TO BIPV OR NOT TO BIPV

One approach with a degree of common-sense appeal is simply to replicate on a building's vertical surfaces the technology that is already in wide use on rooftops. With the population – an estimated 68 percent globally and 89 percent in North America by 2050 (UN Department of Social and Economic Affairs)─shifting toward cities, where the geometry of buildings provides much more facade area than rooftop area, BIPVs could logically expand the energy-harvesting capacity of the urban built environment.

BIPVs have been heralded since the 1990s as a potentially game-changing advance, converting the envelope of a building from an inert insulator and light transmitter to an active contributor to its energy balance, paying back its upfront cost for decades after construction. With no moving parts, a single system to construct (rather than a “PV-ready” facade to which separate solar panels, or building-applied photovoltaics [BAPVs], are attached), and little difference from low-emissivity (low-E) coated glass except for connections to DC-to-AC electrical inverters, BIPVs strike some observers as an approach worth expanding. Why they have not caught on widely in the U.S., beyond their proof-of-concept role in early green buildings like 4 Times Square (originally the Condé Nast Building; Fox & Fowle, 1999), is a point of some contention.

Gabrielle Brainard, an associate principal at SOM, specializes in facade thermal performance in both adaptive-reuse projects and new construction. As a trainer for Passive House Network, she identifies the integrity and construction quality of the envelope as the first priority, promoting “passive strategies around envelope performance and envelope construction quality that help reduce overall building energy use: things like super-insulated facades [and] airtightness.” Moving from energy conservation to generation through BIPV, she says, is “something that's been around for a long time, and it works. I think it's just maybe the economics haven't really made sense in New York City.” In large-scale BIPV installations she has examined, “the PVs are only generating 1 percent or 2 percent” of the building's power. “PV is great, and it's solid state, nothing's moving, shouldn't fail, but a lot of what we need to do is at our fingertips without going to high-tech strategies....Everyone wants the high-tech solution, but I think the low-tech solutions that we need are already here, and it's really more about scaling and applying them.”

There is more to be gained from passive strategies such as selective, operable airtight envelopes and energy-recovery ventilation, Brainard says─“Warming up and humidifying the incoming air without wasting the energy that you just used to heat up the air inside the building, so that type of system provides very high indoor air quality and humidity control throughout the year”─than from advanced technologies that have remained on a demonstration scale. That said, she notes that a tipping point may have occurred to scale up BIPV's potential at last: as of August 2022, the Investment Tax Credit included in the federal Inflation Reduction Act (H.R. 5376) for renewables may increase the financial incentives enough to persuade hesitant developers to take the solar plunge. (The resolution also specifically includes another separate smart-glass technology, electrochromic glass.)

Components of a building eligible for the credit include the structures holding the PVs, Brainard points out. “If you do building-integrated photovoltaics, you can potentially get a tax credit for a good chunk of the entire curtain wall system,” she says, “so it's not just the photovoltaic glass; it's also the balance of systems on the electrical side, but also the support, framing, and anchorage for that glass, which in the case of a curtain wall is the curtain wall. So this is of great interest to developers because it's basically a way for them to reduce the cost of their facade.” The tax credit begins at 30 percent and can rise to 50 percent for projects located at brownfield sites (the Energy Community Bonus); there is also a Domestic Content Bonus (a “Buy American” provision) and a Low-Income Bonus for projects in low-income communities or on Native American land. Projects placed in service in 2022 or later and beginning construction before 2033 are eligible for the credits; the rates taper downward after 2033 (Solar Energy Technologies Office, 2023). “From what I understand, these tax credits are highly liquid, so they're monetizable by developers,” Brainard adds. “They can potentially sell them to a third party even before the end of the year, so they don't need to necessarily wait until the building is built in the end of the fiscal year to actually realize the credit.”

The credit is a potential boon not only to the conventional cellular PV industry but to the manufacturers of various forms of smart glass or transparent PV, the technology that Patterson calls “the holy grail of the photovoltaic glazing products.” However, the tax credit's ability to “accelerate the market” depends on clients having an appetite for tax credits as well as a commitment to sustainability, notes Diego Cuevas-Gómez, vice president for business development at the Spanish firm Onyx Solar. “Whenever you can put that into the equation,” he says, “the ROI is going to be higher, the payback time is going to be lower, so the economics are going to work better. But the technology is already affordable enough without the need for those credits or incentives, depending on the type of application.”

Onyx calculates the return on investment (ROI) in terms of “the delta price between doing a conventional facade and a PV facade,” Cuevas-Gómez says, comprising the price of PV glass, the additional electrical equipment required, and different costs of metalwork or installation labor, including wire routing, since “typically, the PV glass is frameless: it works on conventional framing and mullion systems.” With energy output, the tax credit, and accelerated depreciation factored into the net ROI calculations, “you're going to always get to payback times of less than five years.” Many clients, he adds, fail to consider the ongoing ROI from PV glass against the up-front cost difference from conventional facades. “Whenever we have a client that really wants to understand the math,” he summarizes, “I would say that 99 percent of the time they buy the glass.”

Decisions about types of PVs (including electrochromic glass, which offers both passive and active properties) hinge on aesthetics and efficiency of power generation as well as finances, Cuevas-Gómez says. Amorphous silicon PV glass is more transparent and thus appropriate for vision glass and skylights; crystalline silicon glass using either monocrystalline or polycrystalline cells adds opacity and is more often chosen for spandrel or canopy settings. Some skylight applications can also use crystalline cells spread out to allow some natural light transmission through clear gaps. Crystalline silicon is two to three times more efficient than amorphous silicon, and monocrystalline is slightly more efficient than polycrystalline. Organic PVs, Cuevas-Gómez and others report, are not yet competitive with silicon in efficiency or stability.

Properties aiding passive thermal insulation and natural light control (laminated safety glass, low-E coatings, or double or triple glazing with air or argon spacers) can be combined with the PV component's active property of generating power. A recently developed product, Cuevas-Gómez says, is colored PV glass: “Especially with the crystalline silicon solar cells, you can put a digital print on surface number one on the exterior of the glass, and with a different color, we have to hide completely the solar cells, and the unit will still generate electricity.” The colored glass can be ideal for rainscreen cladding, spandrels, or solid walls.

Because “not all the architects like the look of the conventional crystalline silicon solar cells or a conventional PV panel,” Cuevas-Gómez says, his firm offers a range of options balancing desired colors with energy efficiency. Covering the whole area of the glass with opaque cells can yield 17 to 18 percent efficiency (where an efficiency percentage translates roughly to watts per square foot); a more spread-out arrangement of cells with more light transmission gives about 12 percent, and “when we go with highly customized designs with the crystalline silicon, you may go down to 8 watts per square foot or 8 percent efficiency.” Color on the front surface or thicker glass used to classify the product as a primary building element, he adds, decreases light transmission and PV performance; Onyx provides laminated PV panels with thicknesses ranging from 4 mm to 12 mm, about a half-inch. Double or triple glazing does not decrease performance, because the multiple configuration is behind the solar cells.

Conventional rooftop PV panels, for comparison, offer 21 to 22 percent efficiency, so that “if you want to compare the most efficient solar panel in the market with the most efficient PV glass, I think it's a difference of 5 percent efficiency.” Weighing wattage generated against visibility, color preference, and other architectural considerations, major clients in the U.S. and around the globe have incorporated Onyx's PV glass in airports at Boston and Newark; corporate headquarters for Coca-Cola, Heineken, and Balenciaga; a major museum under construction (the Lucas Museum of the Narrative Arts in Los Angeles); the renovated Bell Labs in Holmdel, NJ (Eero Saarinen's 1962 building repurposed by Alexander Gorlin Architects in 2019 as the mixed-use Bell Works); and others. “Bell Labs was home to seven Nobel Prize winners,” Cuevas-Gómez notes, “and actually the first commercial solar cell was perfected there back in 1954. So the fact that they replaced the entire atrium with PV glass, that was a milestone for them: a way to pay tribute to all the inventions that happened in the building.” Although BIPV may remain a niche technology in the larger scheme of things, its prominence at high-visibility sites may imply that some see its potential as yet to be fulfilled (see Case Studies, “Gioia 22”).

Gioia 22, Milan

Photo courtesy of Ariatta S.p.A., MEP Consultants ; Onyx Solar

Gioia 22 in Milan (left) and original design of Gioia 22, with color-coded table showing seven different dimensions of PV glass.

The city that gave birth to the Italian Futurist movement in the 20th century has a new landmark that ushers it into the 21st. Just northeast of Porta Nuova Garibaldi, the newly developed business district of Milan, an office tower by the New Haven-based firm Pelli Clarke and Partners (formerly Pelli Clarke Pelli) sets a new standard for sustainable design and operations in Italy. Gioia 22 is the first and largest Italian building to approximate net-zero energy standards, thanks in large part to an installation of 55,000 square feet (circa 5,200 square meters) of spandrel glass with monocrystalline silicon Passivated Emitter and Rear Contact (PERC) PV cells.

Pelli Clarke and the client, real estate management firm COIMA SGR, sought to limit consumption of energy, take a cradle-to-cradle approach to use of materials, and respond to local interest in green mobility; the building's other features include three tiers of sunshades on the southwest facade to reduce heating load and glare, a groundwater-based heating and cooling system with no refrigeration unit (“free cooling”), electric-vehicle charging, paths and other facilities for cyclists and pedestrians, and sensor-controlled LED lighting, all contributing to LEED Platinum, WELL, Cradle-to-Cradle, and Nearly Zero Energy Consumption Building certification. The building also received a Facade Engineering Award from the Council on Tall Buildings and Urban Habitat in 2022.

Diego Cuevas-Gómez of Onyx Solar, manufacturer of the glass in collaboration with facade contractor Permasteelisa Group and facade consultant FACES Engineering, notes that the 26-story, 398-foot high-rise's facade can offer far more surface area and energy-harvesting surface than conventional rooftop panels could provide. The unitized-system curtain wall, comprising a total of 3,051 facade glass units, is sloped 6.5º outwards on the south elevation, 4.4º outwards on the east, and 4.4º inwards on the east, creating an angular profile that Pelli Clarke have likened to the peaks of the Swiss Alps some 80 miles away. The PV glazing, 2,500 units in all, generates 65 percent of the energy required for heating and cooling, aiding Gioia 22's overall energy performance 15 percent above the requirements of Milan's energy regulations. The total PV installation generates approximately 800 kilowatts peak power, the estimated equivalent of 300 residences' energy; compared with similarly scaled buildings lacking its renewable systems, Gioia 22 avoids the emission of about 2,260 tons of carbon dioxide per year. On the business side, Onyx estimates the PV facade's internal rate of return at 17.7 percent and its payback time at less than 5.7 years.