|Facade-integrated photovoltaics (PV) is of increasing interest, not only to generate clean renewable energy on-site, but also to send a clear and visible message of environmental responsibility. Several national manufacturers have developed integration packages for PV.
For facade-integrated PV, strings of solar cells are typically captured between two lites of glass using EVA interlayers, creating an unframed "laminate" or "module" that is handed much like conventional laminated glass. Of course, wires exit the laminate either at the back, or at the edges, to deliver DC current, which must be run through inverters to be put back into the utility grid, for net metering and utility "buy-back." PV already enjoys a significant market presence in off-grid applications; boats, road signs, cabins, and of course, satellites. Therefore, the curtainwall designer must deal with all the typical air, water, structural and thermal requirements, while also making sure that the frames are adequately grounded, safe conduits are provided for wiring runs, strain relief and bend radii comply with code, and connectors are accessible for subsequent trades. At Wausau, we have UL-tested our framing systems for PV laminate support on several demonstration projects, the first of which was almost ten years ago.
The most difficult facade-integrated PV design challenge is the "stringing and array" strategy - How many modules will be connected together in series to make a string, and how many strings will be connected in parallel to constitute the array. As module size changes, so can its electrical characteristics, adding another element to this design challenge. Wausau always recommends that a PV systems integrator, such as Solar Design Associates of Harvard, MA, be added to the design team early-on to facilitate an overall solution , and select hard-to-get architectural PV modules.
South-facing vertical PV facades will produce the most output, however, west-facing facades will reduce peak demand in the late afternoon. As a rule of thumb, PV arrays tilted from horizontal at an angle equal to the site's latitude will offer the best stationary output. Designers must be cognizant of shading from adjacent structures (present or future) as well as landscaping, in modeling PV array output. On a smaller scale, curtainwall mullions and building projections must not shade any part of the PV module - If one cell in a series-connected web is shaded, the entire module's output is affected.
Cost of facade-mounted PV modules can be partially offset by the cost of the glazing infill they replace. In many places, there are subsidies or tax credits to further help defray first cost. PV power generation can be significant. In one modeling scenario, where facade-integrated arrays were assumed to cover south-facing spandrel areas of a suburban office building, PV power output equal to 25% of the building's lighting needs was anticipated. Most demonstration projects are of a smaller scale. In the US, we enjoy relatively low electrical power rates, so justification of BIPV must be based on criteria other than payback time. As society starts to recognize the long-term costs of emissions from conventional fossil-fueled power sources, this equation will change.