Overheated cars are constantly parked underground: their waste heat can cause engines to heat up underground car parks to such an extent that the groundwater in the surrounding underground also warms up considerably, researchers report. According to them, this could harm water quality. However, this problem may be ideally solved through geothermal use of the elevated temperature. Using Berlin as an example, the scientists calculated that the underground car parks there release so much energy into the groundwater that they could supply over 14,500 households with heat.
As is well known, there is an extreme lack of space, especially in large cities. That's why there is not only a lot of building going up, but also going down. These areas are often used as parking spaces: the underground of many cities is now characterized by underground car parks, sometimes on a gigantic scale. These rooms are heated up considerably by the waste heat from car engines, so they are usually a few degrees warmer than the surrounding ground. It can therefore be assumed that they play a role in the groundwater temperatures in urban areas that have been rising for decades.
Scientists led by Maximilian Noethen from the Martin Luther University Halle-Wittenberg (MLU) have now devoted more detail to researching this aspect. They examined the temperature in 31 underground car parks in various cities in Germany, Austria and Switzerland. In six of them they also recorded the temperature of the groundwater in the immediate vicinity of the systems. In this way, a heat profile could be created for the locations, which made it possible to draw conclusions about the heat effect of the underground car parks.
On the trail of the heat effect
As the team reports, their results initially made it clear that the groundwater warms throughout the year. As far as the extent is concerned, the scientists were now able to document the importance of important factors in more detail: The traffic volume in the underground car parks had the greatest influence. “Since public underground car parks are often deeper and are used for shorter periods of time, they warm the groundwater more than private systems,” says Noethen. In addition, the proximity to the groundwater and the previous groundwater temperature also play an important role in the effect, explains the geoscientist.
The team examined the effect of underground car parks on groundwater most closely in the case of Berlin. There they integrated all known parameters into a model of the overall effect of the city's 5,040 underground car parks. As the researchers report, a particularly intense effect was clear there. In the central districts, many underground car parks are located in or close to groundwater, which means that a particularly large amount of heat is given off to the medium. Specifically, their calculations showed that around 0.65 petajoules of energy are transferred from the underground systems to the surrounding area in Berlin every year. To estimate this magnitude, the scientists write: Theoretically, around 14,660 households could be supplied with heat.
Energy generation and preservation of water quality
This is exactly where the researchers now see the possible use of the heated groundwater: it could be used through geothermal energy processes. The thermal potential of groundwater alone is not sufficient for heating. But this is possible through the process of “high transformation” using heat pumps. “Of course, the heat from the groundwater is not enough to cover the heating needs of a city like Berlin or even a country. However, we know from previous work that geothermal energy can play a significant role in sustainable heat supply,” says senior author Peter Bayer from MLU.
As the scientists emphasize, using the increased groundwater temperatures would also result in an important second positive effect: “The advantage of this would be that energy is removed from the groundwater and it cools down,” says Noethen. This would bring it back to normal levels, which could benefit drinking water quality. As the team explains, increased temperatures could unfavorably shift the species composition of microorganisms in the water. “This could also change the quality of the groundwater from which we get a large part of our drinking water. This development needs to be controlled through a variety of measures,” concludes Bayer.
Source: Martin Luther University Halle-Wittenberg, specialist article: Science of The Total Environment, doi: 10.1016/j.scitotenv.2023.166572