comparative study based on the first and second law analyses of a typical
solar air heater collector system with and without temporary thermal energy storage
has been carried out at different mass flow rates using hourly solar radiation. From
the present experimental study, some interesting results are found and can be
summarized as below:
I. The optimal outlet temperatures in the case of with thermal energy storage
are found to be higher than those without thermal energy storage. Also in
general, the optimal outlet temperature of arrangement with paraffin wax is
found to be higher than those of with hytherm oil for all the mass flow rates
with thermal energy storage.
98
II. It is found that there is fluctuation in both the efficiencies which is mainly due
to the fact that solar radiation also fluctuates throughout the day as can be
seen clearly from the figures given in this paper. Also as time increases, both
the efficiencies first increase and then decrease in case without temporary
storage material and the similar trend is found for the solar radiation.
III. In case of without temporary heat energy storage material, the efficiency
increases with time, attains its peak in the first half in general (Figs.2-6) and
then decreases after that. However in cases where temporary heat storage
material is used, both the efficiencies increase with time, attain their peak at
near about 16:30hrs with a small fluctuation with flow rate and then decreases
smoothly. This is due to the fact that the solar radiation sharply decreases in
the late afternoon and heat is supplied only by TES for some time. But due to
the limited mass and capacity of the storage material, this supply also
decreases slowly after a certain time. As a result, the stored heat energy
decreases smoothly, and hence, both the efficiencies again decrease towards
the late evening hours.
IV. As the mass flow rate increases peak of both efficiencies slightly shifts
towards origin in case without storage material/fluid. However, in case with
TES, there is a small shift in the peak due to due to different mass flow rate of
the working fluid except in case of 40LPM which is because of shift in the
peak of solar radiation during that particular day.
V. It is also noted that the fluctuation in the efficiencies is in phase where there is
no temporary storage material, while the fluctuation in both the efficiencies is
in the opposite phase where temporary storage is being used, as can be seen
from the graphs. It can be explained with the same physical significance that
99
the temporary heat energy storage material behaves as temperature regulator
by supplying additional heat during the fluctuation of solar radiation.
VI. From the observations, it is also found that first law efficiency is much higher
than that of the second law efficiency, because exergy represents the quality
of energy which is obviously enhances with the increase in the temperature
unlike the quantity of energy. Also as explained by several authors mentioned
in this paper. Thus exergy, which is the quality of energy, once lost is lost
forever and cannot be recovered, unlike energy. Also exergy loss is more in
the collector receiver-assembly and not in the low temperature utility unlike
energy. This results in more losses in exergy than that of the energy and
hence, we found the second law efficiency much less than that of the first law
efficiency.
VII. The curves for second law efficiency are found to be smoother than that of
first law efficiency which may be due to the fact that exergy losses are less
sensitive to the input energy viz. solar radiation, while it is reverse in the case
of energy losses.
Thus the results obtained in this study will be very useful and informative for real
life applications using the temporary storage in both the solar collector and in the
thermal energy utilities for better performance. Most importantly, where there is a
need of thermal energy without much fluctuation in the outlet temperature and heat
content out of the solar collector system for various applications of physical
importance.
Further analysis of direct flow ETC based SWH which was designed and
fabricated with copper tubes inserted inside ETC revealed that:
100
I. After some point of time solar radiation starts decreasing which is an obvious
nature of solar radiation, ideal case is that as solar radiation decreases the
corresponding energy and exergy efficiency should also decrease as there is
no thermal energy storage in the system. But with copper tubes inserted
inside the ETC efficiencies increases for some time which ultimately increases
the overall system efficiency.
II. Optimum flow rate of the present SWH has been found to 15 LPH i.e. this
SWH performs better at 15 LPH among the other flow rates analysed and
presented in this study.
III. The energetic and exergetic efficiencies at 15 LPH flow rate has been found
to be 66.57 % and 13.38 % respectively.
IV. In the present SWH hot water at the outlet is directly collected to well
insulated storage tank and water at inlet is taken from the storage tank
already available at the roof of the building due to which no mixing of water
occurs and we got enhanced efficiency of the SWH.
V. Also heat exchanger is made inside the hot water storage tank so that water
remains hot for a longer duration of time and can be used in the night time
when there is no availability of solar radiation.
VI. This SWH is very useful for domestic applications due to high efficiency also if
this SWH is applied to larger scale may be useful for industrial applications as
well.
Other researchers can use the outcomes of this study for designing of different
solar thermal devices for real life applications depending on the site and
requirements of the end users. Further, The concept of energy and exergy analysis
101
can also be applied to other solar thermal systems to study the realistic performance
of such devices and other energy conversion systems as well.
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