Study of the Mass Flow Rates on the Efficiency of Hybrid Thermal / Photvoltaique Sensor

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1 Universal Journal of Pysics and Application 9(6): , 2015 DOI: /ujpa ttp:// Study of te Mass Flow Rates on te Efficiency of Hybrid Termal / Potvoltaique Sensor Maifi Lyes 1,*, Kerbace Taar 1, Hioual Ouided 2 1 Pysical Cemistry of Semi-Conductor Laboratory, University of Constantine, Algeria 2 Department of Informatics, Faculty of Sciences & tecnology, University of Kencela, Algeria Copyrigt 2015 by autors, all rigts reserved. Autors agree tat tis article remains permanently open access under te terms of te Creative Commons Attribution License 4.0 International License Abstract Hybrid termal/potovoltaic systems associating a solar concentrator wit a eat excanger are an effective way to improve solar energy conversion yield. We present ere an analysis of te effect of te mass flow rates in suc a collector. A numerical simulation of te performance of te termal/potovoltaic sensor wit a eat excanger including fins attaced to te absorber and using air as a coolant is presented. A toroug analysis of te influence of te mass flow rate on te efficiency and te working of a termal/potovoltaic collector is presented. Te analysis is made using te equations of te components of eat transfer cascade into a matrix of four unknown s wic are te glass, cells, fluid and insulation plate temperature. Tis matrix is solved by te fixed point metod and Gauss-Seidel, at te permanent regime. Results sow tat te overall conversion efficiency of te system is increasing from 27% to 65%, and te cell temperatures decreasing from 345 K to 335 K wen mass flow rates varies from 0.02 kg/s to 0.1 kg/s. Keywords Termal Potovoltaic, Cells Solar, Parabolic-concentrator, Permanent Regime, Heat Excanger 1. Introduction Problems of environmental pollution and energy lack in te world ave become muc serious recently. Solar energy as a renewable and environmentally friendly energy as te potential to meet global energy demand in te future. Metods for te conversion of solar energy can be classified into two types: a termal option, tat converts solar energy into eat, subsequently transformed into electricity, and te potovoltaic metods tat converts solar energy directly into electrical energy. Usually, tese two metods, wile perfectly compatible, are used separately leading to a low efficiency. In order to improve conversion yield, a new solar termal combined system, as been developed to arness eat and electricity simultaneously. In 1978, kern and all [1] presented a prototype of a termal/potovoltaic system using air and water as a coolant to reduce te temperature of te solar cells. A number of teoretical and experimental studies to assess te efficiency of ybrid termal/potovoltaic system ave been performed [2-7]. Florscuetz and all [2] used te model of otel-willier to analyze te performance of te ybrid solar system tat could provide domestic ot water. Garg and all [3] ave developed a stable model to simulate te performance of termal/potovoltaic ybrid system. In te recent years muc researc as been reported in te literature on new solar termal systems wit lower costs [4-7]. Wen sunligt is concentrated on te solar cell, most of te energy absorbed by te cell is converted into termal energy wic in turn increases te temperature and reduces te electrical efficiency. Terefore, it is necessary to remove eat from te cell by a eat transfer fluid (air, water ). 2. System Model Te system model consists of two part: te first calculates te termal eat system parameters suc as temperature (T), termal energy (Q) and termal efficiency (ƞ t ) of te sensor, and te second part is devoted to te electrical model, wic calculates te current (I), voltage (V) and te generated electric power. Te ybrid termal/potovoltaic collector considered in tis work is sown in (figure 1). Te glass cover (in plastic) is inserted in order protect te sensor against mecanical damage, te ligt is reflected and concentrated on te solar cell by a reflective cylindro-parabolic concentrator. Te focusing system consists of te cylindro-parabolic concentrators. Eac one as seven panels from (Generic 60 WP polycrystalline), wit 36 cells in series for eac panel. Te panels are connected in series along te direction of te system; tey are glued and sealed to keep surfaces cells clean. Fins are fixed on te eat excanger to improve eat excanger bottom te fluid and te solar cells. Te eat excanger bottom is covered wit a good insulator to minimize eat losses to te ambient [7].

2 Universal Journal of Pysics and Application 9(6): , rpb eat transfer by radiation between absorber/insulating plate. T a ambient temperature. T s ceil temperature. T g,t P,T pv,t b, T f are temperatures glass, absorber, cells solar, insulating plate and fluid respectively. T fi and T fo are fluid temperatures in and out respectively For te glass cover Figure 1. Scematic view of our termal/potovoltaic sensor 2.1. Termal Model (Figure 2) sows te termal model of te system. For te sake of simplicity, te following ypoteses are made A one-dimensional steady state eat transfer in te direction of te flow. Te eat capacities of te glass, concentrator, solar cell, fins, absorber and te insulating plate are negligible. Te parabolic concentrator is ideal and all te incident radiation in te acceptance angle can reac te solar cells. Te solar radiation converted into termal energy is completely absorbed by te panels and solar absorber. Te temperature of te solar cell and te absorber are uniform. Based on tese assumptions, te equations of energy can be written as follows: α g GC 1 + τ g ρ g ρ R 2n = rgs T g T s + cgw T g T w + cpg T g T p + t rpg T g T p (1) Were n is te average number of reflection for radiation inside te acceptance angle, were t, b and are te areas (m 2 ) of reflector, solar cell and convection, respectively, G is solar irradiation (W.m -2 ) and C te concentrating ratio is At te termal/potovoltaic plate ρ pv ρ g ρ R 2n C τ g α p Gρ n R d 1 + ρ 2n pρ g ρ R (1 P) + τ g α p GPρ n R d 1 + C 1 η pv = b pf η p T p T f + b c A rpb T p c T b + t cpg (T P T f ) + t rpg T p T g (2) Te solar reflectivity of te glass cover ρ g, of te potovoltaic cell surface ρ pv and of plate ρ p are assumed to be te same as tat of black absorber (ρ g =ρ p =ρ pv ). Were d is a correction for te gap loss. Te value of te glass transmittance τ g is 0.9, and plate absorptivity α p, and glass absorptivity α g are 0.9 and 0.06, respectively [8,9,10] Te eat excanger m p C f dt f = w dx cpf(t b T f ) + b pf η p T p T f (3) c Were m p is mass flow rate (kg.s -1 ), C f is specific eat (J.kg -1.k -1 ) and w is system widt (m) [11,12] Te insulating plate U b (T b T a ) = cpf (T f T b ) + b A rpb T p T b (4) c Te back loss coefficient U b is W.m 2.k [12] Electrical Model Figure 2. Termal model of our termal/potovoltaic sensor Were: cgw Convective eat transfer between glass/ambient. cpg eat transfer by natural convection between glass/ cells solar. cpf eat transfer by forced convection between fluid, insulating plate and absorber. rgs eat transfer by radiation between glass/ambient. rpg eat transfer by radiation between glass/cells solar. Te semiconductors p-n junction and a current source (potocurrent) wic mimics te electrical energy obtained from te ligt flux. Losses arising from junction leakage, contacts and connections resistance are taken into account by sunt (R s ) and series (R s ) resistances. Te model commonly used for a solar cell or a potovoltaic module in an electrical circuit is sketced in (figure 3). To enable comparisons, te manufacturers give eiter te I versus V or E pv versus V curves depending on ligting and temperature, or typical operating values (I,V) corresponding to sort circuit (I cc,0), open circuit (0, V oc ), and maximal power

3 246 Study of te Mass Flow Rates on te Efficiency of Hybrid Termal / Potvoltaique Sensor (I,V). Tese tree pairs of values were measured under standard test conditions illumination G ref =1000 W.m -2, temperature T jref =25 C. I = I p V+R si I R 0 (exp( q i (V+R si) ) 1) (5) s n s γkt j I p = I ccref G/G ref + c t (T J T jref ) (6) I 0 = I 0ref ( T J ) 3 exp q i Eg nsγk 1 ( 1 )) T jref T J T jref Figure 3. Electrical model of our sensor termal/ potovoltaic Were I P, I ccref and I 0 is te potocurrent, reference current of sort-circuit and te saturation diode current (in A ) respectively, T j is te junction temperature of te cells ( K) and q i is te carge of te electron, K is te Boltzmann constant, E g is te gap energy (ev), γ is te ideality factor of te junction wit values between 1-2 and I Oref is a coefficient dependent on temperature and on te cell tecnology [14]. G is te illumination (W.m -2 ), I cc is te sort-circuit current (A) and c t is te temperature coefficient of te sort-circuit current (A.K -1 ) given by te manufacturer. Following equation (5), it is easy to assume tat te current intensity is mainly proportional to te ligting, wereas te oter parameters (E g,i cc,r s,r s and γ) vary strongly wit te temperature and te tecnology used [15]. 3. Metod of Calculation We can write te equation 3 as: dt f (x) dx (7) + pt f (x) = q (8) Were p and q are constants obtained by algebraic manipulations. Te boundary conditions are : T f (x) = T a, at x = 0 T f (x) = T 0, at x = L Te solution can be obtained as T f (x) = q p + T a q p exp px (9) By grouping te four equations from equation 1 to equation 4, we obtain a four variables matrix. In te equation 9, p and q are te two unknown temperatures functions. An iterative algoritm is applied to determine tese temperatures. In order to calculate te temperature of eac cell of te potovoltaic concentrator, te panels is divided into n=252 units of m lengt (n is also te number of series cells in te collector). To stsrt te calculation, initial values at T g,(t p =T pv ) and T b are introduced. Te temperature T f of te in air flow at x=0, is equal to te ambient temperature. Te new temperatures can be obtained from te matrix. Gauss-seidel metod is used to calculate te temperatures of eac cell by an iterative process wic is repeated until temperature values converge. Tus te components temperatures for te first cell can be determined. Applying it as te intel to te next cell, te components temperatures for te second cell can be similarly calculated. By repeating tis step, all temperatures for te different components can be determined. Using tese temperatures, one can deduce te air mass flow influence on te cells and panel efficiency. 4. Performance Parameters Performance parameters of te ybrid sensor termal/potovoltaic are computed as following: Te termal energy gained by te air flow troug te system of termal/potovoltaic is: nn QQ = jj=1 mm pp. CC ff. (TT 0,jj TT ii,jj ) (10) Te termal efficiency of te system is: η t = n j=1 m pc f (T o,j T i,j ) Te electric power produced by te system is [15]. GC Te electrical efficiency of te system is: (11) E pv =I.V (12) η pv = IV GC (13) Te combined termal/potovoltaic efficiency is te sum of potovoltaic and termal efficiencies of te system. 5. Results and Discussion η tot = η pv + η t (14) Termo-pysical and internal, used in te calculation are sows in (table 1). Te dimension of te system is (w=2m x L= 7.8 m). te dimensions of te fins are (δ=0.0015m x H=0.025 x L=7.8m ). Panel dimensions (w p =1.189m x L p =0.595m). te values of te radiation G and te ambient temperature T a used in tis study are constants in a calculation.

4 Universal Journal of Pysics and Application 9(6): , Table 1. Termo-pysical and internal parameters of te system potovoltaic panels Figure 5. Te mass flow influence on te termal efficiency Te figure 5 and figure 6, sow te effect of te mass flow on termal and electrical efficiency of te system respectively in te air flow direction. Te efficiency termal and electrical increases wit te air flow direction, te electric efficiency increase is linear and we ave a small increase in te curves wen te mass flow increases in te range iger tan 3m along te air flow direction. Te termal efficiency increases in exponential in te air flow direction, and is very important wen te mass flow increases figure 5. Tis is due to te improvement in internal convective excangers at a constant eat loss wen te air flow increases. Te termal conversion efficiency of te system is increasing from 18% to 55%, wen a mass flow rate varies from 0.02 kg/s to 0.1 kg/s. Figure 4. Te mass flow influence on te fluid, cells and insulating plate temperature Figure 4 sows te effects of te mass flow rate on te temperature of te fluid, te cells and insulating plate respectively as a function of te position along te panel lengt. It can be observed tat all te temperature, increase wit te position. Increasing of te air mass flow rate will decrease te temperature of te system, at constant sunligt radiance. Tis is explained by te fact by te increase of air amount at a given eat load, leading to a reduction of its temperature of exit. Figure 6. Te mass flow influence on te electric efficiency

5 248 Study of te Mass Flow Rates on te Efficiency of Hybrid Termal / Potvoltaique Sensor electric power of te sensor in te air flow direction respectively. Te curve of te power electric of sensor is identical in a lengt smaller tan 3m but above 3m tere is small increase wen te mass flow is increasing, and te difference in te total efficiency is important wit te increasing of te mass flow. Tis trend is explained by te fact tat wen te mass flow increases, te cells are cooled wic increases te electric power and te total efficiency. Note tat te power of eac cell decreases in te air flow direction wen te temperature increases, but for a given point in te sensor lengt tis value increases because te mass flow rate increases. te total conversion efficiency of te system is increasing from 27% to 65%, wen mass flow rates varies from 0.02 kg/s to 0.1 kg/s. Figure 7. Te mass flow influence on te total efficiency Figure 10. Te variation of te reference current according in te cell temperatureell Figure 8. Te mass flow influence on te electric power of eac cell Figure 11. Te variation of te panel current according on te panel voltage Figure 9. Te mass flow influence on te electric power of sensor Te figure 7 to 9 sow te mass flow rate influence on te cumulative efficiency te electric power of eac cell and Te variation of te reference current as a function of te cell temperature is sown in te figure 10. Te reference current increase, wit te increase of te cell temperature, te losses by effect of joule, increases, terefore te eac cell electric power decreases. In te same figure if te mass

6 Universal Journal of Pysics and Application 9(6): , flow increases, te reference current at a given position, decreases. Tis is induced from te fact tat te increase of te mass flow rate strongly cools te cells. If we consider figure 11, we deserve te current decreasing wen te temperature increases. Tis is consistent wit te pysical principles governing cells beavior. Indeed, te current (I) is proportional to te mobility and te density of electrons and oles. To te density of tese carrier s is constant in te used temperature domain, te current at constant, voltage is proportional to te carrier s mobility. It is known tat te carrier s mobility decreases wen te temperature increases. Tis is due to te strong diffusion of te carge carrier s by, te crystalline lattice and localized on doping atoms potons. Tis diffusion decreases te carrier s mobility, so te electric current, wic is in agreement wit our results. Were Re and Ra are Reynolds and Rayleig numbers respectively [11] and convective losses due to te wind is assumed to be 25 W/m2. K. All oter radiative eat transfer coefficients are taken as 6 W/m2.K. In equation 6, p and q are te temperature functions for various components of te ybrid termal/potovoltaic system. Were And p = cpf ( b η p + 1)/( m pc f w ) (17) q = b Ac cpfη p T p + cpf T b mpc f w (18) 7. Conclusions Using our model, we ave sown, te amount of eated air rises wit te air mass flow rate increases. Tis causes a decrease in te outlet temperature wic ten cools, te solar panel, owing to an improvement in te internal convective eat transfer. Consequently, te termal and te electrical performance of te system is enanced. Te increase of te electrical efficiency for solar cells following te decrease in teir temperature as a beneficial effect on te electrons and oles mobility. Tis carrier mobility increases as te temperature decreases. Te results obtained suggest tat it is a good alternative to conventional potovoltaic systems. Extracted eat could be used for eating te air or be transformed into anoter energy or elp prevent te problem of (ot spots) in potovoltaic generators. Appendix Ƞp is te total efficiency of te absorber plate [9]. And η fin is fin efficiency defined as: η p = +A fin η fin b (13) η fin = tan (ωh fin) ωh fin And ω = 2 cpf λ fin δ fin (14) Te convective eat transfer coefficients, cpf calculated using te following relationsip [10]. is cpf = λ f D (0.0158R e ( R e )exp X D (15) Te convection eat transfer coefficient between te solar cells and te glass cover is calculated from te relation [11]: cpg = λ f sin(1.8β) H pg R a cosβ R a cosβ (R a cosβ/5830) (16) REFERENCES [1] Kern Jr E C, Russell M C. Combined potovoltaic and termal ybrid collector system. In: Proceedings of 13t IEEE Potovoltaic Specialist, [2] L.W. Florscuetz, Extension of te Hottel-Willier model to te analysis of combined potovoltaic termal flat collector, Sol. Energy. 22 (1979) [3] H.P. Garg, R.S Adikari, Conventional ybrid potovoltaic termal (PV/T) air eating collectors: Steady-state simulation, Renewable Energy. 11 (1997) [4] H.P. Garg and R.S. Adikari, Performance analysis of a ybrid potovoltaic/termal (PV/T) collector wit integrated CPC trougs, Int. J. Energy. Res. 23 (1999) [5] G.R. Witfield, R.W. Bentley, C.K. Weaterby, B. Clive, Te development of small concentrating pv systems, Proc. of 29t IEEE PVSC. New Orleans. (2002) [6] M.Y. Otman, B. Yatim, Performance analysis of a double-pass potovoltaic/termal (PV/T) solar collector wit CPC and fins, Renewable Energy. 30 (2005) [7] X. Cen, Y.M. Xuan, Y.G. Han, Investigation on performance of a solar termopotovoltaic system, Sci. Cina. Ser. E-Tec. Sci. 51 (2008) [8] R. Ari, Optical and termal properties of compound parabolic concentrators, Sol. Energy. 18 (1976) [9] K. Sopiana, H.T. Liu, S. Kakac, T.N. Veziroglu, Performance of a double pass potovoltaic termal solar collector suitable for solar drying systems, Ener. Conv. Manage. 41 (2000) [10] S. Sarples, P.S. Carleswort, Full-scale measurements of wind- induced convective eat transfer from a roof-mounted flat-plate solar collector, Sol. Energy. 62 (1998) [11] S.J. Anand, T. Arvind, Energy and exergy efficiencies of a ybrid potovoltaic termal (PV/T) air collecto, Renewable Energy. 32 (2007) [12] A. Duffine, W.A. Beckman, Solar Engineering of Termal Processes, Jon Wiley & Sons, New York, [13] G. Walker, Evaluating MPPT converter topologies using a

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