Researchers at Georgia Tech tested a combination of two easily available salts to store clean energy in the form of heat. The thermochemical battery produced showed good energy storage capacity, power density and cyclability
Integrated thermal energy storage in buildings
Thermal salt batteries could be applied in buildings to meet heating requirements or to complement heat pumps to keep them cool. This is the goal that a group of engineers at Georgia Tech (USA) is working on, and a good piece of global research on thermochemical accumulation is being tested on it. The new work published by the team in the Journal of Energy Storage shows clear progress in that direction.
Thermochemical materials: hydrates of salts
Let’s start with a concept upstairs. Today, the storage of renewable thermal energy (TES) is an exciting solution for decarbonizing buildings by providing thermal loads on demand. In three different TES technologies, salt batteries can be classified in the category that exploits materials and reversible thermochemical reactions. This is one of the most exciting options for building as these materials can store from 8 to 20 times more energy per unit of volume than other types of thermal storage. In other words, making small and compact devices with high capacity is possible.
The most promising thermochemical materials? Those based on inorganic salt hydrates, i.e., salts within their crystalline structure water molecules combined in a defined ratio. These are economical and safe compounds, which undergo a solid-gas chemical reaction with water vapor at temperatures below 100 °C.
How do salt thermal batteries work?
Thermal batteries based on thermochemical salt hydrates utilize two phases, hydration and dehydration, for their charging/discharge cycle. For precision during charging, heat is used to drive an endothermic reaction that dehydrates the salt-producing water vapour. “Because energy is stored inside the chemical structure (reaction heat), TCMs show negligible self-charge during storage, making them promising for seasonal or long-term energy storage,” Georgia Tech scientists write.
In the discharge phase, the stored energy is transferred through heat through an exothermic reaction between the dehydrated salt and the water vapour (or wet air) that reproduces the original salt hydrate.
Technical challenges
This is an excellent process on paper, but it encounters several problems in practice. For example, as this process of hydration and dehydration occurs, the salt is subjected to physical stress, condemning it to a mechanical yield over time. “After going through some of these cycles […], it disintegrates into tiny particles and completely dusts or overflies and clusters into a block,” explains Professor Akanksha Menon, who leads the team.
Not only that but devices suffer from incomplete and/or slow reactions that manifest themselves as low energy or power density values, especially during hydration.
A double salt thermal battery
It is at this level that new research enters. Menon and researcher Erik Barbosa, after testing different salts for two consecutive years, found two compounds that complemented each other well: magnesium chloride and strontium chromite. The first tends to absorb too much water, and the second to hydrate too slowly, but together, they can exceed their respective limits.
“We didn’t plan on mixing the salts,” Menon says. “It was just one of the experiments we were trying. Then we saw this interactive behaviour, and we spent a whole year trying to understand the cause and if it was something that we could generalize to use it in the storage of thermal energy.”
These materials were characterised using simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a moisture generator. The salt mixture – the researchers write – has achieved a high specific energy density of 1100 J/g and a peak thermal power of 1.4 W/g.
The group’s search has just begun, but the following steps have already been planned. The objective is to develop structures containing these salts to create rechargeable thermal batteries.
A system-level demonstration is also planned, based on filling a drum with salts in a packed bed reactor. By making hot air flow on the salts, you would dehydrate them by effectively charging the drum like a battery. To release that stored energy, it would simply be blown onto the same damp air rehydrating them.
The research results appear in the article “Thermochemical energy storage using salt mixtures with improved hydration kinetics and cycling stability,” published in the Journal of Energy Storage.
