Public defence in Mechanical Engineering M. Sc. (Tech) Konsta Turunen

As renewable energy production increases, availability, security, and reasonable price of energy must be guaranteed. Energy storage can smooth the variations in the production and reinforce these vital aspects. However, use of long-term storage is still slowed down by the large size of these storages and losses during the storage. This dissertation developed materials that enable long-term heat storage for months, even years. The materials utilize the heat involved in melting, which is stored by cooling and released by heating. However, the heat storage ability is affected by the material composition, which consists of sugar alcohol and polymer. This research categorized material properties and upscaled the material idea to a kilogram scale prototype device. As a result, this research determined operational parameters of the prototype, which confirmed the viability of using the materials in heat storage applications. The most significant scientific result was the strong and exceptional impact of the temperature on the crystallization rate of the materials. Determination of this impact explained why the applied method and materials can store heat long-term. Moreover, it forwards the application of these materials because operation and material behaviour can be predicted more accurately. This research also discovered that the efficient storage time can be prolonged by decreasing the storage temperature and adjusting the composition of the materials. For example, a nine-month storage can be efficiently implemented at 10 °C. These materials’ ability to store heat long-term in a compact space could be beneficial in smaller heating systems, such as in buildings or small neighbourhoods. The results of this dissertation show that new heat storage materials and methods are potential options for reducing the carbon emissions of heat production.