Fig. 34 Scroll compressor.
© Viessmann Werke GmbH & Co. KG, Allendorf
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Capacity-controlled heat pumps

The heating capacity of heat pumps with on-off controlled compressors increases with rising heat source temperatures. Particularly air source heat pumps achieve the highest heating capacities when they are not actually needed. Here, the number of cycles increases while operation times decreases, which is non-beneficial for such a heating system in terms of lifetime and efficiency. One possibility to counter this is to use capacity-controlled compressors.

The current state-of-the-art technology in the output range for single-and two-family houses is the so-called scroll compressor, which has almost completely replaced the reciprocating compressor. With the scroll technology, the compression takes place between two meshed spirals.

Irrespective of the type of compressor, until now machines have been mostly used whose compressor capacity only depends on the pressure or temperature level on the evaporator and condenser side respectively. With these on-off controlled compressors there is a direct correlation between the heat source temperature and the heating capacity. Particularly with air source heat pumps, this means that the high heating capacity is provided precisely when the space heating requirement is low. The heating capacity of the heat pump and the heat load of the building only correspond at the design point. For the operation this means an increase in the cycles with increasing external temperatures. Numerous start-ups will reduce the compressor’s durability. At the same time, short cycle times cause longer operation under start-up conditions, which reduces the efficiency.

In order to ensure continuous operation, heating buffer storage systems can be used or minimum running or resting periods can be set in the control system for the compressor. Furthermore, the monovalent use of heat pumps with an unregulated heating capacity (i.e. systems with single heat generators) is more disadvantageous than with conventional heating systems, particularly when external air is used as the heat source. This would suggest the use of an auxiliary system to cover peak loads. In general this is realised by equipping heat pumps with electric back-up heaters that are inefficient in primary energy terms. It is possible to regulate the heat load by on-off controlled heat pumps using capacity stages or a rack of compressors.

There are a diverse range of possibilities for actively matching the compressor’s heating capacity to the heat load. Control systems that require intervention in the cooling circuit and involve losses (e.g. suction pressure or hot gas bypass control systems) play very little role in practice; the building sector actually uses variable-speed control or Digital Scroll™ technology to adjust heating capacity.

The almost exclusively used variable-speed control systems are also known as inverters and have already been successfully used for many years in air-conditioning technology. Using power electronics, a rotation speed corresponding to the current heat requirement is set in the drive motor. The great advantage of the inverter technology is the high COP in the partial load range. The disadvantage of this technology is the continual energy required by the electronics.

The Digital Scroll™ compressor works according to the same basic principle as the on-off controlled scroll compressor. The difference is that the Digital Scroll™ can also be operated in the so-called “unloaded” or “idle” state, whereby the motor continues to work at constant speed while the orbital scroll is disengaged. The compressor capacity is a result of the ratio of the running time in the unloaded state to the duration of a heat pump operation cycle. The disadvantage of this technology is that the capacity can only be modulated by losing efficiency.


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