Development and comparison of measurement techniques for the phase change determination in latent heat storages

Kurzfassung

Latent heat storages exploit the phase change enthalpy of a material to store large amounts of thermal energy at yet small temperature differences over a phase transition. Due to the isothermal nature of the phase transition, the reliable determination of the state of the phase change using temperature measurements poses a challenge, resulting in an erroneous determination of the state of charge. No universally applicable measurement technique exists serving this purpose. A novel measurement technique taking advantage of density changes over the solid-liquid phase transition of the eutectic sodium and potassium nitrate compound is developed. Distance measurements using a triangulation-based laser distance sensor, measuring the distance to the fill level of the PCM, are correlated to the liquid fraction of the phase change material and validated using multiple temperature measurements. Therefore, multiple experiments evaluating charging and discharging cycles of the latent heat storage with different heating and cooling parameters, as well as initial conditions, are tested. The liquid fraction is determined with an uncertainty of ±0.43% and matches well with the validation experiment, but the measurement technique is susceptible to cavity formation for the mostly solid storage material. Thus, if cavity formation is prevented by controlled discharging processes, the general operationality of the evaluated measurement technique is confirmed.