Design and Investigation of Solar Hybrid Electric/Thermal System with Sun-tracking Concentrator, Photovoltaic and Thermoelectric Generators

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1 Design and Investigatin f Slar Hybrid Electric/Thermal System with Sun-tracking Cncentratr, Phtvltaic and Thermelectric Generatrs EDGAR ARTURO CHÁVEZ-URBIOLA, YURI V. VOROBIEV Centr de Investigación y de Estudis Avanzads del IPN, Unidad Querétar Libramient Nrpniente 2, Real de Juriquilla, 7623, Querétar, MÉXICO vrbiev@qr.cinvestav.mx Abstract: - We study slar hybrid system that includes Sun-tracking slar radiatin cncentratr, phtvltaic cell and thermelectric generatr with the heat extracting water-based thermsyphn unit, s that the system prduces electric and thermal energy. Theretical mdeling was made, shwing that the system has a great ptential fr develpment and applicatin, with the ability t prvide higher efficiency and thermal stability than traditinal hybrid PV/Thermal systems. Experimental study was perfrmed with Si mncrystalline phtvltaic cell and thermelectric generatr (TEG) using the classic Bi 2 Te 3 material. The results btained agree with theretical predictins. The system studied can be efficient and ecnmic, especially in cuntries with high inslatin, like Mexic, China and Greece. Key-Wrds: - Slar hybrid system, Electrical and thermal energy, radiatin cncentratin, Phtvltaic cell, Thermelectric generatr Intrductin It is nw well-knwn and generally accepted that the energy prductin and cnsumptin in the near future must be based upn the renewable energy surces (slar energy, first f all), and the efficiency f utilizatin f these surces is f great imprtance. In this aspect, the slar hybrid systems are mst interesting because they are mre efficient and mre stable in relatin t the variatins f the surce intensity than individual slar devices; that is why the great attentin f scientists and technlgists was fcused n these systems during the last decades (see, fr example, [-]). The simplest f the hybrid systems, and the mst widely used and studied, is the PV/Thermal (PVT) system cnsisting f phtvltaic (PV) panel cupled t heat extractr with running water r air. Usually PVT system emplys crystalline Si slar PV mdule, taking advantage f its cling by heat extracting unit thus increasing its efficiency, and prduces 4 W/m 2 f electric energy at peak illuminatin, and 3 6 times as much thermal energy stred in water/air, heated up t 45 5 C. In several publicatins [-3] studying was perfrmed f the pssibilities f use f thermelectric generatrs (TEGs) in slar systems, with the cnclusin that TEGs will bring new features in the field and can be successfully used in hybrid systems, instead f PV panels r tgether with them. It is imprtant t mentin an essential increase in thermelectric cnversin efficiency reached during the last decade and cnnected with the utilizatin f nanstructured materials fr their prductin [4, 5]. In ur previus papers [, 6] the general analysis was made f the different gemetries f slar hybrid systems emplying TEGs. One f the schemes treated there, namely, the system with slar radiatin cncentratr, phtvltaic cell, TEG and water filled heat extractr using thermsyphn effect, was the subject f studying in this paper where theretical estimatins as well as experimental results are presented. 2 Descriptin and Mdeling f the System All parts f the hybrid system studied are shwn in Fig. where slar radiatin flux is dented as. The radiatin cncentratr attached t the 2-axis Suntracking blck (nt shwn) is represented by Fresnel lens 2 that with the help f the plain mirrr 3 creates unifrm illuminatin f the generating stage including PV cell 4, TEG 5 and water-filled heat extractr 6 which is a part f thermsyphn circuit (its ther parts are the strage tank cnnected with the water tubes, as seen in the figure). We dente as C the radiatin cncentratin degree that takes int accunt all ptical lsses in the cncentrating system, s that the radiatin intensity n the generating stage is IC where I is slar radiatin ISBN:

2 Fig.. Scheme f the hybrid system. intensity at the entrance It is knwn [7] that radiatin cncentratin causes increase f the PV cell efficiency with practically linear grwth f shrt circuit current I SC and lgarithmic increase f pen circuit vltage V OC ; n the ther hand, it leads t an increase f the cell s temperature that reduces V OC and the efficiency. These variatins are dminated by the changes f V OC value; fr this, in a gd apprximatin we can use the expressin (see [7]) kt CI SC ln () e I Here k is the Bltzmann cnstant, e - electrn s charge, I SC - shrt circuit current withut cncentratin (C = ), I cell s saturatin current having expnential dependence upn the band gap E g and temperature T: I = A exp (-E g /kt), A = cnstant (actually, it has a weak temperature dependence that is neglected here). Having intrduced the frmulae fr I in (), we can btain an apprximate expressin fr the temperature dependence f V OC fr relatively small variatins f temperature in relatin t the ambient temperature T = 3 K, assuming that T = T + T = T (+ T/T ), T <<T. kt CISC ln e A exp( Eg / kt ) (2) kt CISC = ln e Eg Aexp kt ( + T / T ) Taking int accunt that (+ T/T ) - ( T/T ) when T <<T, we prceed in the fllwing way: kt CISC ln e Eg T Aexp kt T (3) kt CI E SC g T = ln exp e Aexp( Eg / kt ) kt T One can see that the first part f the last lgarithmic expressin actually defines initial value f V OC (withut cncentratin) multiplied by C, s that we finally btain: T kt E = + g T V ( C, T) V (, T ) lnc (4) OC OC T e e T This expressin clearly shws the tendencies f variatin f the pen circuit vltage (and the cell s efficiency) with changes f cncentratin degree and the cell s temperature: since the temperature increase T has apprximately linear dependence upn C value (see experimental data in Fig. 2), the secnd (psitive) term in (4) increases with cncentratin degree in lgarithmic way, whereas the third, negative term, is the linear functin. Therefre, ne shuld bserve increase f V OC fr small cncentratins when the third term is negligible, and the decrease at larger C values. The Fig. 2 shws such a dependence calculated accrding t (4) with the initial vltage V OC (,T ) equal t.52 V that crrespnds t the c-si cell used in experiments; besides, the Si band gap value ISBN:

3 was taken fr calculatins. The dependence fund exactly crrespnds t the predictins. (V) PV Temp Fig. 2. Effects f cncentratin (see text). Under assumptins made, the expressin (4) als will give the temperature/cncentratin dependence f the slar cell efficiency η(c,t), if we multiply it by η /V OC (,T ): T kt E g T η( C, T) = η + lnc (5) T ev OC (, T ) ev OC(, T ) T Here η is the cell s efficiency withut cncentratin at ambient temperature. The TEGs efficiency η* (see [, 6]) is the functin f temperature difference between its plates that we take equal t T, assuming a gd thermal cntact between PV cell and TEG s ht plate and efficient cling f its cld plate by heat extracting system, as well as f the thermelectric figure f merit ZT f TEG s material: T * = T M c h PV Temp ( C) M η (6) h + ZT + ZT + T / T Here T h and T c are the temperatures f the TEG s ht and cld plates crrespndingly (thus we assume that T c = T a, and T h = T a + T), and index M at figure f merit means that it crrespnds t the average TEG s temperature. As it was pinted ut in [6], the classic thermelectric material Bi 2 Te 3 used in ur experiments has the ZT value f.7, whereas the highest value fund in the literature is 4; here we shall use bth values fr the discussin. Taking int accunt that TEG absrbs thermal energy frm PV cell that is less than the incident slar energy (part f that is reflected and anther transfrmed int electricity), we shall write fr the efficiency f TEG as a part f the hybrid system η TH the fllwing expressin: η TH = η ( R)( η) (7) Here R is cell s reflectin cefficient, and η its electrical efficiency, s this parameter characterizes additinal electrical efficiency given by the TEG; the ttal electric pwer generated by the system will be equal t the sum η + η TH multiplied by the incident energy. Figure 3 presents temperature dependence f the cell s efficiency calculated after (5) (curve ) and f the η TH values after (6, 7) fr bth values f ZT abve (curve 2 fr ZT =.7, and curve 3 fr ZT = 4; value f R is taken as.9 assuming the antireflectin cating). Curves 4 and 5 give summary system s efficiency fr these 2 types f TEGs. It can be seen that in bth cases the system s efficiency in certain temperature interval is higher than the efficiency f PV cell alne, thus shwing the thermal stability f parameters and increase f the electric pwer generatin. The thermal efficiency f the system will nt be much different frm that f traditinal PVT system. η (%) T (K) Fig. 3. Calculated electrical efficiency f hybrid system. 3 Experimental: methds, results and discussin Experimental investigatins were carried ut with the mdel using mirrr msaic cncentratr having 55 mirrrs, each ne f the size f the PV cell and psitined in a manner t reflect the slar light t the generating unit; varying a number f mirrrs pen fr illuminatin, we can change cncentratin degree frm t 55 (see pht Fig. 4). One can see that the mdel has all the elements present in a scheme f Fig.. ISBN:

The cncentratr was made in ur labratry. The c-si PV cell 8 X 8 cm 2 used in the experiments was elabrated by factry Krasne Znamya (Mscw, Russia); 4 TEGs 4 X 4 cm 2 each f the type TGM-27-.4-2.

4 The cncentratr was made in ur labratry. The c-si PV cell 8 X 8 cm 2 used in the experiments was elabrated by factry Krasne Znamya (Mscw, Russia); 4 TEGs 4 X 4 cm 2 each f the type TGM were made by Krytherm, Saint Petersburg, Russia. expected, althugh nt in the linear manner. We ascribe this discrepancy t ur chice f the PV cell, and hpe t get mre linear dependence with the ther cells in the nearest future..62 ICC (V) ICC (A) Fig. 5. dependencies f pen circuit vltage V OC and shrt circuit current I SC f the PV cell..5 Fig. 4. Pht f experimental mdel f the hybrid system. T prvide a gd thermal cntact between the elements f the energy generating stage, special thermal paste was used (fr this and the ther experimental details, see [6, 8]). The cnstructin f thermsyphn heat extracting circuit was chsen after calculatin f the water flux parameters crrespnding t the laminar flw and efficient heat exchange between the cpper rear wall f the energy generating stage and running water in the circuit; heat exchanger design was made n the basis f finite element cmputer simulatin mdel in cmmercial sftware (COMSOL Multiphysics 4.2a, [8]. The characteristics f the PV cells at different cncentratin degrees are presented in Fig. 5. It is seen that the pen circuit vltage behaves, in the first apprximatin, as it was predicted in sectin 2: it increases at initial small cncentratins, and then decreases, as the temperature grws. The shrt circuit current increases with cncentratin, as The electric pwer generated by the system as a functin f cncentratin is shwn in Fig. 6. Here we see that the generatin by TEGs is practically linear functin f cncentratin (i.e. f temperature), exactly as it shuld be expected. Generatin f the PV part f the system agrees with the data f Fig. 5, shwing saturatin at C frm 5 t 3, thus giving a mdest increase f the ttal electric pwer generated. Electric pwer (W) PV TEG Ttal Fig. 6. Electric pwer generatin at different cncentratins. The saturatin f electric pwer generatin in PV cell indicates that the c-si mdule used in ur experiment is far frm ideal chice fr this system; it gives expected grwth f generatin up t C = 5 that crrespnds t the incident slar pwer f 3 W ISBN:

5 and the cell s efficiency f arund 6% (the ttal system s electric efficiency at this cncentratin degree is 7.3%). Thus we see that ur experimental mdel wrks better at small cncentratins than at large nes, which is rather unusual. It is easy t shw that with GaAs-type f PV cell, the results fr electric efficiency will be much better, and with cncentratin degree f 3-5 it will be pssible t btain ttal electric efficiency f rder f 2% r mre (fr this estimatin, we use the data fr GaAs slar cell treated in [], and extraplate the btained TEG s efficiency t higher cncentratins and temperatures; we might als nte that the mre efficient materials fr TEG can be used). Besides, it must be mentined (see [6]) that the ptimal use f TEGs in hybrid systems demands reasnable increase f temperature and therefre higher cncentratins, s it is nt practical fr nrthern latitudes; hwever, in cuntries with high inslatin (Mexic, India, China, Greece, Turkey etc.) it will be quite efficient. The thermal pwer generated by the hybrid system and stred in ht water tank as functin f cncentratin is presented in Fig. 7. The dependence is almst linear which is quite nrmal. The thermal efficiency defined as rati f the generated thermal pwer t the incident slar radiatin pwer is given by Fig. 8. Thermal Pwer (W) Thermal pwer Fig. 7. Thermal pwer generatin in the system. It is seen (Fig. 8) that starting frm cncentratin degree C = 5, the thermal efficiency is almst cnstant, staying at the level f arund 45%, which is standard fr the majrity f PVT hybrid systems. The relatin between the thermal and electric pwer generated in ur hybrid system mdel is larger than that in traditinal PVT, and changes with cncentratin: it is 9 at C = 5, and 3 at C = 25. As it was mentined abve, this is a cnsequence f the behavir f c-si PV cell at higher cncentratins, and culd be different fr ther cells. Thermal efficiency (%) Thermal efficiency Fig. 8. Thermal efficiency f the hybrid system. 4 Cnclusin Theretical mdeling and experimental study f the slar hybrid electric/thermal system with cncentrated radiatin, PV cell and TEG prve that it can be a gd alternative t the traditinal PV/Thermal hybrid systems. With ur experimental mdel we have shwn that the system s electrical efficiency is cmparable with that f traditinal system, and its thermal efficiency is practically the same, with larger amunt f energy generated. The use f cncentratin allws emplyments f mre expensive and efficient PV cells like GaAs r the tandem cells withut essential increase in system s cst; thus the system studied can be bth efficient and ecnmic, and therefre is prmising fr develpment and applicatins. Acknwledgement The wrk was supprted by CONACYT, Mexic, prject N Analysis f the factrs that determine the glbal efficiency f the slar hybrid systems. References: [] E.C. Kern, M.C. Russel, Cmbined phtvltaic and thermal hybrid cllectr systems. Prceedings f the 3th IEEE Phtvltaic Specialists, Washingtn DC, USA, 978, pp [2] J. Prakash, Transient analysis f a phtvltaicthermal slar cllectr fr c-generatin f electricity and ht air/water. Energy ISBN:

6 Cnversin Management, Vl. 35, 994, pp [3] B.J. Brinkwrth, Estimatin f flw and heat transfer fr the design f PV cling ducts, Sl. Energy, Vl. 69, 2, pp [4] H.A. Zntag, Flat-plate PV-Thermal cllectrs and systems: A review. Renewable and Sustainable Sustainable Energy Reviews, Vl. 2, 28, pp [5] Y. Tripanagnstpuls, Aspects and imprvements f hybrid phtvltaic/thermal slar energy systems. Sl. Energy, Vl. 8, 27, pp.7-3. [6] A. Ibrahim, M.Y. Othman, M.H. Ruslan, S. Mat, K. Spian, Recent advances in flat plate phtvltaic/thermal (PV/T) slar cllectrs, Renewable and Sustainable Energy Reviews, Vl. 5, 2, pp [7] J. Aschaber, C. Hebling, J. Luther, Realistic mdelling f TPV systems, Semicnd. Sci. Technl., Vl. 8, 23, pp [8] S. Dubey, G.N. Tiwari, Thermal mdeling f a cmbined system f phtvltaic thermal (PV/T) slar water heater. Sl. Energy, Vl. 82, 28, pp [9] A. Luque, A. Marti, Limiting efficiency f cupled thermal and phtvltaic cnverters, Sl. Energy Mater. Sl. Cells, Vl. 58, 999, pp [] Yu. Vrbiev, J. Gnzález-Hernández, L. Bulat, Thermal-phtvltaic slar hybrid system fr efficient slar energy cnversin, Sl. Energy, Vl. 8, 26, pp [] S.A. Omer, D.G. Infield, Design ptimizatin f thermelectric devices fr slar pwer generatin. Sl. En. Mater. Sl. Cells, Vl. 53, 998, pp [2] [2] L. Peng, C. Lanlan, Z. Pengcheng, T. Xinfeng, Z. Qingjie, M. Niin, Design f a cncentratin slar thermelectric generatr. J. Electrn. Materials, Vl. 39, 2, pp [3] D. Kraemer, L. Hu, A. Mut, X. Chen, G. Chen, M. Chiesa, Phtvltaic-thermelectric hybrid systems: A general ptimizatin methdlgy. Appl. Phys. Lett., Vl. 92, 28, [4] A. Tavkhelidze et al., Therminic cnverter with quantum tunneling. Prc. XXI Int. Cnf. n Thermelectrics, August 26-29, IEEE. 22, pp [5] L.P. Bulat et al., Bulk nanstructured p-bi-sb- Te thermeletcrics btained by mechanical activatin methd with ht pressing. J. Electrn. Materials, Vl. 39, 2, pp [6] E.A. Chávez-Urbila, Yu.V. Vrbiev, L.P. Bulat, Slar hybrid systems with thermelectric generatrs, Sl. Energy, Vl. 86, 22, pp [7] A. Luque, S. Hegedus, editrs, Handbk f phtvltaic science and engineering, Jhn Wiley & Sns, 25. [8] E.A. Chávez-Urbila, Y.V. Vrbiev, Slar hybrid electric/thermal system with thermelectric generatr. Prceedings f 6 th IEESE, June 28-3, 22, Izmir, Turkey (in print). ISBN:

Verification of Quality Parameters of a Solar Panel and Modification in Formulae of its Series Resistance

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