On the Design Considerations for Thermal Energy Storage with Phase Change Materials: Material Characterization and Modelling

Licentiate thesis, 2018

Integrating thermal energy storage (TES) technologies to a process enables storing and releasing thermal energy on demand. Depending on the implementation, this may lead to economic and ecological improvements, e.g. by shifting the peak demand to off-peak hours or by increasing the share of utilized renewable energy. Additionally, the utilization of the latent heat of melting and solidification of so called phase change materials (PCMs) as storage materials offers the potential for considerably increased energy storage densities compared to materials storing only sensible heat.

This thesis summarizes current progress for developing a design framework, which covers the selection, utilization and process integration of a PCM TES. For selecting the material based on its phase change temperature and latent heat content, the T-History method has been developed further by studying the method numerically and experimentally. It is shown that adjustments in the data evaluation method have to be made in order to obtain repeatable measurement results. The results are then still subject to systematic errors, which limit the accuracy of the method.

In the next part, the material level results serve as model input to simulate charging or discharging cases of a PCM TES by modeling the heat transfer between the PCM and a heat transfer fluid (HTF). The developed model performs comparable to existing literature models but has to be verified experimentally in future work.

Based on the preliminary model, a process integration case is studied in terms of variations of geometrical and operational parameters relevant for the heat transfer within the PCM TES during (dis)charging. It is shown that restrictions given by the process may limit the effectiveness of a PCM TES considerably, if the PCM TES is operated at too high mass flow rates. It is concluded that the current results have to be placed at least within an economical context given by the process conditions in order to decide which PCM TES design is optimal.