Keywords

1 Introduction

In the era of growing civilization and industrialization, the requirement of energy is growing. Due to this, the demand for the renewable energy and consequently the solar photovoltaics has played a greater role in the energy generation. However, till date, the technological development of photovoltaics is able to achieve a highest efficiency of about 15–20%. Moreover, the part of the irradiation not converted to electricity increases the temperature of the photovoltaics and the efficiency decreases with increase in the temperature. Thus, it is essential to remove heat from the photovoltaics to optimize the electricity generation. For this objective, researchers are continuously investigating and developing technologies for controlling the temperature of the photovoltaic panels. Some of these are heat exchangers, water sprinklers, phase change materials, heat sinks, transparent coating, etc.

The present article presents a concise review of the latest developments made in this field of research for obtaining the best suitable cooling technology. Along with this a greater emphasis is given to the application of the phase change material (PCM) for cooling of PV cell. The comparison of different ways to use PCM for this application is presented.

Sunlight is one of the perpetual renewable energy sources which is converted into electrical energy through the photovoltaic (PV) system. These photovoltaic systems are gaining popularity for electricity generation because of their anti-pollution technology, no effect on global warming, low cost of operation and low maintenance. As already described, a big problem in case of the photovoltaic is the reduction in efficiency with increase in temperature. For each 1 °C temperature rise of the photovoltaic module surface, there is an efficiency reduction of 0.5% [1].

The above discussion leads to the conclusion that there is a strong need for a suitably designed cooling technology with least power consumption for the optimum temperature of the photovoltaic module and best possible efficiency. Thus, the present article presents an discussion on the current trends of the cooling technologies being investigated for cooling of PV modules and more emphasis is given on the use of phase change materials for this purpose.

2 Review of Other Techniques for Cooling of Photovoltaic

Floating-Tracking-Cooling and Concentrating (FTCC) is a concept applied to get the maximum performance of PV modules. In this arrangement, the modules are arranged over the structures made out up of PVC pipes in a water pool. For cooling purpose, the water sprinklers are used to spray water on the panels for cooling purpose. Due to the water spray, the power production increased by 40%. However, the water sprinkling is capable to decrease the temperature of the certain areas of the photovoltaic modules. Another method adopted for cooling of the PV modules is the hybrid solar PV and thermal methodology in which the cooling fluid is usually air or water. The heat recovered by the air or water is used for the domestic purpose. Akbarzadeh and Wadowski [2] have adopted a similar method and observed an increase in the power output by almost 50%. It is also observed that the maximum temperature could be maintained at 46 °C when allowed for power generation for a time span of 4 hour [2]. Tonui and Tripanagnostopoulos [3] have investigated the performance of PV modules with air as the cooling fluid under natural and forced convection conditions. They observed that the arrangement with fins on the panel has performed best considering air as the working fluid [3].

3 Cooling of Photovoltaics Using PCM

By considering these problems facing in case of solar photovoltaic cell efficiency, so many works were carried out in order to solve the problems stated and observed that cooling is needed for solar photovoltaic cell as like heat exchanger, to minimize wasted solar radiation and high system temperature. Less numbers of authors have published an extensive literature review on cooling solar PV modules by using various techniques which can increase the overall efficiency of the solar conversion system by implementing these techniques, we can reduce the unwanted temperature rise. Some cooling techniques are Floating-Tracking-Concentrating-Cooling system (FTCC), heat sink, cooling by spraying water, forced water circulation, forced air circulation, water immersion cooling technique, use of phase change materials (PCM), cooling by transparent coating (photonic crystal cooling) and thermoelectric cooling.

Phase change material can be used as a latent heat storage in the temperature range 0–120 °C for low temperature applications like solar cooling, etc. [4]. Duffie and William [5] have suggested to select such a phase change material which do not have a tendency to supercool. In a suitable temperature range, material undergoes in a phase change and absorbing a large amount of latent heat is known as phase change materials (PCMs) which can used for a passive heat storage [5]. Ismail and Goncalves [6] have stated PCM as a latent heat storage unit by changing its states form solid to liquid and regaining it back without any external sources [6]. Wirtz et al. [7] have formulated the performance of a dry PCM and resulted positive use of PCM for more heat storage and temperature control of electronics. Without showing adequate change in temperature, the thermal energy can be stored into latent energy by heating and cooling of phase change material. The stored energy can be repaired when the process is overturned. Due to the high latent heat of phase change material during phase change process, it is popularly used in such thermal energy storing process.

Sarı and Kaygusuz [8] have used lauric acid with 95% of purity as a phase change material which has a small temperature change and no subcooling during the solidification and they studied temperature distribution and thermal characteristics of phase change material [8]. Khodadadi et al. [9] and Kibria et al. [10] have reviewed thermo-physical properties of phase change material with dispersed several nanoparticles. Some reviews has been concise on thermal energy storage capacity, long-term stability, encapsulation, temperature range and system-related issues of phase change materials [11, 12].

Waqas and Jie [13] have investigated numerically phase change materials’ effect on cooling of photovoltaics in the hot climatic condition in Pakistan during hottest month of summer. They carried out the computational work using the enthalpy method for the melting of PCM and taking mathematical model for the PV panel. It has been observed that the peak module temperature can be lowered by maximum 30 °C which increases the efficiency of the panel by 9–10%. They concluded that the performance will be best when there is a 10–12 °C temperature difference between the melting temperature of the PCM and the atmospheric temperature. Figure 1 shows that with the increase in the melting temperature of PCM, the peak temperature of PV panel decreases. Moreover, higher the thickness of the PCM, the lower will be the peak temperature [13].

Fig. 1
figure 1

Melting temperature effect [13]

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Energy balance equation for the solar cell is given in Eq. (1).

$$S*\alpha \tau *\beta_{c} + S*\alpha \tau *\left( {1 - \beta_{c} } \right) = U_{T} *\left( {T_{c} - T_{g} } \right) + U_{T} *\left( {T_{c} - T_{bs} } \right) + E$$
(1)

In Eq. (1), the first factor represents the solar energy absorbed by the solar cell after transmission, second factor represents the solar energy absorbed after transmission, third factor represents conductive heat transfer between glass and solar cell, fourth section represents rate of energy conducted from solar cell to the back surface of the module, and E suggests the rate of electrical energy available from PV module.

The rate of thermal energy transferred from the conventional PV cell to the back surface of the PV module and then transferred to the ambient can be obtained by following equation

$$U_{T} *\left( {T_{bs} - T_{c} } \right) = h_{w} *\left( {T_{amb} - T_{bs} } \right)$$
(2)

where the rate of energy conducted from solar cell to the back surface of PV module is equal to the rate of heat transferred from the back surface of ambient.

Sainthiya and Benewal [14] have carried out an experimental investigation studying effect of front surface cooling of PV panels by flowing water for different flow rate conditions. During their experimentation, a thin layer of water is allowed to flow over the PV panel from top to bottom. The power output and the efficiency are observed in both summer and winter weather conditions under four flow rate conditions (1–2.5 LPM). They observed an increase in efficiency of about 20–40%. It was observed that in winter the water cooling is more effective than the summer. Figure 2 shows the variation of the module efficiency during the whole day in both winter and summer for a different water flow rate.

Fig. 2
figure 2

Effect of water flow cooling or PVT [14]

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Ahmed et al. [15] have investigated photovoltaic in combination with thermal heat recovery by using water flow at the back surface of the photovoltaic cell in a hot climatic condition of Egypt. The efficiency and the power output showed an improvement because of the water cooling arrangement. They evaluated the overall efficiency of the system by considering the thermal heat gain. One important point needs to be mentioned here that the electrical power consumption for the pump used for the flow of water. The variation of the peak temperature is shown in Fig. 3.

Fig. 3
figure 3

Comparison of the front and back surface temperature of the photovoltaic cell for (1) only PV cell and (2) PV cell with water cooling (PV/T) [15]

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$$\eta_{{{\text{tot}}}} = \frac{{Q_{{{\text{th}}}} + P_{{{\text{ele}}}} }}{{P_{{{\text{act}}}} }}$$
(3)

Abdollahi and Rahimi [16] have investigated a cooling technology for photovoltaics by using water and PCM. The water is allowed to flow at the back side of the PV cell to receive heat. The hot water exits from the cooling path is allowed to pass through a helical coil present in a cylinder for receiving heat by placing phase change material. The PCM consists of 82% coconut oil and 18% sunflower oil leading to a melting temperature of 25–26 °C and latent heat of 308 kJ/Kg. The schematic of the setup is presented in Fig. 4. They compared the performance of the arrangement with that of only PV by using a term COE which is the efficiency considering the power consumption by pump.

Fig. 4
figure 4

Properties of a PCM desired for photovoltaic thermal regulation

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As per the studies, 25–28 °C is the suitable operating temperature of PV cell which provides 80–90% of efficiency. By violating this range and constant solar radiations, it results decrease in conversion rate and efficiency by 0.5% per each degree Celsius rise of surface temperature [17].

Heng et al. [18] have decreased the operating temperature and increase in efficiency of solar panels by using phase change materials (PCM) and developed a 2D finite volume heat transfer model for framing integrated photovoltaic cell with the use of PCM.

Hachem et al. [19] have experimented and stated that pure PCM enhance electrical performance of PV cell by an average of 3%, where transient energy balance presented to analyse thermal behaviour and an average of 5.8% improvement of efficiency while using combined PCM. The thermal conductivity of phase change material which is 0.18 W/m K is enhanced by combining mass ratio of 20% copper powder and 10% of graphite powder with 70% of pure PCM and the enhanced thermal conductivity will be 92.1 W/m K (Table 1).

Table 1 Thermal properties of pure and combined PCM
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A PCM (paraffin-based) with 38–43 °C of melting range is integrated at the backside of the solar PV panel and its cooling effect is monitored. The increased PV power output due to cooling produced by PCM is quantified and PV annual electrical energy enhanced by 5.9% in the hot climatic condition [20].

At an adequately constant temperature, PV panel is maintained with the help of high latent heat capacity of PCM. The stored heat later can be used for water heating, space heating and many more. It is viewed that forced air and water cooling techniques are widely used to cooling PV panels as compared to natural ventilation-based cooling as an inadequate method. Without any additional electricity consumption, PCM has the advantages to delaying the temperature rise of PV panels.

Japs et al. [21] have experimented by considering PV with and without PCM and resulted that the generated energy by the panel with PCM is higher than the panel without PCM for 5 out of 25 days while with PCM+ graphite-PV. They got the results that the average energy and economic yields were positive at peak temperature means at the afternoon while it is negative for the rest. Later, Japs et al. [22] have concluded that PCM rigged with graphite, accomplished better than that have not with it. As graphite improves the conductivity of the phase change material which causes expeditious dissipation of heat results better output.

Thermal management performance and the heat transfer characteristics of dry-packed MEPCM were investigated by Tanuwijava et al. [23] via CFD simulations. And stated that by consolidating the appropriate MEPCM layer in PV module gives improvement in thermal and electrical performance as the output. Ho et al. [24] have concluded that the layer thickness with 5 cm PCM has the best PV cooling performance with a melting temperature of 30 °C. Though, PCM could not fully solidify at night after certain thickness causes poor cooling performance. Experiment was carried out by installing microencapsulated phase change material (MEPCM) which is water insulated, attached to the back of PV panel and floats on a water surface (Table 2).

Table 2 Various techniques used for solar PV cooling
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A noticeable drop in temperature results efficiency improvement while cooling the solar PV module using PCM technique. PCM being restrained by many researchers due to its higher cost for PV module cooling application. Pork fat, a cheaper phase change material have been numerically used by Nižetić et al. [30] and they have developed a numerical model by showing comparison study between conventional PCM and pork fat for cooling PV models. However, not much convincing results have been found, but concluded that the pork fat can be used as a potential PCM. 10–26 °C temperature drop and efficiency increases up to 3.73% have been resulted by Arıcı et al. [31] by developing a numerical model and analysing the performance of PV module cooling using phase change material technique. Many researchers have also used PCM as a cooling agent to improve the efficiency of hybrid PVT. Preet [32] has used water-based PCM cooling technique for cooling of PVT system (Table 3).

Table 3 Summary of current research trends
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Some research articles are mentioned in Table 4 by the use of different types of PCM. They are petroleum jelly, coconut oil and palm oil and RT35.

Table 4 Types of PCM used in some research studies
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The above discussion expounds the use of phase change materials in different ways for the cooling of photovoltaic modules such as directly on the back side, combined PCM, PCM with fins, PCM as secondary heat recovery material, etc. It is also observed that different types of PCMs are also used for different ambient conditions and application methodologies. Still, further research is needed to design and develop a passive cooling system for the photovoltaic modules which is possible by using phase change material.

4 Conclusion

Low heat dissipation rate is the major affecting parameter which increases the temperature of solar PV panel and decreases the system efficiency. Various cooling techniques have been investigated by many researches resulted in the enhanced system efficiency. Latest researchers stated some advantages and disadvantages which is listed in the conclusion section.

  • PCM can store large amount of heat at a tiny temperature change

  • Changing of phase can occur at stable temperature

  • Further, the utilization of absorbed heat can be done

  • Maintenance free

  • During off sunshine also PCM can works

  • High cooling capacity

  • Absorption capability of the material degrades over time

  • May not attain the same output during hot and cold climate

  • Disposal problem after life cycle

  • Due to toxic nature of some PCMs cause fire safety issue

  • Segregation reduces active volume for heat storage.

Future research must focused on to find suitable techniques for cooling a different solar PV module. Finding of suitable phase change materials will also be a challenging work, and of course, the further utility of harvested heat from PCMs in a broader way.