Although there are different types and designs, heat pumps are fundamentally based on one principle. The refrigerant, which is circulated in the heat pump, plays a key role here. It absorbs thermal energy in the evaporator, e.g. from geothermal probes, groundwater, waste heat or ambient air. It then evaporates. A compressor then compresses it and feeds it into the condenser. There it condenses, thus becoming liquid again, and transfers the resulting heat to the water in the heating circuit via a heat exchanger. The refrigerant then passes through an expansion valve, where the pressure and thus the temperature are greatly reduced. The cycle then starts again with the absorption of the ambient heat.
In recent years, heat pumps have taken hold as an alternative to fossil-fuel heat generators and have become the new standard for heat supply, particularly for new buildings. However, they are also suitable for virtually all renovations. They are also flexible in terms of dimensioning. Compact solutions are available for single-family homes and apartment buildings; they can be scaled up for new developments or for operating thermal networks, and can be implemented in plant construction. Heat pumps can use a variety of renewable energy sources, from waste heat (e.g. from industrial processes or sewage treatment plants) to geothermal probes, groundwater, river water and lake water to ambient air. The electricity mix used to operate the heat pump determines how environmentally friendly its operation is. The combination with photovoltaics is a frequently used option that uses renewable electricity to operate the heat pump.
The heat pump’s circulation pump can also be used to supply buildings with cooling. This function, known as ‘free cooling,’ is becoming increasingly important in the face of climate change, which is likely to result in more heat waves. In free cooling, cool water is pumped through a circuit, which otherwise circulates the heating water at a higher flow temperature. Instead of releasing heat, the water absorbs heat from the rooms and cools it down. Free cooling requires significantly less energy than air conditioning, for example. At the moment, it is mainly used for commercial purposes. In future, more and more homes can be cooled in this way to make hot days more comfortable.
A prerequisite for the cooling function of a heat pump is a suitable energy source, such as geothermal probes or lake water, which can guarantee the required low temperature level. In addition, the cooling energy in the building needs to be efficiently distributed, for example via a floor circuit or thermoactive building structures (TABS). Radiators, on the other hand, are not suitable for this.
As a carrier medium for thermal energy, the refrigerant plays a key role in the heat pump cycle. The demands are consequently high. First and foremost, a refrigerant must have suitable thermodynamic properties. This means that it evaporates at low temperatures and that the temperature range between evaporation and condensing matches the heat requirement. The smaller the temperature difference between the evaporator and the condenser, the higher the efficiency of the heat pump. Different refrigerants have different applications, so can be suitable for lower or higher temperatures as well as smaller or larger systems. The choice of refrigerant therefore also always depends on the requirements in each case.
It is important that refrigerants do not have a negative impact on the climate, should they leak. In this respect, natural refrigerants such as ammonia, carbon dioxide and propane perform significantly better than synthetic refrigerants.
The latter are often based on hydrofluorocarbons (HFCs), which have a highly harmful effect on the climate if they escape from the system because they remain stable in the atmosphere for a long time. Here’s an example: two kilograms of the refrigerant R-404A produce the same greenhouse effect as 6.5 tonnes of CO₂ when released. The potential climate impact of refrigerants is described using the GWP (global warming potential) value. As the table shows, the potential environmental impact of natural refrigerants is many times lower than that of synthetic refrigerants.
Natural refrigerants can also have disadvantageous properties. Propane, for example, is highly flammable. Ammonia is less flammable, but toxic. Disadvantages like these can be offset with suitable technical strategies.
These heat pumps are located in an airtight cell that regularly undergoes leak testing. In the event of a leak, sensors would detect the increased concentrations and immediately cut off the current to minimise risk of ignition. In an emergency, the contaminated air from the cell can be neutralised with a special cleaning system. Special ventilation systems (hazard ventilation or storm ventilation) also form part of the safety concept. Though these measures require a certain level of investment, natural refrigerants pose no threat to the climate.
‘Working with natural refrigerants is well established and the handling is well known,’ says our specialist Christoph Bleuler. ‘It requires a healthy dose of respect, but no need for fear.’
Due to their climate-friendly and thermodynamic properties, we have been using natural refrigerants wherever possible for many years. Today, we use it to operate more than half of the installed thermal capacity of our portfolio. We have enormous expertise in the use of natural refrigerants and look forward to using it in future projects – for a pleasant indoor climate and healthy environment.
