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International Journal of Ambient Energy

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Thermal performance improvement of solar air

flat plate collector: a theoretical analysis and an
experimental study in Biskra, Algeria
a a a b
Kamel Aoues , Noureddine Moummi , Miloud Zellouf & Adel Benchabane
a
Laboratoire de Génie Mécanique , Université Mohamed Khider Biskra , B.P. 145 R.P. 07000,
Biskra, Algeria
b
Département de Génie Mécanique , Université Mohamed Khider Biskra , B.P. 145 R.P.
07000, Biskra, Algeria
Published online: 02 Aug 2011.

To cite this article: Kamel Aoues , Noureddine Moummi , Miloud Zellouf & Adel Benchabane (2011) Thermal performance
improvement of solar air flat plate collector: a theoretical analysis and an experimental study in Biskra, Algeria,
International Journal of Ambient Energy, 32:2, 95-102, DOI: 10.1080/01430750.2011.584469

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 International Journal of Ambient Energy
Vol. 32, No. 2, June 2011, 95–102

Thermal performance improvement of solar air flat plate collector: a theoretical

analysis and an experimental study in Biskra, Algeria
Kamel Aouesa*, Noureddine Moummia, Miloud Zelloufa and Adel Benchabaneb
a
Laboratoire de Ge´nie Me´canique, Universite´ Mohamed Khider Biskra, B.P. 145 R.P. 07000, Biskra, Algeria;
b
De´partement de Ge´nie Me´canique, Universite´ Mohamed Khider Biskra, B.P. 145 R.P. 07000, Biskra, Algeria

This article presents the results of the first experimental investigation of the performance of solar air flat
plate collector at Biskra (latitude 34 480 N and longitude 5 440 E), Algeria. The thermal efficiency between
absorber plate and air in flat plate solar collector has been enhanced by introducing obstacle rows in the dynamic
air vein of the collector. For this objective, a flat plate solar collector, of 1.73 m2 area and 25 mm air gap, has
been designed and constructed. These obstacles formed with two parts: first part is perpendicular to fluid flow
and the second part is inclined, they are mounted in a staggered pattern, oriented perpendicular to the fluid flow
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and soldered to the back plate. The solar air heater was mounted on a stand facing south at inclination angle, and
it was tested under the environmental conditions. Moreover, a theoretical approach is employed for
determination of the thermal performances of this collector where the temperatures of all components of the
collector and outlet air are predicted. Comparisons among the experimental and theoretical results considered are
reported.
Keywords: solar energy; flat plate collector; obstacles; heat transfer; oriented flow

1. Introduction roughness is to make the flow turbulent adjacent to

Unlike other sources of energy, solar energy allows the wall.
independent systems to be constructed. This solar A number of studies have investigated the effect of
energy possesses a thermal conversion mode which different geometries of artificial roughness on heat
necessitates a simple technology which is adapted to transfer and friction factor in duct flows (Liu et al. 1984,
the site and to the particular region for many appli- Prasad and Mullick 1985, Ouard 1989, Moummi et al.
cations. Solar energy collectors are devices employed 2004, Youcef-Ali and Desmons 2006, Esen 2008).
to gain useful heat energy from incident solar Prasad and Mullick (1985) utilised a small diameter
radiation. wire in a solar air heater duct to increase the transfer
Biskra is located in a geographical area commonly rate. Liu et al. (1984) found an enhancement of heat
called the solar belt in the south-east of Algeria exchange in a solar air heater by providing extended
(34 480 N and 5 440 E). The town of Biskra is char- surfaces on plate absorber. Ouard (1989) studied the
acterised by a cold climate in winter, and heat and optimisation of the forms and dispositions of obstacles
dryness in summer. Moummi et al. (2010) studied the in dynamic air vein. Moummi et al. (2004) analysed the
collector’s performances in various sites of different energy of a solar air collector with rows of fin obstacles.
climates of Algeria. Ben Slama (2007) compared the results to introduction
The value of the efficiency of these collectors can be of baffles to favour the heat transfer in the air solar
increased by increasing thermal exchange between the collectors. Esen (2008) developed experimental energy
absorber plate and air. The application of artificial and exergy analyses of a double-Fow solar air heater
roughness in the form of different geometries on the having different obstacles on absorber plates. Youcef-
heat transfer surface has been recommended to Ali and Desmons (2006) tested a solar collector
enhance the heat exchange by several investigators equipped with offset rectangular plate fin absorber
(Webb et al. 1971, Joudi and Mohammed 1986, plate, and also those concerning the introduction of
Choudhury and Gary 1993, Zugary and Vullierne obstacles in the dynamic air vein of the collector in order
1993, Yeh et al. 1998). The purpose of the artificial to obtain a turbulent flow that favours the thermal

*Corresponding author. Email: aoues_kamel@yahoo.fr

ISSN 0143–0750 print

ß 2011 Taylor & Francis
DOI: 10.1080/01430750.2011.584469
http://www.informaworld.com
 96 K. Aoues et al.

exchange by convection, and hence the maximal of components as listed below:

extraction of the absorber energy, as shown in
(1) A transparent cover of one layer of glass
Ahmed-Zaid et al. (1999), Abdullah et al. (2003), Ben
material and with a thickness of 5 mm.
Slama (2007), Karsli (2007), Aoues et al. (2008, 2009),
(2) The absorber plate is a galvanised metal sheet
Hikmet et al. (2009a, b), Sopian et al. (2009) and Ozgen
painted black with a thickness of 0.4 mm.
et al. (2009).
(3) A gap maintained between the absorber plate
The choice of the geometrical shapes of the
and the back insulator (back-pass) with a
obstacles to be used has to satisfy some criterion as
thickness of 25 mm.
the form and the disposition of the obstacles affect the
(4) A gap between the cover and the absorber plate
air flow during its trajectory. The obstacles ensure a
is 25 mm.
good air flow over the absorber plate, create the (5) The air stream canal formed by absorber and
turbulence and reduce the dead zones in the collector. by a galvanised metal plate used as the back
In this study, theoretical and experimental studies
side of the collector is 25 mm.
of solar collector are presented. For this objective, a (6) The rear insulation is provided by 40-mm thick
flat plate solar collector, of 1.73 m2 area and 25 mm air polystyrene sheet.
gap, has been designed and constructed. We intro- (7) The solar air collector is provided with fin
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duced rows of fin obstacles in dynamic air vein, which obstacles. The latter is shown in Figure 1. The
are mounted in staggered pattern, oriented perpendic- obstacles are ach ¼ 70 mm in length and are
ular to fluid flow and soldered to the back plate. The mounted perpendicular to the flow.
influence of obstacles on the thermal performance has
been investigated. The experimental setup is schematised by a solar
In the theoretical study, a mathematical method, collector, as shown in Figure 1.
using the approach of Hottel, Whiller and Bliss, is
developed to determine the thermal performances of
flat plate collector. It determines the temperature
profile of the absorber and the outlet air. These results 3. Theoretical analysis
are compared with those obtained by the experiments. The air solar flat plate collector used is with a simple
pass between the absorber and the back in a galvanised
metal plate placed on the insulator; the obstacles
increase the thermal performances of the collector,
2. Solar collector and experimental setup consequently increasing its temperature at the exit.
In this study, a flat plate solar collector is constructed The method selected for modelling and studying
with a length of Lc ¼ 1:95 m and a width of lc ¼ 0:89 m the performances of this collector is the total method
(Figure 1). These collectors are made up of a number which supposes that all the components of this section
are at a constant average temperature, the average
temperature between the inlet and the outlet of this
collector.
The collectors operate under quasi steady-state
conditions (Figure 2). In these conditions, the perfor-
mance of a solar collector is described by an energy
balance that indicates the distribution of incident solar

Figure 2. General heat transfer exchanges in the solar air

Figure 1. Experimental setup.
heater collectors.
 International Journal of Ambient Energy 97

energy into useful energy gain, energy stored and An empirical equation for the loss coefficient
energy losses. through the top of the collector Ut was developed by
Klein following the basic procedure of Hottel and
g ¼ u þ l þ st ð1Þ Woertz (1942) (Duffie and Beckman 1991):
By neglecting the thermal energy stored (thermal 2 31
inertia) in the collector, we obtain 6 N 17
Ut ¼ 4 h ie þ 5
g ¼ u þ l ð2Þ C ðTabs Ta Þ hw
Tabs ðNþfÞ
2 3
The useful heat gain by a collector can be
expressed as 6   7
6  ðTabs þ Ta Þ T2abs þ T2a 7
  6
þ 6( )7 ð10Þ
1 7
_ P Tfs  Tfe
u ¼ mc ð3Þ 4 ð"abs þ 0, 00591  Nhw Þ 5
þ 2Nþ f 1þ0:133"
"v
abs
N
While introducing the collector, the overall loss
coefficient between the absorber and the ambient air
UL, the useful energy gain provided by the collector is f ¼ ð1 þ 0:089hw  0:1166hw "abs Þð1 þ 0:07866NÞ
given by Equation (4) (Duffie and Beckman 1991):  
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C ¼ 520 1  0:0000512
    
u ¼ Sabs  FR IG ðv abs Þ  UL Tfe  Ta ð4Þ 1  100
e ¼ 0:430
The heat removal factor, FR is defined by Tabs
Equation (5). In the above equation, Ta is the ambient temper-
   ature (K) and Tabs the mean absorber plate tempera-
_ p
mc F0 UL Sabs
FR ¼ 1  exp  ð5Þ ture (K).
Sabs UL _ p
mc For 70 hh90 , use  ¼ 70 , then the loss coefficient
The collector efficiency factor F 0 and collector through the bottom of the collector is
overall loss coefficient UL for this studied configura- 1
tion, shown in Figure 1, are obtained from the energy Uar ¼ ð11Þ
eis
is þ ebb þ h1vv
balances on the absorber plate, the fluid and the back
plate like in Duffie and Beckman (1991), as shown in The outlet air temperature of the collector can be
(Figure 2), where ‘Ssup ’ represents the additional area obtained from an energy balance as (Duffie and
of artificial roughness Beckman 1991):
     
habsf Uar þ Ssup  hrabspl þ hrabspl þ Ssup  habsf IG  ðv abs Þeff UL  Tfs  Ta S  F 0  UL
F0 ¼      ¼ exp  abs
Uav IG  ðv abs Þeff UL  Tfe  Ta _ f
mcp
 þ hrabspl þ habsf 
 Uar þ Ssup  habsf þ hrabspl  h2rabspl ð12Þ
ð6Þ
The mean temperatures of the absorber plate are
8 9 obtained by solving the energy balance equations on
> ðUav þ Uar Þ > these plates, where the quantity ‘Ssup ’ takes into
< S  h2
> >
=
sup absf þ habsf  hrabspl account the fin area, which is supposed with the
>
> þS sup  h 
absf hrabspl
 >
> galvanised metal plate temperature.
: ;
þUar Uav Ssup þ 1  habsf  
UL ¼   ð7Þ 1 þ Ssup  Tf þ Sabs hðabsf
u
 Tpl
habsf hrabspl þ Ssup  habsf Uav Tabs ¼ Þ
ð13Þ
þSsup  habsf hrabspl þ Ssup h2absf Ssup

where The instantaneous collector efficiency relates the

useful energy to the total radiation incident on the
Sch collector surface by Equation (14).
Ssup ¼ 1 þ ð8Þ
Sabs  
Tfe  Ta
In the above equation, Sch represents the total  ¼ FR ðv abs Þ  FR UL ð14Þ
IG
surface of obstacles, which is calculated as follows:
Here, FR ðv abs Þ and FR UL are two major param-
Sch ¼ n1  ðach  bch Þ ð9Þ eters that constitute the simplest practical collector
 98 K. Aoues et al.

model. FR ðv abs Þ is an indication of how energy is

absorbed and FR UL is an indication of how energy is
lost. Besides, UL is the collector overall heat loss
coefficient.
The radiation heat transfer coefficient between the
inner wall of the absorber plate and the aluminium
plate, where the temperatures Tabs and Tpl are
expressed in Kelvin, is written as (Sacadura 1980):
 
 1 1

hrðabspl Þ ¼  Tabs  Tpl T2abs  T2pl þ 1
"abs "pl
ð15Þ
With regard to the forced convection, the average
heat transfer coefficient is given as
Figure 3. Variation of the ambient temperature and the
Nuf insulation during the characteristic day of June, at Biskra.
hðabsf Þ ¼ hð plf Þ ¼ ð16Þ
DH
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where the Reynolds number is given by

Vf Dh
Re ¼
f
The average velocity is
m_
Vf ¼

f Sf
The cross-surface area Af in the dynamic air vein of
the collector is defined as follows:
Sf ¼ lc  e  n2  ðach  bch Þ ð17Þ
The hydraulic diameter is given by the following
definition:
4  ½ðlc  eÞ  n1  ðach  bch Þ
Dh ¼ ð18Þ
2  ðlc þ eÞ þ ðn2  1Þ  ach

4. Results and discussion

The experiments took place at Mohamed Khider
University of Biskra (Algeria).
The collector was mounted on a stand facing south Figure 4. Variation of theoretical and experimental temper-
at an inclination angle equal to the ideal collector atures of (a) absorber plate and the (b) outlet air.
slope. It was tested under the environmental condi-
tions. Data regarding these conditions are shown in given. Another important parameter regarding the
Figure 3. environmental conditions is the ambient air tempera-
In Figure 3, we have presented the global hourly ture. The recorded values of the ambient air temper-
insulation of the characteristic day, in June corre- ature are shown in Figure 3.
sponding to the average of years 2008, which we have In Figure 4, the theoretical results of the perfor-
determined using the collector inclination angle value mance of solar collector issues of the developed
(34.48 ). The air flow rate was kept constant, equal to method based on the approaches of Hottel, Whiller
48.11 kg/hm2, during the experiment time. The results and Bliss are confronted and compared with those
depending on the experiment time (7:00–18:30) are experimental ones. All the presented data grant
 International Journal of Ambient Energy 99

a quasi-steady state for each test period (the test period

is the duration in which 10 data points are averaged
and shown as a single point in the presented results).
This is confirmed by the fact that within the test period
(10 min), the maximum variations in ambient, inlet and
outlet temperatures are 1 C, 2 C and 3 C, respec-
tively, while in global radiation, it is 30 W m2.
In Figure 4(a), these curves represent the variations
of theoretical and experimental results of the temper-
ature of the absorber, according to the time for the day
of the test. On the one hand, it is noticed that the two
curves have practically the same profile. This profile is
similar to that of solar irradiance and that the
variations in temperatures are very sensitive with
respect to its disturbances which depend on the
climatic and environmental conditions of the site and
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the experimental day of the test, we take an example of

these disturbances: when solar irradiance decreases up
to 270 W m2 in the neighbourhoods of solar midday,
the evolution of the two curves theoretical and
experimental is the same one with a reduction in the
theoretical temperature of the absorbing plate more
Figure 5. (a) Variation of theoretical temperature of the
significant than that recorded in experiments. absorber plate and outlet air and (b) variation of experi-
In addition, by examining the two theoretical and mental temperature of the absorber plate and outlet air.
experimental curves all along the day of the test, we
can see the difference which exists between the theo-
retical and experimental results. At the beginning of
obtained starting from the theoretical approach and
the test, this difference is about 4 C which starts to
represent profile of the temperature for a fixed flow.
increase, according to the intensity of the solar
We can see that a difference between the temper-
radiation, until reaching its maximum which is about
ature of the absorber plate and that of the air is
30 C in the neighbourhoods of solar midday and which
preserved long during the day. This variation first
corresponds to the highest recorded solar radiation on increases and then decreases according to the intensity
this day equal to 1014 W m2, then decreases gradually of the solar radiation and which is completely logical
until with its minimum which is always about 4 C. considering the physical properties and thermals of the
We can note as there is almost symmetry between the absorbing plate and of the air with the same remarks
two halves of each curve which depends on the referred to above during the disturbances of the energy
symmetry, which exists in the results of the intensity source.
of the solar radiation (Figure 3). On the other hand, in Figure 5(b), relating to the
In Figure 4(b), the theoretical and experimental experimental evolution according to the time of the
results of the outlet air temperature according to the temperature of the absorber plate and that of the outlet
time for a fixed flow are given. This figure exposes us air of the solar collector, the recorded variation is less
the theoretical results by giving the developed significant at the beginning and at the end of the day
approach which has the same profile as that recorded because this variation depends on the intensity of the
during the experimentation. We can note that the two solar radiation.
curves are almost confused all along the day, as The difference observed between the theoretical and
expected, during the disturbances due to the irradiance experimental results concerning the variations in tem-
solar with always the fall of the theoretical temperature peratures is certainly due to the fact that in the
of the outlet air of the collector due to these distur- theoretical approach, one does not take account of the
bances more significant than those of the experimental energy stored in the various components of the solar
results. collector (Equation 2). This assumption is also true for
In Figure 5(a), we illustrate the theoretical results the air which has poor physical properties and the
of the absorbing plate and the outlet air temperature quantity of stored heat is negligible, which is visible in
according to the time. These predicted results are Figure 4(b). On the other hand, in Figure 4(a),
 100 K. Aoues et al.

this assumption does not reflect the reality recorded for 5. Conclusion
the plate absorbing because of its capacity of significant In this study, we made theoretical and experimental
storage and which has a considerable impact on the studies on a flat plate solar collector. For those, a flat
results obtained. This quantity of stored heat which plate air solar collector was constructed of a surface of
represents the thermal inertia of the system compensates collecting equal to 1.73 m2 with the dynamic air vein
the fall of the solar radiation at the end of the day what furnished with thin metal obstacles and tested at the
is quite visible in the curves of Figure 5(b) as well during laboratory under the climatic conditions of the area of
the disturbances in Figures 4(a), (b), 5(a) and (b). Biskra.
These curves provide some information on the The comparison between the results obtained from
quality of the thermal transfer in the solar collector the theoretical and those from the experimental one
and allow us to evaluate the coefficients of convective makes it possible to make the following conclusions:
transfer what makes it possible to calculate the
(1) The theoretical approach presented in this
quantities of useful heat and lost by the system. This
article translates in a satisfactory way the
is very significant because it is starting from these data
thermal performances of the flat plate air
that one will be able to act on such parameter to
solar collector used obstacles in the dynamic
optimise the performances of such a converter of
vein.
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energy.
(2) The theoretical approach does not hold in
In Figure 6, we present the variations of the
account of the energy stored which is also true
efficiency of the collector during the test day. The
for the air with its physical properties poor but
average points of the experimental data are shown in
with regard to the components of the collector
the figure. Scatter of the data around the line of
(mainly the absorber), this assumption has a
average interpolation is mainly attributed to the significant and considerable impact on the
disturbance of the radiation incident and the speed of results obtained.
the wind. Figure 6 shows that there are two transient
regimes relating to the sunrise and the sunset. This is
due to the transient behaviour of the solar flat plate
Acknowledgements
collector as was already observed by other authors
The authors acknowledge the suggestions and the technical
(Esen 2008). After the system stabilisation, the effi- assistance of Belhi Guerira who was responsible for the
ciency will be located around a mean value depending technological hall at the Department of Mechanical
on environmental conditions. The efficiency calculated Engineering in Mohamed Khider university of Biskra,
for the configuration of the tested collector is accept- Algeria.
able and reached 70%. Otherwise, the efficiency
evolution is in agreement with other authors’ result,
some of them were mentioned above (Karsli 2007). Nomenclature
ach length of obstacles (m)
bch absolute height of the obstacles (m)
cp specific heat of air (J kg1 K1)
Dh hydraulic diameter (m)
e height of air tunnel in solar collector
(m)
eb thickness of the insulating (wood) (m)
eis thickness of the insulating (polysty-
rene) (m)
FR heat removal factor of solar collector
F0 efficiency factor of solar collector
G air mass flow rate (kg h1)
habsf convection heat transfer coefficient
between the absorber plate and air
(W m2 K1)
hplf convection heat transfer coefficient
between the channel back and air
(W m2 K1)
hrabspl radiation heat transfer coefficient
Figure 6. Efficiency of the collector during the time of day between the absorber plate and the
tested. channel back (W m2 K1)
 International Journal of Ambient Energy 101

hvv ¼5:67þ3:86Vv convection heat transfer coefficient f cinematic viscosity of air (kg m1 s1)
caused by wind (W m2 K1)  constant of Stefan–Boltzmann
IG global irradiance incident on solar air v transparent cover transmittance (0.90)
heater collector (W m2) f dynamic viscosity of air (m2 s1)
kf thermal conductivity of fin
(W m1 K1)
Lc length of the flat plate collector (m)
lc width of the flat plate collector (m)
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