Overview on recent photovoltaic module cooling methods: advances PVT systems

Received Sep 18, 2018 Revised Mar 24, 2019 Accepted Jul 17, 2019 Renewable energy had been monopolized the research area in these past decade up till nowadays, due to its reliability and future in global production of electrical and thermal energy. Narrowing down the scope to the photovoltaic thermal (PVT) system, lots of improvements had been implied both theoretically and experimentally. One of the most attractive applications of PVT water or air-based collectors is building integrated photovoltaic thermal (BIPVT) system, which has undergone rapid developments in recent years. This review paper comprises the research findings on the improvements that had been integrated by PVT systems as well as well as personal and cited remarks on advancements on cooling techniques on PVT system.


INTRODUCTION
The dependence of human on technologies and augmented living ideals certainly lead to the rise of energy needs. Hence, as the energy demands needs to be fulfilled, consumption of fossil fuels is growing, resulting lots of climate and environmental issues [1][2][3][4][5]. The necessitate today kept flowing, expanding 2.4% annually, interest in renewable technologies which are growing fast [6]. In addition, rising concerns over climate change, environmental sustainability and security of supply have exerted pressure towards initiating reformation in the energy sector. Global efforts had focused on a transition towards sustainable energy provision. A substantial change in the global perceptions of renewable energy has been observed since 2004 and it has shown that their potential is achievable. Renewable energies had been improving and many technologies are at par with conventional energy generation technologies. There are lots of studies had been conducted on the energy demands which can be concluded in order to recognize on how the demands had contributed to the society, surroundings and the economy of one self's country. Solar energy is one of the sustainable energy which promises clean energy production. Regarding to limited conventional fuels, the implementation of solar technology is going into great advancement as one used to produce electrical energy using photovoltaic (PV) solar system. Furthermore, thermal energy can be generated which by utilizing working fluids in an integrated cooling system of PV; known as photovoltaic thermal (PVT) system. PV solar panel is a PV power generation system composes with series of silicon cells, interconnected and then joining together forming a circuit. PV solar panel results higher power output as the higher solar irradiances relatively absorbed. However, cloudy days will contribute to a decrease in sunlight absorption. It is because the clouds reflect some of the rays of sunlight and limit the sun absorption by the panels. An issue involving higher solar absorption which will also results to high temperature of solar panel itself,  16 resulting the efficiency of the panel in generating energy to decrease. This problem of efficiency's deficiency leaded to the improvement of solar energy technology, which the PVT solar panel had been introduced which had been advantaged as the one that can produce hot water with the same time as electricity as the system's operation is at lower temperature. The PVT system had improved the electrical deficiency of the PV system as heat from the panel had been extracted. The studies on PVT system had been anticipated by lots of researchers which intended to study on how it can contribute its performances in generating both electrical and heat energy [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].
Further theoretical approaches had been done such for the use of nanofluids as coolants as had been carried out by Mustafa et al [27] which a theoretical model was performed to evaluate system's performance while studied numerically a new configuration of the PVT system which includes the collector tube and working fluid. In this writing, it will be focusing on the advancements of cooling techniques with the employement of nanofluid, phase-change material (PCM) and heat pipe in PVT system.

CLASSIFICATION ON PV COOLING METHODS
These past years had discovered lots of researches on the advancements in the technology of solar energy in order to establish reliable energy source. The focuses in the studies can be classified into on how the improvements had been worked on the cooling system design and the working fluid used as the coolants for the systems of PVT solar collector.
By recognizing the abilities and contributions of PV system to one's country, tremendous research and study had been conducted in order to attain most advancement which may produce reliable and sustainable PVT system. The cooling system's design referred to the design of the absorber which mostly focused on water and air PVT solar system. Foremost, for air-based system had been developed through different absorber configurations, the air flow mode and using single or double pass design [28]. An optimization on single and double pass PVT system had been designed by Sopian et al. [29] which the performances had been analyzed for both the cases with varied flow rate of air as well as packing factor, collector length and duct depth. The study had concluded that the thermal efficiency generated by doublepass system was higher by 8% as it produced 32-34% compared to single-pass which produced 24-28% while the combined efficiency for double-pass also higher than the single-pass system, producing 30-35% and 40-45%.
As shown in Figure 1, PV cooling methods can be categorised into two types are: (i) base on cooling system's design, and (ii) base on type of coolants. PV cooling method using fluid such air, water, bifluid (air+water) known as conventional photovoltaic thermal (PVT) systems. PV cooling method using phase change material (PCM), nanofluid and heat pipe known as advances PVT systems.

PVT SYSTEM WITH PCM
The integration of phase change material (PCM) to the PVT system often related to the employment of it along nanofluids which acted as coolants. PCM acts to store heat energy. It absorbs sensible heat sensibly until it reaches melting temperature [30]. PCM has been engaged with nanofluid-based PVT to control heat capacitance of the system. This purposed to maintain electrical efficiency and to increase the overall efficiency in the same operation time. Higher efficiency had been attained by employing nanofluid due to the establishment of high thermal conductivity [31].
Delisle and Kummert [32] also stated the validation in assuming that more improvement of the PVT systems can be done by applying different design structures besides employing various materials. However, in the same time, they must be within an acceptable amount as to avoid cause poor costs of energy or extended pay-back periods. Beforehand, there were also more advance research in applying PCM as the coolant for PVT system. There were lots previous and current study on preparation, characterization, properties and applications of nanofluids. One had been conducted by Devinderan and Amir [33] where focusing on preparation of metal, metal oxides nanofluids and hybrid nanofluids as well as the methods applied in studying the features of them, both physically and chemically. An outlook had been conducted which focused on the uses of PCM for PV module thermal regulation and electrical efficiency improvement. Throughout the research, it can be described that the system may not be practicable in economical term to enhance PV conversion efficiency as the main requirement [34]. Beforehand, an indoor analysis as well as computational study had been carried out by Jay et al. [35] which focusing on the performance of PV-PCM system. A honeycomb structure made up of aluminum was used to capsulate the PCM which purposed to boost heat conduction. From the study, it was reported that there was improvement of about 18% in electrical efficiency compared to the stand alone PV panel.
An experimental study had been done by Huang et al.
[36] which validated numerical model of PV/PCM. The analyzed system designed by with and without fins, integrating with RT25 and paraffin wax used as PCM. It was confirmed that, the employment of designing fins to the system was significant on thermal management of PV/PCM. Meanwhile, the effect of PCM thickness on the temperature reduction of the panel module by Indartono et al. [37] had led to an observation of attaining optimum PCM thickness based on CFD simulation results among three PCM thickness. The optimum thickness was 80 mm among the three thickness variables which were considered.

PVT SYSTEM WITH NANOFLUID FLOW
A theoretical study had been conducted by Tyagi et al. [38] with the presence of aluminum/water nanofluids had been implied on direct absorption solar collector and the performance of it was being compared to conventional flat plate solar collector. The studied system involved enclosed space of fluid channel which in the same time, the bottom surface was perfectly isolated. It was also equipped with a transparent glass and it was concluded that small amount of solar irradiance, lost by scattering or transmission through the glass cover. Meanwhile, the major amount of it absorbed by the nanoparticles and converted into useful heat.
A research on direct absorption solar collector by utilizing nanofluids made from different nanoparticles also had been investigated by Otanicar et al. [39] in which they were carbon nanotubes, graphite, and silver. This had resulted an improvement in the collector efficiency up to 5% by utilizing nanofluids as coolants and in the meantime, the using nanofluids has a reduced reflectance as it acted as volumetric based absorption medium. Hence, the absorbance of heat had been increased which was higher paralleled to the surface based absorption.
Besides, thermal performance of a densely packed PV cells cooled by Al2O3/water nanofluid based cooling system also had been conducted by Xu et al [40]. Lee et al. [41] proposed a theoretical study on the feasibility of using plasmonic nanoparticles in which it was suspended in water of direct absorption solar collector in order to improve broad-band solar thermal absorption. Khanjari et al. [42] also had presented a theoretical study on the performance of PVT system, focusing to the effects of utilizing Ag/water and Alumina/water nanofluids as working fluids. The results had been shown that the thermal efficiency and the heat transfer coefficient improved by increasing volume fraction of the nanoparticles. By comparing to the pure water, 12% and 43% of maximum increment of heat transfer coefficients for alumina/water and Ag/water nanofluids were obtained.
Sardarabadi et al. [43] also had investigated and compared the effects of using purewater and silica/water nanofluids on PVT units. Two different concentrations had been prepared and tested at constant optimum mass flow rate, with a tilt angle of 32 o . Through economical assessment on both of nanofluids preparation and silica/water nanofluid suspension, PVT system's performances and exergy assessment of system had been improved. A study by Yun and Qunzhi [44] had employed film of Magnesium Oxide (MgO)/water nanofluid of different concentrations on top of PV cells. The evaluation of the system proved that the thickness of the film had influenced the system's output. Both energy efficiencies had been decreased at fixed light irradiance when thicker film had been used.

PVT SYSTEM WITH HEAT PIPE
Integration of heat pipe guaranteed high thermal conduction, allowing transfer of heat almost without any temperature drop. Gang et al. [45] had investigated on heat pipe PVT system which this system can be applied in cold states without worrying that it may freeze compared to the conventional water-based PVT system. Beforehand, a study in 1995 had confirmed that the thermal efficiency of a heat-pipe collector is comparable with that of a water based solar collector [46]. The implementation of heat pipes in solar collectors had preventing the freezing and backflow of the working fluid during night time. Hence, more stable operating conditions had been achieved [47].
Yang et al. [48] conducted a study of heat pipe which sodium was used as coolant. It had been concluded that the inclination angle and heat input influenced the thermal performance. The system had been able to exhibit good temperature uniformity and excellent thermal conductivity. Beforehand, in an investigation conducted by Boo et al. [49] on loop heat pipes filled with different ratios of sodium. They concluded that the fill ratio has an effect on thermal resistance, effective thermal conductivity, startup time and as well as isothermal characteristics.
Xia et al. [50] probed the impacts of the PVT module size on its performance in heating dominated residential building. The ideal PVT collector size for conducted research had been persisted through an economic analysis. The high initial investment caused the short-term economics even increasing the significance of the system's main design parameters optimization. Recent advance study on this heat pipe implementation had been carried out by focusing on designing optimization plan for ground source heat pump systems which was integrated with the PVT collectors. The study gap on the design optimization of hybrid ground source heat pump systems had been governed and computationally comprehensive [51].

CONCLUSION
PVT solar system had been such a demand during this recent years. Though, it can still be assumed that, the commercialization of this technology in being employed by the industries and communities, are still in trial stage. There are also much factors and main point that can be discussed such as the classification of solar collectors and as well as the gap or some boundaries to the implementation of them in the PVT solar system. By focusing on employment of nanofluids, PCM and heat pipe, even they promised better performances of PVT system, the advancements are still in the mid of lacking in discovery and cost of technology it self. As had been recommended and concluded for upcoming study on PVT, the first is to emphasize its thermal insulation and assess this improvement with the conventional model and by further studies alongside with the sensitivity analysis. Lastly, the PVT's operation in pairing with a liquid to liquid heat pump need to be focused on and by that, potential of PVT module in generating energy can be assessed.