(photo credit Solaris Bus & Coach S.A.): On the go throughout Berlin: the new electric buses of Berliner Verkehrsbetriebe with air conditioning based on CO2 refrigerant and BITZER ECOLINE compressors

Alternative drives is a major topic that has preoccupied the bus sector for years. Especially in the transit bus segment, the trend away from the classic internal combustion engine and towards electrically powered buses has long been a reality, including for Berliner Verkehrsbetriebe (BVG), which in 2019 ordered new electric buses from bus manufacturer Solaris. But that wasn’t the only criterion. The electric buses also needed to feature CO₂ as a refrigerant for cooling and heating – and Konvekta is one of the leading and most experienced manufacturers in the field of bus air conditioning with CO₂, which is why the company puts its trust in reliable BITZER ECOLINE compressors for its CO₂ heat pumps.

Konvekta has more than 30 years’ experience with CO₂ refrigerant and over 1,300 heat pumps operating successfully in the field – with a lightweight design and a focus on service-friendliness. Awards such as the German Environmental Award and the EBUS award are a testament to the success of the transport air conditioning manufacturer based in the German city Schwalmstadt. The Polish company Solaris, on the other hand, is one of the leading manufacturers of transit buses and coaches in Europe and a market leader in electric mobility in European public transport. These two industry giants complement each other perfectly – and, with BITZER, form the perfect trio to fulfil all of BVG’s requirements, as the BITZER ECOLINE reciprocating compressors for transcritical CO₂ applications form the core of the Konvekta heat pumps.

Perfectly coordinated: ECOLINE compressors and Konvekta CO₂ heat pumps

Sebastian Fink, Head of Testing and Validation at Konvekta, explains: ‘We began using the BITZER ECOLINE 4MTE in field testing in 2014, where it more than proved its reliability, smooth operation and stability. We then made the decision to use it in the project with Solaris. We had already used a small number of ECOLINE compressors in railway projects and, of course, were able to draw on our past experiences.’

Konvekta supplies Solaris with the UL 500 EM CO₂ HP CO₂heat pump featuring BITZER ECOLINE compressors for two different bus types: the twelve-metre nE12 electric bus is equipped with just one CO₂ heat pump, whilst the Solaris nE18 articulated bus benefits from two CO₂ heat pumps due to its length of 18 metres. Solaris then installs the compressor with double decoupling to minimize vibrations in the vehicle. But thanks to the exceptionally smooth operation of the BITZER ECOLINE 4MTE, system operation is imperceptible to the passengers inside.

The CO₂ heat pumps are perfectly tailored to the ECOLINE compressors and enable especially efficient, reliable operation of the electric buses. There were, of course, also unique features in the design of the CO₂ heat pumps. Sebastian Fink: ‘The high requirements in terms of tightness required brand-new concepts in production. Key points that we had to take into account included the provisions of the DIN EN 378-2 standard regarding the strength pressure test and leak check as well as safety concepts in industrial production and mobile application.’

The benefits of CO₂

Whilst CO₂ is not all that common for bus applications, as it often requires comparably expensive technologies, the refrigerant is future-proof and boasts long-term availability. CO₂ also fulfils all the eco-friendliness and sustainability criteria – as a natural refrigerant that, with a GWP of 1, has a negligible greenhouse effect.

Even when the outside temperature is 45°C, the Konvekta CO₂ heat pump achieves a good COP in cooling mode – and demonstrates that cooling with CO₂ also works with high temperatures outside. In heating operation, the CO₂ heat pump delivers reliable results down to -20°C. With an outside temperature of -10°C, for example, it achieves a heating capacity of 19.3 kW with a COP value of 2.3. ‘Figures like these cannot be achieved with any other refrigerant,’ says Björn Kern, Head of Technical Applications at Konvekta. ‘It just goes to show that BVG made the right decision with CO₂ for its new electric buses.’

Development of the CO₂ heat pump system began in 2017, with the series first delivered to Solaris in 2019. And even though the project is ongoing, Björn Kern has already made up his mind: ‘We’re more than satisfied with the initial results and our experiences using CO₂ heat pumps in electric buses. Of course, the partnership between Konvekta, Solaris, BVG and BITZER has been very good, which plays a key role.’