100% CO2-Emission Neutral, Solar Power Driven Air Conditioning with Water as Refrigerant

The Customer

Combitherm GmbH is a renowned company for apparatus and refrigeration plant engineering run by the second generation of owners.

Project and implementation

The building heats up under solar radiation and previously window ventilation was the only possibility of cooling. Due to the F-gas regulation no standardized solution based on F-gas-refrigerants should be installed. Compared to other natural refrigerants, the use of water was ideal because it does not restrict the site of installation. Installing the eChiller in a direct adjoining room of the staircase, protects it from the weather and makes it easily accessible without any risk for operators.

The flat roof with sufficient free space was perfectly suited for own power generation by means of a photovoltaic system. As a result, the roof is not only shaded, but generates the electricity for chilled water production when being exposed to sunlight. To compensate fluctuations in power generation, 28 kWh batteries for power storage were installed. This electrical energy is dimensioned for a full summer day. The combination of electrical and thermal storage in conjunction with the eChiller’s part-load-efficient turbo-compressor covers cooling via the daily cycle including pre- and post-cooling of the building structure. At favorable outdoor temperatures, a partial or complete free cooling is driven at night, which significantly extends the lifespan of the battery storage.

Selecting appropriate ceiling cassettes, the chilled water supply temperature of about 16 °C is applied. Later time a second consumer circuit was connected to the office area on the second floor. Thus, during summer a cooling load of 25 kW is given. The design data and the power consumption in the full and partial load range of the eChiller are perfectly in line with the network load capacity of the power storage unit. The starting current of the eChiller is below 1A. The pumps, the buffer tank (500 l) and the cooler were designed based on the hydronic and energy data. In February 2018 the plant was put into operation. Using the office space (circuit 1) led initially to a utilization of only 40-60%. By exploiting the pre-cooling and after-cooling of the building structure, an annual COP (refrigeration) for the eChiller of more than 12 is aimed for. This set-up with mechanical cooling of the office and the unrestricted indoor installation of the refrigeration system is unique.


With the selection of these components, a system was created which is not only completely CO2-neutral, but also provides cooling without temperature and current peaks. The plant generating its own power, storing and using it, prevents the infeed into the public grid during periods of solar driven electrical overload.
After commissioning, the plant became a reference project and has already convinced many experts with its high energy efficiency, performance and operational safety. The previous operating experience is very good and the feedback from the operator extremely positive. However, thorough validation can only be carried out after at least 12 months of operation, but initial evaluations of days with high outside temperatures from April to June have already exceeded all expectations. It was possible to cool the rooms to the setpoint temperatures without any problem and without having to take energy from the supply network.

Technical Facts
Cooling capacity (kW) approx. 25 kW
Chilled water temperature 16 °C
Annual COP (refrigeration) start Feb. 2018, calculated after 12 months, target > 12
Refrigerant R718 (Wasser)
Cooling water inlet 40 °C
Sound level 68.6 dB (A), pressure level in 5m: 42 dB (A)
Cooling Air-cooled dry cooler