Founders of Green City Solutions
Renewable energy

Renewable energy

Clean air for city dwellers

Moss and IoT technology for clean city air – this is the basis for the CityTrees project being undertaken by the founders of the start-up company Green City Solutions.

A CityTree in front of a train station
The air is clean

The CityTrees, made up of over 1,600 moss boxes, absorb as much particulate matter as up to 275 regular trees. Solar cells supply electricity for irrigation, advertising and a Wi-Fi hotspot.

It is Wednesday morning, and students and tradesmen bustle around the almost five hectare site, the 78-metre high Gasometer Schöneberg towering above them. The EUREF campus in Berlin-Schöneberg is considered to be the blueprint for a smart, future-oriented city in which the buildings are networked and built for maximum energy efficiency, and where you can live, research, study and work.

Dénes Honus, the Managing Director of Green City Solutions, moved into an office here last year: a full floor with a long corridor leading to several rooms with glass doors. The walls are freshly plastered, grey felt rugs cover the floor, and cables hang from the exposed concrete ceiling – the building technology is currently being upgraded to the very latest state of the art.

Dénes Honus is 30 years old and founded the start-up Green City Solutions with his three friends Peter Sänger, Victor Splittgerber and Zhengliang Wu in March 2014 in Dresden, where the four found themselves during their studies. Their expertise is drawn from the disciplines of architecture, mechanical engineering, computer science and horticulture/biology.

Office with employees at Green City Solutions
Start-up with vision

The young team and their CityTrees are enjoying success beyond Dresden – the start-up’s invention is already cleaning the air in Paris and Hong Kong. A total of more than 50 installations have now been erected in various countries.

Green City Solutions has developed a product known as CityTrees: four-metre high, vertical plant walls with moss cultures. These moss displays feed on dirty air: they filter CO₂, nitrogen oxides and particulate matter and convert these substances into their own biomass. Mr Honus and his colleagues have great ambitions: “We want to tackle one of the biggest problems threatening the quality of life of people in cities: air pollution”, he explains.

In the conference room, he talks about his start-up’s vision and the problems caused by polluted air. Outside, the sun shines, and the air seems clean – but is it really? “No, you cannot usually see pollution. Humans don’t have an organ for perceiving it. The air pollution is so high, however, that it exceeds the EU thresholds. And not just beyond the windowpanes,” remarks Dénes Honus, pointing to the windows and the open skylight, “but likely in here too.”

Around 90 percent of urban residents breathe polluted air every day. As a result, every seventh death can be traced back to the consequences of air pollution. Increasing urbanisation will further exacerbate the problem.

“Cities are running out of air – we’re doing something about it,” reads the Green City Solutions slogan. “Actually, it’s completely absurd that we haven’t been filtering the air we breathe for years. When we get water, we are used to it being clean. There is a closed circuit: dirty water flows through the sewerage system and into a treatment plant where it is purified. Why should it be any different for air?", Mr Honus asks rhetorically, pointing to the CityTrees.

The founders of Green City Solutions
The founding four

Peter Sänger, Liang Wu, Dénes Honus and Victor Splittgerber (from left to right) founded their start-up Green City Solutions in Dresden in March 2004.

A total of more than 50 permanent and temporary installations have already been put in place throughout the world, including in Germany, Norway, the United Kingdom, France and the Netherlands. “We’ve also installed our first test unit in Hong Kong to see how it operates under the local climatic conditions, so that we might target the Asian market at some point in the future,” explains Mr Honus.

Green City Solutions initially financed itself primarily from its own funds. In addition, the company received a start-up grant and made profits from the first sales of CityTrees to pilot customers. Dénes Honus cites the investment from Coparion GmbH & Co. KG from Cologne as the biggest success in the financing phase. Coparion is a co-investment fund of KfW and the Federal Ministry for Economic Affairs and Energy. Coparion participates in innovative start-ups and young technology companies as an independent company.

“Gaining Coparion as an investor in the last round of financing was a great achievement! Coparion and KfW have a high market standing and a great deal of expertise. We meet regularly to exchange ideas in Cologne and Berlin, we talk on the phone a lot – they’re a very strong partner,” he says. David Zimmer from Coparion is convinced of Green City Solutions: “The start-up offers a unique technology for reducing particulate matter in inner city areas. No other such technology that can be applied in practice exists anywhere in the world. In addition, the company has an excellent multidisciplinary team which has developed CityTrees using its own funds and prize money, and which has already successfully entered the market.”

The customer base of Green City Solutions is currently made up mainly of cities and municipalities, however, it also includes some companies, for example from the real estate sector. “We have a major customer in France, for example, who builds business parks and integrates CityTrees into the grounds to provide clean air,” says Mr Honus.

CityTrees filter city air regardless of the source of pollution. The goal for the future is that each of the vertical plant filters will be able to reduce local air pollution within a radius of up to 50 metres by about one third. They are mobile and can be set up flexibly in an urban environment, requiring no anchoring to the ground. One CityTree has the environmental performance of up to 275 regular trees and can absorb around 240 tonnes of CO₂ equivalents in particulate matter and carbon black per year. At the same time, it is much cheaper and requires only a few square metres of space.

“Purchasing a CityTree costs EUR 25,000 to 50,000 per unit. 275 trees would cost around one million euros – excluding the land and soil,” explains Mr Honus, adding: “Maintenance and care are also significantly cheaper than they would be for an equivalent number of trees.” What’s more, CityTrees ensure their own energy and water supplies using connected solar cells and rainwater collection systems. They therefore require only a few hours of maintenance per year. “Our aim is not to replace any of the city’s trees with CityTrees, however,” says Mr Honus. “Rather, CityTrees should simply serve as an efficient – and, at the same time, green – addition, capable of actively addressing the problem of air pollution.”

KfW Capital takes over

Green City Solutions was looking for an investor and found coparion. coparion was jointly founded in 2016 by KfW and the Federal Ministry for Economic Affairs and Energy to support young high-growth companies in the start-up phase working together with private investors. The fund has a volume of EUR 225 million. With the start of its business operations, the new KfW subsidiary KfW Capital took over the parent company's shares in coparion and other investments.

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A further advantage of CityTrees is that they connect the natural abilities of plants with modern Internet of Things technology: “CityTrees have sensors, hardware and software, as well as a cloud connection. These allow us to monitor the moss cultures from the office and automatically supply them with what they need to absorb as much air pollution as possible.” The water and nutrient supply is thus remote-controlled and automated. Green City Solutions’ 25 employees are now able to access all information in real-time and measure, for example, how much particulate matter a CityTree has already absorbed. “In the future, we’ll be able to see how much the pollution in the surrounding area has been reduced.”

CityTrees can also be used as an advertising space and Wi-Fi hotspot: “We also offer customers the opportunity to advertise on the tree’s surface. You can add lettering or attach displays to communicate content.” The future holds potential for Green City Solutions, particularly in the digital field: “We collect an enormous amount of data: about the installations and their performance, but also about the environment and climate as well as traffic data. We want to tap into new business sectors.”

There might even soon be miniature CityTrees for purifying the air in rooms. Discussing commercially available air filtration devices, Dénes Honus explains: “At present, there are only those big grey boxes that no-one likes – they don’t work properly and you have to change the filter every two weeks. We eventually want to offer something better.”

Looking forward, company founder Dénes Honus is convinced the problem of air pollution will not be quick to disappear from the agenda. To illustrate his point, he cites the example of emerging countries, which are severely affected by the problem of air pollution. Even 50 years from now, Mr Honus expects that there will still be power plants in urban areas generating energy from fossil fuels. "Not even electromobility is much use in this regard; at the moment more than 70 percent of particulate matter originates not from the exhaust itself, but from tyre and brake wear. As long as electric cars continue to have tyres and brakes, the problem of particulate matter will continue to be acute in the future,” acknowledges Mr Honus. “We want to be part of the solution, if not THE solution to helping people live healthy lives in cities!”

Published on KfW Stories on 10 July 2017


All United Nations member states adopted the 2030 Agenda in 2015. At its heart is a list of 17 goals for sustainable development, known as the Sustainable Development Goals (SDGs). Our world should become a place where people are able to live in peace with each other in ways that are ecologically compatible, socially just, and economically effective.