BIPV Applications

8/4/2019 BIPV Applications

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Optimal Building-Integrated

Photovoltaic Applications

Gregory Kiss

Jennifer Kinkead

Kiss & Company Architects

New York, New York

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

A national laboratory of the U.S. Department of Energy

Managed by the Midwest Research Institute

for the U.S. Depar tmen t of Energy

un der Contract N o. DE-AC36-83CH10093

November 1995 NREL/ TP-472-20339


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NOTICE

This report was prepared as an account of work sponsored by an agency of the United

States government. Neither the United States government nor any agency thereof, nor

any of their employees, makes any warranty, express or implied, or assumes any legal lia-

bility or responsibility for the accuracy, completeness, or usefulness of any information,

apparatus, product, or process disclosed, or represents that its use would not infringe pri-

vately owned rights. Reference herein to any specific commercial product, process, or

service by trade name, trademark, manufacturer, or otherwise does not necessarily con-

stitute or imply its endorsement, recommendation, or favoring by the United Sates gov-

ernment or any agency thereof. the views and opinions of authors expressed herein do

not necessarily state or reflect those of the United States government or any agency

thereof.

Available to DOE and DOE contractors from:

Office of Scientific and Technical Information (OSTI)

P.O. Box 62

Oak Ridge, TN 37831

Prices available by calling (615) 576-8401

Available to the public from:

National Technical Information Service (NTIS)

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

(703) 487-4650


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Optimal Building-Integrated

Photovoltaic Applications

Gregory Kiss

Jennifer Kinkead

Kiss & Company Architects

New York, New York

NREL Technical Monitor:

Sheila Hayter

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401-3393

A national laboratory of the U.S. Depa rtmen t of Energy

Managed by the Midwest Research Institute

for the U.S. Depar tmen t of Energy

un der Contract N o. DE-AC36-83CH10093

Prepared un der Subcontract N o. AAE-5-14456-01

October 1995

NREL/ TP-472-20339 UC Category: 1600 DE95013150


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TABLE OF CONTENTS

I. Introduction 1

II. Architectural Applications for PV Integration 4

III. Construction Material Credits 8

IV. Additional BIPV Construction Costs 16

V. Location 21

VI. PV Technology 24

VII. Payback 25

VIII. Payback and Architectural Value 34

IX. Conclusions 35


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Optimal BIPV Applications Kiss and Company Architects 9/29/95 1

I. INTRODUCTION

Photovoltaic (solar electric) modules are clean, safe and efficient devices

that have long been considered a logical material for use in buildings.

Recent technological advances have made PVs suitable for direct integra-

tion into building construction. PV mod ule size, cost, app earance and reli-

ability have adv anced to the p oint where they can fun ction w ithin the archi-

tectural parameters of conventional building materials. A building essen-

tially provides free land an d structural support for a PV mod ule, and th e

mod ule in turn d isplaces standard bu ilding components.

This report identifies the highest-value applications for PVs in buildings.

These systems shou ld be the first markets for BIPV prod ucts in the comm er-

cial buildings, and should remain an important high-end market for the

foreseeable futu re.

Op timizing BIPV app lications is a fun ction of m any variables: construction

methods and materials, photovoltaic technology and module fabrication,

insolation levels and orientation, and electrical costs. This report ad dresses

these variables in the following ord er:

Architectural application (curtain w alls, skylights, etc.).

Construction material credits (the type and value of conventional build-

ing materials displaced).

Add itional BIPV construction costs (wiring, ventilation).

Location param eters (insolation, construction costs, electrical rates).

PV technology (crystalline silicon, amorphous silicon, advanced thin-films).

Using these variables,the most promising BIPV applications, building loca-

tions and PV technologies are selected and evaluated in a simple payback

analysis.

Previous StudyAn ear l ier s tudy by the same auth ors , ent i t led Bui lding-Integrated

Photovoltaics: A Case Study, completed in February 1995, evaluated the

performance and economics of a series of roof-integrated photovoltaic sys-

tems in high-end comm ercial buildings. Results from that case study con-

firmed that infrastructure costs for PV systems are significantly reduced

with building integration. The stud y found , however, that building-inte-

gration introd uces a complex set of issues wh ich greatly affect PV perfor-

man ce and viability. Figu res 1-3 illustrate the primary advant ages and dis-

advan tages identified by th e study for building-mounted and building-inte-


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Fig. 1: Field-mounted PV s.

Advantages

Unconstrained orientation.

Disadvantages:

Land and maintenance costs.

Sup port structure costs.

Fig. 2: Building-mounted PVs.

Advantages:

Si m p l e s u p p o r t s t r u c tu r e

minimizes roof penetrations.

Disadvantages:

O r i en t a t io n p a r t ly c o n -

strained by building position,

structure and roof equipm ent.

Potential comp lications re:

structure and w aterproofing.

P o t en t i a l co d e p r o b le m s

without mechanical at tach-

ment.

Fig. 3: Building-integrated PVs.

Advantages:

Low structural and installa-

tion cost.

Credit for offset construction

materials.

Disadvantages:

Orientation constrained by

architectural requirements.

Potentially higher PV oper-

ating temperatures. Safety, w aterproofing, aes-

thetic risks.



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grated PV installations comp ared w ith traditional field-mou nted installa-

tions.

The case study was designed to evaluate the architectural and economic

implications of integrating PVs into the roof of a comm ercial building. Five

different roof constru ction systems were stud ied, ranging from ballasted

(gravity-moun ted) PVs on a conventional roof (fig. 2), to a fully integrated

PV roof w ith light m onitors and active heat recovery. The five systems

were eva lua ted in s ix d i ffe rent loca t ions a roun d the Uni ted Sta tes .

Analyses included a bu ilding energy balance mod el, PV outp ut calcula-

tions, construction cost estimates, utility rate calculations, and a simple

payback analysis.

Payback results from the case study indicated that with current PV tech-

nologies and u tility rates, some BIPV systems are econom ically viable

today. Und er the right cond itions (insolation and u tility rates), payback

periods for some BIPV roofs are under 20 years, an acceptable return on

investmen t for some long-term institutional and utility investors. The

report conclud ed th at opp ortunities for economically comp etitive BIPV sys-

tems can only increase with advances in PV technology, higher utility rates

and/ or better tax credits and governm ent incentives. These add itional fac-

tors, some of them available today, were not evaluated in the study.

Equally imp ortantly, the stud y demonstrated th e benefits of PV integration

into architectural systems with high displaced material credits and low

add itional construction costs. A BIPV atrium roof, for example, requires lit-

tle or no add itional constru ction to incorpora te PVs and offers a high ma ter-

ial credit for displacing lam inated , overhead glazing. This roof type per-

formed particularly well in the payback analysis.

Although promising BIPV roof types were identified, the previous study

did n ot attempt to seek out the most optimal BIPV app lications. A broad

range of roof types w as chosen an d roofs were compared against each other

und er various conditions. Research from the case stud y provides mu ch of

the background for the choice of construction systems and locations used in

this report. This report builds upon th e assum ptions mad e in the case

study and sets out to identify, optimize and analyze niche applications for

BIPV.



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II. ARCHITECTURAL APPLICATIONS FOR PV INTEGRATION

Any building surface that intercepts the sun is a candidate for PV integra-

tion. Many buildings incorporate semi-attached elements in addition to

walls and roofs, such as awnings, light shelves, canopies and fences. All of

these surfaces can d eliver the m ultiple benefits of BIPV: prod ucing energy

while perform ing other architectura l functions. This report concentrates on

two of the most straightforward app lications: atria/ sloped glazing and cur-

tain walls. In these systems, PVs form the weathering skin for a building

while directly replacing expensive glazing.

Glass-based PVs are the only PV products available today that can be readi-

ly integrat ed into existing constru ction systems. Metal substrate PVs are

being developed, wh ich prom ise to be able to replace sheet metal in build-

ing roofs and skins. Since little cost or performance data is yet available for

these prod ucts, this study focuses on glass-based BIPV installations.

BIPV systems m ay be bu ilt as par t of new constru ction, or retrofit to exist-

ing buildings. This report evaluates only new construction, since there are

many additional constraints and unpredictable costs associated with retro-

fitting an existing structure. Recladd ing a building with a PV curtain w all,

for example, is a very similar process to recladd ing with a conventional

curtain wall, with the exception that wiring must be accommod ated.

Depend ing on the existing construction, wiring may be d ifficult or even

hazard ous. In the best cases, retrofit applications will perform as well as

new installations, but each retrofit project mu st be evaluated individ ually.



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BIPV Atria/Sloped GlazingFor the pu rposes of this report, atria are defined as overhead, semi-trans-

parent glazing systems, framed with aluminum extrusions, containing

tinted, laminated or wire glass or plastic glazing units. Sloped glazing, as

in sun spaces, greenhouses, or tilted walls, is usu ally constructed with sim-

ilar framing an d glazing. Mediu m to large area skylights often fall into this

category but small skylights do not; they are normally prefabricated units

consisting of a metal cur b and a plastic dom e.

Many off-the-shelf PV mod ules are su itable for d irect installation into these

glazing systems, since they are the same size and shap e as tinted, laminat-

ed glazing un its. PVs also transmit a comfortable amou nt of diffuse light,

either through a crystalline pa ttern or scribe lines in thin films. Diffuse day-

lighting is frequently a desirable condition in overhead glazing since too

mu ch sun light will overheat interior spaces and cause excessive glare. At

the APS facility in California, a PV skylight incorporates amorphous PV

mod ules with standard skylight fram ing members (fig 4). The stand ard

amorphous modules transmit 5% daylight a comfortable amount for thework environment below. In other situations, customized m odu les may be

fabricated for specific size, strength, transp arency, color and other criteria.

Atrium systems are p otentially the high est-value app lication for BIPV.

They offer:


Fig. 4. PV skylight at APS facility, Fairfield, California. A standard skylight

framing system mixing amorphous silicon PVs with tinted laminated glass.

The PVs transmit 5% daylight.


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Potential optimal orientation for maximum PV output. Subject to the

building s orientation and geometry, PV atrium u nits can be designed

at any tilt and azimuth.

No additional cost for structure or installation of modu le. Laminated

glass PV mod ules can directly replace standard laminated glass.

Lower costs du e to balance of systems costs that include only wiring

and p ower conditioning.

A high material credit for the replacement of expensive laminated sky-

light glazing.

BIPV Curtain WallsCurtain w al ls have m any of the same construct ion character is t ics as

atria/ sloped glazing, but they suffer from reduced PV output as a conse-

quence of their vertical orientation. Nevertheless, the market size for cur-

tain walls is substantially greater than for atria, and prod ucts developed

for atria should be usable in curtain w alls with little or no m od ification.

In addition, a wider range of PV products is suitable for curtain walls than

for atria. Cur tain walls often contain opaqu e surfaces (spand rel areas),

where non-transp arent mod ules can be used. Vision glass areas will require

highly transp arent PVs with good optical prop erties; n o such modu les exist

yet, but they may be developed in the future. Semitransparent PVs with

med ium op tical quality might be used in p arts of curtain wall glazing, such

as high glazing in tall spaces, wh ere daylighting is the prim ary criterion

and view is secondary.

At the APS facility, amorphous PV modules are combined with vision glasspanels in stand ard cur tain wall fram ing (fig 5). The PV mod ules are sealed

at the back with an opaqu e insulating p anel, much like spand rel panels in a

mu lti-story curtain w all. From the exterior, the clear vision glass and PV

mod ules look the same. Figu re 6 show s an interior view of the vision glass

and sealed PV pan els. Two of the PV pan els are left unsealed to comp are

their transparency with th e adjacent vision glass. In add ition, large-area

amor ph ous m odu les (2.5 by 5) were used to fit standa rd cur tain wa ll fram-

ing dimensions. To penetrate the cur tain wall mar ket, PV mod ules should

be available in dimensions compatitible with curtain wall standards.



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Fig. 5. PV curtain wall cube at APS facility in Fairfield, California. The

BIPV system icludes amorphous silicon PV modules in standard curtain wall

framing. The framing is at a 76cm x 152cm (2.5 x 5) spacing. Ten of the glaz-

ing units at the left center of the cube are tinted vision glass.

Fig. 6. Interior view of PV curtain wall at APS. For comparison, two of the

PV panels to the left of the vision area are left unsealed, showing the relative

transparency of the modules.


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III. CONSTRUCTION MATERIAL CREDITS

The key to th e economics of the highest-value BIPV systems is the m aterial

credit received for the replacement of conven tional building mater ials. For

curtain wall and sloped glazing, there exists a broad ran ge of conventional

glazing materials, construction method s and assemblies. The choice of a

particular material for a project depends on many factors including solar

control, aesthetics and construction bud get. BIPV installations are most

cost-effective in p rojects where h igh-end glazing m aterials are used .

Non-glass materials such as stone or metal panels are also used as building

cladd ing in curta in walls. These materials can be more expen sive than glass

indeed more expensive than PV modules which can result in a PV

cladd ing system which is cheaper than a conventional one. However, since

these m aterials have significantly different aesthetic and material character-

istics from glass and glass-based PVs, they are not consid ered to be directly

replaceable in th is stud y.

Glazing MaterialsWith th e growing focus in the bu ilding comm unity u pon control of energy

flow in building envelopes and with the increased use of glass as a featured

architectural material, the glass industry has introduced an increasingly

soph isticated line of prod ucts. Following is a list of single-pan e glazing

produ cts, most of which can be replaced w ith a standard BIPV module

un der the right conditions:

Transparent Glazing:

Clear float glass, the basic element of glass construction, is applicable

where high visibility and clarity are required and thermal control and

safety are not a priority.

Tempered glass is a treated glass product which provides resistance to

breakage from wind an d therm al loads. It is common ly found in

entrances, storefronts and curtain w alls (approximate cost premium

over clear glass: 36%).

Tinted float glass is colored glass wh ich controls light transm ission w hile

redu cing solar heat gain. Green or blue tints allow more light and are

often used in skylights and atria. Gray and bronze tints are used w here

reduced light transmission is desirable, as in office buildings or hotels

(app roximate cost prem ium over clear float glass: 43%).

Laminated glass is manufactured by combining two or more layers of

glass together with an ad hesive interlayer. Laminating offers add ition-



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al strength and soun d control. Since it is less likely to break or shatter

under loads, it is most suitable for sloped glazing and skylight applica-

tions (app roximate cost prem ium over clear glass: 61%).

Reflective glass has an applied reflective coating which controls light

transmittance and reflectance to varying degrees while reducing solar

heat gain. It is comm only found in ap plications similar to tinted glaz-

ing, but provides higher levels of performance and control (approxi-

mate cost premiu m ov er clear glass: 76%).

Low-emissivity (low-E) glass has a high-performance, neutral-colored

coating which maximizes visual light transmittance, provides good

solar thermal performance and blocks UV transmission. It is applicable

where energy performance and l ight t ransmit tance are pr ior i t ies

(app roximate cost prem ium over clear glass: 100%).

Semitransparent Glazing:

Fritted glass is a specialty glazing material in which an opaqu e ceramic

pain t is silkscreened an d fired onto glass. The pattern red uces heat gain

by blocking d irect radiation. It partly or comp letely obstructs views in

or out and is often used a s a design element. Fritted glass is found in

many high-design or high-tech architectural projects which use glass

extensively. Tw o examp les of fritted glass constru ction: the Un ited

Terminal at O 'Hare ai rp ort in Ch icago an d the Federal Jud iciary

Building in Washington (fig 7). (app roximate cost prem ium over clear

float glass: 120%).


Fig. 7. Example of fritted glass curtain wall at the Federal

Judiciary Building in Washington, DC.


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Opaqu e Glazing:

Spandrel glass is used in curtain walls to cover areas between floors

where no view or light transm ission is required . Some span drel glass is

designed to match the ap pearance of reflective or tinted vision glass; in

these cases the spand rel glass is mad e with sim ilar or identical coatings,

sometimes with a separate opaque layer behind. Other spand rel glass

i s back pa i n t ed o r f r i t t ed t o p r oduce a co l o r ed , opaque un i t .

{App roximate cost p remium over clear float glass: 73%}

Chart 1 illustrates the relative costs of these glazing materials as compared

to current p hotovoltaic technology costs.


0

50

100

150

200

250

300

350

400

450

500550

600

Tempered

Tinted

Laminated

Spandrel

Reflective

Low-E

Fritted

Laminated

andfritted

Tempered

ClearFloat

$/m2

:advancedthinfilm*

:Amorphoussilicon

V:crystallinesilicon

$38

$51

$54

$61

$65

$67

$75

$83 $

130

$130

$156

$216

$616

:advancedthinfilm

Chart 1: Typical glazing materials costs vs. typical photovoltaic module costs in $/m2. Advanced

thin film2=more aggressive prediction for CIS and CdTe technologies in the near term. (Glazing

costs source: RS M eans Inc.; PV costs source: Energy Photovoltaics Inc., Advanced Photovoltaic

Systems, AD Little).

Clearfloat

Tempered

Tinted

Laminated

Spandrel

Reflective

Low-E

Fritted

Laminated&fritted

PV:advancedthinfilm2

PV:amorphoussilicon

PV:advancedthinfilm1

PV:cyrstallinesilicon


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Insulating Glazing:

Due to their superior thermal performance, insulating units are used in

more than 80% of all transparent building glazing.1 These units are fabri-

cated from two layers of glass separa ted by a spacer and sealed. The most

common configuration is a 25mm (1) thick unit consisting of two layers of

6mm (1/ 4) glass separated by a 12mm (1/ 2) air space. Many variations

of the basic un it are possible, including triple glazing, gas-filled units,

un its w ith a thin heat-reflective plastic film in the airspace, and others.

Any of the single pan e glazing produ cts discussed p reviously can be com-

bined into an insu lating un it. For overhead (atrium) app lications, for

example, the outer lite is often tinted, heat-strengthen ed glass while the

inner lite is lamina ted for safety reasons.

Configuring PVs for Architectural GlazingGlass- to-glass PV modules are fabricated in several different ways,

dep end ing on the PV material, encapsu lation method, electrical connectordetail, and oth er factors. For ease of integration into glazing systems,

frameless laminated modules of a standard thickness (6mm for single glaz-

ing) are easiest to accomm oda te.

The following figures illustrate some of the most common PV module types

and their method of integration into insulating un its and into framing sys-

tems.

Crystalline Silicon Modules:

Crystalline and polycrystalline silicon PV mod ules (figs. 8, 9) are character-

ized by high reliability, good efficiency (12-15% cells, 10-12% modules),and costs in the range of $4-5/ Wp.

2 As figure 9 show s, these modu les are

made up of individual cells laminated between two sheets of glass or one

sheet of glass and anoth er encapsu lating film such as Tedlar.


Fig. 8. Diagram of typical crystalline silicon glass-based PV module. Silicon

wafers are encapsulated between two layers of glass and connected in series by

electrical contacts.


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Thin Film Modules:

Thin film modules (fig. 10) include amorphous silicon, a technology whichis present ly avai lable, and CIS (Copper Indium Diselenide) or Cd Te

(Cadmium Telluride), more advanced technologies which will be available

in the near future. These are large-area mon olithic dev ices with a single,

un iform sur face pun ctuated by thin scribe lines. They are either super-

strate-based, where the PV film is applied to the bottom sur face of the top

glass (fig. 11), or substrate-based , where the surface is on the top of the

bottom glass (fig. 12). Am orph ous silicon and CdTe are superstra te-based

mod ules while CIS mod ules are substra te-based. For architectur al applica-

tions, there are advan tages and disadvantages to both configurations.


Superstrate glass.Crystalline silicon wafers (~0.5 mm).Resin encapsulating wafers and applied to superstrate.

Encapsulating substrate glass.Wiring drawn through encapsulating substrate.Electrical contact.

Fig. 9. Typical section detail of crystalline silicon PV module suitable for build-

ing integration (not to scale).

Fig. 10. Diagram of typical thin-film glass-based PV module. A thin film of

photovoltaic material is deposited on a layer of glass and laser-scribed, creating

a series of thin cells connected in series. A second layer of glass encapsulates

the module.


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PVs as Glazing MaterialPVs can replace conventional architectural glazing in a number of applica-

tions.

Opaque PV Glazing:

Any PV can replace span drel glass, prov ided the size and visua l quality of

the modu le are comp atible with the building design. A very close visual

match is possible between existing thin-film amorphous PV modules and

vision g lazing (fig. 5).

Semi-transparent PV Glazing:

To date, no PV produ cts have been developed w hich are sufficiently trans-pa rent to replace vision glass. However, there are man y applications for

semi-transparent glazing for wh ich PVs are well suited . Skylights, atria,

and greenhouse structures often u se heavily tinted or patterned glass to

minim ize heat gain or control glare. Like pa tterned g lass, most large-area

thin-film modules are partly transparent as a result of thin lines scribed

throu gh the cell mater ial. Single crystal or polycrystalline BIPV mod ules


Encapsulating superstrate glass.

Thin film (3-10 m) applied to substrate.Substrate glass.Wiring drawn through substrate glass.Electrical contact.Scribe lines in film yield approx. 5% transparency.

Fig. 12. Typical detailed section of a thin film substrate-type PV module (not

to scale).

Superstrate glass.Thin film (3-10 m) applied to superstrate.

Encapsulating substrate glass.Wiring drawn through encapsulating glass.Electrical contact.Scribe lines in film yield approx. 5% transparency.

Fig. 11. Typical detailed section of a thin-film superstrate-type PV module (not

to scale).


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usu ally consist of a grid of opaque PV cells, laminated between tw o sheets

of clear glass. Light passes throu gh the spa ce betw een cells, and light

transm ission is easily controlled by varying th e space between cells.

The transparency of thin-film BIPV modules can be controlled by etching

add itional patterns in the PV material by the same lasers that pattern the

cells. This process requ ires a modification to the in-line lasers on the m anu -

facturing line, or a separate laser station off-line, either being a significant

capital investm ent on the part of the PV man ufacturer. For either type of

PV mod ule, any significant p enetration of the architectural mar ket will

deman d som e degree of design flexibility.

The highest-cost glazing that can reasonably be replaced by PVs is tinted,

laminated glass with a fritted patter n. This type of glass is used in high-end

buildings for atria and exterior curtain walls where safety issues require

lamination an d aesthetic and/ or solar control issues justify the fritted p at-

tern. The cost of this type of glazing is app roximately $130/ m2 ($12/ sf).3

This num ber is used as the high-end material credit in the cost analysis.

Insulating PV Units:

Packaging thin film or crystalline m odu les into architectural insulating

units will render the modules more thermally effective, and therefore more

attractive to the building market. A PV mod ule may be incorporated into

insulated units either as the exterior glass element (fig. 13) or interior glass

element (fig. 14). Either app roach has certain advan tages and disad van-

tages:

Exterior-lite PV insu lating units allow the PV un it direct exposure to the

sun . How ever, since the air space behind the PV is not ventilated, the

mod ule operates at a higher temperatu re, and efficiency is redu ced.

Furthermore, runn ing w iring from front to back of the sealed un it

requires the penetra tion of the insulating unit seal. Failure of the seal

leads to moisture penetration, causing w indow fog and possible short-

circuiting. In sloped glazing app lications, building codes require that

the inner lite of an insu lated un it be laminated . This increases the cost

of the unit.

Interior-lite PV insulating units will lose some efficiency due to reflec-

tive losses through the outer lite, but w ill run cooler. For code pur pos-

es, the laminated PV unit may replace the standard laminated inner

lite, saving cost.

Cost projections for PV insulating units are more speculative than for PVs



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as single glazing. No stand ard PV insulating u nit exists as a produ ct today,

and no p rototypes have been bu ilt in the US. For the purp ose of this report,

the cost difference between a standard insulating unit and a PV insulating

unit is assumed to be equal to the difference between the PV module and

the stand ard glass lite it replaces in the unit. No ad ditional cost is assum ed

for provision of wiring, because without actual prod uction experience

these costs are difficult to predict. The study assu mes that w iring costs

within an insulated PV modu le will not be significant.


Tempered glass @ exterior.

Spacer.Air space.

Thin-film substrate PV module @ interior.Electrical contact.Wiring.

Fig. 14. Detailed section of PV insu lating unit with PV as interior lite (not to

scale).

Thin-film superstrate PV module @ exterior.Electrical contact.

Spacer with wiring penetration.

Air space.Laminated glass @ interior.

Fig. 13. Detailed section of PV insulating unit with PV as exterior lite (not to

scale).

Exterior

Interior

Exterior

Interior


20/41

IV ADDITIONAL BIPV CONSTRUCTION COSTS

Although BIPV installations offer a number of significant cost savings,

there are add itional costs and comp lexities to consider. Ideally, a BIPV

module behaves exactly like a piece of architectural glazing as far as build-

ing structure and framing systems are concerned, and no add itional costs

are incurred for structure or installation labor. A BIPV mod ule is also an

electrical comp onen t, how ever, and consequently there are other factors

wh ich w ill add costs.

Glazing Construction MethodsTwo basic glazing meth ods exist in conventional glazing constru ction.

Pressure-plate glazing (fig. 15) consists of horizontal an d/ or vertical

framing members that captu re the glass in gaskets and are fastened by

the pressure of exterior mu llion caps. Details and insta llation method svary w ith individu al manu facturer.

Structural silicone glazing (fig. 16), also known as flush-glazing, elimi-

nates the need for a mullion cap by capturing and sealing the glass

using a stru cturally adh esive silicone.


Fig. 15 Pressure plate framing. Fig. 16. Structural silicone framing.


21/41

PVs in Glazing Construction

Glazing Method:

Both pressure-plate and structural silicon glazing methods are suitable for

BIPV app lications. Pressure-plate glazing is the more comm on and less

expensive of the two, but has the disad van tage of a projecting mu llion cap

wh ich casts a shadow on th e modu le. The shad ow can be especially dis-

rup tive to thin film m odu les, wh ere a thin shad ow that comp letely covers

one of the individual mod ule cells can shu t dow n the entire mod ule. These

effects are minimized by using shallow mullion caps and by making a small

inactive region at the edge of the mod ule.

In flush glazed systems, shad owing effects are eliminated , but other p rob-

lems are introd uced . The structural silicone sealant wh ich seals the glass at

the edge may react with the PV mod ule laminate. In add ition, flush -glazed

mod ules are visible in their entirety. Since some PV mod ules have un fin-

ished or different-colored edges, mod ule edges may h ave to be painted or

otherw ise treated. Furthermore, wiring accomm oda tion will be difficult to

conceal.

Because there are fewer potential problems with pressure-plate construc-

tion, i t is the method assumed in performance analyses in this study.

Performance calculations, wh ich are don e on a per-squa re-meter basis, do

not account for reduced efficiency due to inactive edge zones as discussed

above. It is assum ed th at these effects are relatively insignificant.


Mullion (customized for PV).PV wiring housed in m ullion.

Access for wiring maintenance@back of mullion.

Mullion cap.Inactive PV area at cap edge.

PV module.

Fig. 17. BIPV glazing detail: single-glazed PV glazing in pressure plate framing.


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Wiring:

Wiring may be accommodated by conventional conduit or inside the hol-

low framing elemen ts if they have been approp riately customized . Figu re

18 illustrates options for wiring insulating PV units in pressure plate fram-

ing. For insulating units, wiring from a sup erstrate mod ule must penetrate

the air space and seal in ord er to be concealed w ithin the framing members.

Substrate mod ules avoid any m anipulation of the thermal seal, but a wiring

cap m ay be requ ired at the back electrical contact.

Inverters:

Most PV systems use DC wiring to an inverter; there are also small invert-

ers under development (AC modules) which allow AC wiring between

PV mod ules. In an atrium , these inverters wou ld be visible on the inner

surface of the mod ule unless they can be remotely located. This stud y

assum es DC wiring to a large central inverter.

Ventilation:

Heat bu ildu p behind PV mod ules causes reduced PV efficiency, creates

therm al stress which may indu ce cracking, and increases heat gain into the

bu ilding. This pro blem particularly affects curtain wa ll span dr el areas,

which are normally covered by interior finishes. To minimize these prob-

lems, a curta in wall or atrium stru cture requires some degree of ventilation.

Ventilation can be accomplished simply by exposing the back face of the

panels, but this configur ation transm its heat into the interior. An exposed

system may be appropriate for spaces with little need for environmental

control, such as greenhou ses or solarium s.

Spaces which require a greater degree of thermal control may justify venti-

lation. Some examples follow:

Convective ventilation in the framing members is a method used in the

APS curtain wall system (fig. 5, 6). Standard curtain walls framing is

used to frame un insulated PV mod ules. Insulating panels at the back of

the fram ing form a therma lly-sealed air space within the fram e. This

space is then vented to the roof via slots in the mu llions.

Mechanical vent ilation is a more comp lex and expensive alternative. If

properly designed, excess heat gain may be recovered and used else-wh ere in the bu ilding via exhaust fans and du ctwork.

In man y cases, the extra expense of venting w ill not be justifiable, since the

loss in PV efficiency due to heat gain is often negligible and high-end con-

struction considered in this report usually includes sophisticated mechani-

cal systems. In this stud y, no cost allowance is mad e for ventilation.



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Mullion cap.Gasket.Thin-film superstrate module.

Electrical contact.

Spacer with wiring penetration.Air space.

Laminated glass @ interior.Wiring through mullion.Mullion.

Fig. 19. Cross-section detail of PV curtain wall at BWI Air

Terminal (Courtesy of Solar Design Associates).

Fig. 18. Detailed section of exterior PV insulating unit in typical framing.

Exterior

Interior

Exterior

Interior

Integral wireway instructural rafter.

PV module.


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Construction Codes and PVsIf a glass-to-glass PV module is considered the equal of other laminated

glass products by code officials, there is no structural problem in rep lacing

stand ard laminated g lass with PVs. Preliminary p rojects have been con-

structed u sing laminated PVs, but there are no specific code provisions in

US national or local codes governing th e use of BIPV materials. Likewise,

electrical interconnections between modules are typically governed by the

interpretation of local code officials of stand ard electrical codes. No specific

prov ision for BIPV prod ucts has yet been mad e.

The lack of clear code provisions and the rarity of built examples of BIPV

systems w ill cause some code officials to take a conservative atti tude

toward such p rojects.

As an active energy source, PV glazing may offset the energy code calcula-

tions required for a building s mechanical and electrical loads. For large-

scale installations, PVs may provide an opportunity for exemption from

energy code regu lations altogether.

The BIPV ind ustry is still in its infancy. Until there is a greater body of built

BIPV projects, each project will have to be evalu ated by local code officials

individually.



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V. LOCATION

The cost-effectiveness of BIPV systems is as dependent on the value of

avoided electricity as it is on insolation or climate. For this reason, locations

were evaluated whose average commercial electric rates ranked within the

top 15 of average US electricity prices.4 Average electric rates were then

multiplied by the PV output for a latitude-tilt, south-facing 10% efficient

cell. The prod uct was used to ran k these locations by total PV value as fol-

lows:

From this list, six top locations were evalua ted in detail. Two of the

Hawaiian locations were elimina ted to avoid repetition. Detailed rate struc-

tures were obtained from each utility and are d iscussed later in this chapter.

OrientationChart 3 show s the effects of array slope and azimuth orientation PV pow er

prod uction for a samp le city. South -facing arrays perform consistently bet-

ter wh en their slope app roaches local latitud e. For more vertical tilts (e.g.

curtain walls) a southw est (or southeast) orientation prod uces more pow er.

West-biased curtain walls often provide power with a higher energy value,

since man y utilities charge higher rates in the afternoon. In any case, opti-

mizing orientation for BIPV atria and curtain walls is dependent upon inso-

lation levels and utility rate structure and will need to be evaluated on a

case-by-case basis.


Rank by Total PV $/m2/yr

Location U ti li ty Total PV

kWh/m2/yr*Average/kWh*

Total PV$/m2/yr

Honolulu Hawaii Elec Light 202 16.99 $34.32

BIPV Applications

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