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Energy and Buildings 43 (2011) 3592–3598

Contents lists available at SciVerse ScienceDirect

Energy and Buildings

j our na l ho me p age: www.elsev ier .com/ locate /enbui ld

uilding-integrated photovoltaics (BIPV) in architectural design in China

hanghai Penga,b,∗, Ying Huanga, Zhishen Wub

School of Architecture, Southeast University, Nanjing 210096, PR ChinaIIUSE, Southeast University, Nanjing 210096, PR China

r t i c l e i n f o

rticle history:eceived 10 August 2011eceived in revised form0 September 2011ccepted 22 September 2011

a b s t r a c t

Building-Integrated Photovoltaics (BIPV) are one of the best ways to harness solar power, which is themost abundant, inexhaustible and clean of all the available energy resources. This paper discusses issuesconcerning BIPV in architectural design in China, including how to choose between BIPV and building-attached photovoltaics (BAPV), whether it is necessary for photovoltaic components to last as long as

eywords:IPVifetimerchitectural design

buildings and how to design BIPV structures. The paper shows that we should consider the function, cost,technology and aesthetics of BIPV, rather than solely the high integrations. According to developments intechnology and markets, photovoltaic structures and design should be focused on the maintenance andreplacement of photovoltaic cell modules, rather than simply prolonging their lives. To solve problemsassociated with the existing photovoltaic structures in China, we design a building photovoltaic structurethat allows convenient maintenance and replacement of photovoltaic components.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

Global environmental concerns and escalating demands fornergy, coupled with steady progress in renewable energy tech-ologies, are creating new opportunities to utilize renewablenergy resources. To date, solar energy is the most abundant, inex-austible and clean of all the renewable energy resources. The sun’sower reaching the earth is approximately 1.8 × 1011 MW, which

s many times greater than the present energy consumption. Pho-ovoltaic technology is one of the best ways to harness this solarower [1,2]. Photovoltaics generate electrical power by convertingolar radiation into direct current electricity using semiconductorshat exhibit the photovoltaic effect [3]. Photovoltaic power genera-ion employs solar panels composed of a number of cells containinghotovoltaic material. Materials presently used for photovoltaics

nclude monocrystalline silicon, polycrystalline silicon, amorphousilicon, cadmium telluride, and copper indium selenide/sulfide [4].ue to the growing demand for renewable energy sources, theanufacturing of solar cells and photovoltaic arrays has advanced

onsiderably in recent years [5–8].BIPV are photovoltaic materials that are used to replace conven-

ional building materials in parts of the building envelopes, such as

∗ Corresponding author at: School of Architecture, Southeast University, Nanjing10096, PR China. Tel.: +86 25 83792484/13 851682989; fax: +86 25 83793232.

E-mail address: pengchanghai1@yahoo.com.cn (C. Peng).

378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.enbuild.2011.09.032

the roofs, skylights or facades. They are increasingly incorporatedinto the construction of new buildings as a principal or ancillarysource of electrical power, although existing buildings may beretrofitted with BIPV modules as well [9]. The advantage of inte-grated photovoltaics over more common non-integrated systemsis that the initial cost can be offset by reducing normal construc-tion costs of building materials and labor for parts of the buildingreplaced by BIPV modules. These advantages make BIPV one of thefastest growing segments of the photovoltaic industry [10].

For BIPV systems to achieve multifunctional roles, various fac-tors must be taken into account, such as the photovoltaic moduletemperature, shading, installation angle and orientation. Amongthese factors, the irradiance and photovoltaic module tempera-ture should be regarded as the most important factors becausethey affect both the electrical efficiency of the BIPV system andthe energy performance of buildings where BIPV systems areinstalled. The results of basic studies on irradiance and energyoutput of photovoltaic systems have been reported by someresearchers [11–13], while there have been other studies on thetemperature and generation performance of photovoltaic modules[14–16].

Based on this background, this paper aims to discuss some issuesassociated with the following: how to choose between BIPV and

BAPV, whether it is necessary for photovoltaic components to lastas long as buildings and how to design BIPV structures. To resolveproblems associated with the existing photovoltaic structures inChina, the paper describes a building photovoltaic construction that

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maintain and, even without photovoltaic modules, these typesof buildings can function normally. Moreover, BAPV can createa space between photovoltaic arrays and the buildings’ skins.This gap is very important for the performance of photovoltaic

ig. 1. An example of BIPV in which photovoltaic arrays are combined with the roof.

llows convenient maintenance and replacement of photovoltaicomponents.

. BIPV or BAPV?

.1. What are BIPV and BAPV?

Two principal classifications can be defined for building pho-ovoltaic array mounting systems: BIPV and BAPV [17]. BIPV areonsidered a functional part of the building structure, or they arerchitecturally integrated into the building’s design. This categoryncludes designs that replace the conventional roofing materi-ls, such as shingles, tiles, slate and metal roofing. These typesf products can be indistinguishable from their non-photovoltaicounterparts. Aesthetically, this can be attractive if there is a desireo maintain architectural continuity and not to attract attentiono the array. BIPV modules can also be architectural elements thatnhance the building’s appearance and create very desirable visualffects. These types of arrays include custom-made module sizesnd shapes with opaque or transparent spaces between the cellsnd can be used for curtain walls, awnings, windows and skylights17,18]. Thus, BIPV are multifunctional solar products that generatelectricity while also serving as construction materials. Fig. 1 showsn example of BIPV, in which photovoltaic arrays are combinedith the roof.

BAPV are considered an add-on to the building, not directlyelated to the structure’s functional aspects [17]. They rely on auperstructure that supports conventional framed modules. Stand-ff and rack-mounted arrays are the two subcategories for BAPVystems. Standoff arrays are mounted above the roof surface andarallel to the slope of a pitched roof. Rack-mounted arrays areypically installed on flat roofs and are fashioned so that the mod-les are at an optimum orientation and tilt for the application. Theuperstructure is typically attached to the roof through a seriesf brackets or “feet” that are mechanically fastened to a structureegment of the roof system. BAPV arrays can also “float” over theriginal roof without any mechanical connection to the roof. Inhese cases, the array must be ballasted or designed to remain inlace when subjected to wind or other loads that would cause therray to slide, move or overturn [17]. The aim of BAPV is simplyo generate electricity. Fig. 2 shows an example of BAPV, in whichhotovoltaic arrays are attached to the rooftop.

However, sometimes these two classifications cannot be clearlyefined in practice. From the above definition, the main differenceetween BIPV and BAPV is the extent of tightness in the inte-ration of photovoltaic systems and buildings. For example, BAPV

ecomes BIPV when the photovoltaic arrays are integrated tightlyith buildings. The rapid development of photovoltaic technologyakes the integration of photovoltaic arrays and buildings easy and

iversified. Fig. 3 shows the effect of amorphous silicon thin-film

Fig. 2. An example of BAPV in which photovoltaic arrays are attached to the rooftop.

photovoltaic modules. The characteristics of thin-film photovoltaicsystems make them closely integrated with buildings so that thelevel of integration has reached the requirements of BIPV. There-fore, transparent curtain–wall constructions with thin-film solarmodules are typical of BIPV. Yet, this classification is consistent withthe definition of BAPV.

2.2. How to choose between BIPV and BAPV

Based on the previous descriptions, we know that the purposeof both BIPV and BAPV is to generate electricity with solar energy.The differences between them are that BIPV’s level of integration isso high that photovoltaic arrays can act as building envelopes, suchas curtain walls, awnings, windows and skylights. The advantagesof this form are that it is architecturally clean and attractive andoffsets the cost of roofing, fac ade or glazing materials. However,the total cost of BIPV is much higher than BAPV in China because ofBIPV’s complicated structures and difficult mounting and mainte-nance technologies. Conventional building materials and envelopeconstructions have solved many problems easily, such as thoseassociated with building loads, water drainage and thermal prop-erties. Further, their costs are far lower than those of photovoltaicarrays. In particular, damaged BIPV components directly affect theuse of buildings’ internal functions. For example, when the water-proof structures of a photovoltaic component are destroyed, theroom mounted with this photovoltaic array cannot serve its occu-pants any more.

While BAPV simply cause photovoltaic components to overlapwith the envelopes, their structures are simple to mount and

Fig. 3. Effect of amorphous silicon thin-film photovoltaic modules.

3594 C. Peng et al. / Energy and Buildi

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policies. The output of solar cells in 2000 was only 3 MW; in 2007, it

ig. 4. The Center for Renewable Energy at the Department of Architecture and Builtnvironment, University of Nottingham, UK.

omponents and buildings because of diffuse thermal emission byir or liquid. The effects of temperature on electrical performancend the life of crystalline silicon photovoltaic modules and arraysre generally well known [17–20]. The electrical performancef most photovoltaic arrays is strongly affected by temperaturend issues associated with the temperature ratings for electricalomponents. In general, temperature coefficients for power outputf crystalline silicon photovoltaic arrays reduce by approximately% for each 10 ◦C increase in cell operating temperature. Standoffounted arrays typically do not increase heat gain to the building,

nd in most cases, they reduce roof temperatures by shading theoof from direct solar gain. Reduced roof temperatures trans-ate into less conductive heat transfer through the roof section,hereby lowering temperatures of the roof underside and theorresponding radiation heat transfer to the top of conditionedpaces [21].

Therefore, we should choose suitable photovoltaic arraysccording to photovoltaic technologies, architectural forms, costsnd other building site situations. The Center for Renewable Energyhown in Fig. 4 is a two-story office/educational building. Photo-oltaic arrays that can convert photoelectric are used in the fac ade.he photovoltaic systems are used to meet part of the building’slectrical demand and to educate people on the application of BIPVystems. With the assistance of computer simulation and energynalysis, the vertical array was considered to be the best optionecause it would occupy a prominent position on the fac ade [22].he application form of photovoltaic systems for the renewablenergy center does not explicitly classify it as BIPV or BAPV. It isomewhere between the two, acting as a model for the promotionf both functions and forms.

. Is it necessary for photovoltaic components to last asong as the buildings?

.1. Cost of photovoltaic panels

The most important issue with solar panels is cost. Althougholar cells in the early 1950s cost 286 USD/W and reached efficien-ies of 4.5–6% [2], because of greatly increased demand, the pricef silicon used for most panels is now rising significantly. This hasaused developers to start using other materials and thinner sil-con to keep costs down. Due to economies of scale, solar panelsecome less costly as people use and buy more, and as manufac-urers increase production, the cost is expected to continue to dropn years to come. As of early 2006, the average cost per installed

att was approximately USD 6.50–7.50, including panels, invert-

rs, mounts and electrical items. By 2050, the cost of electricityenerated by photovoltaic cells will be close to that of conventionalower generation [23].

ngs 43 (2011) 3592–3598

3.2. Technologies of solar cells (also known as photovoltaic cellsor photoelectric cells)

Solar cell development is often considered to have occurred inthree successive generations, although the third is still being inves-tigated and is not fully developed. The two previous generations arestill in use, and are being developed further [23].

The first-generation technologies are most commonly used incommercial production and account for nearly 90% of all cells pro-duced. They are often described as high-cost and high-efficiency.They involve high energy and labor inputs, which has preventedmajor progress in reducing production costs.

These solar cells are manufactured from silicon semiconduc-tors and use a single junction for extracting energy from photons.They are approaching the theoretical limiting efficiency of 33% andachieve cost parity with fossil fuel energy generation after a pay-back period of 5–7 years. Nevertheless, due to capital-intensiveproduction, it is generally not believed that first-generation cellswill be able to provide energy more cost effectively than fossil fuelsources.

The second generation of solar cells has been intensivelydeveloped since the 1990s and 2000s. These cells are oftendescribed as low-cost and low-efficiency. Second-generationmaterials have been specifically developed to address energyrequirements and production costs of first-generation cells. Theseinclude copper–indium–gallium–selenide, cadmium–telluride,amorphous silicon and micromorphous silicon. Alternative man-ufacturing techniques, such as vapor deposition, electroplatingand use of ultrasonic nozzles are used to reduce needs forenergy-intensive production processes significantly [23].

A commonly cited example of second-generation cells is printedcells that can be produced extremely quickly. Though these cellshave only 10–15% conversion efficiency, the decreased costs meanthat, per unit of energy produced, the tradeoff is favorable. Second-generation technologies have been gaining market share since2008, and it is thought that second-generation solar cells willsurpass first-generation cells in market share sometime in 2012.Second-generation solar cells have the potential to become morecost effective than fossil fuels.

Third-generation solar cells are currently being researched. Noactual products exist yet. Third-generation technologies aim tocombine the high electrical performance of the first generation withthe low production costs of the second generation. The goal is thin-film cells that obtain efficiencies in the range of 30–60% by usingnew technologies. Some claim that third-generation cells couldbegin to be commercialized around 2020, but it is too early to knowdefinitively. Technologies associated with third-generation solarcells include multijunction photovoltaic cells, tandem cells, nanos-tructured cells for improved incident light usage and even infraredcollection during night, and excess thermal generation caused byultraviolet (UV) light to enhance voltages or carrier collection [23].

Fig. 5 indicates that the cost of third-generation solar cells ishigher than that of the second generation cells, but the efficiencyof the third-generation solar cells increases 3–5 times over that ofthe second generation cells [24].

3.3. Photovoltaics in China

In recent years, the photovoltaic power generation industry hasdeveloped quickly in China, beginning with steady improvement ofthe technology, creation of the industrial systems, gradual realiza-tion of the market potential, and the development of more favorable

reached 1088 MW; in 2009, it was up to 4382 MW. Since 2002, theannual average growth rate of China has reached as high as 191.3%[25]. Fig. 6 shows that the Chinese crystalline silicon ingots/wafers

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ig. 5. The “cost-efficiency” curve of three generations of batteries in which I is therst generation of solar cells, II is the second generation of solar cells and III is thehird generation of solar cells.

ndustry has developed rapidly, with an average annual rate ofrowth of 116% from 2004 to 2007 [26].

.4. The lifetimes of photovoltaic panels and buildings in China

According to the idea of sustainable development, a buildinghat saves resources and is environmentally friendly should reachts design life, and the longer it lasts, the better. However, as a newroduct, the life of photovoltaic cells is not long. At present, theower warranty of the domestic production of solar modules is only5 years, which is much shorter than that of common buildings,hich reaches at least 50 years, based on current Chinese building

odes.Previous research indicates that with the rapid development

f photovoltaic technologies, the conversion efficiency of pho-ovoltaic cells increases. Further, the cost diminishes due to

conomies of scale, and solar panels become less costly as peoplese and buy more and as manufacturers increase production. Thisas been true since 2004. Therefore, updates of photovoltaic com-onents occur so rapidly that their lifespans become increasingly

Fig. 6. The development trend of the photovo

ngs 43 (2011) 3592–3598 3595

short. In contrast to photovoltaic cells, the lifetime of buildings isrequired to be 50 years or greater, depending on the importance ofthe building. Thus, it is not necessary for photovoltaic componentsto last as long as buildings.

4. The ease of maintaining and replacing photovoltaiccomponents should be emphasized

Photovoltaic components include photovoltaic cells and steelsupport systems. Through integration, photovoltaic componentsbecome part of buildings, and their efficiency directly affects theperformance of buildings. Especially for BIPV, even if the existingphotovoltaic cells can last as long as buildings, the maintenanceand replacement of photovoltaic components are a current priorityfor the application and popularization of photovoltaics in buildingsbecause the existing photovoltaic cells must be updated as the costof new solar cells continues to drop, while the conversion efficiencyof photovoltaic systems continues to increase. This process is simi-lar to the updating of personal computers and cellphones. Thus, it isessential to design a photovoltaic structure that is easy to maintainand that can be replaced to meet the demand of current markets inthe photovoltaic industry.

4.1. The technology of BIPV structure

Although there are many mounting systems in the currentphotovoltaic market, only a few systems can be used flexibly inbuildings. In general, the existing mounting systems for BIPV typ-ically require attached intermediaries and bolts to join and fasten.As shown in Fig. 7, photovoltaic components are fixed by the hold-down plates of aluminum alloy [27]. Then, these plates are fastenedby bolts to the substructures. The disadvantage of this mountingsystem is that applies only to the frame-exposed BIPV systems,and it cannot be installed quickly due to its complicated wiresand numerous bolt connections. Therefore, the buildings of this

mounting system are not attractive. To make the maintenance andreplacement of photovoltaic components easy and rapid and tomake the building more attractive, the authors here propose a newtype of photovoltaic component.

ltaic industry in the world and in China.

3596 C. Peng et al. / Energy and Buildings 43 (2011) 3592–3598

Fig. 7. The diagrammatic sketch of photovoltaic installation with a middle-pressureplate.

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Fig. 10. Upper-spring connection module.

Fig. 11. Details of the longitudinal beam.

Fig. 8. Photovoltaic component design.

.2. A novel structural design scheme for BIPV

This novel photovoltaic structure, which is very convenient toaintain and replace, includes photovoltaic cell components and a

teel support system, shown in Fig. 8. Fig. 9 indicates that the photo-oltaic cell modules, which contain some photovoltaic panels, two

pper-spring connection models and two under-fixed connectionodels, are integrated closely with buildings through a steel sup-

ort system. The upper-spring connection model is comprised of

Fig. 9. Photovoltaic components.

Fig. 12. Details of horizontal beam.

two springs and a sliding block in which the “anode–cathode” con-tact points of the electric circuit are at the end (Fig. 10). One end ofthe spring is fixed on the sliding block and the other end of it is builton the U-bracket which is welded on the photovoltaic panels. The

Fig. 13. “Anode–cathode” contact points of the electric circuit at the horizontalbeam.

C. Peng et al. / Energy and Buildings 43 (2011) 3592–3598 3597

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Fig. 14. Architectural pictures of a

liding block can glide over the surface of the U-bracket. Moreover,his model is an auto-lock device that can fix photovoltaic cell mod-les to the steel support system when the device is pushed downnd released. The under-fixed model functions to provide spacingnd regularity.

The steel support system includes the longitudinal beams andorizontal beams. As shown in Fig. 11, the longitudinal beams are

ike an I-bar. The electrical circuit box of the I-beam is placedt the side of the horizontal beam. The other side of the I-beamirectly connects with the buildings. The horizontal beam is a C-eam, in which the connectors of photovoltaic cell modules and thelectrical circuit box hide, as shown in Fig. 12. The photovoltaicell modules are mounted on the C-beam by plugging their upper-pring models and under-fixed models into the connectors of the-beam, on which the “anode–cathode” contact points correspondo the upper-spring models (Fig. 13). The I-beam and C-beam havelectrical circuit boxes for placement of cables. The photovoltaicell modules are mounted on the steel support system, which ishen fixed on the buildings. Fig. 14 shows architectural pictures of

photovoltaic roof and photovoltaic wall.

.3. Advantages of the design scheme

) The photovoltaic cell module combines so many separate parts,such as cell panels, connection blocks and electrical circuitboxes, that it is very easy to install and replace photovoltaicmodules quickly.

) The I-beam has sufficient strength and altitude. This character-istic makes cell temperature drop possible through ventilationand long-term installation of photovoltaic modules.

) The C-beam greatly simplifies the installation process becauseit simultaneously solves the problem of connecting componentsand the electrical circuits of photovoltaic modules.

) The upper-spring module is both a fastening structure and aconductive block. This module makes the structure’s installationand wire connection easy.

) The photovoltaic modules can be prefabricated in factories andmounted on site.

. Conclusions

Due to technical and economic constraints, the current form ofIPV is restricted in function and appearance. By analyzing casesnd researching data, we conclude:

) It is not true that tighter integrations of BIPV are preferablebecause the form of high combinations may not be favorable infunction, economy and technology, although it is very attractive

voltaic roof and photovoltaic wall.

architecturally. Therefore, we should choose the appearance ofBIPV according to actual needs.

2) As a high-tech industry, photovoltaic technology has developedby leaps and bounds. The cost of photovoltaics declines contin-uously, the conversion efficiency of photovoltaic cells increasesconstantly, and the forms of photovoltaics are increasing. Underthese conditions, the updating of photovoltaic components isso rapid that existing products could be eliminated in 10 years.However, the lifetime of buildings in China is required to be atleast 50 or more years, depending on the importance of thebuilding. Thus, photovoltaic components do not need to lastas long as buildings, but easy maintenance and replacement ofphotovoltaic components are important.

3) Though the forms of photovoltaic components currently vary,they have not reached requirements of standardization, and theprocesses of installation, maintenance and replacement of pho-tovoltaics are not convenient. Therefore, we propose a novelstructural design scheme for BIPV that is very easy to maintainand replace. The idea comes from the principle of dry batteries,self-locking and the integration of electrical circuits and steelsupport systems. Moreover, these photovoltaic modules can beprefabricated in factories and mounted on site.

Acknowledgments

The work described in this paper is supported by the Natu-ral Science Foundation of Jiangsu Province (BK2010061), The R&DProgram of Ministry of Housing and Urban-Rural Development ofPeople’s Republic of China (2011-K1-2) and National Key Technolo-gies R&D Program of China (2011BAJ03B04, 2011BAJ03B11)

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