Cold storage using latent heat

Electricity is an important energy carrier with many advantages:

• it can be produced from multiple primary fossil and non-fossil sources;

• It can be transported in large quantities and over long distances;

• it can be converted into the main forms of mechanical, chemical and thermal energy.

On the other hand, one of its major drawbacks is that it cannot be stored, except by reverting to another form of energy, such as chemical (batteries), mechanical (water pumping stations) or thermal (the subject of this article). Efforts are therefore needed in the field of energy storage. This is what the public authorities are encouraging in a report on Key Technologies 2005 [1], in which the theme of energy storage is identified as a major one on which research and development efforts should focus. In particular, the report's authors call for "a multiplication, under attractive economic conditions, of mass storage capacities by a factor of 10, which would considerably alter the energy landscape".

In this way, the refrigeration industry is often faced with a non-constant energy demand, with "peaks" and "troughs". It is therefore interesting to store energy for deferred use, taking advantage of better energy pricing for a lower investment based on average use.

After a few general remarks on thermal energy storage, we describe the advantages of latent heat storage through solid-liquid transformations (pure bodies or solutions). We recall the phenomenological aspects, with particular emphasis on the phenomenon of supercooling, which is sometimes a source of trouble or misunderstanding.

The major part of this article is devoted to the description of the three main methods for cold storage using latent heat:

• ice trays, a tried-and-tested technique based on long-standing experience;

• the method using encapsulated nodules, which has been operational for several years;

• ice slurry, which is still at the research and development stage.