Resumen de Passive energy storage materials for building applications
This thesis focuses on the synthesis and development of passive thermal energy storage composites for building applications in order to increase the energy efficiency in dwellings. The development of building composites containing PCMs is being extensively studied, however, some current problems must be addressed, such as the leakage of the PCM from the carrier matrix, low enthalpies of transition, detriment of the mechanical properties or lack of knowledge about the effects of the PCM on the final composite properties, among others.
The developed composites tackled in this PhD thesis are based on diverse building elements to cover the varied requirements which may arise in a sustainable building construction, such as RPU foams, gypsum and bricks. The incorporation of PCMs has been made by using mSD-(LDPE·EVA-RT27) and Polymeric-SiO2-PCMs microcapsules, synthetized in two pilot plants at the Instituto de Tecnología Química y Medioambiental, ITQUIMA (UCLM), based on different techniques. In the case where the microcapsules addition was not possible to accomplish, a new ssPCM was manufacture whose properties make this material ideal to be adsorbed by capillary action and further solidified into a porous matrix.
RPU foams containing between 0 and 50% by weight of mSD-(LDPE·EVA-RT27) were synthesized. The latent heat of this composite was higher than those values reported in literature for thermoregulating RPU foams using MicroPCMs. Moreover, Gibson and Ashby formulations and the Kerner equation for mechanical properties of composites were used to predict the Young’s moduli and collapse stress.
At full-scale the RPU foam composites containing 30 wt% mSD-(LDPE·EVA-RT27) exhibited a high thermal damping behavior during the day, however, a loss of insulating capacity was observed at night related with a large density and hence, high thermal conductivity of the composite foams containing large amount of microcapsules.
In order to decrease the density of the foam composites the foaming formulation was optimized, and different low-pressure operational conditions were tested. Notwithstanding the achieved lower densities, a deleterious impact on the RPU foams thermal conductivity was observed when low-pressure operational conditions were used, probably due to the removal of the CO2 formed during the foaming reaction.
By using the proper amount of surfactant, suitable isocyanate:polyol mass ratio and a combination of Tegoamin BDE and Tegoamin 33 for the synthesis of RPU foams containing 25 wt% of microcapsules was possible to achieve a composite able to work as heat accumulator and thermal insulation both at transient and at steady state. The incorporation up to 45 wt% of Polymeric-SiO2-PCMs/Hemihydrate into gypsums has been achieved by varying the water/hemihydrate mass ratio up to 0.8. The effect of the selected microcapsules on the gypsum properties was previously studied and compared with another three kind of microcapsules. Polymeric-SiO2-PCMs particles have a better distribution on the needle crystals structure than the exhibited by the other particles, in addition the mechanical properties were also less affected by the addition of Polymeric-SiO2-PCMs than by the other kind of studied particles.
The physical, mechanical and thermal properties of the obtained gypsum composites were tested. Finally, a series-parallel model combined with Rayleigh or Lewis-Nielsen models was proposed in order to predict the effective thermal conductivity of the gypsum composites as function of their porosity and microcapsules content.
On the other hand, the incorporation of PCMs into porous bricks was performed with a previous synthesis and study of a ssPCM based on PEG as heat storage material and SiO2 as stabilizer compound obtained by sol-gel method. With a molar ratio H2O:EtOH:H2SO4:PEG1000:TEOS of 2:0.34:0.021:0.50:1 and an equivalent ratio NaOH/H2SO4 of 1.15 for promoting the gel step a ssPCM having a latent heat up to 113.8 J/g was synthetized. The influence of gelation step on the final ssPCM properties and the role of the hydroxyl groups density of the silica matrix on the stabilization of PEGs were addressed.
The long gelation time and low initial viscosity of the proposed ssPCM in the sol form allowed the preparation of a new form-stable PCM. The sol is adsorbed by the brick and further gelled in their inner structure by controlling the pH of the medium, leading to a material of high physical and thermal stability. This way, different composites containing between 15 to 110 wt% of ssPCM respect to the initial dried mass of brick have been obtained, testing the viability of both techniques, immersion and vacuum impregnation.
The effective diffusion coefficient (Deff) of the obtained composites was estimated by means of the Fick´s second law and the orthogonal method as strategy for solving the problem, obtaining a good fitting between experimental and theoretical data. The relationship between Deff and the porosity was also analysed, being possible to obtain only one global diffusion coefficient (DAB) to predict Deff as function of porosity, for porosities below 0.7.
Besides, all the samples exhibited a high long-term thermal stability influenced by the additional stabilizer effect of the ceramic matrix and improved thermal properties that make these composites suitable for being used in passive buildings.
