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http://www.iaeme.com/IJMET/index.asp 102 [email protected]
International Journal of Mechanical Engineering and Technology (IJMET)
Volume 9, Issue 6, June 2018, pp. 102–112, Article ID: IJMET_09_06_013
Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=6
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
EXPERIMENTAL ANALYSIS OF A SOLAR
PARABOLIC TROUGH COLLECTOR
Alka Bharti, Amit Dixit and Bireswar Paul
Department of Mechanical Engineering,
Motilal Nehru National Institute of Technology Allahabad, Uttar Pradesh, India
ABSTRACT
In this study, an experimental analysis of a small-sized solar parabolic trough
collector (PTC) has been done to investigate its performance. A PTC system with
4.075 m2 aperture area was evaluated in this paper. The experimental setup is made
up of stainless steel reflector. The performance of PTC was investigated in two parts.
In the first part, performance investigation was done by using copper and stainless-
steel (1-inch diameter) as receiver tube material at different mass flow rates. In the
second part, a comparison was done using bare receiver tube and receiver tube (0.5-
inch) covered with acrylic cover at different mass flow rates. Both the cases were
studied by using water as the heat transfer fluid. This study was conducted for finding
out the better combination of receiver tube, receiver tube material, diameter of
receiver tube and mass flow rate. From first analysis, it was observed that the copper
receiver tube is showing better performance at both the mass flow rates 0.01 kg/s and
0.02 kg/s in comparison of stainless steel tube. The maximum thermal efficiency of
35.9% is obtained in case of a copper receiver at 0.01 kg/s mass flow rate. From the
second analysis, it was observed that receiver tube with acrylic cover is showing
better performance than a bare tube. The maximum thermal efficiency of 61.4% was
obtained in case of a receiver with an acrylic tube.
Key words: Parabolic trough collector, receiver tube, acrylic tube, thermal efficiency
Cite this Article: Alka Bharti, Amit Dixit and Bireswar Paul, Experimental Analysis
of a Solar Parabolic Trough Collector, International Journal of Mechanical
Engineering and Technology 9(6), 2018, pp. 102–112.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=6
1. INTRODUCTION
Population explosion and advancements in technologies increase world’s energy demand. At
present, most of these energy demands are fulfilling by the non-renewable energy sources
such as fossil fuels: coal, oil and natural gas. These energy sources produce harmful emissions
along with electricity generation which is dangerous for human health and to the environment.
Therefore, consideration of renewable energy sources is very important to reduce harmful
gases and to meet the requirements of the living population. On our planet, renewable
energies are present in various forms such as solar energy, hydropower, geothermal energy,
wind energy, biomass power and others. Solar energy is one of the biggest energy sources
Experimental Analysis of a Solar Parabolic Trough Collector
http://www.iaeme.com/IJMET/index.asp 103 [email protected]
among all these renewable energy sources. It is one of the promising and most proven
renewable energy option to substitute these non-renewable energy sources [1]. Solar energy
on the earth surface can be harness by various concentrating and non-concentrating
technologies such as flat plate collector, linear fresnel collector, parabolic trough collector
(PTC) and parabolic dish collector. Among all the concentrating and non-concentrating solar
energy technologies, PTC is the most suitable and used technology.
Experimental analysis on PTC was conducted by various researchers to investigate and
enhance its performance. Zou et al. [2] experimental study has been done to evaluate the
performance of PTC using mirror reflector and evacuated aluminum receiver. They obtained
67% thermal efficiency even with solar radiation less than 310 W/m2. Results of this study
indicated that when temperature of working fluid is under 100°C, thermal efficiency improves
with increasing fluid temperature because Reynolds number increases due to increase in
thermophysical properties of the heat transfer fluid. Chafieet al.[3]designed and manufactured
a PTC system of a 10.8 m2 aperture area and evaluated thermal performance of PTC. The
receiver tube is made up of stainless steel with a selective coating and surrounded by an
evacuated glass cover and reflector is made up of an aluminum sheet. They selected Transcal
N thermal oil as HTF and obtained 55.1% maximum thermal efficiency, average thermal
efficiency was 41.09% for sunny days and 28.91% for cloudy days. Jaramillo et al. [4]
constructed and evaluated performance of five parabolic trough solar collectors to generate
hot water and low enthalpy steam. Out of the three collectors have 90° rim angle and two
collectors have 45° rim angle. The design of copper receiver of both collectors is without
glass cover and unshelled which reduces manufacturing and transportation costs. They
obtained 35% peak efficiency for solar collectors of 45° rim angle and 67% peak efficiency
for solar collector of 90° rim angle. Valencia et al. [5] presented a paper on design,
construction and analysis of a demonstrative prototype parabolic trough collector made of
aluminum reflector and copper receiver, using water as heat transfer fluid. They reported that
maximum outlet temperature of water was 47.3°C at 783.58 W/m2 of direct solar radiation
and collector’s thermal efficiency is strongly depends on direct solar radiation, cloudiness and
room temperature. QiBin et al. [6] conducted experimental investigation of PTC system for
solar thermal power generation using synthetic oil as HTF. They used silver plated glass
mirror for reflector and evacuated stainless steel as receiver. The influence of varying solar
flux and the heat transfer fluid flow rate on the efficiency of solar collector was identified.
They found that there is a specified delay in temperature response between solar flux and heat
transfer fluid and this delay between temperature responses plays an important role in
designing of a PTC system. They have also studied the effects of heat loss on the efficiency of
the collector. They found that heat loss was 220 W/m2 for temperature difference of 180°C
between atmospheric temperature and collector temperature. Sagade et al. [7] experimentally
investigated performance of a compound prototype of PTC system whose reflector is made of
G.I and silver coated selective surface. They used different types of receiver tubes coated with
black copper and black zinc and top glass cover. They observed that if the temperature of the
receiver tube increases by 1°C, then heat loss increases by 0.21 W/m2, 0.188 W/m
2 and 0.224
W/m2 in case of copper tube coated with black copper and black zinc, and mild steel tube
coated with black copper. They obtained 60% maximum instantaneous efficiency with top
glass cover. Sagade et al. [8] experimentally investigated the performance of prototype PTC
which is made of fiberglass-reinforced plastic with mild steel receiver tube using water as
heat transfer fluid. They reported performance of mild steel receiver tube with and without
glass cover. They concluded that the instantaneous efficiency of the collector increases by
13% and it was 51.67% with receiver tube covered by glass cover. Average temperature of
receiver tube increases by 23% and useful heat gain by the glass covered receiver tube was
Alka Bharti, Amit Dixit and Bireswar Paul
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22% throughout the day. They observed that the outlet temperature of water increases by 29%
and temperature gradient increases by 68% with glass covered receiver tube. When receiver
tube was covered by glass the heat loss coefficient decreased by 70%. Sagade et al. [9]
experimentally tested performance of a prototype PTC system covered with a top cover whose
reflector is made of mild steel and coated with an aluminum foil and copper tube coated with
black copper. They obtained average temperature gradient 35°C throughout the day and
59.8% maximum thermal efficiency. Dudley et al. [10]. They have done experimental testing
of SEGS LS-2 PTC under different operating conditions such as vacuum in annulus, air in
annulus and removed annulus (bare tube) with black chrome and cermet coating at stainless
steel receiver surface. They observed high thermal efficiency and low thermal losses in case
of vacuum in annulus and cermet coating. Yaghoubi et al. [11] reported heat loss after their
study by considering receiver tube with air and vacuum in annulus space and a bare receiver
tube, that heat loss is 40% when air in annulus space which reduces thermal performance of
the collector by 3-5% whereas bare tube produces more heat loss and less thermal
performance. Kumaresan et al. [12] conducted experimental study with mirror reflector and
evacuated stainless-steel receiver tube using therminol VP-1 as heat transfer fluid and
reported maximum instantaneous efficiency of 62.5%. Selvakumar et al. [13] conducted
experimental study with evacuated tube using water and therminol as heat transfer fluid and
reported 40% more efficiency in therminol based evacuated tube than water based evacuated
tube. Arasu et al. [14] designed and manufactured a small-sized PTC consists of fibre
reinforced plastic reflector and copper receiver.
From the literature survey, it is observed that performance of PTC is mainly depend on the
solar radiation, rim angle, receiver tube and material of elements. The experimental study on
PTC using copper and stainless steel was done by various researchers. They reported
performance either by using copper or stainless steel as receiver tube material. Therefore, a
comparison study was conducted for both materials, copper and stainless-steel at different
mass flow rates to observe which is better for a small sized PTC. An evacuated receiver tube
can increase the performance of PTC by reducing heat losses from the receiver surface. Since
it is very expensive and not easily available everywhere. Therefore, an acrylic tube is tested in
this study to find out the performance improvement.
2. PARABOLIC TROUGH COLLECTOR
SPTC is a heat exchanging type of technology that converts solar energy into thermal energy
and ultimately helps to generate electricity and hot water based on the area of application.
Majorly it consists of two parts, a reflector, basically a metal sheet that curved as parabola
with specified dimensions in two directions and straight in one direction and a receiver tube.
Reflector is used to collect solar energy and concentrates towards the focal line of reflector. A
receiver tube is placed at the focal line of reflector which contains a heat transfer fluid,
circulating through it. In this system, receiver tube receives direct solar radiations on its upper
surface and concentrated radiations on its lower surface. Heat transfer fluid circulating
through it, gets heated through heat transferring processes.It is mainly used for high-
temperature application such as electricity generation. Medium temperature applications such
as industrial process heating, domestic uses and residential purposes are also very important
which can reduce the use of electricity produced by conventional methods. It will ultimately
reduce the environmental pollutions caused by fossil fuels.
Experimental Analysis of a Solar Parabolic Trough Collector
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2.1. Geometric Parameters
Figure 1.A Cross-sectional view of PTC
Geometric parameters include rim angle, concentration ratio, focal length, specifications
of the receiver tube and the collector. All these parameters are numerically related to each
other. Some of these are shown in Fig. 1. Rim angle focal length, concentration ratio, aperture
width, aperture area and effective aperture area of PTC can be calculated by the following
equations [15].
Rim angle (ϕ): It is the angle between the optical axis and the line between focal point and
collector rim. It is calculated by using the following relation:
1sin2
a
r
W
r
(1)
Where Wa is aperture width (m) and rr is the radius of a parabola (m).
Focal length (f): It is the distance between the focal point and collector rim.
4 tan2
aWf
(2)
Concentration ratio (C):It is the ratio of collector’s aperture area and receiver’s surface
area.
a
o
WC
D
(3)
Where Do is the outer diameter of the receiver tube (m).
Aperture width of collector:
4tan2
aW
(4)
Aperture area of collector:
*a aA W L (5)
Effective aperture area of collector: Aperture area that receives direct solar radiation:
Alka Bharti, Amit Dixit and Bireswar Paul
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( )*a oA W D L (6)
Where L is the length of the collector (m).
3. EXPERIMENTAL METHODOLOGY
A small-sized PTC system was fabricated on the basis of the designed parameters. The
specifications of experimental setup are listed in Table 1. This experimental setup consists of
parabolic trough (reflector), receiver tube, storage tank, circulating pipes and supporting
stand. The reflector is made of mirror finished stainless steel sheetof 88% reflectivity [16]
which is supported on a supporting frame made of stainless steel ribs. Two materials for the
receiver tube are used in this study, properties are listed in Table 2. Three receiver tubes of
different size are used in this study. Receiver tubes are coated with matte black paint that
absorptivity is 92% [17]. The matte black paint as coating on the receiver tube surface is
selected to increase absorptivity of the receiver surface. The receiver tube is surrounded by a
concentric acrylic tube with an annulus gap of 1.73 cm. The annulus gap is filled with still air
to reduce convection and radiation losses from the surface of the receiver tube by resisting the
movement of air presented in surrounding of the receiver tube outer surface. An acrylic tube
has been used as substitute of glass tube and air is used in annulus gap instead of vacuum used
in conventional PTC systems. An evacuated receiver tube has not been used as it is very
expensive and also it was not easily available everywhere. An acrylic tube has many
advantages over a glass tube as it has transmissivity of 92% [18]. It has low weight and many
times stronger than a glass tube. It is suitable for small sized cost-effective PTC system. The
receiver tube is located at the focal line of reflector in order to receive the concentrated heat
flux from the reflector.
The experiments on PTC system (shown in Fig. 2) was conducted at MNNIT (Motilal
Nehru National Institute of Technology) located in Allahabad city (25.4358° N, 81.8463° E),
Uttar Pradesh, India. The PTC system is oriented in North-South axis and single axis manual
tracking is adopted along east-west direction to track the sun rays to obtain the maximum
amount of incident solar radiation. The experiments were conducted in the months of April
and May 2017. In this present work, we selected water as HTF. The properties of water are
listed in Table 7. The readings were taken from 8:00 am to 5:00 pm and noted at an interval of
30 minutes.
Figure 2. Experimental setup of parabolic trough collector
Experimental Analysis of a Solar Parabolic Trough Collector
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Table 1 Specification of PTC system
Design specification of PTC -
Focal length (f) 0.4856 m
Aperture width (Wa) 1.63 m
Length of collector (L) 2.5 m
Rim angle (ɸ) 80°
Aperture area (Aa) 4.075 m2
Effective aperture area(m2) 4.01
Stainless steel tube-1 (m) Inner diameter: 0.0107, outer diameter: 0.0127
Stainless steel tube-2 (m) Inner diameter: 0.0214, outer diameter: 0.0254
Copper tube (m) Inner diameter: 0.0224, outer diameter: 0.0254
Concentration ratio 40.53(in stainless steel-1) and 20.1(in copper and stainless steel-2)
Table 2 Physical properties of receiver tube material [19] and water [20]
Property Stainless steel Copper Water
Density (Kg/m3) 7900 8930 998.2
Thermal conductivity (W/m-K) 48 384 0.6
Specific heat (J/kg-K) 500 386 4182
Viscosity (kg/m-s) - - 0.001003
3.1. Efficiency Calculation
Thermal efficiency: The thermal efficiency of collector is defined as the ratio of the
instantaneous useful heat gained by the HTF and the instantaneous direct solar radiation
incident (Id) on the given aperture area (Aa) of the collector [21].
u
a
Q
Q
(7)
Useful heat gain: The amount of heat gained by HTF flowing through the receiver tube.
( )u p out inQ m C T T
(8)
Instantaneous solar beam radiation (Id) incident on the given aperture area (Aa) of the
collector:
a a dQ A I
(9)
Where, m is the mass flow rate (kg/s), Cp is specific heat capacity (J/kgK), Id is the direct
solar radiation (W/m2), Tout is outlet temperature of the HTF, Tin inlet temperature of the HTF.
4. RESULTS AND DISCUSSION
4.1. Case-1 Comparison of different materials of the receiver tube at different
mass flow rates (receiver tube diameter: 1-inch)
Direct Solar Radiation (DSR) for stainless steel and copper receiver tube at mass flow rates of
0.01 kg/s and 0.02 kg/s is shown in Fig. 3. It can be seen that the amount of DSR is maximum
of 716.6 W/m2
at 13.00 p.m. in case of stainless steel receiver tube at the mass flow rate of
0.01 kg/s. Variation in the inlet and outlet temperature of HTF with time for four different
conditions is shown in Fig. 4. Since cold water storage tank was kept aside near the PTC
system and it was receiving diffused radiations. Therefore, small temperature increment can
also have seen at inlet condition of the receiver tube. The inlet temperature of HTF is
increasedfrom 8.00 a.m. up to 14.00 p.m. and after that variation become slower due to low
Alka Bharti, Amit Dixit and Bireswar Paul
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radiation up to 16.00 p.m.The outlet temperature of HTF is low in the morning in all the cases
due to low DSR. It can be seen that outlet temperature of water in case of copper tube is
decreased at 8:30 a.m. due to low DSR at that time in comparison to other cases. After that as
the heat flux increases from 8:30 a.m. up to 13.30 p.m. The outlet temperature also increases
in case of copper receiver tube at 0.01 kg/s mass flow rate and the maximum temperature of
44.6°C is observed at 13:30 p.m. at DSR of 623 W/m2. Though the radiation is maximum at
13.0 p.m. The outlet temperature reaches the maximum at 13.30 p.m. As the solar radiation
will decrease the outlet temperature will not decrease immediately due to high heating
capacity of the receiver at that time.In case of the copper tube at a mass flow rate of 0.02 kg/s,
it is also following the same trend and reaches the maximum of 43.5°C at 13.0 p.m. when
DSR is 652 W/m2. The maximum DSR, in this case, is 717 W/m
2 at 11.30 p.m. which is
giving 41.1°C temperature of HTF that is lower than 13.00 p.m. due to high heat losses from
the receiver surface at maximum DSR.If we compare same material (copper) receiver at
different mass flow rates, we observed that at low mass flow rate receiver is performing better
even at high radiation because at low mass flow rate, HTF is having sufficient time for proper
convention heat transfer fromthe receiver inner surface to HTF and that will also reduce heat
losses fromthe receiver outer surface. In case of stainless steel at the mass flow rate of 0.01
kg/s, DSR is maximum, 717.1 W/m2 at 11:30 p.m. and outlet temperature is maximum of
39.6°C at 14.00 p.m.Temperature increment for all conditions is shown in Fig. 5. It is
observed that at same mass flow rate copper tube is showing better thermal performance than
stainless steel tube. This is due to the high thermal conductivity of copper than stainless steel.
Thermal efficiency for all the cases is shown in Fig. 6. It strongly depends on the solar
radiation. Maximum efficiency of 35.9% was obtained in case of a copper tube at a mass flow
rate of 0.01 kg/s. It can observe from Fig. 9 that efficiency in case of a copper tube with 0.02
kg/s mass flow rate is more than stainless steel tube.It is representing that a copper tube has
better thermal performance even at a high mass flow rate in comparison of stainless steel tube.
The curve of the thermal efficiency would show decreasing trend where the variation of the
temperature increment will be lower than the variation of solar radiation. It will happen due to
heat losses from the receiver outer surface. The increasing pattern of thermal efficiency shows
that the variation in temperature is more than the variation in solar radiation.
4.2. Case-2 Comparison of bare and with acrylic receiver tube at different mass
flow rates (receiver tube diameter: 0.5-inch)
Direct solar radiation for a bare stainless-steel tube of 0.5-inch diameter and same receiver
with acrylic cover is shown in Fig. 7 for mass flow rate of 0.01 kg/s and 0.02 kg/s from 8:00
a.m. to 17:00 p.m. It is observed that DSR is low in case of a bare receiver at the mass flow
rate of 0.01 kg/s and almost similar in other cases. The variation of the temperature of HTF is
shown in Fig. 8. The maximum outlet temperature of 49.1°C was obtained in case of a
receiver tube with acrylic cover at 0.01 mass flow rate. The temperature increment for all
conditions is shown in Fig. 9. It is observed that variation in the temperature increment of
HTF in case of the receiver with acrylic cover from 8:00 a.m. up to 11:30 a.m. at a mass flow
rate of 0.01 kg/s is very slow than a bare tube at the same mass flow rate. The possible reason
is that during experimental study there was some manual tracking error that was unable to
track the maximum solar radiation that reduces the amount of concentrated heat flux on the
receiver bottom surface.
The maximum temperature difference of 18.9°C was obtained in case of the receiver with
an acrylic cover at 0.01 kg/s mass flow rate. It is obtained from the curves of temperature
increment of the bare tube and tube with acrylic cover at 0.02 kg/s mass flow rate that
temperature increment is higher in case of a tube with acrylic cover. The thermal efficiency is
Experimental Analysis of a Solar Parabolic Trough Collector
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shown in the Fig. 10. It is observed that the efficiency is higher in case of the receiver with an
acrylic cover in comparison of bare tube for a mass flow rate of 0.02 kg/s. In case of a
receiver with an acrylic cover at a mass flow rate of 0.01 kg/s, efficiency is higher at all the
time except for the hours when there was a manual tracing error.
Figure 3. Variation of Direct solar radiation with time for copper and stainless
steel receiver at different mass flow rates.
Figure 4. Variation of HTF temperature with time for copper and stainless
steel receiver at different mass flow rates.
Figure 5. Variation of temperature increment of HTF with time for
copper and stainless steel receiver at different mass flow rates.
Alka Bharti, Amit Dixit and Bireswar Paul
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Figure 6. Variation of thermal efficiency with time for copper and stainless
steel receiver at different mass flow rates.
Figure 7. Variation of Direct solar radiation with time for bare and with acrylic
receiver tube at different mass flow rates.
Figure 8. Variation of HTF temperature with time for bare and with acrylic
stainless steel receiver tube at different mass flow rates.
Figure 9. Variation of temperature increment of HTF with time for bare and
with acrylic stainless steel receiver tube at different mass flow rates.
Experimental Analysis of a Solar Parabolic Trough Collector
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Figure 10. Variation of thermal efficiency with time for bare and with acrylic
stainless steel receiver tube at different mass flow rates.
5. CONCLUSIONS
The experimental study was conducted on a small-sized solar parabolic trough collector at
MNNIT Allahabad (25.4358° N, 81.8463° E), Uttar Pradesh, India. The performance of PTC
was investigated in two cases. In the first case, performance investigation was done by
conducting experiments using copper and stainless-steel (1-inch diameter) as receiver tube
material at different mass flow rates. In the second case, a comparison was done using bare
receiver tube and receiver tube (0.5-inch) covered with an acrylic cover which is filled with
air in the annulus space at different mass flow rates of the HTF. Both the cases were studied
using water as the heat transfer fluid. This study was conducted for finding out the better
combination of bare and acrylic covered receiver tube, receiver tube material, diameter of the
receiver tube and mass flow rate. From the first case, it was observed that copper receiver
tube is showing better performance at both the mass flow rates 0.01 kg/s and 0.02 kg/s in
comparison of stainless steel tube. The maximum thermal efficiency of 35.9% is obtained in
case of a copper receiver at 0.01 kg/s mass flow rate. It was obtained that copper is better
material for the receiver of PTC system. From the second case, it was observed that receiver
tube with acrylic cover is showing better performance than a bare tube at both the mass flow
rates because an acrylic tube reduces the heat losses from the surface of the receiver. The
maximum temperature difference of 18.1°C and thermal efficiency of 61.4% was obtained in
case of a receiver with an acrylic tube. It was obtained that the study can be conducted by
using copper receiver tube of 0.5-inch diameter with an acrylic cover to enhance the
performance of a PTC system.
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