In each of the project subtasks the work performed led to important achievements as described in the following.
SO1) Characterization and enhancement of electrical properties of PCMs
A wide range of PCMs were characterized via electrical impedance spectroscopy with an in-house developed experimental set up and procedure. Organic PCMs were found to behave as either polar or non-polar. Non-polar paraffin-based PCMs act as dielectrics with high impedance and minimal permittivity change (~0.94) limiting impedance-based liquid fraction sensing. For non-polar organic PCMs, effective functionalization approaches were developed to improve the electrical properties aimed at liquid fraction sensing. Polar organic PCMs exhibit moderate permittivity contrast (1.20–2.71) enabling liquid fraction monitoring with large-area electrodes. Salt hydrates function as electrolytes with up to 100x ionic conductivity contrast, suitable for liquid fraction detection via multiple small electrodes, enhancing spatial resolution in tomographic methods.
SO2) Design of the electrical sensing system
The effect of electrode configuration on the sensitivity of the sensing system was explored for PCMs in vertical enclosures. The melting and freezing dynamics of a PCM in a vertical enclosure were numerically simulated to generate physically meaningful solid and liquid phase distributions inside the vertical enclosure. The phase distributions were used as input for electrical simulations to estimate the response to changes in the liquid fraction of the electrical impedance between electrode pairs in in-line and front-facing configurations. For moderate conductivity contrast, both in-line and front-facing electrodes perform adequately, with front-facing configurations offering more linear responses. High conductivity contrast causes electrical bridging in in-line setups, reducing sensitivity. Front-facing electrodes, which force the current flow across the solid-liquid interface, are more robust and produce simpler, series-based response models. This simplified electrical approach enables reliable liquid fraction estimation without complex algorithms, ensuring robust sensing.
SO3) Experimental implementation and validation of the proposed sensing system
The proposed sensing approach for the liquid fraction was experimentally validated in a custom-designed test section that enables precise thermal control of the side walls of a vertical enclosure, enabling both heating and cooling cycles. The setup incorporated optical access via a front window to observe solid-liquid phase transition dynamics in situ. Also, the test section was instrumented to monitor the temperature of the side walls. Melting and solidification tests were performed under controlled conditions. The liquid fraction estimates from the electrical impedance measurements exhibited adequate accuracy and consistency with thermal and optical methods to monitor solid-liquid phase change processes. Hence, the proposed sensing approach for the liquid fraction is demonstrated to be accurate and feasible to be implemented in real world LTES with minimal intrusion.
SO4) Validation of ET4PCM technology in real LTES
The knowledge developed and accumulated during the project execution was applied to design, implement and experimentally test an innovative liquid fraction measuring approach for a real-world 300 kg. The electrical impedance-based sensing approach estimated the global liquid fraction from local measurements. The sensing system and liquid fraction estimation approaches were successfully tested and provided invaluable insight on the phase change dynamics. The liquid fraction estimated can be used for the improvement of the LTES design and for the implementation of smart control approaches for the maximization renewable energy utilization.
