Simulation of hybrid Photovoltaic-Thermal Collector (PV-TC) Systems for domestic Heating and Cooling Case Study: Island of Rhodes

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1 Simulation of ybrid Potovoltai-Termal olletor (PV-T) Systems for domesti Heating and ooling ase Study: Island of odes N. HISTANDONIS G.A VOKAS. SKITTIDES Department of Meanial Engineering - Management of Energy Systems Tenologial Institute of Eduation - ΤΕΙ Peireaus P. alli and Tivon 5, 144 Aigaleo, Atens GEEE Abstrat: Te idea of ybrid potovoltai-termal olletors (PV-Ts) is based on te simultaneous operation of a potovoltai laminate and a termal olletor attaed as a termal absorber. PV-Ts ave improved eletrial effiieny, due to te derease of te potovoltai panel temperature. Te termal effiieny of PV- Ts is also remarkable. Tis resear aims to simulate and define PV-Ts eletrial and termal effiieny. After te teoretial simulation of a PV-T, its eletrial and termal effiieny is alulated and ompared respetively to te eletrial effiieny of a potovoltai panel and to te termal effiieny of a onventional solar olletor. Tis omparison leads to signifiant onlusions onerning te PV-T overall effiieny in relation to its surfae area. Te teoretial study of a potovoltai-termal system for domesti eating and ooling in te region of odes resulted tat te system an over a remarkable perentage of te domesti eating and ooling demands. Key-words: Potovoltai-termal olletor, Solar eating and ooling 1 Introdution Solar radiation onsists of potons and every poton as a speifi quantity of energy, wi is equal to te produt of its mass and speed. Te material normally used for te manufature of potovoltai panels is silion. Ea eletron of te nuleus of te silion atom possesses a quantity of energy in order to remain in te nuleus. Every poton, wi as energy equal or iger tan te above-mentioned energy, is absorbed by te silion atom and ativates te eletron, wile te rest of te potons onvert teir energy to eat. One very important element is tat ea poton an only ativate and release one eletron from te nuleus. Te potons, wi ave more energy tan te minimum energy required for te ativation of an eletron, onvert it to eat on te potovoltai ell [1]. Te result is tat a great part of solar radiation turns to eat on te potovoltai ell. However, te eletrial effiieny of te potovoltai panel is redued wen te temperature inreases. Tis is evident from equation (1), wi sows ow te effiieny of a potovoltai ell is influened by its temperature []: n mp = n mp,ref + µ P,mp (T T pv,ref ) (1) Equation (1) sows tat ea potovoltai panel produes apart from eletrial energy a remarkable amount of termal energy as well, wen solar radiation falls on its panel. Te idea of improving te eletrial effiieny, by reduing te potovoltai olletor s temperature, as well as taking advantage of te termal energy produed, onstitutes te basi idea in te development of ybrid PV-Ts. Problem ormulation It is well known and proved in several energy audits tat te greatest part of energy onsumption in ouses and buildings is for eating and ooling. Tis part an rea almost 6% of te total energy [3]. Taking into aount tat all reent years sow a rising trend of te average global temperature, te ooling of ouses during summer as also beome an important issue. Te notable inrease in te use of air-onditions espeially at urban areas in European Union during te summer may ause problems overloading te eletrial network leading onsequently to blakouts. Tis problem is more evident in Greek islands, were te autonomous diesel stations are often unable to over te inreased eletrial load during summer. A possible solution to tis problem ould be te extended use of te ybrid PV-Ts in order to partially meet te needs for eletrial energy and for domesti water eating and air ooling.

2 Te subjet of PV-Ts is quite new, so te existing bibliograpy onsists of a few reently publised artiles. Wiebe [4] desribes a proedure for designing a potovoltai-termal system using analytial equations. Sandnes and ekstad [5] ave also developed a ybrid PV-T wit several different absorbing surfaes. A sound paper on PV- Ts as been written by Tripanagnostopoulos et al [6], were te eletrial and termal effiienies of four different settings of PV-Ts wit air and water as an absorption medium are ompared. It is obvious tat not many ybrid potovoltaitermal systems aiming at furter usage of teir produed termal energy ave been developed so far. So, tere is mu to investigate espeially towards te o-operation of PV-Ts wit oter additional equipment (for example absorption illers), teir use instead of flat plate solar olletors and te appliation of omplete ybrid systems in buildings. In tis resear a PV-T at NOT onditions is simulated and ten its eletrial and termal effiieny is alulated. Ten te eletrial effiieny of a onventional potovoltai olletor and te termal effiieny of a onventional solar olletor, wit arateristis similar to tat of te PV-T, are also alulated. Te results of te above alulations for te termal and eletrial effiieny of te PV-T are ompared respetively to te termal effiieny of a onventional solar olletor and to te eletrial effiieny of a onventional potovoltai olletor. In addition, in te present resear te use of a system onsisting of PV-Ts for te overage of domesti eating and ooling load in te area of odes is analysed. urtermore, te solar overage perentage of te potovoltai-termal system is alulated and ten ompared to te solar overage perentage of a onventional solar olletors system also installed in odes. Moreover, te results of te above omparison are indiative of weter te PV-Ts an replae te onventional solar olletors for domesti eating and ooling appliations. 3 Problem solution 3.1 Termal effiieny Te instant termal effiieny of te PV-T is expressed by te following equation: Te arateristi parameters of te PV-T (τα) n and U are alulated using te equations tat refer to te flat plate olletors []. Te equations are modified and adjusted properly in order to inlude te added setion of te potovoltai aording to te resear of Wiebe [4]. Using tese equations te eat removal fator ( ) and overall loss oeffiient (U ) of a PV-T is alulated. Te transmittane absorptane produt of a PV-T was equal to.74 aording to Wiebe [4]. Te alulated values of te PV-T oeffiients are presented in te Table 1, wi also inludes te results of te above alulations of te oeffiients repeated for te same termal olletor, witout taking into aount te laminate of te potovoltai. Table 1. PV-T and onventional solar olletor arateristis parameters Symbol Pv-t Solar olletor U (τα) n Te temperature tat is developed on te PV-T is alulated using te following equation [4]. T pm Q u A ( ) = T fi + 1 (3) U or Nominal Operating ell Temperature (NOT) onditions (irradiane level 8W/m, wind veloity 1m/se, ambient temperature o ) te temperature tat is developed on te PV-T is found to be equal to T pm = o. Aording to te equation () and te above values of U and (τα) n, te termal effiieny of te potovoltai-termal and of te termal olletor are presented in te igure 1. Effiieny n,8,6,4, pv-t termal olletor,5,1,15 (Ti-Ta)/G [W/m^ ] n T ( τα ) U T T i α = n () GT ig. 1. Termal effiieny of te potovoltaitermal and termal olletor

3 Aording to Wiebe [4], at NOT onditions te inlet fluid temperature may be onsidered stable and equal to o. Terefore, te PV-T s termal effiieny is: n t =.678 or 6.78% At NOT onditions te onventional solar olletor s termal effiieny is: n t =.6968 or 69.68% rom tese alulations it is onluded tat te termal effiieny of te PV-T is about 8.9% less tan te termal effiieny of te termal olletor. Tis small differene is due to te insulation of te PV-T. 3. Eletrial effiieny or te investigation of te PV-T te arateristis of te SP15 potovoltai olletor of te Sell ompany are used. All te details for tis potovoltai are inluded in te produt information seet of SP15 [7]. Te olletor is manufatured from 7 series of onneted 15x15 (mm) mono-rystalline silion solar ells. Its dimensions are 1.6 x.814 (m)= 1.3m and its eletrial arateristis at NOT onditions are sown in Table. Table. Eletrial arateristis of te SP15 panel at NOT onditions [7] Temperature T (pv)not 45 o Peak power P mp 19 W Peak power voltage V mp 31. Volts Open iruit voltage V o 39.9 Volts Sort iruit urrent I s 3.9 A Te eletrial effiieny of a PV-T is represented at te equation (1). Tis equation desribes te influene of te temperature T of te potovoltai olletor on te system's effiieny n mp. Te term T pv,ref refers to te temperature of te potovoltai olletor at referene onditions (5 o ). Te maximum power point effiieny of te potovoltai olletor is alulated at NOT onditions and panel temperature 45 o, aording to te equation (4) as desribed in []. I mp Vmp nmp, ref ( pv) = =.13 (4) A GT or irradiane level 8W/m and ambient temperature o, te PV-T s temperature was found to be o. Te derease of te PV-T s temperature affets te olletor s I mp and V mp and aording to te produt information seet, te power (P mp ) is inreased.45% per ο [7]. Terefore, te redution in temperature of 9. ο ( ), results to an inrease of te PV-T s power (P mp(pv-t) ) for.45 x 9. = 4.59%. Hene te potovoltai panel power jumps from 19 W to 113.4W. P mp(pv-t) = I mp V mp = W (5) Te maximum power point effiieny of te PV-T at NOT onditions aording to te following equation is: I mp Vmp nmp, ref ( pv t) = =.174 A G Te potovoltai effiieny temperature oeffiient is µ P,mp (pv-t) = -.46 (1/K) for bot te PV-T and onventional potovoltai olletor. inally, based on te above elements te eletrial effiieny of te potovoltai panel and of te PV-T at NOT onditions is: n mp,(pv) = (45 5) =.94 or 9.4% n mp,(pv-t) = ( ) =.13 or 1.3% Hene, te ybrid PV-T as.83% iger eletrial effiieny in relation wit te plain potovoltai panel. 3.3 A Potovoltai - Termal system for domesti eating in odes Te approximation metod -art eating is used for te study of te potovoltai-termal system for te overage of te domesti eating load. Moreover, te same metodology is used for te onventional solar system, wi is ompared to te potovoltai-termal system. Tis metodology as been extensively developed by Klein, Bekman and Duffie in teir book Solar eating design by -art metod [8]. In te present resear te analytial equations of te -art metod tat are put to use are tose mentioned in te book Analysis of Energy Systems [3]. Aording to tis metod te perentage of montly termal domesti load, wi is overed by solar energy, is determined by te equation (6) for liquid system. f = 1.9 Y Y for <Χ <18, <Υ <3 3 T.45 Y + (6)

4 and Y are two dimensionless parameters. is related to te ratio of te olletor's losses to eating load and Y is related to te ratio of absorbed solar radiation to eating load. Teir values are defined from te equations (7) and (8). Y ( T ref T a ) t k 1 k A = U (7) A τα ( τα ) n H k3 ( ) (8) τα n = β Te montly average inidene solar radiation (H β ) on te PV-T surfae for te region of odes was alulated aording to te equations tat are presented in te book onventional and mild forms of energy [9]. Te domesti eating onsumption ( ) is alulated by using te Degree-days metod of Kreider and abl [1]. rom te use of te above elements arises te perentage of te montly termal domesti load overed by a potovoltai-termal system (4 m ) at te region of odes. Te same proedure was repeated for a system of solar olletors wit total surfae area equal to tat of te PV-Ts (4m ). Te results are presented in te Table 3. Table 3. Montly eating domesti load overed by potovoltai-termal system and onventional solar system during te winder for te region of odes Mont T α [] Η β [kw/m ] [GJ] (PV-T)H (Solar)H o o o o o o o o o o o o Average.8 o Total Te solar overage perentage of te domesti eating load from a potovoltai-termal system and from a system of onventional solar olletors is presented in ig.. Solar overage Nov De Jan eb Mar Apr Mont Pv-t system onventional solar system ig.. Montly solar overage perentage of te domesti eating load from a potovoltai-termal system and from a system of onventional solar olletors 3.4 A Potovoltai - Termal system for domesti ooling in odes Te analysis of te system during te summer season is ompleted by using te metod -art ooling, wi is torougly examined in te researes of Joudi and Abdul-Gafour [11-1]. Te overage of te ooling load during te summer season is aieved by a low power absorption iller. Te oeffiient of performane (OP) of tis system is between.6 and.8 [13] and in te present investigation it is used OP =.6. Aording to tis metod te perentage of montly ooling domesti load, wi is overed by solar energy, is determined using te following equation. f = ( A B + A ) Y 1 + ( A ) + ( B + + B + Table 4. -art ooling parameters ) 1 Y (9) A = A 1 = A = B = B 1 = B = = =.6918 =.41 Were: is te ratio of te referene montly energy loss to te montly energy required for operating te absorption iller,

5 Y is te ratio of te montly energy absorbed on te PV-Ts surfae to te montly energy required to operate te absorption iller. Tese values an be alulated by using te equations (1) and (11). Y OP A = OP A = U ( τα ) n ( T T ) ref H β α t (1) (11) Solar overage May June July Aug Sept Ot Mont Pv-t system onventional solar system Te ooling domesti onsumption ( ) is alulated using te ooling Degree-days metod of Kreider and abl [1]. rom te use of te above elements arises te perentage of te montly termal domesti load overed by a potovoltaitermal system (4 m ) at te region of odes. Te same proedure was repeated for a system of solar olletors wit total surfae area equal to tat of te PV-Ts (4m ). Te results are presented in te Table below. Table 5. Montly ooling domesti load overed by potovoltai-termal system and onventional solar system during te summer for te region of odes Mont T α [] Ηβ [kw/m ] [GJ] (PV-T) (Solar) o o o o o o o o o o o o Average.8 o Total Te solar overage perentage of te domesti ooling load from a potovoltai-termal system and from a system of onventional solar olletors is presented in igure 3. ig. 3. Montly solar overage perentage of te domesti ooling load from a potovoltai-termal system and from a system of onventional solar olletors 4 onlusions Te presented analysis and simulation led to te onlusion tat te termal effiieny of a PV-T is about 9% lower tan te effiieny of te onventional solar olletor. A very important fat is tat a PV-T is operating at lower temperatures tan plain potovoltai panels leading to a iger eletrial effiieny of about.9%. Gatering te results in Table 6 one an observe te differenes in te termal and eletrial effiieny of a PV-T and a system onsisting of a solar olletor and a onventional potovoltai panel. Altoug a PV-T as a lower total effiieny (by almost 8%), it demands only alf of te total surfae area. Table 6. Total effiieny of a PV-T (1.3m ) and of a system onsisting of a potovoltai panel (1.3m ) and a solar olletor (1.3m ) Potovoltai-termal olletor (PV-T) Potovoltai panel and termal olletor n termal n eletrial n termal n eletrial 6.78% 1.3% 69.68% 9.4% 71.1% 79.8% Te ase study worked out for a PV-T system designed to over part of te eating and ooling load of a typial ouse on te island of odes. Using te -art and -art ooling metods it is proved tat te above system an over 63.4% of te domesti eating load and 33.43% of te domesti ooling load. In addition, a onventional solar system an over 69.17% of te domesti eating load and 4.37% of te domesti ooling

6 load. rom te above omparison it is proved tat a potovoltai termal system an replae onventional solar systems in domesti eating and ooling appliations. Te use of potovoltai-termal systems in ouses an ontribute to te redution of te energy onsumption for eating, ooling and to te eletrial energy onsumption wile at te same time it is aieved te redution of te total surfae area of te system. Nomenlature: A : Te surfae area of te PV-T Α : Te PV-T's area OP : Te average oeffiient of performane of te absorption iller f : Te montly domesti ooling load overed by solar energy f : Te montly domesti eating load overed by solar energy : Te eat removal fator : Te olletor-eat exanger effiieny fator G T : Te solar radiation at NOT H β : Te montly average daily radiation inident on te olletor I mp : Te peak power urrent k 1 : Te orreting oeffiient of te apaity for te storage k : Te orreting oeffiient of warm water k 3 : Te orreting oeffiient of te eat load exanger : Te montly domesti ooling load : Te montly domesti eating load n mp : Te eletrial effiieny of te potovoltai olletor n mp,ref : Te maximum power point effiieny of te potovoltai olletor at referene onditions P mp : Te peak power Q u : Te useful energy gain T α : Te ambient temperature T : Te potovoltai olletor temperature T fi : Te inlet fluid temperature T i : Te inlet fluid temperature T pm : Te mean plate temperature T pv,ref : Te potovoltai olletor temperature at referene onditions. T ref : Te referene temperature (1 o ) U : Te overall loss oeffiient V mp : Te peak power voltage t : Te duration of ea mont in (se) µ P,mp : Te potovoltai effiieny temperature oeffiient (generally a negative number) (τα) n : Te transmittane absorptane produt of te PV-T eferenes: [1] T.Markvart, Solar Eletriity, nd ed., Soutampton: University of Soutampton. Jon Willey and Sons TD,. [] J.A.Duffie, W.A. Bekman, Solar engineering of termal proesses, nd ed., New York: Jon Willey and Sons In, [3] K.Balaras, E. Daskalaki, A. Tsagrasoulis, Analysis of Energy Systems, 3 rd ed., Atens: TEI Pireaus, 1. [4] D. Wiebe de Vries, Design a potovoltai/termal ombi-panel, Eindoven University Press, [5] B. Sandnes, J. ekstad, A potovoltai/termal (pv/t) olletor wit a polymer absorber plate. Experimental study and analytial model, Department of Pysis, University of Oslo,. [6] Y.Tripanagnostopoulos, TH. Nousia, M. Souliotis, P. Yanoulis, Hybrid potovoltai/termal solar systems, Solar Energy, Vol.7, No.3,. [7] Sell solar, U: ttp:// ds/produts/usa/sellsp15p_usv1.pdf (aessed September 3). [8] S.A. Klein, W.A. Bekman, J.A. Duffie, Solar eating design by -art metod, New York: Jon Willey and Sons In, [9] A. Argiriou, K. Balaras,. Karagianis, onventional and mild forms of energy, 5 t ed., Atens: TEI Pireaus, 1. [1] J.. Kreider, A. abl, Heating and ooling of buildings. Design for effiieny, New York: MGraw-Hill In, [11] K.A. Joudi, Q.J. Abdul-Gafour, Development of design arts for solar ooling systems, Part I: omputer simulation for a solar ooling system and development of solar ooling design arts, Department of Meanial Engineering, University of Bagdad, 1. [1] K.A. Joudi, Q.J. Abdul-Gafour, Development of design arts for solar ooling systems, Part II: Appliation of te ooling f-art, Department of Meanial Engineering, University of Bagdad, 1. [13] lorides, G.A., S.A. Kalogirou, S.A. Tassou and.. Wrobel, Modelling and simulation of an absorption solar ooling system for yprus, Higer Tenial Institute of yprus, Brunel University,.