BIPV: innovation puts spotlight on solar

Cover story

62 renewable energy focus May/June 200862 renewable energy focus May/June 2008

BIPV: innovation puts spotlight on solarIN A TIME WHEN SOLAR PV IS CHARACTERISED AS BEING A PROHIBITIVELY EXPENSIVE

ALTERNATIVE FORM OF ENERGY SUBSIDIES NOTWITHSTANDING, ONE APPLICATION

AREA THAT COULD MAKE A REAL DIFFERENCE TO PERCEPTIONS IS BIPV BUILDING

INTEGRATED PV. THIS APPLICATION CAN EFFECTIVELY ENABLE BUILDING OWNERS TO

SAVE ON THE CONSTRUCTION COSTS OF NEW BUILDINGS, WHILE AT THE SAME TIME

GENERATE A PORTION OF THEIR OWN ELECTRICITY. FURTHER PROGRESS REQUIRES

A HIGH LEVEL OF INNOVATION TO TRULY BRING SOLAR PV INTO BUILDINGS, WHILE

MAKING THE TECHNOLOGY EVEN MORE AFFORDABLE. FORTUNATELY, THERE ARE

SIGNS THAT SUCH INVENTIVENESS IS NOW MAKING A DIFFERENCE. George Marsh

A time for BIPV

BIPV can either be integrated onto existing structures (retro-fi tting) or incor-

porated into the new build marketplace. For new build, it can make sense to

integrate solar elements into roof spaces, for example, in order to save money

on standard materials that would otherwise have been used (metal sheeting

or other components for example).

BIPV is certainly growing in popularity as more and more architects and

constructors begin to understand the possibilities available to their clients. In

addition, the incentive structures in specifi c European markets can make

larger-scale PV development attractive to both building owners – who can

off set electricity costs/generate money through feed-in-tariff s (FiTs) by

investing in their roof space – as well as equity investors who see the oppor-

tunity to make money from large scale BIPV projects. Various current initia-

tives in Europe off er high levels of subsidies for BIPV, or seek to mandate the

construction industry to integrate more renewables in buildings.

Even in markets where incentive schemes don’t tend to favour PV, BIPV

can help building owners save on their electricity costs. And BIPV seeks to

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renewable energy focus May/June 2008 63

Building/BIPV


create as much function as possible from the building space. One example

is the PV solar facade; these can in many cases be cheaper to construct

than normal building facades (not to mention able to generate electricity),

and the appearance can be attractive and modern, something that over-

comes a key barrier to PV takeup in the eyes of some potential

customers.

BIPV and grid parity

Dr Douglas Dudis, a researcher with the US Air Force Research Laboratory,

Materials and Manufacturing Directorate, told delegates at the 2007 Solar

Conference that lack of building integration to date was one of three

main factors contributing to the present high cost of distributed solar PV

technology. He cited as the other two factors: material availability issues,

in particular the shortage of semiconductor grade crystalline silicon; and

labour-intensive manufacture of wafers, cells, modules and arrays.

BIPV could be a transformational technology, slashing the high proportion of

conventional energy consumption accounted for by buildings, cutting CO2

emissions and easing pressure on fuel reserves. But further progress requires

a high level of innovation to truly bring solar PV into buildings, and make the

technology aff ordable.

The traditional means of capturing solar energy on buildings has been

to place arrays of solar thermal (or solar PV panels) in fi xed frames on

roofs. This solution, while serving well as a fi rst generation, has resulted

in installations that are clearly ‘add-ons’ and out of harmony with the

buildings they sit on. Arrays are also more costly than solar PV needs to

be.

Integrating solar modules into the building envelope saves money

because the modules serve also as structural elements, thereby reducing

building costs. This multifunctional solution also reduces concentrations

of added weight on roofs, avoids roof penetrations required for mount-

ings and wiring, and reduces vulnerability to high winds. And of course

more solar area becomes available if better use is made of building

surfaces.

There are other advantages too. Because BIPV installations are contained

within the building envelope, there are no requirements for extra space or

additional civil engineering. Accordingly, there need be no restrictions in

populous urban areas. Buildings use the electricity they generate on the spot,

minimising distribution needs. All these advantages together should, costs

permitting, make BIPV a crucial component of sustainable architecture.

Technologies for BIPV

The technology a building owner would need to select for BIPV depends

on factors related to the roof’s location. For example, crystalline modules

would be recommended for scenarios where the building in question

has a southern orientation (plus or minus 45%), with an inclination of

between 20 and 60 degrees.

However, on other projects with less than optimal positioning – for example

premises that have fl at roofs, industrial roofs, semi-fl at roofs, or east/west

facing roofs and façades (to name a few examples), thin fi lm technology

could be an eff ective solution to maximise power output available while

off setting the capital investment of installation. Thin fi lm solutions also tend

to be used on large roofs and industrial premises where space and area isn’t


Building/BIPV

64 renewable energy focus May/June 2008

a problem. As a general rule of thumb, thin fi lm technologies need roughly

double the amount of area of modules for the same kW output.

Another current challenge for the BIPV industry is to combine the latest

module technologies with the best roofi ng materials to develop/create a

new solar system – such as solar roofi ng systems that utilise roofi ng

membranes with cables on the underside, for example.

Crystalline mainstay

Today, mono and polycrystalline forms of silicon are the mainstay of the

solar PV array industry. One strand of innovation is to incorporate these

materials into modules that double as building elements, tiles and shin-

gles in particular. This approach is exemplifi ed by the SolarSave roofi ng

tile from the Open Energy Corporation. Claimed to be equally suitable for

new construction and re-roofi ng applications, these solar PV/polycar-

bonate tiles are manufactured in black, red/brown and blue/grey

colours.

Each tile weighs 12lb, measures 17in by 36in by 1in and provides up to

35 watts at 48 VDC. Attributes cited by the company include robust,

weather proof, fi re-rated properties; easy installation, seamless blending

with standard cement tiles, low voltage for safety, water-shedding edge

profi les, 125mph wind rating, and ‘short string’ inverters with 93% conver-

sion ratio of DC to AC. Enclosing the active monocrystalline semicon-

ductor material within a protective composite laminate avoids the need

for external framing, which can raise issues of discontinuity with the

building envelope, maintenance and cost.

Thin silicon

While crystalline silicon remains the dominant building PV technology,

its position is being challenged by thin-film alternatives. Thin-film solar

materials that can conform to the building envelope can potentially

supplant the rigid ‘add-on’ arrays that adorn buildings today. Initially

this trend is based on the exploitation of amorphous (non-crystalline)

and micromorphous forms of silicon. The ability to deposit such mate-

rial extremely thinly onto suitable substrate materials can yield solar

cell wafers many times thinner than those produced from conventional

crystalline silicon, which cannot be sliced from ingots to anything like

the same degree of fineness.

Thin solar materials not only maximise the amount of active surface area

exposed to solar radiation for a given volume of silicon, they also lend

themselves to integration with buildings because they can be made fl ex-

ible and readily bondable to the surfaces of conventional materials. Some

are thin enough to be incorporated into glass while retaining transpar-

ency, eff ectively freeing solar PV from the confi nes of the roof and

bringing it into facades.

Producing thin-fi lm materials in continuous roll-to-roll processes – rather

than the batch step-and-repeat processes associated with conventional

crystalline silicon – off ers the prospect of cost-effi cient production and

reduced system cost per installed power capacity. Producers can leverage

innovations in large-area deposition, roll coating and other processes

used in the fl at panel display and architectural glass industries. Using

amorphous silicon (a-Si or ASI) has the added advantage of sidestepping

diffi culties currently faced by manufacturers regarding the global shortage

of crystalline silicon wafers.

Technology examples

Several companies have developed thin-fi lm ASI-based solar material.

One of these is United Solar Ovonic LLC, which has compounded

its achievement by incorporating triple semiconductor junction

technology into its product, and thereby partly overcoming a downside

to amorphous silicon – that it is a generally a less effi cient energy

converter than crystalline silicon. Each cell of United Solar Ovonic’s

UniSolar BIPV material is composed of three stacked semiconductor junc-

tions, each junction absorbing a diff erent spectral band of light (see image

above).

This results in superior light absorption, especially in low insolation levels,

and diff use light conditions. The material is produced in a roll to roll

process, in which semiconductor material is deposited as a vapour onto

continuous rolls of thin stainless steel substrate. With added anti-refl ective

coating, the overall result is a rugged, fl exible material that is continuous

until subsequently cut into lengths suitable for module production.

Building integrated Photovoltaic (BIPV) – combining function with design using fl exible UNI-SOLAR PV laminates (Image courtesy of UNITED SOLAR OVONIC, a Michigan, US-based company, which manufactures thin fi lm amorphous PV technology). Its proprietary technology has been developed to deposit solar cells simultaneously on to 6 rolls of stainless steel, each 1.5 mile long, using an automated line. According to the company, the fl exible modules off er “nearly complete freedom of design to architects as they can also conform to curved surfaces and hence are meeting the increasing demand for Building Integrated PV (BIPV). The modules also off er advantages in low and diff use light conditions, due to higher absorption of light in the blue wavelength range.”


Building/BIPV


The company says its triple-junction a-Si product performs up to 40%

better in low-light conditions (40-100W/m2) than conventional crystal-

line technology, making it suitable for climatic conditions in much of

Europe and north America. Moreover, whereas crystalline modules can

lose 20%- 30% of their power as their surface temperature rises – some-

thing that can happen easily in buildings – the triple-junction modules

lose only 5% power at 28 deg. c ambient, and 1100W/m2 irradiation

level. Testing has shown the product to be stable over time, while the

energy yield overall is said to be competitive with that of conventional

crystalline modules.

From the building integration point of view, the IEC 61646-certifi ed mate-

rial is described as strong and “walkable” when used in roofi ng. It can,

says the company, be integrated with a range of metal and non-metallic

roofi ng materials. Advantages include a high degree of off -site pre-fabri-

cation, minimal additional weight and no extra wind loading, plus the

ability to be installed using normal roofi ng procedures with clearly defi ned

trade responsibilities on the roof. For example, it is clear that it is the roof-

er’s job to ensure that the fi nished roof is watertight, a situation that may

not pertain with fi rst-generation roof array systems.

Partner companies who have created building elements by bonding UniSolar

laminates to conventional roofi ng materials include ThyssenKrupp with its

Solartec panels, Alwitra with a single-ply roofi ng membrane incorporated in

its Evalon Solar product, Corus with its Kalzip roofi ng, and Solar Integrated

Technologies’ BIPV roofi ng membrane. American Energy Technologies Inc

has installed over 5,000 “peel and stick” Uni-Solar panels on a metal roof of a

large warehouse to generate up to 700kW. Sun Edison LLC is using them

over 74,000ft2 of the metal roof of a large distribution centre in Connecticut

to provide up to 433kW of power, while Rock Systems and Technologies

has utilised the panels on solar school projects in California.

Amorphous silicon can also be deposited onto glass to form a more-or-

less transparent solar surface, which can be integrated with or substituted

for glazing. German company Schott Solar GmbH, for example, points

out that it can electrically deposit amorphous silicon semiconductor

material onto glass in a layer less than a micron thick, whereas wafers of

crystalline silicon are at least 180 microns thick (Schott also produces crys-

talline silicon solar material and BIPV modules.) A fi ne laser is used to

structure the silicon fi lm on the glass substrate into many tiny solar cells.

Transparent conductor pathways conduct electrons from the cells to the

module’s cables. Schott says that its ASI solar panels can be integrated

into a wide range of glazing applications, stimulating new architectural

approaches. Panels can, it claims, be installed just like normal windows.

PV windows appear shaded and admit less light than clear glass, but this

can add to visual interest. Solar Solutions LLC says that one of its prod-

ucts, in which PV material is embedded in glass, allows 10% of natural

light through when generating full power. It adds that the glass panels,

described as attractive, form a good surface for the projection of images

and presentations from projector units.

Suntech Power Holdings says that it uses less than two percent of the

silicon required to manufacture equivalent crystalline silicon PV prod-

ucts in manufacturing its own thin-fi lm solar material by depositing

amorphous and microcrystalline silicon onto glass substrate. Using this

process to make thin fi lm modules almost 6m2 in size results in a highly

cost-competitive product, says the company, which is targeting 6%-9%


Building/BIPV

66 renewable energy focus May/June 2008

solar conversion effi ciency, and production cost of approximately

US$1.20 per Watt.

The advantages of thin film are not lost on producers who have estab-

lished their solar credentials with conventional crystalline silicon.

Sharp Solar Energy Solutions, for one, has developed a solution that

combines two layers of amorphous silicon and one of microcrystalline

silicon, for a module efficiency of some 10%. The company recently

revealed that it is investing in a large thin-film solar cell plant in Osaka,

Japan. In an innovative production move designed to limit costs, it is

co-locating the new plant with a thin-film LCD display facility, so that

infrastructure and technical resources can be shared. It is further

improving efficiency by enlarging its glass substrate size by 2.7 times

(from the original 560) by 925mm. The new plant, which with an

intended eventual capacity of 1,000MW per year is likely to be the

world’s largest thin film solar cell factory, is due to commence opera-

tion in March 2010. Sharp Solar says its material will lend itself to crea-

tive transparency solutions.

Meanwhile, Sharp has also worked hard to provide better integration of

conventional technology, notably with crystalline Si-based modules that

can be secured to roof battens and deck in the same manner as fl at

concrete tiles. Module enhancement has stemmed from such innovations

as advanced surface texturing to increase light absorption, aesthetically

pleasing black-anodised frame fi nishes and trim, and triangular modules

to increase roof design fl exibility.

Sharp Solar is wise to back both crystalline and amorphous horses. Conven-

tional silicon is a more effi cient converter, with typical solar cell effi ciencies

typically in the 10%-20% range, comparing well with less than 10% for amor-

phous thin-fi lm devices. Appropriately engineered with common grounding

and electrical connection arrangements so that multiple roof penetrations

are avoided, modules can double as tiles and other building entities that are

not required to be slim. Nevertheless thin fi lm, with its greater fl exibility, ease

of building integration and ability to form large solar surfaces looks to be a

wave of the future and could pick up in terms of effi ciency as R&D eff orts

around the world bear fruit. A recent pointer to this was the discovery by

researchers at the US National Renewable Energy Laboratory (NREL), in

collaboration with thin-fi lm solar cell developer Innovalight Inc., of a multiple

exciton generation (MEG) eff ect in silicon nanocrystals that is said to be able

to enhance effi ciency by several percent.

Thin compound

Other innovators, unhappy with cost and/or effi ciency limitations of thin-

fi lm silicon, have focused on alternatives to silicon, notably compound

semiconductors such as copper indium di-selenide (CIS), copper indium

gallium di-selenide (CIGS) and cadmium telluride (CdTe). Researchers have

laid the groundwork. The European Union’s fi fth framework High Perform-

ance in Buildings (HIPERB) programme, for instance, specifi cally addressed

the development of thin-fi lm solar CIS modules optimised for stable long-

term performance in BIPV applications. Researchers at the USA’s NREL

have sought reproducible processes for making high-effi ciency CdTe

devices, where an ultra thin semiconductor layer is possible.

Shell Solar made waves a couple of years ago by selling its well established

crystalline silicon PV interests (to SolarWorld AG) in order to concentrate on

CIS, which it says is likely to become cost-competitive with retail energy

before silicon. It claims that CIS is substantially cheaper to produce than

silicon, with a fraction of the material input, and has achieved effi ciencies of

greater than 13.5%. Solutions of CIS materials are sprayed onto glass sheet in

layers to form large solar surfaces, avoiding the need for complex wiring and

assembly. A smooth black fi nish makes the product visually suitable for BIPV

applications. And Avancis, a joint venture with Saint-Gobain Glass Deutsch-

land, is due to begin manufacturing CIS solar panels this year.

Texas-based HelioVolt Corp. hopes to substantially reduce the cost of

BIPV with an ultra-rapid method of producing thin fi lm CIGS semicon-

ductor material. Its major innovation, the patented FASST process, which

is said to be 10 times faster than thin fi lm competitors and has earned the

company several awards, relies on printing the semiconductor material.

Much of the innovative drive is owed to Dr. BJ Stanberry, a leader in

pioneering the process. HelioVolt states that its product can be applied

directly onto conventional construction materials including steel, architec-

tural glass and roofi ng materials to create power generating buildings.

Another PV tile product incorporating CIS thin fi lm is MegaSlate, devel-

oped in Switzerland. Suitable for roofs having an inclination of at least 20

degrees, frameless MegaSlates are laid overlapping, like standard roof

tiles. The material has a wood-like appearance and is marketed as solar

wood by Luxembourg fi rm Solar Wood Technologies SA. The solar tiles

are strong enough to be walked on, and have a biological growth-resistant

fi nish. Wuerth Solar GmbH in Germany uses similar technology in the

70W CIS modules it markets for BIPV use (see main image and front

cover).

First Solar Inc has developed a high-rate vapour transport deposition

process for depositing cadmium telluride-based semiconductor onto

glass substrate pane by pane, and quotes a price of US$1.87/Watt

compared with crystalline silicon cells at around US$2/Watt to US$3/Watt.

There are, however, environmental, health and safety concerns over the

use of heavy metals in commercial devices. Dr Douglas Dudis (quoted at

the beginning of this article) for one would prefer to avoid the possibility

Design Concept for BIPV at Solar Fabrik’s headquarters in Freiburg, Germany: The headquarters houses a 1500 m2 production facility, and 1000m2 of offi ces. The outside of the building is dominated by a glazed, south-facing façade, which lets sun into the building and reduces the need for electricity for lighting and energy for heating. A total of 275m2 of PV modules are integrated into the façade, the insulating-glass windows and the cold façade. A further 300m2 have been mounted on the roof of the building. Together with a CHP unit that runs off plant oil, these components fully cover the energy requirements of the building.


Building/BIPV


that toxic cadmium, tellurium, gallium etc. could leach into the world’s

water courses, and therefore favours silicon-based solutions. Holders of

the opposite view, however, counter that innovations in laminate encap-

sulation can overcome this objection, enabling the potential of these

highly promising solar materials to be realised. The debate continues

even as commercialisation proceeds.

Organic movement

One way round this issue is to go organic, a possibility raised by the discovery

of polymer conducting materials. In 2000 Alan Heeger, Professor of Physics at

the University of Santa Barbara, California, was awarded a Nobel Prize for his

pioneering work in this area, along with Alan MacDiarmid and Hideki

Shirakawa. In 2007 the Nobel Prize winners, together with Korean Kwanghee

Lee presented an organic solar cell which, by virtue of a double layer that

absorbed a broader spectrum of solar radiation than single-layer cells,

achieved an unprecedented (for organic) conversion effi ciency of 6.5%. This

level has since been raised to 10% in laboratories, and some researchers have

claimed that effi ciencies of up to 25% are theoretically possible. Given the

aff ordability that thin-fi lm organic PV (OPV) material also promises, the pros-

pects for their use in BIPV are clearly interesting.

R&D eff orts around the world are now focused on developing OPV. A

particular hot spot is Germany where the Federal Government along with

companies like BASF, Bosch, Merck and Schott expect to invest some

€360m in further development, with the aim of producing thin fi lm mate-

rial commercially by 2015. Scientists at the Free University of Berlin believe

that cost-eff ective thin-layer production techniques such as printing and

effi ciencies in the 5%-10% bracket will make OPV a viable competitor to

established PV technologies. Further progress is needed, however, in

terms of stability, life span and encapsulation of the active material.

Another prospect arising from material innovation is the dye-sensitised

solar cell. Dye cells mimic nature with a photosynthesis-like process that

converts light into electricity. Australian-based company Dyesol Ltd, with

assistance from Australia’s Defence, Science and Technology Organisation

– and universities – is working to commercialise this technology and, using

nanotechnology, has produced cells that are about 8% effi cient. Moreover,

it believes that 12% is achievable with new material combinations. The

company is collaborating with steel maker Corus to develop a steel BIPV

product based on its fl exible dye cell technology. This project involves

detailed materials engineering and process validation at Dyesol, while

Corus undertakes chemical engineering studies aimed at rapid cell assembly

and optimisation of the structure of the metal substrate. Other companies,

including G24 innovations in the UK, are working on similar technology.

As the necessarily limited selection of examples in this article indicate, much

of the innovative thrust in this fi eld is maintained by specialists in materials

and processing science. Nevertheless, if BIPV is to achieve its potential, such

innovation has to be matched by parallel inventiveness in terms of manufac-

turing, electro-mechanical integration (including hybrid thermal/PV solu-

tions), applications, fi nance and market mechanisms. The potential prize is

huge. Buildings, at least in industrialised countries, account for an estimated

20%-30% of total consumption of conventional (non-renewable) energy.

Utilising just a fraction of the 110 terawatts of solar energy received at the

earth’s surface continuously could help transform energy economics and

environmental stewardship. BIPV can achieve this given a continuation,

indeed acceleration, in present innovation trends.


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BIPV: innovation puts spotlight on solar