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Heat pump cascade at KB4 in Düsseldorf

Heat pump cascade at KB4 in Düsseldorf

New non-residential construction like the ultra modern KB4 in the Düsseldorf Business Park “Airport City” is subject to stringent legal requirements. This applies particularly to heating and cooling the structure. Modern technology makes it easy to meet these requirements. Reversible heat pumps that can be used for monovalent heating and cooling.



The KB4, the new home of the Gerresheimer Group’s headquarters, offers 4300 m2 of the most modern office space on five floors. Each floor can be divided into two rental units. The sizes of the two units can be chosen to meet the respective needs of the tenant – from a single office to a business lounge. There are storage and archive rooms as well as underground parking on the lower level. The building in which STRABAG Real Estate GmbH invested around 17 million EURO was finished in mid 2014, after about one and a half years of construction. At the end of last year a company belonging to the Talanx group acquired the building.




As dictated by the current standards governing office buildings, one planning task was climate control of the rooms. In addition to a heat generator, a cooling system also had to be installed. The requirements specified by the German Energy Saving Ordinance (EnEV) had to be met when planning the system. Paragraph 3 of the German Renewable Energy Heat Act (EEWärmeG) dictates that owners of new buildings with more than 50 m2 of usable floor space have to ensure that part of the energy needed to heat and cool the facility comes from renewable energy sources. The law recognises ambient heat, geothermal energy, solar radiation energy and biomass as renewable energy forms.


The planning office Bodzian from Duisburg was committed to not only meeting the legal requirements but also finding the most efficient and sensible solution possible for KB4. Ideas were actively sought, rejected or firmed up, then refined and enhanced. This continued until the engineer Frank Müller found a concept that met the planning office’s demands and the customer’s needs. The manufacturer Mitsubishi Electric supported the planning office throughout the entire development phase of the energy concept. “Ongoing collaboration between the planning consultants, engineers and our technical experts helps us, the manufacturer, to find an optimal solution with a system that provides the highest quality demanded by the planning office and our company,” states Bernd Brinkmann, planning advisor for Mitsubishi Electric in the state of Nordrhein-Westfalen.




The system created with nine heat air-to-water heat pumps interconnected in cascades facilitates heating and cooling at the same time. “Measurement and control technology regulates which of the outdoor units generate heating or cooling energy and how much capacity should be produced at a certain time. This allows us to provide both types of energy as needed and at the same time,” explains Bernd Brinkmann, planning advisor for Mitsubishi Electric in the state of Nordrhein-Westfalen. 

The nine heat pumps, type PUHZ-SHW230YKA from the Mr. Slim series, are designed to achieve a heating capacity of 207 kW. The outdoor units are equipped with full inverter technology, allowing the output to be variably adjusted to the current need. This means that the system works much more efficiently than systems with only on and off modes. Regular compression of a machine is between 30 and 100 percent. This means that modulation of about 7 kW – for one pump in part-load operation at 30 percent – can generate up to 207 kW when nine devices are running at full-load. “The control strategy is to let the single heat pumps work in part-load mode when possible. This is more efficient than, for example, having four heat pumps work at full capacity while leaving the other five switched off,” explains Brinkmann.



The devices are equipped with the Zubadan technology, meaning that they work reliably even at low outdoor temperatures. The so-called “flash injection” ensures a stable refrigerant mass flow even at low outdoor temperatures. This means that 100% heating capacity is possible at temperatures as low as -15 °C without the need for additional heat generators such as an electrical heating rod. Proper functioning of the devices is guaranteed even at temperatures as low as -28 °C. 

“With this technology the heat pumps work economically in the winter as well, because the system can be used as a monovalent heat generator,” says Ulf Berens, managing director of Kältetechnik Berens, the company installing the climate-control equipment. Another advantage is that, because the units heat consistently, they can be smaller than models without Zubadan technology.



The nine outside units are interconnected in cascades, but each machine can work individually in heating or cooling mode. On cold days the nine units can all generate heat, while on very hot days all nine heat pumps can cool the building. During the transitional seasons, when sunlight heats the offices on the south side of the building and they need to be cooled, while the cooler north side is chilly and needs heating, some of the units can generate heat and the others cold. To achieve the optimum energy yield, the outside units installed on the roof of the building are arranged in a U-shaped pattern. This arrangement causes the exhaust air to be blown into the space between the heat pumps, which heats or cools the intake air and facilitates more efficient operation of the system.



A special control technology makes demand management possible. A PAC-IF012B-E connection kit was installed for each heat pump. Multiple heat exchangers can be connected to outdoor units from the Mr. Slim series using this kit. They can be connected e.g. to ventilation units or air curtains. 

In KB4 the heat pump system was integrated into the central building control system using the connection kit. This allows different settings to be made via external signals: The output can be set in eight steps from 30 to 100 percent, heating or cooling mode can be selected, and the compressor can be stopped. The control system also shows all of the relevant operating data, including compressor mode, operating mode and defrosting. Each unit is connected separately to its own heat exchanger via refrigerant hoses. The water hoses transfer the energy to two warm water and one cold water buffer tanks. These constellations mean that there is no refrigerant mass flow inside of the building.



The three buffer tanks dispense the amount of water needed to the hot water and cold water circuit in the building. The buffer tanks prevent unnecessary switching of the heat pumps, enabling them to work more efficiently. Heat is distributed via concrete core activation that bears the basic heat load. Radiators were also installed to provide enough heat at peak times. Cool air is distributed via a central ventilation system. This meets the planning specifications dictating that there be no air conditions in the building.


A result that fully meets expectations: An innovative air-to-water heat pump system was installed in KB4. It not only meets the legal requirements, it also acts as a monovalent heat generator and can heat and cool at the same time. The fact that one system handles both functions means that KB4 benefits from a simple and sustainable solution as compared to the classic equipment with heat generators and an air conditioner.

A system that works efficiently even at lower temperatures was consciously chosen. The devices are switched via a suitable connection set in such a way that they efficiently produce the amount of cold and hot water needed – even at the same time. Since the final result meets virtually all of the expectations, a nearly identical concept is being implemented in the AERO building next door, a twin to KB4.