Combined Experimental and CFD Investigation of the Parabolic Shaped Solar Collector Utilizing Nanofluid (CuO-H2O and SiO2-H2O) as a Working Fluid

Nanoscience application plays a major role in heat transfer related problems. A nanofluid is basically a suspension of fine sized nanomaterials in base fluids like water, Therminol VP-1, ethylene glycol, and other heat transfer fluids. This paper evaluates the possible application of nanofluid in parabolic shaped concentrating solar collector using both experimental and CFD analysis. Different types of nanomaterials used are SiO 2 and CuO of 20 nm average size. Nanofluids of SiO 2 -H 2 O (DI) and CuO-H 2 O (DI) of 0.01% volume concentration are used. Flow rates of 40 LPH and 80 LPH are used. ANSYS FLUENT 14.5 is used for carrying out CFD investigation. 3D temperature distribution of absorber tube is obtained using numerical investigation and the result is compared with the experimental one. Improvement in efficiency of collector of about 6.68% and 7.64% is obtained using 0.01% vol. conc. SiO 2 -H 2 O (DI) nanofluid and 0.01% vol. conc. CuO-H 2 O (DI) nanofluid, respectively, as compared to H 2 O (DI) at 40 LPH while at 80 LPH improvement in efficiency of collector of about 7.15% and 8.42% is obtained using 0.01% vol. conc. SiO 2 -H 2 O (DI) nanofluid and 0.01% vol. conc. CuO-H 2 O (DI) nanofluid, respectively, as compared to H 2 O (DI). Both experimental and CFD temperature results are in good agreement.


Introduction
1.1.Solar Energy.Owing to the increasing rate of development and modernization, great threat has been posed to conventional resources like coal, oil, and so forth as their reserves have marginally become scarce [1].To overcome such problem, different means of powering our life have been sorted which are basically everlasting and above all are eco-friendly.One of the means which fulfill such desired need is solar energy [2].Solar energy is the immense and splendorous gift bestowed upon us by God.It has been estimated that Earth receives about 1.8 × 10 11 MW amount of energy a year [3].Various different devices have been developed from time to time to harness this energy like flat plate collector, parabolic concentrating solar collector, heliostats, and so forth [4].Efficiency of such collector is dependent upon numerous factors like intensity of solar radiation, absorber material, design and concentration ratio of solar collector, and nature and thermophysical properties of the working fluid [5].Solar collector is basically a heat exchanger where solar energy is transferred to the working fluid flowing in the absorber tube [6].Applicability of solar collector depends upon the output received from solar collector [7].Flat plate collector is mainly used for domestic heating purpose while parabolic shaped concentrating solar collector is used for producing steam, which in turn is helpful in producing power [8].
1.2.Nanotechnology in Working Fluid.In order to achieve better heat transfer rate and to have efficient heat exchangers, this can be done by changing the nature and properties of working fluids or by incorporating nanofluid rather than normal working fluid [9].Suspension of fine sized nanomaterials in base fluids is called nanofluid [10].Nanomaterials of different materials like gold, silver, copper, aluminum, or carbon nanotubes or their corresponding oxides are used in base fluids like water, ethylene glycol, and so forth [11].Nanofluid possess enhanced thermal conductivity and better heat transfer coefficient as compared with base fluid [12].Nanofluid is basically prepared by two means: (1) two-step method and (2) one-step method [13].Some of the advantages of using nanofluid in solar collector as working fluid are as follows: (a) nanofluid absorbs energy directly, so there is no intermediate heat transfer [14]; (b) absorptivity value is high in solar range, while in infrared range the nanofluid's emissivity is low [15]; (c) due to enhanced thermophysical properties, system efficiency is enhanced [16].Taylor et al. [17] have experimentally found that by using nanofluid of Al 2 O 3 -H 2 O (0.01% vol.conc.)system performance is enhanced by around 10.12%.Khullar and Tyagi [18] have conducted finite difference technique to evaluate the efficiency of parabolic shaped solar collector having 0.05% vol.conc. of Al 2 O 3 -H 2 O nanofluid and have concluded that the system performance is enhanced by around 6.7% as compared with conventional working fluid.Chaji et al. [19] carried out experimental analysis to study the effect of Al 2 O 3 nanofluid in heat transfer enhancement and have found out that system performance is enhanced when water is replaced by alumina nanofluid.

Experimental Methodology
Detailed schematic diagram showing various components involved in the study is shown in Figure 1.Specifications of the parabolic solar collector are shown in   Reflector is mainly made of mirror strips placed all over the parabolic shaped structure which is shown in Figure 2. Absorber tube is insulated with the help of glass cover so as to avoid the radiation losses.Manual tracking arrangement is used to track the solar collector.Experiment is mainly conducted from 9.30 a.m. to 2.30 p.m.

Working Procedure.
Working fluid (7 liters) from the insulated storage tank is made to pass through absorber tube  which is made of copper tube and is black coated.Flow is regulated via flow control valve.Flow rates of 40 LPH and 80 LPH are used.Working fluid after receiving both incoming solar radiation and concentrated solar radiation which are concentrated in absorber tube with the help of reflector is made to enter into a storage tank from where again flow is circulated with the help of pump placed inside the storage tank.Temperature of both inlet and outlet fluid is measured with the help of thermometer placed at both inlet and outlet of the absorber tube.Solar power meter is used to measure the solar flux, while wind velocity is measured through anemometer.Recording of inlet and outlet temperature, solar flux, and wind velocity is measured from 9.30 a.m.till 2.30 p.m. with an interval of 30 min.Solar collector is tracked throughout the day with the help of manual tracking arrangement incorporated in the system.prepared is then stirred in a magnetic stirrer for around 30 minutes.
Then, the stirred mixture is then placed into a sonicator tank of ultrasonicator, where sonication of nanofluid is done for around 2 hours as shown in Figure 3, where ultrasonic rays are made to traverse through the fluid where nanoparticles are broken down, from where fine dispersed nanofluid is prepared which is then ready to be used in the system.Figures 4 and 5 show the prepared sample of 0.01% vol.conc.SiO 2 -H 2 O and 0.01% vol.conc.CuO-H 2 O, respectively.For better stability of a nanofluid, CTAB (hexadecyltrimethylammonium bromide) as surfactant is used [20,21].Moreover, pump is also placed within the storage tank in order to avoid the settling of the nanoparticles [22].

Governing Equations Used for Evaluating the Various Thermophysical Properties of the Nanofluid
(1) Density of nanofluid [20] is expressed by where  np ,  nf , and  bf are the density of nanoparticle (kg/m 3 ), density of nanofluid (kg/m 3 ), and density of base fluid (kg/m 3 ), respectively, and  V is the volume concentration of nanofluid.
(2) Specific heat of nanofluid [20] is expressed by where  nf ,  np , and  bf are the specific heat of the nanofluid, nanoparticle, and base fluid, respectively, in J/kg⋅K.
(3) Thermal conductivity of nanofluid [20] is expressed by where  nf ,  bf , and  np are the thermal conductivity of the nanofluid, base fluid, and nanoparticle, respectively, in J/kg⋅K.
(4) Dynamic viscosity of nanofluid [20] is expressed by where  nf and  bf are the dynamic viscosity of the nanofluid and base fluid, respectively.

Computational Fluid Dynamics (CFD) Methodology
ANSYS FLUENT 14.5 is used for simulating the absorber tube (HCE) of parabolic shaped concentrating solar collector, where nanofluid is made to flow.Nanofluid is simulated using one-phase modelling techniques [23], while solar load cell and solar ray tracing are used for modelling the solar fluxes.Various different steps adopted for conducting CFD simulation are shown below.

Domain Description and Meshing.
Firstly, the 3-dimensional geometry of heat collector element is created which is shown in Figure 6.Geometry is so created which includes HCE along with glass tube.HCE tube is made to split into two parts, namely, upper and lower part.
Upper part is mainly incident by incoming solar radiation while lower part is incident by reflected and concentrated solar radiation with the help of mirror reflector.Annular fluid zone for vacuum is also included.Three-dimensional geometry is made to orient along -axis where south direction is indicated along positive  while east direction is denoted by positive .Tetrahedral mesh is done over a fluid domain of absorber tube.Figure 7 shows the meshed geometry of absorber tube.

Material Model.
In ANSYS FLENT 14.5, material model is applied.In material model, various thermophysical properties of the material are specified.Table 3 shows the thermophysical properties of the various materials involved with absorber tube.4.

Solar Load
Model.Solar load model is applied for numerical simulation, where typical inputs include day and time of the experiment and longitude and latitude of location.By substituting the input values, CFD solar load cell will calculate the direct and solar radiation.S2S (surface-to-surface) radiation model is applied for modeling the radiation mode of heat transfer between diffuse surfaces involved in the system.

Numerical Methodology.
The following governing equations are applied for carrying out numerical simulation.
(a) Continuity equation is given as follows: (b) Momentum equation is given as follows: The first-order upwind differencing scheme is implemented for the momentum and energy equations.Residual target of 10 −4 is used for monitoring convergence criterion except for the energy equation, for which a target of 10 −7 is used.
3.6.Grid Independence Test.Mesh sensitivity analysis is carried out for each condition of a working fluid.Tables 5-10 illustrate the same.

Governing Equation for Efficiency Calculation.
The following different governing equations are used for evaluating the parabolic solar collector's efficiency with different working fluids at different mass flow rates.(1) Absorbed flux is represented by (2) Convective heat transfer coefficient is represented by where (3) Useful heat gain is represented by (4) Instantaneous efficiency,   , is represented by (5) Thermal efficiency,   , is represented by (6) Overall efficiency,   , is represented by Temperature contour 1 (K)    where   is global solar intensity in W/m 2 ,   is bond resistance,  is absorptivity of the absorber tube,  is glass cover transmissivity for solar radiation, p is specular reflectivity, Υ is intercept factor,   is Nusselt number,  is thermal conductivity in W/m⋅K,   is inner diameter of absorber tube in m, Pr is Prandtl number,  is dynamic viscosity in Pa⋅s,   is specific heat in J/kg⋅K,  is density in kg/m 3 , Re is Reynolds number,  is average velocity in m/s, ṁ is mass flow rate in kg/sec,  is width of the solar collector in m,  is the length of the absorber tube in m,  aper is the aperture area of the solar collector in m 2 ,  is the time duration, and  avg is the average value of solar radiation in W/m 2 .From experimental and simulated results, maximum thermal efficiency of 13.8% and 17.2% is seen, respectively, for 0.01% vol.conc.CuO-H 2 O (DI) nanofluid at flow rate of 80 LPH.Also, from both experimental and simulated values of thermal efficiency, it is seen that maximum value of thermal efficiency (both experimental and simulated) is seen at initial time duration with all working fluids, but afterwards drop in the values of thermal efficiency takes place, which is mainly due to increased radiation heat transfer due to increased temperature of the working fluid.Also, higher value of thermal efficiency is seen when flow rate is varied from 40 LPH to 80 LPH for a particular working fluid; this is mainly due to more heat loss occurring at a lower flow rate.

Figure 1 :
Figure 1: Schematic flow diagram of an experimental study.

Figure 2 :
Figure 2: Parabolic reflector and absorber tube along with glass cover.

Figure 8 :
Figure 8: Temperature contours with water as a working fluid in HCE at 40 LPH.

Figure 11 :
Figure 11: Temperature contour with H 2 O (DI) as a working fluid in HCE at 80 LPH.

Figure 12 :Figure 13 :
Figure 12: Temperature contour with SiO 2 -H 2 O as a working fluid in HCE at 80 LPH.

Figure 14 :
Figure 14: Variation of temperature rise with time of day for water as a working fluid at 40 LPH.

Figure 16 :
Figure 16: Variation of temperature rise with time of day for 0.01% vol.conc.SiO 2 -H 2 O (DI) nanofluid as a working fluid at 40 LPH.

Figure 18 :
Figure 18: Variation of temperature rise with time of day for 0.01% vol.conc.CuO-H 2 O (DI) nanofluid as a working fluid at 40 LPH.

Figure 19 :
Figure 19: Variation of temperature rise with time of day for 0.01% vol.conc.CuO-H 2 O (DI) nanofluid as a working fluid at 80 LPH.

Figure 20 :
Figure 20: Variation of experimental instantaneous efficiency with time of day for all working fluids at 40 LPH.

Figure 21 :
Figure 21: Variation of simulated instantaneous efficiency with time of day for all working fluids at 40 LPH.

Figure 22 :
Figure 22: Variation of experimental instantaneous efficiency with time of day for all working fluids at 80 LPH.

Figure 23 :
Figure 23: Variation of simulated instantaneous efficiency with time of day for all working fluids at 80 LPH.

Figure 24 :
Figure 24: Variation of experimental thermal efficiency with time of day for all working fluids at 40 LPH.

Figure 25 :
Figure 25: Variation of simulated thermal efficiency with time of day for all working fluids at 40 LPH.

Figure 26 :
Figure 26: Variation of experimental thermal efficiency with time of day for all working fluids at 80 LPH.

Table 1 :
Specifications of parabolic shaped concentrating solar collector.

Table 2 :
Amount of nanoparticles required for the preparation of nanofluid (1 liter).

Table 4 :
Various boundary conditions applied over HCE.
3.3.Boundary Conditions.Boundary conditions are applied for carrying out numerical simulation.For numerical simulation of nanofluid based solar collector, the following boundary conditions are imposed which are depicted in Table

Table 5 :
Details of grids used in mesh-independence test for water as working fluid at 40 LPH.

Table 6 :
Details of grids used in mesh-independence test for 0.01% SiO 2 -H 2 O as working fluid at 40 LPH.

Table 7 :
Details of grids used in mesh-independence test for 0.01% CuO-H 2 O as working fluid at 40 LPH.

Table 8 :
Details of grids used in mesh-independence tests for H 2 O as working fluid at 80 LPH.
(c) Energy equation is given as follows:

Table 9 :
Details of grids used in mesh-independence test for 0.01% SiO 2 -H 2 O as working fluid at 80 LPH.

Table 10 :
Details of grids used in mesh-independence test for 0.01% CuO-H 2 O as working fluid at 80 LPH.
Contours.Figures 8-13 show the temperature contour with water, 0.01% vol.conc.SiO 2 -H 2 O (DI) nanofluid, and 0.01% vol.conc.CuO-H 2 O nanofluid as working fluid at flow rates of 40 LPH and 80 LPH, respectively.All the temperature contours are for a simulation conducted from 12 to 12.30 p.m.It has been seen that maximum temperature rise is seen when CuO-H 2 O (DI) nanofluid is used as compared to water and SiO 2 -H 2 O (DI) nanofluid.Also Figure 17: Variation of temperature rise with time of day for 0.01% vol.conc.SiO 2 -H 2 O (DI) nanofluid as a working fluid at 80 LPH.experimental value of temperature rise at all time durations, and maximum value of temperature rise of 5.3 ∘ C and 6.7 ∘ C is seen for a flow duration of 40 LPH and 80 LPH.Also, drop in the value of temperature rise is reported with increasing time duration, which is mainly due to associated heat transfer which takes place due to increased temperature rise of the working fluid.Also, both experimental and simulated values of temperature rise are in close agreement with a difference of 11%.Variations of both experimental temperature rise and simulated value of temperature rise when CuO-H 2 O (DI) nanofluid of 0.01% vol.conc. is used as a working fluid at 40 LPH and 80 LPH are shown in Figures18 and 19, respectively.It is seen that both experimental and simulated values of temperature rise are in close agreement with a difference of