Chistoph Zauner, a research scientist at the Austrian Institute of Technology, has been investigating the use of phase change material for a variety of applications since 2010. His most recent papers include “Experimental characterization and simulation of a hybrid sensible-latent heat storage,” published earlier this year in Applied Energy. He discussed his work in an email interview.
Q: How did you first become interested in phase change material?
A: I worked for quite some time in the field of solar thermal energy and was especially focusing on industrial application with non-standard solar collector, so-called medium temperature collectors. This more efficient class of collectors (concentrating and non-concentrating) can produce temperature up to 250° C heating pressurized water, oil and generate steam. In order to achieve high solar fractions (i.e. cover much more than 10% of the total required process energy by solar energy), one needs to store energy. Standard storages (steam, oil, water) have limitations and latent storage certain advantages. Thus, we started developing such storages. In the meantime, we want to use it for a much broader range of applications (not only solar thermal).
Q: Describe the hybrid sensible-latent heat storage concept you have been working on.
A: In our new concept, we place the PCM inside the tubes of a modified shell-and-tube heat exchanger [shown above]. This is in contrast to the well-known approach of placing it outside. On the shell side we use a heat transfer fluid (in our first prototype we used oil) which at the same time serves as a sensible storage medium. Thus, we achieve a hybrid sensible-latent heat storage, which offers several opportunities:
• Heat transfer fluid (in our case oil) and PCM fractions can be varied over a wide range, i.e. a hybrid sensible-latent heat storage is realized. Advantages of both domains can be exploited, such as high energy density of the PCM and high power density of sensible storages
• Fewer weld seams as for the standard concept (PCM outside many small tubes) leads to storage cost reduction
• Larger heat transfer area between tubes and PCM enables higher storage power
• Our tubes serve as a macroencapsulation of the PCM, which serves as protection and increases storage lifetime.
Q: Is HDPE in use as a PCM in any commercialized application?
A: HDPE is a so-called commodity plastics. It is the kind of polymer used most out of all polymers. As such it is produced in a multi-million-ton scale each year. Also a versatile recycling industry is in place, which allows for further cost reduction potential (we know which types are suitable and which ones are not). Usually HDPE is used to contain PCMs only. There are no commercial applications yet, where HDPE is used as PCM. Currently we are investigating various possible applications.
Q: How do you anticipate the viscosity of the PCM affecting the thermal modeling? At what point does this significantly contribute to the internal convection in the tube?
A: Convection plays a minor role for our HDPE grade. This may be somewhat different for other grades and was analyzed in our lab. Our models can be adapted to incorporate convection, too. If necessary, we also have 3d-CFD models available.
A 40 kWh, 100 kW peak power hybrid latent-sensible storage system was successfully tested at AIT labs at temperatures up to 200 °C.
Beyond manufacturing costs, what are the benefits of the inverted shell-and-tube in comparison to other geometries such as a packed bed?
A: Apart from the advantages mentioned above, there is one particular key advantage over packed beds: packing density. Our storage can achieve up to 90% PCM volume density, whereas the theoretical limit for ideally packed spheres is 74%. In a packed bed, however, one does not have “ideal packing,” but the situation of “random packing,” where PCM volume fractions of 64% are achieved.
We found a certain way, which we do not disclose, how to actually fill up the whole tubes even for the crystallized (shrinked) PCM. Usually, PCM macroencapsulations are filled up to 100% only in the molten state, which further reduces the final volume PCM fraction of the whole storage (i.e. kWh/m3).
So summing up: We achieve much higher energy densities.
Q: How was the DSC data implemented into the thermal modeling?
A: Actual measurement data can be easily implemented in our Dymola model. We use different approaches for the two models described in the paper (Stefan-model, cp(T)-model).
However, it is important to emphasize that one has to perform the DSC measurement “in the correct way.” This means one has to use the correct DSC parameter sets. By comparing the data obtained from different DSC settings to experimental storage data, we found out that very often DSC measurements are done in the wrong way. Wrong DSC settings lead to incorrect material values (especially melting temperature, sub-cooling and phase change enthalpy). However, we know now how to do it correctly and implemented the corresponding curves in the models.
Q: Would a sharper phase change peak be advantageous to the proposed application? How would this also affect the Stefan model?
A: We already designed storages for different applications (various combinations of low/high power, low/high capacity, different temperature levels). Sometimes it is very important to actually have a PCM with a sharp peak and sometimes even large subcooling doesn’t matter. It depends on the application.
Of course, one has to be careful by applying the various models (not only the Stefan model) and not to spoil the underlying assumptions. We learned a lot in that direction by comparing experimental storage data to simulations and know now very well where the limits are.
Q: What are the next steps in your investigation of this storage concept for district heating networks and industrial processes? How close is it to possible commercialization?
A: It is important to emphasize that AIT is not a university, but more like a real company which has to do “real business” and earn “real money.” We do business in various ways and offer different business models.
This ranges from material characterization or simulations directly done (and paid) by customers. We also offer storage engineering using our models and experimental know-how to storage manufacturers. We also demonstrate storages at real demo sites (currently we have projects in polymer extrusion and aluminum die casting) and evaluate their potential in various companies (e.g. we are currently investing a specific process in steel industry using a PCM-steam-storage concept).
We can provide various services or even serve as a “one-stop-shop” for energy optimization of industrial processes using storages. This starts from analyzing the process in detail (incl. monitoring), designing the storage (including integration), organization of storage manufacturing, integration at the plant and commissioning. Also this includes financing aspects (contracting, subsidies, R&D projects etc.).
The storages are permanently optimized but can be bought right away as we are only using industrially available PCMs (we also tested [PureTemp PCMs] and might use them, of course) and heat exchanger/storage manufacturing techniques.
Q: What other projects are you working on that might be of interest to the PCM community?
A: We also work on “overheating solutions” using PCM. Some articles have been published in that direction already, including “High temperature phase change materials for the overheating protection of facade integrated solar thermal collectors.” Also, we employ PCMs in car batteries and developed concepts there. We simulated and tested various prototypes of real batteries.
A related topic is development of new insulations, especially aerogel-based. This is very much needed for storages and also for energy efficiency in industries (“stop wasting energy first, then re-use it!”).
[For more examples, see www.ait.ac.at/en/research-fields/sustainable-thermal-energy-systems/projects/storeitup-if.]
We are very much looking for partners for new PCMs. We do not produce them on our own. However, we do some development with partners on organic PCMs.