The PET rotary heat exchanger research project is being carried out in cooperation between the Gesellschaft zur Förderung technischen Nachwuchses GFTN e.V., Darmstadt, and Klingenburg GmbH, Gladbeck. The main focus of the research project is the substitution of the aluminum by PET for the production of the storage mass. The PET shall use the advantages of plastic and increase the efficiency of the rotary heat exchanger up to 90% by a geometrically optimized shaft structure. The design options for aluminum are limited by the yield strength. This leads to a maximum efficiency of 85%. A further advantage is the recyclability of the PET. The reuse of the material leads to a sustainable and future-oriented alternative to the existing heat exchangers. The aim of the project is to develop the complete production cycle in a process chain. The process chain is to include extrusion, embossing and deep-drawing as well as the joining of the films with subsequent winding. Once the life of the heat exchanger has been reached, it will be taken back and recycled by Klingenburg GmbH.
Rotary heat exchangers are used with permitted circulating air. The main task is the supply and removal of heat energy in computer centers, office buildings, factory halls and cruise ships. Due to the large surface area and the associated large air volume, rotary heat exchangers are considered to be the most effective heat exchangers. During operation, two air streams flow through the storage mass, the supply air stream from the outside and the exhaust air stream from the inside. Each of these two flows transfers its heat energy to the storage mass. The energy absorbed is released again and the heat is transferred by the rotation in the respective other stream.
[1] The functional principle of the rotary heat exchanger is shown in Figure 1. In the further course of this abstract, the function of a rotary heat exchanger made of aluminum is compared with one made of PET on the basis of simulation results. with the thermal resistance:
If the layer thickness in equation 2 is continuously smaller with the same denominator, the thermal resistance also becomes arbitrarily low. This means that the significantly higher thermal conductivity of aluminum (approx. factor 1000 better than PET) continuously has a considerable influence.
The first simulations were created with ANSYS 18.2 Fluent © CFD software. The geometry used for this was 2.1 mm wide, 0.5 mm high and 5 mm long. Two channels were provided with a height of 0.15 mm and a width of 2 mm over the entire length of this geometry. The dividing wall between them had a continuous thickness of 0.05 mm (50 µm).
In Fluent © , a stationary state was simulated with an air velocity of 1 m/s flowing through both channels. The temperature of the supply air flow duct is 0°C and the temperature of the exhaust air flow is 25°C. The thermal boundary conditions on the outer walls were defined as adiabatic and the material properties varied between aluminum and PET.
Looking at the results in Figure 3, it can be seen that PET has a more favourable temperature profile for heat exchangers. In the case of aluminum, a constant temperature has been set over the entire cross-section, while the PET volume body has a higher temperature difference in the air flow channels.
The GFTN research team would like to thank the BMWi and the cooperation partner Klingenburg GmbH for their support and cooperation in this research project.
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