How do you tell how charged a phase change heat storage tank is?

Measuring the state of charge of a phase change heat storage tank means working out how much of the material has melted, and a new infrared camera method does it by watching the melt front directly.

The short answer: measure how much has melted

The state of charge of a phase change heat storage tank is mostly a question of how much of the storage material has turned from solid to liquid. A phase change material, or PCM, stores most of its energy as latent heat: it soaks up heat while it melts at a nearly fixed temperature and gives the same heat back when it freezes. That is what makes it compact, and it is also what makes it hard to read. A water tank gets hotter as it fills with energy, so a few thermometers tell you its charge. A PCM sits at roughly its melting point for the whole time it is half full, so temperature alone says little about how much energy is inside.

Researchers therefore talk about the liquid fraction, the share of the material that has melted. Know the liquid fraction and the sensible heat in the solid and liquid parts, and you know the stored energy.

Why storage tanks are built around finned tubes

Most good storage materials conduct heat poorly. A 2020 modeling study from the German Aerospace Center (DLR) in the Journal of Energy Storage states this as the basic design problem: highly conductive fins are needed to reach practical heat transfer rates. In a typical industrial design a heat transfer fluid, such as thermal oil or water and steam, flows through tubes running through the PCM, and metal fins spread out from each tube to carry heat into and out of the material. DLR has developed finned tubes of this kind to charge and discharge salt storage with steam.

A common storage material for this temperature range is a mixture of sodium nitrate and potassium nitrate. Its eutectic blend melts at about 222 to 223 degrees Celsius, inside the 150 to 350 degree window used for concentrated solar heat. The fins solve the heat transfer problem, but they also make the inside of the tank a tangle of metal and salt that is hard to see into.

How state of charge is usually estimated

A 2020 study in the journal Energies by Zsembinszki and colleagues sorted the existing approaches and then tested three families on a lab heat exchanger filled with a commercial PCM: methods based on temperature sensors, methods based on the mass flow and energy balance of the heat transfer fluid, and methods based on pressure, which works because many PCMs change volume as they melt.

Their finding was practical rather than dramatic. Every method could be used, the results generally differed from each other by less than 20 percent, and the best choice depended on the accuracy needed and the instruments available. The authors suggested combining two or more methods for a more reliable estimate. In other words, the indirect methods work, but none of them is precise enough on its own to be the final word.

Watching the melt front with an infrared camera

A 2026 bachelor thesis supervised at DLR and written at the University of Stuttgart tried a more direct route. Leon Felgentraeger filled a hexagonal enclosure, either around a finned structure or around a bare tube, with the sodium and potassium nitrate eutectic and filmed it with an infrared camera. The aim was to tell solid from liquid pixel by pixel.

The useful surprise was that the material looks almost the same to the camera in both states. Its equivalent emissivity rose only from 0.660 to 0.684 across the phase change, small enough that a single radiance threshold of 204.4 watts per square meter per steradian could separate solid from liquid. From one measurement the method gives both the local phase state and the overall liquid fraction, and so the stored heat.

What is still hard: the edges of the fins

The fins that make the tank work also limit the measurement. The thesis reports a mean uncertainty in liquid fraction of plus or minus 7.0 percent around the fin structure, against 2.0 percent around the bare tube, and attributes 83 to 87 percent of that uncertainty to the boundary of the fin structure. Accuracy is governed by the perimeter of the evaluation mask: the more metal edge there is in view, the more pixels sit in doubt.

That makes the camera method best suited to laboratory test cells, where it can serve as a reference to calibrate the cheaper indirect methods that a real, closed tank has to rely on. For anyone designing latent heat storage, the lesson is that measuring the charge is a design problem of its own, and it gets harder exactly where the heat exchanger gets better.