Towards stable salt hydrates for heat storage

Salt hydrates cannot be used as powder but must be manufactured into larger particles on the millimeter scale. However, these salt hydrate particles tend to agglomerate, crack, shed powder, and go into deliquescence, which leads to stability issues. Therefore, in his PhD thesis, Joey Aarts outlines new ways to manufacture, characterize, and stabilize salt hydrate particles for thermochemical energy storage.

Start with analysis

To start, Aarts analyzed mm-sized particles made of pure K2CO3 sesquihydrate. These particles are subjected to various hydration (discharge) conditions to characterize the kinetic performance.

The particles are found to behave as a diffusion limited system of which the performance is solely dictated by the particle porosity. When these particles are subjected to cyclic charging and discharging, a morphological change occurs within the particles. These morphological changes result in the swelling, cracking, and powder loss of the particles with increasing number of (dis)charging cycles.

In addition, the change in morphology results in slower water vapor transport inside the particles, and consequently lower power output, as would be expected from the particle porosity. There are various possible reasons as to why this takes places such as an increase in tortuosity, an increase in surface adsorption and diffusion, and the formation of isolated pores.

Stabilization methods

Based on these observations, Aarts then looked at previous studies to solve issues related to salt hydrate stabilization. From this, Aarts noted that most stabilization is not targeted towards packed bed application, is not feasible on a large scale, or lacks discharge performance.

Therefore, he presented two new methods of stabilization. First, Aarts looked at the stabilization of salt hydrates in a mm-sized porous polymer. This porous polymer can accommodate salt hydrates for both hydration and deliquescence discharge reactions. The deliquescent composites exhibited high energy density and a constant high power output with cycling. The usage of such polymer matrix provides great flexibility in the design of shape and geometry.

The second stabilization method consisted of compacting salt hydrates together with a binder. This method has the advantage that it is a one pot method and can be used to stabilize salt hydrates with otherwise cannot be stabilized or manufactured into mm-sized particles.

Using this method, the swelling of the composite particles was reduced compared to their non-stabilized counterparts. In addition, using this method for the first time a cyclically stable mm-sized calcium oxalate particle is produced with negligible volume changes and constant discharge performance with cyclic charging and discharging.

Dehydration behavior

Lastly, the dehydration behavior of a promising salt hydrate, K2CO3 sesquihydrate, was also investigated.

Aarts observed that dehydration occurs via two processes determined by the crystal structure. The first process occurring at lower temperature becomes more dominant with higher cycle numbers whereas the second process, starting at higher temperature, becomes less dominant due to morphological changes within the material.

This opens new pathways for material design as it helps in understanding the relation between material properties and performance.

Title of PhD thesis: Towards Stable Salt Hydrates for Heat Storage. Supervisors: Olaf Adan, A.I. Fernández, and Henk Huinink.