Amin Shahsavar

Prabhakar Jha

Muslum Arici

Patrice Estellé
email: patrice.estelle@univ-rennes1.fr

Experimental investigation of the usability of the rifled serpentine tube to improve energy and exergy performances of a nanofluid-based photovoltaic/thermal system

Keywords: Conceptualization, Performing the experiments, Reviewing and Editing PVT system, rifled serpentine tubes, nanofluid, concentrations, energy efficiency, exergy efficiency

This experimental study aims to energetically and exergetically compare the performance of a PVT system with sheet-and-plain serpentine tube collector (base PVT system) with two cases of modified PVT systems. The modified PVT systems are the replacements of plain serpentine tube with rifled serpentine tube with 3 ribs (3-start rifled PVT system) and 6 ribs (6-start rifled PVT system). The electrical parameter of the PV module without cooling is compared with the three cases of the PVT system with cooling. The cooling fluid is water/magnetite nanofluid. The effect of nanofluid flow rates (20-80 kg/hr) and nanoadditive volume concentrations (0-2%) over the three cases of the PVT system is investigated to propose a suitable combination of flow rate and NA concentration offering the best energetic and exergetic performances. Thus, the 6-start rifled PVT system achieved a maximum of J o u r n a l P r e -p r o o f 22.5% and 3.8% higher overall energy efficiency, and 5.9%, and 1.9% higher overall exergy efficiency than the base and 3-start rifled PVT systems at flow rate and concentration of 80 kg/hr and 2%. Finally, the electrical power generated by the base, 3-start rifled, and 6-start rifled PVT systems achieved maximum enhancement of 27.5%, 29.5%, and 31.5% compare to the PV module without cooling.

Introduction

As reported by scientists, non-renewable energy sources will be exhausted by the end of the 22 nd century [START_REF] Zou | Energy revolution: From a fossil energy era to a new energy era[END_REF]. Hence, significant attention is paid to renewable energy sources and development of renewable energy systems. Solar energy is one such renewable energy source that is extensively used to meet energy demand [START_REF] Ameri | An experimental study on a PV/T air collector with direct coupling of fans and panels[END_REF]. The conversion technologies of solar energy have been assiduously investigated and noteworthy advances in recent decades have been carried out in this area, including photovoltaic (PV), photochemical, photothermal, and photovoltaic-thermal (PVT) systems as a third-generation solar systems. The defining feature of all these collectors, that differentiates them from other solar systems, is perhaps the cogeneration of heat and electricity [START_REF] Esfe | Application of nanofluids and fluids in photovoltaic thermal system: An updated review[END_REF]. PV system is one such kind of solar-based power generation unit. In the PV system, the production of electricity is realized due to the falling of sunlight on the semi conductive materials (silicon). These semi conductive silicon materials are referred to as solar or PV cells in the field of PV technology [START_REF] Shahsavar | Energy saving in buildings by using the exhaust and ventilation air for cooling of photovoltaic panels[END_REF]. The electricity generation is the result of potential difference across the negative and positive poles that actuates the flow of electrons. But, the exposed PV cells to the scorching heat from the sun leads to a high PV module operating temperature. The high operating temperature of the PV J o u r n a l P r e -p r o o f module deteriorates the PV electrical efficiency [START_REF] Khaki | Scenario-Based Multi-Objective Optimization of an Air-Based Building-Integrated Photovoltaic/Thermal System[END_REF]. To overcome the demerit of the PV module, the thermal collector was conceptualized in 1976 by Wolf [START_REF] Wolf | Performance analysis of combined heating and photovoltaic power systems for residences[END_REF] and termed as a photovoltaic thermal (PVT) collector system. In a physical form, the PVT system has a thermal collector placed below the PV module. The thermal collector has a fluid channel that enables the fluid to flow through the back surface of the PV module and results in withdrawing the unutilized heat present in the PV module's surface [START_REF] Gao | The feasibility of genetic programming and ANFIS in prediction energetic performance of a building integrated photovoltaic thermal (BIPVT) system[END_REF]. This heat withdrawal leads to improved PV electrical efficiency and simultaneously provides thermal energy. The thermal energy received is employed for low heat applications like industrial and domestic applications [START_REF] Li | Multi-objective energy and exergy optimization of different configurations of hybrid earth-air heat exchanger and building integrated photovoltaic/thermal system[END_REF].

Since the technological evolution of the PVT system, a never-ending attempt is in the process to amplify its efficacy in terms of both thermal and electrical energies. Many pieces of research have explored the possibilities of improving the overall efficiency of air and waterbased PVT systems by employing techniques such as optimization of design parameters [START_REF] Khanmohammadi | Energy analysis and multi-objective optimization of a novel exhaust air heat recovery system consisting of an air-based building integrated photovoltaic/ thermal system and a thermal wheel[END_REF][START_REF] Farshchimonfared | Channel depth, air mass flow rate and air distribution duct diameter optimization of photovoltaic thermal (PV/T) air collectors linked to residential buildings[END_REF], inclusion of heat exchanger in flow channel [START_REF] Mojumder | An experimental investigation on performance analysis of air type photovoltaic thermal collector system integrated with cooling fins design[END_REF][START_REF] Fudholi | Energy and exergy analyses of photovoltaic thermal collector with ∇groove[END_REF][START_REF] Hussain | An improved design of photovoltaic/thermal solar collector[END_REF][START_REF] Elsaf | Comparative study of double-pass flat and compound parabolic concentrated photovoltaic-thermal systems with and without fins[END_REF], flow configurations [START_REF] Slimani | A detailed thermal-electrical model of three photovoltaic/thermal (PV/T) hybrid air collectors and photovoltaic (PV) module: Comparative study under Algiers climatic conditions[END_REF][START_REF] Ooshaksaraei | Performance of four air-based photovoltaic thermal collectors configurations with bifacial solar cells[END_REF], glazing/unglazing [START_REF] Shahsavar | Experimental investigation and modelling of a direct coupled PV/T air collector[END_REF][START_REF] Shahsavar | Energy and exergy analysis of a photovoltaic-thermal collector with natural airflow[END_REF], etc. Apart from air, and water, some other conventional fluids are used as coolants for the PVT system such as ethylene glycol and motor oil. These conventional fluids are also termed as another significant parameter to withdraw heat from the flow channel. Motor oil and ethylene glycol are associated with higher cost and lower economic benefit. Nanofluids are a promising option as cooling fluids because they possess superior thermal properties over ordinary fluids [START_REF] Sokhansefat | Heat transfer enhancement in parabolic trough collector tube using Al 2 O 3 /synthetic oil nanofluid[END_REF][START_REF] Liu | Impact of oscillating magnetic field on the thermal-conductivity of water-Fe 3 O 4 and water-Fe 3 O 4 /CNT ferro-fluids: Experimental study[END_REF][START_REF] Shahsavar | Experimental investigation and develop ANNs by introducing the suitable architectures and training algorithms supported by sensitivity analysis: Measure thermal conductivity and viscosity for liquid paraffin based nanofluid containing Al 2 O 3 nanoparticles[END_REF][START_REF] Shahsavar | A novel comprehensive experimental study concerned synthesizes and prepare liquid paraffin-Fe 3 O 4 mixture to develop models for both thermal conductivity & viscosity: A new approach of GMDH type of neural network[END_REF][START_REF] Shahsavar | Effect of magnetic field on thermal conductivity and viscosity of a magnetic nanofluid loaded with carbon nanotubes[END_REF].

The suitability of a nanofluid as a cooling fluid in PVT systems has been explored by many scientists in recent years. Yazdanifard et al. [START_REF] Yazdanifard | Performance of nanofluid based photovoltaic/thermal systems: a review[END_REF] tested the PVT system under the laminar and turbulent flow for two different nanofluids. This work has shown that the addition of NAs increases the overall efficiency which is more dominant for turbulent flow rather than laminar flow. The investigation further indicated that for a turbulent flow, energy, and exergy J o u r n a l P r e -p r o o f generated from the PVT system were higher for a higher NA diameter, while for laminar flow, an opposite behavior was witnessed. Again, it was revealed that the aluminum oxide (Al 2 O 3 ) nanofluid has a higher efficiency than the titanium oxide (TiO 2 ) nanofluid.

Moradgholi et al. [START_REF] Moradgholi | Experimental study of using Al2O3/methanol nanofluid in a two phase closed thermosyphon (TPCT) array as a novel photovoltaic/thermal system[END_REF] reported an experimental study using methanol mixed Al 2 O 3 nanofluid.

The study showed that the use of Al 2 O 3 /methanol nanofluid decreases the PV module temperature by 14.5°C which simultaneously generated 1.4 W higher power output. A sheet and tube-based PVT system with Al 2 O 3 /water, ZnO/water, and TiO 2 /water nanofluids were numerically studied by Sardarabadi and Passandideh-Fard [START_REF] Sardarabadi | Experimental and numerical study of metaloxides/water nanofluids as coolant in photovoltaic thermal systems (PVT)[END_REF]. This research evidenced that the thermal performance of the PVT system is better when it is operated with ZnO/water nanofluid whereas the better electrical performance is imparted by the Al 2 O 3 /water nanofluid.

Purohit et al. [START_REF] Purohit | Heat transfer and entropy generation analysis of alumina/water nanofluid in a flat plate PV/T collector under equal pumping power comparison criterion[END_REF] numerically investigated a combination of water and alumina as a nanofluid for the PVT system and compared with the water-based PVT system in terms of working performance. They reported a 25% enhancement in overall working performance for the nanofluid-based PVT system as compared to the water-based PVT system. However, this study also revealed that at the same pumping power, this percentage enhancement falls to 13.8% for the water-based PVT system compared to the nanofluid-based PVT system. Al-Waeli et al. [START_REF] Al-Waeli | Mathematical and neural network modeling for predicting and analyzing of nanofluidnano PCM photovoltaic thermal systems performance[END_REF] studied a nanofluid (SiC/water)-based PVT system. The prepared nanofluid was introduced into the phase change material (PCM) tank to check its effect on the PV module temperature. It was found that the PV module temperature reduced considerably.

Another concept of PCM was implemented by Hassan et al. [START_REF] Hassan | Thermal management and uniform temperature regulation of photovoltaic modules using hybrid phase change materials-nanofluids system[END_REF] where a hybrid PCM based nanofluid was used as a coolant for the PVT system. It was shown that the used nanofluid enhances the system's electrical efficiency by 23.9% when compared with the water-based PVT system. A techno-economic study was experimentally carried out by Al-Waeli et al. [START_REF] Al-Waeli | Experimental investigation of using nano-PCM/nanofluid on a photovoltaic thermal system (PVT): Technical and economic study[END_REF] where a paraffin mixed nano-SiC nanofluid was made to pass through the backside of the PV module. The study reported that this modification improved the electrical efficiency J o u r n a l P r e -p r o o f from 7.1 to 13.7%, as well as provided 72% thermal efficiency. On the other hand, the techno-economic section of the study suggested the payback period and electricity cost as 5-6 years and 0.125 $/kWh. A new theoretical approach was established by Fudholi et al. [START_REF] Fudholi | TiO2/water-based photovoltaic thermal (PVT) collector: Novel theoretical approach[END_REF] to evaluate the performance of nanofluid-based PVT system. In this study, an aqueous suspension of TiO 2 was utilized as NA for the nanofluid preparation. The findings reported a 13-25% augmentation in the energy efficiency of the PVT system due to the utilization of NAs in comparison to pure water.

Exergy and entropy are the two important concepts in the thermodynamical systems, the former shows the system's ability to execute useful work whilst the latter indicates the system's inability to execute useful work [START_REF] Sardarabadi | Experimental investigation of the effects of using metal-oxides/water nanofluids on a photovoltaic thermal system (PVT) from energy and exergy viewpoints[END_REF]. The important concept of exergy is used in nanofluid-based PVT system also by various researchers [START_REF] Sardarabadi | Experimental investigation of the effects of silica/water nanofluid on PV/T (photovoltaic thermal units)[END_REF][START_REF] Aberoumand | Energy and exergy analysis of a photovoltaic thermal (PV/T) system using nanofluids: An experimental study[END_REF][START_REF] Hosseinzadeh | Energy and exergy analysis of nanofluid based photovoltaic thermal system integrated with phase change material[END_REF][START_REF] Yu | Numerical study on energy and exergy performances of a microencapsulated phase change material slurry based photovoltaic/thermal module[END_REF][START_REF] Khanjari | Numerical investigation on using of nanofluid in a water-cooled photovoltaic thermal system[END_REF]. Sardarabadi et al. [START_REF] Sardarabadi | Experimental investigation of the effects of silica/water nanofluid on PV/T (photovoltaic thermal units)[END_REF] experimentally investigated the performance of nanofluid SiC/water nanofluid in PVT system. The authors reported the exergy efficiency of pure water (19.36%) increased to 22.61% and 24.31% for 1 wt% and 3 wt% of SiC nanoparticles, respectively. Ag/water nanofluid with 4 wt% in nanoparticle content was studied and compared with the traditional PVT system by Aberoumand et al. [START_REF] Aberoumand | Energy and exergy analysis of a photovoltaic thermal (PV/T) system using nanofluids: An experimental study[END_REF]. They concluded that the PVT system's exergy efficiency and electrical power was enhanced by 30% and 10%, respectively, with nanofluid over the traditional PVT system. Another study to improve the PVT system's exergy efficiency was performed by Hosseinzadeh et al. [START_REF] Hosseinzadeh | Energy and exergy analysis of nanofluid based photovoltaic thermal system integrated with phase change material[END_REF]. For this purpose, they used a combination of nanofluid (ZnO/water) and PCM (paraffin wax), which resulted in an attainment of 13.6% of overall exergy efficiency. With a similar objective of improving the exergy performance, a microencapsulated PCM slurry was used as a cooling fluid by Yu et al. [START_REF] Yu | Numerical study on energy and exergy performances of a microencapsulated phase change material slurry based photovoltaic/thermal module[END_REF]. In their study, the exergy performance was mapped with the variation of the melting point and slurry concentration. It was found that the increase in melting point and slurry concentration increased the exergy efficiency. Khanjari et al. [START_REF] Khanjari | Numerical investigation on using of nanofluid in a water-cooled photovoltaic thermal system[END_REF] investigated a PVT system J o u r n a l P r e -p r o o f with two different nanofluids, Ag/water and Al/water varying NA concentration. It was shown that the higher NA concentration results in higher heat transfer properties for both nanofluids. On the other hand, Ag/water nanofluid resulted in a better PVT system's exergy efficiency as compared to the alumina/water nanofluid for the same NA concentration.

As reported before, some previous works have been conducted to evaluate and improve the energy and exergy generation capacity of the nanofluid-based PVT systems. This is also the objective of the present work to improve the energy and exergy performance of the traditional nanofluid-based PVT system by replacing, in addition, the plain serpentine tube with a rifled serpentine tube. To the best of our knowledge, the energy and exergy analysis of the PVT system with a riffled serpentine tube collector has never been conducted in the past. The effects of nanofluid flow rate (20-80 kg/hr), and NA volume concentration (0-2%) on the energy and exergy performances of the modified PVT systems are studied and compared with that of traditional system. A suitable combination of flow rate and NA concentration that offers the best energetic and exergetic performances is identified.

Experimental set-up

The objective of this research is investigate three experimental cases of the PVT system coupled with the use of nanofluids and evaluate the best option based on energy and exergy analysis. The first case is the plain serpentine tube PVT system which is considered as the base case. The other two PVT systems are the modifications of the base PVT system, which is done by replacing a plain serpentine tube with a rifled serpentine tube with 3 ribs (3-start rifled PVT system), and 6 ribs (6-start rifled PVT system).

The dimension considered for the serpentine tube is 8 mm in inner diameter and 2 mm in thickness, and the collector is made of copper. The collector is positioned at the back of the PV module by applying thermal grease while serpentine tubes are attached to the collector J o u r n a l P r e -p r o o f surface with the help of soldering process. To reduce heat loss from the back surface of collector, the serpentine tubes are insulated with stone wool. Both the height and width of ribs used in the 3-start and 6-start rifled PVT system are 0.4 mm, while the rib pitch is 1 mm. The schematic presentation of the considered PVT systems is illustrated in Fig. 1 As mentioned above, the objective of this work is to compare three cases of the PVT system considering similar input conditions such as radiation intensity and ambient temperature. For such reason, a solar simulator is fabricated to perform the experimental investigation under the laboratory situations. The solar simulator consists of an aluminum box (1 nos.), DC fan (4 nos.), 10 W LED lamps (4 nos.), 400 W reflector-sunlight-dysprosium-lamp (RSDL) also described as metal halide lamp (4 nos.), and an AC phase-cut dimmer (1 no.). The first objective is to achieve a similar radiation intensity (126,582 lux≈1000 W/m 2 [START_REF] Long | Evaluation emerging PV Performance Rating under indoor lighting simulator[END_REF]) and ambient temperature (22 °C) throughout the experiment which has been measured by a lux meter (BF06-Trotec-type) and a thermocouple (K-type). The PV module (Eco-Worthy type) used for the four cases has a dimension of 32 cm×21.5 cm, a power weightage of 10 W and consists of 72 solar cells. A photograph and important specifications of the PV module are respectively given in Fig. 2(a) and Table 1. The temperature at 20 different locations over the PV module (shown in Fig. 2(b)) is also measured with the help of K type thermocouples. The temperature measurements are recorded by a datalogger (JUMO type). To calculate the electrical energy, the current and voltage imparted by the module during its functioning are measured by a multi-meter (LUTRUN-type). The working fluid used in the present J o u r n a l P r e -p r o o f investigation is nanofluid, which requires a pump to push it from the inlet to the outlet of the serpentine tube. The pump operates on a DC source and has a head capacity of 3.5 m. The pump also facilitates different pumping speeds which is another requirement for the present investigation as various mass flow rates are used in the study. The pumping speed is measured by utilizing a rotary flow meter (M10000-Malema). The pressure drop caused by the nanofluid across the surfaces of the tubes at various flow rates is measured by a differential type pressure transmitter (3051CD-Rosemount). The circulation process of the nanofluid takes place in a closed cycle. The nanofluid is placed in a 2L capacity reservoir through which the nanofluid is pumped into the collector with the help of a tube and collects heat from the PV module. This cyclic process continues until steady conditions are achieved.

Another criterion in the comparative analysis is to have a constant inlet temperature which is maintained throughout the trial and error method by passing the nanofluid through a plate type heat exchanger. The nanofluid, when passing through the heat exchanger, rejects its heat to a separate coolant (mixture of water and ethylene glycol) prepared through a constant temperature bath. The schematic diagram and real photograph of the utilized experimental setup are respectively shown in Fig. 3(a) and (b). Important specifications of the utilized equipment are also presented in Table 2.

Table 1. Technical characteristics of the studied PVT units in the current investigation [START_REF] Shahsavar | Experimental evaluation of novel photovoltaic/thermal systems using serpentine cooling tubes with different crosssections of circular, triangular and rectangular[END_REF]. Thermal properties analyzer Decagon-KD2 0.02-4 W/m.K ±5%

Nanofluid preparation and its specification

The co-precipitation method is used to prepare the water/magnetite nanofluid [START_REF] Shahsavar | Effect of magnetic field on thermal conductivity and viscosity of a magnetic nanofluid loaded with carbon nanotubes[END_REF] for the present experimental investigation. The different processes associated with the preparation of the water/magnetite nanofluid are:

1. Addition of 67.58 gm of FeCL 3 .6H 2 O in 100 ml of 2M hydrochloric (HCL) acid.

2. Addition of 39.76 gm of FeCL 2 .4H 2 O in 100 ml of 2M HCL acid.

3. The compositions obtained in the above processes 1 and 2 are mixed in a ratio of 4 to 1 using a magnetic stirrer for 2 minutes at 100 RPM.

4. 50 mL of 0.7 M ammonia hydroxide under intense stirring is introduced dropwise to the solution obtained in process 3 for 10 min at 700 RPM.

5. The solution obtained in process 4 is placed over the permanent magnet with the help of a container. The permanent magnet discharged the magnetic NA and settled it into the bottom surface of the container.

J o u r n a l P r e -p r o o f 6. The magnetic particles are added with 8 ml of 25% tetramethylammonium hydroxide and the final solution underwent a centrifugation process for 1 minute at 4000 RPM.

7. The magnetic NAs prepared in the previous step are sufficiently poured into pure water and the resulting solution is stirred using a magnetic stirrer for 30 min at 300 RPM.

All the above-mentioned processes are also presented in a schematic representation in Fig. 4.

The stability of the prepared nanofluid samples was investigated using the fact that the instability of the fluid led to the deposition of NAs and, therefore, the change in nanofluid viscosity over time. Therefore, the nanofluid viscosity was measured every day for three weeks by means of a density gravity meter at temperature of 23 ℃ (liquid type, DA-130N, KEN Japan), and the results showed that the viscosity changes were negligible within three weeks. The necessary thermophysical properties of the prepared nanofluid are given in Table 3. Thermal conductivity of NF was measured using a KD2-Pro thermal properties analyzer (Decagon devices, USA) that operates based on the transient hot wire technique. This device has a probe (KS-1) that must be placed in the solution for 90 seconds during the test. To be more precise, the thermal conductivity measurement was repeated 5 times at 20-minute intervals (to allow thermal stability), and the value reported in Table 3 is actually the average of the values obtained from the 5 experiments. To calibrate the device, the thermal conductivity of glycerol and water was measured at 20°C and the obtained values (0.274 W/m.K for glycerol and 0.586 W/m.K for water) were compared with the values in a valid source (0.285 W/m.K for glycerol and 0.598 W/m.K for water) [41]. The difference between the experimental results and the values reported in the ref.

[41] was less than 3.9%. In addition, the density ( ) and specific heat ( , ) of nanofluid were computed using the following equations [START_REF] Sheikholeslami | Ferrohydrodynamic and magnetohydrodynamic effects on ferrofluid flow and convective heat transfer[END_REF]:

= 1 - + (1)
J o u r n a l P r e -p r o o f

, = 1 - , + , (2)
where is the NA volume concentration and the subscripts , and stands for the nanofluid, water and NA. The density and specific heat of magnetite NAs are respectively 5200 kg/m 3 and 670 J/kg.K and the specific heat of water is 4179 J/kg.K [START_REF] Sheikholeslami | Ferrohydrodynamic and magnetohydrodynamic effects on ferrofluid flow and convective heat transfer[END_REF].

Energy and exergy formulation

The concept of the PVT system was developed to receive thermal energy and electrical energy simultaneously. Keeping this in mind, the present study is aimed to improve the thermal and electrical performances of the traditional plain serpentine tube collector by some J o u r n a l P r e -p r o o f design modifications as discussed in section 2. The detailed formulation of energy, exergy, and electrical energy are presented in this section.

The thermal behavior of the PVT systems is characterized by its ability to produce total thermal energy with respect to the total solar energy received. This process is termed as thermal energy efficiency of the PVT system [START_REF] Li | Multi-objective energy and exergy optimization of different configurations of hybrid earth-air heat exchanger and building integrated photovoltaic/thermal system[END_REF]:

= , - ! " # $ % &' × 100 (3)
where , , and respectively denote the inlet temperature, outlet temperature, mass flow rate, and specific heat capacity of nanofluid. In addition, " # $ and % &' are the striking radiation intensity and PV module area, respectively.

Similarly, the electrical behavior of the PVT system is characterized by its ability to produce electrical energy with respect to the total solar energy received. This process can be termed as electrical energy efficiency )$ of the PVT system [START_REF] Li | Multi-objective energy and exergy optimization of different configurations of hybrid earth-air heat exchanger and building integrated photovoltaic/thermal system[END_REF]:

)$ = * &' " &' -+ )$ " # $ % &' × 100 (4)
where " &' and * &' are respectively the output current and output voltage of the PV module. + )$ is the fluid pumping power which can be calculated as [START_REF] Yazdanpanahi | Experimental investigation of exergy efficiency of a solar photovoltaic thermal (PVT) water collector based on exergy losses[END_REF]:

+ )$ = ∆- (5)
where ∆and are the pressure drop and density of the nanofluid, respectively.

Additionally, is known as pump efficiency (=75%).

Thermal energy is characterized as low-grade energy whereas electrical energy is characterized as high-grade energy [START_REF] Li | Multi-objective energy and exergy optimization of different configurations of hybrid earth-air heat exchanger and building integrated photovoltaic/thermal system[END_REF]. The conversion of electrical energy into thermal energy is possible with an approximate efficiency of 100% while the opposite behavior is not feasible. The PVT systems can be assessed and compared based on the overall energy efficiency which can be determined as [START_REF] Joshi | Performance evaluation of a hybrid photovoltaic thermal (PV/T)(glass-to-glass) system[END_REF]:

J o u r n a l P r e -p r o o f & = + )$ 0.36 (6)
where 0.36 is the conversion coefficient of a coal-based power plant [START_REF] Joshi | Performance evaluation of a hybrid photovoltaic thermal (PV/T)(glass-to-glass) system[END_REF].

Exergy is one of the important applications of the second-law of thermodynamics and is characterized as the maximum work theoretically possible from a thermodynamic system.

Accordingly, the exergy efficiency of the PVT system is characterized by the percentage of the maximum theoretically possible output that can be achieved as an actual desired output [START_REF] Aberoumand | Energy and exergy analysis of a photovoltaic thermal (PV/T) system using nanofluids: An experimental study[END_REF]. Therefore, it is important to assess the overall exergy scenario of the PVT systems which is the summation of the thermal exergy efficiency and electrical exergy efficiency )$)1 [START_REF] Li | Multi-objective energy and exergy optimization of different configurations of hybrid earth-air heat exchanger and building integrated photovoltaic/thermal system[END_REF]:

&)1 = )1 + )$)1 (7)
where )1 is thermal exergy efficiency which is defined as the ratio of thermal exergy gain from the PVT system to the exergy of sun radiation [START_REF] Aberoumand | Energy and exergy analysis of a photovoltaic thermal (PV/T) system using nanofluids: An experimental study[END_REF][START_REF] Badescu | How much work can be extracted from diluted solar radiation?[END_REF]:

)1 = , - ! 21 -3'4 + 273 + 273 7 " # $ % &' 8 9: 21 - 4 3 < 3'4 #= > ? + 1 3 < 3'4 #= > ? @ 7 × 100 (8)
whereas )$)1 is the electrical exergy efficiency expressed as the ratio of the electrical power gain from the PVT system to the exergy of sun radiation [START_REF] Al-Waeli | Experimental investigation of using nano-PCM/nanofluid on a photovoltaic thermal system (PVT): Technical and economic study[END_REF][START_REF] Badescu | How much work can be extracted from diluted solar radiation?[END_REF]:

)$)1 = * &' " &' -+ )$ " # $ % &' 8 & 21 - 4 3 < 3'4 #= > ? + 1 3 < 3'4 #= > ? @ 7 × 100 (9)
where 3'4 and #= are the ambient temperature and sun temperature, respectively (≅5800 K). In addition, 8 & is the absorptivity of PV panel (=0.9 and > is the interaction factor, which is determined as follows [START_REF] Badescu | How much work can be extracted from diluted solar radiation?[END_REF]:

> = F # G # H G H 8 & I J/@ (10)
J o u r n a l P r e -p r o o f

where G # and G H are the dilution factors for incoming solar radiation and PV panel emitted radiation, respectively. In addition, H is the geometric factor for solar collector and H is the geometric factor for solar radiation defined as the ratio of concentration of solar collector (L)

to the maximum concentration of solar radiation at level of earth orbit (L '31 ).

Uncertainty analysis

To assess the accurate findings of the experimental study, the uncertainty of the used instruments must be considered. In this study, uncertainty analysis is carried out via the method established by Moffat [46] which identifies the uncertainty of the dependent variable (D) as a function of the independent variables (M J , M N , …, M ) as expressed below:

OP = Q F RP RSM J OM J I N + F RP RM N OM N I N + ⋯ + F RP RM OM I N (11)
In the above equation, O represents the parameter's uncertainty. The outcomes demonstrated that in all the experiments conducted in the current contribution, the uncertainty of the parameters was less than 5.21%, which is a justifiable value. The details of the uncertainty calculations are given in "Appendix A".

Results and discussions

The performance of the three cases of is due to the greater thermal conductivity of nanofluid at higher NA concentrations (see Table 3). The explained behavior of thermal energy output with the flow rates and NA concentrations is quantified below.

It is observed that for the base PVT system when the flow rate increased from 20-80 kg/hr at a 0% NA concentration, the thermal energy output increased by 21.9%. At a similar condition, this percentage improvement for the 3-start rifled, and 6-start rifled PVT systems has been evaluated to 24.2% and 24.4% respectively. For a similar increment in the flow rate J o u r n a l P r e -p r o o f (20-80 kg/hr), at a 2% NA concentration, the thermal energy output enhancement for the base, 3-start rifled, and 6-strat rifled PVT systems is 21%, 24.8%, and 25.2% respectively.

On the other hand, when NA concentration increased from 0-2% at a flow rate of 20 kg/hr, the thermal energy output increased by 17.1%, 19.9%, and 19.4% for the base, 3-start rifled, and 6-start rifled PVT system. For a similar increment in the NA concentration (0-2%), at a flow rate of 80 kg/hr, the thermal energy output enhancement for the base, 3-start rifled, and 6-start rifled PVT systems reaches 16.3%, 20.4%, and 20.2% respectively.

The crux of this investigation is to also directly compare the three cases of the PVT system. It is seen that the 6-start rifled PVT system has a higher thermal energy output as compared to the 3-start rifled and base PVT systems at any combinations of flow rate and NA concentration. It is seen that at a 2.0% and 80 kg/hr of NA concentration and flow rate, the 6start rifled PVT system has gained thermal energy output of 39.7 W against 28.49 W and, 37.71W of the base and 3-start rifled PVT system. The 6-start rifled PVT system has a 39.3% and 5.3% higher thermal energy output than the base and 3-start rifled PVT systems at maximum flow rate (2 kg/hr) and NA concentration (2%). It is seen that when the base PVT system is modified into the 3-start rifled PVT system by adding 3 ribs, the thermal power output enhanced. Similarly, when the 3-start rifled PVT system is modified into the 6-start rifled PVT system by adding 6 rifled ribs, the thermal energy output further enhanced as quantified in the above discussions.

Two factors can be considered as the reasons for this behavior: first, the rifled tube has a higher heat transfer surface area compared to the plain tube, and secondly, due to the centrifugal force, the rifled tube entails the fluid rotation and, in consequence, the mixing of near-wall hot fluid with the cold fluid passing through the central region of the tube.

J o u r n a l P r e -p r o o f 384 and 20 kg/hr, while the maximum value is achieved at 2% and 80 kg/hr for all the three cases.

385

The reasons for an increase in the value of thermal energy efficiency and overall energy 386 efficiency with NA concentration and flow rate are similar to the case of thermal power.

In terms of quantitative value, it is evident from Table 4 and Fig. 6 that as the flow rate increased from 20-80 kg/hr at a 0% NA concentration, the thermal energy efficiency of the base, 3-start rifled, and 6-start rifled PVT system enhanced by 21.9%, 24.3%, and 24.4%, while these values are 21%, 24.8%, and 25.2% at a 2% NA concentration for a similar range of nanofluid flow rate. Again, the effect of increasing NA concentration at a given flow rate has also been observed in terms of thermal energy efficiency. It is seen that as the NA concentration increased from 0-2% at 20 kg/hr, the thermal energy efficiency of the base, 3start rifled, and 6-start rifled PVT systems improved by 17.1%, 19.9%, and 19.4%, while these values are 16.3%, 20.4%, and 20.2% at the flow rate of 80 kg/hr for a similar range of NA concentration. On the other hand, when the three cases are compared with each other, it has been observed that the 6-start rifled PVT system achieved highest thermal energy efficiency of 57.7% against 41.4%, and 54.8% of the base and 3-start rifled PVT systems at the combination of maximum flow rate (80 kg/hr) and NA concentration (2%). This indicates that the 6-start rifled PVT system has 39.4%, and 5.3% higher thermal energy efficiency than the base and 3-start rifled PVT systems.

Coming to the overall energy efficiency behavior of the three cases of the PVT system, it has been observed that the base, 3-start rifled, and 6-start rifled PVT systems achieved an overall energy efficiency in the range of 64.66-78.04%, 72.66-92.08%, and 75.05-95.58% for the range of flow rate (20-80 kg/hr) and NA concentration (0-2%). These ranges suggest that the 6-start rifled PVT system has generated 22.5% and 3.8% higher overall energy efficiency when compared with the base, and 3-start rifled PVT system at a maximum flow rate (80 kg/hr) and NA concentration (2%). The reason for the dominance of the 6-start rifled PVT system over the base, and 3-start rifled PVT system in terms of thermal energy and overall energy efficiency is the same as explained for the case of thermal power. While observing the effect of increasing flow rate from 20-80 kg/hr at a given 0% NA concentration, the thermal exergy output for the base, 3-start rifled, and 6-start rifled PVT systems is enhanced by 44.4%, 55.5%, and 60.9% as shown in Fig. 7. On the other hand, at 2% NA concentration, these percentage enhancements are reported as 53.4%, 70.1%, and 76.5% under similar flow rate variations (20-80 kg/hr). Again, when the effect of increasing NA concentration (0-2%) is observed at a given flow rate of 20 kg/hr, the thermal exergy output for the base, 3-start rifled and 6-strat rifled PVT systems is enhanced respectively by 7.4%, 6.3%, and 6.2%, while these corresponding enhancements at 80 kg/hr are observed to be 14.1%, 16.3%, and 16.5%. Again, when these three cases are compared with each other, it has been found that the 6-start rifled PVT system has gained a highest thermal exergy output of 1.2 W against 0.89 W and 1.14 W of the base and 3-start rifled PVT systems at the maximum flow rate and NA concentration of 80 kg/hr and 2%. It can be said that the 6-start rifled PVT system has 34.8% and 5.3% higher thermal energy output than the base and 3start rifled PVT systems.

J o u r n a l P r e -p r o o f

The effect of increasing flow rate at a given NA concentration and vice versa in terms of thermal exergy efficiency is also evaluated in this investigation (Table 5). It is evidenced that as the flow rate increased from 20-80 kg/hr at a given NA concentration of 0%, the thermal exergy efficiency increased by 45.9%, 56.2%, and 60% for the base, 3-start rifled and 6-start rifled PVT systems, respectively, whereas at 2% NA concentration, these percentage enhancements are 53.6%, 72.3%, and 76.4%. Similarly, when NA concentration increased from 0-2% at a given flow rate of 20 kg/hr, the thermal exergy efficiency increased by 8.7%, 6.2%, and 5.4% for the base, 3-start rifled and 6-start rifled PVT systems, whereas at 80 kg/hr, these percentage enhancements are reported as 14.5%, 17.2%, and 16.2%. The comparison of the three cases shows that 6-start rifled PVT system has achieved thermal exergy efficiency of 3.07% against 2.29% and 2.93% of the base and 3-start rifled PVT J o u r n a l P r e -p r o o f systems at 80 kg/hr and 2% NA concentration, which means that the 6-start rifled PVT system has 34%, and 4.7% higher thermal exergy efficiency than the base and 3-start rifled PVT systems. Fig. 8 displays the overall exergy efficiency achieved by three cases of the PVT system at different flow rates and NA concentrations. The base, 3-start rifled, and 6-start rifled PVT systems gained an overall exergy efficiency in the range of 25.99-27.71%, 26.63-28.82%, and 26.96-29.36% for the range of flow rate (20-80 kg/hr), and NA concentration (0-2%). The ranges discussed with the help of the Fig. 8 is a clear representation of an increasing overall exergy efficiency with the increase in flow rate and NA concentration for all the three cases.

These ranges also suggest that the 6-strat rifled PVT system has the highest overall exergy efficiency as compared to the other two configurations of the PVT system at any combination of flow rate and NA concentration. It is seen that at 80 kg/hr of flow rate and 2% of NA concentration, the 6-start rifled PVT system has generated 5.9%, and 1.9% higher overall exergy efficiency than the base, and 3-start rifled PVT systems.

J o u r n a l P r e -p r o o f

Electrical energy and its related efficiency

The concept of the PVT system was developed to reduce the operating temperature of the PV module as the high value of the PV module temperature deteriorates its electrical efficiency.

So, in the present investigation, the temperature of the PV module and electrical power have Fig. 10 and Tables 13 and14 display the electrical power, electrical energy efficiency, and electrical exergy efficiency achieved by the three cases of the PVT system under the influence of the increasing flow rate and NA concentration. It has been observed that these three parameters increase with an increase in the flow rate (20-80 kg/hr) at any given NA concentration. When the flow rate increased from 20-80 kg/hr at 0% NA concentration, the electrical power for the base, 3-start rifled and 6-start rifled PVT systems increased by 3.6%, 3.4%, and 2.8%, whereas at 2% NA concentration, these percentage enhancements are 3.4%, 3.3%, and 2.5%. The similar percentage enhancements are achieved in terms of the electrical energy efficiency and electrical exergy efficiency for the three cases at 0% and 2% NA concentration as the flow rate is increased from 20-80 kg/hr. The reason for the increase in the electrical efficiency at a given NA concentration when the flow rate is increased from 20-80 kg/hr is the higher efficient heat transfer. However, the behavior of the electrical parameters is not consistent when the utilized three cases are compared with each other as the NA concentration is increased from 0-2% at a given flow rate. This is because when the NA concentration increases, both viscosity and thermal conductivity of the nanofluid increases [START_REF] Shahsavar | Effect of magnetic field on thermal conductivity and viscosity of a magnetic nanofluid loaded with carbon nanotubes[END_REF]. The increase in the viscosity results in high-pressure drop which causes high power consumption. On the other side, the increase in the thermal conductivity improves the PV cooling capacity and therefore increases the electrical power capacity of the PVT system. So, it is seen that for the base case when the NA concentration increased from 0-2% at 20 kg/hr, the electrical power, electrical energy efficiency, and electrical exergy efficiency decrease by 0.1%, whereas at 80 kg/hr, these parameters reduce by 0.3% because of higher power consumption at 2% NA concentration compared to at 0% NA concentration, which J o u r n a l P r e -p r o o f outweighed the significance of higher conductivity. However, for the 3-start rifled PVT system, it has been observed that as the NA concentration increased from 0-2% at 20 kg/hr, the electrical power, electrical energy efficiency, and electrical exergy efficiency increased by 0.07% because of a better conductivity capacity of the nanofluid coupled with the effect of 3-start rifled ribs that resulted in a high electrical power from the PV module that outperformed the problem of power consumption. However, these three parameters for the 3start rifled PVT system are decreased by 0.04% at 80 kg/hr because of the same reason as explained for the base PVT system. In the case of the 6-start rifled PVT system, the electrical power, electrical energy efficiency, and electrical exergy efficiency are increased by 1.3% at 20 kg/hr, and 1.1% at 80 kg/hr, as the NA concentration is increased from 0-2%. This is due to the fact that the nanofluid coupled with the 6-start rifled ribs results in a better PV cooling and hence a better electrical behavior.

Finally, this investigation shows that the maximum electrical power, electrical energy efficiency, and electrical exergy efficiency achieved in the case of base PVT system is 9.604 W, 13.958%, and 25.498% at 0% NA concentration, whereas the counterpart values for the 3start rifled PVT system are 9.753 W,14.175%, and 25.498% at a 1% NA concentration, and for the 6-start rifled PVT system are 9.902 W, 14.398%, and 26.291% at a 2% NA concentration. All these maximum values for the three cases have been achieved at the flow rate of 80 kg/hr. This shows that the 6-start rifled PVT system has generated 3.1%, and 1.5% higher electrical power compared with the maximum value achieved by the base and 3-strat rifled PVT systems. A similar percentage enhancement has been observed for the 6-start rifled PVT system over the base and 3-start rifled PVT system in terms of electrical energy efficiency and electrical exergy efficiency.

The main objective of the PVT system is to enhance the electrical performance of the PV module. So, at the beginning of the study, the electrical power generated by the PV module J o u r n a l P r e -p r o o f without cooling was observed and recorded as 7.530 W, and in the end, it is compared with the maximum electrical power generated by the three cases with cooling. So, the base, 3-start rifled, and 6-start rifled PVT system showed the percentage enhancement of 27.5%, 29.5%, and 31.5% of electrical power when compared with the PV module without cooling. J o u r n a l P r e -p r o o f

Conclusions

In the present experimental investigation, three cases of the PVT system were investigated for various nanofluid flowrates and NA concentrations. These three cases are base PVT system, 3-start rifled PVT system, and 6-start rifled PVT system. It has been found that the increase in the flow rate and NA concentration from 20-80 kg/hr, and 0-2% increases the thermal performance of all three cases of the PVT system. Further, it has been found that the 6-start rifled PVT system has achieved a maximum of 22.5%, and 3.8% higher overall energy efficiency, and 5.9%, and 1.9% higher overall exergy efficiency than the base and 3-start rifled PVT system, respectively. In terms of electrical performance, the base and 3-start rifled PVT system, has achieved maximum performance at 0% and 1% NA concentrations at the flow rate of 80 kg/hr, whereas for the 6-start rifled PVT system, the maximum electrical performance has been achieved at 2% NA concentration and the flow rate of 80 kg/hr.

Further, it has been concluded that the 6-start rifled PVT system has achieved 3.1% and 1.5% higher electrical power as compared with the base and 3-start rifled PVT systems. It is further found that the 6-start rifled PVT systems has achieved 31.5% higher electrical power with respect to the PV module without cooling as compared to its counterparts. V ^kl = <V , V H Z,\] , V m \]e , V m \]n , V c deY , V f Zgh ?

= Q _ O O V `N + _ O O , V H Z,\] `N + _ O O V m \]e `N + _ O O V m \]n `N + F O O" # $ V c deY I N + _ O O% &' V f Zgh `N V ^kl = Q _ V `N + _ V H Z,\] , `N + _ V m \]e - `N + _ V m \]n - `N + F V c deY " # $ I N + _ V f Zgh % &' `N = 4.43%
-Electrical exergy efficiency:

)$)1 UUUUUU = <* &' UUUUUU , " &' UUUUUU , + )$ UUUUU , " ̅ # $ , % &' UUUUUUU , 3'4 UUUUUU ?

V ^XYXo = <V : S , V c S , V W XYZ , V c deY , V f Zgh , V m phq ? V ^klXo = <V ' \] , V H Z,\] , V m \]e , V m \]n , V c deY , V f Zgh , V m phq ?

= Q _ O )$)1 O* S V : S `N + _ O )$)1 O" S V c S `N + _ O )$)1 O+ )$ V W XYZ `N + F O )$)1 O" # $ V c deY I N + _ O )$)1 O% &' V f Zgh `N + F O )$)
= Q _ O )1 O V ' \] `N + _ O )1 O , V H Z,\] `N + _ O )1 O V m \]e `N + _ O )1 O V m \]n `N + F O )1 O" # $ V c deY I N + _ O )1 O% &' V f Zgh `N + F O )1 O 3'4 V m phq I N V ^klXo )1 = Q _ V ' \] `N + _ V H Z , , `N + _ V m \]e - `N + _ V m \]n - `N + F V c deY " # $ I N + _ V f Zgh % &' `N + F O )1 O 3'4 V m phq

  .

Fig. 1 .

 1 Fig. 1. Schematic view of the (a) sheet-and-serpentine tube collector, (b) plain serpentine tube, (c) 3-

Fig. 2 .Fig. 3 .

 23 Fig. 2. (a) A photograph of PV module and (b) placement of thermocouples at the front surface of the PV module.

Fig. 4 .

 4 Fig. 4. Various steps of the preparation of water/magnetite nanofluid [40].

  Fig. 5 displays the thermal energy output gained by the three cases of the PVT system under

Fig. 5 .

 5 Fig. 5. Thermal energy output of the base, 3-start rifled, and 6-start rifled PVT systems at different 377

Fig. 6 .

 6 Fig. 6. Overall energy efficiency of the base, 3-start rifled, and 6-start rifled PVT system at different 417

Fig. 7 .Table 5 .

 75 Fig. 7. Thermal exergy output of the base, 3-start rifled, and 6-start rifled PVT systems at different

Fig. 8 .

 8 Fig. 8. Overall exergy efficiency of the base, 3-start rifled, and 6-start rifled PVT systems at different

  been initially observed without cooling it by nanofluid. Under this condition, the value of the PV module temperature and electrical power have been observed as 346 K and 7.530 W. The investigation has been further extended to monitor the difference of the PV module temperature without cooling ( & ) and the PV module temperature with cooling ( & ) for the three cases of the PVT system as shown in Fig. 9. The value of & -& has been found in J o u r n a l P r e -p r o o f increase in flow rate and NA concentration for all the three cases. Fig.

Fig. 9 .

 9 Fig. 9. & -& difference of the base, 3-start rifled, and 6-start rifled PVT systems at different flow rates and NA concentrations.

Fig. 10 .

 10 Fig. 10. Electrical power of the base, 3-start rifled, and 6-start rifled PVT systems at different flow

J o u r

  n a l P r e -p r o o f -Thermal energy efficiency:

  r n a l P r e -p r o o f

Table 2 .

 2 The specifications of instrument used in the experimental study[START_REF] Shahsavar | Experimental evaluation of novel photovoltaic/thermal systems using serpentine cooling tubes with different crosssections of circular, triangular and rectangular[END_REF].

				Precision
	Instrument	Model	Range
				(from manufacturer)
	Luxmeter	TROTEC-BF06	0-40 klx	±5%±10 lx
	Thermometer	K-type	-200-1260 °C	±0.75%
	Temperature sensor	TES-1327 K	-150-1260 ℃	±0.1% -1℃
	Multimeter DC voltage	LUTURON-DW6060	0-600 V	±0.8%
	Multimeter DC current	LUTURON-DW6060	0-10 A	±1%
	Rotary flow meter	Malema-M10000	-0.1 to 225 Lit/min	±2%
	Pressure transmitter	Rosemount 3051CD	0-2.07 MPa	±1%
	Liquid density gravity meter	KEN-DA130N	0-2 g/cm 3	±0.001 g/cm 3

Table 3 .

 3 Thermophysical

	0	4179.0	0.610	997.10
	0.5	4089.39	0.695	1018.11
	1	4003.40	0.873	1039.13
	1.5	3920.83	0.917	1060.14
	2	3841.46	0.961	1081.16

properties of the prepared nanofluid (aqueous magnetite).

NA concentration (%) Heat capacity (J/kg.K) Thermal conductivity (W/m.K) Density (kg/m 3 )

Table 4 .

 4 Thermal energy efficiency of the base, 3-start rifled, and 6-start rifled PVT systems at different flow rates and NA concentrations.

	Mass flow rate		Base PVT system			3-start rifled PVT system			6-start rifled PVT system
	(kg/hr)
	NA concentrations	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0
	(%)	20 40 60 80	29.2 31.2 33.3 35.6	30.6 32.6 34.9 37.1	31.7 33.8 36.1 38.5	32.6 34.8 37.1 39.5 J o u r n a l P r e -p r o o f 34.2 36.6 38.5 40.1 41.5 36.5 39.3 41.6 43.4 45.1 38.8 42.2 44.5 46.5 48.3 41.4 45.5 47.7 50.0 51.7	43.9 47.7 50.9 54.8	38.6 41.5 44.6 48.0	40.6 43.6 47.1 50.3	42.2 45.5 49.1 52.6	43.7 47.1 51 54.3	46.1 49.7 53.7 57.7

  9 further suggests that 493 the maximum & -& difference has been observed at 80 kg/hr and 2% NA concentration 494 for all three cases and the 6-start rifled PVT system has the & -& difference of 37.88 K The reason for the higher & -& difference for the 6-start rifled PVT system over the base 497 and 3-start rifled PVT systems is due to the efficient heat transfer from the back of the PV

	495
		which is 15.6%, and 5.3% higher than the base and 3-start rifled PVT systems, respectively.
	498
	499	module due to the 6 rifled ribs.
	500
		28.73	31.01	33.7	35.28	29.55	31.73	33.91	36.16	30.29	32.44	34.55	36.79	31.09	33.17	35.28	37.37	31.66	33.77	35.89	37.88
		26.53	28.88	31.47	33.44	27.23	29.47	32.04	34.12	28.01	30.31	32.77	34.78	28.74	30.87	33.41	35.43	29.27	31.57	33.85	35.97
		23.56	25.79	28.57	30.67	24.28	26.55	29.27	31.26	24.95	27.24	29.91	31.89	25.5	27.86	30.36	32.29	25.99	28.36	30.66	32.77

496

Table 6 .

 6 Electrical energy efficiency of the base, 3-start rifled, and 6-start rifled PVT systems at different flow rates and NA concentrations. .477 13.478 13.470 13.458 13.700 13.703 13.713 13.719 13.710 13.850 13.903 13.949 14.001 14.035 40 13.623 13.631 13.634 13.632 13.620 13.860 13.855 13.870 13.863 13.867 13.985 14.030 14.075 14.121 14.157 60 13.816 13.820 13.819 13.804 13.777 14.039 14.033 14.039 14.038 14.022 14.152 14.158 14.197 14.243 14.280 80 13.958 13.953 13.951 13.932 13.919 14.171 14.173 14.175 14.174 14.165 14.233 14.291 14.329 14.364 14.393 J o u r n a l P r e -p r o o f

	Mass flow rate		Base PVT system			3-start rifled PVT system			6-start rifled PVT system
	(kg/hr)
	NA concentrations	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0
	(%)
	20	13.471 13

Table 7 .

 7 Electrical exergy efficiency of the base, 3-start rifled, and 6-start rifled PVT systems at different flow rates and NA concentrations. 568 .619 24.623 24.607 24.585 25.027 25.034 25.052 25.026 25.047 25.298 25.395 25.481 25.576 25.637 40 24.887 24.901 24.908 24.902 24.882 25.321 25.312 25.338 25.326 25.333 25.547 25.628 25.71 25.795 25.861 60 25.239 25.246 25.246 25.217 25.168 25.647 25.635 25.647 25.645 25.616 25.85 25.862 25.934 26.017 26.085 80 25.498 25.489 25.487 25.451 25.428 25.887 25.89 25.894 25.892 25.876 26 26.104 26.175 26.237 26.291

	Mass flow rate		Base PVT system			3-start rifled PVT system			6-start rifled PVT system
	(kg/hr)
	NA concentrations	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0	0.0	0.5	1	1.5	2.0
	(%)
	20	24.609 24

J o u r n a l P r e -p r o o f

kg/hr, while the maximum is obtained for 2% and 80 kg/hr.

the range of 23.56-32.77 K, 26.53-35.97 K, and 28.73-37.88K for the base, 3-start rifled, and

-Electrical energy efficiency:

J o u r n a l P r e -p r o o f • Energetic and exergetic performances of a nanofluid-based PVT unit are assessed.

• The unit is equipped with a sheet-and-rifled serpentine tube collector.

• Aqueous suspension of magnetite nanoparticles is considered as coolant.

• Energetic and exergetic performances of the unit intensify with boosting and .

• Overall energy and exergy efficiencies of rifled PVT unit is better than base unit.

J o u r n a l P r e -p r o o f

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:


Jean-Marc Tadié

Vincent Thibault
email: vincent.thibault@chu-rennes.fr

Charlotte Pronier

Arnaud Gacouin

Gisèle Lagathu

Yves Le Tulzo

Vincent Pr

Thibault

Respiratory Influenza viral load as a marker of poor prognosis in patients with severe symptoms

Keywords:

come

Respiratory Influenza viral load as a marker of poor prognosis in patients with severe symptoms

Charlotte Pronier, Arnaud Gacouin, Gisèle Lagathu, Yves Le Tulzo,

INTRODUCTION

Influenza A and B viruses (IV) are respiratory pathogens that cause regular seasonal epidemics, with every year thousands of death in the world [START_REF] Fiers | A "universal" human influenza A vaccine[END_REF][START_REF] Gbd | Mortality, morbidity, and hospitalisations due to influenza lower respiratory tract infections, 2017: an analysis for the Global Burden of Disease Study 2017[END_REF]. Viral infection of the lower respiratory tract may exacerbate a chronic underlying lung disease, may cause pneumonia or acute respiratory distress syndrome (ARDS), all potentially leading to respiratory failure and mechanical ventilation (MV) requirement. Pathogenesis of infections with IV has been extensively described [START_REF] Shieh | pandemic influenza A (H1N1): pathology and pathogenesis of 100 fatal cases in the United States[END_REF][START_REF] Mauad | Lung pathology in fatal novel human influenza A (H1N1) infection[END_REF][START_REF] Short | Pathogenesis of influenza-induced acute respiratory distress syndrome[END_REF]. Disease severity and ARDS evolution could be explained by differences in epithelial-endothelial cell damage, alveolar epithelial cells involvement, immune cell recruitment, cell apoptosis, or cytokine production by endothelial and epithelial cells [START_REF] Camp | A Role for Neutrophils in Viral Respiratory Disease[END_REF]. Disease severity is also linked to the epidemic viral strain as demonstrated for the pandemic 2009 A-H1N1 (H1N1pdm09) virus [START_REF] Shieh | pandemic influenza A (H1N1): pathology and pathogenesis of 100 fatal cases in the United States[END_REF][START_REF] Short | Pathogenesis of influenza-induced acute respiratory distress syndrome[END_REF][START_REF] Herold | Influenza virus-induced lung injury: pathogenesis and implications for treatment[END_REF]. Diagnosis of IV infection relies nowadays mostly on genome detection through molecular amplification. Diagnosis accuracy largely depends on the sample collection quality. In the ICU setting, IV testing is done on nasopharyngeal swabs or nasal washes but also on tracheal aspirates or J o u r n a l P r e -p r o o f bronchoalveolar lavage (BAL) especially in those receiving MV [START_REF] Uyeki | Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenzaa[END_REF]. Because collection modes are different, samples are heterogeneous and comparable determination of IV loads is difficult in real-life settings. In clinical trials, quantification of IV load by RT-PCR has been used to assess the efficacy of antiviral therapies but standardization of nasopharyngeal sampling from all investigators was required, allowing more accurate viral load assessments [START_REF] Beigel | IRC003 Study Team, Oseltamivir, amantadine, and ribavirin combination antiviral therapy versus oseltamivir monotherapy for the treatment of influenza: a multicentre, double-blind, randomised phase 2 trial[END_REF][START_REF] Hayden | Baloxavir Marboxil Investigators Group, Baloxavir Marboxil for Uncomplicated Influenza in Adults and Adolescents[END_REF][START_REF] Dunning | International Severe Acute Respiratory and Emerging Infection Consortium (ISARIC), Antiviral combinations for severe influenza[END_REF]. Despite an absence of standardization, some data suggest a relationship between IV load and clinical outcomes including severity of symptoms, hospital length of stay, and protracted shedding [START_REF] Clark | Viral load is strongly associated with length of stay in adults hospitalised with viral acute respiratory illness[END_REF]. Higher viral loads in patients who died from influenza than in those who survived was also reported for influenza A-H5N1 virus but such relationship seems less obvious with A-H1N1pdm09, A-H3N2, and B viruses [START_REF] Jong | Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia[END_REF]. A recent study from Granados et al. compared different IV loads according to the severity of symptoms and also failed to find a robust association between respiratory IV-A viral loads and severity [START_REF] Granados | Influenza and rhinovirus viral load and disease severity in upper respiratory tract infections[END_REF]. Overall, it seems difficult to find a universal viral load threshold that would allow classifying an infection as severe or mild as it is highly impacted by the delay from the first symptoms and the patient's age [START_REF] Feikin | Is Higher Viral Load in the Upper Respiratory Tract Associated With Severe Pneumonia? Findings From the PERCH Study[END_REF]. The lack of viral load quantification standardization due to sample heterogeneity and technical diversity remains a true challenge.

We performed a retrospective study to assess the respiratory influenza viral load measured at admission to the ICU with the aim to determine whether high viral load could be associated with ICU outcomes, with a focus on patients presenting with an ARDS. To accurately compare viral load between heterogeneous respiratory samples, the amount of virus was determined against the total amount of cells.

METHODS

Patients and setting

Samples and PCR methods

Respiratory samples were collected in universal transport medium. Up to January 2016, influenza viruses were detected using Argene (bioMérieux, Marcilly, France) and after this date, Seegene Allplex Respiratory panel (Eurobio, Les Ulis, France) became the routine method. Internal results and a recent publication indicate a good correlation between both techniques for the detection of influenza viruses [START_REF] Chan | Comparison of the Biofire FilmArray Respiratory Panel, Seegene AnyplexII RV16, and Argene for the detection of respiratory viruses[END_REF]. The population tested with Argene (n=32) and Seegene (n=86) was not different for the tested parameters and the viral load and virus-over-cell ratios were not significantly different.

Retrospectively, on -80°C stored frozen samples, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene amount was quantified in each sample to normalize J o u r n a l P r e -p r o o f for the number of cells contained in respiratory samples as previously described [START_REF] Medhurst | The use of TaqMan RT-PCR assays for semiquantitative analysis of gene expression in CNS tissues and disease models[END_REF].

Cell DNA quantification was done against a standard curve based on standardized genomic DNA (Roche, Meylan, France). By using IV and GAPDH Ct values, the relative amount of virus over cellular DNA was calculated as proposed by Livak and Schmittgen and further described in the supplementary file [START_REF] Livak | Analysis of Relative Gene Expression Data Using Real-Time Quantitative PCR and the 2-ΔΔCT Method[END_REF].

Study endpoints

The main objectives were to assess IV load at admission and to determine whether viral load at admission was associated with ICU outcome in ARDS and non-ARDS patients.

Statistical analysis

Data are expressed as percentages, medians and interquartile ranges (IQR). As appropriate, qualitative data were compared using Chi-square test or Fisher exact test while quantitative data were compared using Mann-Whitney or Kruskal-Wallis test for comparisons between three groups or more. We used a logistic regression analysis to identify variables independently associated with mortality in the ICU as described in the supplementary file.

RESULTS

Patients

During the study period, 118 patients were positive for influenza infection as detected by IV molecular detection on respiratory samples obtained at admission to the ICU (Figure 1 and table 1). Type A IV was found for 82% (n=97) of patients. Fifty-five percent (n=65) of patients presented ARDS criteria. Among ARDS patients, all received neuromuscular blocking agents, 8 (14%) received iNO, 6 (10%) ECMO, and J o u r n a l P r e -p r o o f 29 (49%) prone positioning. Among non-ARDS patients, 22 patients (48%) received invasive MV, 5 (10%) received solely noninvasive ventilation (NIV), 6 (13%) only highflow nasal oxygen therapy, and 13 (29%) standard oxygen therapy. Mortality for the entire cohort was 21% (n=25). As expected, patients with ARDS had statistically more often BAL than nasopharyngeal swab compared to non-ARDS patients (p=0.0017).

Results for RT-PCR and viral load assessment

Results for RT-PCR on respiratory samples are shown in table 2. Due to volume shortage, 13 samples (11%) could not be retrospectively tested for cell DNA content and were not included in the final analysis performed on 105 samples (Fig. 1). IV Ct values ranged from 20 to 40 and did not differ significantly according to the type of sample, endotracheal aspirates (n=70), BAL (n=22) or nasopharyngeal aspirates (n=13). Sample cellular richness, as assessed by GAPDH-DNA measurement, was quite broad ranging from 20 to 39 Ct, corresponding to approximately 10 2 to 10 7 cells/mL of sample and was independent from the sample type (Table 2). Noteworthy, the relative amount of detected virus-per-cell copy number was equal to 1 (virtually representing 1 virus per cell) in BAL and higher than in other samples (0.45 and 0.20 for nasopharyngeal and endotracheal aspirates, respectively). There was no correlation between viral load and cellular richness of the samples and the relative amount of virus-over-cell, expressed as log10 ranged from -5.2 to 3.7. A good correlation was observed between the absolute virus copy number and the virus-overcell ratio (Spearman's -0.876) (figure 2). When considering the entire population, there was a tendency (p=0.07) for a higher viral load in patients who eventually died, compared to those who survived, but no association with any other studied parameters such as the length of stay or additional J o u r n a l P r e -p r o o f treatments was found. Cellular sample richness was higher for ARDS compared to non-ARDS patients, with a mean difference in Ct value of 2, corresponding to approximately 4 times more cells in ARDS patient samples (p=0.001).

As cellular richness was slightly less important in nasopharyngeal samples and to exclude a possible sampling bias, the same analysis was performed without considering nasopharyngeal samples and led to the same findings (Table S1 provided as supplementary file).

Mortality in the ICU

Overall mortality was 21% (n=25), 31% (n=20) for ARDS, and 9% (n=5) for non-ARDS patients (Table 1). Deaths in non-ARDS patients were attributed to preexisting comorbidities. Bilateral injury on chest-ray was associated with mortality after comparison on the entire population (p=0.02). SAPS II score calculated on admission differed significantly (p<0.001) between deceased patients and survivors after comparison on the entire population but also on each group of ARDS (p=0.004) and non-ARDS (p=0.01) patients. No other studied parameters varied significantly between dead and alive patients in the non-ARDS group. Noteworthy, mortality was lower (p=0.03) in ARDS-patients who received oseltamivir treatment (n=60) on the first day in ICU.

Viral load results available for 105 patients were compared between deceased patients and survivors over the entire population and in the subgroups of ARDS and non-ARDS patients (Table 3). Considering the entire population, IV Ct-values were significantly lower, and therefore viral loads higher, in patients who died compared to those who survived (p=0.03). However, when IV Ct-values were normalized to cell DNA content, there was only a tendency (p=0.07) for a higher virus-over-cell ratio in patients who J o u r n a l P r e -p r o o f eventually died (Figure 3). The same conclusions were reached when considering only lower respiratory samples (n=92) (Table S2 provided as supplementary file).

Subgroup of ARDS patients

When considering only ARDS-patients (n=59), IV loads but also virus-over-cell ratios were significantly higher, p=0.01 and p=0.02, respectively, in patients who ultimately died (n=19) compared to those who survived. When considering the relative virus-overcell index, patients who died had more than 5 times virus-per-cell than those who survived (1 vs. 0.17, p=0.02). These differences were not observed in non-ARDS patients (n=46) although the number were limited (n=5 deaths in this subgroup) to draw firm conclusions. Restraining the analysis merely on lower respiratory samples (n=57) led also to the same observation (Table S2).

The variables included in adjusted analyses included age, oseltamivir treatment started on the first day at ICU admission, SAPS-II score (table 2) in addition to IV Ct-values.

Adjusted analyses showed that IV Ct-value (1-point decrement, OR = 1.21, 95% CI 1.04-1.40, p= 0.009), log virus-over-cell ratio difference (1-point increment, OR = 2.19, 95% CI 1.23-3.91, p= 0.008), remained independently associated with mortality in the ICU.

DISCUSSION

In this retrospective study performed on patients admitted to the ICU with a diagnosis of acute respiratory failure due to influenza virus, higher viral load as assessed either by RT-PCR Ct-values or by normalized virus-over-cell ratios on admission to the ICU was associated with mortality. More specifically, this association was highly significant in patients presenting with ARDS at ICU admission.

J o u r n a l P r e -p r o o f

We hypothesized that high IV loads could be associated with poor prognosis in ICU patients admitted with respiratory failure, because they could reflect high replication level and large amount of infected cells in the lung. Indeed, viral load has been shown to be predictive of different clinical outcomes in respiratory tract infections [START_REF] Clark | Viral load is strongly associated with length of stay in adults hospitalised with viral acute respiratory illness[END_REF][START_REF] Granados | Influenza and rhinovirus viral load and disease severity in upper respiratory tract infections[END_REF][START_REF] Feikin | Is Higher Viral Load in the Upper Respiratory Tract Associated With Severe Pneumonia? Findings From the PERCH Study[END_REF][START_REF] Fuller | Association of the CT values of real-time PCR of viral upper respiratory tract infection with clinical severity, Kenya[END_REF][START_REF] Lu | Relationship between respiratory viral load and lung lesion severity: a study in 24 cases of pandemic H1N1 2009 influenza A pneumonia[END_REF]. Persistent detection of virus by RT-PCR is often considered to determine whether combination of new antivirals might be more effective than oseltamivir alone in the treatment of flu and viral load reduction was used to assess the efficacy of baloxavir marboxil in the treatment of patients with uncomplicated influenza [START_REF] Beigel | IRC003 Study Team, Oseltamivir, amantadine, and ribavirin combination antiviral therapy versus oseltamivir monotherapy for the treatment of influenza: a multicentre, double-blind, randomised phase 2 trial[END_REF][START_REF] Hayden | Baloxavir Marboxil Investigators Group, Baloxavir Marboxil for Uncomplicated Influenza in Adults and Adolescents[END_REF]. To our knowledge, few studies have previously assessed the usefulness of respiratory viral load measurement in the prognosis of flu patients. It has been reported that in patients with avian influenza-A H5N1-virus infection, nonsurvivors had significantly higher viral load on pharyngeal and nasal swabs than survivors [START_REF] Jong | Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia[END_REF]. Clark et al. also demonstrated that in patients with influenza-induced acute respiratory illness, as in patients with non-influenza viral-induced acute respiratory illness, high viral loads were strongly associated with a longer duration of hospitalization but not with hospital mortality [START_REF] Clark | Viral load is strongly associated with length of stay in adults hospitalised with viral acute respiratory illness[END_REF].

In our study, high viral load on admission appears associated with bad prognosis in ARDS but not in non-ARDS patients, while viral load did not differ significantly between the two groups of patients. Influenza-mediated upper and lower airway damages, lung injury and pneumonia certainly result from viral pathogenicity but also from the host response [START_REF] Petrova | The evolution of seasonal influenza viruses[END_REF]. Immunologic mechanisms involved in influenza-induced ARDS, including cytokines production, alterations in neutrophils and macrophages functions, or apoptosis of epithelial cells differ from those noted in non-influenza ARDS and from those present in patients without ARDS [START_REF] Short | Pathogenesis of influenza-induced acute respiratory distress syndrome[END_REF][START_REF] Camp | A Role for Neutrophils in Viral Respiratory Disease[END_REF][START_REF] Herold | Influenza virus-induced lung injury: pathogenesis and implications for treatment[END_REF]. Clark et al suggested that clinical illness severity following respiratory viral infection could be predicted by viral load magnitude, J o u r n a l P r e -p r o o f kinetics and persistence [START_REF] Clark | Viral load is strongly associated with length of stay in adults hospitalised with viral acute respiratory illness[END_REF]. Our results could suggest that in patients with acute respiratory failure severe enough to require ICU admission, mortality appears more associated with a great inoculum of influenza virus than with an excessive immune response. Reduction of viral load through antiviral treatment might therefore be a desirable goal, particularly for ARDS patients. Of note, oseltamivir treatment was associated with decreased mortality in ARDS but not in non-ARDS patients.

Measurement of cellular DNA highlighted a higher cell content in samples from ARDS compared to non-ARDS patients. Unfortunately, this study was not designed to identify the type of recruited cells. Yet, this observation could set the path for qualitative studies looking at either transcriptomics or precise identification of recruited cells in such samples.

Reflecting our standard procedures, respiratory samples from our patients were heterogeneous due to upper or lower airway collection. Calculating a virus-over-cell ratio index is an attempt to normalize as much as possible the inherent variability resulting from sample heterogeneity. Although imperfect, this index based on virus and cellular DNA Ct-values through the validated "delta-Ct method" [2 -Ct ], remains at least a homogeneous marker between patients [START_REF] Livak | Analysis of Relative Gene Expression Data Using Real-Time Quantitative PCR and the 2-ΔΔCT Method[END_REF]. This method provides a standardized measurement of the amount of virus-over-cell genome equivalent, independently of the sample quality. Using this approach, we demonstrate a high heterogeneity of sample cellular richness ranging from 100 to more than 10 7 cell/mL, in agreement with the recent work from Piralla et al. that nicely demonstrated the wide range of DNA amount in unselected nasal swabs from clinical practice [START_REF] Piralla | Cellular DNA quantification in respiratory samples for the normalization of viral load: a real need?[END_REF]. Our findings would support the use of such standardized measurement to assess its relevance in virally induced lung pathologies.

J o u r n a l P r e -p r o o f

Several assets of our study can be highlighted. First, it encompasses several flu seasons with the circulation of different strains; noteworthy, no difference of viral loads or virus-over-cell ratio was observed according to the viral strain type. The second is the relative homogeneity of the unselected population recruited in a single center with standardized procedures of care and stringent disease classification. The third is the standardization of respiratory samples by assessing their cellular DNA load and the measurement of viral loads through standardized and commercialized procedures.

Importantly, all sub-analyses performed after removal of nasopharyngeal samples led to the same conclusions, strengthening our findings and eliminating a potential bias due to sample heterogeneity. Obviously, some limitations might be raised such as its retrospective nature and the relative limited number of patients, particularly when performing subgroup analyses. Unfortunately, an extensive validation of this proof of principle was not performed because of the lack of samples and resources.

Complementary efforts should be done to confirm this preliminary set of data.

In conclusion, we report here the first study comparing standardized respiratory IV load at admission to the ICU, in patients with acute respiratory failure according to the development of ARDS or not. A high viral-over-cell content in patients with ARDS was associated with a poor prognosis. This measurement could help to identify patients at higher risk of mortality and justifying closer management and treatment. In patients without ARDS criteria, such link between IV load and clinical outcome or length of stay was not as robust. Our results suggest that viral load reduction through antiviral treatment might be an important goal to achieve, particularly in ARDS patients. J o u r n a l P r e -p r o o f

J o u r

  n a l P r e -p r o o f This retrospective study was performed in a 21-bed ICU admitting mostly medical patients. Patients were identified through Rennes University Hospital ICU and virology laboratory databases. The study was approved by the hospital's ethics committee (N° 16.117). All patients aged over 18 years admitted to ICU between February 1, 2014 and March 30, 2018, for respiratory failure and with a documented positive respiratory sample for IV infection as assessed by RT-PCR detection were included in the study. Patients were classified according to the occurrence or not of ARDS as described in the supplementary file. Data collection Baseline characteristics of patients, severity score, ventilating support received during the ICU stay, and results of IV real-time RT-PCR Cycle threshold (Ct) values were collected prospectively, extracted from medical records through a standardized questionnaire and analyzed retrospectively. Collected biological data are provided in the supplementary file.

J o u r*

  n a l P r e -p r o o f Credit-Author-Statement Charlotte Pronier, Arnaud Gacouin, Gisèle Lagathu, Yves Le Tulzo, Jean-Marc Tadié, Vincent Thibault  Conceptualisation: AG, GL, VT  Methodology: AG, GL, VT  Validation: CP, AG, GL, VT  Formal analysis: AG, VT  Investigation: CP, YLT, JMT, AG, GL, VT  Resources: CP, AG, GL, VT  Data Curation: AG, VT  Writing -Original Draft: AG, VT  Writing -Review & Editing: CP, AG, GL, VT  Visualization: CP, YLT, JMT, AG, GL, VT  Supervision: AG, VT Influenza virus, n (%) See text for definition; continuous variables are presented with (interquartile ranges)

J o u rFigure 1 :

 1 Figure 1: Sample disposal for the 118 patients presenting with influenza virus infection.

Figure 2 :

 2 Figure 2: Correlation between Influenza virus Ct values and virus over cell ratio as

Figure 3 :

 3 Figure 3: box-plot representing virus over cell ratio expressed as log according to

Table 2 :

 2 Results for values obtained from bronchoalveolar lavages and from nasopharyngeal and endotracheal aspirates

		All patients	Nasopharyngeal aspirates Endotracheal	Bronchalveolar	P value
				aspirates	lavages
		n= 105	n= 13	n= 70	n= 22
	Influenza virus, n (%)					0.12
	H3N2 A	52 (50)	5 (38)	34 (48)	13 (59)
	H1N1pdm09 A	33 (31)	4 (31)	20 (29)	9 (41)
	B	20 (19)	4 (31)	16 (23)	0 (0)
	Cell DNA Ct-value (IQR)	25 (23-27)	26 (25-29)	25 (23-27)	25 (24-27)	0.52
	IV Ct-value (IQR)	27 (22-34)	34 (28-35)	27 (22-34)	26 (22-29)	0.07
	Virus-over-cell ratio (IQR)	0.222 (0.005-5.517)	0.451 (0.005-0.518)	0.204 (0.003-6.288)	1.032 (0.097-6.364)	0.08
	log10 Virus/Cell (IQR)	-0.661 (-2.332-0.575)	-1.135 (-2.365--0.490)	-0.730 (-2.470-0.770)	0.005 (-1.010-0.800)	0.08
	IQR, Interquartile Range

Acknowledgments