Highlights

Photovoltaic solar system has emerged as an alternative feasible substitute for energy community in the world. However, suitable thermal regulation or PV cooling strategies are important as the increased module temperature during operation reduces its power conversion efficiency.

Discussions

  • PV systems are modified with copper multi-pipe frame, ZnO powder and PCM.

  • Variation of the module temperature is observed for different PV systems.

  • The best modified PV system drops 5.68% temperature and enhances 5.04% efficiency.

Introduction

At present, the efficiency enhancement of a PV system by integrating different cooling systems has received increasing attention since the conventional silicon PV module being the dominator in the current market is adversely affected by the module temperature enhancement. In recent years, several studies have been accomplished to mitigate overheating and increase resulting power. These include air cooling,1 water cooling,2 hydraulic or refrigerant cooling,3,4 thermoelectric cooling,5,6 cooling with PCM,7,8 nano-fluids.9,10 The attempts taken by different researchers to increase the PV efficiency vary in terms of design, materials, and operating systems. However, sustainable achievements in stabilizing the module temperature via distinct strategies and techniques seems still underway.

To date, crystalline silicon (c-Si)-based PV is the most predominant solar technology commercially available around the globe constituting 95% of the market while the share of monocrystalline silicon is approximately 84% of total c-Si PV production.11 The efficiency of these crystalline silicon PV modules under standard test condition (STC) rated in the range of 14–25%.11,12,13 However, the efficiency observed in practical applications is far below the rated efficiency measured in laboratories. Furthermore, the output of the solar module decreases owing to the increment of module temperature. This temperature impact is reflected by temperature coefficients which usually vary within − 0.3%/°C to − 0.5%/°C.14 SunPower’s PV modules have a temperature coefficient of − 0.37%/°C indicating that the PV module losses 0.37% of efficiency for every degree above 25 °C. According to Browne et al.,15 a 1- °C increment of PV cell temperature can reduce 0.08–0.1% conversion efficiency over a nominal cell operating temperature of 25 °C. Therefore, the employment of a sustainable cooling system for the achievement of suitable thermal stability considering location, surroundings, and the temperature has become a vital and promising method in enhancing the PV efficiency. The commonly practiced active cooling systems use blowers or pumps to maintain the flow of heat transfer fluids (i.e., water, nano-fluids, air and so on) on the back or front of PV module. It requires extra or parasitic power consumption as well as maintenance costs. On the other hand, a passive cooling system depends on natural conduction, convection, and radiation heat transfer mechanisms. Researchers around the globe are putting tremendous efforts into innovating passive cooling system as it does not need extra or parasitic power consumption as well as maintenance cost like active cooling system.

Among others, PCM seems to be a possibly feasible and economical passive cooling technique as it can absorb a considerable amount of latent heat without a rise in temperature.16 In this case, the latent heat property of a phase change material is used as a thermal storage benefit under a specific range of temperature. The abstraction of heat from the solar module by the attached PCM can help in pinning its operating temperature and thus, it stabilizes module temperature. The selection of effective phase change material is primarily dependent upon its melting point in the desired operating temperature of the module. Paraffin wax seems to be attractive since it is typically inexpensive, abundant, and noncorrosive. Furthermore, it seems suitable to apply in high surrounding temperature of tropical regions like the Gulf area due to its high melting temperature. Hasan et al.,17 reported that using of paraffin wax with a PV solar system under the climate of UAE improved electricity yielding around 5.9%. Park et al.,18 examined the PCM’s impacts on the vertical PV solar system for the climate condition of South Korea. They observed that PCM ultimately caused a 3–4 °C reduction in PV module temperature, generating an energy efficiency increment of 3%. However, due to its poor heat conductivity, PCM has also been reported for its adverse effect on generating electricity.19 According to Qureshi et al.,20 the low thermal conductivity of PCM might cause slow heat transfer as well as low heat storage and release rate. Therefore, the use of PCM in some cases is more likely limited owing to its low thermal conductivity and instability of PCM properties. As a result, it seems that the effectiveness of PCM depends on its physical properties as well as PV location, weather conditions and system design.

In some recent studies, different nanoparticles are incorporated into PCM for increasing its conductivity. Wang et al.,21 added 1% TiO2 nanoparticles in paraffin wax and found an improvement in the latent heat capacity. A significant improvement of PCM’s latent heat capacity is recently reported by Babapoor and Karimi22 upon adding distinct metal oxides nanoparticles such as SinO2, ZnO, Al2O3, and Fe2O3. However, the technical difficulties in preparing PCM encapsulation as well as attaching PCM with PV system seem to be significant concerns in this field. In this study, slightly pressed copper pipe filled with ZnO-doped paraffin wax is investigated for improving the efficiency of the PV solar system.

Experimental materials and method

Experimental materials and design

Monocrystalline PV modules (50 W each) with and without passive cooling systems are experimentally evaluated in the present study. The specification of the used PV solar module is encapsulated in Table I. The other required materials including paraffin wax, ZnO powder, copper pipe, high-temperature adhesive (glue), silicone sealant, glass beaker (6 ml each) are purchased from the local market of Jeddah, Kingdom of Saudi Arabia (KSA). The current study is conducted at the University of Jeddah, KSA in the month of December. Three different PV systems as illustrated in Fig. 1 are developed and experimentally evaluated. The used experimental setups include (a) conventional PV system, (b) PV system integrated with PCM (PV/PCM) and (c) PV system integrated with 1% ZnO-doped PCM (PV/PCM/ZnO). It is worth to mention that the data obtained for conventional PV was considered as baseline for comparing those of PV/PCM and P/PCM/ZnO systems. The differences in parameters among the three PV modules were not significant. However, the initial module temperatures before conducing the tests were slightly varied and those were normalized to put the comparison on the same level.

TABLE I Specification of the monocrystalline silicon PV module provided by the manufacturer.
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Figure 1
figure 1

Experimental setup: (a) conventional PV system, (b) PV system integrated with PCM and (c) PV system integrated with ZnO-doped PCM.

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Integration of PCM or ZnO-doped PCM material with PV panel was performed using a copper multi-pipe frame as shown in Fig. 2a. The used copper pipe having 1.9 cm diameter and 0.09 cm wall thickness was cut to fit at the back to the solar module. Each pipe is pressed around 0.2 cm diametrically (Fig. 2b) along with its entire length and then one end is sealed using Loctite clear silicon sealant. The multi-pipe copper frame shown in Fig. 2a is then made by attaching the pipes in parallel with two similar low carbon steel bars. The high-temperature adhesive is used to attach 12 pipes with the steel bars by keeping all the sealed ends on the bottom. The PCM (paraffin wax) is melted and poured into the frame’s pipes, and then sealed on the pouring side too. The frame is then attached at the back of the PV to make it a PCM integrated PV system (PV/PCM). Similarly, another frame was developed and filled with ZnO-doped PCM and then attached at the back of the PV module to make it ZnO/PCM integrated PV system (PV/PCM/ZnO).

Figure 2
figure 2

PV cooling system: (a) Copper multi-pipe frame, (b) Dimension of pressed copper pipe.

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According to Kaddoura et al.,23 the optimal tilt angles for Jeddah city diverge from 51° to − 15° towards the south (negative sign indicates towards the north) throughout the year. They also added that the average tilt angle in winter months such as December, January and February is around 46.56°. In the present study, each PV system having single solar module is placed facing the south direction with a tilt angle of around 45°. The experimental parameters to be examined include irradiance, air temperature, module temperature, wind speed, short circuit current, open circuit voltage, output power and electrical efficiency of each PV setup. The irradiance, I-V curve, module temperature and wind velocity are measured using the solar meter, multi-meter, temperature sensor and digital anemometer, respectively. The accuracy level and the standard uncertainty for each measurement are presented in Table II. In this case, the half of the interval was divided by √3 to calculate the standard uncertainty.24

TABLE II Equipment list, accuracy and the standard uncertainty levels.
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Calculation of PV efficiency

The PV module as shown in Fig. 1 was operated over a wide range of voltage and current for finding the maximum power and then power conversion efficiency (PCE) for a particular level of irradiance at 12 pm. The resistive load at this irradiance is increased from zero (short circuit) to a significantly high value (open circuit) for determining the maximum power point (Pm) through plotting Isc versus Voc curve. To calculate the PCE of the PV module, the electrical output (Pm) was divided by the incident power received from irradiance (Pin). Similar methods were also employed for the calculation of PCEs of PV/PCM and PV/PCM/ZnO systems. These are in good agreement with experimental procedure used by a couple of researchers.25,26

Results and discussion

The changes in ambient temperature, solar radiation, and wind speed with respect to time over the test day starting from 9:00 am to 16:00 pm are illustrated in Fig. 3. The maximum (759 W/m2) and minimum (375 W/m2) solar radiations are observed at 12:00 pm and 4:00 pm, respectively. The air temperature is found to be maximum (31.2 °C) at 12:00 pm and minimum (25 °C) at 9:00 am. The air temperature and solar radiation are gradually increased till 12:00 pm and then both are decreased with time. The air temperature is mainly dominated by solar radiation. The wind speed as shown in Fig. 3b is found to be varied with time ranging from 0.65 to 3.4 m/s. This is in good agreement with the results presented by Al-Amri et al.,7 Due to the fluctuated speed, the impact of wind flow on changing air temperature and solar radiation values could logically be non-smooth. Although it can work positively for reducing the PV module temperature, it could have a relatively larger negative impact on solar radiation reaching to solar module surface. The variation of ambient temperature with time is primarily caused by the varying of incident solar radiation while wind speed, location, season, etc. are expected to be the second most important factors.

Figure 3
figure 3

Changes in (a) ambient temperature, solar radiation, and (b) wind speed with respect to time.

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As per the schematic diagram shown in Fig. 1, there are three different setups consisting of 50 W monocrystalline silicon PV module. The impact of phase change material (PCM) and ZnO-doped PCM on open circuit voltage and power output for those different setups is shown in Fig. 4. The presented data is determined at 12:00 pm in the month of December under 31.5 °C air temperature and 759 W/m2 solar radiation. The open circuit voltage and output powers are found to be depended on the module’s temperature. The maximum output power of ZnO-doped PCM integrated PV system (35.85 W) is found to be higher than those of conventional PV system (35.32 W) and PV system integrated with PCM (35.76 W). This could be attributed to the reduced cell temperature which resulted from the attachment of ZnO-doped PCM with PV module. In this case, the presence of ZnO in PCM enhances the heat transformation through natural convection and conduction inside the frame, thus ensuring, the reduced module temperature. It acts as a passive cooling system to increase the power output by reducing module temperature.

Figure 4
figure 4

Variation of (a) current and (b) output power as a function of voltage of the PV system in the presence of ZnO-doped paraffin wax under the radiation of 759 W/m2 at 12:00 pm.

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Figure 5 presents the variation of module temperature as a function of different PV systems. It is seen that the cell temperature gradually increased from 9:00 am to 12:00 pm and then decreased. The maximum cell temperatures of PV system and PV integrated with ZnO-doped PCM are found to be 52.8 °C and 49.8 °C at 12:00 pm. This demonstrates that the attachment of copper multi-pipe system with ZnO-doped PCM drops around 3 °C, which is approximately a 5.68% reduction from the PV module temperature.

Figure 5
figure 5

Effects of PCM, and ZnO-doped PCM on the PV module temperature.

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Heat is easily transferred from heated module to pressed copper pipe via the contact point as copper is good conductor. The solid paraffin wax molecules near the heated pipe surface starts to absorb heat and then it starts melting once latent heat is crossed. In this case, the heat transferred from module to copper pipe is occurred by conduction mechanism. On the other hand, paraffin wax inside the pipe and air in touch with copper pipe conduct heat by natural convection method. The differences in cell temperature for the different PV systems are primarily due to the attached PCM, and ZnO-doped PCM. In addition, wind speed, design uniformity, contact surfaces between these materials and the back of the PV module and so on are to be influencing factors. The used copper multi-pipe frame has facilitated the cooling process as it helps increase the heat transfer from the PV module to the surrounding by increasing surface area. Further study should be conducted to understand the effect of metal oxides in PCM for different concentration levels. It is worth mentioning that the reduced module temperature greatly influences in increasing the efficiency of the PV solar system.

As seen in Fig. 6, the efficiency of the conventional PV module is 12.29% under the radiation of 759 W/m2 at 12:00 pm. Under a similar condition, the PV system integrated with ZnO/PCM shows an electric efficiency of 12.91% with a filling factor, FF = 76.65%, an open circuit voltage, Voc = 20.2 V, short circuit current, Isc = 2.29 A. The high value of the electrical efficiency observed for the PV system integrated with ZnO-doped PCM demonstrates that the ZnO/PCM integrated PV system has the potential to effectively overcome the adversative impacts of module temperature. Nada et al.,27 observed the maximum decrease in the temperature of about 10.6 °C and the increment in the efficiency of approximately 13.2% for Al2O3-doped phase change materials than without PCM.

Figure 6
figure 6

Electrical efficiency of different PV systems under the radiation of 759 W/m2 at 12:00 pm.

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Figure 5 reveals that the module temperature decreases from 52.8 to 49.8 °C indicating 5.68% reduction in temperature due to the integration of 1% ZnO-doped PCM with the PV system. It is seen from Fig. 6 that the electrical efficiency of PV and PV integrated with ZnO-doped PCM is 12.29% to 12.91% indicating a 5.04% increment in PCE. The PV system with PCM shows limitation in achieving the highest system efficiency. The heat absorption by PCM is slow due to its low heat conductivity. In contrast, adding ZnO in PCM increases heat conductivity resulting in a significant improvement in system efficiency. Implementation of ZnO/PCM integrated PV system effectively overcomes the negative impacts of module temperature and hence, enhances its electric efficiency. Table III summarizes on temperature reduction and electric efficiency increment obtained from different studies. In every case26,27,28,29,30 presented in Table III, the reduction of module temperature works as an essential contributing factor for enhancing electrical efficiency. An increment of module temperature after crossing a crossing a special value causes a reduction in open circuit voltage and a slight increment of short circuit current. The reason for the slight increment of short circuit current with temperature could be owing to the decrease of bandgap energy and the increased number of photons with the required energy for creating electron–hole pairs. However, the open circuit voltage decreases as an increment of temperature is perceived. Senthilraja et al.31 has reported that the diffusion length of semiconductors at higher temperatures increases due to more output current and less output voltage and ultimately, electrical efficiency decreases. Other, important factors that may influence PV efficiency are air temperature, wind speed, solar peak time, zenith angle, air-mass, dust level, slope angle, and the PV materials. It is worth mentioning that wind speed positively impacts on the electrical efficiency of a PV system as it helps reduce temperature. The present study is carried out using clean solar panels. But practically the solar modules during energy production could be exposed to dust, rain, etc., along with different wind speeds, air temperature, and air mass. Therefore, it is important to conduct further studies to analyze the impact of these factors on solar module performance.

TABLE III Comparison of temperature reduction and electrical efficiency increment for different cooling systems reported in literatures.
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Brief economic assessment

It is important to conduct an accurate economic assessment for understanding the reliability of the developed PV cooling system. Though it is difficult to present a precise payback period of the PV system integrated with ZnO-doped PCM at this very early stage, some indications on the used materials and their relative expenses are provided and explained here. In the current study, two copper cooling frames were used to make PV/PCM (PV integrated with PCM) and PV/PCM/ZnO (PV module was integrated with ZnO-doped PCM) systems. The costs of the locally available copper tube and PCM (paraffin wax) used in these two cooling frames were $16 and $12, respectively. The high-temperature adhesive (glue), silicone sealant and steel bars required for the frames were also locally available at a very low cost of $20. The cost of ZnO used in PV/PCM/ZnO system was $6. The total costs incurred for developing the cooling systems (excluding PV cost) for PV/PCM and PV/PCM/ZnO were $24 and $30, respectively. Each PV having surface area of 0.3618 m2 was of 50 W maximum power. Therefore, the estimated total cost for thermal regulation of PV/PCM/ZnO system was approximately $83/m2 of the PV area or $0.6/W of the PV power. The estimated total cost of PV/PCM cooling system (excluding PV) reported in literature32 was $139/m2. In comparison, the estimated cost of the current PV/PCM/ZnO cooling system is approximately 40% lower.

Conclusions

The conclusions drawn from the primary outcomes of the given study are listed subsequently:

  1. (i)

    The copper multi-pipe cooling frame filled with ZnO/PCM has the potential to efficiently reduce the temperature’s adverse impacts and hence, improving the PV system’s energy conversion efficiency. This could be attributed to the faster heat transfer by natural convection and conduction mechanisms from the module to the surrounding through the passive cooling frame attached.

  2. (ii)

    The PV system integrated with ZnO/PCM shows an electric efficiency of 12.91% with 76.65% fill factor, 20.2 V open circuit voltage, and 2.29 A short circuit current. It reduces around 5.68% of module temperature and increases approximately 5.04% of electric efficiency than the conventional PV system.

  3. (iii)

    The enhancement of solar electrical efficiency is predominately owing to the increased output voltage, which is more likely caused by reduced diffusion length in semiconductors due to the drop in temperature. The maximum cell temperature of the PV system is decreased from 52.8 to 49.8 °C due to the attachment of a pressed copper cooling frame filled with ZnO-doped PCM.

  4. (iv)

    The total cost incurred for developing the PV/PCM/ZnO cooling system (excluding PV cost) was $30. It indicates that the estimated cost for developing ZnO-doped PCM cooling system is approximately $0.6/W of the PV power or $83/m2 of the PV area.

Recommendations for future work

The major challenges that need to be addressed for making the PV cooling system more reliable and attractive are listed below.

  1. (i)

    Further study should be conducted to explore the accurate payback period of the PV system integrated with ZnO-doped PCM. To assess a comprehensive techno-economic analysis, a year-round experimental and numerical study considering different environmental factors should be conducted in future work.

  2. (ii)

    Nano-enhanced PCM stability and their physical properties, selection of PCM with appropriate melting point, thermal contact between cooling frame and PV back surface are challenging work and need further research.