In their position paper published in August 2015, the scientists of IEA SHC Task 42 (Compact Thermal Energy Storage) summed up the key results of their work between 2009 and 2015. Operating agent Matthias Rommel sees huge potential for latent heat and sorption materials in the long run – in seasonal solar heat storage for small and medium applications, as well as in the building sector. So-called smart grids will also require more heat storage units when devices such as heat pumps and co-generation plants are based on electricity grid requirements. Rommel views the definition of measurement standards for PCM materials as one of the task’s big achievements, which will help in material development. Furthermore, a research group from German research institute ZAE Bayern has performed a first cost estimate of compact heat storage technologies.
The position paper describes two major compact thermal energy storage technologies:
- Phase Change Materials (PCM) materials store much heat when they change from solid to liquid or from liquid to gas. One limitation of the technology is that its advantage over common sensible heat storage systems is only present when heat is stored within the small range of either the melting or evaporation temperature. PCM products have already been available on the market for a number of (niche) applications.
- Thermochemical Materials (TCM) store heat through the separation of two different substances, usually either two liquids or a solid and a gas – the stronger the binding force, the higher the storage density, because higher temperatures are needed to separate the two materials. TCM processes range from physical sorption caused by forces on the surface with storage temperatures starting at 30 °C to chemical sorption caused by covalent attraction with temperatures above 100 °C to chemical reactions caused by ionic forces with temperatures above 200 °C.
As it is still too early to tell where both technological developments are heading, basic research plays a big role in the IEA SHC Task, which is a joint task with the Task 29 of the IEA Energy Conservation through Energy Storage (ECES) Programme. It brings together material scientists and process engineers (see http://solarthermalworld.org/content/austria-task-42-meeting-compact-thermal-energy-storage-systems) and has resulted in scientists from different research labs worldwide agreeing on a standard for the measurement of the most important properties of phase change materials – melting enthalpy and phase change temperature.
How much energy can be stored in PCMs?
It is difficult to measure the precise amount of latent energy contained in the phase change process and the exact temperature at which the change occurs. In material development, the size of available samples is often limited to a few milligrams. Hence, small differences in sample handling and measuring can have a big impact on results. Under the lead of Fraunhofer ISE, Germany, the scientists agreed on standards, e.g., for sample size, pre-heating of crucibles and the duration of continued temperature measurement after the phase change (see the attached document Standard to determine the heat storage capacity of PCM using hf‐DSC with constant heating/cooling rate [dynamic mode] from January 2015). The procedure was evaluated in a Round Robin test at seven research institutes in four countries with a paraffin called octadecane, which melts at 28 °C. Materials such as this are, for example, used in the component activation of building technology. “A few degrees can make a big difference here,” Rommel points out. If a PCM material is used to bolster room temperatures, a melting point of 24 °C would be good, but a melting point of 26 °C would be too high.
In a next step, further Round Robin tests are planned to show whether the same standards lead to consistent measurement results for paraffin and other phase change materials with different melting points. “The debate and research work on the standardisation has taken four years up until now, and we have improved the consistency of measurements significantly, which is essential for material development,” explains Rommel. With consistent data, the focus on promising materials will be easier to keep in the future, especially regarding the development of special materials tailored to specific applications.
First cost estimations made by ZAE Bayern
In the end, latent heat and sorption storage technology will have to prove their cost effectiveness compared to other storage technologies and to direct heat generation from fossil fuels or electricity. In a top-down approach, the expert group calculated that based on the costs of substituted energy, the maximum acceptable storage capacity costs were in the range of EUR 2 to 4 per kWh of installed storage capacity for seasonal storage systems with a 25-year lifetime and a 1 % interest loan (see paper attached). In a bottom-up approach, the researchers examined 20 existing compact heat storages by sending out a questionnaire, collecting data about the costs of the initial investment, operation, maintenance and installation. Most of these storage systems are still in the R&D phase. The estimated bottom-up costs for those storage systems were in the range of EUR 2 to 15 per kWh of installed storage capacity. Rommel emphasises that it is actually too early for a more detailed cost assessment. “There is a lot of development work to do, mostly on process engineering, but also on the materials,” he says. “To find solutions for all the small challenges one encounters will require a lot of work and therefore, good funding and interdisciplinary cooperation.”
And this is exactly one of the position paper’s urgently needed measures called for to drive market deployment:
- Strong support of R&D work by governmental and international research programmes is necessary to follow a broad and internationally collaborative research approach.
- The companies involved in the development of compact thermal storage systems are often relatively small and highly innovative. They need strong support by governments to be able to apply their technology in the building and industrial process sectors.
- Strong support of a growing number of demonstration projects is needed in order to gather operational experiences, to monitor and evaluate performance and to improve the performance of systems step by step.
Email addresses of the two Operating Agents:
Matthias.Rommel@spf.ch for IEA-SHC Task 49
Andreas.Hauer@zae-bayern.de for IEA-ECES Annex 29