Alleviating Operating Temperature of High Concentration. Solar Cell by Active Cooling

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1 Alleviatin Operatin Temperature o Hih Concentration Solar Cell by Active Coolin Fah Al-Amri Collee o Technoloy PO Box: 765; Dammam 347; Saudi Arabia alamri@tvtc.ov.a Tapa Kumar Mallic Mechanical Enineerin; School o Enineerin and Phyical Science; Heriot-Watt Univerity; Edinburh; UK; EH4HG ABSTRACT It i well now that the hihet poible olar cell eiciency i achieved by multijunction baed concentratin olar cell. However, due to very hih olar lux at the olar cell reion, the operatin temperature o the olar cell become very hih which le to neative power coeicient. In thi wor, a heat traner model or multijunction concentratin olar cell ytem ha been developed. The olar cell me o GaInP, GaA and Ge with active aluminium bac plate and anti-relected la plate wa modeled in thi wor. To alleviate the operatin olar cell temperature an active coolin wa ued. Water i orced to low within the duct behind the olar cell aembly and a inite dierence technique ha been ued to olve the overnin equation. In dition, the parametric analyi o the concentrator cell dein and the eect o the concentration ratio to the operatin temperature o the olar cell have been perormed. Reult how that maximum cell temperature i extremely dependent on water inlet velocity and channel width more than thicnee and thermal conductivitie o the olar cell holder and acceorie. It i alo ound that the thermal conductivity and the thicne o the olar cell aembly have mall eect compared to that o the luid propertie o the active coolin arranement. Keyword: Hih concentration; active coolin; orced convection.. INTRODUCTION The direct converion o olar enery into electricity i.e. Photovoltaic i the mot promiin ource o enery that i expected to ill the need o the world' rowin demand. The converion eiciency o vanced triple-junction cell have recently exceeded 43%. However, it ha been ound that the eiciency decreae with increain the operatin temperature o olar cell []. On the other hand, the operatin cell temperature increae remarably with an increae in the concentration ratio, reachin well over oc at 4 un []. Hence, olar cell need to be cooled durin operation. Paive coolin i o the eicient method or removin heat rom hih-illumination photovoltaic cell. However, uin paive coolin mean that heat enery (which i about 6-9% o the total enery i rejected to the environment a a wate enery. Thu, active coolin ytem enhance the overall eiciency and the economic eaibility. Cui min et al. [] and octal and Frot [3] developed a theoretical thermal model to predict the temperature o the multi junction olar cell baed on paive coolin ytem. icenza et al. [4, 5] ued a ilicon waer with micro channel circulatin water directly underneath the cell in their active coolin ytem. In the preent wor, an in-houe developed thermal model or active coolin o concentrator multi junction olar cell i developed and preented.. CONCENTRATOR TRIPLE JUNCTION CELLS ASSEMBL DESIGN: The chematic o the ytem eometry i hown in Fiure. It conit o a triple junction GalnP/GaA/Ge olar cell, a Cu-A-H ront contact to the olar cell, a mm cover la and a.5 mm thic aluminum Bac plate. The olar cell i attached to the cover la and rear aluminum plate via an heive. The olar cell ha been cooled by orcin water to low within the duct behind the bac plat.

2 3. GOERNING EQUATIONS AND BOUNDAR CONDITIONS: The phyical ituation depicted in Fiure can be decribed by the enery conervation equation in the multi wall and the equation o continuity, momentum, and enery conervation in the luid. Under the uual boundary layer aumption and nelectin the axial conduction o heat in both the olid and luid reion, the correpondin dimenionle equation which overn the conjuate laminar orced low and the heat traner in the entryreion o two parallel plate are: U ( Z Z > and S (i (i S( i ( i, i= and S( i ( i R and S(i (i < Z < Z c or (Z c +L c < Z < L, i=3 and 4 i i S(i (i S( i ( i S ( i ( i S( (6e i ( i KR R ( ( (6d U U dp U U ( Z dz and < Z < Z c or (Z c +L c < Z < L S5 7 U (3 Z Pr (4 The interal orm o the continuity equation ( i F Ud (5 RS 75 (6 S 7 ( 5 ( KR 5 S( i ( i S5 7 and Z c Z (Z c +L c, i=3, 4, and 5 S( i ( i S6 7 R S ( i ( i and Z c Z (Z c +L c S( i ( i (6 The above convection ield equation are ubject to the ollowin dimenionle boundary condition Z = and << U=, = P = = Z > and = U = =, and Z > and = U==, and (6a (6b 6 S 67 R R ( i Z> and 7 S 78 R R ( i S76 S87 S7 8 S 67 S 78 (6h (6i R (6c Z> and S8

3 R Where R and i S 8 8 i i * *...*, R i i R (6j i, (i i i TABLE THERMO PHSICAL PROPERTIES ASSUMED IN THE SIMULATION Lenth (mm Thicne (mm GuInp. Solar cell GuA. Ge. Front contact Cu+A+H.5 Gla cover Low-iron la Adheive.5 Bac plate Aluminium.5 Thermal Aborptivi conductivi ty ty w/m. GuInp 73.4 Solar cell GuA 65.4 Front contact Ge 6.5 Cu+A+H 3.5 Low-iron.4 Gla cover la Adheive 5. Bac plate Aluminium 38 Fiure. Schematic o the ytem eometry. 4. RESULTS AND DISCUSSION The overnin equation ( throuh (4 under the contraint o the interal continuity equation (5 and ubject to the boundary condition 6(a ( j are olved uin a inite-dierence numerical technique. A meh o 56, rid point wa contructed and a tran proram wa written to model the problem. The ytem dimenion and thermo phyical propertie ued in the imulation are ummarized in Table. The temperature o water at channel entrance i aumed to be 7 oc and the eiciency o the triple junction olar cell i.38. Fiure how the variation o the maximum cell temperature with the concentration ratio or three elected value o the inlet water velocity namely,.5,, m/. Cold water which enter the inlet channel remove heat rom the hot P cell, the heated water leave the exit channel which can be ued or water dealination, pace heatin, aborption coolin ytem or other application. A the inlet water velocity increae the convective heat removal rom the ytem reult in a iniicant reduction in maximum cell temperature a hown the iure. Thu, in turn, improve the electrical converion eiciency o the P ytem and increae the collected thermal o water. Fiure 3 how the eect o the channel width on the maximum P temperature. It i clear rom the iure that the maximum temperature decreae a the width decreae ince the convective heat traner coeicient cale inverely with the channel width. Thi mean that a lare amount o heat can be removed by narrowin the channel width to the micro-cale value. However, thi caue a iniicant preure drop that le to hih pumpin power requirement [5]. 3

4 Maximum cell temperature ( O C Maximum cell temperature ( O C Maximum cell temperature ( O C Maximum cell temperature ( O C inlet velocity =.5 m/ inlet velocity = m/ inlet velocity = m/ TABLE. THICKNESSES ALUES (MM USED FOR THE RESULTS GENERATED IN FIGURE 4. Cae I Cae II Cae III Tranparent ront cover 4. Adheive.5.5 Bac plate Fiure. Maximum cell temperature under concentrated illumination or three elected value o water inlet velocity. b= mm b=.75 mm b=.5 mm Fiure 3. Maximum cell temperature under concentrated illumination or three elected value o channel width. To tudy the eect o the thermal conductivitie and thicnee o the holder and acceorie o the olar cell (bac plate, heive, and tranparent ront cover three dierent cae or thicnee value (Table and thermal conductivitie value (Table 3 are tudied. Fiure 3 and 4 how the eect o thicnee and thermal conductivitie o bac plate, heive and tranparent ront cover on the maximum temperature o the cell, repectively. It can be oberved rom the iure that Increain thermal conductivitie or decreain thicnee o thoe component have a mall eect on the eiciency o the P ytem and the heat traner to the coolant. TABLE 3. THERMAL CONDUCTIITIES ALUES (W/M.K USED FOR THE RESULTS GENERATED IN FIGURE 5. Cae A Cae B Cae C Tranparent ront cover 3 4 Adheive 5.5 Bac plate cae I cae II cae III Fiure 4. Eect o thicnee o holder and acceorie o olar cell on maximum temperature. 8 7 cae A 6 cae B 5 cae C Fiure 5. Eect o thermal conductivitie o holder and acceorie o olar cell on maximum temperature. 4

5 5. CONCLUSIONS Finite dierence technique were ued to predict the maximum olar cell temperature cooled actively by water orced convection. It wa ound that the cell temperature can be iniicantly reduced when inlet velocity increae or channel width decreae. In dition, thermal conductivitie and thicnee o the olar cell holder and acceorie (bac plate, heive, and tranparent ront cover were ound to have mall eect on the cell temperature. 6. NOMENCLATURE b channel pacin, m K thermal conductivity, W m K Re Reynold number, T T u u o b temperature at any point, K inlet temperature, K entrance axial velocity,m/ u lonitudinal velocity component at any point, m/ U dimenionle lonitudinal velocity, U=u/u o v tranvere velocity component at any point, m/ dimenionle tranvere velocity, =bv/ y horizontal coordinate,m dimenionle horizontal coordinate, y/b z vertical coordinate,m Z dimenionle vertical coordinate, z/(b Re Gree Symbol inematic luid vicoity ρ luid denity, 3 μ dynamic lu id vicoity, m α aborptivity dimenionle temperature at any point T T q b Subcript heive luid la olid m 7. ACKNOWLEDGEMENTS Thi wor wa upported by Kin Abdulaziz City or Science and Technoloy, Saudi Arabia 8. REFERENCES [] M.J. Huan, P.C. Eame, B. Norton, Thermal reulator o buildin-interated photovoltaic uin phae chane, Int. J. Heat Ma Traner 47 ( [] Cui Min, Chen Nuou, an Xiaoli, Wan u, Bai imin, and Zhan Xinwan, Thermal analyi and tet or inle concentrator olar cell, Journal o Semiconductor, ol. 3, No. 4 (9 [3] H. Cotal and J. Frot, Heat traner modelin o concentrator multijunction olar cell aemblie uin inite dierence technique, 3t IEEE PSC Conerence ( 3-8. [4] D. incenzi, F. Bizzi, M. Steancich, C. Malau, G. L. Morini, A. Antonini, G. Martinelli, Micromachined ilicon heat exchaner or water coolin o concentrator olar cell, Conerence record, P in Europe Conerence and Exhibition-From P technoloy to Enery Solution, Rome, Italy,. [5] Anja Royne, Chritopher J. Dey, David R. Mill, Coolin o photovoltaic cell under concentrated illumination: a critical review, Solar Enery & Solar Cell 86 (