Indirect Coper Metal
Indirect Coper Metal
Indirect Coper Metal
Renewable Energy
journal homepage: www.elsevier.com/locate/renene
a r t i c l e i n f o a b s t r a c t
Article history: In the present work the efficiency of a solar parabolic trough has been investigated experimentally. An
Received 5 August 2016 absorber filled with metal foam is used in order to improve the heat transfer and increase the efficiency
Received in revised form of PTC (parabolic trough collector). The porosity of copper foam is 0.9 and the pore density is 30 PPI
28 December 2016
(pores per inch). The experiments were performed in different volume flow rates from 0.5 to 1.5 Lit/min
Accepted 2 February 2017
Available online 7 February 2017
and the standard of ASHRAE 93 was used to test the solar collector’s performance. Friction factor and Nu
number have been investigated for both cases. It was found that by increasing the mass flow rate, the
efficiency of the collector was enhanced and the same pattern can be seen when absorber filled with
Keywords:
Solar parabolic trough collector
copper foam. When absorber filled with metal foam the overall loss coefficient UL decreases by 45% and it
Metal foam causes to increase efficiency because less energy is lost.
ASHRAE 93 standard © 2017 Elsevier Ltd. All rights reserved.
Thermal efficiency
Thermal conductivity
1. Introduction no limitation of the air speed is assumed. Wang et al. [12] studied
the impact of a secondary reflector which used as a homogenizing
Heat transfer phenomena are applied in many industrial devices reflector on the efficiency of a parabolic trough solar collector. the
such as heat exchangers in petroleum engineering, filtration, geo- Monte Carlo ray-trace (MCRT) method is applied in order to esti-
mechanics, and solar energy [1e5]. Heat transfer in porous media mate the concentrated solar flux distribution and uniform solar flux
has been used for heat transfer enhancement because of its distribution on the absorber tube was found. Results illustrated that
considerable advantages of high solid thermal conductivity and the efficiency of solar collector declines because of enhancement
large specific area [6,7]. Parabolic trough collector (PTC) is one of optical loss. The performance of parabolic trough collector using
the linear concentrating solar collectors which are appropriate for Al2O3-C2H6O2-H2O nanofluid as a working fluid have been done by
working in the range of 150e400 C [8,9]. Mainly, a parabolic Ajay et al. [13]. They observed an improvement in the overall per-
trough collector is made of a parabolic trough mirror and a receiver formance of solar collector by increasing volume flow rate of
in the focal line of the reflector in order to absorb the reflected working fluid and good agreement was found between experi-
radiation from the sun. The concentrated radiation heats the fluid mental and CFD result. The numerical study of the flat and tubular
that moves through the absorber tube; as a consequence, the solar solar collectors was investigated by Nimr and Alkam [14,15]. The
radiation is converted to thermal energy [10]. Kasperski and Nems duct of the solar collectors partially filled with porous media sub-
[11] investigated the effect of an internal multiple-fin array strates at a laminar flow state.
arrangement of solar air heater on a thermo-hydraulic efficiency of Several experimental works consider the heat transfer and
the linear concentrating collector. Results illustrated that there is an pressure drop measurements during single phase forced convec-
optimum value for a thermo-hydraulic efficiency against the tion through metal foams. The effect of copper metal foam on the
volume-flux, fin thickness and duct width. Moreover, 14% heat transfer and pressure drop in tube has been investigated by
improvement in the thermo-hydraulic efficiency is observed when Abadi and Kim [16]. High-porosity metal foams have been used in
order to increase the heat transfer area between the fluid and solid
phase. Results indicated that Reynolds number and metal foam
* Corresponding author. geometry have a significant effect on the pressure drop and heat
E-mail addresses: miladtajik6@gmail.com, m-TajikJamalabad@semnan.ac.ir transfer coefficient. The heat transfer in aluminum foam baffles
(M.T. Jamal-Abad).
http://dx.doi.org/10.1016/j.renene.2017.02.004
0960-1481/© 2017 Elsevier Ltd. All rights reserved.
M.T. Jamal-Abad et al. / Renewable Energy 107 (2017) 156e163 157
installed on the top and on the bottom walls was studied experi- foam. Two PT100 type thermocouples were inserted into the flow
mentally by Hoon Ko et al. [17]. They found that the heat transfer at the inlet and exit of the test section to measure the bulk tem-
enhancement ratio was increased with the baffle thickness and the peratures of the water. Also, a portable thermocouple used to
grade, but the friction factor was slightly decreased with an in- measure the ambient temperature. All thermocouples were cali-
crease of the Reynolds number, and increased with baffle thickness. brated and the error of deviation at measuring temperatures was
The thermalperformance of six different copper alloy foams have around 0.1 C. The flow rate of the fluid in the collector was
been investigated by Zhao et al. [18] and the heat transfer co- measured using a flow meter which was mounted in a vertical
efficients and pressure drops have been measured. They introduce position. Two differential pressure transducers (accuracy of ±2 Pa)
an optimal porosity based on balance between pressure drop and have been used in order to measure the pressure drop. One placed
overall heat transfer and found that the effect of cell size is more is in the inlet of the absorber and the other one is located at the end
effective on the overall heat transfer than porosity. of the absorber. The level of the heat flux was determined using a
Using metallic foams for heat transfer enhancement is a novel Solar Power Meter TES-1333R and the temperature values were
method. Copper foams have great potential in heat-transfer-related measured using a data logger. Also, a pump mounted at the outlet
applications such as solar collectors where the enhancement of of the tank which continuously circulates the working flow from
heat transfer cause to increase the thermal performance of the the outlet and back into the inlet. The detailed specifications of the
collector. Hence, efficient and compact absorber (heat exchanger) data for the mirror reflector are listed in Table 1.
and PTC will be designed which are needed in many engineering The ASHRAE 93 [20] was used for evaluating the testing solar
applications and reduce the initial cost of PTC power plant. In this collector. The specific environmental conditions required by the
study, the performance of a solar trough collector by using a metal ASHRAE 93 in performing the thermal efficiency test are listed in
foam absorber has been studied. A new solar trough collector was Table 2. Furthermore, during the testing period, steady-state con-
designed and fabricated in order to study the collector efficiency ditions should be maintained. Table 3 illustrates the acceptable
and parameters which are affected by it. The absorber of solar maximum variation of variables which define a steady-state con-
collector has been filled by a copper metal foam and its outer sur- dition in accordance to ASHRAE 93. After reaching steady state
face has been painted by black color. The thermal performance of conditions, the data for each the experiment are averaged and used
solar collector has been evaluated under ASHREA 93 standard. The in the analysis as a single point while other data were rejected.
thermal conductivity of porous media, the Nu number, the friction These two conditions are checked throughout the test period.
factor and thermal efficiency of receiver tubes in two different cases The difference between the maximum and minimum solar irradi-
have been analyzed and compared. Finally, the reasons for ance upon the collector must be less than 64 W/m2 during any
enhancement of the solar collector efficiency were explained. 20 min interval within the test period. For the fixed test mount
measurements must be taken symmetric to solar noon according to
2. Experimental setup ASHRAE 93 further reducing the number of test days. Closed loop
collector test rings are shown in Fig. 1.
2.1. Experimental setup Copper Foam is low density permeable material and has excel-
lent thermal and electrical conductivities and good corrosion
A parabolic trough reflector with two types of absorber wit and resistance. This means that copper’s high thermal conductivity al-
without metal foam is investigated experimentally. A parabolic lows heat to pass through it quickly Hence, it can be used in
reflector was designed with the length of 1.28 m and aperture numerous applications and industries.
width of 1 m. The reflector was made of steel mirror with a thick- The foam structure can be described by porosity ε and the
ness of 1 mm (Table 1). The rim angle of this prototype was selected number of pores per inch PPI; the porosity ε is defined as the ratio
as 90 and, as reported by Valan Arasu and Sornakumar [19], this of total void volume to the total volume occupied by the solid
degree represents a suitable rim angle. Seven polyglass fixtures are matrix and void volumes, while PPI is easily obtained by counting
applied in order to support of the parabolic reflector. The reason is the number of pores in 25.4 mm. The length of porous media in
that polyglass density is 1.19 gr/cm3 and very close to water density absorber is 1250 mm which is formed by connecting five pieces of
and have perfect resistance against the wind. copper foam.
A steel tube is used to connect polyglass fixtures to the body of The porous medium that filled the test section is copper foam
the collector. The most important part of a parabolic trough col- with one geometric specification: 30 PPI. The foams are determined
lector is the absorber tube. This part is responsible for absorbing in Fig. 2, and also detailed thermophysical parameters and di-
solar heat and conduct to the fluid. For this aim, the outer surface of mensions are and shown in Table 4.
a copper tube coat with black chrome whereas a glass enveloped it.
A Pyrex glass tube has been used as an envelope. It is clear that The
3. Efficiency calculations and analysis
quality of coating and space between copper and glass tube have a
significant effect on the thermal and optical performances of the
ASHRAE Standard suggests performing the tests in various inlet
collector. In this study, two different absorbers were tested, a
temperatures. The theory of solar parabolic trough collector is well
copper tube with black chrome coating with and without metal
established and can be found in the basic literature [21e23]. The
collector performance test is carried out under steady e state
Table 1 condition, including steady radiation energy falling on the collector
The detailed specifications of the mirror reflector. surface, steady fluid flow rate, constant wide speed and ambient
Parabola length (Lc) (m) 1.28 temperature. It is worthwhile to mention that a constant outlet
Parabola aperture (w) (m) 1 fluid temperature from the collector should be maintained. In this
Thickness (mm) 1 case, the useful energy gain from the collector is calculated from the
Material steel following equation:
Focal distance (f) (mm) 250
Aperture area (Aa) (m2) 1.28
_ p ðTo Ti Þ
Qu ¼ mC (1)
Diameter of absorber, DA (mm) 28
Rim angle (4) 90
The useful energy collector from a solar collector is given by
158 M.T. Jamal-Abad et al. / Renewable Energy 107 (2017) 156e163
Table 2
Environmental conditions required.
Table 3
The allowed maximum variation of key variables.
Table 4
Thermophysical parameters and dimensions of copper foam.
Material Copper
Porosity 0.90
Permeability, K (m2) 1.37 1011
Length, Lf (mm) 250
Diameter of porous media, dp (mm) 28
Thermal conductivity (W/(m.K)) 399
Qu ¼ FR ½Aa Gt ho UL Ar ðTi Ta Þ (2) where, C is the concentration ratio and FR is the heat removal factor
which is defined as
where Qu is the rate of useful energy gained, m_ is the volume flow
rate of fluid flow, Cp is the heat capacity of water, To and Ti are the Table 5
Measurement uncertainties from a specific data point.
outlet and inlet fluid temperature of the solar collector, respec-
tively. Also, Aa denotes in this equation, the appropriate areas for Variable Qty. Unit Uncertainty Conf. (%)
the absorbed solar radiation, FR is the heat removal factor, ho is the Mass flow 1 kg/s ±5% of value 95
optical efficiency, GT is the global solar radiation, UL is the overall Temperature 2
C 0.1 C 95
loss coefficient of the solar collector, and Ta is the ambient Solar Irradiance 1 W/m2 ±32 95
temperature. Anemometer 1 m/s ±5% of value 95
Pressure drop 2 N/m ±2 Pa 95
Moreover, the thermal efficiency is obtained by dividing Qu by
M.T. Jamal-Abad et al. / Renewable Energy 107 (2017) 156e163 159
450
400
350
300
ke (W/(m.K))
250
200
150
100
50
0
0.000001 0.00001 0.0001 0.001 0.01 0.1 1
kf/ks
10
8
Nusselt Number (Nu)
3
present work
2
Analytical results
1
Analytical results-without porous media
0
0.01 0.1 1 10
Darcy Number (Da)
Fig. 4. The Nusselt number for absorber filled with metal foam.
100
full porous
free porous
10
f (friction factor )
0.1
0.01
0 500 1000 1500 2000 2500
Re
Fig. 5. Variation of friction factor versus Re number for free and full porous conditions.
determining the thermal efficiency is given by the square root of calculated based on [24]:
the sum of the variances of the statistical distributions of each
component (i) involved in the process, namely
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u n 2
uX vy
jc ðyÞ ¼ t jðxi Þ2 $ (6)
vx i 1
i¼1 ke ¼ pffiffiffi
2ðRA þ RB þ RC þ RD Þ
The values determined during the test and used in the calcula-
4l
tion of the instantaneous thermal efficiency of solar collector are RA ¼
the following 2e2 þ plð1 eÞ ks þ 4 2e2 plð1 eÞ kf
vpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
Besides the measurement uncertainty described above, it was u ffiffiffi pffiffiffi
u 2 2 ð5=8Þe3 2 2ε
considered the uncertainty emanated from the regression analysis. u
l¼t pffiffiffi
p 3 4 2e e
4. Result and discussions
e ¼ 0:339
4.1. Thermal conductivity (8)
The effect of the using absorber filled with porous media on the which ks and kf are the thermal conductivity of solid and fluid
performance of the PTC solar collector was investigated. Copper phases. Fig. 3 illustrates that the effective thermal conductivity has
metal foam has been used as a porous media. Using copper foam two different manners for kf =ks 103 and kf =ks 103 . Although
cause that the thermal conductivity of absorber increases and the the constant value has been observed for kf =ks 103, there is a
ability of absorber for transferring heat from surface to fluid en- considerable increase when kf =ks 103 . It means that kf is the
hances. The effective thermal conductivity of metal foam is dominant term in the Equation (6) compare with ks.
M.T. Jamal-Abad et al. / Renewable Energy 107 (2017) 156e163 161
0.6
1 lit/min
0.5 Lit/min
0.55 1.5 Lit/min
0.5
Efficiency
0.45
0.4
0.35
0 0.01 0.02 0.03 0.04 0.05 0.06
x=Ti-Ta/Gt
Fig. 7. Variations of collector efficiency versus the reduced temperature.
162 M.T. Jamal-Abad et al. / Renewable Energy 107 (2017) 156e163
0.6
without metal foam
with metal foam
0.55
0.5
Efficiency
0.45
0.4
0.35
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07
x=Ti-Ta/Gt
Table 6 Nomenclature
Efficiency parameters for both cases.
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