Experimental Study of Photovoltaic Performance Under a Experimental Study of Photovoltaic Performance Under a Compound Parabolic Solar Concentrator with a Nanofluid Spectral Compound Parabolic Solar Concentrator with a Nanofluid Spectral Filter Filter

The spectral-splitting technique splits the sun ' s spectrum into two pieces to save photovoltaic cells from overheating. The ﬁ rst component is instantly converted into electricity, while the second is used to generate heat energy. Since nano ﬂ uid simultaneously serves as a heat transfer liquid and a spectrum splitter, several investigations have demonstrated that it is a very effective liquid-based ﬁ lter. This work presents a novel PV/T design incorporating a compound parabolic concentrator (CPC) and a ZnO-water nano ﬂ uid ﬁ lter over the PV cell. In this work, different ZnO concentrations, such as 50, 100, 150, and 200 ppm, were tested to assess their impact on system performance. The average results were then compared to those of a reference PV cell. The results showed that the percentage reduction in PV cells ' temperature rises steadily with concentration, from 9.9% at 50 ppm to 10.7% at 200 ppm. Moreover, the average enhancement percentages in electrical power and ef ﬁ ciency increase consistently from 88% to 89.3% e 93% and 107.5%, respectively, as the nano ﬂ uid ' s concentration climbs from 50 ppm to 200 ppm. In contrast, the electrical ef ﬁ ciency of ﬁ ltered cells rises from 7.1% at 50 ppm to 7.7% at 100 ppm before falling to 6.7% and 6.2% at 150 ppm and 200 ppm, respectively. The thermal and overall ef ﬁ ciencies steadily rise with concentration, climbing from 27.9% to 31.7% at 50 ppm to 31.4% and 34.6% at 200 ppm, respectively.


Introduction
A significant portion of the heat, light, and power required to sustain life on earth comes from solar energy.With high efficiencies and no need for noisy, complicated systems, PV cells have recently become the most effective way to convert sunlight into electricity directly.The primary problem with PV systems is the rise in temperature during operation, which reduces efficiency since the semiconductor material can only convert solar light with the proper spectral response for the PV cell, while the rest is converted to heat.This problem is well-solved by spectral-splitting technology, which excludes UV and IR radiation that would otherwise cause the PV cell to overheat and only allows solar energy compatible with the PV cell's spectral response to reach the cell.These technologies can be classified into many groups, such as reflectivebased (Mojiri et al., 2013), absorbed-based (Wang et al., 2022), and diffractive and refractive-based (Xiao et al., 2016).Many studies have shown that nanofluid can simultaneously act as a heat transfer fluid and a spectral splitter, making it an effective liquid-based filter.Huaxu et al. (2020) investigated the effect of the ZnO nanoparticle concentration in glycol on electrical and thermal performances.When particle concentration was increased from 11.2 ppm to 89.2 ppm, thermal efficiency increased by 47%.The electrical efficiency, however, decreased.In a study presented by Hjerrild et al. (2016) to test the effect of different CNTs-water and AgeSiO 2 nanofluid concentrations, AgeSiO 2 filters with a concentration of 0.026 wt% boosted combined efficiencies by 30% when contrasted with just the base fluid filter.An also studied the effect of particle concentration, Zhang et al. (An et al., 2016a), and the findings showed that while the thermal efficiency dropped with increasing particle concentration, the temperature of the nanofluid and the electrical efficiency rose.The performance of a PV/T system based on an AgeCoSO 4 nanofluid spectral filter was proposed by Han et al. (2019).The findings demonstrate that when Ag nanoparticle mass fractions are equal, Ag/CoSO 4 nanofluid exhibits stronger visible and UV wavelength absorption than a water-based one.
Moreover, at the same mass fraction of Ag nanoparticles, Ag/CoSO 4 nanofluids have a lower electrical output than Ag/water nanofluids but have a greater stagnation temperature.He, Hu, and Li's study (He et al., 2019) used ethylene glycol/water solution (3:2, v/v) as the base fluid because of its low freezing point and suitability for cold areas.It was possible to achieve an overall efficiency of up to 83.7% at 1 kWm À2 with Ag@TiO 2 nanoparticle concentrations as low as 200 ppm.This extraordinary performance was mainly attributable to wideband light absorption, which produces a sizable thermal energy.Li et al. (2019) found that the 200 ppm Ag@TiO 2 -water nanofluid's overall efficiency was around 1.4 times greater than water's.The maximum total efficiency of the PV/T system designed by An, Wu et al. (An et al., 2016b) based on Cu 9 S 5 -Oleylamine nanofluid was 34.2%, 17.9% more than the unfiltered system.Using a blend of indium tin oxide and gold nanoparticles in Duratherm S, Otanicar et al.'s system (Otanicar et al., 2018) achieved a maximum thermal efficiency of 61% with a fluid output temperature of 110 C and an electric efficiency of 4% at a concentration ratio of 14.The PV/T system could create 12% more power than a bare PV cell under the same sunlight, according to an experimental study by Crisostomo et al. (2017), utilizing AgeSiO 2 -water nanofluid.The thermal conductivity and the transmittance of silica/water nanofluids of different particle sizes were examined by Jing et al. (2015).The findings demonstrated that the transmittance of nanofluids with a particle diameter of 5 nm and volume fractions of 2% could reach as high as 97%, which is quite near that of pure water.A 20% improvement in this nanofluid's thermal conductivity was also observed.To address the shortcomings of solar distillation systems, a more advanced system that combines a PV/T system with spectral-splitting nanofluids was developed by An et al. (2018).The upgraded systems with Ag and Au nanoparticles had a thermal efficiency gain of 4.8% and 6.8%, respectively.Additionally, the system's electrical efficiency with Agnanofluid was higher than Au-nanofluid's.
As explained in the previous literature, nanofluids have proven their effectiveness as liquid-based absorbed filters, where the effect of different types of nanoparticles with different concentrations and different types of liquids with different properties on the electrical and thermal performance of the system has been studied.Despite this, previous studies did not study the performance and stability of the system over hours in which the intensity of solar radiation changes throughout the day, as the system performance was only tested for several minutes.In addition, the system performance was not tested when integrating a compound parabolic solar concentrator to concentrate solar radiation with the PV/T system.This work suggests a novel PV/T design that combines a compound parabolic concentrator (CPC) with a ZnO-water nanofluid, a selective absorptive filter over the PV cell.To test the effect of various ZnO-water nanofluid concentrations of 50, 100, 150, and 200 ppm (0.05, 0.1, 0.15, and 0.2 kg/m 3 ) on the system's performance, many tests were conducted over a comparable 4.5-h time span from 10:30 to 15:00 on several days.After that, the average results were compared.

Experimental setup
The setup for the experiment is depicted in Fig. 1.It was executed at Mansoura University in Mansoura,Egypt (31.0449 N,31.3537 E).Several experiments were carried out over a more extended period, from September 4 to October 5, to examine the increase in performance while utilizing varied concentrations of ZnO nanofluid.The research presented here proposes a unique PV/T system that combines a compound parabolic concentrator (CPC) with a ZnO-water nanofluid that acts as a selective absorptive filter over the PV cell.Several experiments were carried out during a comparable 4.5-h period from 10:30 to 15:00 on several different days to examine the impact of different ZnO-water nanofluid concentrations of 50, 100, 150, and 200 ppm (0.05, 0.1, 0.15, and 0.2 kg/m 3 )on the system's performance.
The system consists of a CPC (1) with a 30 tilt that faces south.The significant reflectivity of nickelchrome made it the ideal choice for a reflecting substance.The wooden block (2) was made by stacking many identically sized and shaped layers of MDF and was attached to the CPC using glue.The CPC is made up of two symmetric parabolas.The lower end of any parabola is the focal point of the other (Bahaidarah et al., 2014), as indicated in Fig. 2. The angle created by the axes of the two parabolas, known as the half-acceptance angle, is crucial for designing CPCs.As shown in Fig. 3, only radiation that is incident at an angle less than or equal to the half-acceptance angle ðq i q a Þ may be reflected on the absorber; all other radiation is reflected back into the aperture.
The geometrical concentration ratio (CR) is the proportion of the exit area to the aperture area.Its relationship to the half-acceptance angle is demonstrated by Equation (1) (Zheng, 2017) as follows: Where W ab , W ap are the absorber and aperture widths, respectively.The upper portion of the CPC was trimmed to a third of its original length, which lowered the cost of the reflective material.Equation (2) provides the height for the whole CPC prior to truncation (Zheng, 2017): where f is the CPC parabola's focal length, which is determined by Equation (3) (Zheng, 2017) as follows: Equation (4) provides the height of the concentrator after trimming (Zheng, 2017) as follows: Where W ap;T is the width of the aperture after truncation, q c;T is the angle of concentration after truncation.The CPC's concentration ratio after truncation is given by Equation ( 5) (Zheng, 2017) as follows: As a result of truncation, the CPC concentration ratio in the current study is reduced to 4. The width of the aperture after truncation can be related to that before truncation by Equation ( 6) as follows (Zheng, 2017): A rectangular fluid-based optical filter (4) with a thickness of 0.01 m was made using acrylic (PMMA) slaps.The wooden block's bottom grooves regulate the distance between the concentrated PV cell (5) and the liquid channel.To compare the performance to that of the concentrated cell, a reference PV cell (6) with the exact dimensions (0.275 Â 0.155 m) was placed up next to the wooden block on a metal frame (7).A pump (11) circulated the nanofluid in a closed loop, with a valve regulating the flow rate.A water flow sensor (12) measuring a range of 0.3e6 l/min (5 Â 10 À6 e10 À4 m 3 /s) was used to gauge the flow rate.A solar power meter ( 14) records the sun's irradiance every 10 s.The wind speed is measured using a vane anemometer (9).Digital thermometers were used to gauge the temperatures of PMMA channels, PV cells, and ambient air.Waterproof sensors were used to gauge the temperature of the fluid at the channels' entrance and outflow.Each sensor is connected to an Arduino microcontroller unit (13), which logs measurements to a memory card every 3 s.The cells' current and voltage were also recorded every 3 s.Both an LCD affixed to the Arduino box and a laptop (8) was used to display all the data regularly.The specifications of both PV cells under standard conditions are listed in Table 1.Detailed information about the measuring tools employed in the experimental research is listed in Table 2.

Nanofluid preparation
In the current work, ZnO nanofluid samples were created using the two-step process (Huaxu et al., 2020;Vidhya et al., 2021) at varying concentrations of 50, 100, 150, and 200 ppm (0.05, 0.1, 0.15, and 0.2 kg/m 3 ).The nanoparticles are first created and added to the primary solution in this method.First, samples of ZnO nanoparticles with average diameters of 30 nm were introduced to 1 L of pure water.These samples had weights of 0.05, 0.1, 0.15, and 0.2 g.Then, for 20 min, with one pulse every two seconds, the solution was stirred by an ultrasonic liquid processor [SONICS (Vibra-cell) MODEL: CV334], as shown in Fig. 4, with a maximum output power of 750 W and a frequency of 20 kHz.
The amount of solar radiation that may effectively travel through the nanofluid to the cell without causing PV overheating is greatly influenced by the spectral transmittance of the nanofluid, which is computed by Equation (7) as follows (Han et al., 2019;Yazdanifard et al., 2020): Where e nf is the nanofluid channel thickness, s nf is the extinction coefficient of nanofluid, which is the summation of nanoparticles and base fluid extinction coefficients as illustrated by Equation ( 8), Equation ( 9), and Equation (10) as follows (Taylor et al., 2011(Taylor et al., , 2012)): Where k bf is the base fluid's absorption index, D np is the nanoparticle's average diameter, which for the ZnO particles utilized in this work is equivalent to 30 nm. ∅ v is the nanoparticles' volume fraction, which is given by Equation ( 11).h sc andh abs are the nanoparticles' scattering and absorption efficiencies, which can be given by Equation ( 12) and Equation (13), respectively (Yazdanifard et al., 2020;Jing and Song, 2017), as follows: m bf , m np are the masses of the base fluid and nanoparticles, respectively.r bf , r np are the densities of the base fluid and nanoparticles, respectively.R is the nanofluid's complex refractive index, which is the ratio between the complex refractive indices of the nanoparticles and base fluid, as illustrated by Equation ( 14).The nanoparticle size parameter, S np , is determined by Equation (15) based on Rayleigh scattering approximation that is valid when S np "1 and jRjS np "1.
n bf , n np are the base fluid's and nanoparticles' refraction indices, respectively.k bf , k np are the absorption indices of the base fluid and nanoparticles, respectively.Several research papers derived these indices for water (Hale and Querry, 1973;Otanicar et al., 2009) and ZnO nanoparticles (Huaxu et al., 2019;Royanian et al., 2020;Zarandi and Bioki, 2017;Manoharan et al., 2016) at various wavelengths.

The impact of nanofluid concentration on spectral transmittance
The monocrystalline PV cell utilized in this work should receive solar radiation with energy close to the silicon band gap energy (1.12 eV).The matching wavelength of this band gap, according to studies (Huaxu et al., 2020;He et al., 2019), is close to 1100 nm.Therefore, most thermal radiation has wavelengths longer than 1100 nm and belongs to the infrared (IR) spectrum, which cannot produce electrical power because its energy is lower than the silicon band gap.Conversely, wavelengths shorter than 1100 nm have more energy than the silicon band gap; as a result, only some of this energy is converted into electricity, while    the remainder raises the cell's temperature and lowers its efficiency.Therefore, the optical filter should block all sunlight with energy below the silicon band gap energy, which implies blocking wavelengths longer than 1100 nm.Additionally, a portion of solar irradiance with energy higher than the silicon band gap (wavelengths below 800 nm (Saroha et al., 2015;Rodrigues Fernandes and Schaefer, 2019)) should also be blocked.The spectral transmittance of ZnO-water nanofluid can be determined for different concentrations of ZnO-nanoparticles using Equations ( 7)e(25).Fig. 5 displays the spectral transmittance of ZnO-water nanofluid for concentrations of 0, 50, 100, 150 and 200 ppm with a fluid layer thickness of 0.01 m.The spectral transmittance of the 3 mm thick acrylic slab is also indicated in Fig. 5 (Molded Plastic Aspheric Lenses, 2022).
The spectral transmittance of ZnO-water nanofluid decreases with concentration below a wavelength of 800 nm.However, it practically remains constant above it, as seen in Fig. 5.The average transmittance of the whole optical filter (0.01 m nanofluid layer and two 3 mm acrylic layers) can be calculated by the following equation: Where t l;nf is the nanofluid's spectral transmittance, t l;ac is the acrylic's spectral transmittance, I l is the solar spectral irradiance, which is plotted in Fig. 6  Table 3 provides the average transmittances of optical filters throughout the various wavelength bandwidths.
As shown in Table 3, the optical filter's average transmittance declines gradually over the wavelength range of (400e2500 nm), from 81.4% at 0 ppm to 71.9% at 200 ppm, while it remains nearly constant over the wavelength ranges of (800e1100 nm) and (1100e2500 nm), with average values of 72.5% and 5.5%, respectively.

The comparison of the CPC-PV system with a nanofluid optical filter against the reference PV cell for various concentrations
Since only a fraction of the sun's light is converted into electricity and the rest raises the solar cell's temperature, the solar cell temperature is highly dependent on the amount of solar radiation that enters the system.Therefore, various experiments were carried out on September 5th, 6th, 18th , and October 5. Table 4 lists the dates and the corresponding concentrations of nanofluid.The four testing days' solar radiation is plotted in Fig. 7. Fig. 8 displays the average concentrated and reference PV temperature curves for all tested cases.
As illustrated in Fig. 8, there is an apparent reduction in PV cell average temperature owing to the effect of solar spectral filtration.Therefore, to compare all the tested cases, the drop in concentrated cell average temperature can be expressed as a percentage as follows:   Where T PV j and T PV;ref j are the concentrated and reference cells' average temperatures at any moment, respectively.n t is the total number of time steps over the testing interval where the data was captured every 3 s.According to Fig. 9, which compares the percentage reduction in filtered cell average temperature for all investigated situations, a gradual increase in percent reduction is obtained from 9.9% at a nanofluid concentration of 50 ppm to 10.7% at a concentration of 200 ppm.
In comparison to the reference systems, Fig. 10 shows the increased electric power of the filtered systems over the tested period compared to the reference cell.The reference cell's electrical efficiency, h PV;ref , and that of the filtered systems, h PV , can be calculated as follows (Adam et al., 2019): Where, V are the measured output current and voltage of the PV cell, I ab is the absorbed solar radiation by the cell, A ab is the solar cell's surface area, t ac is the average transmittance of the acrylic layer, t nf is the nanofluid's average transmittance.Fig. 11 displays the variations in electrical efficiency with time for the filtered PV systems and the reference PV cells for all nanofluid concentrations.The electrical power and efficiency enhancement percentages owing to using the nanofluid filter can be expressed by the following equations: Electricalef f iciencyenhancement%¼ Where P PV j , P PV;ref j are the filtered and reference cell's electric power at any time from the start, respectively.h PV j , h PV;ref j are the filtered and reference cell's electric efficiency at any time from the start, respectively.Fig. 12 displays the variations in electrical efficiency improvement percentages over time for all studied cases.The boost in all cases is  attributable to the optical filter's capability to block unwanted solar radiation, which would otherwise heat the cell.Fig. 13 compares the average percent increase in electrical power and efficiency for all studied cases due to utilizing the nanofluid optical filter.As seen in Fig. 12, the percent enhancement in electrical power and efficiency shows a gradual rise from 88% to 89.3%e93% and 107.5%, respectively, with increasing nanofluid concentrations from 50 ppm to 200 ppm.The temperature difference between nanofluid input and output impacts the thermal energy gain.The optical filter's thermal efficiency can thus be calculated as illustrated below (Huaxu et al., 2020): Where _ m nf , c p;nf , T o , T i are the nanofluid's mass flow rate, specific heat capacity, and outlet and inlet temperatures.I ab the solar radiation intensity that reaches the filter.A ab is the optical filter's surface area.The fluctuation of thermal efficiency with time for all tested cases at a constant average flow rate of 0.4 L/min is shown in Fig. 14.The following equation can give the system's overall efficiency (Huaxu et al., 2020): Fig. 15 depicts the system's overall efficiency variations over time for all studied cases.Fig. 16 compares the filtered system's average electrical, thermal, and overall efficiencies for all studied cases.As shown in Fig. 16, although the electrical efficiency enhancement rises continuously with increasing concentration from 50 ppm to 200 ppm, the electrical efficiency rises from 7.1% at 50 ppm to 7.7% at 100 ppm and then declines to 6.7% and 6.2% at concentrations of 150 ppm and 200 ppm, respectively.Moreover, the thermal and overall efficiencies increase with increasing concentration, starting from 27.9% to 31.7% at 50 ppm to 31.4% and 34.6%, respectively.
Table 5 summarizes the main findings of the previous studies related to the current work.Many factors, including the concentration ratio, nanoparticle type, base fluid type, and PV semiconductor material, impact the thermal and electrical efficiency of PV/T systems, as shown in Table 5.It is noted that the effect of increasing the concentration on thermal efficiency using ZnO nanoparticles in glycol in the research done by Huaxu et al. (2020), which was previously mentioned in the introduction, agrees with what has been concluded in the current research using the same nanoparticle type in water, where the thermal efficiency increased with an increase in concentration from 11.2 ppm to 89.2 ppm by 47%.When the concentration was increased from 50 ppm to 200 ppm in the current study, the thermal efficiency increased from 27.9% to 31.40%.However, the study conducted by Zhang et al. (An et al., 2016a) showed a difference from the current study, as the thermal efficiency decreased with increasing the concentration of Cu 9 S 5 in water.However, the effect of increasing the concentration on the electrical efficiency in the current study is different from that of the study done by Huaxu et al. (2020), as the electrical efficiency increased first in the current study from 7.1 to 7.7% by raising the concentration from 50 ppm to 100 ppm before decreasing to 6.2% by increasing the concentration to 200 ppm.In contrast, the electrical efficiency decreased with an increase in concentration in the study conducted by Huaxu et al. (2020).However, the electrical efficiency increased with the increase in concentration in the research conducted by Zhang et al. (An et al., 2016a).
There are anticipated challenges in scaling up the entire physical system because the technological economy is built on a large-scale system.One of these challenges is the potential for nanoparticle sedimentation after just a few hours of operation, which lowers the system's efficiency.To solve this issue, it is necessary to enclose the ultrasonic liquid processor in the nanofluid and use it frequently to keep particles from settling.One of the additional challenges is that using an increased quantity of liquid necessitates a higher pumping power than was used on a small scale.As a result, this power needs to be precisely computed, its quantity needs to be compared to the electrical energy and heat generated by the system, and efforts should be made to decrease it as much as possible.Finally, the possibility of a solar concentrator's shadow falling on the solar cells, which lowers the system's effectiveness, is one of the challenges that might arise when the system's scale is expanded.Thus, manual solar tracking needs to be performed frequently.

Conclusion
In order to prevent photovoltaic cells from overheating, spectral-splitting technology divides the sun's spectrum into two parts.The first part is immediately turned into electricity, and the second produces thermal energy.Numerous studies have shown that nanofluid is a very efficient absorbed liquid-based filter since it simultaneously functions as a heat transfer liquid and a spectral splitter.This study proposes a novel PV/T design that integrates a compound parabolic concentrator with a ZnO-water nanofluid that acts as a selective absorptive filter over the PV cell.Various tests were conducted to determine the effects of various ZnO-water nanofluid doses, including 50, 100, 150, and 200 ppm, on the system performance, and the average outcomes were then compared against a reference PV cell.The key findings are summarized as follows: (1) The temperature percent reduction of filtered PV cells increases gradually with concentration, rising from 9.9% at a nanofluid concentration of 50 ppm to 10.7% at a concentration of 200 ppm.(2) As the nanofluid's concentration rises from 50 ppm to 200 ppm, the average electrical power and efficiency percentage increase steadily from 88% to 89.3%e93% and 107.5%, respectively.(3) The electrical efficiency increases from 7.1% at 50 ppm to 7.7% at 100 ppm and subsequently decreases to 6.7% and 6.2% at concentrations of 150 ppm and 200 ppm, respectively, even though the electrical efficiency enhancement constantly climbs with increasing concentration from 50 ppm to 200 ppm.(4) With increasing concentration, the thermal and overall efficiencies gradually improve, going from 27.9% to 31.7% at 50 ppm to 31.4% and 34.6% at 200 ppm, respectively.

Fig. 8 .
Fig. 8. Average concentrated and reference PV temperature curves for all tested cases.

Fig. 9 .
Fig. 9. Comparison of average filtered PV temperature reduction for all tested cases.

Fig. 11 .
Fig. 11.The variation of electrical efficiency with time for all tested cases.

Fig. 12 .
Fig. 12.The variations in electrical efficiency improvement percentages over time for all studied cases.

Fig. 13 .Fig. 14 .
Fig. 13.A comparison of the average percent increase in electrical power and efficiency for all studied cases.

Fig. 15 .
Fig. 15.The variation of overall efficiency over time for all studied cases.

Fig. 16 .
Fig.16.The average performance of the filtered system for all studied cases.

Table 2 .
Detailed information about the measuring tools employed in this study.

Table 3 .
Average transmittances of the optical filter throughout various wavelength bandwidths.

Table 4 .
Dates of each concentration test.

Table 5 .
Related work comparison.Related work