New Design of a Bimetallic Finned Tube for the Use in Latent Heat Thermal Energy Storage Units

Abstract

The use of finned tubes as enhancement method to increase the heat flow rate into a phase change material, which has in many cases a low thermal conductivity, is a common method. A highly efficient and easy-to-assemble solution for finned heat exchanger tubes is a key component for innovative thermal energy storage systems which play a key-role in electricity production and industrial heat management. In the present article the results of the investigation for different designs of bimetallic heat exchanger tubes is presented. These tube designs are developed for the use in latent heat thermal energy storage systems (LHTES) at a medium temperature range. For the use in latent heat thermal energy storage systems, the probably high pressure of the heat transfer medium and the high temperature differences between the operating temperature and the ambient temperature are challenging. Therefore, the bimetallic finned heat exchanger tube consists of a steel tube, where the heat transfer fluid flows, and an aluminum tube with longitudinal fins, which should improve the heat transfer to the phase change material. Due to different thermal expansion coefficients, displacements of the tubes are given. To guarantee a high heat transfer rate between the two connected tubes the contact between aluminum and steel plays an important role. In the present study 4 prototypes (including the new design) were designed, analyzed and compared on the connection strength. Long-term tests for simulating the application in a LHTES were done to determine the creep rupture properties of the compositions. All prototypes were tested successfully; the new design is convinced in many aspects of that challenge and is submitted to the Austrian patent office. Main advantages of the new design are the simple production and assembling compared to other analyzed prototypes. Furthermore, the new design shows the best results under the analyzed operation conditions and the layout of the geometry has a high optimization potential in terms of stresses.

Transient Numerical Simulation of the Melting and Solidification Behavior of NaNO3 Using a Wire Matrix for Enhancing the Heat Transfer

“…A widely used method to overcome this heat transport limitation in LHTES is to increase the specific surface of the heat exchanger tubes. Various techniques, like fins [4][5][6][7][8][9][10][11], dispersed high-conductivity particles inside the PCM [12,13], metal and graphite-compound matrices [14][15][16], micro-encapsulation [17][18][19] as well as a combination of metal foam made of copper and sandwich structure fins [20] have been suggested and were analyzed.…”