Long-term thermal energy storage with cold-crystallizing materials - Method, properties and scale-up

Thermal energy storage (TES) is an attractive technology for balancing the variations in renewable energy production because currently, half of the global final energy consumption consists of heating, which mainly relies on fossil fuels. If an efficient and compact long-term TES emerged, viability of the renewable energy production would improve as seasonal variations could be smoothened. One way to achieve long-term TES is to utilize supercooling, glass transition and cold-crystallization to store and release the latent heat of melting. This work provides new knowledge on crystallization behaviour of cold-crystallizing materials and their implementation in TES applications. This thesis categorized mixtures of sugar alcohols and polymers, and their crystallization, thermal and morphological characteristics. Additionally, the key storage parameters of potential compositions were determined using a TES prototype system and a thermal chamber measurement procedure. The results reveal that the crystallization mechanism changes below 1.2*Tg (Tg= glass transition temperature, (K)) and the crystallization kinetics drastically reduce below 1.14*Tg. Additionally, reducing the lowest temperature achieved during supercooling accelerated the subsequent cold-crystallization at a constant temperature. The observed crystallization behaviour was explained in terms of energy landscape of the material and conformational flexibility of the sugar alcohol. The material showing the highest potential for long-term storage applications possessed a volumetric melting enthalpy of 200 MJ/m3, which is in the mid-range of typical phase change materials used in TES. Moreover, it demonstrated high storage efficiency after a nine-month storage at 10 °C. However, the materials should be used in a combined short- and long-term storage to yield high round-trip efficiency of 0.50-0.80, which depends considerably on the temperature at which the released heat is used. This work explains and demonstrates experimentally the fundamental changes in the crystallization behaviour occurring below 1.2*Tg, which enables using the supercooling, glass transition and cold-crystallization methods for long-term storing and adequate release rate of thermal energy. Furthermore, the results confirmed that this method may be practically applied to TES systems, indicating that advanced material solutions have potential to replace fossil fuel heating sources.