In tropical cities, afternoon thunderstorms can plunge entire neighborhoods into brief moments of darkness.
When civil engineer Markus Schläpfer moved to Singapore a decade ago, he recognized these thunderstorms as an emerging engineering challenge. For cities that hope to run on solar energy, these short periods without strong sunlight could destabilize urban power grids and undermine reliability.
In a new paper , published April 7 in Nature Communications, Schläpfer and collaborators explain how tropical cities, which will soon contain half of the global population, can address this problem without expensive infrastructure build-outs. For Schläpfer, the solution lies in connecting electric vehicles to the grid.
"If you have a thunderstorm moving over an area with solar energy, you can have your electric cars that are parked serve as the energy source and balance out this lack of energy generation," said Schläpfer, assistant professor of civil engineering and engineering mechanics at Columbia Engineering. "When the thunderstorm moves away, the cars are charged again by the photovoltaics."
The hidden cost of going solar
Solar photovoltaics (PV) have become one of the cheapest sources of energy on the planet. PV energy is inexpensive, carbon-free, and reliable — when the sun is shining. When thunderstorms cut off power generation in one neighborhood, electricity has to travel from neighboring regions that are generating power. While that trip may only be a mile or two, the amount of electricity flowing through powerlines can easily overwhelm the grid's capacity.
Traditionally, fixing a problem like this would require new infrastructure, but that comes with significant drawbacks. In dense cities, such projects can be staggeringly expensive. Underground transmission lines in Singapore, for example, cost around 60 million Singapore dollars per kilometer.
"Building new infrastructure is extremely challenging and expensive in dense cities," Schläpfer said. "This is a way to use the existing network in a more efficient way and integrate more solar photovoltaics, which would otherwise need more transmission line capacity."
Batteries already on the road
Researchers across the world are exploring the possibility of using electric vehicles — namely their batteries — as a substitute for new grid capacity. The idea is simple: since electric vehicles have high-capacity batteries that connect to the grid through charging cables, the grid should be able to use the energy stored in these batteries as a backup during short-lived lulls in PV generation.
"Car batteries can feed in the electricity stored in their batteries to the grid," Schläpfer explained. "We do not need to import the electricity from nearby neighborhoods. Therefore, we do not need to install a new cable."
When a thunderstorm cuts off solar generation in a neighborhood, nearby parked cars discharge stored energy into the local grid, absorbing the shortfall without requiring power to travel from elsewhere. When the storm passes, the panels recharge the cars.
The right scale for the problem
Schläpfer's paper demonstrates the importance of scale in developing a strategy for charging and discharging EV batteries for this purpose. A conventional city-wide optimization strategy can make things worse: by smoothing aggregate demand, it allows local imbalances to accumulate, forcing the system to push large amounts of electricity across longer distances. According to the team's research, loads traveling through some transmission lines more than doubled during thunderstorms.
A better approach is managing charging neighborhood by neighborhood — in this case, across Singapore's 55 urban planning areas — to reduce maximum line loads by roughly 18 percent on storm days while also smoothing the broader daily demand curve.
"It's one of those things that only seems intuitive once you see it," Schläpfer said. "This potential hasn't really been explored before."
Where cars park matters
The method's effectiveness depends on where cars are parked. Residential neighborhoods empty out during the day, leaving fewer batteries available when solar generation peaks. Commercial districts show the reverse. The researchers mapped these patterns using anonymized, aggregated mobile phone data, which provided a level of detail that allowed for more accurate models.
Crucially, the approach works even where car ownership is low. Singapore has roughly one vehicle per eight residents.
"This solution is really working in very car-light environments," Schläpfer said. "We need only a small number of cars, and it works."
