An Experimental Approach to Energy and Exergy Analyses of a Hybrid PV/T System with Simultaneous Water and Air Cooling

Abstract

In this paper, the effects of simultaneous air and water cooling on the energy performances of a single-pass hybrid PV/T system are experimentally investigated. Both air and water are used as coolants and are tested at four different mass flow rates, ranging from 0.0014 to 0.0026 kg/s. It is found that the overall efficiency of the PV/T system is dependent on the solar intensity and mass flow rate of coolants. Maximum PV/T system efficiency is found to be 62.2% for a simultaneous flow of water and air at a 0.0026 kg/s flow rate, which is 35.5% higher than the efficiency found at 0.0014 kg/s coolant flow rate. Maximum thermal energy outputs are 85.8 and 211.84 W/m² while using air- and water-based coolants, respectively. Exergy analysis of the developed system indicates that exergy efficiency increased by up to two times by introducing simultaneous air and water cooling in this PV/T system.

1. Introduction

Solar radiation is usually converted into either thermal energy by using solar thermal collectors (i.e., flat plate, evacuated tubes, solar still and parabolic trough) [1,2] or into electricity by using solar photovoltaic (PV) cells. Most commercial PV cells can convert only 10–20% of the total incident radiation (at 25 °C) into electricity. In contrast, the rest of the incident radiation remains in the form of heat, which in turn causes an increase in PV panel temperature [2,3,4,5]. Furthermore, the cell temperature of the PV panel when it operates at rated power should be kept below 47 °C [6] to maximise the cells’ performance. This is because each degree Celsius increment of temperature over this temperature level exerts a negative impact on PV panel electrical efficiency in the order of 0.45–0.65% [7,8]. Moreover, PV cell materials are sensitive to this increase in ambient temperature, which causes a slight expansion of cell and a reduction in open-circuit voltage and fills factor of PV cells. Solar irradiation and ambient air temperature significantly affect the I-V curve of PV cells by controlling short-circuit current and open-circuit voltage. An increase in module temperature acts linearly with short-circuit current but inversely interrupts open-circuit voltage. Therefore, a significant portion of the incident photon energy, which is likely to be higher than the semiconductor materials bandgap, remains as waste heat without being converted into electrical energy [9,10]. This excess heating drastically reduced the maximum power output of PV panels [11,12].

In addition, a significant portion of incident photon energy is likely to be higher than semiconductor materials bandgap and remains as waste heat without being converted into electrical energy [9,10]. The laws of thermodynamics act as performance indicators to make a quantitative and qualitative approach towards the optimisation of heat transfer and energy conversion parameters of PV-based energy conversion systems. In order to improve the overall performance of PV modules, various cooling techniques such as air, water, refrigerant and heat pipe were introduced to recover the heat generated in the PV modules for thermal applications (i.e., domestic hot water and space heating) while keeping the temperature of the PV panel at the desired level (i.e., below 47 °C) [1,12,13,14]. Apart from the adoption of new energy-efficient materials for PV modules (which also incurred higher costs), the conversion of this incident sunlight can be utilised by adding a flat plate solar thermal collector. Photovoltaic thermal (PV/T) solar collector is one of the modified systems that concurrently generates thermal energy (low-grade energy for hot air and/or water) and electrical energy (high-grade energy to direct current (DC) [2,8,15,16,17].

Several studies have been conducted on PV/T systems focusing on component design modification [18,19,20], integration of individual features [7,21], system optimisation [22,23] and performance evaluation [24,25]. In addition, a glazed absorber plate with good contact between the absorber plate, cooling medium and encapsulated PV panel is a key factor in extracting high thermal efficiency from a hybrid PV/T system [8,12,26,27]. Furthermore, weather conditions, collector design (i.e., arrangement of liquid flow tubes, glazing plate, absorber plate, diffuser, insulator) and modes of operation (flow rate of cooling fluids) are considered as the parameters that determine the efficient conversion of heat and electricity from the solar PV/T system [2,13].

Prakash et al. [25] developed a mathematical model to theoretically investigated the performance of hybrid PV/T systems. Both air and water cooling were taken into consideration in the analysis, and energy balance equations were used to compare the energy efficiency of the PV/T model. They found that the electrical energy efficiency of PV increases when integrated with the thermal collectors. The results also showed that the thermal efficiency of the PV/T system using water cooling increases by approximately 50% to 67%, whereas the efficiency of air-cooling increases by approximately 17% to 51%. An improvement in electrical efficiency to 16.3% was observed by Nižetić et al. [28] when they sprayed water on both sides of the panel to reduce the PV cell temperature from 54 to 24 °C. The efficiency reduces a bit to 14% by cooling the back surface only. Aste et al. [29] developed a mathematical model to investigate the electrical and thermal performances of a glazed PV/T water collector at which a flat plate absorber with roller bonds is integrated with a thin-layer PV cell. The results of this research showed that the overall efficiency of the PV/T system increased from 13.2% (for PV panel only) to over 42%. Instead of using an absorber plate, Saygin et al. [23] incorporated the PV panel with the collector itself. The optimal performance of such a PV/T system was investigated by considering the gap between the panel and cover, as well as mass flow rate of air passing the panel. It was found by Saygin et al. [23] that the highest performance of the PV/T system was achieved when the distance between the panel and cover is in the range between 3 and 5 mm. The increase in air mass flow rate also has the additional benefit of improving the thermal performance of the system.

Abdollahi et al. [30] tested the effectiveness of a cooling system in a PV/T collector by varying the flow rates of water and air. They also reported that the evaporation rate at the air–water interface increased at constant airflow and dropped while the airflow rate was increased. Fakouriyan et al. [31] demonstrated a cooling system to improve the electrical efficiency of a PV panel while simultaneously producing hot water. The electrical and thermal efficiencies were found to be 12% and 49.4%, respectively, thus increasing the overall efficiency up to 61.7%. In another study, Amanlou et al. [19] developed a diffuser for the purpose of controlling excessive local heating of a PV panel that helps to improve the efficiency of an air-cooled low concentrated PV/T system. The thermal efficiency of such a PV/T system was reported to be 42.2% if the rates of airflow are between 0.008 and 0.016 kg/s. Slimani et al. [32] conducted a numerical and experimental investigation of a double pass PV-air collector which was embedded in an indirect solar dryer system. The overall efficiency of the hybrid PV/T air collector was improved by using an additional glass cover and a metal absorber plate separating the passing airflow. The total energetic performance of the hybrid collector (electrical and thermal efficiencies were 10.5% and 70%, respectively) was found to positively improve with an increase in airflow rate (up to 0.0155 kg/s). It is also opined that a mass flow rate of coolant at below 0.10 kg/s can contribute to PV efficiency improvement [33].

PV cells electricity generation and their cooling process involve irreversible energy conversion processes that prevent the system from achieving its full potential for delivering heat and power. Energy/exergy analysis is a helpful tool that is increasingly used to understand the irreversibilities of various energy systems and to explore opportunities for improving their performance [17,34,35]. This approach has also attracted attention to be applied with the aim of improving the performance of PV/T systems [12,36,37]. For example, Evola and Marletta [34] theoretically assessed the energy and exergy performance of a glazed PV/T collector by considering the impacts of operating parameters such as inlet temperature and mass flow rate of the cooling medium (i.e., water). They found that energy efficiency of the system increases by reducing the inlet temperature; however, it leads to a decrease in the exergy efficiency. They recommended that the inlet temperature of cooling water must be kept between 30 °C and 40 °C to optimise the energy and exergy efficiencies of the collector. In another study, Kallio and Siroux [35] developed a model in MATLAB to optimise the design of a PT/V collector by analysing its thermal and electrical exergy efficiencies. The results indicate that thermal energy efficiency of the PV/T collector can reach up to 80%, and the maximum thermal exergy is around 2% depending on the solar irradiation. Recently, Kim et al. [36] experimentally studied the energy and exergy performance of an air-type PV/T collector. The results showed that average thermal energy and exergy efficiencies of the air-type PV/T collector they studied were 35–50% and 8.5–14%, respectively, depending on the flow rate of air passing the collector.

Previous studies on PV/T systems have mostly focused on thermal and electrical efficiency improvement using different models and experimentations [8,11,12,16,18,37,38,39], and most of such studies dealt with a single coolant (air or water) for cooling. Thermal management of hybrid PV/T systems is a well-known technical challenge considering the high heat flux of solar radiation, non-uniform heating, and the nature of flow through cooling channels [40]. A thorough literature review conducted to support this study indicates a clear gap in experimental investigation of the energy and exergy performances to understand the simultaneous application of both air and water as coolants in PV/T systems. Therefore, this study aims to investigate the impacts of simultaneous air and water cooling on the energy performances of a single-pass hybrid PV/T system. It is particularly important to study the performance of a PV/T collector when both air and water are used for its cooling by considering thermal and electrical efficiencies as key performance indicators.

We also spotted the scarcity of experimental data in the literature with respect to the topic covered by this study. Hence, in this study, an experimental approach is used to analyse the energy and exergy of the thermal management side of a hybrid PV/T system. Water cooling flows at the backside of the PV panel, whereas a blast of airflow is used to create an extra cooling effect on the PV/T system. The cooling channel is positioned underneath the PV panel to attain maximum heat transfer between the absorber plate and the cooling medium (water and air). The paper is structured in the following manner, where materials and methods used for this experimental investigation are discussed in Section 2. Experimental results for the energy and exergy analyses are presented in Section 3, and the concluding remarks are discussed in Section 4.

2. Materials and Methods

2.1. Overview of Experimental Setup

This research work aims to explore the effects of cooling on the thermal and electrical efficiencies of a ground-mounted hybrid PV/T system (tube and sheet type). The schematic diagram of the designed hybrid PV/T system is given in Figure 1a. In the designed system, air and water were used as cooling fluids. The schematic of the designed PV/Ts’ front view is provided in Figure 1b, where the PV/T system was equipped with a glazed absorber plate (at the rare face of PV panel), copper tube (for water cooling) and air blower. In this setup, the collector was positioned at the bottom of the PV module to absorb heat generated from the PV panel. There was a gap between the PV panel and absorber plate, and ten copper tubes were placed in this gap (attached to absorber plate) to remove the heat through circulating water. Thus, the total heat captured from the developed PV/T system includes the heat trapped by the glass cover and the heat absorbed by the flat plate (absorber plate). Furthermore, using both air and water for cooling, both the top and bottom surfaces of the PV modules also improve the efficient conversion of solar irradiation into electrical energy. Figure 1c illustrates a cross-sectional view of this PV/T system arrangement that displays the relative position of glass cover, PV panel, absorber plate and copper tubes.

A sheet and tube-type water cooling system are applied to absorb excess irradiation for thermal energy conversion. The PV/T system was tilted at an angle of 31° to match the locations’ latitude. Copper tubes were placed above the glazed absorber plate (arranged in serpentine style), and airflow was supplied by using an air blower. The collector used in this experiment consists of a rectangular wooden box equipped with four single monocrystalline PV panels to convert solar energy to electrical energy. The detailed specification of the experimental setup is given in

Table 1. Specification of the experimental setup.

Items	Description
PV/Ts’ tilt angle	31°
PV cell type	Mono crystalline
PV module dimension	Four panel: 306 mm × 236 mm× 25 mm
Maximum power, Pmax	20 W
Number of cells per panel	36
Open circuit voltage, Voc	26.15 V
Short circuit current, Isc	1.32 A
Voltage at maximum power, Vm	18.70 V
Current at maximum power, Im	1.06 A
Collector effective area, Ac	578,200 mm2
Collector type	Sheet and tube Wooden box dimension: 1200 mm × 800 mm × 100 mm
Absorber plate material	Mild steel Thickness: 3.5 mm
Front glass type	Tempered glass (thickness 4 mm) Transmissivity = 0.84
Gap between absorber plate and PV panel	75 mm
Water tube material	Copper Thermal conductivity: 386 W.m−1.K−1
Number copper tube rows	10
Copper tube diameter	12 mm
Insulator thickness	50 mm
Insulating material	Cork sheet Thermal conductivity: 0.043 W.m−1.K−1
Heat exchanger	Spiral type
Tube material in heat exchanger	Copper
Tube diameter of heat exchanger	6 mm

.

A 10-W pump was used to circulate the water stored in a 30-litre tank through the tubes at different flow rates (0.0014 to 0.0026 kg/s). A spiral heat exchanger was also utilised to cool down the hot water at the outlet of the thermal collector before starting a new cooling cycle.

From a heat transfer point of view, this cooling method has the advantage of taking the infrared energy from incident solar radiation without disrupting electricity generation. Heat transfer performance between a surface and fluid is better at higher mass flow rates of coolant; thus, it improves the electrical energy efficiency of the PV panel. The heat lost to the surrounding ambient is minimised by insulating a 50 mm layer of cork sheet placed underneath the absorber plate. Temperature sensors (LM 31) were used to measure the temperature of inlet and outlet water of the collector. Two thermocouples (TENMA 72-14556) were placed at the top and bottom of the absorber plate to read the temperature. A rotameter (40–400 LPM) was connected to measure the coolant flow rate, and a battery (12 V, 7.5 AH/20 h) was used for storage electricity from the PV panel. A detailed specification of the equipment used for these purposes is given in

Table 2. Specifications of the measurement equipment.

Equipment Name	Model	Error Percentage	Measurement Applications
Digital Multimeter	VEYRON VL9205	DC Voltage: ± (0.5% + 1) AC Voltage: ± (0.8% + 3) DC Current: ± (0.5% + 1) AC Current: ± (1.0% + 3) Resistance: ± (0.8% + 1) Capacitance: ± (4.0% + 3)	Voltage (open circuit and load) Current (short circuit and load)
Anemometer	ML-81Am	≤20 m/s: ±3% FS >20 m/s: ±4% FS	Air flow
Temperature and humidity sensor	DHT 11	5% for humidity ±2% for temperature	Air temperature
Temperature sensor	LM 35	± 5%	Water temperature
Photo diode		±2.25 V to ±18 V	Solar intensity
Thermocouple (K-type probe)	TENMA 72-14556	−0 °C to + 800 °C (±2.5%)	Surface temperature measurement
Rotameter	BES Series 7200 (40–400 LPM)	±5%	Water flow

.

2.2. Experimental Analysis

The following assumptions were made while conducting experimental analysis and helped to conduct the study by overcoming the complexities of experimental design.

• The flow was considered steady for both air and water, whereas any compressible effect was neglected.
• Thermal properties of the absorber plate and tube were considered constant with varying operating conditions (temperature and fluid flow), and perfect insulation was assumed at the bottom of the absorber.
• In this study, total solar irradiation was counted, and independent participation of short- and long-wavelength radiation was not considered.

Based on these assumptions, the amount of heat extracted by water and air as coolant can be given as [3,24,41,42]:

Thermal energy extracted by water,

E_w = ṁ_w × C_w × ΔT_w

(1)

Thermal energy extracted by air,

E_a = ṁ_a × C_a × ΔT_a

(2)

where ṁ_w and ṁ_a are mass flow rate (kg.s⁻¹) of water and air, respectively; C_w and Ca are specific heat (J.kg⁻¹.K⁻¹) of water and air at constant pressure, respectively; ΔT_w and ΔT_a are the temperature difference (K) at outlet and inlet, respectively.

The thermal efficiency of the PV/T system is calculated individually by using air and water and expressed as:

Energy efficiency of water collector,

$${\eta }_w = \frac{{ṁ}_w{C}_w \mathsf{\Delta }{T}_w}{I{A}_c}$$

(3)

Energy efficiency of air collector,

$${\eta }_w = \frac{{ṁ}_w{C}_w \mathsf{\Delta }{T}_w}{I{A}_c}$$

(4)

where I = solar irradiation (W.m⁻²), Ac = collector area (m²)

The electrical power output (E_el) of the system is defined as [3,24,41,42],

E_el = I_sc × V_oc × FF

(5)

where V_oc is open circuit voltage (V), I_sc is short circuit current (A), and FF is the photovoltaic panel fill factor, which can be defined as FF =$\frac{P_{max}}{I_{sc}\times V_{OC}}$ = $\frac{V_{max}\times I_{max}}{I_{sc}\times V_{OC}}$; where P_max, V_max and I_max are the maximum power, voltage and current of PV panel.

Therefore, the electrical efficiency (${\eta }_E$) of PV system can be determined by:

$${\eta }_E =\frac{I_{max}{V}_{max}}{I{A}_p}$$

(6)

where A_p = panel area (m²).

The designed PV/T system is operated from 9.00 to 16.00 h in the daytime, and the maximum thermal and electrical energy outputs are recorded as 297.64 and 28.71 W. m⁻², respectively, during mid-day (12.00 to 13.30 h). In this study, the flow rates of both coolants (i.e., air and distilled water) are regulated from 0.0014 to 0.0026 kg.s⁻¹ to investigate their cooling effect and the impacts on the energy performances of the system.

2.3. Energy Analysis Model

The result of heat and energy transformations in PV/T systems can be assessed by the second law of thermodynamics (exergy analysis) [43]. The exergy analysis emphasises the quality of energies, whereas the other methods focus only on the amount of energy that takes place during the energy conversion processes. As PV/T systems offer both electrical and thermal forms of energy, electrical energy has added an attribute (in terms of energy conversion efficiency) of being better than thermal energy; however, the difference is marginal. Furthermore, it is essential to determine the amount of absorbed solar irradiance and PV/T thermal and electrical power output before conducting an exergy analysis.

The PV/T system is taken as the control volume (as shown in Figure 2) and is assumed to be in a semi-steady state. The amount of solar radiation absorbed by PV/T is determined [44] using Equation (7).

E_sun = I × A_P × τ_g × α_cell

(7)

where τ_g denotes the transmissivity of the glass layer covering the photovoltaic module, and α_cell denotes the absorptivity of the photovoltaic cells. The amount of useful heat absorbed by the coolant fluids (air and water) flowing through the solar thermal collector is estimated by the sum of Equations (1) and (2). Thus, the total energy consumption of PV/T is described by Equation (8) as follows (adjusted for thermal and electrical energy);

E_total = E_th + E_el

(8)

Moreover, the energy equations of the PV/T system can be derived as a single control volume by the following Equations (9)–(11).

∑E_in = ∑E_lost + ∑E_out

(9)

E_mass,in + E_sun = ∑ E_lost + E_el + E_mass,out

(10)

∑E_lost = E_{lost,convection} + E_{lost,radiation} + E_{lost,friction}

(11)

where E_lost,_radiation, E_{lost,convection}, and E_{lost,friction} are the rates of energy loss due to radiation, convection, and friction. Additionally, E_in denotes the rate of energy input, E_out denotes the rate of energy output, and E_lost is the rate of energy loss. By using Equation (9), the following are the energy losses incurred in each system:

∑E_lost = E_sun − E_th − E_el

(12)

2.4. Exergy Analysis Model

Exergy associated with a PV/T system is depicted in Figure 2 for easier comprehension. As a starting point for investigating the performance of a PV/T system from an exergy standpoint, it is necessary to think of the PV/T system as a control volume. In this case, it is assumed that the system is in a semi-steady state. The exergy balance of a PV/T system is represented by the following expression,

∑ξ_in = ∑ξ_lost + ∑ξ_out

(13)

In Equation (13), ξ_in represents exergy input, ξ_out represents exergy output, and ξ_lost represents exergy loss or destruction caused by the irreversible process. The term ξ_in in this Equation (14) stands for net input exergy rate. In solar systems, such as a PV/T system, the input energy is the solar radiation that strikes the facing surface area of the system; as a result, the input exergy is equal to the exergy of incident solar irradiation to the system (ξ_sun). Thus, Equation (13) can also be expressed as in Equation (14):

ξ_in = ξ_sun

(14)

PV/T systems generate net output exergy (ξ_out), which is the sum of thermal exergy (ξ_th) and electrical exergy (ξ_el), then it can be expressed as:

∑ξ_out = ∑ξ_th + ∑ξ_el

(15)

If the thermal exergy is equal to the difference between the flow exergy at the collector outlet and inlet, the exergy balance becomes,

ξ_th = ξ_mass,out − ξ_mass,in

(16)

ξ_sun = ∑ξ_th + ∑ξ_el + ∑ξ_lost

(17)

ξ_sun = (ξ_mass,out − ξ_mass,in) + ∑ξ_el + ∑ξ_lost

(18)

Several numerous approaches have been proposed to determine the exergy of solar irradiation. However, Kabelac [45], Spanner [46], and Petala [47] recommended using Equation (19) for the exergy analysis of absorbed solar irradiation by the PV/T system.

$${\mathsf{\xi }}_{\mathit{sun}}={\mathit{E}}_{\mathit{sun}}[1-\frac{4{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}}}{3{\mathit{T}}_{\mathit{s}\mathit{u}\mathit{n}}}+\frac{1}{3}(\frac{{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}}}{{\mathit{T}}_{\mathit{s}\mathit{u}\mathit{n}}}{)}^4]$$

(19)

where T_sun is the surface temperature of the sun (~5777 K), and T_amb is the temperature of ambient air surrounding the panel. Equations (20) and (21) give the breakdown of thermal (ξ_th) and electrical (ξ_el) exergy of the system, respectively [48,49].

$${\mathsf{\xi }}_{\mathit{el}}=\frac{{\xi }_{\mathit{e}\mathit{l}}}{{\xi }_{\mathit{s}\mathit{u}\mathit{n}}} = \frac{{\xi }_{\mathit{e}\mathit{l}}}{\mathit{G}\left(1-\frac{{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}}}{{\mathit{T}}_{\mathit{c}\mathit{o}\mathit{l}}}\right)} = \frac{{\mathit{I}}_{\mathit{s}\mathit{c}}\times {\mathit{V}}_{\mathit{o}\mathit{c}}\times \mathit{F}\mathit{F}}{\mathit{G}\left(1-\frac{{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}}}{{\mathit{T}}_{\mathit{c}\mathit{o}\mathit{l}}}\right)}$$

(20)

$${\mathsf{\xi }}_{\mathit{th}}=\frac{{\xi }_{\mathit{t}\mathit{h}}}{{\xi }_{\mathit{s}\mathit{u}\mathit{n}}} = \frac{\mathit{m}\mathit{C}\mathit{p} \left[ \left({\mathit{T}}_{\mathit{o}\mathit{u}\mathit{t}}-{\mathit{T}}_{\mathit{i}\mathit{n}}\right)-{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}} \mathit{l}\mathit{n}\left( \frac{{\mathit{T}}_{\mathit{o}\mathit{u}\mathit{t}}}{{\mathit{T}}_{\mathit{i}\mathit{n}}}\right) \right]}{\mathit{G}\left(1-\frac{{\mathit{T}}_{\mathit{a}\mathit{m}\mathit{b}}}{{\mathit{T}}_{\mathit{c}\mathit{o}\mathit{l}}}\right)}$$

(21)

where and T_col is taken as collector surface temperature, and T_amb is the ambient temperature of the panel.

2.5. Feasibility Analysis of the Solar Hybrid PV/T System

The system is designed to convert solar radiation into heat and electricity, which can be used in combined heat and power applications. To maximise the energy conversion, the following algorithm is developed for proper utilisation of energy, as given in Figure 3. Here, P_solar represents the total energy obtained from the PV panel in the form of electrical energy, and P_pump is the power required to drive the air pump for the solar air heater. B_soc is the state of charge (SOC) of the battery, and P_B shows the battery power. When the net power from the solar panel is greater than zero, the algorithm checks the SOC of the battery compared with the maximum capacity of the battery. If the charge is lower than the maximum value, power from the solar panel is used for charging the battery. After charging, it is used for operating an electric air heater. Furthermore, if the SOC of the battery is equal to the maximum, the power from the PV panel is used directly to drive the electric air heater. If the net power is equal to zero, the system runs naturally. However, if the net power is smaller than zero, the algorithm checks the SOC of the battery. If it is greater than that of the minimum, the power is utilised to drive a heat exchanger where heat is extracted from the hot water of the solar water heater to heat the air and eventually use it for those applications. However, if the SOC of the battery is lower than that of its minimum value, the hot air from the solar air heater is directly utilised.

2.6. Uncertainty Analysis

Uncertainty analysis is the internal quantitative assessment of data to estimate inherent uncertainty in the data. This internal standard deviation or standard error can result from minor errors in the measurement of physical quantities (length, width, height for intricate geometry), which generates slightly different values for each experiment. Apart from systematic errors from device calibration, uncertainty analysis helps to estimate random errors that occur from human errors, experimental and ambient conditions, selection of devices used, mistakes related to calibration, reading and measurement, and connection points [50].

If R is the function of ‘n’ independent linear parameter such that R = R (w₁, w₂………w_n), the function is defined as [51],

$$W_R={\left[{\left(\frac{\delta R}{\delta x,1}{W}_1\right)}^2+{\left(\frac{\delta R}{\delta x,2}{W}_2\right)}^2+\dots \dots \dots +{\left(\frac{\delta R}{\delta x,n}{W}_n\right)}^2\right]}^{0.5}$$

(22)

where W_R is the uncertainty function, and $\frac{\delta R}{\delta x,i}$ is the partial derivative of R with respect to x. Considering the experimental results and recalling the technical specifications of measuring equipment listed in Table 1 and Table 2, the maximum absolute uncertainty of this experimental study is calculated as ±0.0188 or 1.88%, which supports the validity of the developed system and experimental results.

3. Results and Discussions

3.1. Efficiency Analysis of PV/T System

The intensity of solar irradiation and duration of sunlight are the dominating factors that control PV/T performance; however, these factors depend on the geographical location and local weather conditions as well. The experimental work is carried out as a case study at Rajshahi (lies at 24°22′26″ N 88°36′04″ E and 23 m above sea level), the northern region of Bangladesh. Rajshahi is marked with having a tropical wet and dry climate. The experiment was conducted for several days during the wintertime (November to December in Bangladesh) when the solar radiation varies from 980 to 1050 W.m⁻² [52]. The performance of the hybrid PV/T is evaluated on the basis of the assumptions mentioned in Section 2.1. The temperature of the PV panel, collectors’ surface, and coolants (air and water temperature at inlet and outlet) were recorded every half hour during the experimental processes. It is noted that part of water circulating through the panel may evaporate when captured heat is generated by PV/T system, and it was observed that the evaporation rate was higher at a lower mass flow rate. The experimental system was carefully sealed to avoid leaking steam.

Figure 4 illustrates the cooling effect on the PV/T system at four different mass flow rates, 0.0014, 0.0018, 0.0022, and 0.0026 kg.s⁻¹ for both cases of air (Figure 4a) and water (Figure 4b) medium. It is observed in Figure 4a,b that higher heat transfer for flowing air and water was obtained at 0.0014 kg.s⁻¹, and the corresponding temperature difference between inlet and outlet of the coolants or carried out by coolants was 26.2 and 41.7 °C, respectively; whereas a 19 and 35.8 °C temperature drop in the system were recorded for 0.0026 kg.s⁻¹ flow rate of air and water, respectively. With the increase in cooling medium flow rate, heat transfer was decreased due to the less available time to exchange heat between the cooling medium and heat-absorbing bodies (absorber plate, PV panel), and a similar pattern was found for both air and water. Therefore, a low mass flow rate offers higher residence time to carry out heat from the system, but it requires much time to make the necessary cooling [52]. Moreover, solar irradiation is also important to improve the system efficiency, as it is detected that maximum cooling happens at mid-day (12.00 to 13.30).

As the cooling of the system has a positive impact on the PV panel performance, it can be seen that a maximum of 19.8 W from the PV panel was obtained when cooling, as well as the temperature difference between the inlet and outlet of the thermal collector, which was maximum. However, the availability of solar radiation (maximum recorded 1050 W.m⁻²) is a dominating factor in having satisfactory PV/T performance. In addition, maximum power was recorded between 12.00 to 13.30 h (as the maximum cooling also happens in that range) because solar intensity remains at its peak at this time. Thus, variation in the performance of thermal collectors and PV panels was attributed to the availability of solar radiation at a different time of day and some loss in the system.

From Figure 5, the individual efficiency of solar PV, solar air heaters and solar water can significantly affect the combined heat and power efficiency of the PV/T system. A steep increase in thermal as well as electrical efficiency can be observed from the beginning of the day to mid-day (as solar intensity is gradually increased during that time period). The maximum combined efficiency was recorded to be 62.21% for a mass flow rate of coolant of 0.0026 kg.s⁻¹ at around 13.00 to 13.30 h. Whereas, at the same time, for 0.0014, 0.0018 and 0.0022 kg.s⁻¹ flow rates, the combined efficiency was recorded as 59.10%, 53.02% and 45.91%, respectively. After then, the solar intensity was decreased with time; hence, the combined efficiency, as well as the individual efficiency, dropped gradually.

3.2. Energy Analysis

Mass flow rate has a direct effect on the cooling of PV system, and it is observed that the PV efficiency was improved at a higher mass flow rate of coolant. Furthermore, thermal efficiency was also enhanced with temperature, which in turn enhanced the PV efficiency by allowing it to operate at a suitable operating temperature. The hourly variation in thermal and electrical power at different mass flow rates of coolants (both air and water) is depicted in Figure 6. Thermal power output was calculated by using Equations (1) and (2) while using air and water as coolants, respectively. It is observed from Figure 6a,b that thermal power was increased linearly with mass flow rate, and a maximum of 85.8 and 211.84 W.m⁻² thermal power was extracted when the mass flow rate was 0.0026 kg.s⁻¹ (during mid-day: 12.00 to 13.30 h). After increasing the mass flow rate from 0.0014 to 0.0026 kg.s⁻¹ (i.e., almost 85.7%), the efficiency of air and water coolant reached up to 47% and 37%, respectively.

Figure 6c,d demonstrate the electrical power output of the designed system (as calculated from Equation (7)) for air and water as coolants, respectively. The results presented in Figure 6c,d also show that electrical power was at its lowest in the morning. As the solar intensity reached the highest at noon, the output power reached a maximum at that time. The highest electrical power was recorded as 10.30 W.m⁻² for air coolant and 28.6 W.m⁻² for water coolant while using the maximum flow rate of 0.0026 kg.s⁻¹. Again, the electrical power output follows the same trend as the thermal output, but the higher specific heat capacity of water helps to attain a higher power density than air.

Figure 7 depicts variation in thermal and electrical efficiencies of the PV/T system while changing the flow rate from 0.0014 to 0.0026 kg.s⁻¹. System efficiency increased with flow rate, and a maximum of 62.21% was recorded (55.54% thermal and 6.67% electrical) when the coolant was circulated at 0.0026 kg.s⁻¹. A similar trend was observed by the previous research works [41,52,53], and this linear relation between efficiency and mass flow rate can be characterised by the effect of an increase in volume flow of coolant per unit time. Sarhaddi et al. [54] also observed an overall efficiency of 45% for a hybrid PV/T air heater, and about 68.4% overall efficiency was reported by Fudholi et al. [15] for a PV/T water collector having a mass flow rate of 0.041 kg.s⁻¹. Moreover, about a 30.7% increase in system efficiency was found while the coolant flow rate increased from 0.0014 to 0.0026 kg.s⁻¹.

3.3. Exergy Analysis

An energy study is a quantitative approach to analysing the performance of PV/T collectors; however, it is not enough to establish a definite conclusion on the performance of a system with only an energy-based assessment [12,36,37,55,56]. Furthermore, exergy analysis provides a qualitative assessment of energy output from PV/T system. The maximum available energy that can be extracted from a PV/T collector is the quality of exergy. Exergy analysis is more concerned with the characteristics or capabilities of the system. As a result, it is possible to estimate the energy potential that can be used in practice.

Figure 8c shows the thermal (combined effect of air and water as coolant) exergies output of the PV/T system. From Figure 8a,b, it can be observed that thermal exergy of the PV/T collector is a function of flow rates of coolant. The maximum thermal exergy efficiency of the collector obtained at 2.89%, of which 2.25% (Figure 8a) contributes from water cooling and 0.64% is covered by air cooling (Figure 8b). Moreover, for the PV system, a maximum of 1.87% exergy was recorded. Due to the higher specific heat capacity of water, more than 90% of the thermal exergy was contributed by water. Moreover, Figure 8d illustrates the effect of introducing cooling in the PV system. As cooling has a positive impact on PV performance and enhances the system efficiency, the combined exergy reached up to 4.78%, which is almost double.

Due to irreversibility, exergy loss is decreased with the reduction of entropy developed in the system. Usually, entropy generation is reduced with an increase in the diameter and length of a collectors’ tube; however, tube length and diameter are kept constant in this study; thus, entropy generation from this piping arrangement is considered as very low. However, when the mass flow rate of the water was increased, the entropy of the collector decreased. Using nanofluid as a coolant might likely reduce this effect of entropy [57], which is the future study focus.

4. Conclusions

The effect of cooling on the performance of a solar PV/T system is studied through an energy and exergy approach. The cooling of the PV panel and performance of the PV/T system are investigated with simultaneous application of both air and water as coolants. Four different mass flow rates, 0.0014, 0.00184, 0.0022 and 0.0026 kg.s⁻¹ are applied to observe the cooling effect on the system. Moreover, the flow rate of coolant in the system has a significant role in heat transfer as well as helps improve the overall efficiency of the system.

The maximum temperature differences between the inlet and outlet of the air collector are 41.7, 40.2, 38 and 35.8 °C for air; and for the water collector, 26.2, 23.8, 22 and 18.3 °C are observed for the mass flow rate of 0.0014, 0.00184, 0.0022¹ and 0.0026 kg.s⁻¹, respectively. Availability and duration of solar radiation act as main contributors to this heat transfer phenomenon. In addition, a low mass flow rate provides a higher residence to convey heat transfer, and a higher mass flow rate accommodates a larger volume of coolant flow throughout the system. As the mass flow rate of coolant increases from 0.0014 to 0.0026 kg.s⁻¹, the efficiency of PV also boosts up to 8.3%. This cooling performance at flow rate also contributes to attaining higher thermal and overall efficiency. The highest thermal and combined efficiencies are 55.54% and 62.21%, respectively, at simultaneous water and air mass flow rates of 0.0026 kg.s⁻¹. A high mass flow rate has a positive impact on the cooling of the PV/T system, and around 35.5% and 43.55% improvement in the combined and thermal efficiency can be observed when the mass flow rate increases from 0.0014 to 0.0026 kg.s⁻¹.

Maximum thermal energy output for air as a coolant is found as 85.8 W/m² from mid-day 12.00 to 13.30 h on the experiment days. The thermal and electrical energy outputs are 211.84 and 28.71 W.m⁻², respectively, when using water as a coolant. Moreover, it is also observed that, as mass flow rate increases, thermal and electrical energy also increase. The maximum thermal exergy for using air and water is found as 2.89% and reached up to 4.78% while introducing the PV/T system. Exergy analysis shows that energetic efficiencies are increased with mass flow rate, which supports the developed systems’ validity.