Dr. Santiago Madruga
Professor, School of Aeronautics and Aerospace Engineering, Universidad Politécnica de Madrid, Spain

Speech Title: Modeling and Simulations of Phase Change Materials for Enhanced Thermoregulation and Energy Recovery

Abstract: The Phase Change Materials (PCM) allow to use the large latent heat of the solid/liquid transition to store large amounts of thermal energy during melting or release it to the environment during solidification, barely changing their temperature. These materials allow more compact and efficient thermal management units than traditional materials based on sensible heat.

We model the PCM using an enthalpy-porosity formulation and identify the main Dynamic regimes occurring during the melting of a PCM heated from below: (i) conductive regime, (ii) linear regime, (iii) coarsening regime and (iv) turbulent regime. We observe that most of the magnitudes of the melting process are ruled by power laws, such as the Rayleigh number or the thermal and kinetic boundary layers. In particular, we show that the Nusselt number scales with the Rayleigh number as Nu ~ Ra^0.29 in the turbulent regime, consistent with theories and experiments on Rayleigh-Bénard convection predicting an exponent 2/7.

A significant issue in thermal regulation and energy storage with PCM is their low conductivity. This leads to very long times during the heat storage and discharge phases, reducing their usability and performance in thermal management and energy storage. We present two mechanisms to enhance the heat transfer rate suitable for engineering applications in microgravity. First, we show how the thermocapillary effects are a very efficient mechanism to develop convective heat transfer in microgravity and strongly enhance the performance of PCM based systems, without increasing their mass and volume. Second, we use dispersed metallic nanoparticles in PCM to enhance the heat transfer rate, and present an empirical model able to predict the performance of nano-enhanced PCM realistically in a wide range of nanoparticle concentrations, sizes, and types.

Finally, we present how PCM can be used to design efficient micro-energy harvesters. The motivation comes from the need to power low consume electronics; such as wireless sensors to monitor environmental variables, industrial processes, health parameters, etc., in places where conventional batteries are impractical. Thermoelectric energy harvesting looks one of the best solutions to create autonomous monitoring sensors. We show how coupling thermoelectric generators with PCMs in micro-energy harvesters can increase the electric energy output an order of magnitude with respect to conventional designs. In particular, we pay special attention to designs of autonomous micro-harvesters to power structural health-monitoring systems in aircraft, as well as humidity, temperature, and acoustic-ultrasonic parameters.

Acknowledgements: Santiago Madruga acknowledges support by Erasmus Mundus EASED programme (Grant 2012-5538/004-001) coordinated by Centrale Supélec, the Spanish Ministerio de Economía y Competitividad under Projects No. TRA2016-75075-R and No. ESP2015-70458-P. ORCID: orcid.org/0000-0002-9996-1287