Experimental Study on a Line-Axis Concentrating Solar Energy

International Journal of Fluid Mechanics & Thermal Sciences
2017; 3(6): 62-69
http://www.sciencepublishinggroup.com/j/ijfmts
doi: 10.11648/j.ijfmts.20170306.11
ISSN: 2469-8105 (Print); ISSN: 2469-8113 (Online)
Experimental Study on a Line-Axis Concentrating Solar
Energy Collector for Water Heating
Frederick Ikpakwu1, Anthony Okoronkwo1, Modestus Okwu2, Emmanuel Anyanwu1
1
Department of Mechanical Engineering, Federal University of Technology, Owerri, Nigeria
2
Department of Mechanical Engineering, Federal University of Petroleum Resources, Effurun, Nigeria
Email address:
Ikpakwo_fredrick@yahoo.com (F. Ikpakwu), mechanicalmodestus@yahoo.com (M. Okwu)
To cite this article:
Fredrick Ikpakwu, Anthony Okoronkwo, Modestus Okwu, Emmanuel Anyanwu. Experimental Study on a Line-Axis Concentrating Solar
Energy Collector for Water Heating. International Journal of Fluid Mechanics & Thermal Sciences. Vol. 3, No. 6, 2017, pp. 62-69.
doi: 10.11648/j.ijfmts.20170306.11
Received: March 29, 2017; Accepted: June 6, 2017; Published: November 28, 2017
Abstract: This paper examines the experimental study on a line axis concentrating solar energy collector for water heating.
The system considered consists of cylindrical solar radiation concentrator with a black coated tubular absorber positioned
along its axis. A cold water tank is placed above the collector and a hot water tank positioned below it such that fluid flows in
and out of the set up. Solar radiation absorber inlet header is connected to the cold water tank while its outlet header is
connected to the hot water tank. These major components are supported by angle iron raised at a distance from the ground that
depends on the location and function. Valves are used at strategic points on the connecting pipe lines to isolate the flow of
water. When water is poured into the cold water chamber, and the control valve turned on, the water flows under gravity into
the receiver/absorber tube. At the absorber section, heat is transferred from the steel tube to the circulating water and is
consequently heated. The heated water, then flows into the returning tube against gravity, thereby restricting the heated water
from flowing into the storage tank. At this stage, thermo-siphoning effect comes into play. As the temperature of the water
increases, its density reduces while the mass remains constant in order to balance the effect of the reduction in density. Thus,
there is a resultant increase in volume which consequently pushes the water level further along the returning pipe. Further
increase in temperature reduces the water density and increases the volume of the water, thereby causing the heated water to
flow into the insulated tank. Several experimental tests were carried out under meteorological condition at the Federal
University of Technology Owerri, Nigeria at three different mass flow rates of 0.001kg/s, 0.002kg/s and 0.003kg/s. The solar
water heater was tested while oriented in the East–West and North –South directions in order to determine the effects of
orientation on the performance. Results obtained showed that a maximum temperature of 69.5°C, corresponding to 34.5°C
increase in water temperature and a maximum instantaneous efficiency of 51.01% is possible. The aim of the study is to design
a cheaper solar energy system capable of reducing energy bill within the developing countries of the world.
Keywords: Cylindrical Solar Water Heater, Solar Intensity, Mass Flow Rate
1. Introduction
Solar energy has been described as the most promising
energy of the future [1]. It is the energy transmitted from the
sun in form of electromagnetic radiation which requires no
medium for its transmission. The earth receives about two
hundred billion megawatts (200 x 109 mW) of the total solar
output of hundred billion megawatts [2]. This form of energy
is finite, abundant, cheap and environmentally friendly. The
demand for energy continues to grow by the day. The
International Energy Agency, predicted that the world energy
consumption if left unchecked would rise by 53% between
2006 and 2030 with over 70% of the increase coming from
the developing countries [3].
Nigeria is located on the West Coast of Africa and lies in
the tropical region. Studies relevant to the availability of
solar energy resources has been reported in Nigeria. [4]
revealed the viability of solar energy for domestic and
industrial uses. In his paper, the annual average solar
radiation for Nigeria is between 3.7 K/Wm-2 day-1 along the
coastal area and 70 K/Wm-2 day-1 in the arid region. It was
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Fredrick Ikpakwu et al.: Experimental Study on a Line-Axis Concentrating Solar Energy Collector for Water Heating
estimated that Nigeria receives 5.08 X 1012 K/Wh of energy
per day from the sun, and if solar appliances with 5%
efficiency are used to cover 1% of the country’s surface area,
then, 2.54 X 106 MWh of electricity will be produced. This
huge amount of electricity is equivalent to 4.556 million
barrels of oil per day. Thus, solar energy if properly
harnessed will in no doubt contribute significantly to the
development of Nigeria.
In view of Nigeria’s current shortage in power supply, it is
appropriate to design a solar collectors system which will
reduce over- dependent on fossil fuel. It is an obvious fact
that 80% of the energy utilized in Nigeria is obtained mainly
from the fossil fuels with its adverse environmental effect. To
address this, it is very appropriate to design systems that will
reduce pollution from hazardous gaseous emission. These
harmful hazardous gas products pose a great threat to the
existence of man on earth. The aim of this study is to design
a cheaper solar energy system capable of reducing energy bill
within the developing countries of the world.
2. Previous Solar Collector Designs
Several publications are available on solar water heating
systems especially on compound parabolic type. Some of the
reported works on compound parabolic concentrator (CPC)
design include the works of [5], [6], [7]. [5] investigated the
performance of new parabolic trough collector hot water
storage tank. The storage tank water temperature is increased
from 35°C at 9.30h to 73.84°C at 16.00h when no energy is
withdrawn from the storage tank. The average beam radiation
during the collection period is 699 W/m2. The useful heat gain,
the collector instantaneous efficiency, the energy gained by the
storage tank water and the efficiency of the system as a whole
are found to follow the variation of incident beam radiation as
these parameters are strongly influenced by the incident beam
radiation. The value of each of those parameters is observed to
be maximum around noon at which the incident beam
radiation is maximum. Another integrated solar water heater
was designed and constructed by [6]. The test unit has an
absorber area of 1.3 m2 and a capacity of 178 liters. During the
experimental test, they extracted 100 liters of water with peak
temperature ranging between 45°C to 50°C. Day time collector
efficiency of 60% and overall efficiency of 40% were
computed for the system. They found out that night radiation
losses were reduced by screen insulation. Similarly, [7]
presented results on a simple low cost integrated collector
storage solar water heater. The system takes the form of a
rectangular shaped tank incorporating the solar collector and
storage tank into a single unit. Experimental and numerical
study on the integral system was undertaken in order to
compare the results with a newly developed macro model.
Numerical results obtained were found to be in close
agreement with the experimental data.
[8] reported an experimental study on the performance of a
compound parabolic collector thermo syphon solar water
heater at the Federal University of Technology, Owerri. The
result of the experiment gave a temperature of 65°C and
collector efficiency of about 51%. However, their work did
not evaluate the effect of mass flow rate and orientation on
the performance of the system designed. Similarly, [9]
reported a simulation on the performance of a thermo siphon
solar water heater. The system consists of flat plate absorber
area of 3.0m2. A total heat gain of 63.36MJ was recorded
while 50.86MJ was absorbed and only 37.35MJ came out as
useful heat giving collector efficiency of 56.51%. It was
concluded that simulation work has shown that with a good
design configuration of solar water heating system, a
reasonable amount of energy can be collected for heating
purpose. Similar designs have been reported by several
authors like [10], [11]; under meteorological condition of
Nigeria. [12] researched on the experimental behavior of a
solar water heating system combined with floating covers
and photovoltaic cells by using two similar ponds for holding
water, one was heated with coil containing an enclosed
circulating fluid and the other unheated. To minimize water
evaporation the exposed surfaces of the two ponds were
covered by floating elements with photovoltaic cells on top
to supply energy for water pumping and to power auxiliary
devices of the system. The daily average water temperature
values was estimated using experimental meteorological data
over eight months. The pond with floating covers water
evaporation reduction was greater than 90% with respect to
an uncovered pond. Also the photovoltaic cells placed on the
floating cover generated up to 68 Wp/m2 equivalent to
electric power. [13] used Monte Carlo analysis to estimate
the techno-economic benefits and reliabilities of solar water
heaters. The study focuses on a product range manufactured
by a local company in Australia, using the historical data
provided by the company The results show that solar water
heaters can offer significantly better long-term economic
viability compared to conventional systems at moderate
auxiliary energy consumptions. [14] reported the basic
components of solar water heating system (SWHS) and their
advancements. Also, [15] conducted an experimental
investigation of an evacuated tube solar air collector coupled
to a latent thermal energy and water was used as the working
fluid transferring solar gain to the air being heated. The
maximum measured temperature differential between the
heated air and the ambient air was 37°C and 20.2°C during
conditions of incident and non-incident solar radiation,
respectively. This occurred using a circular fin configuration
at a flow rate of 0.018 kg s−1. The efficiency at low (0.018 kg
s−1) air flow rates was 0.05–0.50 times less as compared to
high (0.035 kg s−1) air flow rates. This system has advantages
over systems using sensible storage as it can be used after
sunset due to better heat storing capacity of the PCM.
Similarly, Kumar and Yadav (2016) conducted a research by
investigating experimentally air heating system using solar
collector with evacuated tube incorporated with heat
exchangers. However, several publications are available on
CPC collector. This paper takes into cognizance the effects of
orientation and mass flow rate of fluid on the system
System Configuration
Figure 1 illustrates the schematic diagram of the
International Journal of Fluid Mechanics & Thermal Sciences 2017; 3(6): 62-69
configuration of the cylindrical parabolic concentrating solar
water heater. While Figure 2 shows the pictorial views of the
system. The system investigated is consists of cylindrical
solar radiation concentrator with a black coated tubular
absorber positioned along its axis, a cold water tank
positioned above the collector and a hot water tank below it
such that fluid flows into and out of them by gravity. The
solar radiation absorber inlet header is connected to the cold
water tank while its outlet header is connected to the hot
water tank. These major components are supported by angle
iron raised at a distance from the ground that depends on the
location and function. Valves are used at strategic points on
the connecting pipe lines to isolate the flow of water.
Figure 1. Sketch diagram of Cylindrical Solar Water Heater.
Figure 2. Pictorial view of experimental set-up.
The CPC has a compound parabolic reflecting surface
whose line focuses on a cylindrical steel receiver/absorber.
The receiver is used to receive incident radiant energy and is
64
covered with a selective surface of high solar absorptance (αr)
and low emittance (εr), whilst the reflector is highly reflective
aluminum folded into cylindrical shape. The absorber pipe is
made of a steel tube painted black with inner and outer
diameters of 9.5mm and 12.7mm respectively. The location of
the absorber is at the focal point of the CPC collector. This is
to facilitate the heat transfer between incident solar radiation
and the absorber material. To further enhance the heat transfer
from the concentrated ray to the receiver, the receiver was
covered with a thick black coating which is insulated at the
edges to minimize heat conduction to the receiver and the
reflector. The Water used as heat transfer medium flows
through the receiver tube and the tank. To suppress convection
losses from the receiver a glass envelope is placed around it. A
transparent cover is fitted to protect the reflector surface from
deterioration; and also it reduces the rates of heat loss from the
receiver envelope configuration. On the other hand to reduce
heat losses to the ambient the underside of the reflector is
covered with an insulator.
The present system operation is very simple, when water is
poured into the cold water chamber, and the control valve
turned on, the water flows under gravity into the
receiver/absorber tube. At the absorber section, heat is
transferred from the steel tube to the circulating water and is
consequently heated. The heated water, then flows into the
returning tube against gravity, thereby restricting the heated
water from flowing into the storage tank. In this section the
thermo-syphoning effect comes into play. As the temperature
of the water increases, its density reduces while the mass
remains constant in order to balance the effect of the
reduction in density; thus, there is a resultant increase in
volume which consequently pushes the water level further
along the returning pipe. Further increase in temperature
reduces the water density and increases the volume of the
water, thereby causing the heated water to flow into the
insulated tank.
At the beginning of the experiment, the valve at the outlet
end of the cold water reservoir was opened to allow water
flow into the steel pipe and consequently, the initial water
temperature was measured and the second valve at the inlet
of the hot water tank was shut to allow the water in the
absorber tube to heat for about 30 minutes. The readings at
the five thermocouple points were taken at the end of the 30
minutes before which the inlet valve at the hot water tank
was opened and the temperature of the hot water tank was
measured andrecorded.
This exercise was repeated at 30 minutes intervals starting
from 8:00 am to 5 pm on daily basis. The atmospheric
temperature and the solar radiation were also recorded at
each interval of 30 minutes before the temperature of the
solar heated water was taken. The Solar Water heater is made
of a transparent glass-cover material which houses a stainless
steel tube (receiver). The cylindrical parabolic concentrator is
sealed at both ends with 2mm mild steel plate in other to
allow maximum possible solar radiation that will reach the
stainless steel tube which serves as the absorber.
Thermocouples were installed at five different points
65
Fredrick Ikpakwu et al.: Experimental Study on a Line-Axis Concentrating Solar Energy Collector for Water Heating
spaced equally along the length of the stainless steel tube.
The transparent glass-cover serves the same purpose as
glazing in conventional solar water collector. It choice as a
glazing material is its ability to transmit maximum possible
solar radiation; it also reduces heat loss by convention and
radiation, there by acting as a heat trap. It is cheap,
availability and transparent. Figure 1 shows the picture of the
solar water heater. The complete view of the experimental rig
set-up is shown in Figure 2. It consist of a 40 liters mild steel
cold water tank insulated with fiber glass and wrapped in
aluminum foil to reduce heat transfer into the cold water
content of the tank from the ambient air. The tank which
serves as the cold water tank was placed on a 1 inch pipe
square cross –sectioned stand 1.735m height in order to
create enough head to achieve water flow from the cold water
tank through the solar water heater. The tank which serves as
the hot water tank was placed below the cold water tank.
Water flows from the cold water tank through the absorber
pipe under gravity but enters into the hot water tank through
thermo syphon effect.
Three manual valves were installed: Two to the tanks and
one to the water supply pipe. A wedge gate valve (first valve)
at the outlet end of the cold water reservoir, and a parallel gate
valve (second valve), outlet valve on the cold water supply
pipe and allow water to run at required flow rate into the
absorber tube. Another, wedge gate value (third valve) is
located at the right hand side of the storage tank below the cold
water tank, the storage tank below the cold water tank. The
supply pipe was insulated using fiber glass and wrapped with
aluminum foil to ensure relatively constant water temperature
between the cold water tank and inlet of the solar water heater.
3. Experimental Observation
Data were collected at intervals of 30 mins. The most
important parameters measured are the ambient temperature,
temperature of the cold water storage tank, water temperature
variation along the length of the stainless steel tube, the
temperature of hot water storage tank and solar intensity. Ktype thermocouple insulated with Teflon and metal gauze
wire with maximum insulation temperature of 260°C was
used. The probe accuracy of the thermocouple is ± 2.2°C or
+0.7.5% of reading from 0°C to 800°C. Two types of
thermometers were used for measuring temperature, the
Patos 305 digital thermometer which has a temperature
measurement range of minus 50°C to plus 1300°C and a high
resolution of 0.1°C was used to measure the ambient
temperature, temperature of cold and hot water storage tank.
While the other type of thermometer was a temperature
monitoring device system which has temperature
measurement ranges of 0°C to 390°C. This system was used
to measure temperature variation along the length of the
stainless steel tube as the five thermocouple installed to the
length of the stainless steel tube were connected to five of the
temperature monitoring device systems at the same time
throughout the day. The Temperature was measured by
simply switching from one input to the other, which made the
time interval after taking temperature reading along the
length of the stainless steel tube very small, thus reducing
error. The Daystar solar meter which uses a polycrystalline
silicon PV cell as sensor was used to measure solar intensity.
It provides an accurate reading of 3% from 0.0 to 1200 watts
and has a resolution of 1 W/m2.
4. Results And Discussion
Series of performance testing of the cylindrical solar water
heater were carried out from 11th January, to 13th February
2013 under meteorological condition at the Federal
University of Technology Owerri. It was operated on three
different mass flow rates of 0.001kg/s, 0.002 kg/s and
0.003kg/s respectively; In order to ascertain it performance
when operated at these mass flow rates. This was done while
the solar water heater was orientated in two different
directions, the East-west and north-south directions. The
cylindrical solar water heater was operated on the design
mass flow rate of 0.002kg/s from 11th – 13th and 17th –
22nd January while orientated in the east-west direction
respectively. From 23rd-25th January and 1st, 4th and 5th
February, it was orientated north-south direction and eastwest direction respectively and was operated at a mass flow
rate of 0.003kg/s. Finally, at a mass flow rate of 0.001kgs, it
was orientated in the east-west direction and north-south
direction, respectively on the -8th and 11th, 12th and 13th
February, 2013. Seven temperature data points were
measured and recorded accordingly.
Table 1 is the design specification of the system. The
values were obtained from design calculations using the
governing equations. Results obtained from the
measurements are presented in figures 3 - 10. These results
present similar pattern showing a rise and fall in the water
temperature as the insolation changes.
Table 1. Design Specification of the System.
Design Specification
Glazing Material
Thickness of Glazing Material
Length Of Glazing Material
Absorber Tube
Outer Diameter of Tube
Length of Tube
Tilt Angle
Rim Angle
Overall Heat Transfer Coefficient
Concentration Ratio
Reflecting Material
Transparent Glass
0.004m
1.98m
Steel
0.0127m
1.98m
0.99°
45°
0.7541w/M2k
1.98
Aluminum Sheet
Figure 3 illustrate the variation of solar insolation for five
days with the time of the day. The variation of the temperature
with time of the day at 0.02kg/s mass flow rate and east west
orientation is shown in figure 4. The various data points of T1
to T5, ambient temperature Tamb, temperature of the fluid Tf
and temperature of the stored water Ts are plotted against the
time of the day. It could be observed that the temperature
distribution along the absorber tube is not uniform and could
be seen to have varied along the tube progressively.
International Journal of Fluid Mechanics & Thermal Sciences 2017; 3(6): 62-69
66
Figure 3. Measured solar insolations from 11th January to 18th January 2013.
Figure 4. Experimental result on 11th, 14th and 15th Jan., 2013 at 0.002kg /s east – west orientations.
Figure 4 shows the temperature variation with time of the day for the mass flow rate of 0.02kg/s at north-south orientation.
Similarly, it can be seen from figure 4 that the same trend as observed in figure 3 is repeated. it was also noted that the
temperature distributions along the absorber tube followed the same pattern irrespective of the fact that it was carried out on a
different day.
Figure 5. Experimental result on 17th, 18thand 21th Jan., 2013 at 0.002kg /s north –south orientations.
The experimental result obtained from 17th to 21st of January is shown in Figure 5. The figure has the same pattern as
observed in the previous figures shown above.
67
Fredrick Ikpakwu et al.: Experimental Study on a Line-Axis Concentrating Solar Energy Collector for Water Heating
Figure 6. Experimental result on 23rd, 24thand 25th Jan., 2013 at 0.003kg /s north –south orientations.
Figure 7. Experimental result on 1st, 4th and 5th Feb., 2013 at 0.003kg /s east – west orientations.
Figure 8. Experimental result on 7th, 8th and 11thFebruary, 2013 at 0.001kg /s east – west orientations.
Figure 9. Experimental result on 12thFebruary, 2013 at 0.001kg /s North – South orientations.
International Journal of Fluid Mechanics & Thermal Sciences 2017; 3(6): 62-69
Figure 6 presented the poorest performance recorded
during the experimental period. Thus, could be attributed to
heavily overcast weather condition observed for the day
(17/01/2013). The solar intensity recorded during this day
was very low, the minimum value being 23W/m2. On this
particular day it rained, leading to a maximum out let water
temperature of 33°C which corresponds to a water
temperature increase of 6.5°C. When a high insolation level
at 1027W/m2 was recorded, a maximum outlet water
temperature of 69.5°C, corresponding to a 34.5°C increase in
water temperature was achieved. This is shown in figure 5.
This is an expected trend since high insolation level brings
about an increase in energy input into the water. As a result
of this, the energy input into the water becomes directly
proportional to water temperature. The experimental data
presented in these figures 4-9 show constant rise and fall in
water temperature as water flows through the solar water
heater during the days test were carried out. This same water
temperature fluctuation is observed for the solar intensity
during the experimentation period. Thus, the behavior of the
solar water heater as the insulation level changes reveals that
the solar water heater responds quite fast to changing solar
68
intensity. For instance comparing the temperature and solar
insolation profile on Figures 3 and 4. between the hours of 8
am-10:30 am the solar intensity was low and the
corresponding exit water temperature was 35°C.
When the solar intensity increased to 180W/m2, within 30
mins, exit water temperature rose to 39°C. A close look at
Figures 4 – 9, reveals similar trend in temperature and solar
intensity profiles. The cylindrical solar water heater was
operated on different mass flow rate of 0.002 kg/s a
maximum outlet water of 69.5°C was obtained at a solar
intensity of 1027W/m2 on 21/01/2013..
Collector Efficiency
The collector efficiency is parameter used to compare the
performance of various collector types. It can either be
overall or hourly. The hourly efficiency is reported by [18] as
in Eq. 1.
η = Q / A.I
Equation 1 was applied to the experimental data collected
for the purpose of analysis. Results of the analysis are
presented in Figures 10 and 11.
Figure 10. Efficiency against time.
Figure 11. Efficiency against time.
The solar radiation data used in this analysis is presented
in figure 3. It was observed that on days when poor solar
intensity was recorded (i.e., Overcast days), 17/01/2013 and
1/02/2013. The efficiencies obtained were low for each of
69
Fredrick Ikpakwu et al.: Experimental Study on a Line-Axis Concentrating Solar Energy Collector for Water Heating
these days. (See Figure 10 and figure 11) while on clear days
with high solar intensity the efficiency was high. The best
efficiencies obtained were 44.9% (Figure 10) on 21/01/2013,
when a mass flow rate of 0.002 kg/s was used, and 51.10%
figure 11 at a mass flow rate of 0.003kg/s on 4/02/2013.
Thus, best efficiency was recorded at a mass flow rate of
0.003kg/s which ranged between 10-51% with those for
overcast days generally not exceeding 35%.
an option for building energy conservation Energy and
Buildings, 38 (3). pp. 214-219. ISSN 0378-7788.
[7]
Cuurie, J. I, Garnier C., Muneer T., grassie T. and Henderson
D. (2008): Modeling Bulk Water Temperature Integrated
Collector Storage System. Building Services Engineering
Research and Technology 29(3): 203-218.
[8]
Okoronkwo. (2014) Experimental study on the performance of
a compound parabolic collector thermo syphon solar water
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[9]
Adeyemo S. B. (2000): Simulation of the Performance of
Solar Energy for Domestic Heating. Global Journal of
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5. Conclusions
Arising from the foregoing results of the test carried out,
the conclusion of the study therefore is:
i. the cylindrical solar water heater performed
satisfactorily, and is capable of generating hot water at
69.5°C. However, the maximum achievable temperature is
dependent on solar intensity and mass flow rate, with
operation at a low mass flow rate capable of yielding the
hottest water;
ii. the efficiency of the solar water heater ranged from
12.13% to 51.10%, with the best efficiency of 51% occurring
at a mass flow rate of 0.003kg/s;
iii. the cylindrical solar water heater responds to change in
weather condition and functions more effectively on a sunny
weather;
iv. the performance of the solar water heater does not vary
when positioned at different orientations due to its cylindrical
parabolic shape which allows maximum possible
concentration of solar radiation to reach the absorber tube
from all directions.
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