Introduction

One of the greatest important sustainable energy resources is the solar energy which could be utilized in various applications as solar cell, solar heating, solar cooling, heating, and power systems [1]. The cell system is able to alter the energy from solar to electrical by means of photovoltaic system. In solar cells, only twenty percent of the falling solar energy is altered to electricity, nevertheless, greater than fifty percent is transmitted to excess energy as heat. The cell temperature rising is a result of the reduction of both cell efficiency and cell power. Furthermore, the extra heat in diverse positions in the cell can influence the cell silicon layer and yields a small lifetime of the cell system. Consequently, both uniformity and cell temperature must be adjusted by utilizing an appropriate cooling system. The system of cooling could be categorized into active and passive systems based on the inlet power utilized in the system of cooling. Passive cooling systems don’t need inlet power, however, they have low cooling performance whereas active systems need inlet power to carry out the cooling process. Several studies have been investigated for diverse systems of cooling for the solar cells due to the notable impact of utilizing the system of cooling on the cells performance. Moreover, utilizing the cooling in enhancing the performance and working of concentrated photovoltaic (CPV) has paid attention to the sights recently due to the notable influence of solar cell working temperature on its performance [2]. It’s noticed that cell efficiency is reduced by nearly 0.5 percent when the temperature of the cell augments one degree [3]. Besides, the distributions of temperature over the cell which impacts the temperature homogeneity of the cell have a strong influence on its life and performance [4]. The non-homogeneity of the cell causes a decrease in the cell performance attributed to the output power loss from the cell. Moreover, thermal fatigue can be occurred because of thermal stresses augmentation. Therefore, increasing the hot spot within the cell can damage it [4, 5]. Many investigations and researches that deal with the cooling of solar cells have been numerically and experimentally investigated. They stated that the more effective techniques for cell cooling lead to good homogeneity and low temperature of the cells with lower power of pumping for active cooling systems yielding higher cell performance [6].

Various cooling techniques that are active or passive have been applied to enhance the cells performance. As examples of techniques of passive cooling are cooling by applying energy storage of phase change materials (PCMs) [7, 8], cooling by means of heat spreaders and/or heat sinks [9,10,11], immersion or submersion the cells in low-temperature fluids [12,13,14]. Active cooling requests additional compresses air or power of the pump, etc. Active techniques have greater performance in cooling than passive ones [15]. However, cooling by means of water spraying and jet impingement [16,17,18], cooling employing medium flowing through ducts or channels [19,20,21,22], cooling by microchannels (MCs) [4, 23, 24] are some application of passive techniques of cooling. The simple technique of cooling of a concentrated cell at 50 concentration ratio (CR) by applying an aluminum flat plate was suggested by Kenji et al. [25]. They revealed that the applied system has about 10 °C inferior to the traditional system without a flat plate. Additionally, Araki et al. [26] suggested a simple cooling structure of concentrated cell employing a heat sink (HS). It is established that HS technique of cooling reduced the cell temperature by about 21 and 18 °C at CR of 400 and 500, respectively. A cell cooling by applying a cotton wick structure and HS was developed by Chandrasekhar et al. [27]. The cell temperature is reduced by around 6 °C when utilizing the suggested system of cooling. Cell performance by utilizing HS was investigated by Soliman et al. [9]. They specified that applying HS cooling elevated the efficiency of cell and cell power by 9 and 10 percent, respectively. Serag and Kandil [28] presented numerical and experimental work on the cell cooling by using a perforated cell. They stated that the perforated cell temperature is smaller than the cell of the non-perforated form. Additionally, there was a critical value of the diameter of the holes in the perforated cell where his value is lesser or greater than this, at which the cell temperature augmented.

Heat pipe (HP) is one of the passive techniques of cooling that used in the cooling of high heat fluxes systems such as electronic devices [29,30,31], computer components, and recently, solar cells [32]. The flat heat pipe (FHP) [33, 34] is a closed solid domain which its inner surface contains wick structure. Flat heat pipe transfers the energy from a heat source and dissipates it to sink of heat by utilizing the working fluid of the heat pipe through a phase change process. The working fluid inside the wick absorbs heat from the source of heat at the evaporator where it is evaporated, and the created pressure force due to the evaporation process drives the vapor from the evaporator section to condenser section. Then, the vapor condenses at the HP condenser section and released its heat to the heat sink. With the help of the capillary force generated in the wick structure, the condensate liquid is pumped again to the HP evaporator section [32]. Recent studies on using the heat pipes in the solar cell cooling systems have been demonstrated. Tang et al. [35] proposed a cooling technique of solar cells by utilizing a micro-HP array. The condenser section of the heat pipes was cooled by using air and water. In comparison with uncooled cells, the cell temperature lessened by 4.7 °C and 12.7 °C when using air-cooled and water-cooled HPs, respectively. Additionally, the cell efficiency increased by 8.4% for air-cooled system and 13.9% more than the air-cooled system for water-cooled system. Gang et al. [36] developed a theoretical model for photovoltaics thermal (PV/T) model that cooled by HP which transfers the dissipated energy into useful thermal energy. They studied different parameters affecting the system performance, solar absorber coatings, and flow rates. They informed that the cell performance increases with increasing the water flow rate. Anderson et al. [37] examined the performance of utilizing copper and aluminum finned heat pipes in the cooling of the concentrated photovoltaic (CPV). Their findings revealed that heat pipes can be used effectively in the cooling of CPV. The performance of the cooling system of pulsating HP (PHP) for solar cells was studied theoretically by Alizadeh et al. [38]. Their findings proved that the cell temperature reduced by 16.1 °C when PHP cooling system is utilized while the cell temperature decreased by 4.9 °C when a solid copper heat sink is utilized. Furthermore, the cell electrical power increased by 18% at solar radiation 1000 W m−2 when PHP cooling system is used. Modhinou et al. [39] compared the PV performance coupled with encapsulate phase change material system PV/PCM, PV with micro heat pipe system PV/MHP and conventional PV/T. They found that PV/PCM system has the best performance figures for thermal, electrical and combined efficiency with 28.7%, 7.9% and 36.7%, respectively. Moreover, PV/MHP system thermal, electrical and combined efficiency is 27.7%, 7.7% and 35.5%, respectively and the conventional PV/T system efficiencies are 23.9%, 7.8% and 31.7%, respectively. The cooling of an unconcentrated solar cell by using water-cooled heat pipe was carried out by Du et al. [40]. The cell temperature was reduced by 47% when the water-cooled heat pipe was used. Akbarzedh et al. [41] investigated experimentally the heat pipe cooling system for CPV. Their results stated that the output system power is augmented by 50% and the cell temperature did not exceed 320 K.

From the preceding literature review, it can be stated that there are some researches examined the solar cells cooling by utilizing HP, but few works have considered the HP performance as a passive technique for cooling the cells. Moreover, to the best of the authors survey, no former work has been presented for the flat HP (FHP) design and operating parameters on the cooling system performance of the concentrated cells. Therefore, the current study investigates experimentally the performance of FHP cooling technique on the concentrated cell cooling at diverse condenser and adiabatic regions lengths and different concentration ratios that was not presented earlier. The HP condenser region of the present study is cooled by using a HS with forced and free convection air cooling of the HS which also not considered before. Different incident radiation intensities are applied to the cell by using a solar simulator. The advantage of this cooling system (HP plus HS) is that it is simple and has not any excess cost especially for free convection air cooling of the heat pipe condenser except the initial cost of the HP. Moreover, using the flat heat pipe on cell cooling transfers the extracted heat from the cell to a distance far from the surrounding of the solar cell. The influence of the studied previous parameters on the cell temperature, efficiency, output power, heat pipe temperature and I–V characteristics of the cell is offered.

Experimental

Interior experiments are accomplished to examine the performance of a FHP for cooling the concentrated cell. The experimental setup used in this work as shown in the image of Fig. 1 and layout in Fig. 2 consists of a solar cell, flat heat pipe, solar simulator and heat sink. The solar simulator is used to simulate the solar radiation where it supplies the solar cell with the equivalent solar energy. The solar simulator consists of three halogen lamps each of 1000 W, and they are utilized to represent sun solar radiation. The halogen lamps are enclosed inside a reflector as shown in Fig. 1 to concentrate their power on the cell surface. The output power from the lamps is regulated by using a volt regulator to provide the cell with the required radiation intensity. The used solar cell is a polycrystalline cell with 6 V and 12.5 mA and dimension of 65 mm × 165 mm. Three flat heat pipes from AMECTHERMASOL company with 500 mm length and 50 mm width and 75 watts heat capacity are utilized. The material of HP is aluminum (Aluminum 1050), and the HP working fluid is acetone with groove wick structure of the heat pipe wick. The HP is used to cool the cell by transferring the heat from the source of heat (solar cell) to the HS (Aluminum Fins). The used HP length is 500 mm divided into 100 mm for the evaporator section (solar cell), and the rest 400 mm is divided as (1) 200 mm for the adiabatic section and 200 mm for the condenser section (2) 300 mm for the adiabatic section and 100 mm for the condenser section. Acrylic sheet with 5 mm thickness and low thermal conductivity is used to insulate the adiabatic section of the heat pipe. ELEGIANT manufacturer supplied the Aluminum finned HS that is utilized to remove heat from the HP at the condenser section which has a number of fins 18 and the dimensions of the HS are 180 mm × 100 mm × 45 mm. The heat is detached from the HS (HP condenser) by using natural or forced convection air produced by 12 V Dc fan. An air conditioner is used to fix the surrounding temperature around the experimental setup constant. The cell performance was inspected by measuring the cell temperature, output voltage and ampere. Moreover, temperature distribution over the heat pipe. Finally, the cell power and efficiency are calculated to inspect the cooling system performance.

Fig. 1
figure 1

Experimental setup image

Full size image
Fig. 2
figure 2

The layout of the experimental setup (dimension in mm)

Full size image

Measured values

During the experimental work, different measurements are carried out by using different instruments. K-type thermocouples are utilized to measure cell and HP temperatures. The cell temperatures are measured at its center area. The temperatures through the heat pipe length at a line passes through its center are measured at distances of 30, 100, 250, 350, 450 mm from the starting of the evaporator section as specified in Fig. 2. The output signals from the thermocouples are measured by using a data logger of type Squirrel SQ2040 Series GRANT, and the reading data from the data logger are registered on a laptop as stated in Fig. 1. The cell exit voltage is measured by utilizing the data logger, and the output current is measured by a multimeter. Pyranometer of mark LI-200R is utilized to measure the intensity of the falling radiation from the simulator. Moreover, the air speed at the inlet of the HS is measured by thermo anemometer of type PROVA AVM-03 Primo instrument. The measurements are carried out at steady-state conditions when the maximum difference among two successive temperatures of the supreme cell temperature is less than 10−3 °C s−1.

Uncertainty and error analysis

During this work, the uncertainty and error analysis is considered of the measured and calculated values. Moreover, all necessary precautions are considered to decrease as possible the errors yielding from changing the surrounding conditions and other related errors. For example, to carry out the experiments under the same ambient temperature, the ambient temperature surrounding the experimental setup is controlled by using an air conditioning unit. Table 1 indicates the uncertainty of the measuring instruments. Moreover, the uncertainty Δ of any value z as cell efficiency resulted from the experimental data is computed on the base of the next equation [42,43,44].

$$\Delta = \sqrt {\left( {\frac{\partial z}{\partial x}} \right)^{2} \Delta_{\text{x}}^{2} + \left( {\frac{\partial z}{\partial y}} \right)^{2} \Delta_{\text{y}}^{2} }$$
(1)

where the Δx uncertainty of measured value x and Δy of measured values y. Rely on the previous formula, the maximum calculated daily cell efficiency and power errors are found to be ± 0.6% and ± 0.10%, respectively.

Table 1 Uncertainty of the measuring instruments
Full size table

Thermal equations

The electrical cell efficiency [4, 45] is estimated by:

$$\eta_{\text{c}} = \eta_{\text{ref}} \left( {1 - T_{\text{ref}} } \right)\left( {T_{\text{c}} - T_{\text{ref}} } \right)$$
(2)

where \(\eta_{\text{ref}}\) and \(\eta_{\text{ref}}\) are constant which are supplied by the producer datasheet at the given reference temperature Tref = 25 °C and Tc is the average cell temperature in °C.

Electrical power of the cell module [4, 45] is estimated by:

$$P_{\text{e}} = \eta_{\text{c}} \tau_{\text{g}} GW_{\text{c}} L_{\text{c}}$$
(3)

where \(\tau_{\text{g}}\) is the glass transmissivity, G is the falling solar radiation in W m−2, and Lc and Wc, are the cell length and width in m, respectively.

Results and discussion

The cell performance is inspected under diverse solar radiation intensities changing from 500 to 3000 W m−2. The study is performed under constant evaporator region 100 mm length which equals the solar cell width and under two adiabatic and condenser regions. The first (1HS) represents 100 mm of the condenser region length which equals the HS width and 300 mm the adiabatic region length. The second (2HS) where the condenser region length represents 200 mm having a heat sink with the same width and the adiabatic region length is 200 mm. Additionally, the study is performed for forced and free convection airflow through the HS fins. The cooled cell performance by utilizing the HP is compared with the cell performance cooled directly by using free convection cooling over the solar cell surface at a radiation intensity of 500 W m−2 which is called “uncooled.”

Cell temperature and efficiency

Figure 3 illustrates the cell temperature change with falling incident radiation intensity for different cooling techniques. Moreover, the cell temperature for the uncooled case is superimposed on Fig. 3. This figure reveals that the cell temperature increases with rising the incident energy and for all studied cooling cases, the cell temperature is lower than the uncooled case. It also indicated that the forced convection cooling is more performant in cooling the cell than the natural convection cooling. Additionally, it is noted that the cell temperature of natural convection cooling with 2HS is not greater than the case of 1HS with forced convection. This means that decreasing the adiabatic region with increasing the condenser region increasing the cooling performance of the heat pipe. It is observed that increasing the incident energy 6 times for forced convection cooling augments the cell temperature by approximately 57%, and 53.7% for 2HS and 1HS, respectively. Using HP at 500 W m−2 incident radiation decreases the HP temperature by about 36.3%, 40.9%, 43%, 46.3% for natural 1HS, natural 2HS, forced 1HS and forced 2HS, respectively. The variant of the cell efficiency based on cell temperature from Eq. 1 with incident radiation at different cooling types is stated in Fig. 4. This figure demonstrates that the cell efficiency decreases with rising the falling solar energy because of rising the cell temperature despite the increase in the cell power as will be shown later. This signifies that the cell temperature is the dominant factor in the cell efficiency compared to cell power. It also demonstrates that the cell efficiency for forced cooling is greater than natural convection cooling due to reducing the cell temperature like stated formerly. Additionally, in case of using natural convection, cooling the cell with using HP raises its efficiency compared to natural convection cooling without heat pipe where it augments the cell efficiency by about 13.5% and 15.3% for 1HS and 2HS, respectively at incident radiation 500 W m−2. Moreover, using forced convection cooling of the heat pipe raises the cell efficiency by about 6.5% and 5.3% for 1HS and 2HS, respectively, compared to natural convection of the HP at incident radiation of 3000 W m−2.

Fig. 3
figure 3

Variation of cell temperature with incident radiation at diverse cooling rates

Full size image
Fig. 4
figure 4

Variation of cell efficiency with falling radiation at diverse cooling rates

Full size image

The HP temperature at different positions on the HP for different incident radiation and cooling types is shown in Fig. 5 for natural convection cooling and Fig. 6 for forced convection cooling. Figures 5 and 6 reveal that the HP temperature decreases from the evaporator region to the condenser region and the temperature through the adiabatic region is almost constant. They also indicate that the heat pipe temperature in case of forced convection cooling is lower than natural convection cooling. Moreover, it is noted that the heat pipe temperatures lines for the different cases are approximately parallel (maximum HP temperature difference is nearly the same). This signifies that the temperature drop through the HP length is about constant, and the impact of the cooling technique is on the overall heat pipe temperature. This is maybe due to the HP performance depends mainly on the internal HP properties) working fluid, wick type, HP cross section dimensions, region dimensions, etc.).

Fig. 5
figure 5

HP temperature at different positions for free convection cooling

Full size image
Fig. 6
figure 6

Temperature evolution of the HP at different positions for forced convection air cooling

Full size image

Cell power

The evolution of the output cell voltage with its output current at different loads (I–V curve) is shown in Figs. 7 and 8 in case of forced and free convection cooling, respectively. Figures 7 and 8 reveal that the output voltage decreases with increasing the output current depending on the cell load. They also indicate that the output voltage increases with increasing the incident radiation intensity and increasing the cooling rates of the HP condenser because reducing the cell temperature. Moreover, the output cell current increases with increasing the radiation intensity. The figures also illustrate that after a certain solar intensity, the cell current reaches its maximum value which depends on the tested cell characteristics. If the results of Figs. 7 and 8 are compared, it is stated that the I–V characteristic trend of the tested solar cell doesn’t change from forced and free convection cooling of the HS. However, the output voltage increases in case of forced convection compared to free convection due to reducing cell temperature as specified before and the cell output current is nearly the same for forced and free convection cooling. The theoretical maximum power of the cell calculated based on short circuit current × open-circuit voltage at different incident radiation energies is illustrated in Fig. 9. This figure indicates that the maximum cell power rises with increasing the incident radiation power and with increasing the condenser cooling. It is specified that increasing the radiation energy from 500 to 2000 increases the maximum cell power by about 172% and 134% for forced and free air convection cooling of the HS, respectively.

Fig. 7
figure 7

Variation of cell voltage with the current for forced cooling of the condenser

Full size image
Fig. 8
figure 8

Variation of cell voltage with the current for free cooling of the condenser

Full size image
Fig. 9
figure 9

Variation of theoretical maximum cell power output with incident radiation

Full size image

Figure 10 shows the output electrical power from the cell for different incident radiation intensities and at cooling rates based on the cell efficiency in Eq. 3. Figure 10 reveals that the cell exit power in case of cell cooled by using heat pipe for natural or forced convection air is greater than the uncooled cell (Free convection cooling of the cell without using HP). It also indicates that the cell power for forced convection cooling is greater than free convection cooling due to reducing the cell temperature as declared formerly and increasing the HP condenser region rises its output power (Output power is greater in case of 2HS than 1HS), and this effect increases with rising the incident radiation intensity on the surface of the cell. It is noticed that using HP in cooling the cell increases its output power by about 24.3% compared to the uncooled solar cell for incident radiation 500 W m−2. Moreover, using forced convection cooling with a double HP area increases the cell output power by about 9.1% compared to one HS at 3000 W m−2. Increasing the falling radiation intensity by 6 times increases the cell power output by about 5.4 times. By comparing the results of Figs. 9 and 10, it is found that the cell output power for forced convection cooling represents about 24.3% and 47% of the maximum cell power at 500 W m−2 and 3000 W m−2, respectively.

Fig. 10
figure 10

Variation of cell power output with incident radiation at diverse cooling rates

Full size image

Conclusions

An experimental work is carried out on the concentrated solar cell performance cooled by a flat heat pipe. The solar cell represents the heat pipe evaporator while the heat pipe condenser is cooled by using a finned heat sink with rectangular fins. The cell is subjected to different solar radiation incident simulated by a solar simulator. The study is carried out at different condenser and adiabatic regions of the HP and at forced and free convection air cooling of the HS. The findings illustrate that increasing the radiation intensity increases the cell temperature and decreases its efficiency. Using HP with natural convection cooled heat sink of 100 mm condenser length decreases the solar cell temperature and increases its efficiency by about 36.3%, and 13.6%, respectively, compared with natural convection cooling of the cell without using the heat pipe. Increasing the HP condenser region and decreasing its adiabatic region increase the cell efficiency and exit power and reduces its temperature. The trend of the I–V characteristic of the tested cell does not change due to rising airflow through the heat sink. However, the output voltage increases in case of forced convection compared to free convection rising the radiation intensity incident on the cell 6 times increases the cell power output by about 5.4 times. The impact of heat pipe length on the cell cooling performance can also be studied.