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

Abstract

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°C In this paper the results of these investigations are presented and future activities are outlined.

Thermal energy storage – overview and specific insight into nitrate salts for sensible and latent heat storage

“…Similar investigations of natural stones have been performed by Martin [ 29 ]. Basalt, diabas and quartzite were chosen as potential filler materials due to their high density and compressive strength.…”

“…The stability was confirmed by thermogravimetry analysis. Further analyses of the mineral content before and after the thermo-chemical oven test and thermal test in Solar Salt with higher test duration are necessary [ 29 ].…”

Investigation of the long-term stability of quartzite and basalt for a potential use as filler materials for a molten-salt based thermocline storage concept

“…For this concept, a low cost filler material is embedded in the storage tank, replacing Solar Salt. Whilst at the time of writing, costs for molten salt lie in the range of 500-1000 €/t, filler materialssuch as basaltare as low as 50 €/t and their volumetric heat capacity is comparable to that of molten salt (Martin et al, 2014) . Although utilization of these systems is a little lower due to limited heat transfer between filler and molten salt, the overall cost reduction potential can be up to 40 % (Libby, 2010).…”

“…In cost efficient thermocline storage tanks molten salt will be replaced by natural stones which have to be significantly cheaper than the used molten salt. The natural stones should have a high density, high mechanical strength and a sufficiently high specific heat capacity (Martin et al, 2014). The filler material must be available in large quantities at a low price level for a significant cost reduction, stable at high temperatures up to 560 °C and compatible with the selected molten salt.…”

“…In previous studies (Martin et al, 2014) basalt and quartzite were selected as potential inexpensive filler materials. Depending on the mining area the basalt composition varied, so that two different basalt varieties were tested: the basalt from the mining area Rossdorf (abbreviation in that study BaRo) and from the mining area Huehnerberg (abbreviation in that study BaHu).…”