The Ultra-Efficient Farm of the Future Is in the Sky

Visit a rooftop lab where researchers demonstrate how growing crops under solar power can yield clean energy and food.

At Colorado State University, a very unusual garden is located five stories above the ground beneath a lengthy row of rooftop solar panels. It's nine in the morning in late October, thirty degrees Fahrenheit, and the wind is cutting. The last 600 pounds of frost-intolerant crops for the season had been removed by researchers from the substrate beneath the panels not long before I arrived. Cool-season crops like leafy greens (arugula, lettuce, kale, and Swiss chard) continue to grow in their place, protected from the intense sun up here.

Horticulturist Jennifer Bousselot oversees this expansive, sensor-rich outdoor laboratory, unlike any other green roof. The goal of rooftop agrivoltaics is to simulate a forest above a structure. Solar panels can promote plant growth in the same way that towering trees shield undergrowth from sun stress. The ultimate goal is to increase food production to feed the world's rapidly growing urban population while conserving water, producing clean energy, and improving building energy efficiency.

"When you take a step back and consider the fundamental needs of our society, you'll see that they will always be food, energy, and water," says Bousselot. "You can produce, especially in a primarily unused space, two of those things and conserve the third," according to rooftop agrivoltaics.

And strikingly so: Because the crops are shaded, less water evaporates, resulting in a third of the water used in rooftop agrivoltaics compared to full-sun rooftop agriculture, according to preliminary data. (Here, sensors beneath the panels measure the air temperature and the soil's temperature and moisture content.) Even if water does evaporate, the solar panels overhead benefit greatly from it because they lose efficiency with increasing heat. In essence, the soil and plants "sweat" water vapour into the boards to cool them and prevent overheating. If a rooftop system is intentionally collecting rainwater, say in tanks that are to be emptied outside of the rainy season, it can become even more efficient.

In addition to reducing solar radiation on a bare roof, green roofs increase a building's energy efficiency by roughly 10%. In other words, during a heat wave, you don't need to run the air conditioning as much to cool the space adequately. (Although, in the case of a multistory building, the benefit only extends to the top floor.) Given the urban heat island effect, where temperatures in cities can rise significantly above those in nearby rural areas with lots of greenery to absorb heat and cool the air, this is an especially welcome benefit. Urbanization is the process of essentially bringing the country into the city with rooftop agrivoltaics. The more green space we can introduce into cities, the more we can slow the rate at which temperatures rise.

These early experiments demonstrate that crops can thrive on a roof despite exposure to higher temperatures and more wind, mainly because of the panels' shade. According to Bousselot, "we discovered last year, which was our first growing season, that chili peppers do really well on roofs." They dislike high-nutrient environments and have somewhat weak root systems. However, they enjoy the heat.

As you can see here, these scientists are also experimenting with growing plants outside of solar panels. For example, grasses provide flowers that draw pollinators, fertilising crops to produce more food. Various plant species have a range of flower colours, which draw in different pollinators, such as hummingbirds and bees. Because white flowers are the easiest to spot at night, moths are attracted to them and are far more active pollinators than you may think.

Thus, the goal of rooftop agrivoltaics is to support native ecosystems, albeit from a great height, rather than merely producing energy and feeding urban populations. As we make our way back to the stairs at the end of the tour, we spot a massive hawk perched on the edge of the roof. "What a magnificent creature that is!" Whispers to Bousselot. "The South Platte River, which flows through Denver, is directly next to us. The most charming thing about our location is that it allows us to test the hypothesis that green roofs can function as ecological islands in urban areas.

Stated differently, lush roofs can serve as passageways, facilitating the movement of species such as pollinating insects and raptors throughout a city rather than being significant, empty areas. Therefore, rooftop agrivoltaics may simultaneously support ecosystems, food, and energy systems.