Solid State Phase Change Materials for Thermal Energy Storage in Passive Solar Heated Buildings

A class of hydrocarbon compounds is being evaluated for use in thermal energy storage components for passive solar heated buildings. The most promising compounds are the polyhydric alcohols pentaerythritol (C5H1204), trimethylol ethane (C5H12O3), neopentyl glycol (C5H1202), and closely related materials. Solid solution mixtures of these compounds can be tailored so that they exhibitsolid-to-solid phase transformations at any desired temperature from well below room temperature up to 1880 degrees C (3700 degrees F). These crystalline changes reversibly absorb large amounts of thermal energy (80-290 kJ/kg) making the materials suitable for a wide range of energy storage applications. This research has focused upon understanding the molecular mechanisms of the solid statephase transformations in the polyhydric alcohols and upon evaluating their use in thermal storage components for solar buildings. During this last fiscal year, additional research has confirmed that the solid state phase transitions in the polyhydric alcohols depend upon hydrogen resonance bonding within the molecular crystals. Careful density measurements show that mixtures of the polyhydricalcohols, pentraerythritol and trimethylol ethane are probably true solid solutions which undergo crystalline phase changes by the same process as the individual alcohols. Experiments with deuterium isotope substituted pentaerythritol further confirmed the nature of hydrogen bonding in this solid; and x-ray diffraction measurements showed the nature of the crystallographic changes during thesolid state transformation. Thermal conductivity measurements showed that this important parameter could be significantly increased by the addition of certain dispersoids such as graphite or aluminum powders in the solid polyhydric alcohols. Novel building construction materials have been fabricated with large weight fractions of a solid state thermal storage material. These composite materialsexhibit much greater thermal storage capacities, but retain most of the characteristics of conventional construction materials which make them practical and easy to use in traditional building practice. Composites based on concrete, gypsum, wood products and others have been fabricated. Additional systems analyses were conducted in order to define the limits of thermal storage effectiveness inpassive solar heated buildings and to guide the laboratory research toward the most promising applications.