Evaluation of an integrated photovoltaic thermal solar (IPVTS) water heating system for various configurations at constant collection temperature

Transcription

1 Evaluation o an integrated photovoltai thermal solar (IPVTS) water heating system or various onigurations at onstant olletion temperature Rajeev Kumar Mishra 1,*, G.N.Tiwari 1 1 Centre or Energy Studies, Indian Institute o Tehnology Delhi, New Delhi, India * Corresponding author. Tel: , Fax: , bhu.rajeev@gmail.om Abstrat: Photovoltai thermal (PVT) tehnology reers to the integration o a PV and a onventional solar thermal olletor in a single piee o equipment. In this paper, an integrated photovoltai thermal solar (IPVTS) water heating system or various onigurations has been evaluated or onstant olletion temperature. Analysis is based on basi energy balane or hybrid lat plate olletor in terms o design parameters or a solar water heater installed at Solar Energy Park, IIT Delhi, India and limati parameters provided by India Meteorologial Department Pune, India. It is observed that the daily thermal energy gain o IPVTS system dereases with inreasing the onstant olletion temperature. It is also observed that or olletors partially overed by PV modules, daily thermal energy inreases with derease o olletor area o vered by PV module. The exergy analysis o IPVTS system has also been arried out. Keywords: Hybrid PV thermal, Thermal energy, Exergy. Nomenlature A Area...m 2 C Speii heat J/kg K F Flat plate olletor eiieny ator...dimensionless F R Flow rate ator.dimensionless h Heat transer oeiient..w/m 2 PF 1 Penalty ator irst..dimensionless PF 2 Penalty ator seond dimensionless I (t) Inident solar intensity. W/m 2 K Thermal ondutivity..w/m K Rate o low o water mass in olletor...kg/se Q u Rate o useul energy transer.kw T Temperature.... o C U t,a Total heat transer oeiient rom solar ell to ambient through glass over...w/m 2 K U L,m An overall heat transer oeiient rom blaken surae to ambient..w/m 2 K V Air veloity...m/s Subsripts A ambient... solar ell... luid i inlet luid o outlet luid g glass m module. N number o olletors.. Greek letters α absorptivity..- (ατ) e produt o eetive... β paking ator... η i an instantaneous... τ transmittivity Introdution Photovoltai thermal (PVT) tehnology reers to the integration o a photovoltai (PV) module and a onventional solar thermal olletor in a single piee o equipment. The reason behind the hybrid onept is that more than 8% o the solar radiation alling on PV ells is either releted or onverted to thermal energy. This leads to an inrease in the PV ell s 3749

2 working temperature as muh as 4-5 o C above the ambient temperature. Beause o this temperature inrease there an be two undesirable onsequenes: (i).3% to.6 % o eiieny loss per degree C rise in PV ell temperature and (ii) a permanent damage in the struture o PV module i the thermal stress remains or a long period o time. In appliations o PVT system, the prodution o eletriity is the main priority, and thereore, it is neessary to operate the PV modules at low temperature in order to keep the PV ell eletrial eiieny at a s uiient level. The temperature o the PV module in the hybrid PVT system an be redued by ooling the base o PV module by allowing water/air to low below it (Prakash [1], Tripanagnostopoulos et al. [2], Zondag et al. [3], Jones and Underwood [4], Chow [5], and Inield et al. [6]). Thermal energy available rom PV module an be used or many appliations namely water and air heating or domesti, agriultural setors and buildings or thermal heating/ooling. In this paper, the perormane o the N olletors onneted in series is evaluated by onsidering the three dierent ases, namely: Case A: All the olletors are ully overed by glass and onneted in series. Case B: All olletors are partially overed by PV modules and onneted in series and Case C: All the olletors are ully overed by PV module (glass to glass) and onneted in series. 2. Methodology For the present study onventional tube-in-plate-type olletor o area o 2m 2 is onsidered. The design parameters o photovoltai thermal (PVT) olletors are shown in Table 1. The glazing surae o the olletor is either glass or PV module depending upon the requirement o the user. To inrease the absorption o solar radiation the absorber plate o olletor is blakened by blak paint. Energy balane equations: In order to write the energy balane equation o PVT solar water olletors onneted in series, the ollowing assumptions have been made: One dimensional heat ondution is good approximation or the present study. The speii heat o water remains onstant. It does not hange with rise in temperature o water. The system is in quasi-steady state. The ohmi losses in the solar ell and PV module are negligible. Energy balane or solar ells o PV module (glass-glass), ( ) ( ) ( ) ηβ ( ) ατ gβi t Wdx = Ut, a T Ta + h, p T Tp Wdx + I t Wdx From Eq.(1) the expression ro ell temperature is (1) T = ( ατ ) ( ) +, +, 1, e I t U T h T U + h t, a, p t a a p p Energy balane or blakened absorber plate below the PV module, 2 ( 1 ) ( ) +, ( ) =, ( ) α β τi t Wdx h T T Wdx h T T Wdx p g p p p p From Eq. (3), the expression or plate temperature is T p = ( ατ ) ( ) ( ατ ) ( ) I t + PF I t + U T + h T 2, e 1 1, e L1 a p, U + h L1 p, (2) (3) (4) 375

3 Energy balane or water lowing through an absorber pipe below the PV module, dt C dx = F hp, ( Tp T ) Wdx dx (5) In the present study three dierent onigurations o PVT solar water olletors have been onsidered Case A: All olletors are ully overed by glass and onneted is series: Fig. 1. Colletors ully overed by glass and onnetion in series. Following Duie and Bekman [7] and Tiwari [8] the mass low rate or N olletors onneted in series an be obtained as: = F NA U L, ( ατ ) I( t) ( ατ ) I( t), e, e C log Ti + Ta log ToN + T a U L, U L, (6) Case B: The olletors are partially overed by PV modules and onneted in series Fig. 2. Colletors partially overed by PV module and onneted in series. Following Dubey and Tiwari [9] the outlet water rom N suh olletors onneted in series an be given as: ( AF ( )) ( AF U ) T ατ 1 K I t 1 K T T K N N R 1 K R L 1 K N o N = ( ) + a + i K mc 1 KK mc 1 KK (7) where, 3751

4 K K = ( AF U ) 1 1 R L mc and R L, ( AFR ( ατ )) = AmFRmPF ( ατ ) 1 + AFR ( ατ ) ( ) 1 AF U 2 m, e, e mc AF U = A F U AF U 1 + A F U R L, R L 1 m Rm L, m R L, mc Case C: All the olletors are ully overed by glass to glass type PV module and onneted in series Fig.3. Colletors ully overed by PV modules and onneted in series. Following Dubey and Tiwari [9] Mass low rate or N olletors partially overed with PV modules onneted in series an be obtained as, NFAU m Lm, = PF2( ατ ) I ( t) PF2( ατ ) I ( t) m, e m, e C log Ti + Ta log To N + T a U Lm, U Lm, (8) The rate o useul thermal energy obtained rom N idential olletors onneted in series an be given as Q = mc T T (9) ( ) u on a Eletrial Eiieny o solar ell depends on solar ell temperature and an be given by Evans [1] and Shott [11] ( T T ) η = η 1 β o o o (11) 3752

5 Table 1. Design parameters o photovoltai thermal (PV/T) olletor Parameters Values Parameters Values A C 2.m 2 U LC 3. W/m 2o C A m.65m 2 U Lm 3.44 W/ m 2o C C 419 J/kgK U t,a 9.5 W/m 2o C F.968 V 1. m/s F R1.95 W.125 m F R2.94 α.9 F Rm.96 τ.95 h,p 5.7 W/m 2 β.89 h p, 1 W/m 2 η o.12 PF α p.8 PF τ g 95 K 24 W/m o C.6 kg.se 3. Result and Disussions The variation o solar intensity and ambient temperature or a typial day in the summer month (January) is shown in Figure 4.The values o design parameters o lat plate olletor are given in Table 1. Here, the results o the three ases, ase A (ully overed by glass) and ase B (partially overed by PV modules) and ase C (ully overed by PV modules) are disussed in detail. Equations 6, 7 a nd 8 have been omputed using MATLAB sotware or evaluating the mass low rate at dierent outlet water temperatures or a typial day during the month o January or a given design and limati parameters (Table 1). Figures 5a and 5b represent the hourly variation o mass low rate or ase A and ase B respetively at various onstant outlet temperatures. The result shows that or onstant olletion temperatures 3-6 o C the mass low rate o water in tubes dereases rom kg/s in ase A,.8-.1 kg/s in ase B. Figure 5 gives the hourly variation o mass low rate or ase C. Figure shows that in this ase one annot get the outlet water temperature more than 4 o C in January month and the mass low rate dereases rom.4 to.1 kg/s or ase C. I(t) Ta Solar Intensity(W/m 2 ) Ambient temperature( o C) Time(Hours) Fig. 4. Hourly variation o solar intensity and ambient temperature o a typial day in the month o January. 3753

6 Mass low rate(kg/s),12,1,8,6,4,2 To=To=3 deg.c To=To=4 deg.c To=To=5 deg.c To=To=6 deg.c Time(Hours) Fig. 5a. Hourly variation o mass low rate at dierent outlet temperature or ase A.,1 To=To=3 deg.c To=To=5 deg.c To=To=4 deg.c To=To=6 deg.c mass low rate(kg/s),8,6,4, Time(hrs) Fig. 5b. Hourly variation o mass low rate at dierent outlet temperature or ase B. To=To=3 deg.c To=To=4 deg.c Mass low rate(kg/s),45,4,35,3,25,2,15,1, Time(Hour) Fig. 5. Hourly variation o mass low rate at dierent outlet temperature or ase C. Fig. 6a and 6b represent the hourly variation o thermal energy gain and eletrial energy gain respetively or various onigurations o PVT olletors. The igures show that as the 3754

7 olletor area overed by PV modules inreases the thermal energy gain dereases whereas the eletrial energy gain inreases as the olletor area overed by PV modules inreases. Thermal energy gain(kwh) 3,5 3 2,5 2 1,5 1,5 FPC 3% partially overed 6% partially overed 9% partially overed ully overed Time(Hours) Fig. 6a. Hourly variation o thermal energy gain or dierent oniguration. Eletrial energy gain(kwh),6,5,4,3,2,1 3% overed 6% overed 9% overed ully overed Time (Hours) Fig. 6b. Hourly variation o eletrial energy gain or dierent onigurations. 4. Conlusion The maximum thermal energy gain is obtained when olletors ully overed by glass over; however maximum eletrial energy gain is obtained when olletors are ully overed by PV modules. Reerenes [1] Prakash, J., Transient analysis o a photovoltai-thermal solar olletor or o- generation o eletriity and hot air/water. Energy Conversion and Management. 35, 1994 pp [2] Tripanagnostopoulos, Y., Hybrid photovoltai/thermal solar system, Solar Energy 72(3), 22, pp [3] Zondag, H.A., de Vries, D.W. de, van Helden, W.G.J.,van Zolengen, R.J.C., Steenhoven, A.A., The thermal and eletrial yield o a P V-thermal olletor. Solar Energy 72 ( 2), 22, pp

8 [4] Jones, A.D., Underwood, C.P., A thermal model or photovoltai systems. Solar Energy 7 (4), 21, pp [5] Chow, T.T., Perormane analysis o photovoltai-thermal olletor by expliit dynami model. Solar Energy 75, 23, pp [6] Inield, D., Mei, L., Eiker, U., Thermal perormane estimation o ventilated PV aades, Solar Energy, 76(1-3), 24, pp [7] Duie, J.A., Bekman, W.A., Solar Engineering o Thermal Proesses. John Wiley and Sons, New York. [8] Tiwari, G.N., Solar Energy: Fundamentals, Design, Modeling and Appliations. Narosa Publishing House, New Delhi, 24. [9] Dubey, S. and Tiwari, G.N., Analysis o dierent onigurations o lat plate olletors onneted in series, International Journal o Energy Researh, 32, 28, pp [1] Evans, D.L., Simpliied method or prediting PV array output. Solar Energy 27, 1981, pp [11] Shott, T., Operational temperatures o PV modules. In: Proeedings o 6 th PV Solar Energy Conerene, 1985, pp