Design of Thermal Energy Storage with Phase Change Materials - Investigations within Material, Device and System Scale

Doctoral thesis, 2020

The addition of a thermal energy storage (TES) into a process allows the operator to store and release thermal energy on demand. This increased process flexibility leads to potential benefits such as shifting energy demand from peak to off-peak hours. In particular, so called phase change materials (PCMs) have the potential to achieve high storage densities, when their latent heat of melting and solidification can be utilized. However, despite a high research interest in PCMs over the last years, real life implementations of PCM TES are still uncommon. This thesis presents work done on material, device and system scale based on a case study for daily peak shaving of cold energy in an office building using a PCM TES.
On material scale, the T-History method for thermal analysis of PCM samples has been studied with both numerical and experimental methods. It is shown that the current mathematical model is subject to systematic errors which should be corrected in the future. Moreover, the results of the method are shown to be sensitive to a number of different experimental parameters and their trade-offs are discussed. A new data evaluation method, which is more robust to noisy data when forming the first time derivative of the temperature measurements, is proposed in order to achieve a better trade-off between precision and accuracy of the results. The work can be seen as a contribution to the necessary standardization of the method in future work.
On device scale, an experimental test setup has been built to study a commercially available PCM TES design with a salt-hydrate as storage material. The test setup is used to cycle the storage under actual process conditions. The results show that the storage suffered from phase separation of the PCM with continued cycling, which causes the storage capacity to decrease. Sample analysis using the T-History method reveals that supercooling behavior, phase change temperature and storage capacity systematically changes across the vertical height of the storage before and after cycling. While it is shown that the phase separation can be reset, phase separation needs to be prevented when the storage is scaled up.
A techno-economic analysis on system scale is then performed for an actual real-scale installation of the PCM TES in a new office building. First benchmarking shows that the storage capacity is stable but does not reach manufacturer specifications. Future work needs to determine the reasons behind the performance decrease, such as looking into the role of a superabsorbent polymer that has been mixed with the salt-hydrate to prevent phase separation. The PCM TES can nevertheless be used for daily peak shaving. A mixed integer linear programming (MILP) model is used to optimize the storage discharging schedule for a simulated yearly cooling load of the building. The estimated economic benefits are translated into an investment cost limit for a five year payback time. Since the current storage investment costs are significantly higher than this limit, future work should prioritize finding the boundaries of economic feasible storage applications.