Booster heat pumps with zeotropic mixtures in ultra-low temperature district heating networks



Greater Copenhagen area

Denmark is moving towards an energy supply, which is completely based on renewable energy sources, and heat supply by district heating (DH) networks is accordingly considered to play a major role to achieve the climate targets. It has been shown, that operating the DH networks at lowered temperatures, as so called ultra low temperature district heating (ULTDH) networks, can improve the overall performance significantly. ULTDH networks operate at forward temperatures of around 40 °C, which are just as high to directly supply heat for space heating. In order to supply domestic hot water at 60 °C, the temperature needs to be increased.

Several studies have shown that so called booster heat pump is an effective technology to supply the heat at 60 °C, by using part of the forward stream as the heat source. There is currently a prototype being tested at the department and evaluated with respect to the dynamic behavior.

While the currently available prototype operates with the pure working fluid R134a, we have recently shown that a performance increase of up to 45 % can be achieved, if a zeotropic mixture is used as working fluid. The heat pump required to obtain some modifications with respect to the construction.

A possible student project could therefore focus on the following:

-          Analysis of the use of specific zeotropic mixtures in the specific application

-          Identification of required modifications to ensure optimal operation when using mixed refrigerants

-          Simulation based evaluation of the suggested modifications

Based on the progress of the student project and the progress of the ongoing test rig evaluation, the project could furthermore include:

-          Modification of the test rig at the department

-          Experimental validation and evaluation of the suggested modification


The specific focus of the project can be discussed and any suggestions from students are very welcome. The candidate will be supervised and supported by members of the Section for Thermal Energy at the Mechanical engineering department.

It is possible, that some of the required skills (such as e.g. dynamic modeling with Modelica) are learned in a special course.

The project is conducted in collaboration with Danfoss and according communication and coordination with Danfoss during the project is intended.

For questions, please contact Benjamin Zühlsdorf ( and/or Wiebke Meesenburg ( 


Background in Thermodynamics, Heat Pumps

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