# Demonstration of Phase Change Thermal Energy Storage in Zinc Oxide Microencapsulated Sodium Nitrate
## 3 Results
### Availability of High-Temperature Thermal Energy from Microcapsule Storage  
In a typical charge-self-discharge experiment (Figure 5) the highest temperatures are almost always recorded by T4, in the center of the tank, near the heating coil. T1 is placed on the surface of the stainless-steel shell, on the same transversal section as T4, and therefore used in conjunction with T4 to monitor the temperature in radial direction across the storage material. The presumed left-right symmetry from the center of the tank is consistent with T3 versus T5 readings. In Figure 6, plateaus of relatively constant T4 and T1 temperatures are noted shortly after the beginning of the free-cooling phase of cycles. For T4, this corresponds to the liquid to solid phase transition when latent energy is released from microparticle cores at rates that dominate over sensitive heat. After PCM solidification phase, the temperature decay in tank is controlled by two variables: the caloric capacity of the tank and the heat transfer through tank insulation. For a narrow temperature interval, using linear fit slopes of T1 curves corresponding to 19.23 and 22.07 kg (taken near the PCM offset temperature), K value was determined. Based on the T1 plateau, by extrapolating a constant K towards t = 0 h, the heat transfer rate was calculated, thus estimating the latent PCM energy stored in the tank. Figure 5: T1 to T6 temperatures readings (positions in the prototype tank indicated in Figure 1) during a heating-cooling cycle. Figure 6: _Cont_. In the evaluation of the thermal energy storage capacity of the tank, two loads of microcapsules were considered: 19.23 and 22.07 kg. T4 monitors the solidification in the innermost spot of the tank, therefore informative for the end of the latent heat release phase. At high temperature change rates, T4 readings are affected by nonequilibrium, with measured temperature differences up to 15 \({}^{\circ}\)C for the interval of interest. Thus, to protrude with the T3-T5 thermocouples at their deep spots into the storage material, the tank has three thin tubes welded on the longitudinal side. The tubes are closed at the protruding end and they prevent direct contact between thermocouples and microcapsules. In an experiment to determine the magnitude of correction, another thermocouple was inserted into the air-filled storage tank, with the tip measuring the temperature in the same spot as T4, but from inside the tank. In the experiment presented in Figure 7, for T\({}_{\text{real}}\) = 275 \({}^{\circ}\)C, the T\({}_{\text{measured}}\) is 260 \({}^{\circ}\)C - showing up to 15 \({}^{\circ}\)C temperature differences. This is explained by nonequilibrium measurements for T3 - T5 during tank heating. Correction of T4 on this basis is not straightforward, since at plateau temperature (in the solidification phase) the temperature equilibrium between material and sensor is attained. Therefore, a correction was not applied to thermocouple readings. Instead, in Figure 8, when reporting the energy available as latent heat (by adapted calorimetric method) in tank discharge, a 15 \({}^{\circ}\)C uncertainty on T4 readings was explicitly acknowledged for the solidification offset temperature, and the corresponding energy values given as a maximum estimate. Figure 6: T4 (**A**) and T1 (**B**) temperature evolution during charge–self-discharge cycles of the prototype tank, for progressively increased loads of storage material. Figure 7: The accuracy of T3–T5 thermocouple response to temperature dynamics inside the storage tank. Based on the latent heats, a summary is provided in Figure 8 for the estimates of the energy stored as latent heat in the partially filled storage tank. Results are presented for microcapsules in comparison with pure NaNO\({}_{3}\) at equal PCM content mass. Two tank loads were considered: 19.23 and 22.07 kg of microcapsules, with a net NaNO\({}_{3}\) content of 15.38 and 17.65 kg, respectively.