Numerical study on the electrical performance of photovoltaic panel with passive cooling of natural ventilation

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1 International Journal o Smart Grid and Clean Energy Numerical study on te electrical perormance o potovoltaic panel wit passive cooling o natural ventilation Hongbing Cen *, Xilin Cen, Sizuo Li, Hanwan Dinga Scool o Environment and Energy Engineering, Beijing University o Civil Engineering and Arcitecture, Beijing Cina Abstract Many researcers employed air or water or te active cooling o PV to acieve iger electrical eiciency, but additional energy consumption or an or water pump may reduce te net power output. Tereore, tis paper provided a study on te electrical perormance o PV panel wit passive in cooling o natural ventilation wit te consideration o te eect o PV panel tilt angle, solar radiation, air temperature and wind velocity as well as in size on PV energy perormance. Te study sowed tat tere existed a PV tilt angle resulting in te minimum electrical eiciency and te maximum power output. Higer intensity o solar radiation led to lower PV electrical eiciency and te eiciency dropped to te minimum around 11:40 during te day time in Beijing. Te PV electrical eiciency decreased in linear wit te increasing air temperature, but it increased wit te increasing wind velocity. Te eect o in eigt on te electrical eiciency seemed to be very sligt. Te average electrical eiciency o PV panel wit ins was 0.27~1.14% iger tan tat o PV panel witout ins under various conditions in tis study. Keywords:Passive cooling, natural ventilation, PV panel, electrical perormance 1. Introduction Wit te rapid economic development, energy demand and consumption ad been sarply increasing in te past two decades. Fossil uel reserve as been proven to be very limited, tereore, te development and utilization o renewable energy becomes more and more important. Solar energy, as a green and renewable energy, as been widely used or solar-electricity generation wit potovoltaic (PV) panel in recent years. In 2012, te global PV production was 36 GW and 63.9% o tem were produced in Cina. Te electrical eiciency o commercial PV products was mostly less tan 20%. Many studies [1], [2] ound tat PV cell/working temperature ad a great impact on te solar-to-electricity conversion eiciency (PV electrical eiciency), and te iger PV cell temperature was, te lower te electrical eiciency would be. Many studies [3]-[7] o active PV cooling, employing air, water or rerigerant to extract eat rom PV and combining PV panel wit eat storage or eat pump or PV/termal (PV/T) system, ave been carried out to improve PV energy perormance. But tose systems consumed extra energy to drive te cooling luid, and te improvement o electricity production by active cooling was very sligt compared wit te extra energy consumption or an, water pump and compressor. Passive cooling wit no extra energy consumption could be an option to improve PV electrical perormance, even te cooling eect and energy improvement was not as good as expected. Tereore, tis paper provided a study on te electrical perormance o PV panel wit passive in cooling o natural ventilation. 2. Numerical Models Zondag et al. [8] built our numerical models or te simulation o PV/T collector: a 3-D dynamical model and tree steady state models tat are 3-D, 2-D and 1-D. Te study sowed tat te 1-D steady * Manuscript received May 13, 2014; revised July 26, Corresponding autor: Hongbing Cen; Tel.: ; address: cenongbing@bucea.edu.cn doi: /sgce

2 396 International Journal o Smart Grid and Clean Energy, vol. 3, no. 4, October 2014 state model perormed almost as good as te oters. Tereore, or tis work, te 1-D steady state model was used or te simulation based on te ollowing assumptions: Te temperature at eac layer was well distributed and te geometrical center was selected to be te representive node. Te contact resistance between eac layer was neglected. Based on te energy balance analysis o eac layer, matematic models were developed or te numerical simulations o PV energy perormance. Te eat balance at te PV panel was given by 0 I A I 1 A E T T A T T A T T A (1) c c p p c p p m m p p p a a p p p sky sky p p W m p wa L Fin PV panel Metal seet Fig. 1. Cross-sectional view o part PV panel wit cooling in. were I was solar radiation; () c and () p were te eective absorptances o te solar cells c and PV base plate respectively; c was te ratio o cell area to PV panel area; A p was te area o PV panel p ; E was te electricity generation; pm was te eat transer coeicient between p and te metal seet m ; pa was te convective eat transer coeicient between p and te surrounding air a ; psky was te radiative eat transer coeicient between p and te sky; T m, T p, T a and T sky were te temperature o m, p, a and te sky respectively. Te electricity generation (W) was given by E IA (2) c c c p were c was te temperature-dependent electrical eiciency o c. 1 T T c rc p rc (3) were rc was te reerence electrical eiciency at te reerence operating temperature T rc ; was te temperature coeicient [9]. pm 1 (4) R pm Rp mwas te termal contact resistance between p and m (0.04 m 2 K/W). According to Duie and Beckman [9], pa uwind (5) were u wind was te wind velocity. 2 2 T T T T (6) p sky p p sky p sky were p was te emittance o p ; was te Stean-Boltzmann constant.

3 Hongbing Cen et al.: Numerical study on te electrical perormance o potovoltaic panel wit passive cooling 397 T sky T (7) 1.5 a Te eat balance at te metal seet was given by 0 pa Tp Tm Ap m m T Tm Am m wa Twa Tm Am wa (8) were A pm was te contact area between p and m ; A m was te contact area between m and te ins ; A mwa was te contact area between m and te warm air wa ; m was te conductive eat transer coeicient between m and ; mwa was te convective eat transer coeicient between m and wa ; T and T wa were te temperature o and wa respectively. m xm y 1 xm Am y Am 2k L k L m m W 4 (10) L 2 (11) were k m was te termal conductivity o m (200 W/(m K)); m was te tickness o m (2 mm); L was te eigt o (80 mm); k was te termal conductivity o (210 W/(m K)); was te tickness o (2 mm); W was te widt o p (10 mm). mwa 2k m pa 2k m m pa Te eat balance at te in was given by 0 m Tm T Am wa Twa T A wa were wa (13) wa was te convective eat transer coeicient between and wa and was given by 2k pa 2k m pa Te eat balance or te warm air between te ins was given by 0 m wa Tm Twa Am wa wa T Twa A wa mwac Ta Twa (15) were m wa was te mass low o wa ; c was te speciic eat capacity o air. Te location o Beijing was selected or study in tis paper. To simpliy te calculation and analysis, te time point, 12:00 on October 15t, was employed or numerical simulation. Te solar radiation on tilt ront surace o PV panel was given by [10] cos S sin sin cos S cos cos cos sin S sin cos sin I Ib Id cos s sin S sin cos cos cos sin S cos sin cos 2 were I d was te diuse radiation on orizontal surace; I b was te direct radiation on orizontal surace; S was te angle between te inclined plane and te orizontal plane (30 o ); was te geograpical latitude (39.80 o in Beijing); was te solar latitude (9.60); was our angle (0 o at 12:00); was te azimut angle (15 o ); s was te solar elevation. 2 sin c Id C1 s (17) 2 (9) (12) (14) (16)

4 398 International Journal o Smart Grid and Clean Energy, vol. 3, no. 4, October d 23.45sin 365 (18) sin sin sin cos cos cos (19) s 3. Result and Analysis 3.1. Eect o PV panel inclination on te electrical eiciency Fig. 3 sowed te variation o PV electrical eiciency under dierent tilt angle o PV panel. It can be seen rom Fig. 3 tat te electrical eiciency decreased to te lowest point and ten increased sligtly wit te increasing PV panel tilt angle. For PV panel wit ins, te electrical eiciency was 14.7% responding to te tilt angle o 20 o. It decreased wit te increasing tilt angle and reaced te minimum o 14.6% at te tilt angle o 45 o. Ater tat, te electrical eiciency turned to increase and reaced 14.63% as te tilt angle increased to 60 o. Te average electrical eiciency was 14.6%. For PV panel witout ins, te electrical eiciency was 14.5% responding to te tilt angle o 20 o. It decreased wit te increasing tilt angle and reaced te minimum o 14.3% at te tilt angle o 45 o. Ater tat, te electrical eiciency turned to increase and reaced 14.4% as te tilt angle increased to 60 o. Te average electrical eiciency was 14.4%. It can be concluded rom Fig. 3 tat te PV cooling ins led to an average eiciency increase by 0.27% in tis case. Fig. 3.Variation o electrical eiciency wit PV panel tilt angle 3.2. Variation o electrical eiciency during te day time Fig. 4.Variation o electrical eiciency during te day time. Fig. 4 sowed te variation o PV electrical eiciency during te day time. It can be seen rom Fig. 5 tat te PV electrical eiciency decreased to te minimum beore noon and ten increased in te aternoon. For PV panel wit ins, te electrical eiciency reduced rom 15.7% at 8:00 to 14.9% at 12:00,

5 Hongbing Cen et al.: Numerical study on te electrical perormance o potovoltaic panel wit passive cooling 399 wile it increased to 15.7% at 16:00. It reaced te minimum o 14.8% around 11:40. Te average electrical eiciency was 15.2%. For PV panel witout ins, te eiciency decreased rom 15.6% at 8:00 to 14.6% at 12:00, wile it increased to 15.7% at 16:00. At about 11:40, it reaced te minimum o 14.6%. Te average electrical eiciency was 15.1%, 0.17% lower tan tat o PV wit ins relatively Eect o air temperature on te electrical eiciency Fig. 5 sowed te variation o PV electrical eiciency under dierent air temperature. It can be seen rom Fig. 5 tat te eiciency decreased in linear wit te increasing air temperature. As te air temperature increased rom K to K, te electrical eiciency decreased rom 16.0% to 14.0% or PV panel wit ins, wile rom 15.7% to 13.7% or PV panel witout ins. Every 10K air temperature increase led to an electrical eiciency decrease by 0.7%. Te average electrical eiciency o PV panel wit ins was 0.28% iger tan tat o PV panel witout ins. Fig. 5.Variation o electrical eiciency wit air temperature 3.4. Eect o PV panel inclination on electrical eiciency Fig. 6 sowed te variation o PV electrical eiciency under dierent wind velocity. Higer wind velocity led to better in cooling and consequently better PV electrical perormance. Te electrical eiciencies were 14.6% and 14.4% or PV panel wit and witout ins respectively at te wind velocity o 3 m/s. As te wind velocity increased to 6 m/s, te eiciencies increased to 14.9% and 14.8% respectively. Wit every 1m/s increase o wind velocity, te eiciency increased by 0.08% and 0.14%, respectively. Fig. 6.Variation o electrical eiciency wit wind velocity 4. Conclusions Te paper presented a numerical study on te electrical perormance o PV panel wit passive cooling o natural ventilation. Numerical models were developed to investigate te eect o PV panel tilt angle, solar radiation, air temperature and wind velocity as well as in size on te electrical eiciency and power

6 400 International Journal o Smart Grid and Clean Energy, vol. 3, no. 4, October 2014 output. Te study could be concluded tat: 1) As te PV panel tilt angle increased, te electrical eiciency dropped to te minimum at te tilt angle o 45 o and ten rose sligtly. Te average electrical eiciency o PV panel wit ins was 0.27% iger tan tat o PV panel witout ins. 2) Higer intensity o solar radiation led to lower PV electrical eiciency. During te day time, te eiciency dropped to te minimum at around 11:40 and ten went up or PV panels wit and witout ins. Te average electrical eiciency o PV panel wit ins was 1.13% iger tan tat o PV panel witout ins. 3) Te PV electrical eiciency decreased in linear wit te increasing air temperature. Every 10K decrease in air temperature led to te increase o electrical eiciency by 0.68% or PV panels wit and witout ins. Te average electrical eiciency o PV panel wit ins was 0.28% iger tan tat o PV panel witout ins. 4) Higer wind velocity led to better PV cooling eect as well as better electrical perormance. Every 1 m/s increase in wind velocity led to te increase o electrical eiciency by 0.08% and 0.14% or PV panels wit and witout ins, respectively. Te average electrical eiciency o PV panel wit ins was 1.14% iger tan tat o PV panel witout ins. 5. Acknowledgements Te work o tis paper is ully supported by Funding Project or New Star o Scientiic and Tecnical Researc o Beijing ( ), Te Importation and Development o Hig-Caliber Talents Project o Beijing Municipal Institutions (CIT&TCD ) and Beijing Municipal Key Lab o HVAC. 6. Reerences [1] Skoplaki E, Palyvos JA. On te temperature dependence o potovoltaic module electrical perormance: A review o eiciency/power correlations. Solar Energy, 2009; 83(5): [2] Assoa YB, Menezo C, Fraisse G, Yezou R, Brau J. Study o a new concept o potovoltaic-termal ybrid collector. Solar Energy, 2007; 81(9): [3] Tonui JK, Tripanagnostopoulos Y. Improved PV/T solar collectors wit eat extraction by orced or natural air circulation. Renewable Energy, 2007; 32(4): [4] Kumar R, Rosen MA. Perormance evaluation o a double pass PV/T solar air eater wit and witout ins. Applied Termal Engineering, 2011; 31(8-9): [5] Dubey S, Tiwari GN. Termal modeling o a combined system o potovoltaic termal (PV/T) solar water eater. Solar Energy, 2008; 82(7): [6] He W, Zang Y, Ji J. Comparative experiment study on potovoltaic and termal solar system under natural circulation o water. Applied Termal Engineering, 2011; 31(16): [7] Tiwari A, Soda MS. Perormance evaluation o ybrid PV/termal water/air eating system: A parametric study. Renewable Energy, 2006; 31(15): [8] Zondag HA, de Vries DW, van Helden WGJ, et al. Te yield o dierent combined PV-termal collector designs. Solar Energy, 2003; 74(3): [9] Duie JA, Beckman WA. Solar Engineering o Termal Processes. 3rd ed., New York: Wiley, [10] Fang R, Ceng X. Te Solar Energy Application Tecnology. Beijing: Cina Agricultural Macinery Press, 1985.