Turning Renewable Electricity into Stored Heat: HYSTORE Tests Radio-Frequency Heating for Zeolite-Based Storage

Storing thermal energy does not necessarily mean keeping water or other materials at high temperatures. In thermochemical systems, energy can be stored through reversible processes involving materials such as zeolites and water vapour, offering promising opportunities for medium- to long-term energy storage.

But how can the charging process of these systems be made more efficient?

This question is at the heart of a recent study carried out within HYSTORE, in which CNR-ITAE and research partners tested the use of radio-frequency (RF) heating to directly heat zeolite 13X. The experiments achieved charging efficiencies ranging from 89% to 94%, highlighting the potential of this approach for the development of new thermal energy storage systems.

From Electricity to Stored Heat: Why Radio Frequency?

Thermochemical energy storage works differently from conventional systems such as hot-water tanks. In the system investigated by HYSTORE researchers, zeolite 13X interacts reversibly with water vapour: during charging, the material is heated to release the adsorbed water, while during discharge, water vapour is adsorbed again and heat is released.

This approach is particularly interesting for medium- to long-term storage thanks to its high energy density and the possibility of reducing some of the heat losses associated with sensible thermal storage systems.

The study focuses primarily on the charging phase. In conventional thermochemical systems, the storage material is heated indirectly using air or heat transfer fluids. Direct electromagnetic heating instead makes it possible to transfer energy directly to the material.

Previous research has mainly focused on microwaves, which can lead to uneven heating and localised hot spots when applied to larger volumes.

The radio-frequency system used in this study operates at around 27 MHz, corresponding to a wavelength of approximately 12 metres, compared with around 12 centimetres for commonly used industrial microwaves. This offers potential advantages in terms of heating uniformity and scalability, also thanks to relatively simple electrode configurations.

Putting Zeolite 13X to the Test

Before starting the experiments, the researchers compared several candidate materials based on storage capacity, reaction kinetics, stability, availability, cost and scalability.

Commercial zeolite 13X was selected as the best overall compromise for the experimental campaign. In addition to suitable storage characteristics, it is commercially available in large quantities, in different shapes and at relatively low cost.

Three different morphologies were tested:

• 2 mm spherical beads;
• 1.4 mm spherical beads;
• 1.6 mm cylindrical pellets.

The experimental system developed at INOVALAB used a desorption chamber containing approximately 200 grams of zeolite, connected to an evaporator/condenser and equipped with an electrode powered by the RF generator. The setup reproduced operating conditions typical of a closed sorption thermal energy storage system.

Reaching 150 °C with High Charging Efficiency

The experiments showed that all three zeolite morphologies reached the target temperature of 150 °C during tests at 50 W RF power.

The cylindrical pellets heated up fastest, reaching the target in approximately 36 minutes, followed by the larger spherical beads and then the smaller ones. These differences are mainly related to particle geometry and effective volume.

By comparing the energy supplied experimentally with the theoretical energy required to heat and desorb the zeolite, the researchers calculated charging efficiencies of 93.9% for the larger beads, 88.9% for the smaller beads and 93.6% for the cylindrical pellets.

These results indicate that RF heating can transfer electrical energy directly to the thermochemical storage process with limited losses.

From Grid Flexibility to Scale-Up

A system capable of efficiently converting electricity into stored thermal energy could be charged when renewable electricity production is high and release that energy later to meet heating or cooling demand.

The study therefore represents an important step towards understanding whether RF-driven thermochemical storage can move beyond experiments on very small laboratory samples. Further development will be needed before reaching large-scale applications, but the results demonstrate the feasibility of the process under conditions representative of a thermal energy storage system.

For HYSTORE, this research contributes to a broader objective: developing storage solutions capable of connecting thermal demand with the availability of renewable electricity and supporting a more flexible energy system.

The research was conducted by CNR-ITAE, Inova Lab S.r.l. and Sorption Technologies GmbH within the HYSTORE project and was published in the Journal of Energy Storage in 2025.