Experimental Investigation Of Active Cooling Of Photovoltaic Cells


Published on

Published in: Technology, Business
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Experimental Investigation Of Active Cooling Of Photovoltaic Cells

  1. 1. Experimental Investigation of Active Cooling of Photovoltaic Cells ﹡ ﹡ ﹡ ﹡ H.G. Teo, P.S. Lee , MNA Hawlader Department of Mechanical Engineering, National University of Singapore 9 Engineering Drive 1, Singapore 117576 Absorption of solar radiation increases the temperature of photovoltaic (PV) cells, resulting in a drop of electrical efficiency. A hybrid photovoltaic/thermal (PV/T) solar system was designed, fabricated and experimentally investigated in this work. To actively cool the PV cells, a parallel array of ducts with inlet/outlet manifold was designed for uniform airflow distribution and attached to the back of the PV panel. Experiments were performed with and without active cooling. A linear trend between the efficiency and temperature was found. Without active cooling, the temperature of the panel was high and solar cells can only achieve an efficiency of 8 to 9%. However, when the panel was operated under active cooling condition, the temperature dropped significantly leading to an increase in efficiency of solar cells between 12 and 14%. Keywords: Hybrid photovoltaic /thermal system; manifold design; active cooling; operating temperature; cell efficiency 1. Introduction In recent years, renewable energy is widely advocated by many developed country. PV cell is one of the most popular renewable energy products. It can directly convert the solar radiation into electricity which can be utilised to power household appliances. However, during the operation of the PV cell, only around 15% of solar radiation is converted to electricity with the rest converted into heat. The electrical efficiency will decrease when the operating temperature of the PV module increases. Therefore, decreasing the temperature of PV module can boost the electrical efficiency. Generally speaking, some techniques, like air cooling and water cooling, are utilised to cool the PV module to maintain lower operating temperature. Many numerical and experimental studies have been conducted to find out the most efficient and low cost hybrid PV/T system. Sometimes, the thermal energy extracted from the PV module can also be utilised for low temperature applications e.g. water and air heating. ﹡ ﹡Corresponding author. Tel.: +65 65164187 E-mail address: mpelps@nus.edu.sg 1
  2. 2. Nomenclature Ac Aperture area of PV module (m2) p Packing factor G (t) Solar radiation (W/m2) m Mass flow rate (kg/s) cp Specific heat capacity (J/kg.k) Tg Front glass temperature (℃) Tc Cell Temperature (℃) Tb Temperature of backsheet (℃) Ta Temperature of air flow (℃) Tin Inlet temperature of air (℃) Tout Outlet temperature of air (℃) hg Heat transfer coefficient of front glass (W/m2.k) hc Heat transfer coefficient in air duct (W/m2.k) Greek symbols σ Stefan-Boltzmann constant (W/m2.k4) εg Emittance of glass αT Tedlar absorptivity ηo Nominal efficiency of cell αc Cell absorptivity τg Fraction transmitted through the front glass ηe Cell efficiency β Temperature coefficient (℃-1) In some cases of study, attention is focused on modifying the configuration of PV panel. By changing the structure of the panel, the variation of performance of the system can be observed. Dubey et al. [1] reported the efficiency of different configurations of PV/T-air collector (Case A-Glass to glass PV module with duct; Case B-Glass to glass PV module without duct; Case C-Glass to tedlar PV module with duct; Case D-Glass to tedlar PV module without duct). It was indicated that case A can give the highest efficiency among the all four cases. The annual average efficiency of case A and B is 10.41% and 9.75%, respectively. In order to enhance the heat transfer from the PV panel thereby effectively reducing the temperature and improving the efficiency of the PV module, Prasad and Saini [2] artificially increase the roughness of absorber plate and wall of the channel. However, increased roughness of wall and absorber will incur a pressure drop penalty and, therefore, requiring a higher pumping power. Han et al. [3] and Gupta et al. [4] showed that several types of ribs in the air channel can provide a better performance 2
  3. 3. in heat extraction but it is also accompanied by a significant increase in frictional losses. Garg and Datta [5] suggested several practical modifications to enhance the heat transfer in the air duct. Garg et al. [6] presented a study of a PV/T air hybrid system, this system comprised a plane booster and a flat plat collector mounted with PV cells. An optimization study of the absorber geometry for solar air heating collector has been investigated by Pottler [7]. Naphon [8] carried out a study on the performance and entropy generation of the double pass solar air heater with longitudinal fins. The result of this study showed that the thermal efficiency of PV panel increases with increasing the flow rate, as the heat transfer coefficient increases with increased Reynold number. Tonui and Tripanagnostopoulos [9] also reported a study that an improvement of heat extraction has been achieved by modifying the channels of PV/T air system in low cost. Three different configurations of air ducts (simple air channel, thin aluminum sheet and rectangular fin) were investigated experimentally and numerically. Sopian et al. [10] presented a steady state simulation on single and double pass combined PV/T air collector. The results of simulation indicated that the double pass PV/T collector has superior performance during the operation. Joshi et al. [11] carried out an evaluation of a hybrid PV/T system. Two types of PV module (glass to tedlar and glass to glass) were utilized to investigate the performance under the meteorological conditions of New Delhi. The results showed that the overall performance of hybrid thermal collector with PV module glass-to-glass is better than glass-to-tedlar. In this study, the main focus will be on the electrical efficiency of the PV panel with and without cooling. By varying the air flow rate through the duct, the electrical performance will be investigated. 2. Experiments A test set up was designed to investigate the thermal and electrical performances of the PV/T air system. This system was built on the roof top of EA building at the National University of Singapore. A schematic diagram of the complete experimental set-up is shown in Fig. 1. This experimental set up is designed to investigate how the temperature affects the efficiency and power output of PV panel during the operation. Four polycrystalline solar panels were used in the experiment to generate electricity. The electricity generated by the solar panels was stored in four deep cycle gel batteries. An array of air ducts were used for the air to pass through and it was attached underneath the PV 3
  4. 4. panels. Fins were fitted in the duct to increase the heat transfer rate from the PV panel to the moving fluid. A parallel array of ducts with inlet/outlet manifold was designed for uniform airflow distribution. A direct current blower, which was connected to the batteries, extracted air from the surrounding to cool the panels. During the operation, a maximum power point tracker (MPPT) was used to modulate the power output from solar panel to ensure that the maximum electrical power is extracted. Another alternating current (AC) blower was also used in this experiment because it could function as variable speed blower, to control the flow rate passing through the duct. The experiments were normally conducted from 9:30 am to 5:00 pm. In the experiment, PV current, PV voltage, temperature of panels, temperatures air at inlet and outlet manifolds, wind speed and solar irradiation were collected during the operation of system. Figure 1. A schematic diagram of the experimental set-up. 3. Mathematical Formulation The energy balance equations of the PV module are modified from Cox and Raghuraman [12]: Ec = pα cτ g G (t ) (1) Ec is the total energy absorbed by the PV cell Ece = ηe pτ g G (t ) (2) 4
  5. 5. Ece is electrical energy produced by the PV cell. Ect = (1 − ηe / α c ) pα cτ g G (t ) (3) Ect is thermal energy released by PV cell. p is the cell packing factor which defined as the ratio of area of solar cell to the area of blank absorber ηe which is the cell efficiency can be represented as a function of the module temperature [13]. ηe = ηo [1 − β (Tc − To )] (4) ET = τ g (1 − p )α T G (t ) (5) ET is the rate of solar energy absorbed by tedlar (material of backsheet) after transmission from EVA (polymer encapsulatant of solar cell). Principle of energy conservation is applied to the components of the PV module as shown below: (1 − ηe / α c ) pα cτ g G (t ) + τ g G (t )α T (1 − p ) = Eloss + qc (6) Eloss = hg [Tg − Ta (t )] + ε g σ Tg 4 − α g σ [Ta (t ) − 6]4 (7) Eloss is the energy losses from the front glass to environment through the forced and free convection and radiation. The convective heat transfer from the back of module can be presented by Newton’s law of cooling: qc = hc [Tb − Ta (t )] (8) The energy balance of the air flow • dTair m cp dx = qc (9) dx The thermal efficiency can be computed with the following equation: • m c p ∫ (To − Ti )dt ηth = (10) Ac ∫ G (t )dt The electrical efficiency of the PV module can be described as following equation: ηe = ∫ VIdt (11) A∫ G (t )dt 5
  6. 6. The total efficiency of the hybrid PV/T system is: • m c p ∫ (To − Ti )dt + ∫ VIdt ηtotal = ηth + ηe == (12) Ac ∫ G (t )dt 4. Results and Discussion The electrical efficiency of the PV module is presented in Fig. 2. It can be observed that the electrical efficiency is a linear function of module temperature. The electrical efficiency of PV module declines with the rise in PV module temperature. During the experiment, cooling and non-cooling cases were considered. The impact of cooling is also shown in Fig. 2. Under the same meteorological condition, the temperature of non-cooling case is much higher than the cooling one and this is also reflected in the electrical efficiency of the PV module. 41 31 ) gnilooC hti W %( y c 21 n gnilooC tuohti W ei ci f f 11 E al ci r 01 ct el E 9 8 00.53 00.04 00.54 00.05 00.55 00.06 00.56 00.07 )C( erutarepmeT Figure 2.Electrical efficiency as a function of PV temperature. 00.31 05.21 00.21 %) ( y c 05.11 n e ci fi 00.11 f E 2m/W0001=noitaidarrI al 05.01 ci 2m/W009=noitaidarrI rt 00.01 c e 2m/W005=noitaidarrI El 05.9 2m/W007=noitaidarrI 00.9 05.8 00.0 20.0 40.0 60.0 80.0 01.0 21.0 41.0 61.0 81.0 )s/gk( etaR wolF Figure 3. Influence of flow rate on electrical efficiency. 6
  7. 7. The influence of flow rate on electrical efficiency is presented in Fig. 3. The electrical efficiency of the PV module increases with the flow rate until the flow rate reaches about 0.055 kg/s. The electrical efficiency of PV module will be maintained at a relatively constant value after the flow rate reaches about 0.055 kg/s. This could be explained in association with the thermal efficiency of collector. When the flow rate increases to around 0.055 kg/s, the thermal efficiency of the collector will be maintained at a relatively constant level. In other words, the heat extracted by the cooling fluid has reached a saturated level and it can no longer be increased by increasing the flow rate. Thus, the electrical efficiency of PV module will also be maintained at a relatively constant value after the flow rate reached about 0.055 kg/s. Temperature difference between inlet and outlet air is also investigated in this study and is given in Fig. 4. The electrical efficiency of PV module will decrease when the temperature difference between the inlet and outlet air increases. This could be explained by the occurrence of hot spot in the PV module due to the high temperature gradient. Therefore, the overall electrical efficiency can be dragged down. The inlet and outlet air temperature difference should be controlled to an optimum range to ensure that the electrical efficiency of the PV module can be maintained at the desired level. Figure 2 provides an indicative trend in the relation of electrical efficiency and operating temperature. A linear relation can be obtained: ηel = 0.1577 − 0.0009Tpanel (13) The theoretical efficiency of PV module can be obtained from the Eq. 4. From the theoretical deduction, the electrical efficiency of the module can be written as the equation below: ηel = 0.1664 − 0.0007Tpanel (14) Based on the experimental data, Fig. 5 showed that the theoretical electrical efficiency is about 1 to 2% higher than experimental electrical efficiency. This discrepancy can be attributed to the module to module connection which will result in a drop in the electrical efficiency. This result can be used to compare with result from Tonui et al. [14]. A linear correlation the electrical efficiency and module temperature was found from their experiments. η el = 0.147 − 0.0008Tpanel (15) 7
  8. 8. The efficiency of the system shown in the Fig. 6 indicated that the electrical efficiency seems to be more stable than the thermal efficiency. The average electrical efficiency range is around 10.1% to 10.9%. However, the thermal efficiencies of the system are much higher than electrical efficiency of the system; it is about 40% higher. This showed that most of the solar irradiation is converted into the heat and the thermal efficiency which obtained from the experiment is significant compared to electrical efficiency. The total efficiency of the system is around 50% to 70%. The cooling mechanism not only can enhance the electrical performance of PV cell but also increase the total efficiency of the system. Fig. 7 shows that the peak of total energy output will be at second day and fourth day. This can be attributed to the meteorological condition on these days. The ambient temperature on these two days was relatively high and the solar irradiation was also very intense. Therefore, it can be concluded that under the proper operation of the system, the output energy can be generated proportional to the solar power input. To ensure that the household appliances can be operated under the low irradiation meteorological condition, the auxiliary power is needed to supply the power when the power from the PV module is not sufficient to operate the appliances. The battery bank of the system is also needed to be charged when the solar irradiation is very intense as the battery bank can be also supply the power to the appliances during the cloudy day. 53 1. 0 y c 31.0n e ci fi 52 1. 0 f E l 21.0 a c ri s/g k 38 70.0 =etaR w olF ssaM ct 51 1. 0 el s/g k 67 60.0 =etaR w olF ssaM E s/ gk 98 30.0= etaR w olF ssaM 11.0 0 1 2 3 4 5 6 7 8 9 01 )C( ecnereffiD erutarepmeT Figure 4. Influence of temperature difference (To-Ti) on electrical efficiency for different flow rates. 8
  9. 9. 9 five days Figure 7. Electrical and thermal energy and the total energy gain over the syaD 5 4 3 2 1 1 0 0. 0 0 0. 5 E 00.01 n e r g y 00.51 00.51 00.51 ( 0 0 . 5 1M niaG y grenE latoT J ) ygre nE lamrehT 00.02 00.02 00.02 00.02 ygrenE lacirtcelE 00.52 00.52 00.52 00.52 Figure 6. A comparison of thermal and electrical efficiency over 5 days. syaD 5 4 3 2 1 %00.0 %00.01 ycneiciffE latoT %00.02 ycneiciffE lamrehT %00.03 E ycneiciffE lacirtcelE f f %00.04 ci ei n c %00.05 y %00.06 %00.07 %00.08 Figure 5. A comparison between theoretical and experimental results )C( erutarepmeT eludoM 57 07 56 06 55 05 54 04 53 03 80 .0 E 90 .0 el ct 1. 0 ri c a 11 .0 l 21 .0 E f 31 .0 f ci ei 41 .0 n c y 51 .0 tluseR laciteroehT tluseR latnemirepxE
  10. 10. 5. Conclusion Both electrical and thermal energy are generated through the hybrid PV/T system. From the experiment result, it shows that the effect of using the cooling mechanism. Under the situation where no cooling was used, the operating temperature of PV module attained a value as high as 68℃ and the electrical efficiency dropped significantly to 8.6%. By using the blower to cool the PV module, the operating temperature of module could be maintained at 38℃ and the electrical efficiency could also be kept at around 12.5%. Besides, an optimum flow rate was also found in this study. Air flow rate with 0.055kg/s is sufficient to absorb the maximum heat from the PV module. When the flow rate exceeds this value, the thermal and electrical energies are no longer affected. This helps in choosing the power rating of blower in order to avoid wasting unnecessary energy. Temperature gradient over the different PV module when connected together is also a key factor to affect the electrical performance. To boost the electrical efficiency of the PV module, temperature and temperature gradient over the PV module are critical. The increased efficiency of the air cooled PV/T systems will significantly contribute to the applications of PV system. Furthermore, increasing the performance of the PV systems can also reduce the energy supply to buildings leading to lower CO2 emissions. References [1] Swapnil Dubey, G.S.Sandhu, G.N.Tiwari, Analytical expression for electrical efficiency of PV/T hybrid air collector, Applied Energy 2009, (86): 697-705 [2] Prasad, B.N, Saini, J.S, Optimal thermohydraulic performance of artificially roughened solar air heaters. Solar Energy 1991, (47): 91–96. [3] Han, J.C, Park, J.S,. Developing heat transfer in rectangular channels with rib turbulators. International Journal of Heat and Mass Transfer 1988, (31): 183–195. [4] Gupta, D, Solanki, S.C, Saini, J.S, Heat and fluid flow in rectangular solar air heater ducts having transverse rib roughness on absorber plates. Solar Energy 1993, (51):31–37. [5] Garg, H.P, Datta, G., Performance studies on a finned-air heater. Energy 1989, 14:87–92. [6] Garg, H.P, Agarwal, P.K, Bhargava, A.K, The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer. Energy Conversion and Management 1991a, (32): 543–554. 10
  11. 11. [7] Pottler, K, Sippel, C.M, Beck, A, Fricke, J, Optimized finned absorber geometries for solar air heating collectors. Solar Energy 1999, 67: 35–52. [8] Naphon, P, On the performance and entropy generation of the double-pass solar air heater with longitudinal fins. Renewable Energy 2005, 30: 1345–1357. [9] Tonui, J.K, Tripanagnostopoulos, Y, Air-cooled PV/T solar collectors with low cost performance improvements. Solar Energy 2007b, 81: 498–511. [10] K.Sopian, K.S.Yigit, H. T. Liu, S. Kakac and T. N. Veziroglu, Performance Analysis of Photovoltaic Thermal Air Heaters, Energy Convers. Mgmt 1996, 37: 1657-1670. [11] A.S.Joshi, A.Tiwari, G.N Tiwari, I. Dincer B.V.Reddy, Performance evaluation of a hybrid photovoltaic thermal (PV/T) (glass-to-glass) system, International Journal of Thermal Science 2009, 48(1): 154-164. [12] C. H. Cox, III and P. Raghuraman, Design Consideration for flat-plate photovoltaic/ Thermal Collectors, Solar Energy 1985, 35(1): 227-241. [13] Florschuetz, L. W., On heat rejection from terrestrial solar cell arrays with sunlight concentration. IEEE Photovoltaics Specialists Conference Records, Mat 1975, 318-326. [14] Tonui, J.K., Tripanagnostopoulos, Y, Improved PV/T solar collectors with heat extraction by forced or natural air circulation, Renewable Energy 32 2007):623-637 11