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Energy and Buildings 42 (2010) 1472–1481

Contents lists available at ScienceDirect

Energy and Buildings

journa l homepage: www.e lsev ier .com/ locate /enbui ld

ife cycle cost assessment of building integrated photovoltaic thermalBIPVT) systems

asant Agrawal ∗, G.N. Tiwarientre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

r t i c l e i n f o

rticle history:eceived 6 September 2009eceived in revised form 6 March 2010ccepted 16 March 2010

a b s t r a c t

The present paper deals with an analysis of the building integrated photovoltaic thermal (BIPVT) systemfitted as rooftop of a building to generate electrical energy higher than that generated by a similar buildingintegrated photovoltaic (BIPV) system and also to produce thermal energy for space heating. A thermo-dynamic model has been developed to determine energy, exergy and life cycle cost of the BIPVT system.

eywords:pace heatinghotovoltaic (PV)olar thermal system

The results indicate that although the mono-crystalline BIPVT system is more suitable for residential con-sumers from the viewpoint of the energy and exergy efficiencies, the amorphous silicon BIPVT system isfound to be more economical. The energy and exergy efficiencies of the amorphous silicon BIPVT systemare found to be 33.54% and 7.13% respectively under the composite climatic conditions prevailing at New

generer.

nergy and exergy efficienciesife cycle cost (LCC)

Delhi. The cost of powerthe conventional grid pow

. Introduction

The energy consumption can be examined under four main sec-ors namely; industrial, residential, transportation and agriculture.ccording to Cengel [1], the energy required for space heating inuildings has the highest share of about 40% of the total energyonsumed in the residential sector (Table 1). The electricity pro-uction based on fossil or nuclear fuels induces substantial socialnd environmental costs, whereas in case of renewable energyources these are lower. The renewable global status report [2] indi-ates that the power production by way of solar photovoltaic (PV)as grown more than any other renewable energy source. It has areat prospect of cost break-even, with respect to the conventionalrid power for residential consumers. The production capacity hasrown at an average of 48% each year and the cumulative globalroduction is now at 12.5 × 103 MW.

The main technology routes as seen today can be characterizednto silicon based PV, non-silicon based thin film PV and new con-ept devices (Fig. 1). In 2008, worldwide production of PV modules

sed in consumer products includes 42.2% mono-crystalline siliconc-Si), 45.2% poly-crystalline silicon (p-Si), 2.2% ribbon silicon (r-Si),.2% amorphous silicon (a-Si), 4.7% cadmium telluride (CdTe) and.5% copper indium gallium selenide (CIGS) [3]. The reliability and

ifetime of the PV systems are growing steadily. Depending upon

∗ Corresponding author. Tel.: +91 9926404150; fax: +91 11 26592208.E-mail addresses: bas agr@yahoo.co.in, basantagrawal@rediffmail.com

B. Agrawal), gntiwari@ces.iitd.ac.in (G.N. Tiwari).

378-7788/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.enbuild.2010.03.017

ation is found to be US $ 0.1009 per kWh which is much closer to that of

© 2010 Elsevier B.V. All rights reserved.

the production technology used, nowadays the PV manufacturersoffer a 5–30 years service life guarantee.

The performance of a PV can be described in terms of its energyconversion efficiency, the percentage of incident solar energy(input) that the cell converts into electricity under standard ratingconditions. The overall electrical efficiency of the PV module canbe increased by increasing the packing factor (PF) and decreasingthe temperature of the PV module [4,5]. Table 2 presents the con-version efficiency, efficiency correction coefficient and expectedlife of the cell and modules with different production technologies[6,7].

Historically, the stand alone photovoltaic (SAPV) has not beena cost-effective source of power generation. Benemann et al. [8]realised the installation of BIPV system at Aachen, Germany wherethe PV arrays were integrated into a curtain wall facade with isolat-ing glass. Such systems have improved the economics by allowingsome cost of the PV system to be shared by the building. Yet thepurchase, design, installation and maintenance of the BIPV sys-tems cost more than those in the standard contemporary buildingskins.

In typical BIPV applications the increase of solar cell tem-perature results in the decrease of energy conversion efficiency.Air-cooled hybrid photovoltaic–thermal (PVT) systems consist ofPV modules with an air channel at their rear surface and usuallyambient air is circulated in the channel to achieve both PV cooling

and thermal energy output. The thermal energy obtained can beused to fulfil the thermal requirements of the building. A large num-ber of theoretical as well as experimental works has been reportedon hybrid PVT systems [9–15], for extraction of heat from the PVmodules.

B. Agrawal, G.N. Tiwari / Energy and Buildings 42 (2010) 1472–1481 1473

Nomenclature

Aroof area of roof, m2

b width of the BIPVT system, mCair specific heat of air at constant pressure, J/kg-KCf conversion factor of the thermal power plantCRF capital recovery factordx elemental length, mdt elemental time, sE energy, WhEx exergy, Whh heat transfer coefficient, W/m2

i annual interest rateI(t) solar intensity on the BIPVT system, W/m2

j integerK thermal conductivity, W/m2 KL length of a BIPVT system, mLCC life cycle cost, US $M maintenance and repair cost, US $mair air mass flow rate in duct, kg/sMrCr heat capacity of room, W/KN number of sunshine hour in a dayNo number of air change/hourn life of the BIPVT systemnp number of rows of BIPVT systemsns number of BIPVT system in a rowP present value, US $R replacement cost, US $S salvage value, US $T temperature, KU overall heat transfer coefficient, W/m2

unacost annualised uniform cost, US $v velocity of air, m/sV volume of room, m3

(UA)t heat loss capacity through walls, doors and win-dows, W/K

Greek letters˛ absorptivityˇ packing factor� transmissivity� efficiency� efficiency correction coefficient

Subscriptsa ambientair air in the ductairout air outlet from the ductbs back surfacebb insulation platec solar cellca cell actuale energyeff effectiveel electricalG top glassi insulationI initialL tedlar to insulationMR maintenance and repairo outside airr air in the roomR replacementref reference given for Solar cell by manufacturer

s sunS salvageT tedlartair tedlar to airth thermal

Fig. 1. Classification of photovoltaics on the basis of technology.

BIPVT is a relatively new technology which merges hybrid PVTwith BIPV systems, simultaneously providing both the electri-cal and the thermal energy onsite. Due to sharing of resourceslike materials and functions in the integration, the BIPVT sys-tem becomes cheaper than that having four separate products [6].Moreover, the complete system installed by a single team resultsin further cost reduction. Agrawal and Tiwari [16] used an opaquetype BIPVT system fitted as the rooftop of a laboratory over an areaof 65 m2. It was concluded that for a mass flow rate of 0.2 kg/s thesystem at Bangalore produces annual 15766 kWh and 16708 kWhelectrical energy and exergy respectively which is 629 kWh and1571 kWh higher than that of a similar Building Integrated Pho-tovoltaic (BIPV) system. Optimization of the system configurationsunder cold climatic conditions has also been presented [17]. The lifecycle assessment has been applied to SAPV systems, hybrid PVT sys-tems and BIPV systems by several researchers [18–21]. The presentpaper presents the life cycle cost (LCC) assessment methodologyfor the BIPVT systems. It also aims to compare the performanceand economic feasibility of different BIPVT systems.

2. Problem identification

BIPVT systems similar to those installed on the roof of an exper-imental laboratory at the Centre for Sustainable Technology, IndianInstitute of Science, Bangalore has been considered for the roofof the buildings at New Delhi. Fig. 2 shows an orthographic viewof the experimental laboratory with BIPVT systems as the rooftopat New Delhi. As New Delhi is situated at 28◦35′N, the optimumangle for fitting the BIPVT systems is due south inclined at an angleof 30◦ to the horizontal. The system covers an effective area of10.4 m × 6.3 m. For the purpose of analysis, the solar cells of theBIPVT systems are characterized into six different technologiesnamely; (i) mono-crystalline silicon (m-Si), (ii) poly-crystalline sil-icon (p-Si), (iii) EFG ribbon crystalline silicon (r-Si), (iv) amorphoussilicon (a-Si), (v) cadmium telluride (CdTe) and (vi) copper indiumgallium selenide (CIGS).

Underneath the BIPVT system there are six ducts connected

in series to capture the thermal energy associated with PV mod-ules. An air blower consuming 0.72 kWp is used to circulate the airthrough the duct at a constant mass flow rate of 1 kg/s. Output fromBIPVT panel junction box is connected to the power grid. Thus, solarelectric generation would eliminate the huge number of local bat-

1474 B. Agrawal, G.N. Tiwari / Energy and Buildings 42 (2010) 1472–1481

Table 1Distribution of energy consumption in buildings, in % [1].

Space heating Water heating Air condition ventilation Lighting illumination Cooling freezing Other

Houses 40 17 7 7 12 17Commercial 32 5 22 25 – 16

Table 2Cell/module efficiency, temperature coefficient and expected life for different PV technologies.

PV technology Cell efficiency, (%) [7,8] Module efficiency, �ref (%) Efficiency correctioncoefficient, �ref (/◦C) [8]

Expected life(years) [7]

Mono-crystalline Silicon (c-Si) 14–19 16 0.0045 30Poly-crystalline Silicon (p-Si) 13–17 14 0.0045 30Ribbon (EFG) crystalline Silicon (r-Si) 14–16 12 0.0045 30Amorphous Silicon (a-Si) 6–8 6 0.0020 20Cadmium Telluride (CdTe) 7–11 8 0.0025 15Copper Indium Gallium Selenide (CIGS) 8–13 10 0.0036 5

tal roo

tAtseeapegalcma

3

[B

(

(

ate oom Ps the

Fig. 2. Orthographic view of an experimen

eries and replace the costly electricity during the peak demand.s the BIPVT system provides the electrical energy along with the

hermal energy, the overall performance of the system may be pre-ented either in the term of energy or exergy efficiencies. For the netnergy output of the system, the electrical output is converted intoquivalent thermal energy required by the thermal power plantnd added to the thermal output whereas for the net exergy out-ut the thermal gain from the system is converted into equivalentlectrical energy using Carnot theorem and added to the electricalain of the system. In order to investigate the feasibility the costnalysis is taken into account. The evaluation tool is based on theife cycle concept, which is a cradle-to-grave approach to analyzeost of a system. The tool chosen is life cycle cost (LCC) assess-ent which provides effective evaluation to pinpoint cost-effective

lternatives. [Rate of heatreceived bythe solar cell

]+

[Rate of heatreceived by thenon-packing area

]=

[Rfra

. Thermal modelling

Experimental validated methodology of Tiwari and Sodha22–24] were used for thermal modelling of the components of theIPVT system. The assumptions made are as follows:

m with BIPVT systems fitted at New Delhi.

(a) The system is in quasi-steady-state condition.b) The specific heat of air does not change with a rise in its tem-

perature, i.e. it remains constant.(c) The transmissivity of ethylene vinyl acetate (EVA) is approxi-

mately 100 percent.d) The heat loss from the side of the system is negligible, and

(e) The airflow through duct is uniform for the forced mode ofoperation for streamline flow.

Fig. 3a shows the cross sectional view of the BIPVT system. Fig. 3bshows an elemental area b.dx of the BIPVT system over which solarintensity is received. The energy balances for each components ofthe BIPVT system are as follows:

Solar cell: The energy balance for solar cells of the BIPVT systemfor elemental area b.dx is given by

f heat lossV module to airtop loss

]+

[Rate of heat lossfrom PV module to theback surface/tedlar

]+

[Rate ofelectricityproduction

]

[ ( ) ]

�G ˛c ˇc + 1 − ˇc ˛T I(t) b dx

= [UT (Tc − Ta) + hT (Tc − Tbs)] b dx + �ca I(t) b dx (1)

where UT =(

LGKG

+ 1ho

)−1and hT =

(LTKT

)−1

B. Agrawal, G.N. Tiwari / Energy and B

Fb

T

w

t

h

o[+

[

h

w

c

](1

ig. 3. (a) Cross-section of a BIPVT system. (b) Air flow pattern over elementary area× dx of a BIPVT system.

On Simplifying, expression for the solar cell temperature is

c = hT Tbs + UT Ta + I(t) (˛�)eff

UT + hT(2)

here (˛�)eff = �G

[˛c ˇc + ˛T (1 − ˇc)

]− �c

Back plate of the solar cell: Energy balance for back plate (callededlar) of solar cell for elemental area b.dx is given by

[Rate of heat gain from PV module to the tedlar]

= [Rate of heat loss from tedlar to air side in the duct]

T (Tc − Tbs) b dx = hair (Tbs − Tair) b dx (3)

Air flowing in the duct: Energy balance for air flowing in the ductf the BIPVT system for elemental area b.dx is given by

Rate of heat received fromtedlar to air side in the duct

]=

[Rate of heat gain by air flowingin the duct of the BIPVT system

]

air (Tbs − Tair) b dx = mairCair

(dTair

dx

)dx + Ubb (Tair − Tr) b dx (4)

f (t) = 1Mr Cr

[{(UA)t + 0.33 NoV

}Ta +

{np · mair Cair

[Utair Ta + hp2 hp1 I(t)(˛�)eff

UL

here Ubb =(

1hair

+ LiKi

+ 1hr

)−1

On solving Eqs. (3) and (4) and integrating with boundaryondition at x = 0, Tair = Tr,; at x = ns × L, Tair = Tairout, the outlet air

uildings 42 (2010) 1472–1481 1475

Rate of heat loss fromair through insulation

]

temperature (Tairout) of the air flowing in the duct is

Tairout =[

Ubb Tr + Utair Ta + hp2 hp1 I(t) (˛�)eff

UL

](1 − e− ns ·bL×UL

mair Cair )

+ Tr e− ns ·bL×ULmair Cair (5)

where Utair = UtT ×hairUtT +hair

=(

1hair

+ 1UtT

)−1; UtT = UT ×hT

UT +hT=(

1hT

+ 1UT

)−1

hp1 = hT

UT + hT; hp2 = hair

UtT + hair; UL = (Ubb + Utair)

Average air temperature of the BIPVT system is given by

Tair =[

Ubb Tr + Utair Ta + hp2 hp1 I(t) (˛�)eff

UL

](1 − e− ns ·bL×UL

mair Cair )

+Tr(1 − e− ns ·bL×UL

mair Cair )ns ·bL×ULmair Cair

Building: The available useful thermal energy is partly used toheat the room and the rest is lost. The energy balance for the build-ing is given by

np · mair Cair

[Ubb Tr + Utair Ta + hp2 hp1 I(t) (˛�)eff

UL− Tr

](1 − e− ns ·bL×UL

mair Cair )

+ Ubb

(Tair − Tr

)Aroof = Mr Cr

(dTr

dt

)dt + (UA)t (Tr − Ta)

+ 0.33 NoV (Tr − Ta) (6)

where (UA)t = Awall(1

ho+ Lwall

Kwall+ 1

hr

) + Awindow(1

ho+ Lwindow

Kwindow+ 1

hr

) + Adoor(1

ho+ Ldoor

Kdoor+ 1

hr

)On solving Eq. (6) and integrating with initial condition at time

t = 0, Tr = Tri, the room air temperature (Tr) is given by

Tr = f (t)a

(1 − e−at

)+ Tri e−at (7)

where,

a = 1Mr Cr

[{(UA)t + 0.33 NoV

}−{

np · mair Cair

(Ubb

UL− 1

)(1 − e− ns ·bL×UL

mair Cair )

}

−Ubb

{Ubb

UL

(1 − 1 − e− ns ·bL×UL

mair Cair

ns ·bL×ULmair Cair

)+ 1 − e− ns ·bL×UL

mair Cair

ns ·bL×ULmair Cair

− 1

}Aroof

]

− e− ns ·bL×ULmair Cair )

}+ Ubb

{Utair Ta + hp2 hp1 I(t)(˛�)eff

UL

}(1 − 1 − e− ns ·bL×UL

mair Cair

ns ·bL×ULmair Cair

)Aroof

]In addition to the above equations the values of design param-

eters are shown in Table 3.

4. Energy analysis

The actual electrical efficiency of the BIPVT systems is given by[25],

�ca = �ref

[1 − �ref

(Tc − Tref

)](8)

1476 B. Agrawal, G.N. Tiwari / Energy and Buildings 42 (2010) 1472–1481

Table 3Design parameters of BIPV system.

Parameters Values

Cair , J/kgK 1005Cf 0.38ho , W/m2 5.7 + 3.8 × va

hi , W/m2 2.8hT , W/m2 2.8 + 3 × vair

Kc , W/m2K 0.039KG , W/m2K 0.8Ki , W/m2K 0.035KT , W/m2K 0.38Lc , mm 0.3LG , mm 32Li , mm 10LT , mm 3˛c 0.7˛t 0.7

vfli

=

i

E

atto

EL

r )

{

i

E

w

aoo

�

5

i

ˇc 0.9�c 0.16�g 0.85�a , kg/m3 1.29

Quantities �ref , Tref and �ref are usually given by the photo-oltaic module manufacturers but they can be also obtained fromash tests. Thus, the hourly electrical output of the BIPVT systems

s given byEel = actual cell efficiency × solar insolation

�ca × I (t) × bL × ns · np (9)

The rate of useful thermal energy obtained from BIPVT systemss given by

˙ th = np × mair Cair (Tairout − Tr )

= np × mair Cair (1 − e− ns ·bL×ULmair Cair )

[Utair (Ta − Tr ) + hp2 hp1 I(t) (˛�)eff

UL

](10)

The conventional electrical output can be converted to an equiv-lent thermal output by dividing it with conversion factor (Cf) ofhe thermal power plant [14]. The hourly overall thermal output ofhe BIPVT system can be calculated by adding equivalent thermalutput to the hourly thermal gain.

˙ hourly = Eel

Cf+ Eth = np ×

[�ca × I (t) bL × ns

Cf+ mairCair (1 − e− ns ·bL×U

mair Cai

Therefore, the daily overall thermal output of the BIPVT systems given by

˙ daily =N∑

j=1

(Eel

)j

Cf+

N∑j=1

(Eth

)j

(11)

here N is the sunshine hourAdding daily overall thermal output for a year will give the

nnual overall thermal energy output. Overall energy efficiencyf the BIPVT system can be calculated by dividing overall thermalutput to the overall thermal gain from the solar energy. Thus,

e =

N∑j=1

(Eel)jCf

+N∑

j=1

(Eth

)j

N∑j=1

[I (t)]j × bL × ns · np

(12)

. Exergy analysis

According to Coventry [26], exergy (sometimes called availabil-ty) is defined as the maximum theoretical useful work obtainable

Utair (Ta − Tr) + hp2 hp1 I(t) (˛�)eff

UL

}]

Fig. 4. Cash flow diagram for life cycle cost assessment of the BIPVT system.

from a system as it returns to equilibrium with the environment.With the exergy approach, it becomes possible to assign coherentvalues to the different energy forms (work, heat, electrical energy,etc.) that take into account the two key energy parameters namelyquantity and quality. According to Patela [27], total exergy inflowto the system is given by

Exin = I (t) ×[

1 − 43

×{

(Ta)j

(Ts)j

}+ 1

3×{

(Ta)j

(Ts)j

}4]

× bL × ns · np (13)

The exergy outflow from the system is in the form of thermaland electrical which are given by [17]

N∑j=1

Exth =N∑

j=1

(Eth

)j×

{1 − (Ta)j

(Tairout)j

}(14)

andN∑

j=1

Exel =N∑

j=1

(�ca)j × [I (t)]j × bL × ns · np (15)

Taking into account the total exergy inflow and outflow of the sys-tem, the overall exergy efficiency for a BIPVT system is given by,

�Ex =

N∑j=1

Exout

N∑j=1

Exin

=

N∑j=1

Exth +N∑

j=1

Exel

N∑j=1

Exin

(16)

6. Life cycle cost assessment

There are numerous costs associated with acquiring, operating,maintaining and disposing of a system. In Life Cycle Cost (LCC)assessment, all relevant present and future costs associated withthe system are summed in present value during a given life period.The purpose is to estimate the overall cost of project alternativesand to select the design that ensures the facility will provide thelowest overall cost of ownership consistent with its quality andfunction. Fig. 4 shows the line diagram of the cash flows at differ-ent interval of time for the BIPVT system. The LCC assessment ofthe BIPVT system takes into account

(i) Initial costs (PI): It is the sum of the costs involved in the systemtechnologies (such as BIPVT system, charge controller, battery

bank, inverter, etc.), utility interconnection costs and associ-ated costs for building permits. Table 4 gives the initial capitalcosts of various items of the BIPVT system.

(ii) Maintenance and repair costs: It is defined as the costs incurredduring the operational phase. Supplier quotes and published

B. Agrawal, G.N. Tiwari / Energy and Buildings 42 (2010) 1472–1481 1477

Table 4Capital costs of various items of the BIPVT system in US $.

PV technology Initial cost of thesystem

Maintenance andrepair cost in term ofpresent value

Replacement cost interm of present value

Salvage value in termsof present value

Net present value

c-Si 28989.49 2593.80 1913.01 104.86 33391.45p-Si 24208.84 2593.80 1661.44 87.72 28376.37r-Si 19796.79 2343.31 1229.60 134.16 23235.53a-Si 8251.36 2038.55 545.85 158.39 10677.37CdTe 10640.37 1667.76 522.41 279.72 12550.82CIGS 13015.86 667.77 0 644.52 13039.12

Table 5Maximum, minimum and average solar intensity for different months.

Classification ofdays of month

Specification Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Type A: Clear days(ratio of dailydiffuse to dailyglobal radiation≤0.25; sunshinehours ≥9)

Maximumintensity, W/m2

1013 997 991 973 921 886 843 868 952 921 878 863

Minimumintensity, W/m2

211 235 298 306 283 289 267 248 276 219 187 158

Daily averageintensity, W/m2

711 697 704 708 664 652 615 604 669 625 599 610

Number of cleardays

3 3 5 4 4 3 2 2 7 5 6 3

Type B: Hazy days(ratio of dailydiffuse to dailyglobal radiation0.25–0.50;sunshine hours7–9)

Maximumintensity, W/m2

984 1020 1060 1030 909 933 885 875 928 849 760 870

Minimumintensity, W/m2

184 238 331 293 279 319 283 274 267 117 186 154

Daily averageintensity, W/m2

680 710 757 735 661 679 629 633 656 572 492 571

Number of hazydays

8 4 6 7 9 4 3 3 3 10 10 7

Type C: PartialHazy and cloudydays (ratio ofdaily diffuse todaily globalradiation0.50–0.75;sunshine hours5–7)

Maximumintensity, W/m2

642 655 725 729 855 804 736 687 782 715 524 715

Minimumintensity, W/m2

75 127 198 215 285 284 234 253 249 77 66 93

Daily averageintensity, W/m2

415 433 502 526 613 595 541 516 563 466 335 437

No. of hazy andcloudy days

11 12 12 14 12 14 10 7 10 13 12 13

Type D: Cloudydays (ratio ofdaily diffuse todaily globalradiation ≥0.75;sunshine hours≤5)

Maximumintensity, W/m2

429 437 654 771 727 641 581 567 587 543 536 452

Minimumintensity, W/m2

52 91 159 223 234 211 209 179 151 107 70 54

Daily averageintensity, W/m2

255 271 439 535 517 452 423 411 408 350 322 272

Number of fullcloudy days

9 9 8 5 6 9 16 19 10 3 2 8

Available monthly solar radiation W/m2 14433 12566 17598 18016 19063 17070 15295 14528 16361 15953 13178 13684

1 and Buildings 42 (2010) 1472–1481

(

(

P

v

L

m

C

U

dc

7

emBscfld

478 B. Agrawal, G.N. Tiwari / Energy

estimating guides sometimes provide information on main-tenance and repair costs. However there is great variation inthese costs even for the BIPVT systems of the same type andlife. For the present analysis considering on an average M is theannual maintenance and repair cost of the BIPVT system. Thenthe maintenance and repair costs in terms of present value isgiven by

PMR = M ×[

(i + 1)n − 1i(i + 1)n

](17)

iii) Replacement costs: It is defined as the costs incurred during thereplacement of batteries and components. The number andtiming of capital replacements of components of the BIPVTsystem depend on the life of the components and system. IfR5, R10, R15,. . . Rn is the replacement cost incurred in batteriesand other components made in every five years then the netreplacement costs in terms of present values is

PR = R5 ×[

1

(i + 1)5

]+ R10 ×

[1

(i + 1)10

]+ R15 ×

[1

(i + 1)15

]

+... + Rn ×[

1

(i + 1)n

](18)

iv) Salvage value: It is defined as the costs incurred in demolitionand disposal of the system. If S is the salvage value at the endof the system then the net salvage value in terms of presentvalue is

S = S ×[

1

(i + 1)n

](19)

Thus, the overall LCC of the BIPVT system in terms of presentalue is given by

CC = PI + PMR + PR − PS = PI + R ×[

(i + 1)n − 1i(i + 1)n

]+ R5

×[

1

(i + 1)5

]+ R10 ×

[1

(i + 1)10

]+ ... + Rn

×[

1

(i + 1)n

]− S ×

[1

(i + 1)n

](20)

The capital recovery factor over the lifetime expressed mathe-atically by Raman and Tiwari [19] as

RF = i(i + 1)n

(i + 1)n − 1(21)

Therefore, the annualized uniform cost (unacost) is given by

nacost = LCC × CRF (22)

The cost per unit electricity generated by the BIPVT system isetermined as the ratio of annualized uniform cost and the electri-al energy consumed by the load in a year.

. Methodology

The approach uses individual BIPVT system to compute thenergy, exergy and the outlet air temperatures using basic ther-al modelling and heat transfer relations. The outlet air from one

IPVT system in a column is used as the inlet air in the next BIPVTystem. For the first BIPVT system, the inlet air is the mixture of cir-ulated air of the room and the fresh ambient air. Since the net massow rate of air inside the duct is 1 kg/s, the velocity of air inside theuct is 3.2 m/s. The following methodology is used on the software

Fig. 5. Annual thermal and electrical output from the BIPVT system using differentsolar cells.

“Matlab 7” to evaluate the efficiency and life cycle cost of the BIPVTsystem.

1. The intensity of the solar radiation that arrives at the earth’ssurface is reduced due to existing gases in the atmosphere, thereflection of clouds and the environmental pollution [28]. Asatmospheric situations are not predictable, the solar radiationand climatic condition data of New Delhi for past eleven yearswere obtained from Indian Metrological Department (IMD),Pune. The hourly variation of solar intensity on the inclined sur-face is obtained with the help of Liu and Jordan formula [29].Table 5 shows maximum, minimum and average solar intensitiesfor different months. To summarise solar intensities in a techni-cal manner in the table, the days are classified on the basis of theratio of daily diffuse to daily global radiation and the number ofsunshine hours.

2. The hourly variation in room air temperature (Tr) is obtainedby substituting the design parameters of the BIPVT systems(Table 3), the hourly ambient temperature and the hourly solarintensity in Eq. (7). The hourly variation in temperature of the airat the outlet (Tairout) flowing through the duct is obtained usingEq. (5).

3. The hourly variation in back surface temperature of the photo-voltaic panel and average air temperature of the air flowing inthe duct are determined from Eqs. (3) and (4). The hourly varia-tion of solar cell temperature (Tc) is obtained by substituting thevalues in Eq. (2).

4. The actual cell efficiency is obtained by substituting the solar celltemperature (Tc) in Eq. (8). The electrical output is obtained bysubstituting the solar cell efficiency in Eq. (9). The useful thermalenergy of the BIPVT system is computed by substituting the airoutlet and room air temperatures in Eq. (10). Fig. 5 shows theannual electrical and thermal output from the BIPVT system withdifferent solar cell technologies.

5. The net exergy output is obtained by substituting the values ofelectrical and useful thermal exergies in Eq. (14) and (15) respec-tively. Fig. 6 shows the net exergy output for the BIPVT systemwith different technologies. It also shows the exergy output fromthe BIPV system for comparison purpose.

6. The overall energy efficiency is obtained from Eq. (12) as the

ratio of thermal energy output to the solar thermal energy input.Similarly, the overall exergy efficiency is obtained from Eq. (16)as the ratio of net exergy output to the exergy input. Figs. 7 and 8show the overall energy and exergy efficiencies, respectively, ofthe BIPVT system with different technologies. The figures also

B. Agrawal, G.N. Tiwari / Energy and Buildings 42 (2010) 1472–1481 1479

Fig. 6. Annual exergy output from the BIPV and BIPVT systems using different solarcells.

Fig. 7. Overall thermal efficiency of the BIPV and BIPVT systems using different solarcells.

Fig. 8. Overall exergy efficiency of the BIPV and BIPVT systems using different solarcells.

Fig. 9. Annualized uniform cost of the BIPV and BIPVT systems using different solarcells.

Fig. 10. Unit power generation cost of the BIPV and BIPVT systems using differentsolar cells.

show the energy and the exergy efficiencies for the BIPV systemfor comparison purpose.

7. For the life cycle cost analysis the annual maintenance and repaircost has been assumed to be US $ 150 and battery life of 5 years.The annual interest rate usually offered by government sectorsin India to promote the use of renewable energy applicationsis 4% [30]. In addition to these, the BIPVT cost, battery replace-ment cost and the salvage value given in Table 4 were substitutedin Eq. (20). Annualized uniform cost (unacost) is obtained fromEqs. (21) and (22). Fig. 9 shows the annualised uniform cost ofthe BIPVT systems with different technologies. It also shows theannualised uniform cost of the BIPV for comparison purpose.

8. Dividing the annualized uniform cost with net exergy outputgives the cost per unit power generation. Fig. 10 shows the costof unit power generation from the BIPVT with different tech-nologies. It also shows the cost of unit power generation fromthe BIPV for comparison purpose.

Table 6 summarizes the annual electrical output, thermal out-puts, overall thermal and exergy efficiencies, annualized uniform

cost and cost per unit power generation for the BIPVT systems withdifferent solar cell technologies.

1480 B. Agrawal, G.N. Tiwari / Energy and B

Tab

le6

The

ann

ual

outp

uts

,ove

rall

effi

cien

cies

and

ann

ual

ized

cost

ofp

ower

gen

erat

ion

thro

ugh

BIP

VT

syst

ems.

PVte

chn

olog

yEl

ectr

ical

outp

ut

from

BIP

VT

Ther

mal

outp

ut

from

BIP

VT

Ove

rall

ther

mal

effi

cien

cyof

BIP

VT

syst

em

Ove

rall

exer

gyef

fici

ency

ofB

IPV

Tsy

stem

An

nu

aliz

edco

stof

BIP

VT

Cos

tp

eru

nit

pow

erge

ner

atio

nfo

rB

IPV

Tsy

stem

Cos

tp

eru

nit

pow

erge

ner

atio

nfo

rB

IPV

syst

em

Red

uct

ion

inco

st

kWh

kWh

%%

US

$U

S$/

kWh

US

$/kW

h%

c-Si

1513

116

764

51.9

914

.91

1931

.03

0.11

900.

1338

12.4

4p

-Si

1314

117

535

47.8

813

.19

1641

.01

0.11

430.

1309

14.5

1r-

Si11

179

1830

643

.84

11.5

014

87.3

50.

1189

0.13

9517

.35

a-Si

6066

2061

533

.54

7.13

785.

660.

1009

0.12

6024

.97

Cd

Te79

5819

845

37.4

18.

7511

28.8

30.

1182

0.14

1619

.77

CIG

S95

7819

074

40.6

510

.13

2928

.94

0.26

540.

3195

20.4

1

uildings 42 (2010) 1472–1481

8. Results and discussions

Fig. 5 shows that the annual electrical output from the mono-crystalline silicon (c-Si) BIPVT is maximum (15131 kWh) while thatof the amorphous silicon (a-Si) BIPVT is minimum (6066 kWh). Thisis owing to the higher electrical conversion efficiency of the c-SiBIPVT than the other systems. The system with higher electricalenergy uses higher amount of solar isolation and thereby remainswith smaller portion to get converted into thermal energy. There-fore, the annual thermal output of the mono-crystalline silicon(c-Si) BIPVT is minimum (16764 kWh) while that of the amorphoussilicon (a-Si) BIPVT is maximum (20615 kWh). Fig. 6 shows thatthe annual exergy output is maximum (16225 kWh) in case of c-SiBIPVT system and minimum (7790 kWh) in case of a-Si BIPVT sys-tem. Moreover, the annual exergy output of the BIPVT systems are15–30% higher than that of the similar BIPV system. It is so becausethe solar cells of the BIPVT system are cooled which helps in pro-ducing higher electrical energy than the BIPV system. Also, thermalgain from the BIPVT system helps in space heating. Figs. 7 and 8show that the overall energy efficiency of the BIPVT system is nearly17–20% higher and the exergy efficiency is nearly 1.5–2% higherthan those of the similar BIPV system. This is owing to the coolingeffect produced by the air flowing through the duct which increasesthe electrical efficiency of the BIPVT system and the hot air is usedinside the living space which helps in thermal gain. For the c-SiBIPVT the overall thermal and exergy efficiencies are 51.99% and14.91% respectively which are in consequence with the experimen-tal results of Chow [5], Tiwari et al. [23] and Joshi et al. [14]. For theother solar cells BIPVT systems, both the thermal and the exergyefficiencies are lower than those in the c-Si BIPVT. Thus, from theefficiency point of view the c-Si BIPVT systems are most suitablefor the rooftop.

Fig. 9 shows that the copper indium gallium selenide (CIGS)BIPVT system has a relatively higher annualized cost (US $ 2928.94)owing to higher initial investment made that for a short life span,whereas the a-Si BIPVT system has relatively lower annualised cost(US $ 785.66). The figure also shows that the annualized cost ofsuch systems are 2–7% higher than the similar BIPV systems. Fig. 10shows that the use of BIPVT system reduces the unit power gen-eration cost by 12–25% than that of the similar BIPV systems. Thefigure also shows that the unit power generation cost of the CIGSBIPVT system is highest (US $ 0.2654/kW) while that of a-Si is low-est (US $ 0.1009/kW). Thus from the economic point of view thea-Si BIPVT are more suitable for the rooftop. Also, the cost of unitpower generation from the a-Si BIPVT system is quite closer to thecost of unit power generation through conventional grid. There-fore the application of such systems in residential and commercialbuildings will help in reducing greenhouse gases emission whichis necessary for sustainable development.

9. Conclusions

Performance analysis and life cycle cost are evaluated for theBIPVT systems with different solar cells under the composite cli-matic condition of New Delhi and compared with the similar BIPVsystem. The results show that the use of BIPVT systems is alwaysadvantageous both from the efficiency and the economic point ofview than similar BIPV systems. The mono-crystalline silicon BIPVTsystems have higher energy and exergy efficiencies and are suitablewhere energy and exergy demands are higher and space for mount-

ing such systems are limited, like multi-storey buildings. However,the amorphous silicon BIPVT systems are better option from theeconomic point of view and are more suitable for the urban andremote places. The cost of unit power generation through amor-phous silicon BIPVT systems is US $ 0.1009 per kWh which is quite

and B

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B. Agrawal, G.N. Tiwari / Energy

loser to the cost of power generation through the conventionalrid. The higher use of such systems will also help in sustainableevelopment.

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