The Impact of Tilt Angle and Flowrate on Performance of Nanofluid 1 Based Photovoltaic/ Thermal (PV/T) Solar Collectors

Today, by the arrival of new sustainable energy technology, the provision of energy for the global population has turned into a significant issue for societies. Meanwhile, photovoltaic/thermal 8 (PV/T) solar collectors, as one of the most advanced types to produce electricity and heat simultaneously, can be applied with nanofluid as the working fluid. In this research, PVP coated silver nanofluid was prepared in three volume concentrations being 12 250, 500 and 1000 ppm by two-step method to determine the stability and thermal conductivity, experimentally. Then, the performance of PV/T solar collector is analyzed by TRNSYS software to study electrical and thermal efficiency and also output electrical and thermal energy in different 15 months, flowrates (25, 50, 75, 100, 150 and 200 l/h) and nanofluid’s concentration. Based on the results, the optimum flowrate and nanofluid’s concentration are obtained 50 l/h 18 and 1000 ppm PVP coated silver nanofluid. At last, the effect of tilt angle on the output thermal 19 and electrical energy is determined. According to the results, by changing tilt angle in different 20 months, the performance of PV/T solar collector can be ameliorated. This paper can be heeded as a novel approach to the lack of solar radiation in by improving the performance of

The main step in using nanofluids as the working fluid in PV/T solar collectors is to examine 50 the properties of them. Stability, thermal properties and radiant properties that should be 51 considered for any nanofluid. Khan et al. review some significant researches about the properties 52 of diverse nanofluids and determine the effect of some parameters such as particle size on the 53 performance of them [11]. Over the past decade, many researches have been done on the properties 54 of nanofluids for use in solar collectors [12]. In separate studies, Karami  polyvinylpyrrolidone and sodium dodecyl sulfate and study the size of the particle and the 111 surfactant characteristics. They showed that by increment of particle size, the stability would be 112 decreased [34]. Koca  the stability and energy performance of water-based MXene nanofluids in hybrid PV/thermal solar 120 systems. They optimized and improved the performance of this type of collector [37]. 121 In this paper, stability, thermo-physical and thermal properties of PVP coated silver nanofluid 122 which was prepared and stabilized in three different volume concentration are investigated, 123 experimentally. Then, the performance of PV/T solar collector is determined by TRNSYS tools, 124 numerically and the implication of flowrate and nanofluid's concentration on the performance of 125 collector is studied. Furthermore, the implication of tilt angle of collector on the amount of 126 produced thermal and electrical energy is determined. This project can be deemed as the first study 127 of the application of PVP coated silver nanofluid in PV/T solar collectors and the impact of 128 determinative parameters on its performance, such as flowrate and tilt angle to overcome the lack 129 of solar radiation in winters. This can engender higher efficiency for residential application of 130 PV/T solar collectors which pave the way of using solar energy in houses in future. 131

2-1-Materials and Methods for Preparing Nanofluid 133
In this study, Silver nanoparticle coated with PVP (US Research Nanomaterials, Inc., USA) has 134 been used as the main particle of deionized water-based nanofluid. The properties of nanoparticles 135 are elaborated in Table 1. The volume fraction of nanofluid samples were 250 ppm, 500 ppm and 136 1000 ppm (Ag1, Ag2 and Ag3, respectively). 137 Amongst different processes of preparation of nanofluids, in this research, two steps method 140 have been employed to be used. In his process, nanoparticles with volume fractions of 250, 500 141 and 1000 ppm are dispersed in deionized water as the fluid. In order to better dispersion of 142 nanoparticles in the base fluid, an Ultrasonic device with power of 400 W (Hielscher Ultrasonic 143 UP400D -Teltow, Germany) has been utilized which is depicted in figure 2a. The main features 144 of PVP is the avoidance oxidation of silver nanoparticles and also rising the stability which is 145 added 0.2%wt of silver nanoparticle. Accurately weighed PVP-coated silver nanoparticles and 146 surfactants were dispersed in 1000 ml of deionized water to make sample nanofluids. Data for 147 complete agitation of the nanofluid mixture are as follows: 60 min at 50% amplitude using a 400 148 W, 20 kHz probe. The sample is displayed in figure 2b. It is worth to mention that after 15 min the 149 ultrasonic probe was turned off to prevent the increment of nanofluid's temperature. 150

2-2-Stability 152
Agglomeration and sedimentation phenomena overshadow the nanofluid's performance as the 153 working fluid engendering prevention of using them in thermal systems. Consequently, the 154 consideration of nanofluid's stability in design procedures is consequential. Assessing the stability 155 and dispersal of nanofluids can be carried out using several methods. Zeta Potential method has 156 been used here in as the dispersal assessment method. Calculating the zeta potential and particle 157 sizes has been done by using ZEN 3600, Malvern Instruments Ltd., UK. The measurement results 158 of nanoparticle diameters for the 1000 ppm nanofluid sample have been demonstrated that the size 159 of nanoparticles has been measured as 82.38 nm. Also, regarding analyzing zeta potential of 160 nanofluid, the stability boundary of this potential has been determined as 25 mV (negative or 161 positive) [38]. Based on the results, PVP coated silver nanofluid's zeta potential is calculated -41.6 162 V leading the fact that this nanofluid has an adequate range of stability.  and thermal diffusion of them. It is worth to mention that nanoparticles have larger surface area 176 compared to micro particles and other bigger particles which leads to escalate the heat transfer, in 177 this section, the enhancement of thermal conductivity coefficient by increasing the volume fraction 178 of nanofluid and also temperature has been discussed. Thermal conductivity of nanofluids depends 179 on temperature strikingly. To the extent that, by surging the temperature from 25 to 55°C thermal 180 conductivity of base fluid and 1000 ppm silver nanofluid's growth would be 0.06 and 0.49 W/mK, 181 respectively. By increment of temperature from 25 to 55°C, the amount of K is increased for all 182 samples with different range. The thermal conductivity of water and 1000 ppm PVP coated silver 183 nanofluid are 0.54 and 0.6 W/mK respectively in 25°C. Whereas, these quantities are increased to 184 0.594 and 1.098 W/mK in 55°C. 185

3-2-Validation of Model 200
The final data which is extracted from TRNSYS software is compared with another research by 201 for both study is same and it can be concluded that the extracted data from TRNSYS, is reasonable 208 and also acceptable. 209 is worth mentioning that, each parameter is averaged of all minutes during a day.

4-1-Electrical Efficiency 219
The amounts of electrical efficiency for different flowrates are presented in table (3). As it can 220 be found in table (3), electrical efficiency has a constant trend during a year and also it witnesses 221 negligible fluctuations by changing the flowrate. 222 To assess the effect of nanofluid's concentration, the electrical efficiency of PV/T solar collector 224 for different working fluid (water, Ag1, Ag2 and Ag3) and 50, 150 l/h is illustrated in table (4). 225 According to the table (4), the electrical efficiency increases with the increment of nanofluid's 226 concentration, slightly. This trend repeats for all flowrates, to the extent that 1000 ppm PVP coated 227 silver nanofluid has the highest electrical efficiency. 228

4-2-Thermal Efficiency 230
One of the most significant implications of applying nanofluid as the working fluid is the 231 escalation of thermal efficiency. By consideration of the PV/T solar collector's schematic (figure 232 1) and also the information in figure (3) According to the figure (6), the ascending trend of thermal efficiency over the nanofluid's 239 concentration is repeated for all flowrates and months. Furthermore, a parabolic behavior can be 240 detected by the surge of flowrates. This is to say that, thermal efficiency is increased to more than 241 50% in 50-150 l/h and then it starts to level off and decrease to lower than 50% in next flowrates.
The higher efficiency of collector in winter months is also acceptable because of lower solar 243 radiation in these months engendering lower heat losses. The highest efficiency is gained by Ag3 244 nanofluid and 150 l/h flowrate in November being 71%. Whereas, the lowest amount is 35% 245 (Water, 25 l/h and April). 246

4-3-Electrical Energy 247
Alongside the prominence of the efficiency of collector, the output energy is more valuable for 248 residential applications. Indeed, the effectiveness of PV/T solar collectors depends on the amount 249

4-4-Thermal Energy 263
The most determinative parameter in this study is output thermal energy decreasing the 264 consumption of fossil fuels in conventional hot water supply system. The total amount of thermal 265 energy producing in each months by PV/T solar collector is demonstrated in figure (8).  As it can be found in table (5)    -Ethical Approval and Consent to participate: "Not applicable" 367 -Consent for publication: "Not applicable" 368 -Availability of supporting data: "Not applicable" 369 -Competing interests: "Not applicable" 370 -Funding: "Not applicable" 371 -Authors' contributions: "Not applicable" 372 -Acknowledgements: "Not applicable" 373