Advocates for renewable energy tend to forget the largest and most accessible source of it. The sun sends 86,000 terawatts through space and the atmosphere to the Earth’s surface each year. This energy warms the planet, sustains every ecosystem, and serves us wheat and wine. But sunshine could do more. Even without being captured for electricity through solar panels, it could still heat and illuminate our buildings. For decades the U.S. Green Building Council (author of LEED standards) and the Passive House Institute have advocated for panels plus panel-less sunlight-friendly architecture. But few are listening. Today’s most popular plans for solar energy focus on only the sliver of it—0.078 terawatts and growing—that is converted into electricity.
Princeton’s Net-Zero America project maps utility-scale solar and wind farms countrywide. However, the engineers who wrote the plan completely ignore non-electrical uses of renewable energy. Another engineer, John Doerr, does mention non-electrical uses of sunlight in his book Speed & Scale: An Action Plan for Solving Our Climate Crisis (2021). Wildly hyped by Amazon, the book celebrates Amazon’s chairman, Jeff Bezos, and other venture capitalists. Doerr also writes about Amory Lovins, a longstanding advocate for renewables, whose Colorado house uses direct sunlight for 99 percent of its heating needs. That seems like a workable, highly efficient technology. It’s called “passive solar.” The passive refers misleadingly to all non-electrical means of harvesting photons. It’s a pity that Doerr devotes only half of a page to passive solar, as compared to six pages about Sunrun, the largest dedicated residential solar company in the United States (and seventeen pages on so-far unworkable schemes to capture carbon). Passive, panel-less sunshine is barely noticed in both policy and activism alike today.
Markets, too, ignore daylight. Because it has no price, states and corporations are happy to share the sunlight. Unless incarcerated, one can usually obtain an hour or a patch of outdoor photons. Electrons—the electrical particles—are entirely different: some of them live amid markets. Corporations pushed them through wires and put a price tag on them a long time ago. Now, advocates of renewables demand that we “electrify everything.” Of course, an infrastructure of photovoltaic panels and electric cars is better for the planet than oil wells and Hummers. But electrifying everything will deepen inequality. Panels convert free, public sunlight into private, metered kilowatts. Except within the rare community-owned solar project, people don’t share those kilowatts. On the contrary, we buy kilowatts from Sunrun, and Sunrun gets richer and bigger still. There is a better, supplementary alternative. What would the energy transition look like if—at least, in part—we tried to make free, public light even freer and more democratic?
Current forms of passive solar technology arose as the antidote to urban disease in the nineteenth century, when families packed into the dark tenements of Paris and other European cities. For the purpose of sanitation alone, the authorities needed to open housing to daylight, breeze, and modernist architecture. Today’s metropolis must “compose with light,” wrote Le Corbusier, the most influential modernist. In 1914 he designed the “Dom-Ino” structure, a building almost without walls. Floors and pillars of reinforced concrete carried the entire load, freeing the exterior walls for other purposes. In 1923 he enthused about “apartments opening on all faces to air and sunlight.” And twenty years later he recommended legislation requiring that the sun be able to enter each room at least two hours per day. Even without the force of the law, sealed boxes gave way to open, airy apartments. Facades spouted balconies and transparent doors. Skylights broke through roofs. The age of glass—a sifter for warm light and against cold air—arrived.
Americans pushed this solar technology further during the oil shortages of World War II and immediately thereafter. Eugene Ayres and other scientists at MIT designed experimental houses with long, south-facing exposures, double-paned glass, and overhangs of just the right length and angle. In the summer the overhangs or adjustable awnings block high, nearly vertical sunshine from passing through the glass and overheating interior spaces. In Arizona, for example, this shade keeps model dwellings cool in July with little or no air conditioning. In the winter low, nearly horizontal sunshine strikes and passes through the glass of passive houses, illuminating and warming the rooms. Sunset doesn’t interrupt these effects of daylight. A thermal mass—usually a sunlit stone or concrete floor—heats during the winter day and re-radiates that warmth to interior spaces at night. The floor, in other words, acts as a battery, solving the so-called “intermittency problem” of solar energy. Even first-floor apartments shaded from direct sun can collect solar energy through rooftop structures of sun-warmed water that is then piped down to heat the inside. “This will be a major method of heating houses in the future,” Ayres wrote in Scientific American. Unfortunately his prediction was completely wrong.
Ayres’ estimate—that the full array of passive solar measures would cut fuel-fired heating and electric air conditioning by 90 percent—has been borne out (beaten by Lovins, in fact). But fossil fuels soon became almost too cheap to count. Energy historians refer to the 1950s as the Great Acceleration, the period of rapid economic expansion and consumerism in the United States. Middle-class Americans consumed electricity and every gadget powered by it. They built houses thinly insulated, poorly windowed and, consequently, utterly dependent on energy shipped through wires. Ayres’ proposals died before they could mature. The average American—living in a small house or apartment—never appreciated the solar opportunity missed. Instead, she stayed in her dim studio, paying the fossil-electric bill as utilities grew rich and fat.
By the end of the 1950s, passive solar had become the province of boutique suburban and rural houses and a handful of urban projects, mostly in Europe. Little has changed. In 2020 a beautiful eighteen-unit building of subsidized housing opened in Vienna. Along the entire five-story south façade, glass and automatic sliding doors afford winter gardens, thermal masses, and terraces for each unit. The structure is—to quote a review of its ground-floor café and bike shop—“a wonderful place where lovely and open people stand up for the quarter and have great visions for the public space!” Meanwhile, back in New Jersey, I teach in a top-floor classroom with two small panes of glass. They cut horizontal light to a shard while the solid roof without skylights blocks overhead sunlight. Two feet below that roof and various ducts, fluorescent bulbs illuminate my students and me. This is nonsensical and wasteful: the building throws away free energy to replace it with costly electrons generated somewhere else. This is the windowless, all-electric, not-so-great vision that we are supposed to embrace.
Let’s imagine an infrastructure bill for sunlight: for photons, rather than for electricity alone. Solar energy fits within even the most traditional understanding of infrastructure: roads, bridges, tunnels, pipelines, wires, and other conduits that move things from place to place. Sun-paths move sunlight. A sun-path, as defined by that first generation of U.S. solar designers in the 1940s and ’50s, runs across the “sky-vault” from our star to windows and walls. Sun-paths move by the hour and season, setting the angles for indoor illumination in December and artificial shade in June (and vice-versa south of the Equator). Any architect can sketch a passive house in Boulder or Berlin. It is harder to design a neighborhood of such houses and harder still to create and manage sun-paths at ground level and neighborhood scale. Those photonic conduits require an urban infrastructure. What would it look like?
As a first principle, solar infrastructure would do no harm. Each new building would come with a “daylight envelope”: the share of sunlight it may absorb without depriving adjoining real estate of adequate light. At noon a penthouse should not overshadow a ground-floor apartment across the street. Of course, urban people have been fighting for rights to light for decades—and largely losing. Those rights are private. Residents fight for their own sunlit windows. In so doing, they defend the surface rather than the path—a local, limited claim.
The paths make a much larger grid. We understand that expansive, inclusive logic for the internet. Thanks to activism, net neutrality prevails and keeps megabytes flowing to rich and poor users alike. No one casts a shadow (yet) on the internet. Cities are different, unfortunately. New York’s largely empty luxury towers monopolize and restrict access to sunshine. The studio-dweller must turn on the lights at midday. Perhaps tall, thin palaces produce more tax revenue than the added electricity costs the dimly lit neighborhood below. But the neighbors don’t collect that subsidy directly. And—to think beyond solely energy—money will not substitute for the daily rays from our star. We should tear down billionaire’s lofts, recycle their materials, and restore the sunshine grid.
A second principle of solar infrastructure would regulate and enhance the indirect sun-paths passing between surfaces. Reflection is a form of infrastructure. My bedroom lies on the northern side of my New Jersey house. During much of the day, light strikes the southern, white siding of my northern neighbor and bounces into my bedroom. It is bright enough to wake me up, illuminate the space, and even enable a game of shadow puppets. At noon in the winter, I receive natural illumination and warmth from both the south and the north. No one designed the block that way. Architects do plan “solar neighborhoods” of terraced edifices and interdigitated short and tall structures, but they almost never get to build them. The social obstacles are too great. Neighbors compete for sunlight and views. When the stakes are high enough—over an ocean view in California, for example—some residents will agree to mutual constraint. Few, however, who can afford to remodel consider someone else’s claim on neighborhood energy. Reflectivity has hardly even entered our urban vocabulary. The tiny city-state of Liechtenstein thinks a little differently: its new Parliament building draws light bounced off a pale concrete retaining wall. This is one example of smart solar infrastructure.
The technological trick here is to make light go around corners, as pipelines do for water and roads do for cars. Scientists at Purdue University have invented a white paint that reflects 98 percent of light (as opposed to the usual 80-90 percent). They recommend it for roofs, as a way of reducing heat absorption and the need for air conditioning. Why not paint exterior walls this color so they reflect onto the neighbors? Mirrors would do the same work better, and one could direct and dim them remotely (using minimal electricity). Within a building, a one-foot-wide tube lined with reflective material will bring sunlight from the roof, around a curve, and through an attic or utility layer to habitable space. Without expending any electricity, a dome at the top harvests light from every angle, and a diffuser at the bottom sprays it throughout the room. We could extend these “sun tunnels” as a two-floor infrastructure and pipe photons down from the roof, though the top story, and to the apartments below.
Solar infrastructure has not yet figured in policy discussions because it is profoundly unprofitable. In other words, windows reverse the incentives driving every other form of investment in energy. Electric wires occupy public space while moving a private good. The utility gets the ground it needs rent-free and charges customers for the energy it ships: a double advantage. Now Sunrun and other firms want to push sunlight, as electricity, through the same profitable wires. Conceivably, this fraction of capital could also invest in solar architecture by building houses with the best access to photons. But passive solar energy is a bad economic bet. The builder buys private land and provides free access to the energy. This is a double disadvantage for any investor in solar architecture. Maybe they can sell the passive apartment building or house at a premium, but the revenue stream stops there. There is no meter for photons passing through windows. Unless converted to electricity, sunlight is democratic, and thus unprofitable, by nature.
To keep this energy system democratic—and to contribute it toward reducing carbon emissions—we don’t need new technology. We just need to tweak our institutions. Zoning should encourage southern orientations and setbacks to maximize light. Building codes should facilitate bay windows (now made as triple-panes), overhangs, thermal masses, and solar tubes. To be sure, these features do cost something—but much less over time than electricity for equivalent purposes. For reflective surfaces, community associations should adjudicate among adjacent residents. A second-generation bill for renewables would enable and support such solar neighborhoods. Above all, we cannot mistake panels for the sun. There is much more energy—and much more available—raining from the sky into a well-glassed apartment. Long windows are the “main protagonist of the house,” argued Le Corbusier. In the long struggle for energy justice, they need human allies.
