Heat and mass transfer behaviours of building materials and structures

Abstract

Heat storage as a means to respond to the requirements for improved energy efficiency motivated this study. The objective was to evaluate the impact of thermal energy storage systems in dwellings under Mexican climatic conditions. In the first part of this work thermal behaviors of adobe traditional architecture is discussed; in the second part a latent heat storage system using phase change materials (PCMs) is proposed and assessed. The high thermal mass structural elements of adobe traditional architecture have been charactefted as heat wave modulators. Nevertheless, the moisture content in these structures also plays a significant role as a means for heat storage and potentially enhancing thermal lag. The objective of this part of the study was to assess the scope of existing coupled heat and mass transport models regarding water contained latent heat storage on porous structures. The significant contribution of latent heat storage recognized in adobe structures, led to the study of a solar-thermal storage system using (PCMs). The objective of this part of the study was twofold: 1) Enhance the existing computational models on the Stephan problem by considering the effect of regional variations (weather conditions imposed) on the boundary conditions. 2) Evaluate the impact of the solar-thermal system proposed when applied in dwellings in view of regional variations under Mexican weather conditions. Solar-thermal storage systems independent of the structure offer the possibility to be applied to existing buildings as well as new constructions. The proposal is a storage element that constitutes internal blinds in windows. The computational model of the Stephan problem was solved with the enthalpy method. Simulations were run under different sets of climatic conditions. For the first time the main factors for promoting system's optimisation, when gathered in a single comparison study, provided a more general insight on system's performance. Experimental work was also carried out regarding the charging of the heat storage unit by heat gains other than direct radiation, and the storage unit's performance as insulator. A large-scale solar simulator was constructed. Statistical analysis of experimental results showed interesting findings including: The important role that internal heat gains play on the charging of the latent heat storage unit proposed. A larger effect on the discharging ratio was found with lower air temperatures than with faster air flow rates. The faster discharging rate tests also released slightly more energy. PCM volume was found to be the most critical factor on system performance. The importance of providing the means to discharge the total quantity of heat stored was pointed out. For the cooling mode, elements to enhance discharging might be required. For system control, thermal insulation was found to be an effective measure when the discharging is required to occur over a longer period. The multiple PCM unit was found to be more efficient during the charging process (storing more energy) than units containing a single PCM. Nevertheless the single PCM unit performed better for cooling than the multiple PCM unit. The question was raised as to what extent PCM thermal conductivity actually influences system's performance. The thermal storage system proposed in this study reduced the heating system energy consumption requirements for an experimental room by 28.6%.