If solar energy is to be the primary or only source of heat for houses in the future, there will arise a need for storing it more efficiently. Materials have to be found that are able to hold more energy than water, but with less volume and higher loss. Task 32 of the International Energy Agency's (IEA) Solar & Cooling programme was working on this issue. The scientists presented their results at the Eurosun 2008 in Lisbon, at the beginning of October.
They weren’t too optimistic. Simulations of several combi systems with storages based on Phase Change Material (PCM) revealed unforeseen limitations: The volume of PCM one can introduce into a water tank is limited to 20 to 30 % of the total storage volume. "This is due to the necessary containment of the material. And also, the heat extraction power from PCM to water is limited by the diffusion process inside the material", explained Task 32-operating agent Jean-Christophe Hadorn from BASE Consultants SA, Switzerland.
According to him, sodium acetate, as a PCM, did not show a decisive advantage over water when put in a combi tank. Fine-tuning of adequate PCMs and a proper way to build an efficient heat exchange should therefore remain on the agenda of international solar research.
The situation with sorption storage is no better one. Silica gel and zeolite showed limitations in terms of usable temperature lifts during discharge when used as particles in beds. Nevertheless, an extruded solid zeolite with air channels showed interesting properties at a laboratory level. Using chemicals for storage purposes requires finding an adequate reaction. A promising material is called magnesium sulphate heptahydrate (MgSO4 7H2O). The underlying principle is to dry the material in summer by using solar heat. Rehydration will deliver back the energy in wintertime.
According to Task 32 partner Chris Bales from the Swedish Solar Energy Research Center, the work's main conclusion is that there are a number of promising technologies and materials for seasonal storage of solar heat regarding single family houses, but that a lot of research still has to be done before it can be of practical and commercial use. He named the problems that emerged in Lisbon: "The current materials are either too expensive, do not have the correct properties or have not yet proven to work in prototypes with realistic boundary conditions." Therefore, fundamental research would be necessary to get a better understanding of the physicochemical processes. To improve future research work, Marco Bakker from the Energy research Centre of the Netherlands (ECN) announced a new working group that will bring together material experts from the IEA programme "Energy Conservation through Energy Storage" with solar specialists from the IEA programme "Solar Heating & Cooling". "Several groups around the world are working either on thermal energy storage materials or applications. However, activities are not sufficiently coordinated. They are either limited to specific applications or to specific materials", said Bakker at the Eurosun 2008. The new assignment will bring together the continuing work on both.
More information: www.iea-shc.org/task32
The text was written by Joachim Berner, a Munich-based journalist and solar thermal expert.
Jean-Christophe Hadorn's conference contribution is available for download: Advanced storage concepts for active solar energy Chris Bales' conference contribution is available for download: Chemical and Sorption Storage – Results from IEA-SHC Task 32