A time for BIPV
BIPV can either be integrated onto existing structures (retro-fitting) or incorporated 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 specific European markets can make larger-scale solar PV development attractive to both building owners – who can offset electricity costs / generate money through feed-in-tariffs (FiTs) by investing in their roof space – as well as equity investors who see the opportunity to make money from large scale BIPV projects. Various current initiatives in Europe offer 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 solar PV, BIPV can help building owners save on their electricity costs. And BIPV seeks to 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 overcomes 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 solar 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 affordable.
The traditional means of capturing solar energy on buildings has been to place arrays of solar thermal (or solar PV panels) in fixed frames on roofs. This solution, while serving well as a first 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 mountings 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 20-60º.
However, on other projects with less than optimal positioning – for example premises that have flat roofs, industrial roofs, semi-flat roofs, or east/west facing roofs and façades (to name a few examples), thin-film technology could be an effective solution to maximise power output available while offsetting the capital investment of installation. Thin-film solutions also tend to be used on large roofs and industrial premises where space and area isn't a problem. As a general rule of thumb, thin-film 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 roofing materials to develop/create a new solar system – such as solar roofing systems that utilise roofing 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 shingles in particular. This approach is exemplified by the SolarSave roofing tile from the Open Energy Corporation. Claimed to be equally suitable for new construction and re-roofing applications, these solar PV / polycarbonate tiles are manufactured in black, red/brown and blue / grey colours.
Each tile weighs 12 lb, measures 17 in by 36 in by 1 in and provides up to 35 W at 48 VDC. Attributes cited by the company include robust, weather proof, fire-rated properties; easy installation, seamless blending with standard cement tiles, low voltage for safety, water-shedding edge profiles, 125 mph wind rating, and ‘short string’ inverters with 93% conversion ratio of DC to AC. Enclosing the active monocrystalline semiconductor 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 solar 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 material 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 flexible and readily bondable to the surfaces of conventional materials. Some are thin enough to be incorporated into glass while retaining transparency, effectively freeing solar PV from the confines of the roof and bringing it into facades.
Producing thin-film materials in continuous roll-to-roll processes – rather than the batch step-and-repeat processes associated with conventional crystalline silicon – offers the prospect of cost-efficient 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 flat panel display and architectural glass industries. Using amorphous silicon (a-Si or ASI) has the added advantage of sidestepping difficulties currently faced by manufacturers regarding the global shortage of crystalline silicon wafers.
Technology examples
Several companies have developed thin-film 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 efficient energy converter than crystalline silicon. Each cell of United Solar Ovonic's UniSolar BIPV material is composed of three stacked semiconductor junctions, each junction absorbing a different spectral band of light (see image on the right).
This results in superior light absorption, especially in low insolation levels, and diffuse 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-reflective coating, the overall result is a rugged, flexible material that is continuous until subsequently cut into lengths suitable for module production.
The company says its triple-junction a-Si product performs up to 40% better in low-light conditions (40–100 W/m2) than conventional crystalline 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 – something that can happen easily in buildings – the triple-junction modules lose only 5% power at 28ºC ambient, and 1100 W/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-certified material is described as strong and “walkable” when used in roofing. It can, says the company, be integrated with a range of metal and non-metallic roofing materials. Advantages include a high degree of off-site pre-fabrication, minimal additional weight and no extra wind loading, plus the ability to be installed using normal roofing procedures with clearly defined trade responsibilities on the roof. For example, it is clear that it is the roofer's job to ensure that the finished roof is watertight, a situation that may not pertain with first-generation roof array systems.
Partner companies who have created building elements by bonding UniSolar laminates to conventional roofing materials include ThyssenKrupp with its Solartec panels, Alwitra with a single-ply roofing membrane incorporated in its Evalon Solar product, Corus with its Kalzip roofing, and Solar Integrated Technologies' BIPV roofing membrane. American Energy Technologies Inc has installed over 5000 “peel and stick” Uni-Solar panels on a metal roof of a large warehouse to generate up to 700 kW. Sun Edison LLC is using them over 74,000ft2 of the metal roof of a large distribution centre in Connecticut to provide up to 433 kW of power, while 3rd 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 crystalline silicon solar material and BIPV modules.) A fine laser is used to structure the silicon film 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 products, 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 products in manufacturing its own thin-film solar material by depositing amorphous and microcrystalline silicon onto glass substrate. Using this process to make thin film modules almost 6m2 in size results in a highly cost-competitive product, says the company, which is targeting 6-9% solar conversion efficiency, and production cost of approximately US$1.20/W.
The advantages of thin film are not lost on producers who have established 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 925 mm. The new plant, which with an intended eventual capacity of 1 GW per year is likely to be the world's largest thin-film solar cell factory, is due to commence operation in March 2010. Sharp Solar says its material will lend itself to creative 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 flat concrete tiles. Module enhancement has stemmed from such innovations as advanced surface texturing to increase light absorption, aesthetically pleasing black-anodised frame finishes and trim, and triangular modules to increase roof design flexibility.
Sharp Solar is wise to back both crystalline and amorphous horses. Conventional silicon is a more efficient converter, with typical solar cell efficiencies typically in the 10%-20% range, comparing well with less than 10% for amorphous thin-film 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 film, with its greater flexibility, 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 efficiency as R&D efforts 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-film solar cell developer Innovalight Inc., of a multiple exciton generation (MEG) effect in silicon nanocrystals that is said to be able to enhance efficiency by several percent.
Thin compound
Other innovators, unhappy with cost and/or efficiency limitations of thin-film 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 fifth framework High Performance in Buildings (HIPERB) programme, for instance, specifically addressed the development of thin-film 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-efficiency 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 efficiencies 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 finish makes the product visually suitable for BIPV applications. And Avancis, a joint venture with Saint-Gobain Glass, 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-film CIGS semiconductor material. Its major innovation, the patented FASST process, which is said to be 10 times faster than thin-film 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, architectural glass and roofing materials to create power generating buildings.
Another PV tile product incorporating CIS thin-film is MegaSlate, developed in Switzerland. Suitable for roofs having an inclination of at least 20º, frameless MegaSlates are laid overlapping, like standard roof tiles. The material has a wood-like appearance and is marketed as solar wood by Luxembourg firm Solar Wood Technologies SA. The solar tiles are strong enough to be walked on, and have a biological growth-resistant finish. Wuerth Solar GmbH in Germany uses similar technology in the 70 W CIS modules it markets for BIPV use.
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/W compared with crystalline silicon cells at around US$2-$3/W.
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 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 encapsulation 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 efficiency of 6.5%. This level has since been raised to 10% in laboratories, and some researchers have claimed that efficiencies of up to 25% are theoretically possible. Given the affordability that thin-film organic PV (OPV) material also promises, the prospects for their use in BIPV are clearly interesting.
R&D efforts 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-film material commercially by 2015. Scientists at the Free University of Berlin believe that cost-effective thin-layer production techniques such as printing and efficiencies 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% efficient. 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 flexible 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 field 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 manufacturing, electro-mechanical integration (including hybrid thermal / PV solutions), applications, finance 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 TW 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.