A carefully calibrated combination of rooftop gardens and solar panels could meet 15% of a city’s vegetable needs and provide 5% of its electricity on average, according to a new China-wide study.

Growing food on urban rooftops is a potentially powerful sustainability strategy. So is generating electricity from rooftop solar panels. Alas, any given rooftop spot can host only one of those two options. So how to decide which is best?

In the past, research and planning have usually considered rooftop agriculture and solar separately. Those that have looked at the tradeoffs between the two possibilities have tended to be limited to a single city or small region.

In the new work, researchers analyzed the potential of rooftop agriculture and rooftop solar across 124 Chinese cities with a population of more than 1 million people.

The researchers first built a database of 13 million buildings from real estate websites and Google and Baidu Maps. This enabled them to identify flat roofs on low-rise buildings that would be suitable for either rooftop agriculture or solar panels, as well as pitched roofs and high-rise buildings that could host solar panels but not gardens.

Their analyses assume that half of the theoretically suitable area would actually be used for rooftop agriculture or solar panels (some rooftop space is needed for other purposes, such as fire escapes). Maximizing the rollout of rooftop agriculture could meet on average 24% of a city’s vegetable needs, the researchers report in the journal Nature Cities. Using rooftops for solar production only would provide 10% on average of a city’s electricity.

 

 

Recommended Reading:

 

Which roof beats urban heat better, the white one or the green one?

 

 

“Urban rooftops can play a significant role in enhancing the self-sufficiency of urban food (vegetable) and clean energy demand,” says study team member Xuemei Bai, an urban sustainability researcher at Australian National University in Canberra.

The potential of each strategy varies from once city to another, depending on factors such as the local climate, population density, and what kinds of buildings a city has and how close together they are. For example, maximum rooftop agriculture could provide between 3 and 163% of vegetable needs, depending on the city, and maximum rooftop solar between 1 and 47% of electricity.

The researchers also calculated the economic and environmental benefits of rooftop solar and rooftop agriculture. “We found that agricultural production in most cities yields better economic benefits than power generation per unit area, primarily owing to the significantly higher price of vegetables,” the researchers write. Overall the economic benefit of rooftop agriculture is more than three times that of rooftop solar.

The relatively low productivity and economic of rooftop solar are somewhat surprising, Bai says. These findings may be due to the expense of solar panels and the high industrial energy demand in Chinese cities, she adds. On the other hand, when it comes to reducing greenhouse gases, rooftop solar shines: its benefits are fully two orders of magnitude greater than those of rooftop agriculture.

This push-pull prompted the authors to analyze how to combine the two strategies for maximum benefit. If gardens were planted on 61% of the flat rooftop area on average (ranging from 15-99%, depending on the city) and solar panels on the remaining flat rooftops, as well as the pitched ones, this would preserve at least half the maximum potential benefit of each strategy, they found.

This setup would meet 15% of a city’s vegetable needs on average (ranging from 0.5-99%) and 5% of a city’s electricity needs on average (ranging from 0.5-27%). The economic benefits of this strategy amount to 1.7% of national GDP and would save 1.6% of national greenhouse gas emissions.

The strategy would require significant resources too, including up to 15% of urban residential water use in some cities, which could pose a strain on water-stressed cities for example.

Still, the basic method could be used to analyze the optimal way to balance different rooftop uses in other cities around the world, the researchers say. “Some national and city governments have mandated the installation of solar [panels] in new construction projects,” says Bai. “Our work shows that cities need to find out the most suitable ways of utilizing their rooftop spaces, considering & optimizing multiple options, their benefits and costs.”

Source: Yang R. et al.  “Urban rooftops for food and energy in China.” Nature Cities 2024.