CHAPTER IIC-2

Installation Guidelines:ConstructionBruce CrossPV Systems Ltd, Cardiff, UK

Contents

1. Roofs 8061.1 Roof Types 806

1.1.1 Integral 8061.1.2 Over Roof 8071.1.3 Tiles 807

1.2 Substructure 8081.3 New Build vs. Retrofit 8081.4 Mechanical Strength 8091.5 Loading 8101.6 Fixing Systems 8101.7 Weatherproofing 810

1.7.1 Sublayer Membranes 8101.8 Interfaces with Traditional Roof Types 8101.9 Standards 811

2. Facades 8112.1 Facade Types 811

2.1.1 Rain Screen 8112.1.2 Curtain Wall 812

2.2 Substructure 8132.3 New Build vs. Retrofit 8142.4 Mechanical Strength 8142.5 Weatherproofing 8142.6 Cooling 8142.7 Maintenance 8152.8 Site Testing 816

3. Ground-Mounted Systems 817References 817

805Practical Handbook of Photovoltaics. 2012 Elsevier Ltd. All rights reserved.

1. ROOFS

1.1 Roof Types1.1.1 IntegralAn integral roof has the potential advantage of saving of material of the

roof envelope that has been replaced. However, there are arduous require-

ments for water-tightness of roofs, which must be met. This can be

achieved with regard to traditional roofing practice (which is country spe-

cific) and realistic engineering design (see Figure 1). A general overview

of this field as well as examples of projects can be found in references

[15].The solutions that have been proved fall into the following categories:

Interlocking panel systems, which either use panels that mimic roofing

tiles with the PV element embedded in the surface or have a frame

bonded to the PV panel which provides the sealing interlock.

Adaptations of standard face-sealed sloping glazing systems, where the

PV may be built into a double-glazed sealed unit. These are particu-

larly suitable to buildings where the PV is visible from the inside, and

must also provide a degree of transparency for internal lighting needs.

Internally drained, secondarily sealed systems, where the high cost of a

face-sealed system cannot be justified.

Figure 1 Corncroft development, Nottingham, UK.

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1.1.2 Over RoofThe over-roof mounting of PV panels has been the normal practice in

early installations. It is simple in concept, and has been proven provided

that the attachment through the traditional roof is performed well.

A range of standard frame systems have been tried, and provided that

durable materials (hot dip galvanised steel, aluminium, or stainless steel)

are used, have been successful. The time taken to install these varies

greatly, and has the largest impact on the final cost of the mounting. The

mounting brackets are generally most successful when they are standard

roofing products, rather than special PV made items, and should be rigid

engineered mounts rather than the flexible strap type of fixing sometimes

used for solar thermal collector mountings (Figure 2).

1.1.3 TilesPV roof tiles have been manufactured in several countries. The advantage

of using a traditional roof product is that normal building trade practice

can be used, and there is little resistance to the concept from the naturally

conservative building trade. However, tiles are small components and a

large number are required for an installationthis implies are large num-

ber of interconnections, and also a mix of building trades is needed (e.g.,

electricians must work on a roof, or roofers must perform an electrical

function). The use of tiles or slates frequently requires special tiles for

edges, angled valleys, chimney joins, ridges, etc., and slates are generally

cut to shape on site, and nail holes made to suit. This implies that the PV

tile must link seamlessly into the standard roofing material (Figure 3).

Figure 2 Domestic PV systems for Mr. Treble. Farnborough, UK.

807Installation Guidelines: Construction

1.2 SubstructureMost types of roof have been used for a PV system at some time. The

overall construction must be capable of taking the additional load of the

PV (or indeed survive the additional uplift when the PV replaces a much

heavier roof surface such as concrete tiles). A more arduous requirement

may be the local loads of a frame mounted system that has relatively few

attachment points. The structure must also be able to accommodate any

fixed, or temporary, access structures for maintenance or repair. A trans-

parent or semi-transparent roof has a different set of requirements as it has

to provide all the elements of the roof within a single system. This will

also have to meet any aesthetic requirements from below as well as from

outside. Thus an integral wiring pathway within the mounting system is

an advantage (Figure 4).

1.3 New Build vs. RetrofitIn a niche market such as PV at present, there is a need to be able to fit

PV to both new and existing houses. In many countries the building

stock is old and not being replaced, so the mechanisms for retrofitting PV

are necessarily being developed. The costs of fitting PV to new buildings

is significantly lower than for a retrofit due to access and adaptation at

design stage being zero cost and the one-off costs being absorbed in the

larger building project costs (Figure 5).

Figure 3 Electraslate PV slate system.

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1.4 Mechanical StrengthThe PV elements of the roof have to fulfil the requirements of wind load-

ing, snow loading, fire resistance, and possible traffic for maintenance.

This means that a PV panel made for ground mounting may not always

be suitable for a BIPV application. The grab zone of a standard PV

Wiring Cover Strip

Setting Block

Cross Rail

Main RailFixing Clamp

Roof Baton

Cap Strip

EPDM Gasket

Cap Strip

RivetPV Laminate

Figure 4 RIS PV roof integration system.

Figure 5 PV retrofits for New Progress Housing, UK.

809Installation Guidelines: Construction

laminate is small, and the glass thickness may also be inadequate. A pur-

pose designed laminate or alternatively, a mounting system designed to

transform the standard laminate into a building component may often be

required to meet local codes.

1.5 LoadingMany standard PV laminates are fairly lightweight in roofing terms. The

panels themselves may only weigh 5 kg/m2, and, say, another 5 kg/m2 for an

aluminium mounting structure. However, a double-glazed panel with a dou-

ble glass front PV in a structural roofing system may add up to 40 kg/m2.

1.6 Fixing SystemsTraditional roofs are fitted together with nails and screws. High-technology

fixings are rare in roofing, and PV systems that require precision fitting

with specialist components will be expensive compared to those that are

adapted for the trades already in use on site.

1.7 WeatherproofingThe requirement for the roof is to resist the ingress of water, and also to

resist the loss of heat from the building, and to provide the degree of pro-

tection against fire for the type of building in which it is used. The exter-

nal surface will have to resist degradation from UV, wind, and rain for

3060 years. This can be achieved for roofs with traditional materials,but is hard to demonstrate for new materials. Hence, most PV on roofs

has a glass external surface.

1.7.1 Sublayer MembranesTraditional interleaving roof coverings, such as tiles, slates, etc., have a

need for a secondary membrane to stop water penetration in times of

extreme weather. This requirement remains where PV equivalents are

used. The high value of such roofs makes it a good investment to use

high-specification membranes for additional security.

1.8 Interfaces with Traditional Roof TypesThe join between the specialist PV part of a roof, and the traditional area

of a roof, is an area for greatest care. Here a complete engineered system

meets with a system developed over centuries and implemented by a

craftsperson. If the understanding between these two is not clear, there is

potential for the project to fail (Figure 6).

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1.9 StandardsThere are requirements for all building components to meet certain stan-

dards. For new products these take time to develop, and these are not yet

in place for PV products. There are IEC standards that cover the opera-

tion of a PV product in itself, but not yet one that covers its operation in

a building. Early moves have been made to establish a European test stan-

dard against which products can be tested in order to receive a CE mark,

but this goal is still several years away [6]. The USA has several UL tests

against which products can be tested (Figure 7).

2. FACADES

2.1 Facade Types2.1.1 Rain ScreenThe simplest type of facade integration is the rain screen. Here the object

is to keep the direct impact of the rain from the waterproofing layer,

which is some distance behind. The intermediate space is ventilated with

the ambient air, and an allowance is made for moisture to drain out from

the space. Rain screen panels are generally a simple sheet product within

a frame that hooks onto the lattice substructure. PV has been successfully

embedded into such frames using the PV laminate to replace the sheet

material (Figure 8).

Figure 6 Flashing detail.

811Installation Guidelines: Construction

2.1.2 Curtain WallIn a curtain wall the external surface is the waterproof layer, and hence,

all parts of the structure behind are considered dry. This is not to say that

there is no chance of moisture, as condensation must still be considered.

Normal practice would be to allow a small amount of air movement

behind the outer skin, but in the case of PV it should be increased, if pos-

sible, to provide some cooling from the rear surface of the PV. The

Figure 7 Shell PV roof undergoing prescript test in Solar Simulator at Cardiff, UK.

Figure 8 Bowater House PV facade, Birmingham, UK.

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advantage of the curtain wall is that it allows a continuous skin incorpo-

rating all the facade elementswindows, PV, and blank panels within a

proven design. These systems are complex and expensive without the PV

and so the additional cost may be more readily absorbed into such a

facade (Figure 9). It should be noted that the use of terms rain screen

and curtain wall varies internationally.

2.2 SubstructureThe substructure for a facade may use any type of normal construction

material. The extra requirements caused by having PV as the external skin

are that temperatures may be higher (larger expansion may occur) and

that a path must be found for the cabling and access provided to any con-

nections or marshalling boxes within the building envelope. These cable

Figure 9 William Jefferson Clinton Peace Centre PV facade, Enniskillen, NorthernIreland, UK.

813Installation Guidelines: Construction

paths must be accessible for maintenance and must not provide a path for

moisture ingress into the building fabric.

2.3 New Build vs. RetrofitA retrofit facade has more potential areas of risk than a new-build. The

uncertainties of dimensions may require significant site work, and the

locations for cabling and electrical plant may have to be defined late in

the project, when the old materials are removed. In a new-build facade

there is likely to be a long time delay between fitting the PV elements as

part of the external envelope, and installation and wiring of the electrical

plant. The design of the system must ensure that these operations can be

separated (e.g., external stringing may not be accessible for checking at

the time the system is finally commissioned).

2.4 Mechanical StrengthBuilding designs often require the use of large glazing elements. This has

the effect of increasing the glass thickness, which, in a PV panel, reduces

output. In many areas the facade must withstand impact from foot traffic,

and may have to provide a degree of security also. Standard laminates

rarely have sufficient thickness, so more expensive custom made panels

are needed. In general BIPV facade panels are 5080% more expensivethan standard modules (Figure 10).

2.5 WeatherproofingThe PV is just one element of a facade system and this total system must

provide the weatherproofing. Care is required that the cabling and the

increased expansion allowance do not compromise the performance of

the mounting system.

2.6 CoolingThe output of a PV panel decreases with rising temperature. It is there-

fore advantageous to provide for the flow of air over the rear surface of

the PV if at all possible. For rain screen systems there are usually ventila-

tion slots around the periphery of each panel, so these need only to be

checked for adequate passage of air. However, for curtain walls, it will be

necessary to allow an entry for air at the lowest part of the facade and to

allow the exit of air from the top, if the maximum output is to be

achieved. This design requires care if the waterproofing of the system is

814 Bruce Cross

not to be compromised. An EU project to assist this design process is

underway (pvcool-build.com). When a fully face-sealed double-glazed

transparent facade utilises PV, there is frequently no opportunity to venti-

late the rear, and so the loss of performance (510%) must be accepted.

2.7 MaintenanceAn important part of the design of the building is its future maintainabil-

ity. Physical cleaning is not any more arduous for PV than for glass panels

in general. Access should be provided for inspection and testing to any

cable marshalling box, and a system should be in place to allow the test-

ing, and possible replacement of any PV module in the system. In a large

facade this may require simultaneous access externally, and internally, to

test and to identify a fault.

Figure 10 Office of the Future PV facade, Building Research Establishment. UK.

815Installation Guidelines: Construction

2.8 Site TestingThere are standards for site testing the water tightness of facade systems.

These have been developed by the centre for window and cladding tech-

nology [7]. A high-pressure water jet is used at susceptible joints and the

system is inspected for water penetration. An addition to this test is to

perform a high-voltage isolation test immediately after, in order to evalu-

ate whether any water has compromised the DC cabling or the PV mod-

ules (Figure 11).

Figure 11 Pluswall PV facade, UK.

816 Bruce Cross

3. GROUND-MOUNTED SYSTEMS

When PV is mounted directly on a frame, pole, or concrete block,

the requirements are to provide a rigid attachment that will resist gravita-

tional, wind or impact forces, while protecting the PV from undue twist-

ing or deflection. A typical frame uses standard sections which are

assembled on-site.

Since such systems are not usually part of a building and perform no

weather protection function, there are few constraints on their design.

However, car parking shelters, and entrance canopies are favoured loca-

tions for PV and often fall into the scope of building codes by association.

REFERENCES[1] S. Roaf, EcohouseA Design Guide, Architectural Press, Elsevier Science, Oxford,

2001.[2] F. Sick, T. Erge, PVs in BuildingsA Design Handbook for Architects and

Engineers, International Energy Agency, Solar Heating and Cooling Programme, Task16, 1996.

[3] M.S. Imamura, P. Helm, W. Palz (Eds.), Photovoltaic System Technology:A European Handbook, U.S. Stephens & Associates, Bedford, 1992.

[4] Max Fordham & Partners in association with Feilden Clegg Architects, Photovoltaicsin Builidings. A Design Guide, Department of Trade and Industry, UK, 1999, ReportETSU S/P2/00282/REP.

[5] Studio E Architects, Photovoltaics in Buildings. BIPV Projects, Department of Tradeand Industry, UK, 2000, Report ETSU S/P2/00328/REP.

[6] Prescript, Final Report on EU Project, European Commission, Brussels, 2000,JOR3-CT970132.

[7] Test Methods for Curtain Walling, second ed., Centre for Window and CladdingTechnology, Bath University, 1996.

817Installation Guidelines: Construction

IIc-2 Installation Guidelines: Construction1. Roofs1.1 Roof Types1.1.1 Integral1.1.2 Over Roof1.1.3 Tiles

1.2 Substructure1.3 New Build vs. Retrofit1.4 Mechanical Strength1.5 Loading1.6 Fixing Systems1.7 Weatherproofing1.7.1 Sublayer Membranes

1.8 Interfaces with Traditional Roof Types1.9 Standards

2. Facades2.1 Facade Types2.1.1 Rain Screen2.1.2 Curtain Wall

2.2 Substructure2.3 New Build vs. Retrofit2.4 Mechanical Strength2.5 Weatherproofing2.6 Cooling2.7 Maintenance2.8 Site Testing

3. Ground-Mounted SystemsReferences