Development of nanoencapsulated phase change material slurry for residential applicationsntial applications

Resumen

One of the main problems of the current society is the increase in energy demand, which can lead to a number of problems such as the shortage of resources and the emission of polluting gases to the atmosphere, among others. For this reason, the society is demanding scientific advancements for renewable energy use and energy saving that can be already implemented in an easy and affordable way. The developments for energy storage based on phase change materials (PCMs) are advancing quickly to be a real solution for the energy saving in our daily life. These materials can be applied in the residential sector through active and passive thermal energy storage systems in order to take advantage of the solar energy and increase the energy efficiency of the dwelling. These systems allow improving insulation, reducing thermal jumps, as well as the energy consumption reduction of domestic heating and cooling. Therefore, this doctoral thesis is focused on the synthesis and development of thermoregulating slurries of high stability that allow to develop building materials with high insulating and thermal storage capacities. Besides, this thermal fluids can be also used in active storage systems. Due to the great interest in the research topic, it has been funded by the Ministry of Economy, Industry and Competitiveness through the project called "Development of Slurries Based on Thermoregulating Microcapsules for Residential Applications" from 2016 to 2018, and by the Ministry of Science, Innovation and Universities through the project named "Production of Thermoregulating Sub-Micron Slurries and Low-temperature Thermoelectric Materials for a Profitable Transformation of Solar Radiation into Household's Energy" from 2019 to 2021, as well as the granting of a FPU fellowship for PhD studies (FPU16/02345) by this same Ministry. There are different kinds of thermoregulating fluids, but this research aimed the synthesis of a polystyrene nanoparticle slurry (PSS) and a new class of encapsulated PCM slurries (PCSs), and more specifically of nanoencapsulated PCM slurries (NPCS) obtained in a one single process. For the NPCS++ synthesis, a commercial organic PCM called Rubitherm@RT27 (RT27) with a melting point of 27 ºC and latent heat of 168.4 J g-1 was used. First of all, a stable nanoparticle slurry with low viscosity consisted in polystyrene nanoparticles was synthesized in one single process by suspension polymerization. A significant advantage of this process is the direct obtention of the slurry, avoiding the removal of the reaction medium and particle purification and their further redispersion in a proper media. For that purpose, the concentration of sodium dodecyl sulfate (SDS) and the amount of cosurfactant (CS) employed, as well as the concentration of nanoparticles was optimized. The CS was synthesized in situ by sol-gel method from Tetraethyl orthosilicate and Vinyltriethoxysilane, in order to functionalize the nanoparticles to increase their affinity with water. It was possible to obtain a slurry up to 52 wt% in nanoparticles by using this method. It was found that the use of a 3.0 wt% of SDS together with a 1.5 wt% of CS and 40.0 wt% of styrene allowed to produce a slurry with single-spherical nanoparticles (< 100 nm) and improved and long-term colloidal stability. For the encapsulation of the PCM, gum arabic (GA) was selected as cosurfactant, instead of the silica based stabilizer employed before, because he had previously been successfully used in the encapsulation of PCMs. The in situ polymerization method with a co-polymer of styrene (S) and divinylbenzene (DVB) as shell was employed. The initial PCM dispersion (PCD) was achieved by sonication. For the optimization of the PCD preparation as precursor of the PCS production, the GA/SDS mass ratio was studied in the range from 100/0 to 0/100. It was found out that is possible to produce a PCD with a large colloidal stability by using an energy per liquid volume of ~ 800 kJ L-1, after 9 min of sonication. The PCS prepared with the optimal GA/SDS mass ratio (40/60) resulted to be a homogeneous nanoencapsulated PCM slurry (NPCS) formed by a nanometric capsules (dn0.5 = 94.7 nm). It exhibited a large solids content (28.1 wt%), latent heat (32.8 J g-1) and a Newtonian behavior with low viscosity (10.7 mPa s). In addition, this NPCS maintain its colloidal stability over time (|ζ| =53.4 mV freshly and |ζ| =55.8mV after two years). Clearly, a new method for producing stable and durable NPCSs with large thermal energy storage (TES) capacity had been developed. Nevertheless, the encapsulation parameters, i.e., the PCM content (C_PCM), encapsulation efficiency (EE) and reaction yield to capsule (η_capsules) could be improved (C_PCM = 44.3 %, EE = 48.1 % and η_capsules = 72.3 %). Therefore, it was decided to optimize the polymeric shell that formed the nanocapsules by varying the S/DVB mass ratio, as well as look for a better initiator using the best S/DVB mass ratio obtained. Through this study, it was possible to improve the above mentioned parameters by using an S/DVB mass ratio of 75/25 and using AIBN as initiator (C_PCM = 62.1 %, EE = 91.4 %, η_capsules = 98.2 %), maintaining the nanometric size of the capsules (dn0.5 = 73.0 nm), as well as the colloidal stability (|ζ| = 55.5 mV). Furthermore, the robustness of this method for the synthesis of thermoregulating slurries from other paraffinic PCMs was tested encapsulating hexadecane, n-octadecane, rubitherm®RT50 and parasur 130. Ultimately, the solids content was increased in order to obtain the NPCS with the highest possible thermal storage capacity, obtaining an optimal NPCS with a solids content of 40.5 wt%, maintaining the colloidal stability (|ζ| = 52.8 m V) and an improved heat capacity of 129.1 % in the PCM melting point range with respect to water. Then the process was scaled up from a 0.25 L reactor to pilot plant scale (100 L reactor), passing through a 2 L reactor. For this purpose, the sonication process was modified, going from a discontinuous sonication where the whole volume of PCD was sonicated, to a continuous sonication, where part of the PCD volume was recirculated through a sonication reactor by means of a peristaltic pump. The scale-up process was carried out satisfactorily, obtaining similar properties (thermal and particle size distribution, colloidal stability) in the three scales. Besides, it was observed a reduction in the viscosity for the NPCS synthesized in the pilot plant scale. The synthesized slurries were used for the manufacture of gypsum composites with improved properties. The nanoparticle polystyrene (NPS) slurry (PSS) was applied for the manufacture of lightweight gypsum composites with improve insulating and waterproof properties. It was possible to produce a gypsum composite with up to 0.42 NPS/Hemihydrate mass ratio with homogenous distribution of the NPS through the building material. Although the maximum compressive and flexural strengths, as well as the density, of the new lightweight gypsums decreased with the increase of NPS load, all the synthesized gypsum composites satisfy the European standard regulation EN 13279-1 for gypsum plasters and gypsum binders. Finally, thermoregulating gypsum composites were produced with the best NPCS. It was possible to obtain a nanocapsules/hemihydrate gypsum composite with a mass ratio up to 0.41. The mechanical, physical and thermal properties of all the manufactured gypsum materials were determined. All these new composites had improved TES capacity and showed a reduction in the thermal conductivity up to 142.0 and 40.6 % for the composite with a 41 wt% of nanocapsules, respectively. The use of a 1 m3 of the thermoregulating gypsum panels manufactured with this new synthesized NPCS allow to save 13.5 kWh m-3 and reducing the CO2 emissions in a 3.4 kg per operating cycle.