Abstract
Bio-based glass-forming materials are now considered for thermal energy storage in building applications. Among them, Xylitol appears as a biosourced seasonal thermal energy storage material with high potential. It has a high energy density and a high and stable undercooling, thus allowing storing solar energy at ambient temperature and reducing thermal losses and the risk of spontaneous nucleation (i.e., the risk of losing the stored energy). Generally when the energy is needed, the discharge triggering of the storage system is very difficult as well as reaching a sufficient power delivery. Both are indeed the main obstacles for the use of pure Xylitol in seasonal energy storage. Different techniques have been hence considered to crystallize highly undercooled Xylitol. Nucleation triggering of highly undercooled pure Xylitol by using an air lift reactor has been proven here. This method should allow reaching performances matching with building applications (i.e., at medium temperatures, below 100 °C). The advantages of this technique compared to other existing techniques to activate the crystallization are discussed. The mechanisms triggering the nucleation are investigated. The air bubble generation, transportation of nucleation sites and subsequent crystallization are discussed to improve the air injection operating conditions.
Highlights
One of the key elements to optimize the use of renewable energies and to improve building performances is the development of thermal energy storage [1,2,3,4,5]
Xylitol high and stable undercooling allows long-term thermal energy storage at low to medium temperatures with reduced thermal losses and with negligible risk of spontaneous energy discharge
This paper provides a better understanding of the effect of primitive bubbles burst and small bubbles transportation on nucleation triggering of the whole highly undercooled Xylitol in a small container
Summary
One of the key elements to optimize the use of renewable energies and to improve building performances is the development of thermal energy storage [1,2,3,4,5]. The transition from liquid to solid comes with a conformation change (from a linear structure in the liquid phase, to a bent one in the solid phase) which increases the required energy to overcome the activation energy barrier These are the reasons why Xylitol crystal growth rates are low at ambient temperature. The nucleation triggering, i.e., of the discharge triggering when the energy is needed, is very difficult and the crystal growth rates are too low for an efficient power delivery If these two main obstacles are overcome, Xylitol would be a very competitive phase change material for seasonal storage applications at low to medium temperatures, in building field. This understanding should help to identify the parameters triggering nucleation at any time (or temperature) and to crystallize the entire product in due time
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