INTRODUCTION:
The electricity consumption rate in
residential sector has been increased in the word from year to year. A lot of
studies are recorded in the literature about the energy consumption
optimization in buildings in hot regions using passive concepts such as thermal
insulation, natural ventilation, building orientation and underground
constructions1-4. Other researchers studied the use of solar cooling
systems to provide comfort in the building. Based on flat-plate collector (FPC)
dynamic simulation of a solar heating and cooling system was developed by
Bahria et al.5 in different regions of Algeria. The obtained results
from TRANSOL software shows that 28 m² collector areas for the south regions is
suggested to promote cooling production during summer periods.
Evangelos et al.6, compared four different
collector types in Athens (Greece), flat plate collectors, evacuated tube
collectors (ETC), compound parabolic collectors (CPC) and parabolic trough
collectors (PTC) for covered 100 Kw cooling load. It is apparent from this
study that ETC was the most suitable compared to PTC with 200 m² collector
area. According to the authors, this result may be explained by the utilization
of diffuse radiation by ETC. However, Al-Falahi et al.7 points out
that for PTC with absorption lithium bromide-water solar cooling system (PTC/H2O–LiBr)
gives remarkable results comparing to ETC/H2O–LiBr in the case of
700–800Kw cooling load.
The parabolic trough collector was used to provide the
necessary heat required to drive the absorption cooling machines in the hot
regions by a number of researchers in the last year. Because of
his higher thermal performance compared to the traditional solar collectors8-10.
A single effect absorption lithium bromide-water (LiBr-H2O) with
63.18m² PTC aperture area and 8m3 storage tank volume was installed
on the roof of buildings in Kuwait in order to supply the cooling load of
10.55KW11. Mazloumi and al.12 simulate a single effect
LiBr-H2O absorption cooling system with 17.5KW cooling load for
typical house in Iran, it was concluded that the minimum required collector
area is 57.6m² with storage tank in the range of 0.68-1.23m3. Nidal
et al.13 developed a model of a solar adsorption cooling and
refrigeration based on parabolic trough collector in Saudi Arabia, a
statistical optimization method revealed that the optimum tank volume varied
between 0.2 and 0.3 m3 and the required collector area is about
3.5-5m² in the case of equivalent ambient temperature of 27°C.
Several theoretical and experimental published studies
have examined the impact of nanofluids in various fields of energy14-17, the research to date has tended to
focus on many nanofluid types such as Graphene18,19, CNT/H2O20, Al2O3/Water, CuO/Water
and Cu/Water21,22. Sergio
et al.23 have showed that the solar water heating system (collector
and sensible heat storage tank) is the mean element in the single-effect solar
absorption cooling systems. The use of nanofluid-based PTC has been examined in
many applications by various researchers24,25, and notable paucity
of studies in solar cooling and refrigeration systems26. According
to the simulation results, the use of water/ZnO and water/Al2O3
can increase the heat transfer rates up to 50%. The main objective of this
paper is studying the effect of nanofluid technology in increasing the
efficiency of the solar water heating system, in order to reduce starting
operating time for single-effect LiBr-H2O absorption cooling systems
under real summer weather conditions of Adrar city, south of Algeria.
System Description:
The system is illustrated in Figure1, it consists of a
solar parabolic trough collector, heat storage tank and single effect LiBr-H2O
absorption unit. The systems use water as the storage media with operating
temperature of 65-95°C25. The hot water in the storage tank is used
to ensure the necessary heat of the absorption chiller generator. As discussed
in the previous section, the most common part in the solar cooling system is
solar heating system. The present study is focused on the modeling of the
coupled parabolic trough collector and storage tank, in order to achieve the
chiller starting operating temperature (65°C) in small time.
Figure 1. Typical solar cooling system
The size of collector area and
storage tank capacity depends on the cooling load, solar energy and pumps mass
flow. In this study and based on the previous studies12,27, the
storage tank’s volume was taken as 0.3 m3 and higher collector masse
flow rate of 0.2kg/s is considered. The characteristics of small size PTC used
in the simulations are presented in the Table 1. The collector area of 6 m2
give a storage ratio of 0.05m, extensive research has shown that the perfect
storage tank to collector area ratio is given between 0.01- 0.13m in the
literature10,12.
Table 1. Characteristics of solar PTC28
|
Absorber length
(L)
|
4m
|
|
Collector width
(W)
|
1.5 m
|
|
Focal distance
(F)
|
0.43 m
|
|
Absorber pipe
external diameter Dab2
|
0.02858 m
|
|
Absorber pipe internal
diameter Dab1
|
0.02756 m
|
|
Glass envelop
external diameter Dg2
|
0.03628 m
|
|
Glass envelop
internal diameter Dg1
|
0.03398 m
|
|
Absorber pipe
thermal conductivity (kab)
|
389 W/mK
|
|
Glass envelop
thermal conductivity (kg)
|
1.2 W/mK
|
|
Absorber pipe thermal
absorptance (αab)
|
0.88
|
|
Glass envelop
thermal absorptance (αg)
|
0.05
|
|
Glass envelop
transmitance (τg)
|
0.92
|
|
Transmittance-absorptance
factor (α0)
|
0.817
|
|
Absorber pipe
specific heat (Cpab)
|
380 J/kgK
|
|
Glass envelop
specific heat (Cpg)
|
750 J/kgK
|
|
Absorber pipe
density (ρab)
|
8940 kg/m3
|
|
Glass envelop
density (ρg)
|
2500 kg/m3
|
|
Absorber pipe
emittance (εab)
|
0.49
|
|
Glass envelop
emittance (εg)
|
0.85
|
|
Reflected surface
reflectivity(ρ0)
|
0.83
|
|
Shape factor
(γ)
|
0.823
|
Models development:
PTC model:
The absorber receiver is the main component
in solar PTC with a metallic inner pipe and outer glass envelope, the absorber
of P layers is represented in Figure 2. The absorber model is based on one
dimensional implicit finite difference method with energy balance approach in
each section of the glass envelope, absorber pipe and water.
Figure 2. Longitudinal division of the absorber
receiver29
In the control volume, the heat balance on
the glass envelope, absorber pipe and water can be written as follows29:
(1)
(2)
(3)
In order to solve these partial
differential equations a finite difference method was adopted, after
rearranging the glass envelope algebraic equations was found29:
(4)
Where
(5)
(6)
The same procedure mentioned above is
employed for the absorber tube and liquid water. The system of (3P+3)
equations are solved simultaneously using Gausse-Seidel iterative method. The
PTC model was validated by Marif al.29 using measured data obtained
by Sandia National Laboratories (SNL) and the
simulations results presented in30.
Storage tank model:
The operation conditions cited in section 2
eliminate the thermal stratification in the storage tank and ensure turbulent forced convection in the parabolic trough collector. The energy
balance in the storage tank is given by the first order differential equation:
(7)
mw is the heat source mass flow rate and (Aiso
hiso) is the overall heat loss coefficient between the tank and
environment. The tank hot water temperature Tt can be calculated
using Euler method:
(8)
Where
Nanofluids model:
Cu/Water nanofluids and Cu-Al2O3/Water
hybrid nanofluids are used in this study. A considerable
amount of literature have been published on the correlations used to estimate
thermal properties of the mono nanofluids and hybrid nanofluids31,32.
These studies illustrated that the nanofluid thermal
properties are a function of nanoparticle volume fraction, base fluid and
nanoparticle thermal properties (see Table 2). Equations 9 to 14 determine the density, specific heat
capacity and thermal conductivity of single and hybrid nanofluids respectively31:
(9)
(10)
(11)
(12)
(13)
(14)
Where the total volume fraction
The dynamic viscosity of single and hybrid
nanofluids were estimated by using Eq(15)31:
(15)
Furthermore, the correlation developed by
Maiga et al.33 is recommended for calculating the Nusselt number in
case of turbulent flow (Re > 2300):
Nu = 0.085 Re0.71
Pr0.35 (16)
Table 2. Thermal properties of water and
Nanoparticle used in simulations26,29
|
Base
fluid
|
Water
|
|
pw
= -4.95626 ×10-4T2
– 0.23291 T + 1001.83736 pw
|
|
kw = -5.96341 × 10-6T2
+ 1.68 × 10-3 T + 0.56821
|
|
Cpw
= 0.01378 T2 – 1.42026 T + 4218.2371
|
|
µw
= -4.28265 × 10-10 T3 + 1.88979 ×10-7 –
2.77774 × 10-5 T + 15.6 × 10-4
|
|
Nanoparticle
|
Density
(Kg/m3)
|
Specific heat
Capacity
(J/KgK)
|
Thermal
Conductivity
(W/mK)
|
|
Cu
|
8933
|
397
|
393
|
|
AL2 O3
|
3960
|
773
|
40
|
|
|
|
|
|
RESULTS AND DISCUSSION:
PTC performance:
As previously mentioned parabolic trough
solar collector received only the direct irradiation, full tracking system is
chosen for its high optical efficiency. The ambient temperature and direct
normal irradiation (DNI) received by the collector during typical clear summer
day have been recorded at the New Energy Algeria station (NEAL) installed at
the Research Unit in Renewable Energies in the Saharan Medium (URERMS) located
in Adrar city (27◦53’ N, 0◦17’ E and 264 m) (see Figure 3). It can
be seen that maximal ambient temperature is around 48°C in the afternoon, and
the maximal value of DNI is around 800 W/m².
Figure 3. Variation of meteorological parameters during
15 July in Adrar
Figure 4 presents the temperature variation at the output of the
absorber for PTC with pure water as heat
transfer fluid. When the
direct solar radiation reflected by the PTC reflector, most of this radiations
heat is absorbed by the absorber tub placed in collector focal line and
transmits this useful heat to the heat transfer fluid. It is displayed that the collector area of 6 m2 can produce higher
output water temperature. In addition, higher glass envelope temperature
fluctuation was observed, this higher fluctuation has been caused by the
convective heat loss between glass envelope and exterior air due to wind speed.
Figure 4. Temperature variation at the output of the absorber for
PTC with water
Solar heating system performance:
Physical properties variations with the working
temperature in the case of 0.1%
volume fraction are presented
in Figure 5. Provide the observation that Cu-Al2O3/Water hybrid nanofluid has high thermal conductivity than Cu/Water single
nanofluid and pure water. However, pure water has a higher calorific capacity (ρ.Cp) followed by single nanofluid and hybrid nanofluid in
decreasing order. For these
reasons the useful convection
heat transfer coefficient in the case of hybrid nanofluid as a working fluid is higher. This result in a much
faster increase of the storage tank temperature (Figure 6), the cooling
starting operating time (at Tt=65°C) can be achieved before 13h
compared to the other cases.
Figure 5. Fluids properties with operating temperature
Figure 6. Variation of tank temperature during 15 July
in Adrar
The tank temperature has been simulated for
different volume fraction of single and hybrid nanofluids, as shown in Figure
7. It can be seen that the tank temperature in case of hybrid nanofluid
increased fast then single nanofluid case. The reason for this great increase
is based on the dependency of the nanofluids thermal properties especially
thermal conductivity by small volume fraction, thermal conductivity enhance due
to convection currents between base fluid and solid particles. The cooling
starting operating time is between 12h and 12h30 with 0.2% Cu-Al2O3/Water
volume fraction. Beside, 0.8% Cu/Water volume fraction provide a cooling
starting operating time between 12h30 and 13h.
Figure7. The effect of volume fraction on the tank
temperature during 15 July
CONCLUSION:
Solar parabolic trough collector is one of
the most mature solar collectors for operation in medium temperature. The aim of this paper is to critically examine the effects of single nanofluids of Cu/H2O and hybrid
nanofluids of Cu-Al2O3/H2O in small size solar
heating system (Solar parabolic trough collector and storage tank).The design
was done for supply heat for the absorption lithium bromide-water solar cooling
unit with starting operating temperature equal to 65°C. Based on the
simulations results obtained in summer representative day in Adrar region,
south of Algeria, the solar PTC aperture area of 6m² and storage tank volume of
0.3m3 are sufficient to reach the starting operating temperature
after 13h in the case of pure water. On the other hand, the integration of
nanofluid technology can increase the useful heat transfer
coefficient and decrease starting operating time compared with pure water. The
starting operating time diminished as the volume fraction of the single and
hybrid nanofluids increases, the cooling unit can start before 13h with hybrid
nanofluid and single nanofluid of 0.2% and 0.8% volume fraction respectively.
CONFLICT OF INTEREST:
The authors have no
conflicts of interest regarding this investigation.
ACKNOWLEDGMENTS:
The authors gratefully thank the Research Unit in
Renewable Energies in the Saharan Medium situated in Adrar city for providing
the data used in this study.
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