Public defence, Energy Technology, M.Sc. Aleksi Barsk

Title of the thesis: Microencapsulation of calcium chloride into hydrophobic nanosilica capsules for seasonal thermochemical energy storage

Thesis defender: Aleksi Barsk
Opponent: Prof. Micheal Steiger, University of Hamburg, Hamburg
Custos: Prof. Annukka Santasalo-Aarnio, Aalto University School of Engineering

In Finland and other northern countries, solar heat is mostly available in summer, while buildings need heat mostly in winter. Storing heat over several months is difficult: hot-water tanks lose heat over time and take up a lot of space. A promising alternative is to store energy in a chemical form. Calcium chloride is a salt that releases a large amount of heat when it absorbs water vapor. It can be “recharged” by drying it with solar heat. If kept dry in a closed container, it holds its stored energy for months with minimal losses. It is also cheap and widely available.

However, calcium chloride readily dissolves in the water it absorbs. The powder turns into a liquid, clumps together and stops working due to loss of surface area. Earlier solutions have placed the salt inside porous host materials or capsules, but these composite materials take up so much space that the energy stored per volume has remained too low for practical heat storage in homes.

This doctoral thesis presents a new way to encapsulate calcium chloride inside water-repellent shells made of silica nanoparticles. A concentrated salt solution and silica particles are mixed at high speed, and the mixture is then shaken into a fine powder of tiny capsules that can contain only 1% silica. Water vapor passes through the porous silica shell, but the salt solution stays inside, so the capsules do not stick together even when the salt fully dissolves.

The doctoral research also revealed a previously unreported air-liquid structure formed during preparation, and showed that the water absorption process in small calcium chloride particles is partly random. The latter helps explain conflicting results reported in earlier studies.

Under conditions suitable for home heating, the developed calcium chloride capsules are estimated to store more than 1.5 gigajoules of heat per cubic meter, clearly more than previously reported materials. In a modern single-family house, a storage covering the whole winter’s heating demand could therefore take up roughly a tenth of the building’s floor area.

So far, however, the capsules have only been tested in milligram-scale laboratory experiments. Further research is needed to scale up production, improve the capsules’ low heat conductivity and demonstrate a complete storage system. Still, the high energy storage density achieved at material level brings year-round solar heating of buildings one step closer.
Key words: thermochemical energy storage, microencapsulation, calcium chloride, water vapor absorption, fumed silica, phase inversion

Thesis available for public display 7 days prior to the defence at Aalto University's public display page. 

Contact information: 
Aleksi Barsk, aleksi.barsk@aalto.fi