Boosting power of salt hydrates for heat storage

January 24, 2023

Natalia Mazur defended her PhD thesis at the department of Applied Physics on January 23rd.

Heat is one of the most commonly used energy forms in the domestic environment. The most sustainable way to provide it is to use solar, waste, or other available heat sources. Unfortunately, there is often a mismatch between heat demand and production in space and time. Therefore, to solve this issue, energy storage is needed. The power output of salt hydrates can be boosted by deliquescent additives. The extra water that those additives attract increases mobility of the system which increases reaction kinetics.

Emerging energy storage technology

Thermochemical heat storage in salt hydrates is an emerging energy storage technology. Salt hydrates are salts that can react with water and incorporate it into their structure without dissolving. When solid salt reacts with water vapour, heat is released. To reverse this process, heat must be applied to the hydrated salt, effectively drying it as it releases the water molecules.

Both reactions could be seen as discharge and charge of the heat storage. The heat generated during discharge could be supplied to a family home to provide hot tap water or space heating. To charge the system, solar heat could be harvested with the help of roof panels, industrial waste heat, etc.

Storage without loss

The significant advantage of using thermochemical reactions to store heat is the possibility to store it indefinitely without any loss, provided salt and water are separated. Moreover, both are cheap and non-toxic, which is crucial for the domestic environment.

Nevertheless, as a developing technology, challenges still need to be overcome. One of those challenges is the limited power output connected to slow reaction speed. The present research tackles this challenge.

Most promising salts

First,  over 1000 reactions and over 400 salt hydrates have been evaluated, confirming that potassium carbonate and strontium bromide are the most promising for domestic heat storage. It is due to the large amounts of energy they can store at low costs and temperatures suitable for this application. The desired increased reaction speed was achieved by mixing these salts with a minor amount of secondary salt that has a strong affinity for water. A method was developed to preselect the most optimal salt combinations.

Our findings show that to achieve the best enhancement, the second salt should readily form a salt solution when the other salt remains solid. Furthermore, it is beneficial when this second salt cannot react with the main salt. If an undesired, so-called side reaction occurs, it can decrease the amount of energy stored in the system or have the opposite effect, i.e. slowing down the reaction speed. Therefore, we accredit the power boost for heat storage to excess water attracted by the second salt that acts as a lubricant for the reaction, thus making it go faster.

Title of PhD thesis: Boosting power of salt hydrates for heat storage. Supervisor: Olaf Adan


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