Efficient energy storage and transfer is essential in order to meet the continuously increasing need of energy in a sustainable manner. Thermal energy covers a large part of the energy use: in Finland, the share of heating, ventilation and air conditioning is 22% of the total energy use. This dissertation focuses on the material development of heat transfer fluids (HTFs) and long-term thermal energy storages (LTES) based on phase change materials (PCMs). Many recent publications have demonstrated that nanofluids, dispersions of nanoscale particles and HTF, have higher thermal conductivity and improved convective heat transfer performance as compared to conventional HTFs. However, the results are contradictory and in some publications these abnormalities are not observed at all. This dissertation researches novel type of nanofluid: suspensions of nanoscale PCM particles. In PCM suspensions, both the latent heat of the PCM and the sensible heat of the fluid are exploited for heat transfer. PCM nanofluids are produced with high- and low-energy emulsification methods. The heterogeneity, viscosity and specific heat affect most the thermal properties and convective heat transfer performance of the fluids. No anomalies in the convective heat transfer of the PCM nanofluids are observed if the heterogeneity and changed material properties are taken into account. Efficient LTES that is able to store heat for several months or even for years would be highly beneficial, particularly in cold-climate countries. In principle, thermal energy can be stored long-term almost lossless in supercooled PCMs. The stored heat is released from the supercooled PCM on demand by crystallization. So far, the metastability of the supercooled state has prevented the use of PCMs in large-scale LTES: supercooled melts are always prone to spontaneous crystallization that dissipates the stored heat. Two novel LTES materials with modified supercooling and crystallization properties are developed in this dissertation. Both innovations are based on a supercooling polyol PCM and a polymer additive. The first material is a microstructured polyol-polystyrene composite that is prepared by a new method utilizing polymerization of high internal phase emulsion. The novel microstructured erythritol crystallized more efficiently and in a more reproducible manner than the bulk polyol. The other LTES innovation of this thesis is a glass-forming mixture of a polyol PCM and cross-linked polyelectrolyte. The strong intermolecular forces of the mixture restrain the crystallization of PCM on cooling. Instead, the PCM supercools and vitrifies. The new material releases the stored heat by cold-crystallization. For a first time, heat can be stored long-term in supercooled PCM as low temperatures as desired without the risk of spontaneous crystallization, and released from the material by a slight heating.
|Publication status||Published - 2018|
|MoE publication type||G5 Doctoral dissertation (article)|
- phase change material, heat transfer, thermal energy storage, nanofluid, supercooling