CO₂ heat pump for a residential complex with 28 homes

At the InSite residential complex in Bassersdorf, a modern CO2 air/water heat pump, supported by an isobutane heat pump, acts as a reliable source of heat and hot water for the 28 homes spread across three buildings. The installation shows how fossil fuel heating systems can be replaced efficiently and sustainably even in large residential complexes.

Instead of using oil, the 28 homes in these three blocks of flats are now heated using renewable energy.

CO₂ as a solution for renovations

Thanks to modern technology, two heat pumps running on natural refrigerants are in use at the InSite residential complex. They have proved to be an efficient and fully sustainable solution for the replacement of the previous fossil fuel heating system, especially as district heating, stockpiling wood and using geothermal or ground heat are not viable options for the site.
Zurich cantonal electricity company EKZ (Elektrizitätswerke des Kantons Zürich) is supplying the three buildings, containing a total of 28 freehold flats, with heat for heating and hot water as part of a contracting scheme. At the heart of this system is a CO₂ heat pump with an output of around 120 kW, supported by a compact isobutane heat pump.

The heat pump heating system in use

1 × CO2 air/water heat pump with 120 kW heat output

1 × isobutane water/water destratification heat pump with 10 kW heat output

3 × decentralised water heaters with freshwater modules

High heating output temperatures possible

Transcritical CO₂ installations have proved their worth in industry and commerce and also offer benefits in residential construction. They allow for high heating output temperatures, meaning they are very well-suited to providing hot water for multiple buildings. CO₂ also has the advantage over other natural refrigerants that it is neither inflammable nor volatile, and is non-toxic in small quantities. However, CO₂ can pose a risk of suffocation when present in too high a concentration. This is why the heat pump on the InSite residential complex is located in a separate machine room and fitted with gas sensors and emergency ventilation in case any refrigerant gas leaks out.

Robust technology for high system pressures

CO₂ heat pumps operate at a very high system pressure of 80 to 130 bars. The requirements for using the devices are correspondingly strict. Via the targeted optimisation of various components and processes, such as ejectors and partially flooded vaporisers, the developers at CTA optimised their CO₂ heat pump for larger housing developments. This means that the costs of the CO₂ installation remain economical throughout its service life.

Denis Vres (r.),Renovations Director at EKZ

Working together to find the right solution

“Projects like this show convincingly what can be achieved with technology when everyone pulls together.” → More about energy contracting at EKZ

Focus on hot water

The heat is generated via a cooler, which draws in heat from the surrounding air. This heat is required by the CO₂ heat pump to vaporise and heat the refrigerant to over 100 °C. Unlike systems that use other refrigerants, transcritical CO₂ installations use a gas cooler instead of a condenser. This cools the superheated CO₂ from 110 °C to 32 °C without it precipitating. The remaining 70 °C heat is then transferred to a buffer storage tank and into the local heating network for the other buildings.

Heat distribution in the buildings

Each building has a heat reservoir with two 700-litre tanks to store heat for heating and hot water. Freshwater stations are used to provide drinking water. They heat the domestic hot water via a heat exchanger using the water at 70 °C from the storage tank as soon as it is actually consumed. In this way, the thermal energy is used optimally and it is also the right choice from a hygiene perspective. During the winter, the heating also draws from this storage tank. This allows for underfloor heating at a flow temperature of 35 °C

The isobutane heat pump – the joker in the pack

A large temperature spread is required for the CO₂ heat pump to function optimally. The return temperature should be below 32 °C. The lower the return temperature, the more efficiently the heat pump operates. This is where the isobutane heat exchanger pump, the ‘joker in the pack’, comes into play: If the return temperature from the houses is over 35 °C, the isobutane heat pump ensures that the return temperature to the CO₂ machine does not rise any further and that the operating range remains under 45 °C. The refrigerant circuit of the heat pump is filled with only 600 grammes of isobutane. Nonetheless, the heat pump has a power output of 10 kW for return cooling. The heat extracted is then transferred to the heat storage tank. On days when the return temperature from the houses is under 30 °C, the isobutane pump is not required and the return is sent directly from the return reservoir to the CO₂ heat pump.

Heating centre and substations in blocks of flats: Should the return temperature become too high, the isobutane heat pump ensures that the return temperature to the CO heat pump’s gas cooler always remains below 45 °C.

Positive experience

The initial experience of generating heat in this way in Bassersdorf has been promising. Using CO₂ as a refrigerant and the transcritical use of heat pumps is a trendsetting use case when replacing fossil fuel heating and hot water generation in housing developments. The system design is based entirely on renewable energy, and is highly efficient and economical.

Contributors

Planning and installation: EKZ Contracting AG
Heat pump and system supplier: CTA AG

Comprehensive range of services

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