Impact of cycling on the performance of mm-sized salt hydrate particles

Potassium carbonate is shown to be a promising salt for thermochemical heat storage. For a thermochemical reactor application, the salt hydrate is manufactured in mm-sized particles. It is known that salt hydrate particles undergo swelling and cracking during cyclic testing. Therefore, in this work the influence of cycling on structural and morphological evolution is investigated and the resulting impact on the hydration performance. It is found that the incremental volume increase during cycling is independent of the density at which a particle is produced. With lower starting relative density particles are found to be stable for more cycles compared to particles produced with high starting relative densities. Powder formation at the particle surface starts as soon as the particle density is close to values reported for percolation thresholds. The morphological changes during cycling result in formation of isolated pores and a highly tortuous pore system. As a result, the effective diffusion coefficient for cycled particles is lower compared to what is predicted for as produced particles with similar porosity resulting in lower power output than expected based on porosity. The results from this work help in understanding the reasons for swelling, cracking, powder formation and decreased performance with cycling, laying the foundation for mitigating these unwanted effects.</p

Impact of cycling on the performance of mm-sized salt hydrate particles

Potassium carbonate is shown to be a promising salt for thermochemical heat storage. For a thermochemical reactor application, the salt hydrate is manufactured in mm-sized particles. It is known that salt hydrate particles undergo swelling and cracking during cyclic testing. Therefore, in this work the influence of cycling on structural and morphological evolution is investigated and the resulting impact on the hydration performance. It is found that the incremental volume increase during cycling is independent of the density at which a particle is produced. With lower starting relative density particles are found to be stable for more cycles compared to particles produced with high starting relative densities. Powder formation at the particle surface starts as soon as the particle density is close to values reported for percolation thresholds. The morphological changes during cycling result in formation of isolated pores and a highly tortuous pore system. As a result, the effective diffusion coefficient for cycled particles is lower compared to what is predicted for as produced particles with similar porosity resulting in lower power output than expected based on porosity. The results from this work help in understanding the reasons for swelling, cracking, powder formation and decreased performance with cycling, laying the foundation for mitigating these unwanted effects.</p

Stabilization of salt hydrates using flexible polymeric networks

The use of salt hydrates for thermochemical energy storage is associated with mechanical instabilities during cyclic hydration/dehydration. On the other hand, some salt hydrates do not suffer from these drawbacks, but manufacturing of mm-sized particles is still a challenge. In this work a one pot synthesis method is presented which results in composites using poly (dimethyl siloxane) (PDMS) as binder. Energy densities of 1.14 GJ/m3 and 0.67 GJ/m3 are achieved for a K2CO3 and CaC2O4 composite, respectively. Swelling upon hydration decreases compared to non-stabilized particles. The best K2CO3 composite shows mechanical stability for at least 35 cycles, and the average power output at 50 % conversion increases with cycling to 50–55 kW/m3 at 20 °C and 33 % relative humidity. Also, a stable CaC2O4 composite is made suitable for heat storage. The particle volume and hydration kinetics remain constant for at least 20 cycles. An average power output at 50 % conversion of 5 kW/m3 at 20 °C and 33 % relative humidity is generated. The results from this work show how a one-pot fabrication method can be used to obtain mm-sized particles with enhanced mechanical stability during cycling. Stabilization can be achieved independent of the salt hydrate solubility or material properties.</p

How Chain Transfer Leads to a Uniform Polymer Particle Morphology and Prevents Reactor Fouling

The effect of adding diethyl zinc as a chain transfer agent during the polymerization of propylene in heptane performed at 80 degrees C was studied. Although it was expected that the chain transfer would stop after precipitation of the polymer, the polymer molecular weight continued to increase throughout the whole of the polymerization. The presence of diethyl zinc had an additional effect that the polymerizations were devoid of reactor fouling. To unravel this phenomenon, the polymer particle morphology was studied. Under the conditions applied, surprisingly, uniform platelet-shaped polymer particles were formed. At high polymer content, these particles aggregate into microfibrillar structures consisting of nematic columnar strands of the same uniform platelets. The polymer particle morphology, as a result of controlled crystallization, is believed to play a crucial role in preventing reactor fouling

Elucidating the Dehydration Pathways of K₂CO₃·1.5H₂O

Potassium carbonate sesquihydrate has previously been identified as a promising material for thermochemical energy storage. The hydration and cyclic behavior have been extensively studied in the literature, but detailed investigation into the different processes occurring during dehydration is lacking. In this work, a systematic investigation into the different dehydration steps is conducted. It is found that at higher temperatures, dehydration of pristine material occurs as a single process since water removal from the pristine crystals is difficult. After a single cycle, due to morphological changes, dehydration now occurs as two processes, starting at lower temperatures. The morphological changes open new pathways for water removal at the newly generated edges, corners, and steps of the crystal surface. The observations from this work may contribute to material design as they elucidate the relation between material structure and behavior.</p

Polymeric stabilization of salt hydrates for thermochemical energy storage

Non-stabilized thermochemical materials impose several limitations on their use. These include swelling/shrinkage, cracking, and agglomeration over cycles. In addition, the deliquescence transition cannot be used and is even considered an unwanted side effect. In this work several salt hydrates for low temperature heat storage (K2CO3, CaCl2 and LiCl) are stabilized within a highly porous mm-sized polymer matrix. The composites containing wetting salt solutions are shown to be stable towards deliquescence. Three different composites were cycled. A K2CO3-polymer composite was cycled for 50 hydration/dehydration cycles and found to be kinetically and mechanically stable over all cycles, with swelling at higher cycle numbers. A LiCl-polymer composite was cycled for 40 cycles after which the composite became unstable. The composite containing CaCl2 was found to be kinetically and mechanically stable for 15 cycles. Composites with energy densities up to 2.4 GJ·m-3 and a peak power output of 325 W·kg-1 were fabricated which is equal or higher compared to previously reported systems. All composites have power outputs which are sustained at higher levels throughout the full discharge cycle. This work opens new pathways to stabilize salt hydrates as well-defined mm-sized particles exhibiting cyclic stability, while maintaining a high energy density and power output

Diffusion limited hydration kinetics of millimeter sized salt hydrate particles for thermochemical heat storage

Potassium carbonate is a promising salt for thermochemical heat storage. For an application mm-sized salt hydrate particles are manufactured to be loaded inside a reactor. The step towards larger particles is necessary to prevent a large pressure drop over the reactor bed during hydration/dehydration in a given air flow. Therefore, in this work a systematic study on the hydration kinetics of mm-sized disc shaped salt hydrate (K2CO3) particles is presented for the first time. The effect of density, primary particle size and driving force on the hydration kinetics was evaluated using a 1D diffusion model. The main conclusions are that the hydration kinetics of mm-sized salt hydrate particles is diffusion limited and that the particle density (porosity) and tortuosity are the main parameters controlling its performance. On the contrary, the primary powder size did not affect the particle performance in any way. It is shown that the calculated transport mechanism is unaffected by changes in driving force whereas the power output decreases with decreasing driving force. Lastly shape and size optimization is discussed which can possibly improve the hydration kinetics of salt hydrate particles in view of thermochemical heat storage. Since the particle hydration is expected to be similar for various other salts, the model from this work offers opportunities to predict and optimize particles made from different salts as well

Elucidating the Dehydration Pathways of K₂CO₃·1.5H₂O

Potassium carbonate sesquihydrate has previously been identified as a promising material for thermochemical energy storage. The hydration and cyclic behavior have been extensively studied in the literature, but detailed investigation into the different processes occurring during dehydration is lacking. In this work, a systematic investigation into the different dehydration steps is conducted. It is found that at higher temperatures, dehydration of pristine material occurs as a single process since water removal from the pristine crystals is difficult. After a single cycle, due to morphological changes, dehydration now occurs as two processes, starting at lower temperatures. The morphological changes open new pathways for water removal at the newly generated edges, corners, and steps of the crystal surface. The observations from this work may contribute to material design as they elucidate the relation between material structure and behavior