Zeolite Comparison Reveals Which Crystal Best Stores Heat in Water

Storing heat in porous crystals instead of batteries or hot water tanks has long promised a leap in energy density, and a new study from researchers at ZAE Bayern in Garching, Germany, has now shown just how much the choice of crystal matters. In work published in Frontiers in Thermal Engineering, a team led by Henri Schmit put two commercially available zeolites, known as CWK 13XBFK and CWK NaYBFK, through a four-stage evaluation designed to mimic the exact conditions they would face inside a real thermal storage device. The outcome was striking: after 140 charging and discharging cycles, one material lost more than a third of its water storage capacity, while the other retained almost all of it. The result is a reminder that a material that looks brilliant on paper can quietly crumble under the humid heat of a working machine.

Adsorption thermal energy storage works by exploiting the fact that water molecules bind strongly to the enormous internal surface area of microporous solids such as zeolites. When humid air flows over a dry zeolite bed, water vapour is taken up and the heat released during binding can be delivered to a process or a building. Charging the store reverses the process: heating the bed drives the water out, and the energy is effectively locked in the desiccated crystal until needed. Because the energy is stored chemically rather than as sensible heat, storage densities of around 180 kilowatt hours per cubic metre are theoretically achievable, several times what a hot water tank can manage. Open adsorption systems have already found commercial niches, notably in an energy-efficient dishwasher developed by Bosch Siemens Hausgeräte together with ZAE Bayern, but researchers in the field concede that adsorption heat storage remains in the early stages of commercialisation.

The central problem the German team set out to solve is one of matching. An adsorbent that performs superbly at one combination of temperature and humidity may be mediocre at another, and the conditions are dictated entirely by the application. A mobile storage unit harvesting industrial waste heat operates under very different temperatures than a sorption system built into a tumble dryer. The team therefore proposed a method in four steps: first compare candidate materials under application conditions using characteristic curves, then measure dynamic breakthrough curves in a realistic fixed bed, third validate a computer simulation of that bed against the experiments, and finally test the hydrothermal stability of the shortlisted materials by cycling them hundreds of times under the actual operating conditions. The novelty, the authors argue, lies in folding hydrothermal stability testing, which previous studies often omitted, into the design workflow itself.

The first step hinged on measuring adsorption equilibria with unusual precision. The team used a simultaneous thermal analysis instrument operated in thermogravimetric analysis mode, coupled to a humidity generator and a high-accuracy dew point mirror calibrated over a range from minus 50 to plus 95 degrees Celsius with an uncertainty of no more than plus or minus 0.1 degrees. Roughly 300 milligrams of each zeolite were exposed to carefully controlled combinations of temperature and dew point, generating the equilibrium data needed to construct so-called characteristic curves. These curves, rooted in adsorption potential theory going back to Dubinin, plot the volume of water adsorbed against the adsorption potential, a measure of the free energy change as vapour binds to the pore walls. Once a characteristic curve is known, the volumetric water uptake for any pair of adsorption and desorption conditions can be read off directly, allowing materials to be compared before a single prototype is built.

The measurements proved reliable. For the 13X material the characteristic curve was valid across adsorption potentials from 105 to 3,495 kilojoules per kilogram, and for the NaY material from 106 to 3,524 kilojoules per kilogram, with combined measurement uncertainties of only a few kilojoules per kilogram and about 0.001 cubic centimetres per gram in adsorbed volume. Crucially, adsorption and desorption measurements coincided almost perfectly, indicating that true equilibrium was reached and that neither material shows meaningful hysteresis. The curves also agreed closely with published data for the same and similar zeolites, confirming that the thermal analysis route can replace bulkier methods that require nearly 50 grams of sample. When the team then read the curves against two application scenarios, a mobile sorption storage and an industrial tumble dryer, the NaY zeolite came out slightly ahead in both cases, delivering a volumetric water turnover of 0.270 cubic centimetres per gram for the mobile scenario compared with 0.240 for the 13X, and 0.266 against 0.235 for the dryer scenario.

Static equilibrium, however, says nothing about how fast a real storage bed charges and discharges. For that, the researchers built an open-air test rig in which a blower pushes air through a countercurrent humidification column and an electric heater capable of reaching 350 degrees Celsius, before the flow enters a fixed bed of zeolite pellets. The column, 148 millimetres in diameter and filled to a height of 300 millimetres, is double-walled with evacuated perlite filling to form vacuum super insulation, minimising heat losses and temperature gradients across the bed. Dew point mirrors and Pt100 sensors continuously recorded the air conditions immediately before and after the bed. The resulting breakthrough curves, which track how temperature and humidity at the outlet evolve over time, revealed clear differences between the materials. During desorption, the NaY bed showed sharper, more pronounced transitions, which are generally preferred in applications, whereas during adsorption the 13X bed changed state more abruptly and cleanly.

These experimental curves then served as the benchmark for a one-dimensional fixed bed simulation built on four coupled partial differential equations describing the mass and energy balances of gas and sorbent. The model treats the pellets as isothermal, accounts only for mass transfer resistance in the gas boundary layer around the grains, and was run in Matlab with spatial steps of 0.25 to 0.5 centimetres and time steps of one to two minutes. When adjusted for heat losses, the simulation reproduced temperature levels and phase durations of the measured breakthrough curves very well, and even captured a striking dew point peak in the first hour of NaY adsorption runs, confirming that this behaviour was genuine start-up dynamics rather than a measurement artefact. Transitions in the simulation were somewhat sharper than in the experiments, a discrepancy attributed to airflow maldistribution and thermal capacitances not covered by the model. With validation complete, the tool can now be used to scale column geometry and operating parameters to real systems without repeating costly experiments.

The final and, in this case, decisive step was the cycling test. A purpose-built rig subjected both zeolites to 140 full cycles under conditions derived from the tumble dryer integration: desorption at 300 degrees Celsius with a dew point of 60 degrees, and adsorption at 42.5 degrees Celsius with a dew point of 30 degrees. Reference cycles at milder conditions were run periodically, and the samples were weighed to track water uptake. The 13X zeolite began with an adsorbed water mass of 0.281 grams per gram of dry material, but lost 4.6 percent within just ten cycles and finished at 36.5 percent below its starting capacity after 140 cycles, consistent with earlier reports that zeolite 13X loses roughly half its capacity when exposed to 300-degree humid conditions. The NaY zeolite, starting slightly higher at 0.292 grams per gram, lost only 1.1 percent after ten cycles and just 3.3 percent after 140, with the rate of decline slowing as cycling continued.

For the envisioned application the conclusion is unambiguous: 13XBFK should not be used under those conditions, while NaYBFK shows the hydrothermal resilience needed to sustain storage capacity over a service life. The team is careful to note that stability verdicts cannot be transferred between applications, since lower or higher temperatures and dew points change the ageing chemistry, and that economics and adsorption enthalpies, which feed into the coefficient of performance, must also enter the final decision. The authors acknowledge that their methodology is time-consuming and demands up to three separate test setups, a drawback they consider nonetheless essential for responsible sorption system design. The work was part of the AMThES project supported by the German Federal Ministry for Economic Affairs and Climate Action.

The broader significance of the study goes beyond one dryer or one storage vessel. It demonstrates a template for screening adsorbents in a way that mirrors reality: equilibrium data first, dynamics second, validated simulation third, and brutal long-term stability testing last. As industrial waste heat recovery and electrified heating push the demand for compact, high-density thermal storage, the difference between a material that fades by a few percent and one that collapses by a third could determine whether adsorption technology finally moves from the laboratory bench to the factory floor. For zeolite NaY, at least under these punishing conditions, the future looks considerably drier, and that is precisely why it lasts.

Subject of Research: Application-oriented characterisation and simulation of zeolite materials for adsorption thermal energy storage

Article Title: Application oriented material characterisation and simulation for adsorption thermal energy storage

Article References: Schmit, H., Pöllinger, S., Schubert, T., Lävemann, E., & Hiebler, S. (2025). Application oriented material characterisation and simulation for adsorption thermal energy storage. Frontiers in Thermal Engineering, 5, Article 1513507. https://doi.org/10.3389/fther.2025.1513507

Image Credits: AI Generated

DOI: 10.3389/fther.2025.1513507

Keywords: adsorption thermal energy storage, zeolite, characteristic curve, breakthrough curves, fixed bed simulation, hydrothermal stability, thermogravimetric analysis, water adsorption, thermal energy density, tumble dryer, mobile sorption storage, cycling stability