1 Introduction

Over the past few decades, population explosion and industrialization are the major factors responsible for the shortage of energy supply across the globe. Most of the energy required depends upon fossils or conventional fuels. The demand for mineral-based fuel is high, but the utilization of these fuels creates severe problems such as increasing pollution, high carbon emission, and climate change [1]. These adverse impacts of fossil-based fuels shift the focus toward renewable energy resources, which are environment friendly and can capable enough to reduce the problem of the energy crisis to a large extent [2].

Solar, wind, hydropower, biomass, and geothermal energy are the main components of renewable energy. Out of these available renewable sources, solar is the most prominent source to cater to the global energy demand. In recent times, several countries like Germany, China, Japan, Italy, and the USA encourage the utilization of solar for electricity and thermal energy production [3, 4]. Solar thermal collectors and electrical energy through photovoltaic panels help to transform solar radiations into thermal energy. The output of a photovoltaic (PV) cell depends on several environmental factors (e.g., solar irradiance, ambient temperature, humidity, and dust). The energy production from the PV cell is a function of cosmic intensity, but the cosmic intensity also leads to a rise in the cell temperature. As per literature a 1 °C rise in cell temperature above nominal temperature (25 °C) causes the electrical outcome of the cell to reduce by 0.5% [5]. Therefore, commercial solar cells are very less efficient in converting solar energy to electrical energy, as an abundance of the energy is transformed into heat. The excessive heat in the cell leads to permanent structural damage to the cell. Therefore, in hybrid photovoltaic thermal (PVT) systems, the thermal management system must be incorporated with the PV module to maintain the cell temperature within a threshold value and simultaneously enhance electrical efficiency [6]. The hybrid PVT generates the thermo-electrical energy at the output by utilizing the excess heat inside the cell. The recovered heat is called useful thermal energy and can be used in various domestic and agricultural applications.

So far, several experimental and simulation works are carried out all around the globe to analyze the impact of coolant type on the outcome of a hybrid PVT system. Usually, the coolants used in the PVT system are classified as liquid coolant (water, refrigerant, and nanoparticles), air, and a mixture of air–water. The thermal performance of the PVT is a function of efficient heat transfer. The usage of PCM is adopted by many researchers to keep the cell temperature below a threshold limit. The PCM helps in controlling the cell temperature by extracting and storing the thermal energy at high temperatures and release the stored heat at low temperatures. But the thermal conductivity of PCM is usually poor (0.1–1 Wm−1 K−1). Hence, various problems exist with PCM in the form of heat extraction [7].

Therefore, for the researchers, the use of fluids with high thermal conductivity is a relevant domain to pursue their research. The heat conductivity of coolants may be enhanced by dispersing metal-oxide particles in the conventional fluid [8]. Al-Waeli et al. [9] analyzed and presented the impact of nanofluids and surfactants on the PVT system. This study employs water as basefluid, further, nanosilicon carbide as an additive is deployed to enhance the thermal conductivity of the resulting nanofluid. The thermophysical properties have been examined with several surfactants. The outcomes of the study reveals the time of the suspended nanoparticles in the emulsion is a function of surfactants. The thermal characteristics of the basefluid can be improved by incorporating nanoparticles. Consequently, the use of nanofluid as a coolant is a promising and frequently used technique to improve system output. However, the price of nanoparticles, less stability time, and high pressure are certain factors that hinder the adoption of this technology. Numerous research has been conducted to examine the impact of different nanoparticles and their concentration on the thermo-electrical outcomes of PVT.

Wahab et al. [10] discussed the effect of various nanofluids on the outcome of various solar systems like solar collectors, PVT systems, and solar ponds. Al-Waeli et al. [11] worked upon techno-economic evaluation for PVT collector connected with a nano-PCM/nanofluid tank. Farhana et al. [12] discussed the impacts of nanofluid on the thermal properties of different solar collectors.

Ghadari et al. [13] examined the effect of Fe3O4 nanoparticles dispersed in water (3 wt%) on the hybrid PVT system outcomes. They observed enhancements of 46.29% and 4.93%, in thermo-electrical outcomes of the PVT system. Rejeb et al. [14] examined the sheet-tube type PVT collector performance using Al2O3 and Cu nanoparticles.

Bianco et al. [15] produced a numerical analysis to test the PVT collector outcome with Al2O3 as a nanofluid in a channel. A numerical model was developed using COMSOL software. The authors presented an interesting analysis that nanofluids are having the potential of improving the performance of PVT. Therefore, careful estimation of the PV output and input power is necessary. Al-Waeli et al. [16] worked upon a PVT collector with SiC nanoparticles mixed with water to evaluate the performance under indoor and outdoor conditions.

Fudholi et al. [17] examined the PVT collector outcomes with TiO2/water nanofluid under different intensities of solar radiations. An improvement of around 20% is found in the overall system performance by using 1 wt% of TiO2 in water as a coolant concerning the traditional PVT water collector. Hissouf et al. [18] carried out numerical investigation on PVT water collectors in the climate conditions of Agadir, Morocco. The PVT collector outcome has been tested by mixing Al2O3 and Cu nanoparticles with water for cooling. By using a 2% volume fraction of Cu nanoparticles with water, the thermo-electrical efficiencies increased by 4.1% and 1.9% concerning the pure water as a coolant.

To augment the heat exchange and thermal conductivity, Al-Shamani et al. [19] examined the impact of parameters like module temperature and PV temperature on the outcome of PVT collectors with several nanofluids. It has been concluded that the thermo-electrical and overall performance of the PVT collector is raised by a value of 68.21%, 13.52%, and 81.73%, respectively, with SiC nanofluid. Moradgholi et al. [20] examined the performance of two-phase closed thermosyphons (TPCT) coupled with the PV module. Methanol and Al2O3/methanol are deployed as nanofluids to assess the system’s thermo-electrical output. The study analyzes the impact of filling ratio and nanofluid concentration on the system output. Khanjari et al. [21] presented the numerical simulation to test the outcome of PVT with Ag-water, Al2O3-water, and pure water as a working fluid. The impacts of concentration of nanofluid and inlet fluid velocity on the system outcome were examined. Nasrin et al. [22] experimented with MWCNT-water nanofluid. It is observed by using a 1% weight fraction of MWCNT the thermo-electrical efficiencies have been enhanced by 3.67% and 0.14%, respectively.

Hader et al. [23] developed a hybrid module in which a heat exchanger with fins of different lengths is connected with the PV module. Al2O3 with water is deployed as a working fluid. The numerical analysis is executed with COMSOL software to examine the impact of thermal properties on system outcomes. The outcome reveals the overall output of the proposed system grows with the rise in Reynolds number, fin length, and volume fraction of nanosolid particles in the basefluid.

Lari and Sahin [24] designed a PVT water collector to cater to the energy demand in residential buildings in Saudi Arabia. The outcome demonstrates the PVT thermal output with nanofluid is 13% greater than traditional PVT systems. Alous at el. [25] analyzed the exergetic outcome of PVT systems with MWCNT nanoparticles and graphene nanoplatelets to base fluid. The total exergetic efficiency improved to 12.1% and 20.6% using MWCNT-water nanofluids and graphene nanoplatelets water nanofluid, respectively.

Hosseinzadeh et al. [26] presented an experimental analysis on three distinct systems, i.e., traditional PV system, PVT with nanofluids, and nanofluid-based PVT-PCM system. The observed outcome of the nanofluid-based PVT-PCM was higher than other systems. Kandilli [27] emphasized the use of natural zeolites as thermal storage materials in the PVT collector. The PVT system performance with natural zeolite was compared with the PVT with paraffin wax and stearic acid as a PCM. According to the experimental investigation, energy efficiency was found to be 40%, 33%, and 37% for PVT with natural zeolites, paraffin wax, and stearic acid, respectively. Manigandan & Kumar [28] experimentally examined the PVT collector performance with different nanofluids (ZnO & CuO) and PCM. The thermal performance of five different modules, such as PT, PVT-ZnO, PVT-CuO, PVT-PCM-ZnO, and PVT-PCM-CuO, were compared. The thermal efficiency of the PVT-PCM system using CuO nanofluid is found to be better than the other modules under consideration. Rajaee et al. [29] prepared Co3O4/water nanofluid employing several steps like (1) gradual addition of the nanoparticles to the base fluid at the same time a magnetic stirrer is deployed to disperse the nanaoparticles. (2) Further, the stability of the nanoparticles has been enhanced using the arabic gum as a surfactants for alleviating the agglomeration in the nanoparticles. (3) At last the mixture is finally placed in a ultrasonic device that enables the nanoparticles to homogenously disperse in the water-based fluid.

Jia et al. [30] proposed the mathematical modeling of a PVT collector to examine the effects of several operating parameters like type and volume concentration of nanofluid on thermos-electrical performances.

Throughout the reviewed research, it is found that the deployment of different nanofluids as coolants is an area of interest among researchers. Table 1 shows the comparison of various studies on PVT systems using different nanofluids under different environmental conditions.

Table1 Performance comparison with different studies
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The reported thermo-electrical performance of the PVT system depends upon the MFR of coolant and the concentration of nanoparticles added in the base fluid.

2 Research gap and main contributions

It has been observed from existing state-of-the-art analysis that several experiments are performed to test the impact of several nanofluids as coolants on the thermal and electrical performances of the hybrid PVT systems. Still, very few researchers have worked upon the experimental setup of PVT collectors with nanofluids as a cooling medium. It is also observed from the literature that the impact of nanofluid on the thermo-electric efficiency of PVT collectors is still not conclusive.

The present paper investigates the thermo-electrical performance of serpentine tube-based PVT collector with Cu/water and TiO2/water nanofluid in real outdoor conditions for a clear sky day in Ghaziabad City, India. The influence of nanofluid type and concentration on PV temperature (TPV), electrical efficiency (ηel), thermal efficiency (ηTh), and overall efficiency (ηPVT) is evaluated experimentally. The energy balance equations of PVT collectors with nanofluid are derived from analyzing the impact of operating parameters.

The rest of the manuscript is categorized as follows: The configuration of the PVT module with serpentine tube and the description of measurement tools used in the analysis are presented in Sect. 2. The thermodynamic analysis and the nanofluid properties are discussed in Sect. 3. The detailed PVT collector performance with nanofluids is presented in Sect. 4. At last Sect. 5 concludes the entire work.

3 Experimental arrangement

3.1 Study location

The experimental setup was designed and manufactured to test the impact of two distinct nanofluids on the thermo-electrical outcomes of a hybrid PVT system. The serpentine tube-based PVT setup was installed in Ghaziabad city, India (28.6692° N, 77.4538° E). The weather of Ghaziabad city is mostly similar to New Delhi (Capital of India). In the month of July and August, the city receives its maximum rainfall. Because the city has plenty of sunshine hours throughout the year, the solar PV installations have grown steadily in the last few decades. The good sunshine hours support the investigation of PVT technology.

3.2 System description

The experimental setup comprises a 40 W PV module of poly-crystalline silicon type, serpentine tube, heat exchanger, heat storage, measuring equipment, and data acquisition unit. The schematics of the experimental setup are portrayed in Fig. 1.

Fig.1
figure 1

Schematics of the experimental arrangement

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The dimensions of the PV module are 537*457*35 mm with a no-load voltage of 21.8 V and a short circuit current of 2.74 A. The PV module was purchased from Vikram Solar Company. A serpentine tube made of copper used for fluid circulation is welded at the backside of the PV module. The diameter of the copper tube is taken as 5 mm for the study. In addition, a layer of silicone oil is applied over the connection part to annihilate the air gaps and aggravate the heat exchange.

The heat storage system consists of a water tank, connection pipes (for connection between the hot water tank and heat exchanger), and an external insulation layer. The water circulation pump is used to provide power to the whole circulation unit.

3.3 Measuring instruments

The measurement unit mainly consists of instruments for measuring ambient temperature, solar irradiance, temperature across various layers of PVT unit, and fluid flow rate. All the sensing instruments are connected with the data acquisition unit (DAQ). The data obtained from the DAQ are transferred to a laptop for analysis. The class of instruments, operating range, and accuracy of measuring instruments are shown in Table 2.

Table 2 Specifications of measurement devices
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Three different K-types thermocouples were employed to measure the PV cell temperature and inlet–outlet temperature of tubes. The class-2 type pyranometer is deployed to measure the sunlight intensity. Its measurement range is 0–2000 W/m2, and response time is less than 5 s.

4 Thermodynamic analysis

The components of a hybrid PVT collector are expressed by using the energy balance equations. The developed mathematical model allows estimating heat transfer characteristics and temperature variation across each element of the hybrid PVT collector. To establish the energy balance equations across the different PVT collector layers, a sectional cut of sheet-tube absorber design is considered, as shown in Fig. 2.

Fig.2
figure 2

Schematic diagram of multilayer PVT collector

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Following hypotheses are assumed to develop the mathematical equations presenting the heat exchange process in the PVT systems:

  • Since we have ignored any variation in solar intensities, ambient temperatures, and wind speeds over a shorter span of time (less than 10 min), the quasi-static states are supposed to be valid.

  • The thermal resistance between the solid material interfaces is neglected.

  • A constant temperature is supposed to be maintained across various PVT collector layers.

  • The effect of sunlight and radiant heat transfer are neglected.

  • The frictional resistance between the tubes used for fluid circulation and nanofluid is neglected.

  • Each PVT component is supposed to have a constant thermal conductivity.

  • The optical module surface losses are not considered. The transmittance–absorbance product coefficient (τα) is considered one.

Following the above assumptions, the energy balance equation across each layer of the hybrid PVT is formulated. The portion of radiations incident on the glass cover is returned to the environment and the remaining part of solar energy is consumed by the photovoltaic module through heat transfer. The significant portion of solar radiation received by the PV panel is sent to the absorber tube assembly while the remaining energy is transformed into electrical energy as illustrated in the thermal resistance model shown in Fig. 3.

Fig. 3
figure 3

Thermal circuit model of PVT module

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4.1 Energy balance equation

The energy balance equation across each component is shown in Table 3.

Table 3 Energy balance equations across each layer of PVT module
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As depicted in the thermal circuit model, heat transfer through the components of the hybrid PVT system. Hence, heat transfer coefficients (HTC) play a crucial role in the energy balance equation across each component. Table 4 shows the expressions of HTCs across each layer.

Table 4 Expressions of heat transfer coefficients (HTCs) [8]
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4.2 Equations for thermophysical properties

The thermophysical characteristics of nanofluids are the prominent factors to consider when evaluating their heat transfer capabilities. Because of the huge inaccuracies associated with measuring various thermophysical parameters, thermal conductivity is often regarded as the most difficult attribute to measure. In the present study, nanofluid is prepared by adding nanoparticles into water (basefluid). The thermal properties of basefluid considered for the mathematical models are temperature dependent. Table 5 shows the expression of thermophysical properties of nanofluids as well as basefluid which are used to solve the energy balance equations.

Table 5 Expression for thermophysical properties of nanofluids and basefluid
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Nanofluid helps in enhancing the thermo-electrical outcome of the hybrid PVT system. When compared to conventional cooling approaches, nanofluid could offer a considerable enhancement in heat transfer. Nanofluids as HTF can directly affect the PVT system performance by decreasing the PV module temperature and hence enhance the PV efficiency. In this study, Cu/water and TiO2/water are considered nanofluids. Nanofluids are prepared by adding 0.5% and 1% volume fractions of Cu and TiO2 nanoparticles to basefluid. Table 6 indicates the thermos-physical properties of Cu and TiO2 nanoparticles and base fluid.

Table 6 Thermophysical parametric values of nanofluid and base fluid
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4.3 Performance parameters

Following the first law of thermodynamics, the serpentine tube hybrid PVT collector performance is measured as thermal, electrical, and thermo-electrical efficiencies. The thermal efficiency depends upon the difference in the inlet–outlet temperature of the tube [33].

$$ \eta_{Th} = \frac{{Q_{th} }}{{G.A_{c} }} $$
(26)
$$ \eta_{Th} = \frac{{mc_{p} \left( {T_{nf.o} - T_{nf.i} } \right)}}{{G.A_{c} }} $$
(27)

The PVT electrical efficiency depends on the solar cell temperature and is calculated as [18]

$$ \eta_{el} = P\eta_{0} \left[ {1 - \beta_{0} \left( {T_{PV} - T_{{{\text{air}}}} } \right)} \right] $$
(28)

where P is the packing factor, η0 is the efficiency at STC, and β0 is the temperature coefficient.

The PVT electric power output (Eel) is calculated as:

$$ E_{el} = \tau_{g} G\eta_{el} $$
(29)

The overall efficiency (ɳPVT) is given as:

$$ \eta_{{_{PVT} }} = \eta_{Th} + \eta_{el} $$
(30)

Exergy is a quantitative state that is determined by the surrounding conditions. It is the energy which is available by subtracting unavailable energy from total energy. The instantaneous electrical exergy is calculated as per the relation given by Sopian et al. [7].

$$ Ex_{el} = \eta_{el} G $$
(31)

According to the second law of thermodynamics, the exergy analysis accounts for total exergy inflow, outflow and exergy of destruction [26].

$$ \sum {{\dot{\text{E}}\text{x}}_{{{\text{in}}}} } { - }\sum {{\dot{\text{E}}\text{x}}_{{{\text{out}}}} } { = }\sum {{\dot{\text{E}}\text{x}}_{{{\text{dest}}}} } $$
(32)
$$ \sum {\dot{E}x_{{{\text{in}}}} } - \sum {\left( {\dot{E}x_{Th} + \dot{E}x_{el} } \right)} = \sum {\dot{E}x_{{{\text{dest}}}} } $$
(33)

The net input exergy is calculated as:

$$ \sum {\dot{E}x_{{{\text{in}}}} } = G.A_{c} .\left[ {1 - \frac{4}{3}.\left( {\frac{{T_{{{\text{air}}}} }}{{T_{{{\text{sun}}}} }}} \right) + \frac{1}{3}\left( {\frac{{T_{{{\text{air}}}} }}{{T_{{{\text{sun}}}} }}} \right)^{4} } \right] $$
(34)
$$ \sum {\dot{E}x_{{{\text{Thermal}}}} } = Q_{th} \left[ {1 - \frac{{T_{{{\text{air}}}} + 273}}{{T_{nf.o} + 273}}} \right] $$
(35)
$$ \sum {\mathop E\limits^{.} x_{ele} } = I_{m} .V_{m} $$
(36)

The total exergy is the sum of thermal and electrical exergies shown in Eq. (37).

$$ \dot{E}x_{{{\text{Total}}}} = \dot{E}x_{Th} + \dot{E}x_{el} $$
(37)

The exergy efficiency (ɳEx) of the electrical and thermal components shows the net ɳEx of the system as shown in Eq. (38).

$$ \eta_{Ex} = \frac{{\sum {\dot{E}x_{{{\text{out}}}} } }}{{\sum {\dot{E}x_{{{\text{in}}}} } }} $$
(38)

4.4 Uncertainty analysis

It is necessary to conduct an uncertainty analysis to avoid measurement errors and to ensure accuracy in results. The error in measurement affects the thermo-electrical performance of the system. Holman et al. [34] suggested the mathematical equation for predicting the uncertainties in the thermodynamic performance.

$$ e_{R} = \sqrt {\left( {\frac{\partial R}{{\partial V_{1} }}e1} \right)^{2} + \left( {\frac{\partial R}{{\partial V_{2} }}e2} \right)^{2} + \cdots + \left( {\frac{\partial R}{{\partial V_{n} }}en} \right)^{3} } $$
(39)

where eR denotes the uncertainty in the function R, (V1,V2……Vn) and (e1, e2…..en) are the uncertainties in the independent variables. By applying the maximum range of uncertainties provided in Table 2 over the range of measurement, the maximum uncertainties in calculated electrical, thermal, and overall efficiency were found to be 3.52%, 1.23%, and 3.34%, respectively.

5 Results and discussion

Based upon the mathematical modeling discussed in Sect. 3, the PVT thermo-electrical outcome is investigated in real-time outdoor conditions. A serpentine tube-based PVT collector is designed to examine the effects of adding copper (Cu) and titanium oxide (TiO2) in water as the heat transfer fluid (HTF). The PVT collector outcome is examined for the climatic conditions of Ghaziabad (28.66856° N, 77.4538° E), India. The solar radiation and atmospheric temperature are recorded for every hour starting from 08:00 to 17.00 h. on the clear sky day in the month of August as indicated in Fig. 4.

Fig. 4
figure 4

Incident solar radiation and ambient temperature data

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Figure 5 indicates the average temperature (TPV) of the PVT was reduced by using Cu/water and TiO2/water nanofluid as compared to basefluid. These observations were recorded when the nanoparticle concentration (ϕ) in the basefluid is 1%, the mass flow rate (MFR) of nanofluid in the serpentine tube is 0.03 kg/s.

Fig. 5
figure 5

PV module temperature for all cooling methods with the time of a day

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It is clear that TPV follows a similar pattern as that of the incident solar radiation pattern. The TPV is maximum at optimal solar radiation. By using Cu/water nanofluid, higher temperature reduction is observed as compared to TiO2/water nanofluid and basefluid. The peak TPV obtained were 47.8 °C, 46.1 °C, and 45.2 °C using basefluid, TiO2/water nanofluid, and Cu/water nanofluid respectively. As mentioned in Table 3, the thermal conductivity of Cu is greater than the other fluids considered in this investigation. Therefore, a higher reduction in TPV was observed using Cu/water nanofluid concerning the TiO2/water nanofluid and basefluid.

Figure 6 indicates the variation in electrical efficiency (ɳel) of PVT system with time for different cooling approaches. The ɳel varies inversely with TPV as mentioned in Eq. (28). As observed from Fig. 6, the ɳel is better when Cu/water nanofluid is used as a coolant as compared to the other two methods considered. This is because a steeper reduction in TPV was observed for Cu/water nanofluid.

Fig. 6
figure 6

Electrical efficiency for different cooling methods

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Figure 7 reveals the thermal efficiency (ɳTh) of the serpentine tube-based hybrid PVT collector for different cooling approaches at a constant MFR of 0.03 kg/s. As mentioned in Eq. (27), the ɳTh of the PVT collector is proportional to the inlet–outlet temperature difference of the tube. As observed from Fig. 7, the ɳTh is maximum when the intensity of sunlight is at its peak. The tube inlet–outlet temperature difference is higher at higher sunlight intensity. The average value of ɳTh recorded for Cu/water nanofluid, TiO2/water nanofluid, and basefluid were 43.62%, 42.97%, and 42.10%, respectively. Therefore, the thermal performance is better for Cu/water nanofluid in comparison to TiO2/water nanofluid and basefluid.

Fig. 7
figure 7

Thermal efficiency for different cooling methods

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Figure 8 portrays the ɳEx of serpentine tube-based hybrid PVT collector for different cooling approaches at a constant MFR of 0.03 kg/s. As observed from Fig. 8, the ɳEx is highest for Cu/water nanofluid.

Fig. 8
figure 8

Exergy efficiency for different cooling methods

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As discussed in Eq. (30), the overall efficiency (ηPVT) of the hybrid PVT collector is the combination of ɳTh and ɳel. Figure 9 shows the variation in ηPVT for various cooling techniques. The average ηPVT recorded for Cu/water nanofluid, TiO2/water nanofluid, and basefluid were 58.12%, 57.20%, and 56.64%, respectively. It is clear that the ηPVT of hybrid PVT collector is high when Cu/water nanofluid is deployed as HTF in comparison with other two fluids used for investigation.

Fig. 9
figure 9

Overall efficiency for different cooling methods

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From the above analysis, it is evident that the performance of PVT collector with Cu/water nanofluid is considerably better concerning TiO2/water nanofluid and basefluid. The thermophysical properties of basefluid change by adding nanoparticles to it which leads to heat transfer enhancement. More is the thermal conductivity of Cu more is the heat transfer when used as a fluid. Hence, Cu/water nanofluid is chosen as HTF to further investigate the impact of MFR and volume concentration of nanoparticles on the effectiveness of the PVT collector.

5.1 Effect of volume fraction (vol%) of nanoparticles on the PVT system performance

The copper nano-sized particles were mixed with base fluid in different volume concentrations, i.e., 0%, 0.5%, and 1% to prepare the Cu/water nanofluid [29]. The effect of the volume fraction of nanoparticles on the thermo-electrical performance of the PVT collector was investigated while keeping the other parameters at their reference values. Figure 10 portrays the variation in TPV and solar radiation intensity with day hours for different concentrations of Cu/water nanofluid. This is evident that TPV decreases with a rising volume concentration of nanoparticles in the base fluid.

Fig. 10
figure 10

Cell temperature with different volume fractions of nanoparticle

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The peak value of TPV is 48.01 °C, 47.08 °C, and 46.12 °C using basefluid, 0.5%, and 1% nanoparticle concentration. As seen from Fig. 9, a 4.1% reduction in TPV was observed using Cu/water nanofluid (1 vol %) concerning the basefluid. Figure 11 portrays the ɳel and TPV trend over day time for the distinct volume concentrations of Cu/water nanofluid.

Fig. 11
figure 11

Electrical efficiency with different volume fractions of nanoparticle

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As observed from Fig. 11, the ɳel reduces as the TPV increases as per Eq. (28). Using (1 vol %) of Cu/water nanofluid, the average ɳel is 14.745% while using basefluid, the average ɳel is 13.864%. Hence, the average ɳel improves by 5.98% using Cu/water nanofluid (1 vol %) in comparison with the basefluid.

Figure 12 illustrates the behavior of ɳTh with experimental time at distinct volume concentrations of Cu/water nanofluid. The ɳTh increases until 13:00 and then decreases for the rest of the experimental hours.

Fig. 12
figure 12

Thermal efficiency with different volume fractions of nanoparticle

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As mentioned in Eq. (27), the ɳTh of PVT is a function of the specific heat capacity (Cp), the inlet–outlet temperature difference of tube, and the intensity of sunlight. But the Cp of the fluid falls with the rise in the nanoparticles concentration in the base fluid. At the same time, the outlet temperature (T0) rises with an increase in nanoparticles concentration; hence, the inlet–outlet temperature difference (ΔT) of the circulating tube increases. The effect of temperature difference is more dominating on the ɳTh as compared to Cp. Therefore, the ɳTh rises with a rise in nanoparticle volume concentration in basefluid.

The impact of increasing nanoparticles concentration in basefluid on the ɳEx of serpentine tube-based PVT collector is shown in Fig. 13. The ɳEx increases with increase in intensity of radiation. Further, the ɳEx is highest for Cu/water nanofluid (1 vol %) as the ɳEx increases with the increase in nanoparticle concentration in basefluid.

Fig. 13
figure 13

Exergy efficiency with distinct volume fraction of nanoparticles

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Figure 14 illustrates the variation of ηPVT with experimental hours for different volume fractions of Cu/water nanofluid. According to Eq. (30), \(\eta_{{_{PVT} }} = \eta_{Th} + \eta_{el}\). Since both ɳel and ɳTh increase with a rise in (vol %) of nanoparticles in the base fluid, the ηPVT is higher for (1 vol %) of Cu/water nanofluid concerning other cooling approaches.

Fig. 14
figure 14

PVT efficiency with distinct volume fraction of nanoparticles

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The outcomes show the average ηPVT of the PVT collector increases by 3.23% using Cu/water nanofluid (1 vol %) as compared to basefluid. Cu/water nanofluid caused an increase in HTC, and hence the overall performance of the PVT system improves.

Figure 15 outlines the variation of PVT electric power with experimental hours for various cooling approaches. The electrical power follows a similar pattern followed by solar intensity. Thus, the electrical power increases to a peak of 13:00 h and then falls gradually. This is evident that the electric power increases with a rise in the volume fraction of nanoparticles in the basefluid. The highest electrical power is obtained for Cu/water nanofluid (1 vol%), while the lowest electrical power is obtained for basefluid.

Fig. 15
figure 15

Electrical power variation with different volume fractions of nanoparticles

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5.2 PVT Thermo-electrical performance with different mass flow rate (MFR)

This section investigates the impact of changing MFR on the TPV, PV power, and thermo-electrical performance of PVT collector with Cu/water nanofluid. Figure 16 illustrates the variation in TPV with an increase in MFR. The MFR is varied from 0.005 to 0.03 kg/s. As illustrated in Fig. 16, the TPV falls with a rise in MFR of the fluid through the tube. This effect can be analyzed as, at the higher MFR, more heat can be extracted from the collector, which helps in the cooling of the PVT collector. The highest reduction in TPV was observed when Cu/water (1 vol %) is used as the fluid. The TPV decreases from 51.2 to 42.4 °C using Cu/water nanofluid (1 vol %) when the MFR increases from 0.005 kg/s to 0.03 kg/s. Hence, a 17.18 °C reduction in PV temperature was observed by increasing the MFR from 0.05 to 0.03 kg/s. As discussed above, the TPV directly affects the electric outcome of the PVT system; therefore, rising the MFR also has an additive effect on the electric outcome of the PVT system.

Fig. 16
figure 16

Variation in cell temperature with changing the MFR

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Figure 17 outlines the variation in PV power with MFR for different cooling approaches. The PV power of the module increases with increasing MFR. Further, the PV power is higher when Cu/water nanofluid (1 vol%) is used as fluid, while the PV power is lowest for water cooling (basefluid).

Fig. 17
figure 17

Variation in PV power with changing the mass flow rate

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Figure 17 outlines the variation in PV power with MFR for different cooling approaches. The PV power of the module increases with rising MFR. Further, the PV power is higher when Cu/water nanofluid (1 vol%) is used as fluid, while the PV power is lowest for water cooling (basefluid). This is evident that the optimal PV power achieved from the PVT system was 36.4 W and 35.5 W with MFR of 0.03 kg/s using Cu/water nanofluid (1 vol%) and basefluid, respectively. At an MFR of 0.005 kg/s, the highest PV power obtained were 34.3 W and 33.5 W s using Cu/water nanofluid (1 vol%) and basefluid respectively.

Figure 18 presents the variation of ɳel with MFR using Cu/water nanofluid in different proportions and basefluid. As discussed, the PV temperature reduces with an increase in MFR of fluid which causes a rise in the ɳel of the PVT module. Further, the ɳel is optimal for Cu/water nanofluid (1 vol %) as concerning the other cooling approaches. The highest and lowest values of ɳel were 14.55% and 13.76% obtained at the MFR of 0.03 kg/s and 0.005 kg/s, respectively. Hence, the ɳel improves by 5.43% as the MFR increases from 0.005 to 0.03 kg/s.

Fig. 18
figure 18

Deviation in electrical efficiency with changing MFR

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Similar observations were recorded for ɳTh variation with MFR as depicted in Fig. 19. Higher MFR leads to enhancement in ɳTh of the PVT collector. The ɳTh increases from 42.67 to 43.8% when the MFR increases from 0.005 to 0.03 kg/s.

Fig. 19
figure 19

Variation in thermal efficiency with changing MFR

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Hence, the ɳTh improved by 2.58% at the MFR of 0.03 kg/s. Initially, there is a fewer temperature gradient. When the MFR increases, a higher temperature gradient is maintained that leads to a rise in the convective heat transfer rate amid the working fluid and copper tube. The ηPVT improves by 8.01% when the MFR increases from 0.005 to 0.03 kg/s.

As observed from Figs. 17–19, the PV power, ɳel and ɳTh increases with the increase in MFR. Further, the ɳel and ɳTh are highest at 0.03 kg/s MFR. Similar patterns were observed for ɳel and ɳTh by Jia et al. [30] with MFR. Hence, the numerical simulation results obtained by Jia et al. [30] are closely aligned and synchronized with the experimental results obtained in this study. This validates the experimental outcomes, and hence, the experimental model is accepted.

6 Conclusion

The influence of the type of nanoparticles, nanoparticles concentration, and MFR on the thermo-electrical outcome of serpentine tube-based PVT collector has been investigated experimentally. Parameters influencing the PVT collector’s outcome were registered, and the thermo-electrical outcomes are calculated. The experiments are carried out on a selected day in the month of August 2020 in Ghaziabad, India. The key outcomes of this investigation are as follows:

  • The PVT collector exhibit a better outcome with Cu/water nanofluid as a coolant as compared to TiO2/water nanofluid.

  • The thermo-electrical performance of PVT enhances with the rising volume fraction of nanoparticles in basefluid.

  • Using Cu/water nanofluid (1 vol%), the 4.1 °C reduction in TPV leads to a 5.98% enhancement in ɳel in comparison to basefluid.

  • The increase in MFR from 0.005 to 003 kg/s results in 17.18 °C reduction in TPV using Cu/water nanofluid (1 vol%).

  • The ɳel, ɳTh, and ηPVT improve by 5.43%, 2.58%, and 8.01%, respectively, when the MFR rises 0.005–0.03 kg/s with Cu/water nanofluid (1 vol%).