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OpticalSoC - Optical State of Charge methods for phase changing materials

Using OpticalSoC one can better control thermal storage techniques using phase change material (PCM) at a reduced total cost of ownership.

Phase change materials (PCM) can store and release large amounts of energy during melting and freezing. They typically store 5 to 14 times more heat per unit volume than the more commonly used water storage tanks. Hence PCMs are attractive as they offer high-density energy storage and store heat within a narrow temperature range. For space heating or domestic water heating, organic PCMs (e.g. paraffin) additionally offer high heat of fusion and chemical stability without  health issues. Unfortunately, the thermal conductivity in their solid state is rather poor and costs involved in obtaining reliable phase change points are high.
To make PCM storage tanks economically profitable, a highly cost-efficient design should be considered. This can be obtained with smart tank design. Also, innovative and precise methods to determine the energy content, i.e. limiting the amount of system redundancy, could be included. The present solution focuses on the latter.
Effective and efficient use of a PCM type storage tank requires a reliable estimate of the available energy (state of charge). This is, however, hindered by the low thermal conductivity resulting in a non-uniform melting and freezing process. Moreover, the change in volume between these states often results in pressure induced local cavities, further hindering uniformity of the melting and freezing process and hence resulting in a strong infufficient estimation of the state of charge.


VITO solution

VITO developed the OpticalSoC technology, exploiting the changes in refractive index ocurring when paraffin based PCMs transfer from opaque (solid) to opal (liquid). OpticalSoC allows measuring an average value for the state of a PCM inside an entire vessel which allows a State of Charge value to be determined quickly without extensive testing of many sensor readings. OpticalSoC equally enables the calibration of the measurement system for optical measurements on the phase change material and calibrates the state of the PCM as well.

Using OpticalSoC, pressure induced problems in the vessel are avoided. It is cost efficient, easy to control, involves only a limited amount of components and does not use any heated nor moving parts. What's more, the buffer does not need to be of a symmetric design, i.e. a PCM buffer could have a bespoke shape and is suitable to fit specific spaces such as the engine bay of a car. The OpticalSoC  methodology is based on differences in refractive indexes, measured as intensities by a diode. The measured intensities are proportional to the ratio of liquid/solid in the vessel, regardless of any local degree of (non-)uniformity. Within the temperature region of the phase change, OpticalSoC provides a fast and cheap estimation of the state of charge.


  • Cheap to apply;
  • Simple to control;
  • Few components;
  • No moving/heated parts;
  • Non-intrusive;
  • Straightforward data processing and analysis;
  • Complete buffer vessel scan.


  • Heating and cooling systems;
  • Thermal energy storage systems;
  • All areas where heat storage is relevant and space limited, e.g. cars (A/C).


  • Applications requiring accurate information about energy available in storage units;
  • Households;
  • Heat and cooling industry;
  • Smart grids.
"It is inspiring to create advanced technologies that can be implemented easily at low cost."
Robbe Salenbien - R&D

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