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Evacuated tube solar heat pipe collector model and associated testsFarzad Jafarkazemiand Hossein AbdiCitation: J. Renewable Sustainable Energy 4, 023101 (2012); doi: 10.1063/1.3690958View online: http://dx.doi.org/10.1063/1.3690958View Table of Contents: http://jrse.aip.org/resource/1/JRSEBH/v4/i2Published by theAmerican Institute of Physics.Related ArticlesTheoretical and experimental investigation of Alfa type bio mass Stirling engine with effect of regeneratoreffectiveness, heat transfer, and properties of working fluidJ. Renewable Sustainable Energy 4, 043126 (2012)Performance analysis and parametric optimum criteria of the nanothermoelectric engine with a single-levelquantum dot at maximum powerJ. Appl. Phys. 111, 094318 (2012)Optimization of power and efficiency of thermoelectric devices with asymmetric thermal contactsJ. Appl. Phys. 111, 024509 (2012)Performance of copper coated two stroke spark ignition engine with methanol-blended gasoline with catalyticconverterJ. Renewable Sustainable Energy 4, 013102 (2012)
A high performance thermoacoustic engineJ. Appl. Phys. 110, 093519 (2011)Additional information on J Renewable Sustainable EnergyJournal Homepage: http://jrse.aip.org/Journal Information: http://jrse.aip.org/about/about_the_journalTop downloads: http://jrse.aip.org/features/most_downloadedInformation for Authors: http://jrse.aip.org/authors
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Evacuated tube solar heat pipe collector model andassociated tests
Farzad Jafarkazemia) and Hossein Abdib)
Department of Mechanical Engineering, South Tehran Branch, Islamic Azad University,P.O. Box 1584743311, Tehran, Iran
(Received 10 December 2011; accepted 8 February 2012; published online 6 March 2012)
In this paper, an evacuated solar heat pipe collector is investigated theoretically
and experimentally. Heat transfer formulas were used for theoretical modeling, and
a test method was adopted from ISO 9806-1 to compare the theoretical model with
the experimental results. The collector efficiency and useful heat gain were com-
pared between the theoretical and experimental methods. The effect of the working
fluid flow rates and collector area were also investigated and discussed. The com-
parison shows that the theoretical model is in good agreement with the experimen-
tal results and is capable of predicting the efficiency, useful heat gain, and working
fluid outlet temperature of an evacuated heat pipe collector with good accuracy.VC 2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.3690958]
I. INTRODUCTION
Currently, most of the global energy demand is met by fossil fuels, but the massive exploi-
tation of fossil fuels leads to a real threat to the environment from global warming and acidifi-
cation of the water cycle. Rapidly increasing energy requirements, reduced availability of tradi-
tional sources of energy, and environmental pollution have forced scientists to search for
alternative energy sources. Sun and wind are among those energy resources that are effectively
unlimited and are available in abundant amounts and all over the world at no cost.
The average solar irradiation for the whole of Iran is approximately 5.3 kWh per square
meter per day, and it is even higher in the central part of the country. The amount of useful so-
lar radiation hours in Iran exceeds 2800 h per year.1
To convert the solar radiation energy into a more usable or storable form, solar collectors
are used. These devices constitute the core of solar heating systems and are available in differ-
ent forms and designs. Evacuated solar heat pipe collectors represent a novel design concept
that has a low loss coefficient.
The literature contains studies on evacuated and heat pipe solar collectors and the effect of
different parameters on their performance. Mahdjuri2 first introduced a tubular evacuated solar
collector with rectangular performance characteristics. He introduced a heat pipe cycle to trans-
fer heat from the absorber to the water tubing. Ortabasi and Fehlner3 used a solar thermal heat
pipe collector based on an internal cusp concentrator. The performance of the collector was
compared to that of an evacuated, selectively coated flat-plate absorber equipped with flow-
through heat transfer. Garg et al.4 have discussed the state of the art for evacuated tubular solar
energy collectors. They have also attempted to analyze the optical and thermal behavior of this
evacuated collector. Ezekwe5 employed a mathematical model to analyze and compare solar
energy systems that useheat pipe absorbers with systems that use conventional solar collectors.
Zambolin and Del Col6 and Fischeret al.7 tested different collectors with standard testing meth-
ods, such as EN, ISO, and ASHRAE. EN 12975-2 (Ref. 8) standard is the latest standard in
these series. Ng et al.9 conducted performance tests under steady state conditions for two types
a)Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: 0098(912)
3499232. Fax: 0098(21)66572717.b)
Electronic mail: [email protected].
1941-7012/2012/4(2)/023101/13/$30.00 VC 2012 American Institute of Physics4, 023101-1
JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 4, 023101 (2012)
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http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958http://dx.doi.org/10.1063/1.3690958 -
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of evacuated tube solar heat pipe collectors. A theoretical model was also presented to predict
the collector efficiency. Azad10 designed and constructed a heat pipe solar collector and meas-
ured its performance in an outdoor test facility. The thermal behavior was investigated theoreti-
cally and experimentally. The theoretical model was based on the effectiveness-NTU method.
Ayompeet al.11
presented year-round energy performance monitoring results for two solar waterheaters (SWHs) with flat plate and heat pipe evacuated tube collectors (ETCs). The energy pe r-
formance of the two systems was compared on a daily, monthly, and yearly basis. Hayek et al.12
conducted an experimental investigation of the overall performance of two kinds of evacuated
tube solar collectors, specifically, the water-in-glass tube and the heat-pipe designs. Jafarkazemi
et al.13 reviewed the international standards for determining the thermal performance of solar
thermal collectors and solar water heating systems and introduced the details of a test center
which was under construction in Iran. Badaret al.14 investigated the overall heat loss coefficient
(U-value) of a vacuum tube solar collector experimentally and theoretically with regard to the
pressure of the remaining gas inside the evacuated glass envelope. Tang et al.15
constructed and
tested two sets of water-in-glass evacuated tube SWH for performance comparative study. Both
SWHs were identical in all aspects but had different collector tilt-angle from the horizon with
the one inclined at 22 and the other at 46. Zambolin and Del Col16 introduced an improved
procedure for the experimental characterization of optical efficiency in evacuated tube solar col-lectors. The new method does not require a minimum number of data points for each data subset
and thus it is less demanding in terms of required number of tests.
In this study, an evacuated heat pipe solar collector with a circular fin and a dry condenser
has been theoretically modeled by heat transfer formulas, and its efficiency and heat gain dia-
grams are compared with the results of experimental tests. The test procedure was adapted
from the ISO 9806-1 (Ref. 17) test procedure. Additionally, the variation of the working (cool-
ing) fluid flow rate and its effect on the efficiency and useful heat gain has been discussed, and
the experimental and theoretical results are also presented.
II. THERMAL ANALYSIS
The heat pipe is a device with very high thermal conductance. The main components of
the heat pipe are the evaporator, the condenser, and the contained working fluid. When theevaporator is heated, the working fluid is evaporated as it absorbs an amount of heat equivalent
to its latent heat of vaporization. In the condenser section, the working fluid vapor is condensed
by a cooling fluid. The method of condensate return is dependent on the heat pipe structure.
Condensate return techniques include capillary force and gravity, among others. This cycle con-
tinues as the evaporator is heated.18,19 A solar heat pipe collector consists of a row of heat
pipes that are connected to a manifold on the top that transfers the heat produced in the pipes
to the working fluid. A schematic of a wickless heat pipe that uses gravity for condensate return
and an evacuated heat pipe collector schematic are shown in Figs. 1and2, respectively.
In this study, an evacuated tube solar collector with a heat pipe has been modeled. The con-
denser part of the collector in the manifold was of the dry type. There are many advantages to the
use of heat pipes for solar thermal applications. They have no moving parts, and if the condenser is
of the dry type, maintenance difficulties are reduced because a damaged heat pipe can be changed
while the working fluid is circulating. Moreover, leakage problems are reduced because of the indi-rect contact between the heat pipe and the condenser.9 Water was chosen as the working (cooling)
fluid because the local ambient temperature did not drop below its freezing point during the test. For
simplicity, the following assumptions were made in the theoretical model:
The collector absorber area is always at a uniform temperature. The temperature gradient across the absorber thickness and over its perimeter is neglected. Because the evaporator section is subjected to a constant heat flux and the phase change occurs
at a constant temperature, the temperature gradient along the longitudinal direction is negligible. The thermal contact resistance between the absorber area and the evaporator tube and between
the condenser section and the condenser manifold is neglected. The connection of the evaporator and the condenser of the heat pipe is assumed to be adiabatic.
023101-2 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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The thermal resistance model that has been used is shown in Fig. 3. In the figure, I repre-
sents the solar radiation that reaches the collector surface, and I (sa) represents the radiation
that reaches the absorber. A small part of the gained heat is wasted by radiation, which is
shown by Q loss.rad. The useful heat gain, Qu, is calculated by subtracting the heat pipe collector
heat gain, Qhp, from the manifold heat loss from the top of the collector, Qloss,ma. Rloss,radis the
radiation thermal resistance, which can be calculated and represented as follows:
Rloss;rad TpTa
erT4p T4a Ar
: (1)
The evaporator thermal resistance is represented by Rhp and is a combination of three resistan-
ces. These resistances are the circular fin resistance, the copper pipe resistance, and the boiling
film condensation resistance inside the evaporator pipe. Thus, Rhp may be written as follows20:
RhplnDo;fin=Di;fin
2pkfinLevap
lnDo;evap=Di;evap
2pkevapLevap
1
hhpAevap; (2)
where
hhp 0:555gqlql qvk
3lhfg
llTpTkDevap
14
: (3)
Rcond,w is considered to be the thermal resistance of the condenser section, which is the sum of
the condensing film resistance in the condenser, the condenser copper pipe section, and the dry
manifold condenser resistance. The thermal contact resistance in the dry condenser is assumed
to be negligible. Rcond,wmay be written as:20
Rcond;w 1
hcondAcond
lnDo;cond=Di;cond
2pkcondLcond
lnDo;ma=Di;ma
2pkmaLma; (4)
FIG. 1. Heat pipe schematic.
023101-3 Evacuated tube solar heat pipe collector J. Renewable Sustainable Energy 4, 023101 (2012)
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where
hhp 0:555gqlql qvk
3lhfg
llTpTkDcond
14
: (5)
The manifold resistance is also calculated by the common heat transfer formulas, as
1/UmaAma,loss.
The heat transfer balance is written using the thermal resistance model shown in Fig. 3, as
follows:
_qin _qloss;rad _qhp _qloss;rad _qu _qloss;ma: (6)
The expressions for the temperatures, resistances, and radiation are used in Eq. (6), and it yields
the following:
FIG. 2. Evacuated heat pipe collector schematic.
FIG. 3. Thermal resistance model for an evacuated heat pipe collector.
023101-4 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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IsaArTpTa
Rrad _qu
TfTaRloss;ma
: (7)
To simplify the derivation process, Rcond;w=Rrad is replaced by Rcrand Rhp=Rrad is replaced by
Rhr. From the thermal resistance model, the following expression may be written:
_qhp TpTf
RhpRcond;w; (8)
_qhp _qu _qloss;ma _quTfTaRloss;ma
: (9)
Using the energy balance in the thermal resistance model and Eqs. (8) and (9) to simplify the
useful heat gain equation, the following may be written,
_qu IsaAr
1Rhr Rcr
TfTa
1RhrRcr
1
Rrad
1
Rloss;ma1RhrRcr : (10)
Equation(10)may be written as
_qu F0ArIsa ULTfTa; (11)
where
F0 1
1RhrRcr; (12)
and
UL 1
RradAr
1
Rloss;maArF0: (13)
The collector heat removal factor can be calculated by the following equation:21
FR _mCp
ArUL1exp
ULArF0
_mCp
: (14)
FR is similar to heat exchanger effectiveness and is defined as the ratio of the actual heat trans-
fer to the maximum possible heat transfer. The maximum possible heat transfer occurs when
the collector is at the working fluid inlet temperature.21
The actual collector useful heat gain may be calculated from the following:
_qu ArFRIsa ULTf;iTa: (15)
By incorporating the useful heat gain value, the heat gain of the working fluid in the manifold
can be expressed as _qu _mCpTfTo, where To represents the outlet temperature of theworking fluid and will be determined.
The instantaneous efficiency of a collector, which is the ratio of the useful energy to the
solar radiation, can be expressed as follows:21
g _qu=ArI FRsa FRULTf;iTa=I: (16)
023101-5 Evacuated tube solar heat pipe collector J. Renewable Sustainable Energy 4, 023101 (2012)
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III. EXPERIMENTAL TEST AND PROCEDURE
Fig.4shows a schematic of the experimental configuration used for the efficiency and per-
formance evaluation of an evacuated heat pipe collector. The test facility and weather station
are located on the roof of solar energy laboratory building of the Islamic Azad University
South Tehran Branch. The collector tilt angle was set to 45 , facing south. The latitude and lon-
gitude of the test center are 35.6 N and 51.4 E, respectively. A picture from the test facility
is shown in Fig.5.
The test plan for testing the collectors was adopted from ISO 9806-1 because it is known as
an accepted standard in Iran. The specifications of the system components are described below.
The reservoir tank volume was 150 l, and the tank was made of galvanized steel. Calibrated Pt-
100 temperature sensors were used to measure the inlet and outlet fluid temperatures of the col-
lector and the reservoir tank. In order to control the tank fluid temperature, two 2 kW electric
heaters and one 1 kW heater were used. A calibrated flow meter with a range of 20-200 l/h was
used to measure the inlet working fluid flow rate. The heaters were controlled by a solid state
relay (SSR) controller. A proportional integral derivative (PID) temperature controller was used
for this purpose. The pyranometer, ambient temperature probe, and wind velocity sensor are all
calibrated against reference instruments and supplied by Soldata Instruments. The weather datawere logged at intervals of 10 min during the tests. The general specification of the tested collec-
tor, which is a heat pipe solar collector with 19 heat pipes, is presented in Table I.
The test procedure which is used to test the performance and efficiency, according to ISO
9806-1, is as follows. First, the temperature of the reservoir tank is kept constant with the heat-
ers and controller. The water tank temperature is set to different values during each test period.
The minimum temperature is set to the ambient temperature, and the other temperatures are set
higher for the subsequent data points. Using the flow control valve, the flow is set to a constant
rate, and after 10 min, when the outlet temperature is constant, the collector outlet temperature
FIG. 4. Schematic diagram of the test set-up.
023101-6 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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is recorded. This procedure is repeated for five other temperatures in each test period with steps
of approximately 8 C. The test procedure is also repeated for two other flow rates. The time
periods are chosen such that the data points represent times symmetrical to the solar noon. In
this study, the test is performed for two collectors, one with 19 heat pipes and the other with 4
heat pipes. The experimental efficiency and the useful heat gain of each solar collector are
compared to the theoretical calculated values to evaluate the accuracy of the model. The effect
of the three flow rates on the efficiency and heat gain is discussed after the test procedure was
completed.
IV. RESULTS AND DISCUSSION
The test for evaluation of the heat pipe performance was performed in August 2011. The
climate conditions on August 8 and August 15, 2011 when the tests were performed are shown
in Fig.6.
The tests were performed for 19 heat pipes and 4 heat pipes on days 1 and 2, respectively.
The flow rates were set to 0.056, 0.042, and 0.028 kg/s for the 19 heat pipe collector and 0.014,
0.010, and 0.007 kg/s for the 4 heat pipe collector for the tests. These are based on requirement
of ISO 9806-1, which recommends a flow rate of 0.02kg/s/m2 for collector testing. Other flow
rates were chosen to compare the flow rate effect on heat gain and efficiency.
FIG. 5. Test facility.
TABLE I. Specifications of an evacuated collector.
Parameter (unit) Value
Aperture area (m2
) 2.50
Absorber area (m2) 1.78
Emittance 0.07
Tube spacing (m) 0.078
Tube length (m) 1.7
Condenser O.D. (m) 0.024
Condenser length (m) 0.065
Evaporator O.D. (m) 0.008
Absorber absorptance 0.93
Glass cover transmittance 0.90
023101-7 Evacuated tube solar heat pipe collector J. Renewable Sustainable Energy 4, 023101 (2012)
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The theoretical and experimental efficiencies, useful heat gains and inlet and outlet temper-
atures for the collector with 19 heat pipes and a flow rate of 0.042 kg/s of working fluid is
shown in Table II as an example. For each working fluid flow rate, this table should be madeavailable before processing the data.
The collector flow factor, F00, which is the ratio ofFR to F0, is a function of a single vari-
able, the dimensionless collector capacitance rate _mCp=ArULF0. This factor is shown in Fig. 7
for a working fluid flow rate of 0.028 kg/s and a collector with 19 heat pipes. The theoretical
(calculated) collector outlet temperatures and experimental (measured) collector outlet tempera-
tures are shown in Fig. 8. The data points shown correspond to the collectors with both 4 heat
pipes and19 heat pipes and all six flow rates. The predicted values are in good agreement with
the experimental results to within an error of less than 5%, which means that the theoretical
model is acceptable.
Figs.9and10 show the rise in the working fluid temperature for different flow rates. These
figures indicate that the experimental outlet and inlet temperature difference increases as the
working fluid flow rate decreases. Additionally, while the ratio of the temperature difference to
the incident radiation increases, the experimental outlet and inlet temperature difference also
decreases. This result indicates that in order to reach the maximum outlet temperature, the opti-
mum condition is to have a reduced working fluid flow rate and a smaller difference between
the inlet and ambient temperatures. The slopes of the diagrams are nearly the same, which
means that the results do not depend on the collector area or the number of heat pipes.
The theoretical collector heat gain and the experimental results during the testing day for
different flow rates are shown in Figs. 11 and 12. The theoretical and experimental heat gains
are shown by lines and rectangular diagrams, respectively. The model prediction is close in
value to the experimental results at most data points. As expected, when the difference between
the inlet and ambient temperatures increases, the useful heat gain decreases, and vice versa.
FIG. 6. Climate conditions for the two test days (a) August 8 and (b) August 15.
TABLE II. Data table for the collector with 19 heat pipes and a working fluid flow rate of 0.042kg/s.
Time Temp (C) Efficiency (%) Useful heat gain (W) Radiation
Hour Tinlet
Experimental
Toutlet
Theoretical
Toutlet Tamb Theoretical Experimental Theoretical Experimental
I
(W/m2
)
11:25 75.1 77.4 78.69 34.4 46.39 29.67 625.87 400.22 757.9
12:00 67.9 71.4 71.93 36 48.88 42.43 701.57 609.04 806.4
12:25 59.7 63.4 64.01 36.5 50.98 43.76 750.13 643.84 826.6
13:00 52.5 56.7 56.93 37.8 52.76 49.91 772.47 730.85 822.6
13:30 44.7 48.5 49.33 37.7 54.54 44.72 806.50 661.24 830.7
14:10 40.7 44.8 45.19 39.4 55.63 50.72 782.45 713.45 790.2
023101-8 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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FIG. 7. Collector flow factor.
FIG. 8. Theoretical model and comparison with experimental results for outlet temperature.
FIG. 9. Variation in the working fluid temperature in a collector with 19 heat pipes for different flow rates.
023101-9 Evacuated tube solar heat pipe collector J. Renewable Sustainable Energy 4, 023101 (2012)
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FIG. 10. Variation in the working fluid temperature in a collector with 4 heat pipes for different flow rates.
FIG. 11. Comparison of theoretical and experimental heat gain for a collector with 19 heat pipes.
FIG. 12. Comparison of theoretical and experimental heat gain for a collector with 4 heat pipes.
023101-10 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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This result indicates that more heat can be gained from the collector when the inlet and ambient
temperature difference is kept near zero.
Figs. 13 and 14 show a comparison between the efficiency predicted using the theoretical
model vs. that obtained using the experimental results for the two collectors at different flow
rates. The experimental results are close in value to the predicted results of the model for both
collectors. As the flow rate of the working fluid and the number of heat pipes increase (Fig.
13), efficiency decreases. On the other hand, as shown in Fig. 14, for a collector with smaller
area and reduced working flow rate, the rate of efficiency decrease (slope) is higher. This result
indicates that it is possible to reach higher efficiencies by decreasing the collector aperture
area, but the substantial rate at which the efficiency decreases should be considered. The instan-
taneous efficiency decreases as the ratio of the temperature difference to the incident radiation
increases in both diagrams. The values of FRUL and FR(sa) are estimated for both collectors.FRUL is the slope of the straight regression line, and FR(sa) is the value of the point of intersec-
tion with the y-axis in the region of maximum efficiency. By linear curve fitting of the data
points for the collector with 19 heat pipes, the experimental values of FRUL for flow rates of
0.056, 0.042, and 0.028 kg/s are equal to 3.77, 3.36, and 3.45, respectively, while the theoretical
model yields 1.87, which was approximately constant for all flow rates. The FR(sa) values that
FIG. 13. Predicted efficiency from the theoretical model vs. efficiency determined from the experimental results for the
collector with 19 heat pipes.
FIG. 14. Predicted efficiency from the theoretical model vs. efficiency determined from the experimental results for the
collector with 19 heat pipes.
023101-11 Evacuated tube solar heat pipe collector J. Renewable Sustainable Energy 4, 023101 (2012)
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are determined from the experimental results are equal to 0.55, 0.52, and 0.51 for flow rates of
0.056, 0.042, and 0.028 kg/s, respectively, while the theoretical model yields 0.56, which was
approximately constant for all flow rates. For the collector with 4 heat pipes, the experimental
value of FRUL for flow rates of 0.014, 0.010, and 0.007 kg/s are 8.86, 8.85, and 10.03, respec-
tively, while the theoretical model yields 8.5,which was approximately constant for all flowrates. The experimental FR(sa) values are 0.73, 0.72, and 0.76 for flow rates of 0.014, 0.010,
and 0.007 kg/s, respectively, while the theoretical model yields 0.79, which was approximately
constant for all flow rates.
V. CONCLUSIONS
In this study, a comparison between the theoretical and experimental results for two collec-
tors with different numbers of heat pipes and different aperture areas were made. The results
led to the following findings.
The theoretical model is in good agreement with the experimental results and is capable of
predicting the efficiency, useful heat gain, and working fluid outlet temperature of an evacuated
tube heat pipe collector with good accuracy.
The effect of the flow rate on the efficiency and heat gain were also discussed theoreticallyand experimentally. It is found that decreasing the flow rate leads to a higher outlet temperature
for an evacuated tube heat pipe collector.
It has been shown that the efficiency of the collector decreases as the ratio of the inlet tem-
perature to the incident radiation increases. It is recommended that the inlet water temperature
is kept as near as possible to the ambient temperature to gain more heat and higher efficiency.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial support of Islamic Azad University, South
Tehran Branch (under Contract No. B/16/561).
NOMENCLATURE
Ar collector area (m2
)Cp specific heat capacity (J/kg K)
D diameter (m)
F0 collector efficiency factor
F00 collector flow factor
FR collector heat removal factor
h heat transfer coefficient (W/m2 K)
I incident solar radiation (W/m2)
k thermal conductivity (W/m K)
_m mass flow rate (kg/s)
_q heat transfer rate (W)
R resistance (W/K)
t wall thickness (m)
T temperature (K)UL overall heat transfer coefficient of collector (W/m
2 K)
emittance coefficientg efficiency
l dynamic viscosity (N s/m2)
q density (kg/m3)
sa transmittance absorbance product
SUBSCRIPTS
a ambient
cond condenser
023101-12 F. Jafarkazemi and H. Abdi J. Renewable Sustainable Energy 4, 023101 (2012)
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evap evaporator
exp experimental or measured
f working fluid
g gas
hp heat pipei inlet
l liquid
ma manifold
o outlet
p plate
rad radiation
u useful
w wall
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