Phase change materials (PCM) are a future technology with great potential for thermal energy storage systems. In early July, experts from the fields of research, teaching and enterprise came together to discuss the challenges of PCM technology. All researchers face a similar problem: encapsulation.
Latent heat storage systems use the high melting heat of paraffins, salt hydrates, salts or water. That means that with minor temperature differences, more heat can be stored than via temperature changes in conventional storage materials such as water or concrete. Applications for PCM range from construction materials up to thermal storage in buildings or industry.
In research and development, diffusion-tight encapsulation is a key aspect. Recent research findings show that even minor deviations of the moisture content of the PCM lead to significant enthalpy losses (PC_Cools_S project). Different material combinations (currently largely empirically compiled) often cause undesirable leaks of phase change material. In stable capsules, temperatures over 100 degrees Celsius cause unexpected corrosion. Experts still consider the mechanical stability of PCM unsatisfactory for practical use. Another obstacle is the transfer of the results from laboratory production to technical scale or commercial production. In the PC_Cools_V project, researchers are currently producing mesocapsules for salt hydrates. From a commercial perspective, capsules are still the key cost factor. For example, while smaller capsules improve thermodynamic properties, they also increase the system costs – the MetPCM research project is an example of this.
Alternative concepts focus on technically available and cost-effective capsule materials such as taped bags from the foodstuffs sector (TU Dresden). In addition to this, the researchers are investigating how they can join the capsules permanently, e.g. by welding or adhesion. In the MALATrans research project, researchers are showing how they can increase the thermal performance and reduce the manufacturing costs.
Innovative approaches create visionary PCM technologies with materials which remain dimensionally stable, even in liquid form. That would eliminate the need for encapsulation. With phase change slurries (PCS) in particular, the materials currently used must be analysed for degradation and environmental sustainability. In the Kolan project, scientists are investigating how to achieve a higher thermal capacity, for example. If PCS systems are to be used on a wider scale, prices for products and materials must drop.
Other projects on thermal energy storage
In order to harness PCM technology and make it economical for practical applications, the German Federal Ministry for Economic Affairs and Energy (BMWi) and the German Federal Ministry of Education and Research (BMBF) are funding various research projects in the thermal energy storage research field. Other projects on PCM:
Equipping modular storage units for temperatures from 150 to 350 degrees Celsius, combined with high energy and performance densities, is the stated goal of the researchers in the MOSPEDRA project. To achieve this, the storage modules must be made more dynamic. They aim to implement it using a metallic thermal conduction structure embedded in a phase change material.
In the BERTI project, scientists at DLR are testing chemical thermal storage with calcium oxide. The Stuttgart-based researchers’ laboratory has a unique moving bed reactor. This enables them to configure the thermal heat capacity of the storage system and its heat output independently from one another.
Domestic appliances that buffer thermal energy consume less power, allowing some even to shift their electricity consumption to low-load times. A domestic appliance manufacturer is developing concepts for these kind of products with project partners from industry and research.
In this project, researchers are analysing the commercial and technical potential of cooling units with phase change storage systems as shiftable loads in the electricity grid. They intend to demonstrate the feasibility and advantages compared with other storage technologies and determine their storage costs.
With micro-encapsulated or emulsified organic phase change materials, the solidification temperature is frequently below the melting temperature. This undercooling is a hindrance for many applications. By means of systematic investigations, the researchers want to achieve a fundamental understanding of the physical principles. Based on the experimental findings, models are to be developed to facilitate forecasting of crystallisation properties.
- PCM-DEMO II
This project integrates many PCM systems in suitable demonstration objects and measures them technically under realistic conditions. The goal is to generate reliable measurements for every individual system under real conditions.