Characterization of polymers and Microencapsulated Phase Change Materials used for Thermal Energy Storage in buildings

Abstract

The use of renewable heat decreases the consumption of fossil resources, although its usage is intermittent and usually does not match the demand. A proper thermal energy storage system design can eliminate this problem by reducing the consumption of non-renewable resources and improving energy efficiency where used. In buildings, thermal energy storage using phase change materials (PCM) is a useful tool to achieve reduction in energy consumption. These can be incorporated into passive or/and active systems. Thus, a proper selection of materials and extensive characterization for its usage in thermal energy storage is critical for new construction systems and for those already constructed.

This Thesis is divided in two blocks and presented as a compendium of published articles in scientific journals indexed in Materials, Engineering, and Energy areas. The emphasis is made in the chemical, physical, thermal, mechanical and environmental characterization of PCM, MPCM (microencapsulated phase change materials), and PCS (phase change slurries). The main purpose is to perform an exhaustive characterization of this kind of materials because several scientific studies have highlighted that PCM mixed with construction materials can suffer leakage. Polymeric encapsulation is an alternative for retaining PCM inside building materials, resulting in a system named MPCM.

- Macroscopic samples: the nanoindentation tecnique has been used to characterize thermoplastics that, so far, have had few precedents. As mechanical properties of materials are an important criterion for their selection and nanoindentation allows their evaluation, we have studied the hardness and elastic modulus of different polymeric materials through Loubet and Oliver & Pharr methodologies, to discern which is the most suitable concerning the viscoelastic properties. The obtained values by Oliver & Pharr method are based on the unloading curve analysis; in case of Loubet methodology, these values are a function of the penetration depth of the indentation. Also, we have studied the mechanical changes that occur when a polymer that contains a flame retardant is immersed in PCM. It has been observed that using Mg(OH)2 increases rigidity and mechanical strength while reducing the degradation effect and improving the properties against fire. This block contains two scientific published papers.

- Microscopic samples: This block is based on MPCM studies. A review of publications related to PCM, MPCM and slurries (PCS) (same shell and different PCM; different shell and same PCM; same shell and same PCM, but different encapsulation ratios) was prepared. Then, the evaluation of the chemical, physical, thermal, mechanical, and environmental properties of different MPCM and PCS samples was performed, concluding that AFM is a useful tool to characterize the stiffness and Young's modulus of MPCM. Because temperature is a key parameter in PCM systems, AFM experiments were carried out at different temperatures, in order to simulate the PCM in solid and/or liquid state. PCS samples were observed using SEM device coupled to a cryogenic system. Besides, environmental properties of PCS have been studied by gas chromatography (VOC’s). In addition, PCS were cycled for the evaluation of the polymeric shell durability after pumping the sample several cycles. Also, the chemical and thermophysical properties before and after pumping the sample were compared. Finally, due to the thermal behavior results of PCS in some performed studies, and depending on the liquid or dried PCS sample, the optimum conditions by means thermogravimetric analysis were evaluated. The second block contains five scientific published articles, one article under review after its first revision, one article finished without being submitted to a journal, and one unfinished research. Finally, the contribution in the state of the art of this PhD Thesis related with thermal energy storage in buildings using PCM, MPCM, and PCS is presented.

Un correcto diseño del sistema de almacenamiento de energía térmica (TES) puede eliminar un uso discontinuo y que habitualmente no coincide con la demanda. El TES mediante materiales de cambio de fase (PCM) en climatización pasiva y activa en edificios es un instrumento útil para alcanzar un descenso del consumo de energía.

La Tesis se divide en dos bloques y se presenta como compendio de artículos publicados en revistas científicas indexadas en las áreas de Materiales, Ingeniería, y Energía, haciendo émfasis en la caracterización química, fisica, térmica, mecánica y ambiental de PCM, MPCM (materiales de cambio de fase microencapsulados) y PCS (pulpas con cambio de fase).

- Caracterización de diferentes termoplásticos mediante nanoindentación. a través de los métodos de Loubet y Oliver & Pharr. También se han estudiado los cambios mecánicos que se producen cuando un polímero que contiene carga ignifugante en su formulación se sumerge en PCM. Este bloque contiene dos artículos científicos.

- Estudio de MPCM. Se ha llevado a cabo una revisión de publicaciones por otros autores. Se han caracterizado con AFM diferentes MPCM y PCS, a diferentes temperaturas. Se han observado muestras de PCS mediante el uso de SEM acoplado a un sistema de crionizado, y se han estudiado las propiedades medioambientales por cromatogyafía de gases. Además, se han ciclado PCS para ver la durabilidad de la pared polimérica después de ciertos ciclos de bombeo. Se han investigado las condiciones óptimas mediante análisis termogravimétrico en PCS. Este segundo bloque contiene cinco artículos científicos publicados, un artículo aceptado en primera revisión, un artículo finalizado sin enviar a revista, y un estudio en investigación.

Finalmente, se presentan las conclusiones principales de la contribución de esta Tesis Doctoral en el estado del arte de los PCM, MPCM, y PCS para almacenaje de energía en edificios.