The multifunctional green façade reduces the temperature on site in the summer and improves air quality while also relieving the burden on the city’s sewer system by cleaning grey water. The solar panels attached to the façade generate electricity. The VertiKKA has been running as a »Reallabor« on the façade of the Bauhaus-Institute for Infrastructure Solutions (b.is) at Coudraystraße 10 since 5 September 2024. Maria Hartmann M.Sc., scientific employee at the Professorship of Building Physics, and Gloria Kohlhepp M.Sc., scientific employee at the Professorship of Urban Water Management and Technologies for Urban Material Flow Utilization, are overseeing this testing phase and will continue to develop the VertiKKA using their findings.
 
Ms. Hartmann, Ms. Kohlhepp, it goes without saying that numerous residential buildings have already been outfitted with photovoltaic units, grey water purification systems, and façade greening. The innovative essence of the VertiKKA is the interaction of these three elements — how does this work, and why are they more effective together than separately?
 
Gloria Kohlhepp: The VertiKKA is made up of three layers. Closest to the building wall is the cleaning and irrigation layer, then come the plant and photovoltaic layers. All three layers are connected to one another work in synergy. The cleaning and irrigation layer removes pollutants from grey water. At the same time, it provides irrigation to the hanging vegetation via a hydroponic fabric. This fully eliminates having to water plants with drinking water. A major advantage of this is the nutrient content in grey water; elements that are normally considered pollutants provide important nutrients to the plants. By absorbing nutrients from the grey water, the water is purified and fertilising the plants is no longer necessary.
 
Maria Hartmann: We were able to show in earlier VertiKKA projects that these are good growing conditions for plants. But a south-facing green façade where the plants are exposed to intense and continuous sunlight is also possible thanks to semi-transparent photovoltaic modules that not only protect the plants from »heat stress«, but also produce electricity. The plants are also able to cool themselves through transpiration. The hydroponic fabric pockets help with this. This combination, referred to as evapotranspiration, cools the photovoltaic modules, which increases their temperature-dependent efficiency. Additionally, a wind speed sensor linked to an automated control system ensure that the movable modules fold back during storms. This creates a closed surface in front of the plants and protects them from being torn out.

Would VertiKKA make sense in dense neighbourhoods that receive less sunlight?
 
GK: In principle, living spaces in cities should be designed so that sufficient light and access to fresh air are guaranteed. That said, a densely structured space could actually be advantageous for VertiKKA in some respects. Daily temperatures tend to fluctuate less in denser areas; these stable conditions benefit the microorganisms responsible for, among other things, purifying grey water. On the plant level, most plants will be comfortable even if the VertiKKA is east- or west-facing in the shade for longer periods. The trick is to choose the right plants. It’s perfectly reasonable to integrate species that do well in predominantly shady and damp conditions.

MH: We are currently testing two different models in the »Reallabor« to monitor the effects on plant vitality, one with a solar module and one without. Installing the VertiKKA without photovoltaic modules on façades that receive little sunlight would of course also be an option if it no longer made sense to include this layer.
 
How is surplus electricity produced by the solar modules used?
 
GK: The photovoltaic modules in the »Reallabor« in Coudraystraße are being put to use as a balcony power plant. The system is directly connected to the power grid with a normal plug. We are, however, currently using all the electricity generated to power the VertiKKA itself.
 
MH: Technically, the electricity generated by the »Reallabor« is significantly lower than that of a balcony power plant outfitted with a similarly-sized photovoltaic module; for the sake of the plants, the VertiKKA solar modules are 50% transparent, and only the opaque parts can produce electricity – in other words, the decision results in minor advantages and minor disadvantages. But electricity production is only one function of the system.
 
GK: That’s right. Another long-term perspective is that if the photovoltaic area is expanded, the surplus electricity produced can either be used in the building itself, just like other solar energy systems, or fed back into the grid. This will make the energy use even more efficient and contribute to decentralising power production.

Which plants are most suitable for the VertiKKA? Are crop plants an option?
 
MH: As Ms. Kohlhepp mentioned, the selection of plants should be appropriate for the location. Generally speaking, moisture-loving marsh plants are always a good fit. This is due to what is referred to as the leaf-area index, which is decisive in the intended improvement to the microclimate and for cooling the photovoltaic modules. The index specifies the (single side) leaf area above the substrate; the greater the leaf area is, the stronger the cooling effects are.
 
GK: In addition to a high leaf area index, the plants should fulfil other requirements. They shouldn’t grow high enough that they bump into the photovoltaic modules and they should require minimal maintenance, especially when it comes to regular pruning. It is also important to choose hardy plants that don’t need to be replanted every spring. Evergreen plants are especially advantageous, as they stay attractive even in the colder months. Crop plants – and we get this question a lot – aren’t appropriate for safety reasons. Although some of them can tolerate the system’s damp conditions, they also absorb pollutants from the grey water. These pollutants can accumulate in the roots of the plants, potentially making any fruiting plant unsafe to consume. To protect passers-by and prevent fruit from being picked and eaten, we decided against using any crop plants.
 
How is grey water from showers, sinks, kitchens, and washing machines treated before it enters the VertiKKA system?
 
GK: Grey water is collected in two IBC tanks that are connected at the top. The tanks are installed in the basement of the building specifically for this purpose. The grey water flows into the first tank and the overflow then enters the second tank. Solids, such as food waste or lint from the washing machine, settle at the bottom of the first tank. Kitchen grease floats on the surface and is channelled into the second tank. The water for the VertiKKA is taken from the bottom of the second tank and the grease is left behind. This system allows the grey water to be cleaned mechanically without requiring any additional effort. The VertiKKA »Reallabor« in Coudraystraße is home to two additional tanks for intermediate storage of the grey water where any remaining solids can settle and residual grease can float to the surface.  The pre-cleaned water is taken from the »clean« middle water layer of the tanks and passes through a coarse filter before reaching the VertiKKA. Once it reaches the VertiKKA, mechanical-biological purification occurs using microorganisms, which further break down pollutants and treat the water.

The VertiKKA »Reallabor« on the façade of Coudraystraße 10 has officially been in operation since 5 September 2024. Are there any preliminary findings or plans for further developing the prototype?
 
GK: The long-term reading from the VertiKKA only started coming in a few weeks ago. Luckily, we will continue to measure these results for at least another three years as part of a follow-up project, giving us meaningful data on the performance of the system. We have, however, already gained initial insights on the construction and operation of the system. For instance, we will continue developing the design of the modules so that the cleaning substrate can be replaced more easily. Our regular plant monitoring has also shown that two of the eight plant species being used are unsuitable. We will be adjusting our selection. Based on smaller test modules, we also collected initial results on the cleaning performance of the system with regard to certain pollutant parameters. For example, we observed a reduction in total nitrogen in the substrate as a result of cleaning.
 
MH: In addition to building physics measurements, we have also carried out various energy and microclimatic simulations, the results of which have been positive. For example, VertiKKA offers shade to city façades that heat up quickly and intensely. This positively influences the perceived thermal comfort of residents in the summer. At the same time, VertiKKA reduces heat transfer through the outer wall and in turn the amount of heating or cooling energy required. We have also already been able to show that - contrary to our initial expectations - there is no increased risk of condensation on the façade, despite the fact that the VertiKKA is of course a source of moisture in the immediate vicinity of the building wall. One final thing I want to mention is that we have already carried out sustainability analyses and concluded that the expanded glass granulate, which makes up the largest part of the substrate, could be replaced by alternatives including construction waste such as bricks, aerated concrete, or masonry rubble.
 
The prototype is quite striking. Have reactions been more positive or sceptical in regards to residents wanting such a module on their home?
 
MH: So far, we've had a few opportunities to talk to a variety of interested parties - most recently at the Long Night of Science - since the VertiKKA opened at the beginning of September 2024. General interest is pretty high and people are interested in how the system works. We have also received numerous questions indicating that people are really thinking about its practical use. We are always happy to discuss these questions. So far, response to the project has been consistently positive. We have even been offered test walls on houses and have received two requests from engineering firms who want to integrate green façades into their planning.
 
Ms. Hartmann, Ms. Kohlhepp, thank you so much for the interesting interview!

The BAUHAUS.INSIGHTS questions on the »VertiKKA« project were asked by freelance editor Franz Löbling.