Development of Latent Heat Storages using Polymers as Phase Change Material for Applications in Industry, Solar Energy and Heat Networks

Without doubt we are currently in the midst of a fast worldwide climate change which is mainly caused by human activities related to industrialization and the reckless consumption of fossil energy carriers. Combustion of coal, oil and natural gas results in emission of vast amounts of carbon dioxide that cannot be absorbed in the natural carbon cycle quickly enough and thus accumulate within the earth atmosphere. It is well known that rising CO2 levels in the atmosphere are correlated with rising earth average temperatures. In order to keep global warming below 2 C above the pre-industrial level vast global e orts must be undertaken such as a global shift to renewable energies (mainly solar, wind, hydro, biomass). In addition the energy e ciency in all sectors (residential, commercial, industrial) has to be greatly increased via saving and re-use of energy. Renewable energies are often intermittent and waste heat that could be re-used is often not available in time and quantity when needed. Thus, it is clear that energy storage which allow for a temporal shift as well as capacity and power peak shaving are a key technology to achieve global climate targets. Heat accounts for roughly 50 % of the nal end energy use and thus versatile thermal energy storage are necessary. While water serves well as energy storage for many applications below 100 C there is urgent need for technologically and economically better solutions above 100 C than currently available technologies (steam, thermal oil, pressurized water, sensible solid). Latent heat storages (also known as phase change storages (PCM storages)) o er many advantages in terms of high energy capacity, versatile power pro le and good economics and may thus be such a solution. Water ice storage are already a mature PCM technology operating around the freezing point of water. However above 100 C latent heat storage are not widely spread and only a few demonstrators have been realized. Especially this is true for PCM storages using polymers as phase change material, although this versatile class of materials would be ideally suited as PCM. This is because there are many di erent polymer types and grades available on a global industrial scale for a reasonable prize, they can be manipulated (compounded) to improve their physical properties, they are even available as recyclates, which are even cheaper and they are very well compatible with the best suited storage and heat exchanger materials (steel, aluminum). Thus, the main goal of this PhD thesis was to nd answers to the following central research objective: How can a high-capacity, exible-power, widely applicable, scalable, mass-producible, durable and cost-competitive polymer PCM storage be developed and which technological tools and frameworks have to be developed to enable a wide market penetration? As a rst step we rephrased this complex single objective into six more detailed subobjectives along the lines of PCM storage development and application: phase change material, heat exchanger, modeling and design, experimental characterization, application and system integration and durability. We translated the central research objective for each category into more detailed and speci c research objective which could be tackled within this PhD thesis. A systematic polymer material screening including a detailed experimental thermophysical characterization revealed that there are di erent types and grades of polymers that are suitable as phase change material. Especially, we con rmed that various polyethylene types are technically and economically interesting as was already indicated in some earlier studies. We also found that polyoxymethylene and some polyamide types are suitable too, especially for temperature up to 300 C. Also, we could proof that recyclates are contrary to all expectations, especially also from industry side, were suitable too and performed almost as good as new polymers, but being potentially much cheaper. Finally, we conducted studies for compounded polymers with improved thermal conductivity which allow the design of high power latent heat storages. As important as the PCM itself is the heat exchanger of a polymer PCM storage, both in terms of technological as well as economical performance. We screened many di erent heat exchanger geometries and designs and evaluated di erent materials and semi- nished products. Most importantly we concluded that in order to ensure a wide-spread market penetration it is necessary to rely on mature and widely available manufacturing processes and proven and reliable materials. Thus, we nally found that various steel and aluminum alloys are best suited and that manufacturing methods and designs adapted from classical heat exchanger and container industries are best suited. Especially, we selected the n-tube, tube bundle and shell and tube heat exchanger designs for our next steps. Another key issue for a wide market penetration are fast, reliable and accurate technical design and simulation tools. Thus, we developed simpli ed models of the storages that were able to describe the melting and crystallization as well as all relevant heat transfer processes within a polymer PCM storage to a su cient detail. We implemented the models into the multi-physics-simulation tool Dymola/Modelica which contains a fast and reliable algebraic di erential equation solver and a wide range of libraries that can be used for a later system design and controls implementation. In addition, for some more complicated design problems, we employed 3d computational uid dynamics simulations, which together with our construction software completes the portfolio of polymer PCM storage modeling and design as was initially looked for. The next crucial step to answer our primary research objective was to actually demonstrate the capabilities of polymer PCM storage on a lab-scale and to proof that the polymer PCMs, heat exchanger designs and modeling portfolio that we found and developed was actually working. Thus, we designed, constructed, modeled and simulated three di erent lab-scale polymer PCM storages. We evaluated di erent manufacturers and built the storage with them. In parallel, an appropriate storage characterization test rig was designed and set-up in the storage laboratory at the AIT Austrian Institute of Technology. Finally, the storage were characterized in great detail and we proofed that the polymer PCM storage we design actually worked as planned to our great satisfaction. Of course, a storage needs to be integrated into an appropriate application to actually be able to save energy. Currently there are no real scale industrial use cases of PCM Storages above 100 C. In our view this is in part because there is no versatile and fast design software for the integration of latent heat storages in energy or industry systems available. This was also a reason why we chose Dymola/Modelica for implementing our models. We were able to gure out an industrial use case in pulp and paper industries. In paper production frequently paper tearing events occur where a considerable amount of excess steam has to be dumped. We designed a polymer PCM storage using a thermally conductive HDPE and a tube bundle heat exchanger and developed a proper hydraulic system and control integration scheme to store this excess energy within our storage. In a later step when paper production starts up again and a peak steam demand occurs, we can discharge the storage and actually use it for peak shaving. The nal research objective that we investigated concerns the durability of the polymer PCM storages. At the beginning of the PhD thesis is was far from obvious if a polymer could be found and a heat exchanger could be designed which is able withstand the harsh conditions within a polymer PCM storage, i.e. cyclic charging and discharging, which means regular use of the polymer above it melting range. We performed thermal stress tests within a di erential scanning calorimeter and using hot plates and ovens to proof cyclic stability of the polymer itself. However, it was also necessary to investigate the behavior of a lab-scale storage comprising all geometric and design features of a real-scale storage under cyclic loads. This was performed in our storage characterization laboratory. After the experiments we inspected the storage by cutting it into pieces via a water-jet cutter and were able to proof for two di erent lab-scale storages that the polymer as well as the heat exchanger both remained fully functional. The detailed results culminated in four peer-reviewed publications which are the main part of this PhD thesis. The research was mainly funded by two FFG projects (StoreITup! FFG No. 838669 and StoreITup-IF FFG No. 848914) that were mainly conceived, submitted, lead and to a large extent worked out by the author of this thesis, Christoph Zauner. In addition, many talks and publications as well as three patent applications are related to this work, which are described in a separate section. Finally, we can conclude that we were able to positively answer the central research objective and proof that polymers are very well suited for polymer PCM storages. We developed appropriate heat exchanger designs and design tools validated by experimental lab-scale prototypes as well as suitable integration and control schemes that in future work, in our view, there are no major obstacles left to actually employ polymer PCM storage in real applications and develop a full-scale industrial production.