Heating is one of the main energy drivers in European households, accounting for more than 60% of its energy with an additional 15% going to heat water. Fluctuating electricity prices, driven by weather, time-of-day and availability of local energy production through solar or wind also affects the cost of heating and use of hot water for washing and shower. Replacing existing electricity-based heating with water-based methods, the need for hot water will increase making latent heat storage technologies increasingly relevant.

COWA Thermal Solutions produces latent heat storage units with higher energy storage capacity than traditional water based hot water tanks enabling smaller units for the same heat capacity. Moreover, these units are charged by electrically based heating when the prices and power demands are lowest resulting in a peak shaving effect on the overall power consumption, given enough units are installed. Furthermore, a significant amount of such units will reduce the need for additional lower electricity costs for consumers, reduced CO₂ emissions and reduce environmental impact of building out new electricity producing infrastructure.

Salt Hydrates for High‑Density Thermal Storage

Sure2Coat has applied a type of salt hydrate that can store more heat than pure water for the same volume, effectively reducing the size of the units. The salt hydrate will crystallize when cooled down and transfer the heat to a water phase transported in pipes through solution.

A challenge with salt-hydrates for domestic heating units delivering hot water at 50-70 °C  is the  potential corrosive properties towards steel or aluminium. To avoid this, Sure2Coat developed new PEO (plasma electrolytic oxidation) coatings and methods for applying these to allow for extensive use of cheaper, lighter and more sustainable aluminium compared to i.e stainless steel. Also, aluminium transfers heat much more effectively, making the heat storage units smaller.

Addressing Corrosion and Heat Transfer Challenges

Two issues needed to be solved, the first being the challenge of salt hydrates potential corrosive properties towards steel and aluminum and the second is the low heat transfer due to little movement in the salt hydrate solution during crystallization.

For the first challenge Sure2Coat aimed to solve this trough novel surface treatment developing PEO coatings with higher corrosion and heat transfer properties. For the second Sure2Coat developed novel complex shapes for rapid extraction of the heat from the salt hydrate solution into the water phase transported in pipes.

Enhanced Heat Transfer Through Advanced Aluminium Profiles

Using complex shaped aluminium profiles with a high surface area heat can be led more rapidly the tank  fluid in the tank to the water in the pipes. Tests showed that the efficiency in both storing and extracting heat for use by the household increased by more than 75 % with the complex shapes installed. This makes the units relevant for showers, hot water extraction and rapid heat storage when electricity prices are low andcan save electricity costs related to domestic hot water for a household by up to 70 %.

Validation, Scaling and Industrial Implementation

SINTEF demonstrated the technology by conducting long term tests on coated complex shaped heat transfer enhancers in salt hydrate solution, exposing them to relevant temperature cycling for more than 1600 cycles. HEREON developed PEO  and MicroArc developed the coating process to industrial scale for complex aluminum profiles. The profiles were  developed by WUST and SINTEF COWA developed and improve the salt hydrates with new additives, so that the materials are stable and can function for decades without losing their special properties. Finally, SUPSI integrate the surface treatment of complex shapes into manufacturing line using advanced robotics.

Household marked is the first approach for COWA, with a natural continuation of expanding this its use to larger buildings like hospitals, shopping malls, office buildings, industries, swimming pools, etc.

More work could be done to improve the coatings to withstand more types of salts that are more corrosive. It is also relevant to further improve the coating application to make it cheaper to make the units. In addition, wider acceptance for such units could be explored, in comparison with hot water units and gas-water heaters, as well as increasing the operational temperature of the units as such.

References

https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Energy_consumption_in_households

Contact

For more information, please contact Martin Fossen, Senior Research Scientist, SINTEF: