Development Line C

Supercooling Phase Change Material

Researchers from the COMTES project are developing a system designed to solve one of the key problems associated with the otherwise extremely sustainable solar power: how to store solar energy for the cold, dark months. In Line C of the COMTES project researchers developed a compact thermal storage based on salt hydrates.

The heat from the solar panels is stored in a salt hydrate, a solution of sodium acetate that features a number of special properties. For example, when heated to melting point, the salt hydrate can store energy without losing it—which is what tends to happen at present, when the ‘storage solution’ is simply a tank of water that is heated up.

The melted salt retains a large volume of energy which, in principle, can remain there forever because the energy is not released until a crystal is added to the salt. In this way, solar panel owners can put some of the summer sunshine aside for the winter months, as Jakob Berg Johansen, Research Assistant at DTU Civil Engineering explains: As soon as the salt reaches a temperature of 58°C, it melts, and the energy we have stored in it stays there. When the salt cools down again, it tries to crystallize. It cannot do so, however, until it has something to form crystals around, which is what actually triggers the process.

When we add even a tiny crystal to the solution, the entire volume crystallizes in around a minute—releasing the energy and the heat in the process. It instantly returns to the 58 degrees again, and that is sufficient to heat hot water and homes.

A range of prototypes have been built and installed at DTU. The team now plans to build what corresponds to a single family home next year, and establish whether they can use solar energy to heat it all year long.

The purpose is to test the system thoroughly before manufacturing and marketing it to the numerous households in Denmark that do not have access to the district heating network and are therefore dependent on other sources of heat during the winter months.

There is still a fair amount of development work to be done before the idea is viable as regards price and at a technological level. But we can see that the market’s there—or that it is sure appear—because people are keen to get rid of their old oil-fired boilers.

The ventilation and heat pump company Nilan has contributed to development and production of the storage modules that are to contain the salt solution.

The storage modul developed in line C

Supercooling Phase Change Material

Photos © DTU Technical University of Denmark