European research consortium Empa says that it has demonstrated how to store heat energy generated during sunny summer days for use in winter. The laboratory-scale system is one of three competing system designs developed in response to the COMTES-sponsored thermal energy storage challenge.

Heat storage test apparatus. Credit: EmpaThe project, a joint venture of researchers from Switzerland and Northern Ireland, studied the use of sodium hydroxide (NaOH) as a thermal storage medium. This approach converts thermal to chemical energy.

Adding water to a container of NaOH releases chemical energy and causes the mixture to heat. The researchers say the reverse is true: heating a solution of NaOH and water concentrates the solution, storing heat in the form of chemical energy.

The sodium hydroxide solution is highly viscous, a characteristic that led to failure of an initial storage/heat exchange design. Empa researcher Benjamin Fumey conceived of a method of heat exchange that exploits viscosity: the concentrated fluid drips slowly through a spiral pipe, absorbing water vapor along the way and transferring heat to the pipe.

Furney further optimized the lab-based system by adjusting NaOH concentrations and temperatures of water entering and leaving the pipe. Heating water vapor with a geothermal probe prior to adding to it to the pipe. The NaOH concentration diminishes from 50% to 30%, releasing heat to the pipe. The pipe’s heat is around 50 degrees Celsius.

One practical application for this system could be floor heating. A homeowner could store heat energy during the summer and release it in the winter, using spiral pipes in the floor. The system regenerates after heat release as the NaOH and water recirculate. The pipes themselves are a stock item, possibly reducing system construction cost.

The Empa team says it is searching for industrial partners to build a household system based on the successful laboratory model.