Experimental investigation of energy and exergy characteristics of a novel solar collector with swirling reversed circular flow jet impingement

Abstract

Even though Photovoltaic technology is considered to be among the most convenient sources of electricity production, surface heat accumulation has consistently reduced its conversion efficiency. Therefore, many cooling strategies have been proposed, among which the Reversed Circular Flow Jet Impingement RCFJI is the most recent. This study seeks to address challenges associated with RCFJI, such as low turbulence generation and a high friction penalty, by developing a new design featuring Swirling Reversed Circular Flow Jet Impingement. The study was performed employing a solar simulator under air mass flow rates of (0.01–0.13) kg/s and an irradiance of (500–900) W/m². The principal results showed that mass flow rate positively influences the efficiency of the proposed prototype, whereas solar irradiance has the opposite effect. The new design enhanced electrical energy efficiency by 12.42%, thermal energy efficiency by 4.33%, electrical exergy efficiency by 11.46%, and thermal exergy efficiency by 4.81%. Power production increased by 22.00% compared to a bare PV module. These data provide evidence that the proposed study design can better manage the thermal challenges of PV modules than PV alone or standard RCFJI.

Introduction

Much like water and air, energy is not deemed to be a pure commodity, but rather it is a fundamental need for all other commodities in the modern world¹. Recent literature indicates that global energy dependence has risen sharply over the past four decades, driven by demographic expansion, rapid technological advancement, and the increasing complexity of modern society, including advanced transportation systems, agricultural foundations, and industrial needs^2,3. Although all of these factors are considered good signs, indicating improvement in how human beings manage their lives, the main concern is that fossil fuels still account for about 80% of the energy consumed on Earth⁴. It should be noted that, despite their extreme effectiveness, energy sources such as fossil fuels have disadvantages, including being exhaustible, contributing to global warming and the greenhouse effect, and posing many other challenges, directly influencing human well-being^5,6. Given the catastrophic impacts of fossil fuels, the problem is further exacerbated by the expectation that reliance on them will increase in the coming years⁷. Therefore, alleviating the problem requires communities of governments, scientists, and international organizations to unite their endeavours to shift the emphasis from reliance on fossil fuels to depending more on sustainable energy sources⁸.

One way to reduce reliance on fossil fuels is to adopt Photovoltaic (PV) technology, which can produce a convenient amount of energy by converting sunlight, with minimal economic and environmental side effects⁹. Even though the last three decades have witnessed PV’s tremendous impact as an alternative energy source, research has reported a fundamental problem that significantly reduces its efficiency. As the surface temperature of PV increases, its efficiency decreases by 5% for every 10-degree rise¹⁰. Although this problem has been prominent, causing PVs to lose efficiency and shorten their lifespans, numerous solutions to mitigate it have also been proposed¹¹. The literature suggests incorporating passive and active cooling techniques, as they can play a significant role in cooling PV surfaces. It should be noted that passive cooling techniques are more commonly used because they require no external energy sources. More importantly, the recent literature indicates a common trend toward a hybrid system^12,13.

The use of either a passive or an active cooling method almost always requires a working medium, such as water or air. Nevertheless, the adoption of air has seemed very advantageous, as it is naturally abundant and relatively cheap. With that said, directing the air to the back of the PV in an optimized manner remains the main concern¹⁴. To mitigate this problem, many solutions have been proposed, such as single-pass, double-pass, concentrated, and jet configurations, all of which have demonstrated phenomenal performance¹⁵. Single- and double-pass are common methods for directing air toward the rear of a PV¹⁶. In a single-pass configuration, air enters and exits the system immediately. In contrast, in a double-pass configuration, the air flow encounters a partition and exits the system in two stages¹⁷. It should be noted that a double-pass typically performs better because the air stays in the system for a longer period¹⁸. Even though these two methods are proven effective, recent literature has introduced several modifications to their designs, such as integrating Phase Change Materials (PCM) to enhance storage capacity, passive cooling techniques to increase turbulence, and twisted tapes to generate more vortices^19,20. A good example of such modified systems is a study conducted by Maarof et al.³, who employed a PCM storage component in their newly designed double-pass solar collector. At the optimized geometrical and operational conditions, the modified design achieved electrical efficiency enhancements of 61% and thermal efficiency enhancements of 72%²¹.

Moreover, the literature has conducted extensive research in which fins have played a prominent role in manipulating flow. Machi et al.²² have designed a new solar collector featuring V-angled perforated fins. The study includes mass flow rates of 0.00864 to 0.1317 kg/s, with an overall energy efficiency of 9.83% compared to the standard step-up. In another essential experiment, Srivastava et al.²³ examined the performance of an air collector modified with transverse, inclined, V-shaped, and arc-shaped ribs. The primary conditions of the test were Reynolds numbers of 3000–15,000 and a heat flux of 1000 W/m². The conclusion shows that all the modifications outperformed the standard design, with the arc-shaped rib recording the highest enhancement of 60% compared to the standard collector. In addition, the use of grooves, which are repeated patterns of small fins, can have a significant impact, as shown by a study by Patwari et al.²⁴. The study employed triangular grooves integrated with CuO nanoparticles in a solar collector and found that the newly proposed design outperforms the standard one, achieving a 77.13% increase in thermal efficiency, a remarkable result. Additionally, Chaouch et al.²⁵ obtained yearly average electrical and thermal efficiencies of 33.7% and 12.27%, respectively, using a nonlinear dynamic roll bond PVT solar collector. Finally, twisted tapes are considered an effective flow enhancer because they promote the formation of vortices in the airflow, thereby increasing heat transfer²⁶.

Two additional effective approaches for manipulating airflow pathways involve the use of concentrated channels and micro-channels, both of which have become increasingly prominent in recent literature^27,28. For instance, Djebara et al.²⁹ compared the performance of a folded mini-channel heat sink-based collector with a bare PV and a standard mini-channel-based collector. The experiment was conducted under a mass flow rate of (0.3–1 m/s) and solar irradiance of 1000 W/m². The result shows that the newly proposed design outperforms the standard mini channel, increasing power output from 0.913 to 0.953 W. In a similar study, Noui et al.³⁰ proposed a novel configuration incorporating wavy mini-channel heat sinks, evaluated with Reynolds numbers ranging from 200 to 900 and subjected to a constant heat flux of 100 kW/m². The study’s conclusions demonstrated that the new design achieves a 27% enhancement in the (THPF) compared to a conventional design³¹.

Finally, jet impingement is also considered an effective way to cool a PV surface^32,33. Nevertheless, adopting jet impingement requires detailed optimization studies as random selection can lead to unexpected results. For instance, Srivastav et al.³⁴ enhanced the performance of a solar heater using a vertical submerged pipe, varying shapes (circular, square, triangular, and rectangular), jet diameter ratio $d/{D}_{h}$ of 0.044–0.11, jet spacing ratio ${S}_{j}/{D}_{h}$ of 0.108–0.433, fixed pitch ratios $X/{D}_{h}=Y/{D}_{h}=0.867$, and Reynolds numbers ranging from 3000 to 18,000. The numerical study shows that the optimized design, which features a circular pipe with, ${S}_{j}/{D}_{h}=0.217$, and $Re=\text{15,000}$ recorded the optimal THPF of 2.69 which is considered to be an outstanding record. The same authors in a work referenced in³⁵ developed the analysis further and drew several important conclusions regarding the nature of the effect of geometrical parameters on the performance of jet impingements in solar heater applications. To better align with the design proposed in this study, it is necessary to examine two previous designs that share similarities with the novel design. The first design was proposed by Ewe et al.³⁶, who developed a bifacial photovoltaic thermal solar air collector with jet impingement (JIBPVT). The study determined the primary condition of the test to include mass flow rates of (0.012 to 0.036) kg/s and solar irradiances of 700 to 900 W/m². The newly proposed design outperformed the standard designs, achieving thermal, electrical, and thermo-electro-hydraulic efficiencies of 57.3%, 10.36%, and 83.93%, respectively, which are considered remarkable compared with previous studies. The same design was later modified by Ishak et al.³⁷, who added the effect of a reversed circular flow jet impingement (RCFJI). They discovered the common problem of jet interference. The first problem was the distance between the jet and the target, and the second problem was the creation of more turbulence while the pressure drop remained the same or even decreased. Therefore, this study will introduce the effect of swirling. The investigation will focus on experimental testing of a new design at mass flow rates of 0.01 to 0.14 kg/s and solar irradiance levels of 500 to 900 W/m² using an indoor solar simulator.

Methodology

Research Synopsis

The research begins with the development of an innovative PVT collector configuration, the swirling reverse circular jet impingement system. Manufacturing is based on meticulous geometrical parameter optimizations. Once optimization and manufacturing are finalized, the study employs an indoor solar simulator to test the new module experimentally against a bare PV experiment and a reversed circular-flow-jet impingement-based collector, using mass flow rates of (0.01 to 0.13) kg/s and solar irradiances of (500 to 900) W/m².

The physical module

The newly proposed design, as shown in Fig. 1, comprises two main parts: five PV panels detailed in Table 1 and a collector featuring swirling, reversed circular-flow jet impingement as detailed in Table 2. The cups were made using an Elegoo 3D printer and placed on the collector upside down. The air enters from the side hole, circulates, encounters flow enhancement, and leaves the cup toward the back of the PV. The cups were manufactured from a PLA + filament, and each cup took about 1.5 h, bringing the total to 54 h. Table 3 contains all the required properties of the used material. It should be noted that the temperature of the cup is significantly lower than the temperature of the PV surface, and it did not reach a point where any distortion was countered.

Fig. 1

The physical model manufactured for this study.

Table 1 Physical and electrical parameters of the PV used in the research.

Table 2 Geometrical dimensions of the cup and the collector.

Table 3 The properties of the materials (PLA +) used in making the cups³⁸.

The experimental rig

Figure 2a, b show the experiment conducted using an indoor solar simulator with halogen light bulbs. The experiment starts with an air blower that draws ambient air and sends it through a flexible hose attached to the inlet of the swirling, reversed circular cup. From there, the air flows into the upper duct of the PVT module, between the reflective jet plate and the rear of the bifacial PV panel, where it strikes the panel’s back surface to cool it. It then exits the collector through the air outlet. The difference between inlet and outlet air velocity is measured using an Anemometer MT-4615, while Data Logger AT4824 and K-type thermocouples measure surface and air temperatures. The electrical details of the panel are calculated using an I-V tracer MP11.

Fig. 2

The physical picture of the rig (a) and the schematic diagram (b).

The study structure

The experiment was conducted in three main stages. The first stage was devoted solely to testing a bare PV module, with all the features in Table 1 demonstrated. The experiment measured surface temperature, electrical energy, exergy efficiency, and electrical details of the panel under irradiance ranging from 500 to 900 W/m². In the second stage, the same PV was used with a reversed circular jet impingement technique. The collector included 36 cups. Under mass flow rates of (0.01 to 0.13) kg/s and an irradiance of 500 to 900 W/m², the experiment calculated surface temperature, electrical energy, exergy efficiency, thermal energy, and electrical details of the tested collector. The last stage repeated the second stage, but this time a swirling, reversed-circular-jet impingement technique (swirling cup) was integrated.

The energy and exergy models used in the calculation of this study.

This study employs the energy balance equations. Equations 1–6 show that the total of inlet and outlet energy must be equal^39,40. The inlet energies are energy from solar irradiance and energy from the inlet fluid. In contrast, the outlet energies are electrical output energy, energy wasted, and energy due to outlet fluid. In Eqs. 1–6, E is energy, I is solar irradiance, and A is area, ${\tau }_{{{\text{gl}}}}$ is the transmissivity of the glass, ${\upalpha }_{{{\text{cell}}}}$ is the absorbability of the cell, ${\text{I}}_{{{\text{max}}}} {\text{V}}_{{{\text{max}}}}$ are the maximum current and voltage, and ${\text{I}}_{{{\text{sc}}}} {\text{*V}}_{{{\text{oc}}}}$ are the currents of short circuit and open voltage.

$$\sum {\dot{\text{E}}}_{{{\text{in}}}} = \sum {\dot{\text{E}}}_{{{\text{out}}}} + \sum {\dot{\text{E}}}_{{{\text{loss}}}}$$

(1)

$${\dot{\text{E}}}_{{{\text{sun}}}} + {\dot{\text{E}}}_{{{\text{mass}}\;{\text{in}}}} = {\dot{\text{E}}}_{{{\text{el}}}} + {\dot{\text{E}}}_{{{\text{mass}}\;{\text{out}}}} + {\dot{\text{E}}}_{{{\text{loss}}}}$$

(2)

$${\dot{\text{E}}}_{{{\text{sun}}}} = {\text{I}} \times {\text{A}}_{{{\text{pv}}}} \times {\uptau }_{{{\text{gl}}}} \times {\upalpha }_{{{\text{cell}}}}$$

(3)

$${\dot{\text{E}}}_{{{\text{el}}}} = {\text{I}}_{{{\text{sc}}}} \times {\text{V}}_{{{\text{oc}}}} \times {\text{FF}}$$

(4)

$${\text{FF}} = \frac{{{\text{I}}_{{{\text{max}}}} { } \times {\text{ V}}_{{{\text{max}}}} }}{{{\text{I}}_{{{\text{sc}}}} { } \times {\text{ V}}_{{{\text{oc}}}} }}$$

(5)

$${\upeta }_{{{\text{el}}}} = \frac{{{\dot{\text{E}}}_{{{\text{el}}}} }}{{{\dot{\text{E}}}_{{{\text{sun}}}} }} = \frac{{{\text{I}}_{{{\text{sc}}}} *{\text{V}}_{{{\text{oc}}}} *{\text{FF}}}}{{{\text{I}}*{\text{A}}_{{{\text{pv}}}} *{\tau }_{{{\text{gl}}}} *{\upalpha }_{{{\text{cell}}}} }}$$

(6)

Equation 9, which is devoted to calculating the total energy of the system, is comprised of the electrical energy calculated previously and the thermal energy calculated by Eqs. 7 and 8, where ${\dot{\text{m}}}$ is the mass flow rate, CP is specific heat capacity, ${\upeta }$ is energy efficiency, and T is temperature^39,40.

$${\dot{\text{E}}}_{{{\text{mass}}\;{\text{out}}}} {-}{\dot{\text{E}}}_{{{\text{mass}}\;{\text{in}}}} = {\dot{\text{E}}}_{{{\text{th}}}} = {\dot{\text{m}}} \times {\text{C}}_{{\text{p}}} \left( {{\text{T}}_{{\text{out }}} - {\text{T}}_{{{\text{in}}}} } \right)$$

(7)

$${\upeta }_{{{\text{th}}}} = \frac{{{\dot{\text{E}}}_{{{\text{th}}}} }}{{{\dot{\text{E}}}_{{{\text{sun}}}} }} = \frac{{{\dot{\text{m}}} \times {\text{C}}_{{\text{p}}} \left( {{\text{T}}_{{\text{out }}} - {\text{T}}_{{{\text{in}}}} } \right)}}{{{\text{I}} \times {\text{A}}_{{{\text{pv}}}} \times {\uptau }_{{{\text{gl}}}} \times {\upalpha }_{{{\text{cell}}}} }}$$

(8)

$${\upeta }_{{\text{t}}} = {\upeta }_{{{\text{el}}}} + {\upeta }_{{{\text{th}}}}$$

(9)

As with the energy balance, the exergy balance equation can also be employed using Eqs. 10–14. The same principles adopted above can be replicated here, with the only difference that here the analysis is conducted for exergy^41,42. From Eq. 10–14, Ts is the sum temperature, ${\Delta P}$ is the pressure drop, ${\upvarepsilon }$ is exergy efficiency, and ${\uprho }$ is density.

$$\sum {\dot{\text{E}}\text{X}}_{{{\text{in}}}} = \sum {\dot{\text{E}}\text{X}}_{{\text{out }}} + \sum {\dot{\text{E}}\text{X}}_{{{\text{loss}}}}$$

(10)

$${\dot{\text{E}}\text{X}}_{{{\text{sun}}}} + {\dot{\text{E}}}_{{{\text{mass}}\;{\text{in}}}} = {\dot{\text{E}}\text{X}}_{{{\text{el}}}} + {\dot{\text{E}}\text{X}}_{{{\text{mass}}\;{\text{out}}}} + {\dot{\text{E}}\text{X}}_{{{\text{loss}}}}$$

(11)

$${\dot{\text{E}}\text{X}}_{{{\text{sun}}}} = {\text{I}} \times {\text{A}}_{{{\text{pv}}}} \left( {{1} - \frac{{{\text{T}}_{{\text{a}}} }}{{{\text{T}}_{{\text{s}}} }}} \right)$$

(12)

$${\dot{\text{E}}\text{X}}_{{{\text{el}}}} = {\dot{\text{E}}}_{{{\text{el}}}} - {\dot{\text{E}}}_{{\text{p}}}$$

(13)

$${\upvarepsilon }_{{\text{e}}} = \frac{{{\dot{\text{E}}}_{{{\text{el}}}} - {\dot{\text{E}}}_{{\text{p}}} }}{{{\text{I}} \times {\text{A}}_{{{\text{pv}}}} \left( {1 - \frac{{{\text{T}}_{{\text{a}}} }}{{{\text{T}}_{{\text{s}}} }}} \right)}} = \frac{{\left( {{\text{I}}_{{{\text{sc}}}} \times {\text{V}}_{{{\text{oc}}}} \times {\text{FF}}} \right) - \left( {\frac{{{\dot{\text{m}\Delta P}}}}{{{{\uprho \upeta }}_{{\text{p}}} }}{ }} \right)}}{{{\text{I}} \times {\text{A}}_{{{\text{pv}}}} \left( {1 - \frac{{{\text{T}}_{{\text{a}}} }}{{{\text{T}}_{{\text{s}}} }}} \right)}}$$

(14)

Here again, the total exergy efficiency is the sum of the electrical and thermal exergy efficiencies, calculated using Eqs. 15–20, where the new terms (h) and (S) represent enthalpy and entropy, respectively^41,42.

$${\dot{\text{E}}\text{X}}_{{{\text{mass}}\;{\text{out}}}} - {\dot{\text{E}}}_{{{\text{mass}}\;{\text{in}}}} = {\dot{\text{E}}\text{X}}_{{{\text{th}}}} = {\dot{\text{m} *}}\left[ {\left( {{\text{h}}_{{{\text{out}}}} - {\text{h}}_{{{\text{in}}}} } \right) - {\text{T}}_{{\text{a}}} \left( {{\text{S}}_{{{\text{out}}}} - {\text{S}}_{{{\text{in}}}} } \right)} \right]$$

(15)

$$\left( {{\text{h}}_{{{\text{out}}}} - {\text{h}}_{{{\text{in}}}} } \right){ } = {\text{ C}}_{{\text{p}}} \left( {{\text{T}}_{{\text{out }}} - {\text{ T}}_{{{\text{in}}}} } \right)$$

(16)

$$\left( {{\text{S}}_{{{\text{out}}}} - {\text{S}}_{{{\text{in}}}} } \right) = {\text{C}}_{{\text{p}}} {\text{ln}}\left( {\frac{{{\text{T}}_{{\text{out }}} }}{{{\text{T}}_{{{\text{in}}}} }}} \right)$$

(17)

$${\dot{\text{E}}\text{X}}_{{{\text{th}}}} = {\dot{\text{m}}} \times \left[ {{\text{C}}_{{\text{p}}} \left( {{\text{T}}_{{\text{out }}} - {\text{T}}_{{{\text{in}}}} } \right) - {\text{T}}_{{\text{a}}} {\text{C}}_{{\text{p}}} {\text{ln}}\left( {\frac{{{\text{T}}_{{\text{out }}} }}{{{\text{T}}_{{{\text{in}}}} }}} \right)} \right]$$

(18)

$${\upvarepsilon }_{{{\text{th}}}} = \frac{{{\dot{\text{E}}\text{X}}_{{{\text{th}}}} }}{{{\dot{\text{E}}\text{X}}_{{{\text{sun}}}} }} = \frac{{{\dot{\text{m}*}}\left[ {{\text{C}}_{{\text{p}}} \left( {{\text{T}}_{{\text{out }}} - {\text{T}}_{{{\text{in}}}} } \right) - {\text{T}}_{{\text{a}}} {\text{C}}_{{\text{p}}} {\text{ln}}\left( {\frac{{{\text{T}}_{{\text{out }}} }}{{{\text{T}}_{{{\text{in}}}} }}} \right)} \right]{ }}}{{{\text{I*A}}_{{{\text{pv}}}} \left( {1 - \frac{{{\text{T}}_{{\text{a}}} }}{{{\text{T}}_{{\text{s}}} }}} \right)}}$$

(19)

$${\upvarepsilon }_{{{\text{tota}}}} = {\upvarepsilon }_{{{\text{th}}}} + {\upvarepsilon }_{{\text{e}}}$$

(20)

The uncertainty analysis

It was guaranteed that the experiment must be conducted with high precision. However, there are still two sources of error that can occur during any experiment: random and systematic errors. This experiment ensured that the recorded parameters had no random sources of error by repeating data collection for each parameter at least three times. However, because the devices used in the experiment are subject to errors, some systematic errors were observed. The details of the experimental errors are recorded in Table 4, and the whole experiment’s percentage of errors is calculated using Eq. 21. Despite all that has been said above, the study did not conduct any data manipulation, nor did it encounter any statistical variations. Thus, the authors confirm that the experiment can be duplicated under the same conditions elucidated in the above sections.

$$W_{R} = \left[ {\left( {\frac{\partial R}{{\partial X_{1} }} W_{1} } \right)^{2} + \left( {\frac{\partial R}{{\partial X_{2} }} W_{2} } \right)^{2} + ... \left( {\frac{\partial R}{{\partial X_{n} }} W_{n} } \right)^{2} } \right]^{0.5}$$

(21)

$$W = \left[ {\left( {0.1} \right)^{2} + \left( {0.1} \right)^{2} + \left( {0.1} \right)^{2} + \left( {1.45} \right)^{2} + \left( {1.3} \right)^{2} + \left( {0.65} \right)^{2} } \right]^{0.5}$$

$$W = 2.06\% {\text{W}}$$

Table 4 The percentage of error found in each device.

Results and discussion

Surface temperature analysis

As elucidated previously, the study conducted experiments on bare PV, RCFJI, and SRCFJI under solar irradiances of 500 to 900 W/m² and flow rates of 0.01 to 0.13 kg/s. As shown in Fig. 3a, the surface temperature increases from 55.4 to 79.8 °C as the solar irradiance rises from 500 to 900 W/m². This trend is rational, and the values obtained in this study can be confirmed by comparison with the results reported in references^43,44. It should be noted that this increase in surface temperature will negatively impact the overall functioning of the PV module, as explained in the following sections.

Fig. 3

Surface temperature details of PV, (a) RCFJI (b), and SRCFJI (c).

Transitioning to the RCFJI-based collectors requires several clarifications. For instance, as the flow rate increases, the surface temperature decreases, as shown in Fig. 3b. At a solar irradiance of 700 W/m², the surface temperature drops from 56.2 to 52.59 °C. This is estimated to be about 6.4% of the enhancement in surface temperature reduction. This phenomenon occurs because, as the mass flow rate increases, a greater volume of air reaches the surface in a shorter time, which enhances the cooling rate. It should be said that the percentage of enhancement recorded due to the effect of mass flow rate is rational, as references^32,45 report very similar values. As expected, the relationship between solar irradiance and surface temperature is directly proportional. For instance, surface temperature increases from 43.8 to 70.9 °C as solar irradiance rises from 500 to 900 W/m² at a mass flow rate of 0.01 kg/s. However, it should be noted that during the transition from the PV to the RCFJI, there is a significant reduction in surface temperature. For instance, when comparing the surface temperatures of the PV and the RCFJI at a solar irradiance of 900 W/m² and a mass flow rate of 0.13 kg/s, the surface temperature drop is from 79.8 to 65.6 °C, corresponding to a 21.6% reduction. Such an enhancement is considered a significant accomplishment and consistent with the literature references⁴⁶.

Finally, there is even greater surface temperature reduction when transitioning from the PV and RCFJI to the final design, the SRCFJI. It should be noted that the relationships between mass flow rate and surface temperature, as well as between solar irradiance and surface temperature, remain consistent with those observed in the RCFJI cases, with the only difference being that the effects are more obvious, as indicated in Fig. 3c. In the same case used as an example in the previous paragraph, namely at a solar irradiance of 900 W/m² and mass flow rates of 0.13 kg/s, the surface temperature of the SRCFJI further declines to 63.71 °C, increasing the surface temperature reduction to 25.3%. These recorded reductions in surface temperature will have a significant positive impact on the overall performance of the collectors, as demonstrated in the following sections.

It should be noted that the main reason why SRCFJI has a lower surface temperature compared to the two other cases, which will result in better performance as will be shown in the coming sections is that the swirling added will create more turbulence in the flow, induce vortexes, contribute to better uniformity of flow, and expanded area, all of which increase heat transfer.

Thermal energy efficiency analysis

Since there is no thermal energy efficiency for the bare PV, this section will compare only the thermal energy efficiency of the RCFJI and SRCFJI. As expected, the thermal energy efficiency of the SRCFJI is always superior when compared to that of the RCFJI. Exploring the correlation between flow rates and thermal energy efficiency, there is a direct proportionality, as is the case in many published articles in the literature, such as those reported in reference^33,46. For instance, when employing a solar irradiance of 600 W/m², as flow rates increase from 0.01 to 0.13 kg/s, the RCFJI increases from 63.9 to 65.38%, and the SRCFJI increases from 67.8 to 69.61%. The same reasoning applies here: as the flow rate increases, a larger volume of air impinges on the surface, leading to faster cooling. Additionally, the correlation between irradiance and thermal energy efficiency is also directly proportional, as is the case for references such as those reported in⁴⁶. Specifically, as the irradiance increases from 500 to 900 W/m², thermal energy efficiency increases from 62.3 to 68.8% for RCFJI and from 66.11 to 71.77% for SRCFJI. It should be noted that this proportionality primarily arises because, as solar irradiance increases, additional heat is introduced into the system, which in turn amplifies the resulting temperature differences. As the temperature difference between the inlet and outlet increases, the thermal efficiency becomes more pronounced. Applying the same case considered above, the enhancement in thermal energy efficiency between the two cases is estimated at 4.3%. This enhancement stems from the new design’s ability to generate more turbulence in the flow while reducing the friction penalty through its more appropriate configuration. All the numbers obtained in this study are validated and compared with other designs in the literature, such as those reported in references^34,47, which are rational and consistent. Figure 4a, b depict the primary trends and detailed values of thermal energy efficiency recorded in this study, where the thermal efficiency peaked at about 70% with a maximum flow rate of 0.14 kg/s and an irradiance of 900 W/m² for the RCFJI system, as shown in Fig. 4a. In comparison, the thermal efficiency peaked at about 74% with a maximum flow rate of 0.14 kg/s and an irradiance of 900 W/m² for the SRCFJI system, as shown in Fig. 4b.

Fig. 4

The thermal energy efficiency values calculated in this study for RCFJI (a) and SRCFJI (b).

Electrical energy efficiency analysis

As demonstrated above, there were significant reductions in surface temperature during transitions from bare PV to RCFJI and SRCFJI. Therefore, it is expected that the modules’ electrical performance will also improve, as a cooler surface leads to better performance. Considering the bare PV, an inverse proportionality is evident between solar irradiance and electrical energy efficiency. The primary reason is that, although power production increases with irradiance, the rate of increase is insufficient to offset the more rapid rise in irradiance. The slow increase in power output can be traced to the PV surface becoming hotter, leading to a drop in power output. As a result, electrical energy efficiency declines. Numerically, the electrical energy efficiency of the bare PV declines from 11.21 to 11.07% as solar irradiance increases from 500 to 900 W/m². The obtained values are compared to those shown in references^35,47 for validation. Again, the effect of mass flow rates shows a positive trend on the electrical energy efficiency of both the RCFJI and SRCFJI. Specifically, for a solar irradiance of 900 W/m², electrical energy efficiency increases from 12.1 to 12.16% for RCFJI and from 12.34 to 12.42% for SRCFJI. The same reason given in the two sections above can be used to explain the enhancement here. Similarly, as solar irradiance increases, the electrical energy efficiency of both RCFJI and SRCFJI declines for the same reason outlined above. What is more significant is that there are substantial improvements in electrical energy efficiency as transitions are made from bare PV to RCFJI and SRCFJI. For example, at an irradiance of 900 W/m², the PV electrical energy efficiency is 11.07%. This value increases to 12.16% for RCFJI, representing a 9.8% increase. The enhancement increases to 12.2% for SRCFJI when electrical energy efficiency further rises to 12.42%. As elucidated above, this enhancement is due to the reduction in surface temperature recorded in the previous section. Moreover, the validity of the obtained values for the electrical energy efficiency of all three tested modules can be confirmed by referring to data reported in references such as⁴⁶. Figure 5a–c seeks to capture the electrical energy efficiency values and common trends against varying mass flow rates and solar irradiances determined within the scope of the study.

Fig. 5

The electrical energy efficiency values recorded for PV (a), RCFJI (b), and SRCFJI (c).

Thermal exergy efficiency analysis

As is obvious, exergy analysis is conducted to reveal how much of the energy produced by the analyzed systems can be converted into useful work. The literature reports a very high rate of thermal exergy loss in PVT systems, primarily due to frictional and heat transfer losses⁴³. Consistent with previously published articles in the field, this study also recorded very low thermal exergy efficiency for both RCFJI and SRCFJI systems. However, the relationship between mass flow rates and thermal exergy efficiency continues to show a positive trend. For example, as flow rates increase from 0.01 to 0.13 kg/s under 800 W/m² of irradiance, the thermal exergy efficiency of the RCFJI increases from 2.21 to 2.25%. The recorded data can be compared to those reported in reference⁴⁵ for validation, and the reason behind the enhancement has been provided earlier in this article. The same goes for the SRCFJI case; thermal exergy efficiency increased from 2.33 to 2.37% for the same reasons.

Like thermal energy efficiency, thermal exergy efficiency shows a positive trend with irradiances for both tested systems. For instance, at a flow rate of 0.01 kg/s, the thermal exergy efficiency increases from 0.91 to 2.61% for the RCFJI system, and from 1.09 to 2.71% for the SRCFJI system. Nevertheless, what is more important is that thermal exergy increases when transitioning from RCFJI to SRCFJI. When solar irradiances of 800 W/m² and flow rates of 0.13 kg/s are used, the RCFJI recorded a thermal exergy efficiency of 2.25%, while the SRCFJI recorded 2.37%. The enhancement is 4.8%, which is reasonable compared to the data reported in reference⁴⁵ regarding the same phenomenon. The new design transfers heat more efficiently without increasing friction. Figure 6a, b contain detailed data about the thermal performance of the tested systems at varying flow rates and solar irradiances, aside from the trends.

Fig. 6

The thermal exergy efficiency values recorded for RCFJI (a) and SRCFJI (b).

Electrical exergy efficiency analysis

The literature reports that PVT systems generally have a very high efficiency in converting electrical energy into effective work. Therefore, the electrical exergy efficiency of PVT systems is almost always extremely high, indicating that none of the electrical power produced can be wasted; instead, nearly all of it can be transformed into effective work⁴⁶. Consistent with the literature, the electrical exergy values recorded in this study are also high. For instance, in the case of PV alone, the recorded data show electrical exergy values ranging from 11.27 to 11.35%. As shown in Fig. 7a, the correlation between irradiances and electrical exergy efficiency is inversely proportional for the same reasons given in the section on electrical energy efficiency.

Fig. 7

The electrical exergy efficiency values recorded for PV (a), RCFJI (b), and SRCFJI (c).

Mass flow rates are directly proportional to electrical exergy efficiency in both cases, as indicated in Fig. 7b, RCFJI and SRCFJI. For example, at 900 W/m², the electrical exergy efficiency surges from 12.25 to 12.39% for RCFJI and from 12.41 to 12.55% for SRCFJI as the flow rate increases from 0.01 to 0.13 kg/s. The same rationale given in the section on electrical energy efficiency can be applied to explain the improvement, and the percentage of enhancement can be confirmed using data reported in references^32,46. Like electrical energy efficiency, electrical exergy efficiency also has a negative proportional relationship with solar irradiance. Specifically, when the flow rate is held constant at 0.01 kg/s, the electrical exergy efficiency decreases from 12.25 to 11.9% for RCFJI and from 12.41 to 12.09% for SRCFJI. The same reasons and references given in the section on electrical energy efficiency can be used to explain and validate the values obtained in this section. Finally, as the transition is made from the bare PV to the RCFJI system, for example, at a constant irradiance of 900 W/m² and a mass flow rate of 0.13 kg/s, the electrical exergy increases from 11.26 to 12.39%, representing a significant improvement. The enhancement is a future increase to 12.55% for the SRCFJI system. Figure 7c captures the details of electrical exergy efficiency across all tested systems as solar irradiance and mass flow rate vary.

Power production analysis

Before concluding the energy analysis of the systems, the power production analysis should be presented as well. Although the study recorded the power capacity of all the modules tested, the power capacity of one will be analyzed to be more accurate and to require less technical work. It should be noted that presenting data from a single PV module should not be interpreted as implying deficiencies in the others. The authors guaranteed that all PV modules demonstrate the same power output during the tests; however, it was more practical to collect and report data from a single module.

Power production showed a positive trend with increasing flow rates and solar irradiance. As flow rates increase, the panels cool down, allowing them to produce more power. Similarly, higher solar irradiance means more light, increasing the power output rate. When transitioning from the bare PV to the RCFJI, power output increased from 1.26 to 1.48 W, a 19% increase. The enhancement further increased to 22% when the power production rate reached 1.54 W in the case of SRCFJI. The reason is that the new design, SRCFJI, can manage thermal challenges and maintain a cooler surface, which is the main driver of increased power production. Figure 8a–c captures the main trends and detailed values of maximum power, current, voltage, short-circuit current, and open voltage. Table 5 shows how this study’s results are evaluated in relation to previous research.

Fig. 8

The power production details of the three modules tested namely PV (a), RCFJI (b), and SRCFJI (c).

Table 5 Evaluating the results in relation to previous research.

Conclusions

Conducting experiments on three modules, namely a bare PV, RCFJI, and SRCFJI under flow rates of 0.01–0.13 kg/s and solar irradiances of 500–900 W/m², the following results are obtained.

• The overall performance of the three tested modules, bare PV, RCFJI, and SRCFJI, increased proportionally with increasing flow rates.

• There is a negative, inverse relationship between irradiance and the performance of the three tested modules: bare PV, RCFJI, and SRCFJI.

• The surface temperature reduction is 21.6% when comparing SRCFJI to bare PV.

• The redesigned system improved electrical energy efficiency by 12.42%, thermal energy efficiency by 4.33%, electrical exergy efficiency by 11.46%, and thermal exergy efficiency by 4.81%.

• In comparison with a standard PV module, the upgraded setup also delivered a 22% increase in power output.

Even though the accomplishments presented above demonstrate the effectiveness of the new design and its suitability for several applications, such as PV cooling, the following tasks can be pursued in future studies.

• Providing a comprehensive qualitative analysis of the system by showing temperature and velocity contours of the design.

• Conducting numerical analysis to optimize the geometrical parameters of the new design so that it has maximum performance.

• making the system hybrid by adding a water-cooling or thermal-storage component.