A large untapped source of energy could be right underneath our wheels, a new study shows.

Cars parked in underground parking lots throw off so much heat that it warms up groundwater beneath them. This wasted energy could be enough to supply over 14,600 households with heat in Berlin alone, researchers report in the journal Science of the Total Environment.

Harnessing the wasted heat would also help cool down groundwater, which would benefit biodiversity and water quality. Higher groundwater temperatures have been shown to be a threat to groundwater fauna and increase bacterial growth in drinking water, the team writes.

Dense urban areas are known to be warmer than surrounding rural areas because building materials like asphalt and concrete tarp heat. This urban heat island effect will increase as the world becomes increasingly urbanized. More than half of the world’s population lives in urban areas today, a share that is expected to go up to two-thirds by 2050, according to the United Nations.

Cities typically deal with rising populations by increasing the height and depth of buildings, which leads to a steady growth in the number of underground car parks, Maximilian Noethen, a geoscientist from Martin Luther University Halle-Wittenberg and his colleagues write in the new paper. Waste heat from car engines warms up these underground lots, so they are typically several degrees warmer than the surrounding subsurface, making them a heat source for ambient subsurface and groundwater.

So the researchers investigated how these hot spots can impact urban subsurface warming. They installed small temperature-logging devices in 31 underground car parks across Germany, Austria, and Switzerland. For a year or more, these devices measured the temperature of the ambient air and groundwater every few hours.

The results showed that the parking lots heat up groundwater all year round. The volume of traffic, the proximity of the lots to the groundwater, and ambient groundwater temperatures influenced how much the subsurface water got heated. “Public underground car parks heat up the groundwater more than private facilities as they are often deeper and the cars park there for shorter periods of time,” said Noethen in a press release.

Then, using their recorded data along with openly available data on air temperatures, the team used computer modeling to extrapolate their analysis to 5040 underground car parks across Berlin. Over the year on average, all underground lots in Berlin acted as a heat source, they calculated, emitting 652.6 Terajoules of heat energy into the groundwater. That energy is equivalent to the heating demand of 14,660 average German households or 29,639 people.

Integrating heat pumps into existing ventilation systems might be a way to utilize this waste heat energy from underground parking lot air, the researchers say.

Source: Maximilian Noethen et al. Thermal impact of underground car parks on urban groundwater, Science of The Total Environment, 2023.