Nils Breidenbach

High-Temperature Solid-Media Thermal Energy Storage for Solar Thermal Power Plants

Solid sensible heat storage is an attractive option for high-temperature storage applications regarding investment and maintenance costs. Using concrete as solid storage material is most suitable, as it is easy to handle, the major aggregates are available all over the world, and there are no environmentally critical components. Long-term stability of concrete has been proven in oven experiments and through strength measurements up to 500 °C. Material parameters and storage performance have been validated in a 20-m3 test module with more than 23 months of operation between 200 °C and 400 °C and more than 370 thermal cycles. For an up-scaled concrete storage design with 1100-MWh capacity in a modular setup for a 50 MWel parabolic trough power plant of the ANDASOL-type, about 50 000 m3 of concrete is required and the investment costs are approximately 38 million euro. The simulation of the annual electricity generation of a 50 MWel parabolic trough power plant with a 1100-MWh concrete storage illustrates that such plants can operate in southern Europe delivering about 3500 full load hours annually; about 30% of this electricity would be generated by the storage system. This number will increase further, when improved operation strategies are applied. Approaches for further cost reduction using heat transfer structures with high thermal conductivity inside the concrete are analyzed, leading to a 60% reduction in the number of heat exchanger pipes required. For implementation of the structures, the storage is build up of precast concrete blocks.

Screening and Analysis of Potential Filler Materials for Molten Salt Thermocline Storages

Solar thermal power plants are a promising option for future solar electricity generation. Their main advantage is the possibility to utilize integrated thermal storage capacities, allowing electricity generation on demand. In state of the art solar thermal power plants, two-tank molten-salt thermal energy storages are used. Significant cost reductions are expected by using thermocline thermal energy storage by storing the liquid storage material inside a single tank when compared to a two tank storage system. By embedding a low cost solid filler material inside the storage tank further cost reductions can be achieved.In earlier studies [1, 2] several potential filler materials have been investigated. In these study quartzite turned out to be a promising candidate due to its satisfying thermal stability and availability. At a temperature of approx. 573°C the crystal structure of quartzite changes from trigonal α-quartz phase to the hexagonal β-quartz phase [3]. This quartz conversion results in a volume change [4] that may cause cracking of the quartzite crystals due to weight loads in a packed bed. Since these thermal tests of the study mentioned were limited to 500°C this dunting was not considered. Thus, despite of the published studies there is a need for further, more detailed analysis.One trend in today’s development of solar thermal power plants is to use molten salt as storage material and heat transfer fluid at operating temperatures of 560°C and above. Accordingly, the quartz inversion might limit the applicability of quartzite as a filler material at elevated operating temperatures. Due to this concern, an investigation has been started to investigate the utilizability of natural rocks as low cost filler materials.In the first phase of this investigation a comprehensive literature survey was conducted. Based on this study, magmatic and sedimentary rocks turned out to the most promising rock classes for this application. For the further investigation, basalt was chosen as a suited representative for magmatic and quartzite for sedimentary rocks. In lab-scale tests, these candidate materials were investigated with respect to their:• Calcite content• Thermal stability up to 900°C in air• Thermal stability up to 560°C in molten salt• Cyclic stability between 290°C and 560°C in molten salt• Specific heat capacity up to 600°CIn this paper the results of these investigations are presented and future activities are outlined.Copyright

Modelling and Operation Strategies of DLR's Large Scale Thermocline Test Facility (TESIS)

In this work an overview of the TESIS:store thermocline test facility and its current construction status will be given. Based on this, the TESIS:store facility using sensible solid filler material is modelled with a fully transient model, implemented in MATLAB®. Results in terms of the impact of filler site and operation strategies will be presented. While low porosity and small particle diameters for the filler material are beneficial, operation strategy is one key element with potential for optimization. It is shown that plant operators have to ponder between utilization and exergetic efficiency. Different durations of the charging and discharging period enable further potential for optimizations.