Professor Klärle, will we soon be using cities to generate solar power?

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The outlook is bright: Martina Klärle sees great potential for urban solar energy.

"Yes, absolutely! We already use roofs in cities to generate solar energy, but we need to do a lot more. There is still huge potential to be exploited here. As part of my research at the Frankfurt University of Applied Sciences we once calculated that Germany has on average just under 100 square metres of roof area per inhabitant. This is taking all the buildings in the country into consideration, from residential buildings to garages and schools, town halls and industrial facilities. What is more, we know that it would be enough to use only 25 percent of this roof area. This would produce sufficient solar power to meet the country’s total energy demand.

The only question is which roofs to choose, and which offer the ideal conditions. To find this out, I am conducting research in the overlapping field of geoinformatics and renewable energies. This involves using georeferenced data – data relating to specific physical locations – to paint a clearer picture. Rather than doing this for each location individually, however, we want to use the same formula every time. To this end, during a research project that was concluded in 2008 we developed the very first algorithms capable of calculating the solar potential fully automatically.

Is my roof suitable? Increasingly, information can be found online

This works as follows: we already have knowledge of the three-dimensional composition of the roofs thanks to aircraft scanner data and stereoscopic aerial images. And we know how high the sun will be on any given day of the year, and at what angle its rays will hit the slanting roofs. Furthermore, we are able to calculate the time of day and the time of year that the roof will be in the shade, for example because of leaves on trees. From Germany’s National Meteorological Service we also have good data about the local microclimate: which areas are prone to fog, where does the weather tend to be clear, and where are hazy conditions more likely? We then feed all of this data into our algorithms – and obtain very precise information about how much sun a particular square metre of roof will receive per year.

In recent years, we have used this information to create digital solar cadastres (only in German) that are publicly accessible online. The cadastres – a kind of map showing geographical data – provide politicians and ordinary people in over 1,000 cities with information about the suitability of their roofs for solar power generation. The German state of Hesse has even been mapped in this way in its entirety. And the good news is that the data needed to do this are available not only in Germany and Europe, but exist for many regions worldwide.

Wind can also be used in cities, not only sun

Such site-specific analyses can be used not only for solar energy systems, but also for wind power. If small wind turbines are to be used successfully in cities, it is important to know which areas are particularly windy. Typically, these will be along the ridges of roofs or in underpasses: this is known as the Venturi effect. In my research project WIND AREA we use 3D modelling and algorithms to identify the best sites for harnessing wind energy in a city. Again, we feed large quantities of data into the system and the results are calculated automatically.

I wanted to use my research to show that cities, although they consume a great deal of energy, also offer considerable potential when it comes to producing energy. In our cities we have roofs that catch the sun during the day, and corners of the cities that experience strong winds, especially at night. It would therefore make good sense to use a combination of the two. In the heart of the city, we could set up a power station that would harness different renewable energies and generate more power than any solar farm in the countryside."

Martina Klärle has been a professor of land management since 2007 and vice president for research, continuing education and transfer at Frankfurt University of Applied Sciences since 2019. Her research gave rise to the SUN AREA project, an innovative method of analysing the potential for solar energy production. Professor Klärle’s research project WIND-AREA (only in German) is concerned with identifying precise wind speeds with a view to using small wind turbines in cities. Martina Klärle has won a number of awards, including the 2015 European Prize for Architecture, the 2014 German Sustainability Award and the 2014 European Solar Prize.