Improving the Performance of A Flat Plate Solar Collector Using Nanofluid as Working Fluid

Solar energy is the best alternative to limited fossil fuels. The foremost important means of utilizing solar energy are solar collectors. The most common types of solar collectors are flat plate solar collectors. A great deal of theoretical research has been conducted to enhance the flat plate solar collector efficiency. One effective way to increase the efficiency is by using an improved thermal properties fluid as nanofluid. In this paper, a numerical study has been made to enhance the efficiency and improve the performance of solar collector via the use of (Multi-Wall Carbon Nano Tube-water) MWCNT-H2O nanofluid instead of traditional fluid like water as working fluid under the weather of Mosul city / Iraq (36.3489° N, 43.1577° E). One dimensional dynamic model using implicit finite difference method was used. Solving energy conservation equations through the various layers of the solar system is the basis upon which the current model was based. Effects of nanoparticle volume fraction and mass flow rate on the working fluid stream temperature differences, and the thermal efficiency were studied. The mass flow rate was varied from 0.004 kg/s to 0.03 kg/s, while the volume fraction was varied from 0% to 6%. The results of "Nanofluid Based Flat Plate Solar Collector" were compared with the experimental results presented by [1]. The comparison established a good match between the current results and experimental results. The results showed an increase in the working fluid outlet and inlet temperature difference and collector thermal efficiency due to the addition of MWCNT nanoparticles. The temperature difference of "Nanofluid Based Flat Plate Solar Collector" was found 28.80 C at 0.014 kg/s and 6% in April, while it was 25 0 C at the same condition in the "Water Based Flat Plate Solar Collector case". Also, the thermal efficiency of "Nanofluid Based Flat Plate Solar Collector" was found 2.41% to 6.68% more than the "Water Based Flat Plate Solar Collector case".


INTRODUCTION
In the last decade, as a result of low fuel resources and environmental concerns about air pollution and international warming, increased interest in the use of solar energy has been aided by continued decline and a rapid rate of total cost of solar energy applications. Among the applications that are widely used are solar water heating systems, where solar radiation is converted into heat transmitted to the flowing water for heating. The most common collector types for solar water heating systems are flat plate solar collectors [2]. The standard flat plate solar collector consists of an insulation box with a high conductivity metal plate inside it, such as aluminium or copper called absorber plate, connected to the absorber plate tubes, and sometimes the tubes are integrated with the plate. The single or double clear glass layer is mounted on the box. The insulation box reduces heat loss from the sides of the collector or behind it. These collectors are wont to heat a liquid or air to temperatures under eighty °C [3]. The overall performance of the sun's collector depends on several factors, the most important of which are the thermal properties of the liquid that flows through it. Conventional fluids used in solar collectors have bad thermal properties. So many studies have tended to use another class of working fluids called 'Nanofluid' as a substitute for conventional fluids, which have improved thermal properties and thus increase the thermal performance of the solar collector [4]. Nanofluids consist of nanoparticles of a specific size less than 100 nanometers suspended in conventional liquids. Nanofluids have unique mechanical and thermal properties [4]. Numerous experiments were conducted by different researchers on the flat plate solar collector to test the effect of the use of nanofluids as a substitute for conventional fluids, where the results showed an increase in the rate of heat transfer in the solar collectors as a result of the use of nanofluids [5]. A study of the effect of the use of nanofluids on the thermal efficiency and size of the flat plate solar collector was conducted by Faizal et al. [6]. The researchers noted that the use of the nanofluid will increase thermal efficiency and reduce the area of solar collector. Experiments to verify the heat transfer through the flat plate solar collector using the Al2O3 nanofluid and at different concentrations were carried out by Said et al. [7]. Yousefi et al. [8] experimentally studied the effect of the use of Al2O3-H2O nanofluid with different nanoparticle mass fraction on the flat plate solar collector efficiency. Another experimental study on a flat plate solar collector using the MWCNT-H2O nanofluid was conducted by Yousefi et al. [ 9 ] . Kabeel et al [10] theoretically studied the solar water heating system to perform the effect of adding Cu nanoparticles on improving the working fluid heat transfer coefficient. In this study, an estimation of the outer fluid temperatures was done. The study observed a noticeable effect of the nanofluid volume fractions on increasing the temperature of the fluid leaving the solar collector as well as the thermal efficiency. Another theoretical study was presented by Alper M. G. [11]. They proposed a transient model of flat plate solar collector at different climatic condition to study the effect of a number of factors on the performance of the solar system, including: Al2O3-water nanofluid volume fraction and nanofluid mass flow rates. In this study, at a mass flow rate of 0.004 kg/s and a nanofluid volume fractions of 3%, the maximum increase in the temperature of the nanofluid leaving the solar collector was 7.2%. The effect of adding two types of Al2O3 and CuO nanoparticles with volumetric fractions between 1% and 4% was theoretically studied by M. R. Saffarian et al [12]. According to the results of the researchers, it is possible to improve the coefficient of heat transfer up to 78.25%. Another factor that has been studied is the direction of nanofluids flowing into the solar collector. The present paper aims to study the performance of the flat plate solar collector theoretically using MWCNT-water nanofluid with different mass flow rates and volume fraction under the weather of Mosul city, Iraq using a transient onedimensional model.

Mathematical model
The dynamic model proposed by [13] for "Water Based Flat Plate Solar Collector" (WBFPSC) are going to be adopted during this section after being modified to suit the "Nanofluid Based Flat Plate Solar Collector" (NBFPSC). During this model, the control volume is chosen during the flat plate solar collector to incorporate one tube and divides the control volume perpendicular to the direction of the fluid flow into the following five nodes: glassing, air space, absorbent, working fluid, and insulation, Fig. 1. While N is taken from the nodes within the direction of the flow, therefore the pattern contains (5 × N) nodes.
The most important assumptions adopted in the current model are as follows: 1-The fluid flow is equal in all tubes. 2-One-dimensional heat transfer through the solar collector layers. 3-There is no heat transmitted from the edges of the solar collector. All governing equations are derived by applying the total energy balance of each layer within the control volume [13]: -The glass cover: (1) Where: Cp g ρ g V g , are the specific heat, density, and volume of the glass cover. α ∶ the absorption coefficient. G ∶ the incident solar radiation. A ∶ the collector area. T g ∶ the temperature of glass cover. T amb ∶ the temperature of ambient. T air ∶ the temperature of air gap. T abs ∶ the temperature of absorber. h r : the heat transfer coefficient by radiation [14].

− 1]
Where: R a ∶ the Rayleigh number. β ∶ the thermal expansion coefficient. L ∶ the length of the tube.
The equivalent heat transfer coefficient at the glass external surface, h o1 , can be obtained from the following formula [14]: Where: T sky : the sky temperature, T sky = 0.0552 T amb 1.5 . h c2 : the external surface convection heat transfer coefficient.
Where: Cp abs ρ abs V abs , are the specific heat, density, and volume of the absorber. A s ∶ the pipe cross sectional area. τα ∶ the effective transmittance absorption coefficient. T f ∶ the temperature of working fluid. T ins ∶ the temperature of insulation. k ins ∶ the insulation thermal conductivity. th ins ∶ the insulation layer thickness. h nf ∶ the nanofluid heat transfer coefficient.
-The working fluid Where: A f ∶ the cross sectional area of the tube. d in ∶ the inner diameter of the tube. m f : the nanofluid mass flowrate per tube. Cp nf ∶ the specific heat of nanofluid. ρ nf ∶ the density of nanofluid.
These nanofluid properties are evaluated using the following correlations: Density of nanofluid can be estimated from Pak and Cho's equation [16] as: ρ nf = (1 − ) ρ bf +  ρ p Where:  ∶ the nanofluid volume fraction.  bf ∶ the density of the water.  b : the density of the MWCNT nanoparticle′s. The specific heat of nanofluid given as [16]: Where Pe is the Peclet number which is defined as: P e = U m d p α nf Where: d p ∶ the nanoparticle diameter. U m : the nanofluid mean velocity α nf ∶ the thermal diffusivity for the nanofluid. The thermal conductivity of nanofluid can be estimated from the following equation [16]: Where: k p ∶ the thermal conductivity of nanoparticles. k bf : the thermal conductivity of of the base fluid. n : the shape factor (n = 3  ⁄ ), and  is the particle sphericity. Where: Cp ins ρ ins V ins , are the insulation layer specific heat, density, and volume. h o2 : the equivalent heat transfer coefficient at the outside surface of the insulation.
The implicit finite difference method was used to solve the system of differential equations, and the forward and backward difference scheme were used instead of the dimensional and time derivatives. The final formula of the equation system is: The criterion of performance of the solar collector in current study is the difference between the temperature of the working fluid outlet and inlet temperatures difference, in addition to the efficiency of the solar collector. The thermal efficiency () of the solar collector can be defined as the heat that the fluid gained during the solar collector to the total solar radiation incident on the surface area of the collector.

Solution Method:
An itemized description of solution steps of the flat plate solar collector model is presented below: 1. Stepping along the length of the wheel channel (z direction), the temperatures of: glass cover (eq.6), air gap (eq.7), absorber (eq.8), working fluid (eq.9), and insulation layers (eq.10), are computed incrementally. 2. The temperatures are stored. These temperatures will be used as "previous" temperature in the next time step.
4. The final temperature is recorded and used to find the performance. The variables estimation is stopped when achieve convergence with the convergence criterion set to (error=1.0x10 -4 ). During this work, FORTRAN 90 (Microsoft Developer studio Copy right© 1994-1995) has been utilized to develop the code.

Model Validation.
A comparison of simulation results with experimental results presented by [1] was made to confirm the validity of the current model. Experimentally investigated at the Razi University of Kermanshah, Iran (latitude is 34°20 ′ 56 ′′ N and longitudeis 7°9 ′ 20 ′′ E). The major design information used in the comparison, are given in Table 1. Results of the comparison are pictured in Fig. 2. Fig. 2 show that the present model gives good congruence with experimental data. The maximum difference in the values between the results of the present model and experiment of data is about (8 %), which confirms accuracy of the present model.

Simulation results
This section explores the simulation results and the remarks concluded from those result. The design descriptions of the flat plate solar collector and the operating conditions will be presented before discussing the simulation results.  Fig. 3, it can seen in a typical day in January, the average ambient temperature varies between 3 to 12°C, where the incident radiation varies between 0 to 570 W/m 2 . Fig.4 show that in a typical day in April the average ambient temperature varies between 13.5 to 25.5°C, where the incident radiation varies between 0 to 846 W/m 2 . In a typical day in July Fig.5 show that the average ambient temperature varies between 29 to 42.5°C, where the incident radiation varies Fig. 2    January, April, July and October respectively, whereas in the case of (WBFPSC), maximum temperature differences were (14°C), (25°C), (31.9°C) and (22.8°C) at a volume fraction of (6%) in a typical days in January, April, July and October respectively. The explanation for this can be illustrated by the following: The increase in nanoparticle volume fraction will increase both the density of nanofluid, thermal conductivity, and viscosity, which will increase the heat transfer coefficient. The temperature differences lines in the case of (NBFPSC) are very close to each other, for example the maximum temperature differences were (1°C), (1.8°C), (2.29°C) and (1.8°C) in a typical days in January, April, July and October respectively In Fig. 8, thermal efficiencies are given at the working fluid mass flow rate between 0.004 kg/s to 0.03 kg/s in April with different nanoparticle volume fractions. It can be seen that the efficiency increase with increasing mass flow rate for (NBFPSC) and (WBFPSC) cases. At high flow rate, the addition of nanoparticles to the water leads to an improvement in efficiency with a higher rate than the low flow status. For example, the addition of nanoparticles led to an increase in efficiency between (2.41 %) to (2.91 %) at flow rate (0.004 kg/s), while adding nanoparticles led to an increase in efficiency between (4.67 %) at flow rate (6.68 %) at flow rate (0.03 kg/s).

Conclusion
In this study, the one dimensional dynamic model of MWCNT-H2O nanofluid based flat plate solar collector for different nanoparticles volume fraction and mass flow rate at Mosul city, Iraq climatic condition. The main conclusion are summarized as follow:  A noticeable increase in the working fluid outlet and inlet temperature differences through solar collector obtained by adding nanoparticles for the four selected months.  Changing the values of nanoparticle volume fraction between 1% to 6% led to a limited increase in the working fluid outlet and inlet temperatures difference and thermal efficiency.  The working fluid mass flow rate has a significant impact on increasing the (NBFPSC) and (WBFPSC) efficiencies, but the rate of increase varies according to the flow rate. Whereat high flowrate, adding nanoparticles to water improved efficiency at a rate higher than low flow condition.