Electrical impedance solid fraction sensor for a panel filled with nanocomposite phase change material

Monitoring the solid fraction in a latent thermal energy storage system can enable the implementation of smart control strategies for improved performance. However, measuring the solid fraction in a non-invasive, easy-to-implement and accurate manner is challenging. The present study analyzes a parallel plate impedance sensor to measure the solid fraction in a panel filled with phase change material (PCM). The use of nano-additives is proposed to increase the contrast in electrical properties between the solid and liquid phases, which increases the sensor sensitivity. An equivalent electrical circuit model is used to estimate the sensor impedance. The sensor sensitivity is assessed for a nanocomposite PCM (NPCM) of hexadecane with functionalized carbon nanotubes. The electrical properties of the solid and liquid NPCM are drastically different at low frequencies, but the impedance for the melted PCM can be unpractically large. As the frequency increases, the impedance and sensor sensitivity decrease simultaneously. Hence, proper sensor design requires a balance between sensor size and excitation frequency.