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An experimental study on using natural vaporization for cooling of a photovoltaic solar cell
International Communications in Heat and Mass Transfer 65 (2015) 22–30
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
An experimental study on using natural vaporization for cooling of a photovoltaic solar cell☆ Morteza Ebrahimi a, Masoud Rahimi a,⁎, Alireza Rahimi b a b
CFD Research Center, Chemical Engineering Department, Razi University, Kermanshah, Iran Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Available online 20 April 2015 Keywords: Cooling Natural vapor Photovoltaic cell Efficiency Cooling enhancement
a b s t r a c t This study attempts to investigate a new way for cooling PV cell using natural vapor as coolant. The performance of solar cell was examined on simulated sunlight. The natural vapor encountered backside of PV cell vertically in various distribution and different mass flow rates. Also, the effect of natural vapor temperature in cooling performance was analyzed. Results indicated that the temperature of PV cell drops significantly with increasing natural vapor mass flow rate. In detail, the PV cell temperature decreased about 7 to 16 °C when flow rate reaches 1.6 to 5 gr min−1. It causes increasing electrical efficiency about 12.12% to 22.9%. The best performance of PV cell can be achieved at high natural vapor flow rate, low natural vapor temperature and the obtained optimum distribution condition. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With growing concerns about the damages to the environment caused by burning fossil fuels, and due to ever increasing oil demand, recently there have been numerous attempts to find out an energy source which can serve as an alternate energy for fossil fuel. Harnessing solar energy holds great promise for the world's energy demands, and it will be heavily called upon as fossil fuels are depleted. Photovoltaic solar cells (PV) are used to convert some part of solar energy to electricity and they are the best choices for utilizing solar energy. Another technology to utilize the solar energy is the hybrid photovoltaic/thermal (PV/T) technology. This technology can simultaneously generate electrical and thermal energies by integrating a photovoltaic cell and a solar thermal collector. In the recent years numerous studies were carried out to investigate the applications of solar technology. There are many papers that report the effects of temperature on the electrical efficiency of the PV cells [1–3]. The efficiency of PV cells decreases with increasing the temperature of PV module [4]. Researchers proposed practical ways to reduce the temperature of PV cell. Tripanagnostopoulos et al. [5] experimentally investigated the improvement in the electrical performance of a photovoltaic. It was achieved by using fins and naturally circulating air. Chen et al. [6] used the refrigerant R134a for cooling PV modules insulated by glass vacuum tubes. The PV panel was coupled with a heat pump system. Some researchers have shown an increased interest in the development of micro cooling technology for various applications ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author at: Chemical Eng. Dept. Razi University, Taghe Bostan, Kermanshah, Iran. E-mail addresses: [email protected], [email protected] (M. Rahimi).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.04.002 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
because of the ability of microchannel heat sink to remove a large amount of heat from a small area [7–11]. Karami et al. [12] used Boehmite nanofluid in microchannel heat sinks to achieve as high thermal performance as possible. In hybrid photovoltaic/thermal solar energy systems cooling the PV module can be combined with a useful fluid heating [13–15]. Raghuraman [16] performed two separate onedimensional numerical methods to predict the performance of PV/T flat plate collectors using water and air as heat carrier. They tried to maximize the energy extracted from the collectors. Teo et al. [17] employed an active-cooled hybrid PV/T system to cool down the PV cell. Their results indicated that active cooling improves the efficiency considerably. Tchinda et al. [18] studied theoretically thermal processes in a CPC collector with a flat one-sided absorber. They concluded that the selectively coated CPC with flat one-sided collector is more efficient than the black painted one with the same condition. Othman et al. [19] studied both theoretically and experimentally the performance of hybrid photovoltaic–thermal solar collector. They improved the performance of PV/T solar air collector by the use of a double-pass collector and fins. In another study [20], they designed a double-pass PV/T solar collector with fins and compound parabolic concentrator (CPC) to increase the performance and reduce the cost of photovoltaic electricity. The CPC was used to increase the radiation intensity falling on the solar cells. However traditional water-type PV/T collectors/systems are practical, but they are not used in regions with natural climates because the freezing of water to ice can break up the collectors. In the regions with natural climate, the integration of a heat pipe and a solar collector can be incorporated into a practical design for a PV/T collector [21]. Many other studies on PV/T systems have been conducted on heat pipe PV/T systems [22–25]. Jie et al. [26] represented numerical and experimental
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Nomenclature As PV cell area (m2) area covered with natural vapor (m2) Ah photovoltaic arrays current (A) IPV maximum power of PV arrays (W) Pmax Pcooling PV power by cooling (W) P without cooling reference power (W) PV photovoltaic Q mass flow rate (kg/s) PV cell temperature (°C) Tc natural vapor temperature (°C) Tv velocity (m/s) Uv photovoltaic voltage (v) VPV
investigations on the performance of the photovoltaic solar assisted heat pump (PV-SAHP). Their results implied that the PV-SAHP has a better coefficient of performance and photovoltaic efficiency in comparison with the separate units. Installation of panels on the river canals was done practically in some places [27,28]. Those studies showed that the canal-top solar power equipment produces 15% more power than the plant set up on land as the water vapor flowing underneath of the panels keeps the solar panels relatively cool and helps more power generation. The main objective of this research is to illustrate a new method to reduce the temperature of PV module and improve the efficiency of PV cell. In order to achieve the best performance of PV cell, the PV panel has been cooled using natural vapor as coolant. The vapor has a temperature in the range of temperature of natural vapor escape from rivers, water channels, etc. The effects of the natural vapor flow rate, natural vapor temperature and natural vapor distribution on the performance of PV cell have been discussed. Because of the limitations that exist in real states during the test, such as wind and variable in intensity,
Fig. 2. The thermocouple positions.
vapor flow rates and vapor temperature, all experiments are done in laboratory. Therefore the situations are controlled in each step to highlight the effect of parameters. 2. Experiment setup The schematic illustration of testing setup is depicted in Fig. 1. According to the figure, this system consists of natural vapor generator and photovoltaic systems. Solar simulator, natural vapor simulator and
Fig. 1. The scheme of experimental setup.
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PV cell are three basic components of this setup. The temperature measurement system and V–I measurement system were employed to measure and record data. The vapor simulator employed to simulate natural vapor that is vaporized from the surface of rivers and dams in laboratory scale. In order to distribute the natural vapor in the backside
of PV cell, 12 circulated outlet holes with a diameter of 8 mm were drilled on the natural vapor simulator. The PV cell dimensions were 290 mm × 325 mm. The backside of PV cell exposed to natural vapor at specific points with the same velocity. Three 400 W metal halide lamps (Reflector Sunlight Dysprosium Lamp, RSDL) were used to
Fig. 3. Influence of vapor flow rate on (a) the average temperature of the PV cell. (b) I–V curves during cooling process. (c) Output electrical efficiency.
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
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Table 1 The measured and average values of temperature, and maximum power of the cell module under different flow rate at constant temperature of vapor (Tv = 18 °C). Flow (gr min−1)
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
Tave
Pmax(w)
0 1.6 3 5
60 58.4 53.6 43.8
53.3 41 36.9 33.7
49.8 41.3 36.9 33.8
54.2 52.4 47.1 43.3
57 53.6 52.6 43.4
59.4 48.2 48.6 34.9
53.6 48.3 41.7 42.5
55.2 45.2 43.5 39.9
58 46.6 42.5 38.9
53.2 48.3 44.8 39.3
55.4 48.3 44.8 39.3
3.008 3.367 3.528 3.698
Table 2 The measured and average values of temperature, and maximum power of the cell module under different vapor temperature at constant flow rate of vapor (q = 5 gr min−1). Vapor temperature
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
Tave
Pmax(w)
Without cooling Tv = 18 °C Tv = 23 °C Tv = 38 °C
60 43.8 34.8 52
53.3 33.7 33.3 46
49.8 33.8 35.6 43.5
54.2 43.3 52 47
57 43.4 40.8 51
59.4 34.9 44.7 52.5
53.6 42.5 43.8 48.2
55.2 39.9 46 54
58 38.9 47.8 46.1
53.2 39.3 42 44.4
55.4 39.3 42 48.5
3.008 3.698 3.567 3.51
simulate solar radiation. These lamps produce more pure white light than the popular HPS lamps, close to daylight frequencies. The fabricated solar simulators are evenly loaded on aluminum heat sink plate. In order to measure the PV cell temperature, 10 thermocouples were fixed at 10 specific points on the surface of PV cell. The thermocouples were connected to a digital thermometer (Lutron, BTM-4208SD) to record the temperatures. The locations of thermocouples at the surface of PV module are shown in Fig. 2. To record the I–V data, an electrical load system plugged into the PV electrical output was used. 2.1. Experimental work These experiments have been carried out on various vapor temperatures and various mass flow rates to find out the effect of cooling on the performance parameters of PV cell. The constant intensity of incoming radiation was set at 1000 W m−2 during the test. All the experiments were conducted at steady state condition and temperature (32 °C) considered as an environmental temperature. The vapor simulator was operated at specified flow rates (0–5 gr min−1) controlled by the simulator during the tests. The flow rate q = 0 gr/min indicates the state without cooling. The vapor temperature was in range of 18– 38 °C. Ten K-type thermocouples were used to measure the temperatures at 10 separate points at the surface of PV module and measured the temperature of vapor. The I–V values were recorded at steady state condition, with electrical load system to determine the power of PV module and find the maximum power point (P max).
figure, it can be seen that in all recorded vapor temperature, increase in flow rates of vapor caused a sharp decrease in the average temperature of the PV cell. The reason for this reduction could be related to the higher velocity of vapor that touched the backside of PV cell in higher flow rates. Moreover, in specific period of time, the mass of vapor that stays in the backside of PV cell will be increased. Increase in the velocity of vapor results in higher heat transfer coefficient and increase in the mass of vapor causes higher heat transfer rate. More details were also presented in Table 1. As can be seen in this table, the variation of temperatures is comparatively considerable. A significant decrease in average temperature was ascertained for PV cell, by further increase of the flow rates of vapor. Particularly, the PV cell temperature decreased about 7 to 16 °C when flow rate reaches 1.6 to 5 gr min−1 by the reason
3. Results and discussion 3.1. The effect of natural vapor flow rate
Fig. 4. Variation of the average PV cell temperature with the numbers of vapor outlets.
The temperatures of 10 points on the surface of PV cell were measured in order to plot the variation of the average temperature at different condition. The average value of the temperatures of 10 specific points was considered as the PV cell temperature. Fig. 3a shows the average temperature of PV cell in different flow rates (0–5 gr min−1) and different vapor temperatures (18, 23, 38 °C). According to this
Table 3 The percentage increase in the PV electrical output in T (air) = 32 °C under different flow rates. Flow (gr min−1)
Tv = 18 °C
Tv = 23 °C
Tv = 38 °C
0 1.6 3 5
0 12.12 17.2 22.9
0 11.2 14.15 18.9
0 11.3 13.1 16.6
Fig. 5. The vapor velocity at various number of outlet streams.
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mentioned above. Fig. 3b shows the experimental I–V characteristics of PV cell at constant vapor temperature (18 °C) over the various ranges of the vapor flow rates. As illustrated in this figure, the area under the I–V curve increases at higher flow rates. Since the area under the I–V curve represent produced electrical power, the comparison between obtained results obviously shows a rise in output power by increasing the vapor flow rate. As mentioned above, increasing the vapor flow rate causes enhancement in heat transfer from PV cell and reduction of PV temperature, so the output power increases due to the reduction of PV cell temperature. Fig. 3c illustrates the results for different vapor flow rates at constant vapor temperature (18 °C). As can be seen in the figure, increase in vapor flow rates caused considerable enhancement in
electrical efficiencies. The comparison between results of output power obtained at high and low vapor flow rates could highlight the effect of vapor flow rate on the performance of PV cell in better way. According to Table 1, it is clear that for q = 1.6 gr min−1, the maximum power moved up from 3.008 to 3.332 W, and in the case of q = 5 gr min−1 this value attained from 3.008 to 3.698 W. Therefore, increase in the maximum power can be observed in two mass flow rates (1.6 and 5 gr min−1). While, increase in output power is noticeable in q = 5 gr min− 1. The percentages of increase in the electrical PV cell output at various vapor flow rates with the three different vapor temperatures are tabulated in Table 3. As shown in this table, the highest vapor flow rate had the best cooling performance. In detail,
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125
T4 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
T6 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A a) 4 outlet ( Ah = 0.12 , Uv = 0.103 m.s -1 ) s
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125 100 75 50
50
b) 6 outlet (
100
Ah As
=
150 Length of PV Cell (mm)
200
0.18 , Uv = 0.069 m.s -1 ) Fig. 6. Temperature contour plots of PV cell at various vapor outlet streams.
250
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
27
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125
T8 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A c) 8 outlet ( Ah = 0.24 , Uv = 0.052 m.s -1 ) s
T10 < 35 – 37 – 39 – 41 – 43 – 45 – 47 – 49 – 51 – 53 – >
200
Widt h of PV Cell ( mm)
175 150 125
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
Ah
d) 10 outlet (A
s
=
150 Length of PV Cell (mm)
200
250
0.3 , Uv = 0.041 m.s -1) Fig. 6 (continued).
for q = 5 gr min−1, increase in the performance was 10.78%, 7.7% and 5.3% higher, in comparison with q = 1.6 gr min−1, at Tv = 18, 23 and 38 °C, respectively. The maximum power calculated from Eqs. (1) and (2) was also used to compute increase in the maximum power. P max ¼ V m Im
%P increase ¼
P cooling −P witout cooling P without cooling
ð1Þ
ð2Þ
3.2. The effect of natural vapor temperature According to Fig. 3a, with decreasing the vapor temperature at specific flow rate, average temperature decreased due to increasing heat transfer between the PV cell and vapor. As illustrated in this figure, the maximum decrease in the average temperature of PV cell occurred at the flow rate about 5 gr min−1 and temperature of 18 °C. It is clear that the difference between the temperature of PV cell and vapor is the driving force in heat transfer, thus the highest difference caused highest heat transfer and consequently improved the efficiency of PV cell.
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M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
T12 < 35 – 37 – 39 – 41 – 43 – 45 – 47 – 49 – 51 – 53 – >
200
Widt h of PV Cell ( mm)
175 150 125
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A e) 12 outlet ( Ah = 0.36 , Uv = 0.034 m.s -1 ) s
Fig. 6 (continued).
Moreover, in order to show the effect of vapor temperature, the trend of changing in maximum power and temperature of surface PV module is presented in Table 2. It can be seen that the average temperature of PV cell drops from 48.5 to 39.3 °C, when the temperature of vapor decreased about 20 °C. In addition, the maximum output power increased about 5% when the vapor temperature decreased from 38 to 18 °C. By referring to Table 3, it is obvious that increase in vapor temperature has a significant effect on enhancement in efficiency in high vapor flow rate (5 gr min−1), whereas this effect can be negligible in low flow rate (1.6 gr min−1). The possible reasons for this behavior are that in high flow rate, the ability of heat transfer increases due to increasing the vapor velocity and the difference between the temperature of PV cell and vapor causes higher driving force in heat transfer. 3.3. The effect of natural vapor distribution 3.3.1. Average temperature of the PV cell Uniformity in temperature of PV cell surface caused considerable enhancement in electrical efficiency. The main purpose of the present section is to find the best situation for distributed natural vapor in the backside of PV cell, in order to decrease and make the surface temperature of PV cell uniform. Fig. 4 shows the module average temperature in different numbers of outlet points (number of vapor outlets that exposed backside of PV panel) in constant vapor flow rate (5 gr min−1) and various vapor temperatures (18 and 23 °C). According to Fig. 4, the module average temperature drops with increasing the numbers of outlets. However, the slope of falling in the cell temperature with growth in the numbers of outlets is reduced and finally is changed when the outlets passed over 8. This could be related to the influence of two main parameters, heat transfer surface and vapor velocity. The trends of changing velocity and heat transfer surface are presented in Fig. 5. It can be observed that rising the heat transfer surface and reduction in vapor velocity occurred simultaneously. It can be concluded that
increase in heat transfer surface for layouts less than 8 outlets has a convincing effect on cooling PV cell, whereas growth of heat transfer surface for more than 8 outlets has an inverse effect. The reason for this inverse effect can be related to importance of vapor velocity and the heat transfer area. In layouts with less than 8 outlets, the heat transfer increases due to more efficient vapor spreading. Although the velocity of vapor decreases, the surface of heat transfer plays the dominant role in the heat transfer and the effects of heat transfer surface are more significant in this case. In layouts with more than 8 outlets, increase in the numbers of outlet causes the sharp decrease in the velocity of vapor and the heat transfer rate decreases. Fig. 6 shows the temperature contour plot of PV cell versus the distances from thermocouples at constant vapor flow rate (5 gr min−1), and vapor temperature (23 °C). It can be seen that temperature distribution leads to become uniform and decreased when the numbers of vapor outlets increased to 8. The obtained results implied that for the case with 4 vapor outlets, the mean temperature is 46.39 °C and the standard deviation of recorded temperatures at 10 points from mean temperature is 5.12. Moreover, according to Fig. 6a, it is clear that most parts of PV cell surface were covered with regions with a temperature more than 45 °C. Also, maximum difference in temperature is about 20 °C. Fig. 6b shows that using of 6 outlets causes more efficient temperature distribution in comparison with 4 outlets. It can be seen that over 50% of PV cell surface have temperatures less than 45 °C. With increasing the numbers of outlets to 8 outlets, the mean temperature and the deviation have been recorded to be 40.91 °C and 4, respectively. Fig. 6c indicates that all regions of PV cell surface have temperatures less than 45 °C, except the small regions that have temperatures between 45 and 51 °C. In addition, maximum difference in temperature reduced about 5 °C in comparison with 4 and 6 outlets. Fig. 6d shows that increase in vapor outlets to 10, caused more regions with temperatures more than 45 °C. Furthermore, maximum difference in temperature reaches over 20 °C. After 10 outlets, increasing the numbers of outlets to 12 outlets, leads to rising the mean temperature
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
and the deviation to the values of 45.33 °C and 12, respectively. According to Fig. 6e it is obvious that cooling with 12 vapor outlets has the worst result. The deviation of obtained temperatures was calculated from Eq. (3):
δ¼
sX ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðxi −xi Þ n−1
ð3Þ
By the reason mentioned above, increasing the numbers of vapor outlets to 8 outlets has a beneficial effect on the cooling of PV panel and reduces the temperature of PV panel. However, when the numbers of outlets reach 8 outlets, ever increase in the numbers of vapor outlets has a conflicting effect on the cooling of PV panel. 3.3.2. Electrical performance Fig. 7 shows the increase in maximum power output of the PV cell in different numbers of separate points, constant vapor flow rate (5 gr min− 1) and two different vapor temperatures (18, 23 °C). It can be seen that the efficiency increases significantly at 8 vapor outlets compared with 4 vapor outlets. In detail, about 100% increase in the vapor coverage on the backside surface caused 7.3% increase in the output power. However, for the vapor outlets higher than 8, the efficiency begins to level off and decreases slightly with increasing vapor outlets. Regarding to the 8 outlets, the maximum output power in 10 outlets decreased 1.1%, whereas the vapor coverage on the backside surface increased about 50% and reduced velocity of vapor was 0.011 m s−1. As mentioned above, it could be related to increasing the heat transfer rate before 8 outlets and the reduction of PV cell temperature. This decision manifests that the performance of cooling PV cell with natural vapor was affected by some independent factors, which are the vapor temperature, the vapor flow rates, distribution of vapor and vapor velocity. 4. Conclusions This study introduced a new technique for cooling the PV cell, using the natural vapor as heat transfer fluid, to enhance the electrical performance. Therefore installation of panels on the river canals and other places where vaporization exists could improve the performance of PV panels. Moreover in this way of cooling performance doesn't need any external energy. It was found that, increasing natural vapor flow rate improved heat transfer from PV module. This caused enhancing the electrical efficiency. Based on the obtained data, it was observed that the natural vapor flow rate affected the average PV cell temperature. At low natural vapor temperature, the corresponding average temperature was 48.3 °C in low flow rate (1.6 gr min−1) and 39.3 °C in high flow
Fig. 7. Comparison between the maximum generated powers of various vapor outlets.
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rate (5 gr min−1). In this study, the best performance of the PV cell was achieved in low natural vapor temperature. This result indicates that the natural vapor temperature is one of the significant parameters which must be considered. Moreover, distribution of the natural vapor in the backside of the PV cell has an important effect on improving the heat transfer surface and enhancing cooling performance. Whereas, in constant natural vapor flow rate, with the growth of vapor distribution, the velocity of the natural vapor reduced and the heat transfer to natural vapor decreased. It can be concluded from this result that both distributions of vapor and velocity have a significant effect on cooling performance. In contrast, in this experiment the best PV cooling performance was found at high natural vapor flow rate, low natural vapor temperature and the optimum numbers of outlets (5 gr min−1, 18 °C and 8 respectively). Moreover, the worst case was the natural vapor in low flow rate and high temperature with minimum distribution of vapor. Finally it could be concluded that natural vapor temperature, natural vapor flow rate and distribution of vapor are independent factors which have an effect on the PV cooling performance.
References [1] E. Skoplaki, J.A. Palyvos, On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations, Sol. Energy 83 (2009) 614–624. [2] Y.B. Assoa, C. Menezo, G. Fraisse, R. Yezou, J. Brau, Study of a new concept of photovoltaic–thermal hybrid collector, Sol. Energy 81 (2007) 1132–1143. [3] R. Vaillon, L. Robin, C. Muresan, C. Ménézo, Modeling of coupled spectral radiation, thermal and carrier transport in a silicon photovoltaic cell, Int. J. Heat Mass Transf. 49 (23-24) (2006) 4454–4468. [4] S. Essalaimeh, A. Al-Salaymeh, Y. Abdullat, Electrical production for domestic and industrial applications using hybrid PV–wind system, Energy Convers. Manag. 65 (2013) 736–743. [5] Y. Tripanagnostopoulos, P. Themelis, Natural flow air cooled photovoltaics, 7th International Conference of the Balkan Physical Union, American Institute of Physics, Melville, N.Y, Alexandroupolis, Greece 2009, pp. 1013–1018. [6] Hongbing Chen, Saffa B. Riffat, Yu. Fu, Experimental study on a hybrid photovoltaic/ heat pump system, Appl. Therm. Eng. 31 (2011) 4132–4138. [7] M. Rahimi, E. Karimi, M. Asadi, P. Valeh-e-Sheyda, Heat transfer augmentation in a hybrid microchannel solar cell, Int. Commun. Heat Mass Transf. 43 (2013) 131–137. [8] P.-S. Lee, S.V. Garimella, D. Liu, Investigation of heat transfer in rectangular microchannels, Int. J. Heat Mass Transf. 48 (2005) 1688–1704. [9] J.H. Ryu, D.H. Choi, S.J. Kim, Three-dimensional numerical optimization of a manifold microchannel heat sink, Int. J. Heat Mass Transf. 46 (2003) 1553–1562. [10] H.A. Mohammed, P. Gunnasegaran, N.H. Shuaib, The impact of various nanofluid types on triangular microchannels heat sink cooling performance, Int. Commun. Heat Mass Transf. 38 (2011) 767–773. [11] O.M.P. Nitiapiruk, A.S. Dalkilic, S. Wongwises, Performance characteristics of a microchannel heat sink using TiO2/water nanofluid and different thermophysical models, Int. Commun. Heat Mass Transf. 47 (2013) 98–104. [12] N. Karami, M. Rahimi, Heat transfer enhancement in a hybrid microchannelphotovoltaic cell using Boehmite nanofluid, Int. Commun. Heat Mass Transf. 55 (2014) 45–52. [13] D.J. Yang, Z.F. Yuan, P.H. Lee, H.M. Yin, Simulation and experimental validation of heat transfer in a novel hybrid solar panel, Int. J. Heat Mass Transf. 55 (4) (2012) 1076–1082. [14] B. Moshfegh, M. Sandberg, Flow and heat transfer in the air gap behind photovoltaic panels, Renew. Sustain. Energy Rev. 2 (1998) 287–301. [15] A.K. Bhargava, H.P. Garg, R.K. Agarwal, Study of a hybrid solar system—solar air heater combined with solar cells, Energy Convers. Manag. 31 (1991) 471–479. [16] P. Raghuraman, Analytical predictions of liquid and air photovoltaic/thermal flat plate collector performance, Sol. Energy Eng. 103 (1981) 291–298. [17] H.G. Teo, P.S. Lee, M.N.A. Hawlader, An active cooling system for photovoltaic modules, Appl. Energy 90 (2012) 309–315. [18] R. Tchinda, N. Ngos, A theoretical evaluation of the thermal performance of CPC with flat one-sided absorber, Int. Commun. Heat Mass Transf. 33 (6) (2006) 709–718. [19] M.Y. Othman, B. Yatim, K. Sopian, M.N. Abu Bakar, Performance studies on a finned double-pass photovoltaic-thermal (PV/T) solar collector, Desalination 209 (2007) 43–49. [20] M.Y.H. Othman, B. Yatim, K. Sopian, M.N. Abu Bakar, Performance analysis of a double-pass photovoltaic/thermal (PV/T) solar collector with CPC and fins, Renew. Energy 30 (2005) 2005–2017. [21] P. Gang, F. Huide, Z. Tao, J. Jie, A numerical and experimental study on a heat pipe PV/T system, Sol. Energy 85 (2011) 911–921. [22] P. Gang, F. Huide, Z. Huijuan, J. Jie, Performance study and parametric analysis of a novel heat pipe PV/T system, Energy 37 (2012) 384–395. [23] P.G. Charalambous, G.G. Maidment, S.A. Kalogirou, K. Yiakoumetti, Photovoltaic thermal (PV/T) collectors: a review, Appl. Therm. Eng. 27 (2007) 275–286.
30
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
[24] H.M.S. Husseina, M.A. Mohamada, A.S. El-Asfouri, Optimization of a wickless heat pipe flat plate solar collector, Energy Convers. Manag. 40 (1999) 1949–1961. [25] H.P. Garg, R.K. Agarwal, Some aspects of a PV/T collector/forced circulation flat plate solar water heater with solar cells, Energy Convers. Manag. 36 (1995) 87–99. [26] J. Jie, L. Keliang, C. Tin-tai, P. Gang, H. Wei, H. Hanfeng, Performance analysis of a photovoltaic heat pump, Appl. Energy 85 (2008) 680–693.
[27] A. Chalaskuzhy, Gujarat goes green with solar panels on Narmada Canal, Global Council and Chairman at Global Renewable Energy Investment. BE(IT), MBA (Finance &Marketing) PSG Tech, Coimbatore India, 2012 (http://abhilashchalakuzhy.blogspot.com). [28] Solar panels as canal tops in India, Curenature.com (Nature care & AwarenessEvidence Based), 2013.
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
An experimental study on using natural vaporization for cooling of a photovoltaic solar cell☆ Morteza Ebrahimi a, Masoud Rahimi a,⁎, Alireza Rahimi b a b
CFD Research Center, Chemical Engineering Department, Razi University, Kermanshah, Iran Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Available online 20 April 2015 Keywords: Cooling Natural vapor Photovoltaic cell Efficiency Cooling enhancement
a b s t r a c t This study attempts to investigate a new way for cooling PV cell using natural vapor as coolant. The performance of solar cell was examined on simulated sunlight. The natural vapor encountered backside of PV cell vertically in various distribution and different mass flow rates. Also, the effect of natural vapor temperature in cooling performance was analyzed. Results indicated that the temperature of PV cell drops significantly with increasing natural vapor mass flow rate. In detail, the PV cell temperature decreased about 7 to 16 °C when flow rate reaches 1.6 to 5 gr min−1. It causes increasing electrical efficiency about 12.12% to 22.9%. The best performance of PV cell can be achieved at high natural vapor flow rate, low natural vapor temperature and the obtained optimum distribution condition. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With growing concerns about the damages to the environment caused by burning fossil fuels, and due to ever increasing oil demand, recently there have been numerous attempts to find out an energy source which can serve as an alternate energy for fossil fuel. Harnessing solar energy holds great promise for the world's energy demands, and it will be heavily called upon as fossil fuels are depleted. Photovoltaic solar cells (PV) are used to convert some part of solar energy to electricity and they are the best choices for utilizing solar energy. Another technology to utilize the solar energy is the hybrid photovoltaic/thermal (PV/T) technology. This technology can simultaneously generate electrical and thermal energies by integrating a photovoltaic cell and a solar thermal collector. In the recent years numerous studies were carried out to investigate the applications of solar technology. There are many papers that report the effects of temperature on the electrical efficiency of the PV cells [1–3]. The efficiency of PV cells decreases with increasing the temperature of PV module [4]. Researchers proposed practical ways to reduce the temperature of PV cell. Tripanagnostopoulos et al. [5] experimentally investigated the improvement in the electrical performance of a photovoltaic. It was achieved by using fins and naturally circulating air. Chen et al. [6] used the refrigerant R134a for cooling PV modules insulated by glass vacuum tubes. The PV panel was coupled with a heat pump system. Some researchers have shown an increased interest in the development of micro cooling technology for various applications ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author at: Chemical Eng. Dept. Razi University, Taghe Bostan, Kermanshah, Iran. E-mail addresses: [email protected], [email protected] (M. Rahimi).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2015.04.002 0735-1933/© 2015 Elsevier Ltd. All rights reserved.
because of the ability of microchannel heat sink to remove a large amount of heat from a small area [7–11]. Karami et al. [12] used Boehmite nanofluid in microchannel heat sinks to achieve as high thermal performance as possible. In hybrid photovoltaic/thermal solar energy systems cooling the PV module can be combined with a useful fluid heating [13–15]. Raghuraman [16] performed two separate onedimensional numerical methods to predict the performance of PV/T flat plate collectors using water and air as heat carrier. They tried to maximize the energy extracted from the collectors. Teo et al. [17] employed an active-cooled hybrid PV/T system to cool down the PV cell. Their results indicated that active cooling improves the efficiency considerably. Tchinda et al. [18] studied theoretically thermal processes in a CPC collector with a flat one-sided absorber. They concluded that the selectively coated CPC with flat one-sided collector is more efficient than the black painted one with the same condition. Othman et al. [19] studied both theoretically and experimentally the performance of hybrid photovoltaic–thermal solar collector. They improved the performance of PV/T solar air collector by the use of a double-pass collector and fins. In another study [20], they designed a double-pass PV/T solar collector with fins and compound parabolic concentrator (CPC) to increase the performance and reduce the cost of photovoltaic electricity. The CPC was used to increase the radiation intensity falling on the solar cells. However traditional water-type PV/T collectors/systems are practical, but they are not used in regions with natural climates because the freezing of water to ice can break up the collectors. In the regions with natural climate, the integration of a heat pipe and a solar collector can be incorporated into a practical design for a PV/T collector [21]. Many other studies on PV/T systems have been conducted on heat pipe PV/T systems [22–25]. Jie et al. [26] represented numerical and experimental
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Nomenclature As PV cell area (m2) area covered with natural vapor (m2) Ah photovoltaic arrays current (A) IPV maximum power of PV arrays (W) Pmax Pcooling PV power by cooling (W) P without cooling reference power (W) PV photovoltaic Q mass flow rate (kg/s) PV cell temperature (°C) Tc natural vapor temperature (°C) Tv velocity (m/s) Uv photovoltaic voltage (v) VPV
investigations on the performance of the photovoltaic solar assisted heat pump (PV-SAHP). Their results implied that the PV-SAHP has a better coefficient of performance and photovoltaic efficiency in comparison with the separate units. Installation of panels on the river canals was done practically in some places [27,28]. Those studies showed that the canal-top solar power equipment produces 15% more power than the plant set up on land as the water vapor flowing underneath of the panels keeps the solar panels relatively cool and helps more power generation. The main objective of this research is to illustrate a new method to reduce the temperature of PV module and improve the efficiency of PV cell. In order to achieve the best performance of PV cell, the PV panel has been cooled using natural vapor as coolant. The vapor has a temperature in the range of temperature of natural vapor escape from rivers, water channels, etc. The effects of the natural vapor flow rate, natural vapor temperature and natural vapor distribution on the performance of PV cell have been discussed. Because of the limitations that exist in real states during the test, such as wind and variable in intensity,
Fig. 2. The thermocouple positions.
vapor flow rates and vapor temperature, all experiments are done in laboratory. Therefore the situations are controlled in each step to highlight the effect of parameters. 2. Experiment setup The schematic illustration of testing setup is depicted in Fig. 1. According to the figure, this system consists of natural vapor generator and photovoltaic systems. Solar simulator, natural vapor simulator and
Fig. 1. The scheme of experimental setup.
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PV cell are three basic components of this setup. The temperature measurement system and V–I measurement system were employed to measure and record data. The vapor simulator employed to simulate natural vapor that is vaporized from the surface of rivers and dams in laboratory scale. In order to distribute the natural vapor in the backside
of PV cell, 12 circulated outlet holes with a diameter of 8 mm were drilled on the natural vapor simulator. The PV cell dimensions were 290 mm × 325 mm. The backside of PV cell exposed to natural vapor at specific points with the same velocity. Three 400 W metal halide lamps (Reflector Sunlight Dysprosium Lamp, RSDL) were used to
Fig. 3. Influence of vapor flow rate on (a) the average temperature of the PV cell. (b) I–V curves during cooling process. (c) Output electrical efficiency.
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Table 1 The measured and average values of temperature, and maximum power of the cell module under different flow rate at constant temperature of vapor (Tv = 18 °C). Flow (gr min−1)
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
Tave
Pmax(w)
0 1.6 3 5
60 58.4 53.6 43.8
53.3 41 36.9 33.7
49.8 41.3 36.9 33.8
54.2 52.4 47.1 43.3
57 53.6 52.6 43.4
59.4 48.2 48.6 34.9
53.6 48.3 41.7 42.5
55.2 45.2 43.5 39.9
58 46.6 42.5 38.9
53.2 48.3 44.8 39.3
55.4 48.3 44.8 39.3
3.008 3.367 3.528 3.698
Table 2 The measured and average values of temperature, and maximum power of the cell module under different vapor temperature at constant flow rate of vapor (q = 5 gr min−1). Vapor temperature
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
Tave
Pmax(w)
Without cooling Tv = 18 °C Tv = 23 °C Tv = 38 °C
60 43.8 34.8 52
53.3 33.7 33.3 46
49.8 33.8 35.6 43.5
54.2 43.3 52 47
57 43.4 40.8 51
59.4 34.9 44.7 52.5
53.6 42.5 43.8 48.2
55.2 39.9 46 54
58 38.9 47.8 46.1
53.2 39.3 42 44.4
55.4 39.3 42 48.5
3.008 3.698 3.567 3.51
simulate solar radiation. These lamps produce more pure white light than the popular HPS lamps, close to daylight frequencies. The fabricated solar simulators are evenly loaded on aluminum heat sink plate. In order to measure the PV cell temperature, 10 thermocouples were fixed at 10 specific points on the surface of PV cell. The thermocouples were connected to a digital thermometer (Lutron, BTM-4208SD) to record the temperatures. The locations of thermocouples at the surface of PV module are shown in Fig. 2. To record the I–V data, an electrical load system plugged into the PV electrical output was used. 2.1. Experimental work These experiments have been carried out on various vapor temperatures and various mass flow rates to find out the effect of cooling on the performance parameters of PV cell. The constant intensity of incoming radiation was set at 1000 W m−2 during the test. All the experiments were conducted at steady state condition and temperature (32 °C) considered as an environmental temperature. The vapor simulator was operated at specified flow rates (0–5 gr min−1) controlled by the simulator during the tests. The flow rate q = 0 gr/min indicates the state without cooling. The vapor temperature was in range of 18– 38 °C. Ten K-type thermocouples were used to measure the temperatures at 10 separate points at the surface of PV module and measured the temperature of vapor. The I–V values were recorded at steady state condition, with electrical load system to determine the power of PV module and find the maximum power point (P max).
figure, it can be seen that in all recorded vapor temperature, increase in flow rates of vapor caused a sharp decrease in the average temperature of the PV cell. The reason for this reduction could be related to the higher velocity of vapor that touched the backside of PV cell in higher flow rates. Moreover, in specific period of time, the mass of vapor that stays in the backside of PV cell will be increased. Increase in the velocity of vapor results in higher heat transfer coefficient and increase in the mass of vapor causes higher heat transfer rate. More details were also presented in Table 1. As can be seen in this table, the variation of temperatures is comparatively considerable. A significant decrease in average temperature was ascertained for PV cell, by further increase of the flow rates of vapor. Particularly, the PV cell temperature decreased about 7 to 16 °C when flow rate reaches 1.6 to 5 gr min−1 by the reason
3. Results and discussion 3.1. The effect of natural vapor flow rate
Fig. 4. Variation of the average PV cell temperature with the numbers of vapor outlets.
The temperatures of 10 points on the surface of PV cell were measured in order to plot the variation of the average temperature at different condition. The average value of the temperatures of 10 specific points was considered as the PV cell temperature. Fig. 3a shows the average temperature of PV cell in different flow rates (0–5 gr min−1) and different vapor temperatures (18, 23, 38 °C). According to this
Table 3 The percentage increase in the PV electrical output in T (air) = 32 °C under different flow rates. Flow (gr min−1)
Tv = 18 °C
Tv = 23 °C
Tv = 38 °C
0 1.6 3 5
0 12.12 17.2 22.9
0 11.2 14.15 18.9
0 11.3 13.1 16.6
Fig. 5. The vapor velocity at various number of outlet streams.
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mentioned above. Fig. 3b shows the experimental I–V characteristics of PV cell at constant vapor temperature (18 °C) over the various ranges of the vapor flow rates. As illustrated in this figure, the area under the I–V curve increases at higher flow rates. Since the area under the I–V curve represent produced electrical power, the comparison between obtained results obviously shows a rise in output power by increasing the vapor flow rate. As mentioned above, increasing the vapor flow rate causes enhancement in heat transfer from PV cell and reduction of PV temperature, so the output power increases due to the reduction of PV cell temperature. Fig. 3c illustrates the results for different vapor flow rates at constant vapor temperature (18 °C). As can be seen in the figure, increase in vapor flow rates caused considerable enhancement in
electrical efficiencies. The comparison between results of output power obtained at high and low vapor flow rates could highlight the effect of vapor flow rate on the performance of PV cell in better way. According to Table 1, it is clear that for q = 1.6 gr min−1, the maximum power moved up from 3.008 to 3.332 W, and in the case of q = 5 gr min−1 this value attained from 3.008 to 3.698 W. Therefore, increase in the maximum power can be observed in two mass flow rates (1.6 and 5 gr min−1). While, increase in output power is noticeable in q = 5 gr min− 1. The percentages of increase in the electrical PV cell output at various vapor flow rates with the three different vapor temperatures are tabulated in Table 3. As shown in this table, the highest vapor flow rate had the best cooling performance. In detail,
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125
T4 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
T6 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A a) 4 outlet ( Ah = 0.12 , Uv = 0.103 m.s -1 ) s
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125 100 75 50
50
b) 6 outlet (
100
Ah As
=
150 Length of PV Cell (mm)
200
0.18 , Uv = 0.069 m.s -1 ) Fig. 6. Temperature contour plots of PV cell at various vapor outlet streams.
250
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
27
200
35 37 39 41 43 45 47 49 51 53
Widt h of PV Cell ( mm)
175 150 125
T8 < – – – – – – – – – – >
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A c) 8 outlet ( Ah = 0.24 , Uv = 0.052 m.s -1 ) s
T10 < 35 – 37 – 39 – 41 – 43 – 45 – 47 – 49 – 51 – 53 – >
200
Widt h of PV Cell ( mm)
175 150 125
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
Ah
d) 10 outlet (A
s
=
150 Length of PV Cell (mm)
200
250
0.3 , Uv = 0.041 m.s -1) Fig. 6 (continued).
for q = 5 gr min−1, increase in the performance was 10.78%, 7.7% and 5.3% higher, in comparison with q = 1.6 gr min−1, at Tv = 18, 23 and 38 °C, respectively. The maximum power calculated from Eqs. (1) and (2) was also used to compute increase in the maximum power. P max ¼ V m Im
%P increase ¼
P cooling −P witout cooling P without cooling
ð1Þ
ð2Þ
3.2. The effect of natural vapor temperature According to Fig. 3a, with decreasing the vapor temperature at specific flow rate, average temperature decreased due to increasing heat transfer between the PV cell and vapor. As illustrated in this figure, the maximum decrease in the average temperature of PV cell occurred at the flow rate about 5 gr min−1 and temperature of 18 °C. It is clear that the difference between the temperature of PV cell and vapor is the driving force in heat transfer, thus the highest difference caused highest heat transfer and consequently improved the efficiency of PV cell.
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T12 < 35 – 37 – 39 – 41 – 43 – 45 – 47 – 49 – 51 – 53 – >
200
Widt h of PV Cell ( mm)
175 150 125
35 37 39 41 43 45 47 49 51 53 55 55
100 75 50
50
100
150 Length of PV Cell (mm)
200
250
A e) 12 outlet ( Ah = 0.36 , Uv = 0.034 m.s -1 ) s
Fig. 6 (continued).
Moreover, in order to show the effect of vapor temperature, the trend of changing in maximum power and temperature of surface PV module is presented in Table 2. It can be seen that the average temperature of PV cell drops from 48.5 to 39.3 °C, when the temperature of vapor decreased about 20 °C. In addition, the maximum output power increased about 5% when the vapor temperature decreased from 38 to 18 °C. By referring to Table 3, it is obvious that increase in vapor temperature has a significant effect on enhancement in efficiency in high vapor flow rate (5 gr min−1), whereas this effect can be negligible in low flow rate (1.6 gr min−1). The possible reasons for this behavior are that in high flow rate, the ability of heat transfer increases due to increasing the vapor velocity and the difference between the temperature of PV cell and vapor causes higher driving force in heat transfer. 3.3. The effect of natural vapor distribution 3.3.1. Average temperature of the PV cell Uniformity in temperature of PV cell surface caused considerable enhancement in electrical efficiency. The main purpose of the present section is to find the best situation for distributed natural vapor in the backside of PV cell, in order to decrease and make the surface temperature of PV cell uniform. Fig. 4 shows the module average temperature in different numbers of outlet points (number of vapor outlets that exposed backside of PV panel) in constant vapor flow rate (5 gr min−1) and various vapor temperatures (18 and 23 °C). According to Fig. 4, the module average temperature drops with increasing the numbers of outlets. However, the slope of falling in the cell temperature with growth in the numbers of outlets is reduced and finally is changed when the outlets passed over 8. This could be related to the influence of two main parameters, heat transfer surface and vapor velocity. The trends of changing velocity and heat transfer surface are presented in Fig. 5. It can be observed that rising the heat transfer surface and reduction in vapor velocity occurred simultaneously. It can be concluded that
increase in heat transfer surface for layouts less than 8 outlets has a convincing effect on cooling PV cell, whereas growth of heat transfer surface for more than 8 outlets has an inverse effect. The reason for this inverse effect can be related to importance of vapor velocity and the heat transfer area. In layouts with less than 8 outlets, the heat transfer increases due to more efficient vapor spreading. Although the velocity of vapor decreases, the surface of heat transfer plays the dominant role in the heat transfer and the effects of heat transfer surface are more significant in this case. In layouts with more than 8 outlets, increase in the numbers of outlet causes the sharp decrease in the velocity of vapor and the heat transfer rate decreases. Fig. 6 shows the temperature contour plot of PV cell versus the distances from thermocouples at constant vapor flow rate (5 gr min−1), and vapor temperature (23 °C). It can be seen that temperature distribution leads to become uniform and decreased when the numbers of vapor outlets increased to 8. The obtained results implied that for the case with 4 vapor outlets, the mean temperature is 46.39 °C and the standard deviation of recorded temperatures at 10 points from mean temperature is 5.12. Moreover, according to Fig. 6a, it is clear that most parts of PV cell surface were covered with regions with a temperature more than 45 °C. Also, maximum difference in temperature is about 20 °C. Fig. 6b shows that using of 6 outlets causes more efficient temperature distribution in comparison with 4 outlets. It can be seen that over 50% of PV cell surface have temperatures less than 45 °C. With increasing the numbers of outlets to 8 outlets, the mean temperature and the deviation have been recorded to be 40.91 °C and 4, respectively. Fig. 6c indicates that all regions of PV cell surface have temperatures less than 45 °C, except the small regions that have temperatures between 45 and 51 °C. In addition, maximum difference in temperature reduced about 5 °C in comparison with 4 and 6 outlets. Fig. 6d shows that increase in vapor outlets to 10, caused more regions with temperatures more than 45 °C. Furthermore, maximum difference in temperature reaches over 20 °C. After 10 outlets, increasing the numbers of outlets to 12 outlets, leads to rising the mean temperature
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
and the deviation to the values of 45.33 °C and 12, respectively. According to Fig. 6e it is obvious that cooling with 12 vapor outlets has the worst result. The deviation of obtained temperatures was calculated from Eq. (3):
δ¼
sX ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ðxi −xi Þ n−1
ð3Þ
By the reason mentioned above, increasing the numbers of vapor outlets to 8 outlets has a beneficial effect on the cooling of PV panel and reduces the temperature of PV panel. However, when the numbers of outlets reach 8 outlets, ever increase in the numbers of vapor outlets has a conflicting effect on the cooling of PV panel. 3.3.2. Electrical performance Fig. 7 shows the increase in maximum power output of the PV cell in different numbers of separate points, constant vapor flow rate (5 gr min− 1) and two different vapor temperatures (18, 23 °C). It can be seen that the efficiency increases significantly at 8 vapor outlets compared with 4 vapor outlets. In detail, about 100% increase in the vapor coverage on the backside surface caused 7.3% increase in the output power. However, for the vapor outlets higher than 8, the efficiency begins to level off and decreases slightly with increasing vapor outlets. Regarding to the 8 outlets, the maximum output power in 10 outlets decreased 1.1%, whereas the vapor coverage on the backside surface increased about 50% and reduced velocity of vapor was 0.011 m s−1. As mentioned above, it could be related to increasing the heat transfer rate before 8 outlets and the reduction of PV cell temperature. This decision manifests that the performance of cooling PV cell with natural vapor was affected by some independent factors, which are the vapor temperature, the vapor flow rates, distribution of vapor and vapor velocity. 4. Conclusions This study introduced a new technique for cooling the PV cell, using the natural vapor as heat transfer fluid, to enhance the electrical performance. Therefore installation of panels on the river canals and other places where vaporization exists could improve the performance of PV panels. Moreover in this way of cooling performance doesn't need any external energy. It was found that, increasing natural vapor flow rate improved heat transfer from PV module. This caused enhancing the electrical efficiency. Based on the obtained data, it was observed that the natural vapor flow rate affected the average PV cell temperature. At low natural vapor temperature, the corresponding average temperature was 48.3 °C in low flow rate (1.6 gr min−1) and 39.3 °C in high flow
Fig. 7. Comparison between the maximum generated powers of various vapor outlets.
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rate (5 gr min−1). In this study, the best performance of the PV cell was achieved in low natural vapor temperature. This result indicates that the natural vapor temperature is one of the significant parameters which must be considered. Moreover, distribution of the natural vapor in the backside of the PV cell has an important effect on improving the heat transfer surface and enhancing cooling performance. Whereas, in constant natural vapor flow rate, with the growth of vapor distribution, the velocity of the natural vapor reduced and the heat transfer to natural vapor decreased. It can be concluded from this result that both distributions of vapor and velocity have a significant effect on cooling performance. In contrast, in this experiment the best PV cooling performance was found at high natural vapor flow rate, low natural vapor temperature and the optimum numbers of outlets (5 gr min−1, 18 °C and 8 respectively). Moreover, the worst case was the natural vapor in low flow rate and high temperature with minimum distribution of vapor. Finally it could be concluded that natural vapor temperature, natural vapor flow rate and distribution of vapor are independent factors which have an effect on the PV cooling performance.
References [1] E. Skoplaki, J.A. Palyvos, On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations, Sol. Energy 83 (2009) 614–624. [2] Y.B. Assoa, C. Menezo, G. Fraisse, R. Yezou, J. Brau, Study of a new concept of photovoltaic–thermal hybrid collector, Sol. Energy 81 (2007) 1132–1143. [3] R. Vaillon, L. Robin, C. Muresan, C. Ménézo, Modeling of coupled spectral radiation, thermal and carrier transport in a silicon photovoltaic cell, Int. J. Heat Mass Transf. 49 (23-24) (2006) 4454–4468. [4] S. Essalaimeh, A. Al-Salaymeh, Y. Abdullat, Electrical production for domestic and industrial applications using hybrid PV–wind system, Energy Convers. Manag. 65 (2013) 736–743. [5] Y. Tripanagnostopoulos, P. Themelis, Natural flow air cooled photovoltaics, 7th International Conference of the Balkan Physical Union, American Institute of Physics, Melville, N.Y, Alexandroupolis, Greece 2009, pp. 1013–1018. [6] Hongbing Chen, Saffa B. Riffat, Yu. Fu, Experimental study on a hybrid photovoltaic/ heat pump system, Appl. Therm. Eng. 31 (2011) 4132–4138. [7] M. Rahimi, E. Karimi, M. Asadi, P. Valeh-e-Sheyda, Heat transfer augmentation in a hybrid microchannel solar cell, Int. Commun. Heat Mass Transf. 43 (2013) 131–137. [8] P.-S. Lee, S.V. Garimella, D. Liu, Investigation of heat transfer in rectangular microchannels, Int. J. Heat Mass Transf. 48 (2005) 1688–1704. [9] J.H. Ryu, D.H. Choi, S.J. Kim, Three-dimensional numerical optimization of a manifold microchannel heat sink, Int. J. Heat Mass Transf. 46 (2003) 1553–1562. [10] H.A. Mohammed, P. Gunnasegaran, N.H. Shuaib, The impact of various nanofluid types on triangular microchannels heat sink cooling performance, Int. Commun. Heat Mass Transf. 38 (2011) 767–773. [11] O.M.P. Nitiapiruk, A.S. Dalkilic, S. Wongwises, Performance characteristics of a microchannel heat sink using TiO2/water nanofluid and different thermophysical models, Int. Commun. Heat Mass Transf. 47 (2013) 98–104. [12] N. Karami, M. Rahimi, Heat transfer enhancement in a hybrid microchannelphotovoltaic cell using Boehmite nanofluid, Int. Commun. Heat Mass Transf. 55 (2014) 45–52. [13] D.J. Yang, Z.F. Yuan, P.H. Lee, H.M. Yin, Simulation and experimental validation of heat transfer in a novel hybrid solar panel, Int. J. Heat Mass Transf. 55 (4) (2012) 1076–1082. [14] B. Moshfegh, M. Sandberg, Flow and heat transfer in the air gap behind photovoltaic panels, Renew. Sustain. Energy Rev. 2 (1998) 287–301. [15] A.K. Bhargava, H.P. Garg, R.K. Agarwal, Study of a hybrid solar system—solar air heater combined with solar cells, Energy Convers. Manag. 31 (1991) 471–479. [16] P. Raghuraman, Analytical predictions of liquid and air photovoltaic/thermal flat plate collector performance, Sol. Energy Eng. 103 (1981) 291–298. [17] H.G. Teo, P.S. Lee, M.N.A. Hawlader, An active cooling system for photovoltaic modules, Appl. Energy 90 (2012) 309–315. [18] R. Tchinda, N. Ngos, A theoretical evaluation of the thermal performance of CPC with flat one-sided absorber, Int. Commun. Heat Mass Transf. 33 (6) (2006) 709–718. [19] M.Y. Othman, B. Yatim, K. Sopian, M.N. Abu Bakar, Performance studies on a finned double-pass photovoltaic-thermal (PV/T) solar collector, Desalination 209 (2007) 43–49. [20] M.Y.H. Othman, B. Yatim, K. Sopian, M.N. Abu Bakar, Performance analysis of a double-pass photovoltaic/thermal (PV/T) solar collector with CPC and fins, Renew. Energy 30 (2005) 2005–2017. [21] P. Gang, F. Huide, Z. Tao, J. Jie, A numerical and experimental study on a heat pipe PV/T system, Sol. Energy 85 (2011) 911–921. [22] P. Gang, F. Huide, Z. Huijuan, J. Jie, Performance study and parametric analysis of a novel heat pipe PV/T system, Energy 37 (2012) 384–395. [23] P.G. Charalambous, G.G. Maidment, S.A. Kalogirou, K. Yiakoumetti, Photovoltaic thermal (PV/T) collectors: a review, Appl. Therm. Eng. 27 (2007) 275–286.
30
M. Ebrahimi et al. / International Communications in Heat and Mass Transfer 65 (2015) 22–30
[24] H.M.S. Husseina, M.A. Mohamada, A.S. El-Asfouri, Optimization of a wickless heat pipe flat plate solar collector, Energy Convers. Manag. 40 (1999) 1949–1961. [25] H.P. Garg, R.K. Agarwal, Some aspects of a PV/T collector/forced circulation flat plate solar water heater with solar cells, Energy Convers. Manag. 36 (1995) 87–99. [26] J. Jie, L. Keliang, C. Tin-tai, P. Gang, H. Wei, H. Hanfeng, Performance analysis of a photovoltaic heat pump, Appl. Energy 85 (2008) 680–693.
[27] A. Chalaskuzhy, Gujarat goes green with solar panels on Narmada Canal, Global Council and Chairman at Global Renewable Energy Investment. BE(IT), MBA (Finance &Marketing) PSG Tech, Coimbatore India, 2012 (http://abhilashchalakuzhy.blogspot.com). [28] Solar panels as canal tops in India, Curenature.com (Nature care & AwarenessEvidence Based), 2013.