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Experimental investigation of the performance of a double pass solar water heater with reflector
Journal Pre-proof Experimental investigation of double pass solar water heater by using reflector Soumya Mandal, Subir Kumar Ghosh PII:
S0960-1481(19)31869-5
DOI:
https://doi.org/10.1016/j.renene.2019.11.160
Reference:
RENE 12708
To appear in:
Renewable Energy
Received Date: 9 August 2019 Revised Date:
1 November 2019
Accepted Date: 30 November 2019
Please cite this article as: Mandal S, Ghosh SK, Experimental investigation of double pass solar water heater by using reflector, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2019.11.160. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Author Contributions Manuscript title: Experimental Investigation of Double Pass Solar Water Heater by Using Reflector
Author 1: Soumya Mandal Conceived and designed the analysis The structure of the manuscript and related analysis was done by the author. Wrote the paper Editing and Graphs were drawn by the author Supervision The author investigated and supervised all the factors that enhance the quality and impact of the manuscript. Verification This author verified the analytical methods helped with technical details. This author contributed to the design and implementation of the research, to the analysis of the Results and to the writing of the manuscript with Author 2.
Author 2: Subir Kumar Ghosh Conceived and designed the analysis Designs of the arrangement and related analysis were done by this author Collected the data Data collection and calculation was done by this author Performed the analysis Developed the theoretical framework and some of the analysis was done by this author with the help of author 1. Wrote the paper The manuscript was constructed and written by this author with the help of author 1.
All authors discussed the results and contributed to the final manuscript
1
Experimental Investigation of Double Pass Solar Water Heater by Using
2
Reflector
3
Soumya Mandala, b, Subir Kumar Ghosha, a
4
Department of Mechanical Engineering, Rajshahi University of Engineering and Technology,
5 6
Rajshahi 6204, Bangladesh. b
Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater-
7
74078, USA
8 9 10 11 12 13 (*) Corresponding author E-mail
:[email protected]
Tel
: +8801773246797
Postal Address
Department of Mechanical Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh
14 15 16 17 18 19 20 21 22 1|Page
1
Abstract
2
A new concept of double pass solar water heater using reflector has been introduced in this paper to
3
improve its thermal efficiency. Here the design, construction, and performance scrutiny of double
4
pass solar water have been discussed. The consequence of the mass flow rate on the outlet
5
temperature, thermal performance, and overall performance has been analyzed. Moreover, the
6
performance result has been compared with the single-pass solar water heater. The data are taken for a
7
different mass flow rate of 0.0022kg/s, 0.0028kg/s, 0.0033kg/s, 0.0039kg/s and 0.0044kg/s from 9
8
AM to 4 PM at an interval of 30 minutes. The maximum efficiency obtained for a higher mass flow
9
rate of 0.0044kg/s is 50.26%. The maximum efficiency for other mass flow rates is 47.61%, 47.98%,
10
48.82%, and 48.89%, respectively. All these efficiencies are found in the middle of the day around
11
12.00 PM to 1.00 PM as the maximum solar radiation is observed in that period. Though the sunlight
12
radiation is not much higher in this month for the same mass flow rate, the efficiency of double pass
13
solar water heater using reflector is much greater than a conventional single-pass solar water heater.
14 15
Keywords: Solar radiation, Double pass solar water heater, Reflector, Mass Flow Rate,
16
Efficiency, Temperature Difference.
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 2|Page
1
1. INTRODUCTION
2
Energy is one of the most basic elements of the universe, without which nothing is possible in the
3
present days. The most interactive form of energy that is being given careful attention is renewable
4
and non-renewable energy [1][2]. As the non-renewable sources (Coal, Gas, Oil, etc.) are limited in
5
nature and can’t be compensated once these are used, which shifted the attention towards the
6
alternative energy sources [3][4]–[6]. World energy demand growth has a very close relationship with
7
the energy demand of emerging countries [7]. Bangladesh is a developing and overpopulated country
8
in South Asia [8]. The energy demand is increasing day by day. Figure 1 shows the future energy
9
projection of Bangladesh.
10 11
12 13
Figure 1: Future energy projection in Bangladesh[NP=Nuclear Power; RE=Renewable Energy;
14
B&W=Biofuel and Waste; P(Export)=Power(export)] [8]
15 16
Figure 1 predicted the future energy demand of Bangladesh concerning the present condition. From
17
the Figure, it is seen that in the future, the gas and oil will be in peak demand. Bangladesh has one
18
Nationwide Network to supply the electricity with a mounted capacity of 15,379 MW as of February
19
2017 [9]. Recently the construction of 2.4 GW capacity Rooppur nuclear power plant has been started
20
which will start operation in 2023 [10]. Bangladesh is very much dependent on gas, which fulfills
21
more than 70% of its energy needs [11]. But the gas resources are limited, and consequently, the
22
energy crisis is increasing day by day. Some of the energy comes from oil and coal. But it is not so
3|Page
1
significant as the reserve of oil and coal is limited [12]. That’s why it is high time to find alternative
2
energy sources for a sustainable future [13]. In this esteem, from the perspective of Bangladesh, solar
3
energy seems to be the most available and efficient form of renewable energy [14]. Figure 2 shows
4
the solar energy-based power installation in Bangladesh.
5 6
Figure 2: Solar energy power installation in Bangladesh [15].
7 8
Though the installation of solar energy in Bangladesh is increasing with the advent of time, it is facing
9
trouble to meet the demand. By the end of the year, a solar installation can fulfill almost 190.6 MW
10
demand throughout the country.
11
Bangladesh is situated in the northeastern part of South Asia, and that is why it gets sufficient sunlight
12
[16]. The solar radiation in Bangladesh varies from 3.8kWh/m2/day to 6.4 kWh/m2/day [17]. The
13
maximum solar radiation is recorded during the month of March-April and lowest in December-
14
January[18]. The optimistic sunshine time in Bangladesh varies from 4.7-7.6 hours over the year-
15
round from dry to monsoon season [22]. A study that was held in 2012 has reduced the sunshine time
16
by 54% to the justification for rainfall, cloud, and fog [23]. This larger amount of solar radiation can
17
be used to produce at least 22.2-28.5×108 MW power per annum [24]. So, solar energy has the
18
potential to ensure an eco-friendly environment for future growth [25]. Bangladesh's government has
19
already taken some initiatives to produce 10% of total power generation by 2020 from renewable
20
energy sources like solar energy, wind & waste [26] [27]. The study area Rajshahi is situated on the
21
northern side of Bangladesh. Figure 3 shows the solar radiation in different divisional cities in
22
Bangladesh.
4|Page
1
2 3
Figure 3: solar radiation in a different division in Bangladesh [19]
4 5
Figure 3 shows the solar contamination scenario in the six divisional areas in Bangladesh.
6
Bangladesh has a very impressive position concerning solar radiation. The average solar radiation in
7
Bangladesh varies from 1200-1800 KWhr/m2. This potential can help its population to fulfill its
8
energy crisis from this form of renewable energy [20]. Though some division in Bangladesh shows
9
higher values of solar radiation, the experimental area Rajshahi shows a very uniform change rate
10
throughout the year than another area. Figure 4 shows the average sunshine time in the different
11
divisional areas in Bangladesh.
12
5|Page
1 2
Figure 4: Variation of sunshine time in Different division in Bangladesh [19]
3 4
Solar energy is a form of electromagnetic energy which comes in the form of electromagnetic
5
radiation [21][22]. A solar air heater is a popular form of utilization of solar energy, which absorbs
6
this solar radiation and heating air flowing through it [23] [24] [25]. Thus, the temperature of the air
7
increased by a substantial amount for using it for various purposes, like space heating for small
8
households and the drying of different required elements [26][27]. Moreover, a solar air heater has an
9
extensive field for using it in the winter dominated country [28]. But due to low compactness and
10
small heat conductive capacity, the performance of the conventional single-pass solar air heater is
11
relatively low [29]. For this reason, the double pass solar air heater has been developed [30]. In the
12
case of a double pass, the air gets sufficient time to be heated by moving in the path [31]. But this
13
development also fails to utilize solar energy to the desirable amount [32].
14
Solar water collector is another kind of application of solar energy [33]. In case of it, the water flows
15
through a copper or other type of heat conductive metal pipe. The heat from the solar radiation heated
16
the pipe, and thus the water gets heated [34]. This heated warm water can be utilized in various ways,
17
such as in the bathroom, kitchen, laboratory, household purposes [35]. Moreover, as the conductive
6|Page
1
heat coefficient of water is high, so the efficiency of the system is quite impressive and far better than
2
the solar air heater [36]. The conventional single-pass solar water heater is generally used to heat the
3
water. It uses some circular or rectangular types of copper tube to run the water and to be heated [37].
4
The solar water heater is a device that absorbs heat from sunlight radiation & consequently heating
5
water [38]. This preheated water can be used for normal cooking, hospitals, hotels, and hot water, also
6
used for textile & paper industries [39]. The double-pass solar water heater consists of two parts the
7
top and bottom parts [40]. An absorber plate separates these two parts. Pipes or tubes are mounted on
8
the upper and below the plate [41]. A glass or plastic cover is used on both sides. Sunlight which is
9
coming from the sun directed towards the upper plate through the glass shelter [42]. Thus, the
10
absorber plate absorbs the sunlight radiation and consequently heating the water inside the pipe.
11
Moreover, reflectors can be used to provide the additional heat to the inside water by placing it at the
12
bottom part.
13
Many research and experiments have been done to improve the thermal effectiveness of the solar
14
water heater. Some of the prominent research are listed in Table 1 below, with the key features and
15
key findings.
16 17
Table 1: Literature review of the solar water heater. AUTHOR
KEY FEATURE
KEY FINDINGS
REFERENCES
NAME Sivakumar et.al
Various design parameter
59.09% efficiency was obtained [43]
variation was done, such as a
for the variation of the number of
number of riser tubes and
riser
arrangement of riser tube in a
efficiency was obtained for the
zig-zag pattern from the
zig-zag arrangement of the tubes.
tubes,
and
62.90%
existing collector. Mongre and
A circulating pump, Glass,
The efficiency was increased by [36]
Gupta
Insulator, Aluminum tube,
55% with increasing the glazing
water container was used for
area.
the experiment. Himangshu
A new approach of the
By using reflector increases the [44]
Bhowmik and
reflector was used in the
reflectivity of the collector and
Ruhul Amin
experiment with the ability to
thus obtained the improvement
change its position with the
of the collector efficiency around
sun’s position.
10%.
Shahidul Islam
Two solar water heaters of
It was found that the incoming [45]
Khan and Asif
100 liters and 200 liters were
hot tap water was about 300C
7|Page
Islam
used.
higher than the room temperature during the winter months.
Sathish. D et.al.
An analysis was done on the
The maximum efficiency was [37]
improvement of the
observed at the time of 1.00 PM
performance of the solar water on an average of 57% heating system on different tube arrangements. The literature was reviewed to improve thermal efficiency by varying the tube arrangement and number of riser tubes S. Ramasamy and
an experiment of thermal
The
main
objective
of
P.
performance analysis of the
experiment was to maintain the
Balashanmugam
solar water heater with
velocity at the outlet and also
circular and rectangular
maintaining
absorber fin was done.
reduction, to improve the heat
the
the [46]
pressure
transfer rate by increasing the area. M. S. Hossain
An investigation of the effect
It was found that the thermo- [47]
et.al.
of thermal conductivity of the
siphon
absorber plate of a thermo-
efficiency of 18% higher than
siphon solar water heater was
that of the conventional system
done.
by reducing heat loss.
Xinru Wang
Different types of existing
After the review, it was found [48]
et.al.
ASHP (air source heat
that the PV-ASHP has shown
pump) were reviewed based the
system
best
possesses
an
techno-economic
on their boundary
performance
and
conditions, System
payback
configuration, Performance
PV/T-ASHP and ST-ASHP
time
moderate concerning
indicators, Research methodologies, and system performances. M. Sudhakar
Optimization and
Among the four types of cell [49]
et.al.
parameters were varied to
arrays,
improve the performance of junction 8|Page
the
GaAs
shows
the
triple best
trough concentrating
performance. Also, the mirror
photovoltaic thermal
reflectivity 0.92 shows better
system. Different types of
solar power than 0.69
receiver shapes were used to verify the impact. For performance improvement, four types of solar cell arrays were used. result
were [50]
María Herrando
A combined solar cooling,
The
found
et.al.
heating, and power
compared with the evacuated
application are used to meet tube collector. It was found the energy demand based
that the system can meet up
on a hybrid PV/T collector.
20.9%, 55.1% and 16.3% of
Other energy costs, such as
the space heating, cooling, and
electricity cost, Gas bills,
electrical
and investment costs, were
experimental place. Moreover,
taken into consideration for
it can prevent 911 tons of CO2
calculating the payback
emission per year.
demands
of
the
period. Brice Lecœuvre
A new system was
Astronomical parameters were [51]
et.al.
introduced named
considered to
Orientable Blades
operate the panel to optimum
Reflective System (SRLO).
the
In this system, thermal and
parameters
electrical energy can be
control the command of the
produced combinely.
position.
Nurul Shahirah
A review has been done on
Different types of water-based [52]
Binti Rukman
the exergy and energy of
PV systems were studied and
et.al.
the water-based PV system.
also the review was done on
output.
control
Also,
were
used
and
the to
the efficiency of the PV system conditions.
9|Page
for
different
Ashish Saxena
Photovoltaic thermo control For an intermittent cooling [53]
et. al.
was done in a laboratory by
state, the flow rate was 3
varying the parameter
lit/min, 5.3 lit/min, 6.2 lit/min
which controls the solar
and for continuous cooling the
panel temperature and
flow rate was 0.6 lit/min. It
performance. During the
has been seen that during
experiment, the irradiance
intermittent
over the PV panel was
performance was increased by
varied from 87.38 W/m² to
18%
359.17 W/m² with a
cooling. which was 29% in
different flow rate.
case of continuous cooling.
with
cooling
respect
the
to
no
1 2
According to the above literature review, very limited research work has been carried out to upgrade
3
the thermal efficiency of the solar water heater. Till now, most of the studies are done concentrating
4
on the single and double-pass solar air heater. To the best of author knowledge, no such research work
5
has been done previously to improve the thermal efficiency considering the configuration of a double
6
pass solar water heater. Additionally, the reflector has been utilized to boost up the performance of a
7
double pass solar water heater. This new research work could be proved beneficial for the
8
policymaker to come up with some new strategies to fight against the world energy crisis. By using
9
the double pass, the water gets sufficient time to be heated properly, and the reflector adds additional
10
heat to the collector. As a result, the efficiency of the system has been increased by a remarkable
11
amount. The conventional solar water heater that was used in previous researches are lack of this type
12
of improvement.
13
The main objectives of this article are (i) to introduce a new concept of double pass solar water heater
14
using reflector in order to improve its thermal efficiency, (ii) to discuss the design, construction and
15
performance scrutiny of double pass solar water heater briefly, (iii) to analyze the performance of
16
double pass solar water heater theoretically and experimentally, (iv) to analyze the consequence of the
17
mass flow rate on the outlet temperature, thermal performance, and overall performance of double
18
pass solar water heater (v) to compare the efficiencies of single pass and double pass solar water
19
heater. In fulfilling this, the present article goes further through the following section: firstly, the
20
theoretical analysis of experimental design has been discussed in section 2. Secondly, Section 3 talks
21
about the experimental setup and procedure. Thirdly, section 4 briefly discussed the results and
22
discussion of the experiments. Finally, section 5 is all about the final summarization.
23 24
2. THEORETICAL ANALYSIS OF EXPERIMENTAL DESIGN 10 | P a g e
1
2.1 Experimental Design
2
The design and the experimental expectation are based on some assumptions, which made the
3
experiment possible to exclude some complex terms. So, the assumptions are, •
4
Absorber plate and bulk fluid are functions of flow directions, and both glass protection and
5
fluid are not involved in any solar radiant energy absorption [54].
6
•
The model only considers total irradiation.
7
•
Consideration is not bound to long and short wavelengths.
8
The governing equation required for the experimental work are listed in Table 2 as below,
9 10
Table 2: Governing equations for the design and calculation of solar water heater performance Equations =
−
=
!"
=
No.
?@
. 1 ).+,+ + /0 # $ (
2.
−
%&'
+
7
1
1 E 1 + +G + ABℎD ABF
=
−
K =mL
(1)
(
M
-
N
−
34
+2
$
+
5
6 $
8 +%&'#8 6 9< 9 &).);+,%48#9: 5 :
1 HB + ? − B
(2) =
(3) J I
(5)
)
(6)
∑ Ac × P
(4)
O th= ∑ R0 T
(7)
S
11 12
Various equations are used for calculating the design criterion. Equation (1) is used for governing the
13
calculation. Where,
14
U
15
=
+
+
and
=heat absorbed by the absorber
16
2.2 Top, Bottom and Side Loss Coefficient
17
After that, the top loss is calculated by using the equation (2) which is Hottel and Woertz, Klein
18
empirical equation [55]. Here,
11 | P a g e
VWX.XVY MZ[ ).+,+
1
N= number of glass cover=1; C=
2
angle).
3 4
).+;
; where collector tilt from horizontal \ = 30W (tilt
L= 3 inch = 0.0762 m 4Distance between the glass cover and absorber plate 5Now, C=365.705 C is the constant depending on tilt angle & space between collector plate & glazing material. Y /0
bW $ ! ! /0 + b.c.Y
−
1 + 0.091e
5
In
6
(8)
7
collector= 3 m/s, hw = Heat transfer co-efficient of the wind (W/m2-K); f =emmittance of absorber
8 9 10
equation
(2),
f=
Where, ℎK =heat transfer co-efficient of wind= 2.8+3V=2.8+3×3=11.8; V=velocity of air over plate =0.05; fg =emmitance of glass=0.07; σ= Stephen-Boltzmann constant= 5.67×10–8 W/m2-K4; Assume,
=ambient temperature =32° C,
11
From equation (2),,
12
=1.811 W/m2. Z =0.034
=mean temperature=51° L,
=outlet temp=70° C,
=1.811 W/m2; Consider bottom side heat loss is equal to top the plate.
W/m2;
=1.811+1.811+.034=3.656 W/m2.
13
Assume
14
The outlet temperature is calculated by using overall heat loss coefficient, flow factors, and collector
15
efficiency. Along the flow direction, the overall heat loss coefficient is assumed constant. For design
16
and fluid-flow rate, the collector efficiency factor is considered constant. It is denoted by Fp.
17 18
Collector efficiency factor is defined as Fc =
19
collection of actual useful energy useful energy collected when local fluid temperature is maintained at the entire absorber surface (Considering Aluminum as in absorber material)
20 21
2.3 Fin efficiency factor
22
For determining the plate efficiency factor, first, have to determine the fin efficiency factor from
23
equation (3) [56].
24
Here, Fin efficiency factor is given as,
25 26 27 28
= (9)
$ ~#• + $ ~#• +
/}
€
‚ƒ
Where, •V = „
: …:
; † = Thermal The conductivity of plate, ‡ = Plate thickness
b.‹YVb
ˆ‰, •V = bŒ×.WWb ;
30
So, • = 6.088 E2.; W= 0.0254 m, D = 0.0127 m, F = 0.90 W/m2K,
31
So, after calculating all the values, plate efficiency factor
29
12 | P a g e
= 0.902.
1
=Solar
2
After that, equation (4) is used to calculate the heat absorbed by the absorber plate where,
3
radiation. Equation (5) is used to calculate the heat taken by the water. Then, the total heat absorbed
4
by the collector is calculated by using the equation (6). Finally, equation (7) is used to calculate the
5
collector efficiency Where,
= Collector area and
= Solar Intensity [28].
6 7
2.4 Design calculation of collector plate
8
For doing the design calculation, some parameters have been assumed, and some values are taken into
9
consideration.
10
Assume,
11
Mass flow rate 0.00097kg/s;
=ambient temp. =32° L;
°
12
temp=70 C; Heat took away by water
13
material); And collector efficiency factor
14
heat absorbed by the collector,
15
As
>
16
K ,
2.5 Specific dimension
18
For design consideration
20
=outlet
’ =0.90;
Amount of
W/m ; (Considering, Aluminum as in absorber
= 0.90; Now, plate efficiency factor
= 619.60 W/m2
the design is considered to be correct [57].
17 19
K =558.6
2
=mean temperature=51° L;
=
K;
area
=0.90 E2 , Length = 0.7493 m, Width = 1.2065 m (This is
available in the market), Diameter of copper pipe=0.0127 m (12.7 mm), Design Efficiency, O th= 65.33%,
21 22
23 24
(a)
25
Figure 5: Design (a) and Cross-sectional view (b) of the collector.
26 27
(b)
Here, Distance between two copper pipe = 0.127 m (5 inches)
13 | P a g e
1
Gap between absorber plate and glass cover= 0.0762 m (3 inch)
2
Length of the collector =1.2065 m (47.5 inch)
3
Width of the collector = 0.7493 (29.5 inch)
4 5
Table 3: The specifications of double-pass solar collector components Components
Dimensions
Remarks
Collector
1.2065m × 0.7493 m × 0.1778 m
Gross area=0.1607m2
Absorber plate
1.2065m × 0.762 m × 0.0007m
Material: Aluminum
Reflector
1.2192m × 0.762 m × 0.007 m
5 mm thick mirror with wooden frame
Transparent glass cover
3.5 mm thick
Material: Window glass 2 Glass cover (Top and bottom side)
Copper tube
Edge insulation
12.7 mm in dia,127 m distance
Number of each side contains 11
between two pipes
tube
0.0127 m thick cork sheet
Material: Cork sheet
6 7
3. EXPERIMENTAL SETUP AND PROCEDURE
8
3.1 Experimental set up
9
The schematic view of the experimental set up is shown in Figure 3, which contains a solar collector,
10
reflectors, closed working fluid system, and measurement devices. The working fluid system has a
11
tank, a pipes system and a simple control valve that has been used to control the flow rate of working
12
fluid. The solar collector consists of an aluminum sheet as an absorber plate mounted in the middle of
13
a wooden box shown in Figure 6. A total of 22 copper tube is placed attached on both sides of the
14
aluminum sheet. Each side contains 11 numbers of the tube. The transparent glass cover of 3.5 mm
15
thickness has fitted both sides of the absorber plate at a gap of 75 mm between the absorber plate and
16
glass cover inside of the wooden box. Walls are insulated with cork sheets to minimize conduction
17
losses.
14 | P a g e
1 1 2 3 4 5
Transparent Cover Absorber plate Copper plate Insulating Material Solar and Reflector Support
6 7 8 9
Coldwater inlet port Hot water outlet port Reflector surface Anti-refraction coating
2 3
(a)
4 1 2 3 4
Water flow direction Copper pipe Glass Solar Collector
5 6
(b)
15 | P a g e
1 2
(c)
3
Figure 6: Schematic diagram of the (a) experimental set up with appropriate dimension (b) upper
4
channel and lower channel with proper dimension (c) experimental set up (three-dimensional view).
5 6
The collector is moved with the position of the sun and kept an angle of 30° with the horizontal [58].
7
The reflectors are placed behind the collector at the same angle with the collector to reflect the
8
maximum solar irradiance [59]. With the change of the collector position, the reflector position is also
9
changed accordingly. Eight thermometers are installed at different points on the collector to calculate
10
the temperature of the absorber plate, glass, and copper tube and the air opening between the glass
11
cover and plate [60]. A thermocouple measures inlet and outlet temperatures. The collector is
12
supported on a frame made of wood of appropriate size according to the collector area. The inlet and
13
exit pipe of the collector is extended to simplify the dimension of water temperature, respectively
14
[61]. The inlet section is connected to a tap of a gravity water reservoir to draw the ambient water
15
over the entry unit. The mass flow rate of the water has been measured by volume basis in respect of
16
time by a simple beaker [62].
17 18
3.2 Experimental procedure
19
The experiment is done at Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh,
20
in November 2017. Different mass flow rates of 0.0022 kg/s,0.0028 kg/s,0.0033 kg/s,0.0039 kg/s and
21
0.0044 kg/s are used.
16 | P a g e
1
Water passes from the top of the absorber plate and then goes to the bottom of the absorber
2
plate as a result water gets heated inside the collector due to continuously falling sun rays on
3
the top glass cover of collector and water also gets heated on the bottom absorber plate
4
because sun rays are reflected on the bottom glass cover with a reflector Figure 7 [63].
5 6
Figure 7: Heat gain phenomena in the solar water heater.
7 8
The solar collector collects direct and diffused solar radiant from the sun. A reflector collects
9
direct radiation and reflects the diffused radiation portion onto the bottom part of the double-
10
pass solar water heater. The absorber plate and pipe first absorb heat. Then heat is passed to
11
the water. Water first gets heat from the top absorber plate and again gets heat from the lower
12
absorber plate. As a result, water is getting more heat than the single-pass solar water heater
13
[64].
14
Finally, the hot water is exhausted through the exit port. The data is taken in every 30
15
minutes’ interval when water is circulated continuously through the collectors. All the tests
16
are taken between 9.00 AM and 4.00 PM.
17
The reflector and collector area are taken as 0.90 m2 and 0.929 m2, respectively. The mirror
18
glass reflectivity is taken as 0.9, and the total intensity factor and total occurrence radiation on
19
the bottom of the collector surface area calculated as 0.9×0.7 or 0.63 and around 70%,
20
respectively [65]. The average solar intensity for November is taken as 4.2 kWh/m2 or 600
17 | P a g e
1
w/m2 [66]. The total heat absorbed by water and collector is evaluated by the equation (3) and
2
(4), (6). The thermal efficiency of the collector is obtained using equation (5).
3 4
4. RESULTS AND DISCUSSION
5
The solar radiation mainly depends on location and local weather. The experiments are performed in
6
November with mostly clear sky conditions. But the solar intensity is not so much because November
7
is in the winter season in Rajshahi [67]. The performance of a double-pass solar collector is
8
represented based on efficiency which is obtaining by measuring some parameters such as inlet and
9
outlet temperature, flow rate. The half an hourly variation of the temperature difference between the
10
inlet and outlet water, collector efficiency, and comparison between single pass and double pass solar
11
water heater is shown in the respective figures below.
12 13
14 15
Figure 8: variation of the highest efficiencies in double-pass solar water heater with a different mass
16
flow rate of water.
17 18
Figure 8 shows the embedded relation between mass flow rate and the highest efficiency of the solar
19
water heater in 4 experimental days. In day 1, the mass flow rate of the water is considered 0.0022
20
kg/s. The water is flowing through the copper pipe and got heated by solar radiation, and it gives an
18 | P a g e
1
efficiency of 47.61% which is highest. After the first experimental day, the mass flow rate of water is
2
increased to observe the change in the thermal efficiency of the system. It has been seen that the
3
thermal performance of the system has been increased by a significant amount. On day 2, the
4
efficiency is 47.98% at a mass flow rate of of0.0028 kg/s. At days 3 and 4, with a mass flow rate of
5
0.0033 kg/s and 0.0039 kg/s, the efficiency is recorded as 48.82% and 48.8%, respectively. At day 5,
6
the highest predictable efficiency is obtained by 50.26% with a mass flow rate of 0.0044 kg/s. The
7
efficiency of the double pass solar water heater generally depends on the solar radiation, mass flow
8
rate, and the geometry of the collector[68] (ref:). The increasing mass flow rate causes the increase of
9
the heat transfer coefficient and thus results in higher efficiency [69]. Figure 9 shows the temperature
10
variances ∆T = Tout − Tin versus day time for various mass flow rates on a clear day in Rajshahi for a
11
double pass solar water heater with reflectors.
12
13 14
Figure 9: Double pass solar water heater outlet and inlet water temperature difference vs. day time for
15
different mass flow rates.
16 17
The above figure shows that the temperature difference of the collector enhances with the increment
18
of solar radiation,
19 | P a g e
(W/m2) as expected and reached a peak at 12.30 PM of the day as the solar
1
intensity is maximum at this point. The temperature variance increases with decreasing mass flow
2
rates. The temperature difference is a maximum of 44.9 °C for the mass flow rate of 0.0022 kg/s and a
3
minimum of 8.6°C for the mass flow rate 0.0044 of kg/s. As the mass flow rates increase from 0.0022
4
kg/s to 0.0044 kg/s, the outlet temperature of the flowing water in the collector decreases. The reason
5
behind this reducing nature is the increasing volume of water flow inside the tube at a time.
6
Efficiency versus daytime at various water flow rates for the double pass solar water heater is shown
7
in Figure 10. The efficiencies are improved to a maximum level in between 12:00 PM to 1:00 PM.
8
9 10
Figure 10: Double pass solar water heater Efficiency vs daytime for diverse mass flow rate.
11 12
Figure 10 shows the variation in the efficiency of double pass solar water heater at different mass
13
flow rates ranging from 0.0022 kg/s to 0.0044 kg/s. The efficiency of collector increases with the
14
increasing time along with higher mass flow rates. The maximum efficiency is about 50.26% at 12:30
15
PM for the mass flow rate of 16 kg/ hr. The minimum efficiency is about 14.84% at 9:00 AM for the
16
mass flow rate of 0.0022 kg/s. The graph shows that the thermal proficiency of the double pass solar
17
water heater is improved with increasing water mass flow rate. The cause of increasing thermal
18
efficiency for a DPS is due to using the reflector on the bottom side of the collector. The maximum
20 | P a g e
1
temperature reached by the set up with a glass as the cover is 74.7°C at that time the inlet temperature
2
is 29.8 °C. The maximum efficiency of the collector is 50.26%, which is less than our design
3
efficiency; due to some losses are occurred. Again, the fluctuation of solar radiation concentration
4
affects the performance of the system.
5 6
7 8
Figure 11: Highest temperature difference observation in the double-pass solar water heater with a
9
variation in mass flow rate.
10 11
Figure 11 shows the highest temperature difference of the solar water heater in 4 experimental days
12
with the respective mass flow rate. The temperature difference is an important factor in evaluating the
13
performance of the system. The more the temperature difference of the water heater, the more
14
efficiency can be obtained. It has been seen that, with an increasing rate of mass flow rate, the
15
temperature alteration of the system is decreased. This type of decrement happens because, with the
16
increasing rate of mass flow rate, the water in the copper tube gets less time to captivate the solar
17
radiation. So, the water of the system did not get the time to rise to the desired amount. At day 1, the
21 | P a g e
1
highest temperature difference is obtained 44.9 °C. On day 2 and day 3, the difference is recorded,
2
respectively 35.6 °C and 30.7 °C. The lowest difference is obtained at 23.7 °C on day 5 with a mass
3
flow rate of 0.0044 kg/s.
4
The following Figure 12 shows the efficiency concerning the time of the day curve at a specific mass
5
flow rate of 0.0033 kg/s for a double pass and single-pass water heater. The data for the single-pass
6
solar water heater is collected in April 2016. The solar intensity in April is generally higher than in
7
our experimental month of November. The maximum efficiency is 38.1% at 1:00 PM for the mass
8
flow rate of 0.0033 kg/s for a single pass water heater.
9
10 11
Figure 12: Efficiency difference between a single pass and double pass solar water heater.
12 13
For a double pass solar water heater, the highest efficiency is 47.98% for the same mass flow rate of
14
0.0033kg/s. Although the data from our experiment is collected in November 2017, where the solar
15
intensity is less than in April. But the efficiency is higher than the single-pass water heater for the
16
same mass flow rate. From the above Figure, it is clear that the efficiency of the double-pass solar
17
water heater is higher than the single-pass water heater. The efficiency of the double pass solar water
22 | P a g e
1
heater would show better efficiency if the data were taken in April 2017. Moreover, utilized sensors
2
have some errors that affect the efficiency of the double-pass solar water heater. Errors of different
3
sensors used in these experiments are enlisted in Table 4. Although the individual error percentage is
4
not that much combined, it has a non-negligible effect on the efficiency. Apart from that, the top,
5
bottom, and side loss of heat are also responsible for not getting actual efficiency.
6 7
Table 4: List of various sensors used in this system along with their Error percentage List of sensors
Error percentage
DHT 11 (temperature and humidity sensor) for air
5% for humidity and ±2% for temperature
Digital Multimeter (VEYRON VL9205)
DC Voltage: ±(0.5%+1dgt). AC Voltage: ±(0.8%+3dgt). DC Current: ±(0.5%+1dgt). AC Current: ±(1.0%+3dgt). Resistance: ±(0.8%+1dgt). Capacitance: ±(4.0%+3dgt).
Anemometer (Model: ML-81Am)
≤ 20 m/s: ± 3% FS > 20 m/s: ± 4% FS
Photodiode (intensity measurement)
±2.25V to ±18V
LM 35 (temperature sensor) for water
±0.5%
8 9
5. CONCLUSIONS
10
Double pass solar water heater with a reflector at the bottom of the collector is investigated
11
experimentally and compared with recorded data of conventional single-pass solar water heater. From
12
this, it can be concluded as follows:
13
1. The efficiency shows a proportional relationship with the mass flow rate. Although the
14
contribution from the reflector is not that much, it is not negligible. A maximum of 50.26%
15
efficiency at flow rates of 0.0044 kg/s has been obtained from the system. The mass flow rate
16
exhibits inversely proportional relation with the temperature difference (in between the inlet
17
and outlet flow), i.e., the highest temperature difference is found at the mass flow rate of
18
0.0022 kg/s. However, the temperature difference directly related to solar radiation and it
19
increases with the increasing solar radiation. Moreover, the experimental results depict that
20
the highest efficiency has been found at 12:30 PM because the solar radiation is at the highest
21
peak at that particular time of the day.
23 | P a g e
1
2. An in-depth comparison has been made between the double pass and single-pass solar water
2
heater at a mass flow rate of 0.0033 kg/s. The results indicate that the double pass provides
3
better efficiency than the single-pass, although the data were not taken at the same time of the
4
year. The potential reason is the addition of the reflector in the experimental system. In a
5
word, it can be concluded that the productivity of the experimental double pass solar water
6
heater with reflector is greater than a conventional single-pass flat plate solar water heater.
7 8
Acknowledgment
9
The support of the Rajshahi University of Engineering & Technology (RUET) is gratefully
10
acknowledged, in particular, authors are thankful to the Department of Mechanical Engineering for
11
financial support. The authors are also thankful to Rasel Hossain and Md. Adnan Hossain.
12 13
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28 | P a g e
HIGHLIGHTS • • • •
A new method with reflector is introduced in double pass solar water heater. Design, Modelling and Construction is done for the new method with reflector. The reflector addition shows 9%-15% better results in appraisal to the single-pass. The highest efficiency obtained by using reflector is 50.26% at a mass flow rate of 0.0044kg/s.
Authors have no potential conflict of interest
S0960-1481(19)31869-5
DOI:
https://doi.org/10.1016/j.renene.2019.11.160
Reference:
RENE 12708
To appear in:
Renewable Energy
Received Date: 9 August 2019 Revised Date:
1 November 2019
Accepted Date: 30 November 2019
Please cite this article as: Mandal S, Ghosh SK, Experimental investigation of double pass solar water heater by using reflector, Renewable Energy (2020), doi: https://doi.org/10.1016/j.renene.2019.11.160. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Author Contributions Manuscript title: Experimental Investigation of Double Pass Solar Water Heater by Using Reflector
Author 1: Soumya Mandal Conceived and designed the analysis The structure of the manuscript and related analysis was done by the author. Wrote the paper Editing and Graphs were drawn by the author Supervision The author investigated and supervised all the factors that enhance the quality and impact of the manuscript. Verification This author verified the analytical methods helped with technical details. This author contributed to the design and implementation of the research, to the analysis of the Results and to the writing of the manuscript with Author 2.
Author 2: Subir Kumar Ghosh Conceived and designed the analysis Designs of the arrangement and related analysis were done by this author Collected the data Data collection and calculation was done by this author Performed the analysis Developed the theoretical framework and some of the analysis was done by this author with the help of author 1. Wrote the paper The manuscript was constructed and written by this author with the help of author 1.
All authors discussed the results and contributed to the final manuscript
1
Experimental Investigation of Double Pass Solar Water Heater by Using
2
Reflector
3
Soumya Mandala, b, Subir Kumar Ghosha, a
4
Department of Mechanical Engineering, Rajshahi University of Engineering and Technology,
5 6
Rajshahi 6204, Bangladesh. b
Department of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater-
7
74078, USA
8 9 10 11 12 13 (*) Corresponding author E-mail
:[email protected]
Tel
: +8801773246797
Postal Address
Department of Mechanical Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh
14 15 16 17 18 19 20 21 22 1|Page
1
Abstract
2
A new concept of double pass solar water heater using reflector has been introduced in this paper to
3
improve its thermal efficiency. Here the design, construction, and performance scrutiny of double
4
pass solar water have been discussed. The consequence of the mass flow rate on the outlet
5
temperature, thermal performance, and overall performance has been analyzed. Moreover, the
6
performance result has been compared with the single-pass solar water heater. The data are taken for a
7
different mass flow rate of 0.0022kg/s, 0.0028kg/s, 0.0033kg/s, 0.0039kg/s and 0.0044kg/s from 9
8
AM to 4 PM at an interval of 30 minutes. The maximum efficiency obtained for a higher mass flow
9
rate of 0.0044kg/s is 50.26%. The maximum efficiency for other mass flow rates is 47.61%, 47.98%,
10
48.82%, and 48.89%, respectively. All these efficiencies are found in the middle of the day around
11
12.00 PM to 1.00 PM as the maximum solar radiation is observed in that period. Though the sunlight
12
radiation is not much higher in this month for the same mass flow rate, the efficiency of double pass
13
solar water heater using reflector is much greater than a conventional single-pass solar water heater.
14 15
Keywords: Solar radiation, Double pass solar water heater, Reflector, Mass Flow Rate,
16
Efficiency, Temperature Difference.
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 2|Page
1
1. INTRODUCTION
2
Energy is one of the most basic elements of the universe, without which nothing is possible in the
3
present days. The most interactive form of energy that is being given careful attention is renewable
4
and non-renewable energy [1][2]. As the non-renewable sources (Coal, Gas, Oil, etc.) are limited in
5
nature and can’t be compensated once these are used, which shifted the attention towards the
6
alternative energy sources [3][4]–[6]. World energy demand growth has a very close relationship with
7
the energy demand of emerging countries [7]. Bangladesh is a developing and overpopulated country
8
in South Asia [8]. The energy demand is increasing day by day. Figure 1 shows the future energy
9
projection of Bangladesh.
10 11
12 13
Figure 1: Future energy projection in Bangladesh[NP=Nuclear Power; RE=Renewable Energy;
14
B&W=Biofuel and Waste; P(Export)=Power(export)] [8]
15 16
Figure 1 predicted the future energy demand of Bangladesh concerning the present condition. From
17
the Figure, it is seen that in the future, the gas and oil will be in peak demand. Bangladesh has one
18
Nationwide Network to supply the electricity with a mounted capacity of 15,379 MW as of February
19
2017 [9]. Recently the construction of 2.4 GW capacity Rooppur nuclear power plant has been started
20
which will start operation in 2023 [10]. Bangladesh is very much dependent on gas, which fulfills
21
more than 70% of its energy needs [11]. But the gas resources are limited, and consequently, the
22
energy crisis is increasing day by day. Some of the energy comes from oil and coal. But it is not so
3|Page
1
significant as the reserve of oil and coal is limited [12]. That’s why it is high time to find alternative
2
energy sources for a sustainable future [13]. In this esteem, from the perspective of Bangladesh, solar
3
energy seems to be the most available and efficient form of renewable energy [14]. Figure 2 shows
4
the solar energy-based power installation in Bangladesh.
5 6
Figure 2: Solar energy power installation in Bangladesh [15].
7 8
Though the installation of solar energy in Bangladesh is increasing with the advent of time, it is facing
9
trouble to meet the demand. By the end of the year, a solar installation can fulfill almost 190.6 MW
10
demand throughout the country.
11
Bangladesh is situated in the northeastern part of South Asia, and that is why it gets sufficient sunlight
12
[16]. The solar radiation in Bangladesh varies from 3.8kWh/m2/day to 6.4 kWh/m2/day [17]. The
13
maximum solar radiation is recorded during the month of March-April and lowest in December-
14
January[18]. The optimistic sunshine time in Bangladesh varies from 4.7-7.6 hours over the year-
15
round from dry to monsoon season [22]. A study that was held in 2012 has reduced the sunshine time
16
by 54% to the justification for rainfall, cloud, and fog [23]. This larger amount of solar radiation can
17
be used to produce at least 22.2-28.5×108 MW power per annum [24]. So, solar energy has the
18
potential to ensure an eco-friendly environment for future growth [25]. Bangladesh's government has
19
already taken some initiatives to produce 10% of total power generation by 2020 from renewable
20
energy sources like solar energy, wind & waste [26] [27]. The study area Rajshahi is situated on the
21
northern side of Bangladesh. Figure 3 shows the solar radiation in different divisional cities in
22
Bangladesh.
4|Page
1
2 3
Figure 3: solar radiation in a different division in Bangladesh [19]
4 5
Figure 3 shows the solar contamination scenario in the six divisional areas in Bangladesh.
6
Bangladesh has a very impressive position concerning solar radiation. The average solar radiation in
7
Bangladesh varies from 1200-1800 KWhr/m2. This potential can help its population to fulfill its
8
energy crisis from this form of renewable energy [20]. Though some division in Bangladesh shows
9
higher values of solar radiation, the experimental area Rajshahi shows a very uniform change rate
10
throughout the year than another area. Figure 4 shows the average sunshine time in the different
11
divisional areas in Bangladesh.
12
5|Page
1 2
Figure 4: Variation of sunshine time in Different division in Bangladesh [19]
3 4
Solar energy is a form of electromagnetic energy which comes in the form of electromagnetic
5
radiation [21][22]. A solar air heater is a popular form of utilization of solar energy, which absorbs
6
this solar radiation and heating air flowing through it [23] [24] [25]. Thus, the temperature of the air
7
increased by a substantial amount for using it for various purposes, like space heating for small
8
households and the drying of different required elements [26][27]. Moreover, a solar air heater has an
9
extensive field for using it in the winter dominated country [28]. But due to low compactness and
10
small heat conductive capacity, the performance of the conventional single-pass solar air heater is
11
relatively low [29]. For this reason, the double pass solar air heater has been developed [30]. In the
12
case of a double pass, the air gets sufficient time to be heated by moving in the path [31]. But this
13
development also fails to utilize solar energy to the desirable amount [32].
14
Solar water collector is another kind of application of solar energy [33]. In case of it, the water flows
15
through a copper or other type of heat conductive metal pipe. The heat from the solar radiation heated
16
the pipe, and thus the water gets heated [34]. This heated warm water can be utilized in various ways,
17
such as in the bathroom, kitchen, laboratory, household purposes [35]. Moreover, as the conductive
6|Page
1
heat coefficient of water is high, so the efficiency of the system is quite impressive and far better than
2
the solar air heater [36]. The conventional single-pass solar water heater is generally used to heat the
3
water. It uses some circular or rectangular types of copper tube to run the water and to be heated [37].
4
The solar water heater is a device that absorbs heat from sunlight radiation & consequently heating
5
water [38]. This preheated water can be used for normal cooking, hospitals, hotels, and hot water, also
6
used for textile & paper industries [39]. The double-pass solar water heater consists of two parts the
7
top and bottom parts [40]. An absorber plate separates these two parts. Pipes or tubes are mounted on
8
the upper and below the plate [41]. A glass or plastic cover is used on both sides. Sunlight which is
9
coming from the sun directed towards the upper plate through the glass shelter [42]. Thus, the
10
absorber plate absorbs the sunlight radiation and consequently heating the water inside the pipe.
11
Moreover, reflectors can be used to provide the additional heat to the inside water by placing it at the
12
bottom part.
13
Many research and experiments have been done to improve the thermal effectiveness of the solar
14
water heater. Some of the prominent research are listed in Table 1 below, with the key features and
15
key findings.
16 17
Table 1: Literature review of the solar water heater. AUTHOR
KEY FEATURE
KEY FINDINGS
REFERENCES
NAME Sivakumar et.al
Various design parameter
59.09% efficiency was obtained [43]
variation was done, such as a
for the variation of the number of
number of riser tubes and
riser
arrangement of riser tube in a
efficiency was obtained for the
zig-zag pattern from the
zig-zag arrangement of the tubes.
tubes,
and
62.90%
existing collector. Mongre and
A circulating pump, Glass,
The efficiency was increased by [36]
Gupta
Insulator, Aluminum tube,
55% with increasing the glazing
water container was used for
area.
the experiment. Himangshu
A new approach of the
By using reflector increases the [44]
Bhowmik and
reflector was used in the
reflectivity of the collector and
Ruhul Amin
experiment with the ability to
thus obtained the improvement
change its position with the
of the collector efficiency around
sun’s position.
10%.
Shahidul Islam
Two solar water heaters of
It was found that the incoming [45]
Khan and Asif
100 liters and 200 liters were
hot tap water was about 300C
7|Page
Islam
used.
higher than the room temperature during the winter months.
Sathish. D et.al.
An analysis was done on the
The maximum efficiency was [37]
improvement of the
observed at the time of 1.00 PM
performance of the solar water on an average of 57% heating system on different tube arrangements. The literature was reviewed to improve thermal efficiency by varying the tube arrangement and number of riser tubes S. Ramasamy and
an experiment of thermal
The
main
objective
of
P.
performance analysis of the
experiment was to maintain the
Balashanmugam
solar water heater with
velocity at the outlet and also
circular and rectangular
maintaining
absorber fin was done.
reduction, to improve the heat
the
the [46]
pressure
transfer rate by increasing the area. M. S. Hossain
An investigation of the effect
It was found that the thermo- [47]
et.al.
of thermal conductivity of the
siphon
absorber plate of a thermo-
efficiency of 18% higher than
siphon solar water heater was
that of the conventional system
done.
by reducing heat loss.
Xinru Wang
Different types of existing
After the review, it was found [48]
et.al.
ASHP (air source heat
that the PV-ASHP has shown
pump) were reviewed based the
system
best
possesses
an
techno-economic
on their boundary
performance
and
conditions, System
payback
configuration, Performance
PV/T-ASHP and ST-ASHP
time
moderate concerning
indicators, Research methodologies, and system performances. M. Sudhakar
Optimization and
Among the four types of cell [49]
et.al.
parameters were varied to
arrays,
improve the performance of junction 8|Page
the
GaAs
shows
the
triple best
trough concentrating
performance. Also, the mirror
photovoltaic thermal
reflectivity 0.92 shows better
system. Different types of
solar power than 0.69
receiver shapes were used to verify the impact. For performance improvement, four types of solar cell arrays were used. result
were [50]
María Herrando
A combined solar cooling,
The
found
et.al.
heating, and power
compared with the evacuated
application are used to meet tube collector. It was found the energy demand based
that the system can meet up
on a hybrid PV/T collector.
20.9%, 55.1% and 16.3% of
Other energy costs, such as
the space heating, cooling, and
electricity cost, Gas bills,
electrical
and investment costs, were
experimental place. Moreover,
taken into consideration for
it can prevent 911 tons of CO2
calculating the payback
emission per year.
demands
of
the
period. Brice Lecœuvre
A new system was
Astronomical parameters were [51]
et.al.
introduced named
considered to
Orientable Blades
operate the panel to optimum
Reflective System (SRLO).
the
In this system, thermal and
parameters
electrical energy can be
control the command of the
produced combinely.
position.
Nurul Shahirah
A review has been done on
Different types of water-based [52]
Binti Rukman
the exergy and energy of
PV systems were studied and
et.al.
the water-based PV system.
also the review was done on
output.
control
Also,
were
used
and
the to
the efficiency of the PV system conditions.
9|Page
for
different
Ashish Saxena
Photovoltaic thermo control For an intermittent cooling [53]
et. al.
was done in a laboratory by
state, the flow rate was 3
varying the parameter
lit/min, 5.3 lit/min, 6.2 lit/min
which controls the solar
and for continuous cooling the
panel temperature and
flow rate was 0.6 lit/min. It
performance. During the
has been seen that during
experiment, the irradiance
intermittent
over the PV panel was
performance was increased by
varied from 87.38 W/m² to
18%
359.17 W/m² with a
cooling. which was 29% in
different flow rate.
case of continuous cooling.
with
cooling
respect
the
to
no
1 2
According to the above literature review, very limited research work has been carried out to upgrade
3
the thermal efficiency of the solar water heater. Till now, most of the studies are done concentrating
4
on the single and double-pass solar air heater. To the best of author knowledge, no such research work
5
has been done previously to improve the thermal efficiency considering the configuration of a double
6
pass solar water heater. Additionally, the reflector has been utilized to boost up the performance of a
7
double pass solar water heater. This new research work could be proved beneficial for the
8
policymaker to come up with some new strategies to fight against the world energy crisis. By using
9
the double pass, the water gets sufficient time to be heated properly, and the reflector adds additional
10
heat to the collector. As a result, the efficiency of the system has been increased by a remarkable
11
amount. The conventional solar water heater that was used in previous researches are lack of this type
12
of improvement.
13
The main objectives of this article are (i) to introduce a new concept of double pass solar water heater
14
using reflector in order to improve its thermal efficiency, (ii) to discuss the design, construction and
15
performance scrutiny of double pass solar water heater briefly, (iii) to analyze the performance of
16
double pass solar water heater theoretically and experimentally, (iv) to analyze the consequence of the
17
mass flow rate on the outlet temperature, thermal performance, and overall performance of double
18
pass solar water heater (v) to compare the efficiencies of single pass and double pass solar water
19
heater. In fulfilling this, the present article goes further through the following section: firstly, the
20
theoretical analysis of experimental design has been discussed in section 2. Secondly, Section 3 talks
21
about the experimental setup and procedure. Thirdly, section 4 briefly discussed the results and
22
discussion of the experiments. Finally, section 5 is all about the final summarization.
23 24
2. THEORETICAL ANALYSIS OF EXPERIMENTAL DESIGN 10 | P a g e
1
2.1 Experimental Design
2
The design and the experimental expectation are based on some assumptions, which made the
3
experiment possible to exclude some complex terms. So, the assumptions are, •
4
Absorber plate and bulk fluid are functions of flow directions, and both glass protection and
5
fluid are not involved in any solar radiant energy absorption [54].
6
•
The model only considers total irradiation.
7
•
Consideration is not bound to long and short wavelengths.
8
The governing equation required for the experimental work are listed in Table 2 as below,
9 10
Table 2: Governing equations for the design and calculation of solar water heater performance Equations =
−
=
!"
=
No.
?@
. 1 ).+,+ + /0 # $ (
2.
−
%&'
+
7
1
1 E 1 + +G + ABℎD ABF
=
−
K =mL
(1)
(
M
-
N
−
34
+2
$
+
5
6 $
8 +%&'#8 6 9< 9 &).);+,%48#9: 5 :
1 HB + ? − B
(2) =
(3) J I
(5)
)
(6)
∑ Ac × P
(4)
O th= ∑ R0 T
(7)
S
11 12
Various equations are used for calculating the design criterion. Equation (1) is used for governing the
13
calculation. Where,
14
U
15
=
+
+
and
=heat absorbed by the absorber
16
2.2 Top, Bottom and Side Loss Coefficient
17
After that, the top loss is calculated by using the equation (2) which is Hottel and Woertz, Klein
18
empirical equation [55]. Here,
11 | P a g e
VWX.XVY MZ[ ).+,+
1
N= number of glass cover=1; C=
2
angle).
3 4
).+;
; where collector tilt from horizontal \ = 30W (tilt
L= 3 inch = 0.0762 m 4Distance between the glass cover and absorber plate 5Now, C=365.705 C is the constant depending on tilt angle & space between collector plate & glazing material. Y /0
bW $ ! ! /0 + b.c.Y
−
1 + 0.091e
5
In
6
(8)
7
collector= 3 m/s, hw = Heat transfer co-efficient of the wind (W/m2-K); f =emmittance of absorber
8 9 10
equation
(2),
f=
Where, ℎK =heat transfer co-efficient of wind= 2.8+3V=2.8+3×3=11.8; V=velocity of air over plate =0.05; fg =emmitance of glass=0.07; σ= Stephen-Boltzmann constant= 5.67×10–8 W/m2-K4; Assume,
=ambient temperature =32° C,
11
From equation (2),,
12
=1.811 W/m2. Z =0.034
=mean temperature=51° L,
=outlet temp=70° C,
=1.811 W/m2; Consider bottom side heat loss is equal to top the plate.
W/m2;
=1.811+1.811+.034=3.656 W/m2.
13
Assume
14
The outlet temperature is calculated by using overall heat loss coefficient, flow factors, and collector
15
efficiency. Along the flow direction, the overall heat loss coefficient is assumed constant. For design
16
and fluid-flow rate, the collector efficiency factor is considered constant. It is denoted by Fp.
17 18
Collector efficiency factor is defined as Fc =
19
collection of actual useful energy useful energy collected when local fluid temperature is maintained at the entire absorber surface (Considering Aluminum as in absorber material)
20 21
2.3 Fin efficiency factor
22
For determining the plate efficiency factor, first, have to determine the fin efficiency factor from
23
equation (3) [56].
24
Here, Fin efficiency factor is given as,
25 26 27 28
= (9)
$ ~#• + $ ~#• +
/}
€
‚ƒ
Where, •V = „
: …:
; † = Thermal The conductivity of plate, ‡ = Plate thickness
b.‹YVb
ˆ‰, •V = bŒ×.WWb ;
30
So, • = 6.088 E2.; W= 0.0254 m, D = 0.0127 m, F = 0.90 W/m2K,
31
So, after calculating all the values, plate efficiency factor
29
12 | P a g e
= 0.902.
1
=Solar
2
After that, equation (4) is used to calculate the heat absorbed by the absorber plate where,
3
radiation. Equation (5) is used to calculate the heat taken by the water. Then, the total heat absorbed
4
by the collector is calculated by using the equation (6). Finally, equation (7) is used to calculate the
5
collector efficiency Where,
= Collector area and
= Solar Intensity [28].
6 7
2.4 Design calculation of collector plate
8
For doing the design calculation, some parameters have been assumed, and some values are taken into
9
consideration.
10
Assume,
11
Mass flow rate 0.00097kg/s;
=ambient temp. =32° L;
°
12
temp=70 C; Heat took away by water
13
material); And collector efficiency factor
14
heat absorbed by the collector,
15
As
>
16
K ,
2.5 Specific dimension
18
For design consideration
20
=outlet
’ =0.90;
Amount of
W/m ; (Considering, Aluminum as in absorber
= 0.90; Now, plate efficiency factor
= 619.60 W/m2
the design is considered to be correct [57].
17 19
K =558.6
2
=mean temperature=51° L;
=
K;
area
=0.90 E2 , Length = 0.7493 m, Width = 1.2065 m (This is
available in the market), Diameter of copper pipe=0.0127 m (12.7 mm), Design Efficiency, O th= 65.33%,
21 22
23 24
(a)
25
Figure 5: Design (a) and Cross-sectional view (b) of the collector.
26 27
(b)
Here, Distance between two copper pipe = 0.127 m (5 inches)
13 | P a g e
1
Gap between absorber plate and glass cover= 0.0762 m (3 inch)
2
Length of the collector =1.2065 m (47.5 inch)
3
Width of the collector = 0.7493 (29.5 inch)
4 5
Table 3: The specifications of double-pass solar collector components Components
Dimensions
Remarks
Collector
1.2065m × 0.7493 m × 0.1778 m
Gross area=0.1607m2
Absorber plate
1.2065m × 0.762 m × 0.0007m
Material: Aluminum
Reflector
1.2192m × 0.762 m × 0.007 m
5 mm thick mirror with wooden frame
Transparent glass cover
3.5 mm thick
Material: Window glass 2 Glass cover (Top and bottom side)
Copper tube
Edge insulation
12.7 mm in dia,127 m distance
Number of each side contains 11
between two pipes
tube
0.0127 m thick cork sheet
Material: Cork sheet
6 7
3. EXPERIMENTAL SETUP AND PROCEDURE
8
3.1 Experimental set up
9
The schematic view of the experimental set up is shown in Figure 3, which contains a solar collector,
10
reflectors, closed working fluid system, and measurement devices. The working fluid system has a
11
tank, a pipes system and a simple control valve that has been used to control the flow rate of working
12
fluid. The solar collector consists of an aluminum sheet as an absorber plate mounted in the middle of
13
a wooden box shown in Figure 6. A total of 22 copper tube is placed attached on both sides of the
14
aluminum sheet. Each side contains 11 numbers of the tube. The transparent glass cover of 3.5 mm
15
thickness has fitted both sides of the absorber plate at a gap of 75 mm between the absorber plate and
16
glass cover inside of the wooden box. Walls are insulated with cork sheets to minimize conduction
17
losses.
14 | P a g e
1 1 2 3 4 5
Transparent Cover Absorber plate Copper plate Insulating Material Solar and Reflector Support
6 7 8 9
Coldwater inlet port Hot water outlet port Reflector surface Anti-refraction coating
2 3
(a)
4 1 2 3 4
Water flow direction Copper pipe Glass Solar Collector
5 6
(b)
15 | P a g e
1 2
(c)
3
Figure 6: Schematic diagram of the (a) experimental set up with appropriate dimension (b) upper
4
channel and lower channel with proper dimension (c) experimental set up (three-dimensional view).
5 6
The collector is moved with the position of the sun and kept an angle of 30° with the horizontal [58].
7
The reflectors are placed behind the collector at the same angle with the collector to reflect the
8
maximum solar irradiance [59]. With the change of the collector position, the reflector position is also
9
changed accordingly. Eight thermometers are installed at different points on the collector to calculate
10
the temperature of the absorber plate, glass, and copper tube and the air opening between the glass
11
cover and plate [60]. A thermocouple measures inlet and outlet temperatures. The collector is
12
supported on a frame made of wood of appropriate size according to the collector area. The inlet and
13
exit pipe of the collector is extended to simplify the dimension of water temperature, respectively
14
[61]. The inlet section is connected to a tap of a gravity water reservoir to draw the ambient water
15
over the entry unit. The mass flow rate of the water has been measured by volume basis in respect of
16
time by a simple beaker [62].
17 18
3.2 Experimental procedure
19
The experiment is done at Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh,
20
in November 2017. Different mass flow rates of 0.0022 kg/s,0.0028 kg/s,0.0033 kg/s,0.0039 kg/s and
21
0.0044 kg/s are used.
16 | P a g e
1
Water passes from the top of the absorber plate and then goes to the bottom of the absorber
2
plate as a result water gets heated inside the collector due to continuously falling sun rays on
3
the top glass cover of collector and water also gets heated on the bottom absorber plate
4
because sun rays are reflected on the bottom glass cover with a reflector Figure 7 [63].
5 6
Figure 7: Heat gain phenomena in the solar water heater.
7 8
The solar collector collects direct and diffused solar radiant from the sun. A reflector collects
9
direct radiation and reflects the diffused radiation portion onto the bottom part of the double-
10
pass solar water heater. The absorber plate and pipe first absorb heat. Then heat is passed to
11
the water. Water first gets heat from the top absorber plate and again gets heat from the lower
12
absorber plate. As a result, water is getting more heat than the single-pass solar water heater
13
[64].
14
Finally, the hot water is exhausted through the exit port. The data is taken in every 30
15
minutes’ interval when water is circulated continuously through the collectors. All the tests
16
are taken between 9.00 AM and 4.00 PM.
17
The reflector and collector area are taken as 0.90 m2 and 0.929 m2, respectively. The mirror
18
glass reflectivity is taken as 0.9, and the total intensity factor and total occurrence radiation on
19
the bottom of the collector surface area calculated as 0.9×0.7 or 0.63 and around 70%,
20
respectively [65]. The average solar intensity for November is taken as 4.2 kWh/m2 or 600
17 | P a g e
1
w/m2 [66]. The total heat absorbed by water and collector is evaluated by the equation (3) and
2
(4), (6). The thermal efficiency of the collector is obtained using equation (5).
3 4
4. RESULTS AND DISCUSSION
5
The solar radiation mainly depends on location and local weather. The experiments are performed in
6
November with mostly clear sky conditions. But the solar intensity is not so much because November
7
is in the winter season in Rajshahi [67]. The performance of a double-pass solar collector is
8
represented based on efficiency which is obtaining by measuring some parameters such as inlet and
9
outlet temperature, flow rate. The half an hourly variation of the temperature difference between the
10
inlet and outlet water, collector efficiency, and comparison between single pass and double pass solar
11
water heater is shown in the respective figures below.
12 13
14 15
Figure 8: variation of the highest efficiencies in double-pass solar water heater with a different mass
16
flow rate of water.
17 18
Figure 8 shows the embedded relation between mass flow rate and the highest efficiency of the solar
19
water heater in 4 experimental days. In day 1, the mass flow rate of the water is considered 0.0022
20
kg/s. The water is flowing through the copper pipe and got heated by solar radiation, and it gives an
18 | P a g e
1
efficiency of 47.61% which is highest. After the first experimental day, the mass flow rate of water is
2
increased to observe the change in the thermal efficiency of the system. It has been seen that the
3
thermal performance of the system has been increased by a significant amount. On day 2, the
4
efficiency is 47.98% at a mass flow rate of of0.0028 kg/s. At days 3 and 4, with a mass flow rate of
5
0.0033 kg/s and 0.0039 kg/s, the efficiency is recorded as 48.82% and 48.8%, respectively. At day 5,
6
the highest predictable efficiency is obtained by 50.26% with a mass flow rate of 0.0044 kg/s. The
7
efficiency of the double pass solar water heater generally depends on the solar radiation, mass flow
8
rate, and the geometry of the collector[68] (ref:). The increasing mass flow rate causes the increase of
9
the heat transfer coefficient and thus results in higher efficiency [69]. Figure 9 shows the temperature
10
variances ∆T = Tout − Tin versus day time for various mass flow rates on a clear day in Rajshahi for a
11
double pass solar water heater with reflectors.
12
13 14
Figure 9: Double pass solar water heater outlet and inlet water temperature difference vs. day time for
15
different mass flow rates.
16 17
The above figure shows that the temperature difference of the collector enhances with the increment
18
of solar radiation,
19 | P a g e
(W/m2) as expected and reached a peak at 12.30 PM of the day as the solar
1
intensity is maximum at this point. The temperature variance increases with decreasing mass flow
2
rates. The temperature difference is a maximum of 44.9 °C for the mass flow rate of 0.0022 kg/s and a
3
minimum of 8.6°C for the mass flow rate 0.0044 of kg/s. As the mass flow rates increase from 0.0022
4
kg/s to 0.0044 kg/s, the outlet temperature of the flowing water in the collector decreases. The reason
5
behind this reducing nature is the increasing volume of water flow inside the tube at a time.
6
Efficiency versus daytime at various water flow rates for the double pass solar water heater is shown
7
in Figure 10. The efficiencies are improved to a maximum level in between 12:00 PM to 1:00 PM.
8
9 10
Figure 10: Double pass solar water heater Efficiency vs daytime for diverse mass flow rate.
11 12
Figure 10 shows the variation in the efficiency of double pass solar water heater at different mass
13
flow rates ranging from 0.0022 kg/s to 0.0044 kg/s. The efficiency of collector increases with the
14
increasing time along with higher mass flow rates. The maximum efficiency is about 50.26% at 12:30
15
PM for the mass flow rate of 16 kg/ hr. The minimum efficiency is about 14.84% at 9:00 AM for the
16
mass flow rate of 0.0022 kg/s. The graph shows that the thermal proficiency of the double pass solar
17
water heater is improved with increasing water mass flow rate. The cause of increasing thermal
18
efficiency for a DPS is due to using the reflector on the bottom side of the collector. The maximum
20 | P a g e
1
temperature reached by the set up with a glass as the cover is 74.7°C at that time the inlet temperature
2
is 29.8 °C. The maximum efficiency of the collector is 50.26%, which is less than our design
3
efficiency; due to some losses are occurred. Again, the fluctuation of solar radiation concentration
4
affects the performance of the system.
5 6
7 8
Figure 11: Highest temperature difference observation in the double-pass solar water heater with a
9
variation in mass flow rate.
10 11
Figure 11 shows the highest temperature difference of the solar water heater in 4 experimental days
12
with the respective mass flow rate. The temperature difference is an important factor in evaluating the
13
performance of the system. The more the temperature difference of the water heater, the more
14
efficiency can be obtained. It has been seen that, with an increasing rate of mass flow rate, the
15
temperature alteration of the system is decreased. This type of decrement happens because, with the
16
increasing rate of mass flow rate, the water in the copper tube gets less time to captivate the solar
17
radiation. So, the water of the system did not get the time to rise to the desired amount. At day 1, the
21 | P a g e
1
highest temperature difference is obtained 44.9 °C. On day 2 and day 3, the difference is recorded,
2
respectively 35.6 °C and 30.7 °C. The lowest difference is obtained at 23.7 °C on day 5 with a mass
3
flow rate of 0.0044 kg/s.
4
The following Figure 12 shows the efficiency concerning the time of the day curve at a specific mass
5
flow rate of 0.0033 kg/s for a double pass and single-pass water heater. The data for the single-pass
6
solar water heater is collected in April 2016. The solar intensity in April is generally higher than in
7
our experimental month of November. The maximum efficiency is 38.1% at 1:00 PM for the mass
8
flow rate of 0.0033 kg/s for a single pass water heater.
9
10 11
Figure 12: Efficiency difference between a single pass and double pass solar water heater.
12 13
For a double pass solar water heater, the highest efficiency is 47.98% for the same mass flow rate of
14
0.0033kg/s. Although the data from our experiment is collected in November 2017, where the solar
15
intensity is less than in April. But the efficiency is higher than the single-pass water heater for the
16
same mass flow rate. From the above Figure, it is clear that the efficiency of the double-pass solar
17
water heater is higher than the single-pass water heater. The efficiency of the double pass solar water
22 | P a g e
1
heater would show better efficiency if the data were taken in April 2017. Moreover, utilized sensors
2
have some errors that affect the efficiency of the double-pass solar water heater. Errors of different
3
sensors used in these experiments are enlisted in Table 4. Although the individual error percentage is
4
not that much combined, it has a non-negligible effect on the efficiency. Apart from that, the top,
5
bottom, and side loss of heat are also responsible for not getting actual efficiency.
6 7
Table 4: List of various sensors used in this system along with their Error percentage List of sensors
Error percentage
DHT 11 (temperature and humidity sensor) for air
5% for humidity and ±2% for temperature
Digital Multimeter (VEYRON VL9205)
DC Voltage: ±(0.5%+1dgt). AC Voltage: ±(0.8%+3dgt). DC Current: ±(0.5%+1dgt). AC Current: ±(1.0%+3dgt). Resistance: ±(0.8%+1dgt). Capacitance: ±(4.0%+3dgt).
Anemometer (Model: ML-81Am)
≤ 20 m/s: ± 3% FS > 20 m/s: ± 4% FS
Photodiode (intensity measurement)
±2.25V to ±18V
LM 35 (temperature sensor) for water
±0.5%
8 9
5. CONCLUSIONS
10
Double pass solar water heater with a reflector at the bottom of the collector is investigated
11
experimentally and compared with recorded data of conventional single-pass solar water heater. From
12
this, it can be concluded as follows:
13
1. The efficiency shows a proportional relationship with the mass flow rate. Although the
14
contribution from the reflector is not that much, it is not negligible. A maximum of 50.26%
15
efficiency at flow rates of 0.0044 kg/s has been obtained from the system. The mass flow rate
16
exhibits inversely proportional relation with the temperature difference (in between the inlet
17
and outlet flow), i.e., the highest temperature difference is found at the mass flow rate of
18
0.0022 kg/s. However, the temperature difference directly related to solar radiation and it
19
increases with the increasing solar radiation. Moreover, the experimental results depict that
20
the highest efficiency has been found at 12:30 PM because the solar radiation is at the highest
21
peak at that particular time of the day.
23 | P a g e
1
2. An in-depth comparison has been made between the double pass and single-pass solar water
2
heater at a mass flow rate of 0.0033 kg/s. The results indicate that the double pass provides
3
better efficiency than the single-pass, although the data were not taken at the same time of the
4
year. The potential reason is the addition of the reflector in the experimental system. In a
5
word, it can be concluded that the productivity of the experimental double pass solar water
6
heater with reflector is greater than a conventional single-pass flat plate solar water heater.
7 8
Acknowledgment
9
The support of the Rajshahi University of Engineering & Technology (RUET) is gratefully
10
acknowledged, in particular, authors are thankful to the Department of Mechanical Engineering for
11
financial support. The authors are also thankful to Rasel Hossain and Md. Adnan Hossain.
12 13
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28 | P a g e
HIGHLIGHTS • • • •
A new method with reflector is introduced in double pass solar water heater. Design, Modelling and Construction is done for the new method with reflector. The reflector addition shows 9%-15% better results in appraisal to the single-pass. The highest efficiency obtained by using reflector is 50.26% at a mass flow rate of 0.0044kg/s.
Authors have no potential conflict of interest