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Designing and constructing a net-zero-energy building requires an envelope that provides effective insulation and controls solar gain. An associated goal is that the envelope contributes to the interior environmental quality enjoyed by the occupants. Technologies in use over several decades have added another goal that informed observers advocate integrating into the design process: making a positive contribution to a building’s energy balance via integration of active technology into the facade. Though not yet adopted into mainstream use in the U.S., their promising use overseas and the potential researchers are uncovering could bring about constructive changes.
Photo courtesy of Onyx Solar Group
The spandrel glass of Milan’s Gioia 22, an office tower by Pelli Clarke and Partners, includes approximately 55,000 square feet of monocrystalline silicon photovoltaic cells.
Because buildings account for 40 percent of global energy consumption, informed and responsible architects, engineers, and owners recognize that every measure that can enhance their energy performance is worth considering. An adaptive facade not only can aid in controlling solar energy input but in some cases can transform sunlight and rainwater from adversarial elements to useful resources. In a three-tier approach to energy efficiency (load reduction, passive systems such as shading, and active energy-harvesting systems), adaptive facades can address at least the second and third of these strategies (Lechner 2008), and in certain cases all three. Generating at least some of a building’s power locally, rather than relying on transmission lines, also carries the promise of mitigating the power-grid inefficiencies that hinder decarbonization efforts on every scale.
An envelope that not only conserves energy but harvests it can function as an active organ within the building-organism, as the leaves or petals of plants or the skin of humans and other animals process sunlight in their different ways. This biological metaphor, viewing a building as an organic system and its facade as resembling a living skin, membrane, or botanical component more than an inert shell, can in some cases literalize the metaphor: increasing numbers of advanced buildings have incorporated living materials into their facades, not only emulating nature’s energy-transferring processes but marshaling them directly in the form of green facades or “biofacades” (Bonham and Kim 2022; Patterson 2022).
Net zero remains an asymptotically approachable ideal for some buildings while becoming an increasingly realistic goal for others (with the customary caveats about calculations, questionable offsets, and greenwashing). To date, most adaptive facade systems help reduce a building’s energy burden and carbon footprint incrementally toward net zero, though certain proof-of-concept projects demonstrate that crossing the zero line and reaching net-positive energy balance is possible, at least under research conditions. There is no building category that cannot benefit from analysis and upgrade of its facade's energy management: retrofitting an existing building with an active envelope reduces the waste of embodied energy and carbon involved in demolition, while designing a new building with its energy profile included in the planning from the early stages ensures that the facade and other components add up to a purposefully integrated system.
The relevant terms and philosophies proliferate (active facades, adaptive facades, smart facades, green facades, net zero, Passive House/Passivhaus, Aktivhaus, and others), yet the common goal is clear: to improve on the practices that have made too much of the American built environment a profligate user of resources, an intensifier of the urban heat-island effect, and a contributor to buildings’ share of global energy consumption.
Some specialists in advanced facades recommend that the design, engineering, and construction fields rethink assumptions that are long overdue for drastic change. Professionals working in the transition area between research and day-to-day practice advocate fundamental reconception, not maintenance of a steady state, given the growth in world populations, the proportion of urban residents, and the demands for new housing, transportation, and infrastructure. “Sustainability is no longer an option,” says Enrica Oliva, COO and partner of Werner Sobek New York (WSNY), a pioneering firm at the intersection of engineering and design. “We can no longer afford to just build the way that we used to—to just build the way that we know because it’s faster or because they may be perceived as cheaper, because resources are running out.”
Oliva emphasizes a specific definition of the challenge: it involves reduced reliance on fossil-fuel-based energy systems, not energy use generally, and it requires a close degree of collaboration between architects and engineers. “We’re not running out of energy,” she says. “We’re running out of resources. The sun is still there. It’ll be there for a very long time. So if we use the right sources of energy, we will not run out of that, but the problem is, we’re using resources that are getting depleted, and that very soon will put us in a position where we can no longer afford to live here.” Her firm’s work on lightweight and adaptive structures is guided by a concept that its founder Sobek defines as “Triple Zero” (buildings with zero emissions, zero energy, and zero waste generated), with the goal of moving beyond Passivhaus efficiency standards so that a building becomes an energy-producing Aktivhaus.
Methods of decreasing a building's dependence on the external power grid through local energy harvesting include a technology that has become broadly familiar, rooftop-mounted photovoltaic (PV) cells. The amount of available roof area limits their contribution, however, particularly in urban settings; some buildings manage to gather low-hanging fruit by extending PVs to the larger surface area of the exterior envelope. Along with conventional glass curtain walls reaping thermal energy and offering increasing control of daylight, advanced glazing systems are available using dynamic solar shading and photochromic, electrochromic, or thermochromic glass. Building-integrated photovoltaics (BIPV) have been explored and promoted, more extensively overseas than within the United States, though opinions differ over whether this approach can be scaled up economically to fulfill the promise its early advocates have envisioned. Additional mechanisms to consider include supplemental hydronic heating, rainwater collection, and biophilic air treatment.
Photo courtesy of View Glass; rendering by Neoscape
The retrofit project at 111 Wall Street, a 25-story Class A office tower in New York’s Financial District redesigned by STUDIOS Architecture, reclads the building with electrochromic glass, which controls thermal input and energy consumption along with allowing high-definition transparent displays.
The U.S. lags behind Europe in applying advanced facade technologies in the field, commentators observe. This discrepancy reflects a convergence of factors, including the higher cost of energy in Europe, which increases economic incentives for thermal performance; the cost of American certification programs (e.g., Underwriters Laboratories or FM Approvals) for European firms that have developed advanced technologies but have not gained a foothold in the American market; the relative unfamiliarity of U.S. tradespeople with the newer systems; and the generally more rigorous codes abroad, except for certain bellwether cities or states where environmental concerns are expressed in codes like New York City’s Local Law 97 and California’s Title 24.
Leading American architects and engineers are well aware of the higher standards and openness to innovation that are more prevalent in different regulatory and business environments. The urgency of the need to accelerate the transition to renewable forms of energy is no mystery to any informed observer of the American built environment, though the structure of economic incentives, some observe, has been a barrier to conversion to better-performing envelope technologies, not only on the cutting edge of research but in areas that are relatively well known.
“There are other strategies for energy harvesting other than solar PV,” says Mic Patterson, ambassador of innovation and collaboration at the Facade Tectonics Institute (FTI). “Wind is another familiar strategy, though its use in buildings has been limited. Thermal energy harvesting is another interesting strategy. A liquid piped through the facade system can harvest or shed heat, store it and deploy it when needed.” The various forms of adaptive facades are currently in a transitional state between the research and development (R&D) realm and broader commercial implementation. “It's an issue of adoption,” Patterson finds. “No innovation, regardless of how brilliant, is successful without adoption.”
TECHNOLOGIES WE ALREADY HAVE
“There is an opportunity for significant improvements in facade-system performance with the products that are on the shelf right now,” Patterson continues. Although he has long advocated more R&D in the field, the chief problem he sees at present is slow acceptance of technologies that are commercially available and known to be effective. High-performance buildings that meet or even exceed net zero energy and carbon metrics are already a reality, he says, with a caveat regarding scale. “If you look at the history there and the projects built to date, they're all smaller buildings,” he notes. “Most of them are single-story buildings or else low-rise. There are very, very few exceptions, and most of them succeed through rooftop-mounted PV.” Existing unitized curtain-wall systems, he says, including the variant known as the closed-cavity facade with a compact double-skin modular unit and shading devices sealed in the dust-free interior, dramatically improve a building's performance, yet in larger commercial buildings, too few owners have chosen them on account of cost, unfamiliarity, and other factors.
“Having a product is only part of the deal,” Patterson says; “you've got to have a supply chain to take that product all the way through installation and warranty, all the way through its service life.” For many owners, “faced with a new approach, a new product, [or] a new assembly strategy for the facade system, they'll go for the old tried-and-true one, in spite of the fact that it doesn't have as good performance because they associate it with risk.” Few people lose contracts or jobs over selecting an industry-standard product, and “current industry standard, unfortunately, is code-minimum.”
At the core of this reluctance to support the best-performing technologies, Patterson contends, is a common form of short-term thinking: “the tendency for these decisions to be made in a totally inappropriate context, which is first cost.” He points to large buildings in New York City that are now beginning their second century, yet many of today's owners continue to disregard the payoff from high-performing facade technologies beyond upfront costs or, at best, to assess energy-saving benefits over a compressed period of three years or so─and “society has to live with the results of this over the life of the building.” Today's new buildings, he says, may also stand for the next century or longer, and calculations of their facades' amplified performance should reflect that lifespan.
Other nations, he observes, are setting standards worth emulating in this area: “Europe has been in the lead when it comes to facade system development. Canada is ahead of the U.S., and I think the primary driver there is their weather is a little more severe.” Canada's national building code is evolving, and more progressive step codes in Toronto, Vancouver, and Montreal are driving energy-saving measures; New York, Seattle, and other American cities are moving in a similar direction. Still, “Adoption is the big problem; the industry, I think, is in a good position to respond to these things, and to some extent has responded, but you can understand their reluctance to invest in further R&D when the stuff that they've already developed is not being adopted.”
Designing and constructing a net-zero-energy building requires an envelope that provides effective insulation and controls solar gain. An associated goal is that the envelope contributes to the interior environmental quality enjoyed by the occupants. Technologies in use over several decades have added another goal that informed observers advocate integrating into the design process: making a positive contribution to a building’s energy balance via integration of active technology into the facade. Though not yet adopted into mainstream use in the U.S., their promising use overseas and the potential researchers are uncovering could bring about constructive changes.
Photo courtesy of Onyx Solar Group
The spandrel glass of Milan’s Gioia 22, an office tower by Pelli Clarke and Partners, includes approximately 55,000 square feet of monocrystalline silicon photovoltaic cells.
Because buildings account for 40 percent of global energy consumption, informed and responsible architects, engineers, and owners recognize that every measure that can enhance their energy performance is worth considering. An adaptive facade not only can aid in controlling solar energy input but in some cases can transform sunlight and rainwater from adversarial elements to useful resources. In a three-tier approach to energy efficiency (load reduction, passive systems such as shading, and active energy-harvesting systems), adaptive facades can address at least the second and third of these strategies (Lechner 2008), and in certain cases all three. Generating at least some of a building’s power locally, rather than relying on transmission lines, also carries the promise of mitigating the power-grid inefficiencies that hinder decarbonization efforts on every scale.
An envelope that not only conserves energy but harvests it can function as an active organ within the building-organism, as the leaves or petals of plants or the skin of humans and other animals process sunlight in their different ways. This biological metaphor, viewing a building as an organic system and its facade as resembling a living skin, membrane, or botanical component more than an inert shell, can in some cases literalize the metaphor: increasing numbers of advanced buildings have incorporated living materials into their facades, not only emulating nature’s energy-transferring processes but marshaling them directly in the form of green facades or “biofacades” (Bonham and Kim 2022; Patterson 2022).
Net zero remains an asymptotically approachable ideal for some buildings while becoming an increasingly realistic goal for others (with the customary caveats about calculations, questionable offsets, and greenwashing). To date, most adaptive facade systems help reduce a building’s energy burden and carbon footprint incrementally toward net zero, though certain proof-of-concept projects demonstrate that crossing the zero line and reaching net-positive energy balance is possible, at least under research conditions. There is no building category that cannot benefit from analysis and upgrade of its facade's energy management: retrofitting an existing building with an active envelope reduces the waste of embodied energy and carbon involved in demolition, while designing a new building with its energy profile included in the planning from the early stages ensures that the facade and other components add up to a purposefully integrated system.
The relevant terms and philosophies proliferate (active facades, adaptive facades, smart facades, green facades, net zero, Passive House/Passivhaus, Aktivhaus, and others), yet the common goal is clear: to improve on the practices that have made too much of the American built environment a profligate user of resources, an intensifier of the urban heat-island effect, and a contributor to buildings’ share of global energy consumption.
Some specialists in advanced facades recommend that the design, engineering, and construction fields rethink assumptions that are long overdue for drastic change. Professionals working in the transition area between research and day-to-day practice advocate fundamental reconception, not maintenance of a steady state, given the growth in world populations, the proportion of urban residents, and the demands for new housing, transportation, and infrastructure. “Sustainability is no longer an option,” says Enrica Oliva, COO and partner of Werner Sobek New York (WSNY), a pioneering firm at the intersection of engineering and design. “We can no longer afford to just build the way that we used to—to just build the way that we know because it’s faster or because they may be perceived as cheaper, because resources are running out.”
Oliva emphasizes a specific definition of the challenge: it involves reduced reliance on fossil-fuel-based energy systems, not energy use generally, and it requires a close degree of collaboration between architects and engineers. “We’re not running out of energy,” she says. “We’re running out of resources. The sun is still there. It’ll be there for a very long time. So if we use the right sources of energy, we will not run out of that, but the problem is, we’re using resources that are getting depleted, and that very soon will put us in a position where we can no longer afford to live here.” Her firm’s work on lightweight and adaptive structures is guided by a concept that its founder Sobek defines as “Triple Zero” (buildings with zero emissions, zero energy, and zero waste generated), with the goal of moving beyond Passivhaus efficiency standards so that a building becomes an energy-producing Aktivhaus.
Methods of decreasing a building's dependence on the external power grid through local energy harvesting include a technology that has become broadly familiar, rooftop-mounted photovoltaic (PV) cells. The amount of available roof area limits their contribution, however, particularly in urban settings; some buildings manage to gather low-hanging fruit by extending PVs to the larger surface area of the exterior envelope. Along with conventional glass curtain walls reaping thermal energy and offering increasing control of daylight, advanced glazing systems are available using dynamic solar shading and photochromic, electrochromic, or thermochromic glass. Building-integrated photovoltaics (BIPV) have been explored and promoted, more extensively overseas than within the United States, though opinions differ over whether this approach can be scaled up economically to fulfill the promise its early advocates have envisioned. Additional mechanisms to consider include supplemental hydronic heating, rainwater collection, and biophilic air treatment.
Photo courtesy of View Glass; rendering by Neoscape
The retrofit project at 111 Wall Street, a 25-story Class A office tower in New York’s Financial District redesigned by STUDIOS Architecture, reclads the building with electrochromic glass, which controls thermal input and energy consumption along with allowing high-definition transparent displays.
The U.S. lags behind Europe in applying advanced facade technologies in the field, commentators observe. This discrepancy reflects a convergence of factors, including the higher cost of energy in Europe, which increases economic incentives for thermal performance; the cost of American certification programs (e.g., Underwriters Laboratories or FM Approvals) for European firms that have developed advanced technologies but have not gained a foothold in the American market; the relative unfamiliarity of U.S. tradespeople with the newer systems; and the generally more rigorous codes abroad, except for certain bellwether cities or states where environmental concerns are expressed in codes like New York City’s Local Law 97 and California’s Title 24.
Leading American architects and engineers are well aware of the higher standards and openness to innovation that are more prevalent in different regulatory and business environments. The urgency of the need to accelerate the transition to renewable forms of energy is no mystery to any informed observer of the American built environment, though the structure of economic incentives, some observe, has been a barrier to conversion to better-performing envelope technologies, not only on the cutting edge of research but in areas that are relatively well known.
“There are other strategies for energy harvesting other than solar PV,” says Mic Patterson, ambassador of innovation and collaboration at the Facade Tectonics Institute (FTI). “Wind is another familiar strategy, though its use in buildings has been limited. Thermal energy harvesting is another interesting strategy. A liquid piped through the facade system can harvest or shed heat, store it and deploy it when needed.” The various forms of adaptive facades are currently in a transitional state between the research and development (R&D) realm and broader commercial implementation. “It's an issue of adoption,” Patterson finds. “No innovation, regardless of how brilliant, is successful without adoption.”
TECHNOLOGIES WE ALREADY HAVE
“There is an opportunity for significant improvements in facade-system performance with the products that are on the shelf right now,” Patterson continues. Although he has long advocated more R&D in the field, the chief problem he sees at present is slow acceptance of technologies that are commercially available and known to be effective. High-performance buildings that meet or even exceed net zero energy and carbon metrics are already a reality, he says, with a caveat regarding scale. “If you look at the history there and the projects built to date, they're all smaller buildings,” he notes. “Most of them are single-story buildings or else low-rise. There are very, very few exceptions, and most of them succeed through rooftop-mounted PV.” Existing unitized curtain-wall systems, he says, including the variant known as the closed-cavity facade with a compact double-skin modular unit and shading devices sealed in the dust-free interior, dramatically improve a building's performance, yet in larger commercial buildings, too few owners have chosen them on account of cost, unfamiliarity, and other factors.
“Having a product is only part of the deal,” Patterson says; “you've got to have a supply chain to take that product all the way through installation and warranty, all the way through its service life.” For many owners, “faced with a new approach, a new product, [or] a new assembly strategy for the facade system, they'll go for the old tried-and-true one, in spite of the fact that it doesn't have as good performance because they associate it with risk.” Few people lose contracts or jobs over selecting an industry-standard product, and “current industry standard, unfortunately, is code-minimum.”
At the core of this reluctance to support the best-performing technologies, Patterson contends, is a common form of short-term thinking: “the tendency for these decisions to be made in a totally inappropriate context, which is first cost.” He points to large buildings in New York City that are now beginning their second century, yet many of today's owners continue to disregard the payoff from high-performing facade technologies beyond upfront costs or, at best, to assess energy-saving benefits over a compressed period of three years or so─and “society has to live with the results of this over the life of the building.” Today's new buildings, he says, may also stand for the next century or longer, and calculations of their facades' amplified performance should reflect that lifespan.
Other nations, he observes, are setting standards worth emulating in this area: “Europe has been in the lead when it comes to facade system development. Canada is ahead of the U.S., and I think the primary driver there is their weather is a little more severe.” Canada's national building code is evolving, and more progressive step codes in Toronto, Vancouver, and Montreal are driving energy-saving measures; New York, Seattle, and other American cities are moving in a similar direction. Still, “Adoption is the big problem; the industry, I think, is in a good position to respond to these things, and to some extent has responded, but you can understand their reluctance to invest in further R&D when the stuff that they've already developed is not being adopted.”
Triple glazing is now standard in Europe because of code requirements, Patterson notes, and some applications there are using quad, but in the U.S., efforts to strengthen codes encounter opposition: “Industry shows up. There's a lot of people with vested interests that don't want to see these changes, and they show up in numbers and argue against the adoption of these codes, and they do it successfully.” Despite performance goals for 2030 or 2050 set by public agencies, “they are also mandated through the legislature to not do anything that's going to cost more money.” When industry representatives claim during code-modification discussions that they can meet performance goals without substantial initial investment, FTI recurrently argues that this corner-cutting approach is impossible.
Vacuum-insulated glazing (VIG) is another case in point, he says. After proof-of-concept studies in the early 1990s and the initial appearance of a commercial product in 1996 (Kocer 2020), manufacturers publicized this approach, but VIG is “still only very marginally competitive in the marketplace, and only in very special circumstances.” VIG offers exceptionally low thermal conductance – as low as 0.5 watts per square meter per kelvin or potentially lower – but has made more headway in Asia and Europe, according to industry observers. Patterson describes VIG and other technologies as constrained by “a chicken-and-egg problem,” where wider adoption could lead to more investment in fabrication plants, scaled-up production, and lower costs, but current costs hamper adoption; some form of subsidies may be required to overcome this conceptual bottleneck. The glazing technologies he calls “X-chromics” (photochromics, electrochromics, and thermochromics) have been more competitive, he notes, after years of product development and incremental progress in the marketplace. BIPVs need to follow a similar path if they are to attract demand and attain a market share commensurate with their energy-harvesting potential (see “To BIPV or Not to BIPV,” below).
Because early curtain-wall systems from the mid-20th century are difficult to replace, Patterson notes, “many of them are sitting un-renovated in the New York City marketplace despite the recent boom economy, because it's just too expensive and too disruptive to ongoing building operations to change.” Given that a facade often accounts for about 25 percent of the total cost of a building, he adds, “the building industry is very conservative, very risk-averse” in this area, and the argument for higher-performing technologies needs a key persuasive point: assuring building owners that the long-range payback will be sufficient incentive.
Patterson has a vision of a modular product, yet to be developed, that could make such an upgrade as simple as changing a tire. “A fully integrated plug-and-play window system could be developed with relative ease and be a real game-changer,” he suggests. All the needed technology is there on the shelf, but not as an assembly available from a single-source manufacturer: a dynamic, automated, operable window assembly including solar-control glazing, shading and glare control, sensors and controllers, software-ready to link into the building management system to provide an interface between the facade system and the building mechanical, HVAC, and lighting systems to provide coordinated optimum efficiencies between these systems. The window system [would be] provided as a single-source product, serviceable as necessary and with a minimum five-year warranty. There is no such product in the market. Architects and building owners would love it. This is an easier thing to accomplish than what the lighting industry did in developing LED technology. But, as with the development of the LED, it takes an investment.”
FROM R&D TO THE MAINSTREAM
Value engineering (VE) may be an inescapable part of negotiations as a building passes from design to construction; it is also often the point where useful innovations go to die. “The architects,” Patterson says, “want to do the BIPV thing. I saw a lot of projects come out that included photovoltaics in the facade system, and it almost invariably got VE'd out. You could bet that if your project came out and had a lot of BIPV in it, it wouldn't last through the design process.”
Oliva has had similar experiences with BIPV: “In the U.S., the projects that I have been following... we've tried to implement that solution. As it turns out, it would either be VE'd or be forgotten or no longer be considered interesting a third of the way through the process.” As the American office of a German-based firm with strong links to academia, WSNY is positioned between Europe's leading-edge research world and the cautious American market, striving to foster technology transfer from the former to the latter. The University of Stuttgart's Institut für Leichtbau Entwerfen und Konstruieren (ILEK, a.k.a. Institute for Lightweight Structures and Conceptual Design), currently headed by Werner Sobek managing partner Lucio Blandini, was founded by Frei Otto and run by Sobek for decades; ILEK's research explores adaptive facades and materials-saving efficiency in load-bearing and non-load-bearing structures. “When it comes to innovation, we are a little bit spoiled at Werner Sobek, because we see what's coming that maybe for a year or two people won't hear about as widely, unless you go to Facade Tectonics and to all these symposia.”
Approaches that have succeeded in the realms of R&D and competitions, like WSNY's 2020 Metals in Construction Design Challenge finalist entry in collaboration with SOM and Atelier Ten, using VIG with titanium spacing rings in the ultra-lightweight overcladding of a hypothetical commercial retrofit at 63 Madison Avenue in New York, ideally lay the groundwork for the firm's commercial projects. The VIG/titanium concept emerged from the firm's work on the prototype house built in Stuttgart's Weißenhof development, Aktivhaus B10, whose rooftop PV system generates twice as much energy as the house itself uses, powering a neighboring museum and two electric cars as well (Heinlein 2015). “You often see technologies that are completely unattainable,” Oliva says, “and then at a certain point, a few years later, they become actually affordable, and they can be implemented in a real project.” Some projects have not yet overcome cost-cutting concerns even after studies that the engineers consider persuasive, including return-on-investment studies and analyses of building geometry and optimal PV-panel placement. In other instances, WSNY has developed a new facade strategy on a smaller project and extended it to a larger one (see Case Studies, “320 South Canal Street”).
Photo © Christina Eisenbarth/ILEK
The pneumatically actuated origami sun shading (PAOSS) facade system, a research work by the Institut für Leichtbau Entwerfen und Konstruieren (ILEK), uses folding textile elements incorporated within cushions of ethylene tetrafluoroethylene (ETFE).
Another facade technology with a long history of availability and increasing construction applications, Oliva notes, is inflatable ethylene tetrafluoroethylene (ETFE) cushions. First used on a large scale architecturally in 2001 at Grimshaw's Eden Project in Cornwall, UK, and subsequently included in prominent projects like the Beijing Olympics Water Cube (PTW Architects and colleagues, 2008) and The Shed at Hudson Yards, New York (Diller Scofidio + Renfro, 2019), ETFE can offer variable shading, glare protection, and thermal performance along with its well-known high ratio of strength to weight. WSNY, having worked with prominent ETFE supplier Vector Foiltec and John Ronan Architects on the Ed Kaplan Family Institute for Innovation and Tech Entrepreneurship at Illinois Institute of Technology in Chicago (2018), among other projects, is studying further uses of ETFE at “the largest and first demonstrator building in the world, which is a 12-story building that we built at the ILEK; it hosts 12 potential new facade technologies, one per story” and uses springlike hydraulic actuators to measure the building's response to wind, seismic, and other forces.
“What if our shading could be customizable,” Oliva asks, “and then virtually disappear when we don't need it and we want to take maximum advantage of some solar light and heat?” The object of one current investigation at ILEK's demonstrator building is Pneumatically Actuated Origami Sun Shading (PAOSS), “an adaptive facade system that can be incorporated inside of an ETFE cushion and folds back when you want to flatten the ETFE cushion with a technology related to origami” (Eisenbarth et al. 2021). Inspired by NASA's Starshade pneumatic device for managing glare from starlight for space telescopy, the textile-based PAOSS opens into floral-shaped patterns and “pulls back to virtually no thickness,” Oliva says. PAOSS builds on the known properties of ETFE to suggest new ways for adaptive facades to extend a building's thermal versatility while minimizing its material footprint.
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”).
FROM BIOMIMESIS TO BIOLOGICALS AND BEYOND
Facades that include plants or other life forms, along with inorganic systems inspired by them, have drawn substantial attention, though not all have enjoyed performance or longevity to match their aesthetic or symbolic value. Green roofs have proven easier to support and maintain than green walls, observers note. “A lot of the early ones failed,” Patterson says; “it is not trivial to integrate biological systems into the building skin. I see architects conceptually interested in it, but the biology has to drive the design, and that's what they don't get. They want to design their facade system and do their aesthetic thing, and then plug plants into it; it doesn't work that way.” Seasonal cycles, the weight of soil for structural systems, high wind speeds at tops of buildings, the need to deliver water and attract pollinators, and the complexity of local ecosystems all create challenges for architects seeking to emulate the Hanging Gardens of Babylon.
Yet specialist/theorists such as Ken Yeang and Glen Small have established vertical gardens and eco-highrises as recognized typologies, and examples of viable biofacades are no longer rare. Salient recent cases, Patterson points out, include Thomas Heatherwick's 1000 Trees mixed-use development in Shanghai (the first phase of which opened in 2022), with exoskeletal supporting columns doing double duty as podiums supporting trees and other vegetation, and Stefano Boeri's Bosco Verticale (Vertical Forest, 2014) in Milan, a two-tower residential complex featuring 800 trees mounted on balconies. The latter project's sustainability has drawn critiques involving the long-range carbon footprint of its concrete elements and the challenges of maintenance (Alter 2020); nevertheless, the architects' careful attention to managing the elements of the buildings' microclimate, including deploying “flying gardeners” who maintain the plants by descending from the roof using mountaineering equipment, have made it globally iconic: a multiple award winner as well as a host to some 1,600 species of birds and butterflies.
Green facades, adaptive in the most direct sense, require exceptional attention to detail and substantial up-front investment. In Düsseldorf, Germany, Werner Sobek and Ingenhoven Architects' Kö-Bogen II commercial building has “a green facade that was actually thought through from the beginning of the process to the end,” Oliva says. “One can tell that thought was not put only into the nature of the greenery, but into how weight, the operations, [and] everything that had to do with the building was implemented into the main structure, the secondary structure, and the maintenance plan” (see Case Studies, “Kö-Bogen II”).
As facade designers and engineers strive to tread more lightly on the Earth, technologies that approximate natural forms both make intuitive sense and carry the promise of a measurable step up in performance from their predecessors. Anna Dyson, founding director of the Yale Center for Ecosystems in Architecture (CEA) and previously a co-founder of the Rensselaer Polytechnic Institute/SOM Center for Architecture, Science and Ecology (CASE), sometimes invokes sunflowers as an exceptional evolutionary adaptation to the task of drawing energy from the sun. Early generations of PVs, she finds, have severe limits; developers of newer systems could learn a great deal from these plants and the natural principles underlying them.
“The first generation of building-integrated PV uses a lot of semiconductor material, and it coincidentally is very low-efficiency,” Dyson says. Estimating the efficiency range of early BIPVs at 14 to 25 percent – “and 25 is the absolute upper end if it were perfectly maintained at all times” – she breaks down what happens to the other 75 percent or more of solar energy reaching a building. Much of it “lingers around the building envelope as waste heat, which can be a problem in certain hot, humid climates that don't have adequate ventilation and trapped urban corridors and urban canyons.” It also exacerbates the urban heat island effect and glare. The problem is inherent in passive systems' strategy of rejecting energy rather than transforming it. “For this next wave of building envelopes,” she says, “we need to be optimally using the incoming energy for multiple purposes, not just for electricity, but to mitigate the building cooling or heating loads, to supply hot water to applications, and to daylight the building.”
Passive solar, Dyson believes, is appropriate for smaller-scale buildings such as residences, particularly in winter. Yet “even passive solar oftentimes is allowing heat into the building in an uncontrolled fashion, not in an optimal fashion,” she says. “In large commercial buildings, passive solar is oftentimes a real problem, because at some point during the day, even in the coldest months, you'll go from heating degree to cooling degree; that is, that the internal loads for the live loads will ultimately be producing too much heat energy, and we will transition into cooling. At that point, the passive solar becomes a problem; it's unwanted solar heat gain. So what we really are doing basically with these next-generation building systems is we're treating them as transfer functions.” Two systems that Dyson and colleagues have developed, the Integrated Concentrating (IC) Solar Facade System and Solar Enclosure for Water Reuse (SEWR), are under proof-of-concept patents (Dyson et al. 2007 and 2012); her team plans to demonstrate SEWR at Conference of the Parties (COP) 28, the 2023 United Nations Climate Change Conference, in Dubai later this year.
In tomorrow's multifunctional adaptive envelopes, as Dyson's team envisions, optical methods of concentrating sunlight on multiple solar modules, including Fresnel lenses, thin films, and assorted micro-optic elements, “fly-eye-type geometries” and others (Dyson 2012), respond more effectively to the sun than flat panels can, behaving more like heliotropic plants. She and her colleagues have measured efficiencies in the 40-percent range in tests of their systems; “we can go all the way to 70 with optimized concentrating solar-cell types like gallium arsenide and gallium nitride,” she says.
Another important feature of her systems is that they are “designed for disassembly and reassembly,” so that component parts can be sustainably reused in future life cycles, keeping semiconductors out of the waste stream. (“Toxic materials are OK to use as long as you can reuse them,” she notes, “and you're not distributing them into the ecosystem.”) Semiconductors are troublesome aspects of a building's systems for several reasons, not only because of their toxicity, nonrenewability, and energy-intensive manufacturing processes but because “we're accepting a slavery discount on solar panels” that keeps their prices artificially low because the materials are often sourced from parts of the world that use forced labor. As an alternative, “what we want to do is really replace the semiconductor with a much more viable, better, abundant Earth material, like glass.” Her integrating solar facade system uses only a small percentage of the semiconductor material found in conventional flat-plate PVs. SEWR, which processes a building's graywater along with transforming energy, “is actually adding plant-based biochemistry: just plant dyes, extractions from common plants that we grow for agriculture, like quinoa and kale and garlic. And we put those extractions into the water that flows to the system in order to have on-site water capture and sterilization, so you have complete 100 percent on-site water, and that is the system that we're demonstrating now at scale.”
When Dyson began developing solar facades around 1999 and 2000, “the solar cell was more than $2,000 per unit. Now it's under $5.” Advances in multiple fields were necessary for these costs to drop and for the elements of her multifunctional systems to converge. Now may be the time, her work implies, for technologies that behave more like nature, where every component has a place in cycles and there is no such thing as waste.
CONCLUSION
Because the U.S. built environment contains billions of square meters of glass surfaces, the prospect of converting a substantial amount of it into transparent PVs is tantalizing. Firms like California-based Ubiquitous Energy, which patented a transparent solar coated glass based on research performed at MIT and Michigan State, have attracted national publicity with the possibility of scaling up an aesthetically acceptable PV application. The chief barrier to such a scenario to date, Patterson points out, is that “the building industry is notoriously slow in adopting new design and product developments, a reflection of the extreme risk aversion characteristic of the industry. Incentives, codes, and policies can help new technologies get over the adoption hump by encouraging initial tiers of early adopters.”
Controlling solar gain and harvesting energy are both achievable. “The emerging transparent architectural glazing technologies hold the potential to accomplish both in a single product,” Patterson comments. “But a layered facade assembly that separates functions is already viable with existing technology, and may prove to be more flexible and efficient in application. All that's required is an appropriate configuration of PV cells, insulated glass, low-E coatings, and a shading system, all currently available, cost-effective, and with proven performance.”
“The facade development that occurred in Europe, Germany in particular, in the wake of the 1970s energy crisis pushed their facade technology 20 years ahead of the U.S., and we're still playing catch-up,” Patterson summarizes. “Government-mandated performance improvements drove rapid adoption of new, higher-performing facade technologies. The development effort in the U.S. has been effectively hobbled by artificially low energy prices and lax building codes. The building industry in the U.S. is gridlocked by the protection of vested interests, the concerted effort to keep things as they are. The glass industry is no exception. The industry could move forward by embracing the constraints embedded in the goals established for resilience and sustainability by entities such as Architecture 2030 and the UN's Intergovernmental Panel on Climate Change. Constraints drive innovation.” Code by code, competition by competition, and project by project, this process can drive the architectural and engineering fields in the direction planetary conditions require them to go.
WORKS CITED:
Alter, Lloyd. Another look at Stefano Boeri's Vertical Forest. Treehugger, August 13, 2020.
Bonham, Mary Ben; Kim, Kyoung Hee. Biofacades: integrating biological systems with building enclosures. Skins, July 15, 2022.
Dyson, Anna; Jensen, Michael K.; Borton, David N. Concentrating type solar collection and daylighting system withn glazed building envelopes. United States Patent and Trademark Office, Patent 7,190,531, March 13, 2007.
Dyson, Anna; Vollen, Jason; Mistur, Mark; et al. Solar enclosure for water reuse. World Intellectual Patent Organization, Patent WO 2012/075064A2, 2012.
Bill Millard is a New York-based journalist who has contributed to Architectural Record, The Architect's Newspaper, Oculus, Architect, Annals of Emergency Medicine, OMA’s Content, and other publications.
