How the brine/water heat pump works briefly explained

To better understand how a brine/water heat pump works, it is worth taking a look at the structure of the system. The actual heat pump is located above the earth's surface and has two heat exchangers in addition to the compressor – the evaporator and the condenser. In practice, either geothermal probes or geothermal collectors are used to extract heat from the ground.

Collectors and probes supply the heat pump with thermal energy

Geothermal probes are sunk deep into the earth with the aid of boreholes. Geothermal collectors, on the other hand, are located just below the surface, but have larger surfaces that can absorb the thermal energy. Both consist of a closed circuit in which a frost-proof liquid (the brine) can circulate. An integral pump ensures that the brine keeps moving and transfers the heat from the ground to the heat pump.

A refrigerant is evaporated with the thermal energy obtained through geothermal collectors or probes. Due to its special thermal properties, its physical state changes at low temperatures. With the added heat, the refrigerant becomes vapour-like and its temperature increases. To raise this to the required level, the refrigerant vapour is compressed with a compressor. This not only increases its pressure, but also its temperature. In a second heat exchanger (condenser), the refrigerant vapour transfers the previously generated heat to the heating system and condenses. Before the recondensed refrigerant can absorb geothermal heat again, it is firstly expanded in an expansion valve. In the process, both its temperature and pressure fall. Once the latter has reached its initial state, the process can start again.

Efficiency depends on many factors

The heat pump generates heat by firstly evaporating the refrigerant with the ambient energy and then compressing it. The heat pump requires drive current for this compression process.

VDI Guideline 4650 is used to predict the efficiency of a system of this kind. This calculation method calculates a seasonal performance factor based on the COP of the heat pump and different system parameters. The COP represents the instantaneous ratio of useful heat generated to the drive energy used in the form of electricity under standardised conditions. The SPF, in turn, is the sum of all COPs that have occurred within one year. To determine the actual SPF, the heat and electricity quantities (read off from the heat and electricity meters) need to be considered together.

The differential between the heat source and the flow temperature of the heating system has a decisive impact on the efficiency of the system. If, for example, the initial temperature is ten degrees Celsius and the flow temperature is 30 degrees Celsius, the brine/water heat pump only has to increase the refrigerant by 20 degrees Celsius. If the heating system consists of radiators with small surfaces and a minimum flow temperature of 50 degrees Celsius, the compressor must use twice as much energy to achieve the temperature required.

Brine/water heat pumps for mono mode operation

Thanks to the relatively constant and high level heat source, brine/water heat pumps work very efficiently all year round. Depending on the area of application, they usually provide sufficient heating and domestic hot water as the sole heat generator. In some cases, combined operation with an existing heating system can also make sense. The latter can be switched on during peak loads and will guarantee a high level of comfort within living spaces at all times. To ensure economic operation, several points need to be considered in advance. You can find detailed information and tips on this in the section entitled Heat pump.