Picture if you will, an antipodean solar engineer’s dream world. Every roof faces north with a pitch roughly equal to the local latitude angle; building-block homes all in a row. Unfortunately, centuries of homebuilders and decades of town planners did not consult a solar engineer before scattering homes without regard to optimal solar access. The resulting urban environment is more aesthetically pleasing and arguably more liveable, but constantly faces solar designers with suboptimal outcomes.
Fortunately, while many retired engineers insist on their own solar power system being optimally oriented, most roof-owners are prepared to accept that the cost of optimal azimuth rarely justifies the gains in solar yield. In most Australian locations, losses are kept to 10-15% for roofs oriented 90° either side of north, and the cost of side-pitch mounting typically adds more than 10% to the project cost. Panels mounted parallel with the roof pitch have the key benefit of achieving maximum power density, with what is lost in sub-optimal orientation is typically far exceeded by gains in total energy yield. Regardless, sensible solar array placement can be quite a sophisticated artform, and Australia has many highly capable designers to choose from.
By way of comparison, integrating solar power into the building fabric adds volumes of complexities. The solar designer must integrate with the design and construction team to be able to successfully integrate solar panels into a buildings walls or roof. Installations are invariably sub-optimal, and shading is often unavoidable. This makes Building Integrated PV (BIPV) a specialist design area with highly stimulating, innovative projects, unique challenges, and thoroughly satisfying outcomes.
Application and Product Selection
BIPV is not a typical retrofit solution, and consequently BIPV projects are invariably new buildings. The detailed façade engineering that is required typically means that the project must be a minimum 10kW to be practically viable, which requires vast areas of glazing that instantly excludes most of the residential market. Commercial projects aiming for Green Star accreditation can benefit from the demand reduction and emissions-reduction benefits of solar power, with façade integration necessary once the rooftop is filled with a standard solar array.
The BIPV technology choice depends upon the application. When visibility isn’t critical or glazing is distant from bystanders, as occurs in the Metricon Stadium and Varsity Lakes train station , it is acceptable to use crystalline silicon cells sandwiched between glass panes. Such product is available from a growing number of manufacturers, though the invariable need for high-strength glass in custom sizes typically dictates sourcing from BIPV specialists. Some degree of transparency is key for functional windows, with Schott Solar’s ASI-Thru providing 10-20% transparency in single or double glazing and taking on the appearance of a fly-wire screen (see picture). Pythagoras Solar also recently launched its high-transparency, high-efficiency PV window . Both products can significantly reduce glare and thermal gain, thereby reducing air conditioner size and running costs.
In some applications, opaque appearance is preferred – such as on the Tullamarine-Calder Interchage Solar Noise Wall – while innovative Building Integrated Solar Thermal and Building Integrated Hybrid PV-Thermal applications are also possible with Heliopan . An alternative to façade integration is roof integration, in which the solar panels form part of the roof membrane. Australian designed PV Solar Tiles are one noteworthy product, and Solon has recently introduced a roof-integrated module into Australia .
Azimuth
Involve an architect in a building design and expect a fantastic appearance, to come at the expense of ideal solar orientation. Windows are invariably vertical for good reason, and a 30° sloped façade can add extraordinary amounts to standard building costs. For example while a vertical façade may suffer 40% performance loss, its financial outcome may be superior to that of a 30°-sloped array that costs at least twice as much for the same architectural function – this was part of the reason for a vertical solar noise wall in Victoria. The Metricon Stadium is visually magnificent, though its 270 different azimuths presented significant challenges, especially as solar performance in every moment is limited by the least illuminated panel. The BIPV engineer has to be able to gently influence an architect towards a practically achievable outcome, but ultimately be able to work with the situation that is architecturally-driven.
Shading
Shade is the nemesis of solar performance. Whereas one usually has the luxury of placing retrofit roof-mount solar arrays in the least shaded location, BIPV invariably encounters unavoidable shade. BIPV facades are more affected by nearby shading obstacles from the built and natural environment. Although a lot of energy can be spent articulating the need to avoid shade, for functional reasons projects will encounter shade from trees, light poles, entrance canopies, and flashing (waterproofing) – all of which were overcome at Ballarat University . Inevitably a BIPV design will take account of known shading obstacles, but must be robust enough to handle surprise shade. In an ironic example, the Solar Noise Wall design specifically ensured that the shading effects from overhead wayfinding signs were contained to a small section of the array, but fortunately shade-tolerant amorphous silicon panels coped well with a last minute unavoidable surprise placement of an emergency phone (complete with its own solar panel) in front of the array.
Technical Issues
To achieve a sophisticated BIPV design, one of two approaches can be taken. The easiest, optimal-performing solution is to use a micro-inverter or power optimiser. However, power optimisers’ reliability is not yet fully proven, and the customised BIPV panels often have electrical characteristics that preclude the use of off-the-shelf devices. The second approach is to group onto the same string panels with similar performance – clustering by same orientation and by similar proximity to shade. By similarly grouping together poorer-performing parts of the solar array, the system yield is less compromised. The use of multiple smaller inverters, multiple maximum power-point trackers, and multiple strings of fewer panels can also produce a robust design. Even considering the internal wiring configuration of the panels can improve yield – the characteristic of the shading pattern can determine whether one cell is curtailed or whether the entire panel is bypassed.
The system design is only one part of the overall project, and much more could be said about the construction phase. A successfully implemented project requires that attention be given to facilitating ease of connections and integrating wiring runs into the building structure, and that the system can be easily erected and maintained, particularly as working at heights on a platform is often involved. Metricon stadium was able to be quickly erected by performing panel inter-wiring on the ground, and lifting full bays of 14-18 panels into place by crane.
Each BIPV project has its unique challenges, and the design typically involves considering the relationship of each individual panel to its surroundings, in the context of the string, input, and inverter to which it is connected. Design becomes part science and part art form, though project success requires excellent teamwork between the various disciplines involved. However, through each exciting, innovative project the vision of covering every surface with a solar panel grows one step closer. Then we’d truly have a solar engineer’s dream world.
This article first appeared at Solar Progress, the official journal of the Australian Solar Energy Society.