Understanding PCM Storage Through a Hands-On Mock-Up

Phase change materials (PCM) are at the heart of the HYSTORE project’s innovative thermal energy storage solutions. However, explaining how they actually work — namely, the invisible transition between solid and liquid states that enables large amounts of heat to be stored or released — can be challenging when the technology is enclosed within an insulated casing. To bridge this gap, RUBI developed a transparent PCM storage mock-up specifically designed for public engagement at fairs, conferences, and outreach events.

A Window Into Thermal Storage

The mock-up consists of a double-cylinder vessel made entirely from transparent acrylic, allowing visitors to observe the PCM directly during charge and discharge cycles. The space between the cylinder walls is filled with a capillary tube mat, which acts as the heat exchanger between the heat transfer medium (HTF), in this case water, and the PCM itself.

Crucially, the storage area is sized to hold only a single layer of capillary tubes. This deliberate design choice keeps the energy required to charge and discharge the unit small enough for use in a demonstration setting, without sacrificing visual clarity. The capillary tube mat connects to a single inlet and outlet, which are in turn connected to a thermostat that supplies hot or cold water on demand.

Figure 1 — Empty storage mock-up with blue capillary tube mat (left); thermostat unit used to charge and discharge the PCM on-site.
Figure 1 — Empty storage mock-up with blue capillary tube mat (left); thermostat unit used to charge and discharge the PCM on-site.
Figure 2 — Mock-up filled with SP31 PCM in its liquid (charged) state.
Figure 2 — Mock-up filled with SP31 PCM in its liquid (charged) state.

Observing Phase Change in Real Time

As the PCM begins to solidify from its liquid state, opaque crystalline regions appear alongside still-transparent liquid zones. This reveals a key characteristic of solid–liquid PCM systems: solidification does not proceed uniformly from a single front, but rather nucleates at multiple locations simultaneously, spreading outward from several nucleation sites distributed across the storage volume.

This behaviour has real implications for storage system design, as it affects how quickly and uniformly thermal energy can be extracted. The mock-up makes these dynamics tangible in a way that diagrams or simulations cannot fully replicate.

The PCM in Action: SP31

The mock-up is filled with SP31, a commercially available salt hydrate PCM with a melting point of approximately 31 °C and solidification onset at around 29 °C. These temperatures are close to room temperature, making the phase transition easy to trigger without specialised equipment.

During the demonstration, the thermostat supplies water at around 50 °C to charge (melt) the PCM, and switches to water at approximately 10 °C to discharge (solidify) it.

Figure 3 — Partial solidification: opaque (solid) and transparent (liquid) regions coexist, revealing multiple nucleation sites within the storage volume.
Figure 3 — Partial solidification: opaque (solid) and transparent (liquid) regions coexist, revealing multiple nucleation sites within the storage volume.
Figure 4 — The mock-up on display at a trade fair. The thermostat is positioned below the table, keeping the demonstration area uncluttered.

Deployment at Trade Fairs

The mock-up has been successfully deployed at trade fairs, where it has proven to be an effective conversation starter with potential customers, partners, and the general public. Its compact form factor fits comfortably on a standard tabletop, while the thermostat unit is conveniently concealed beneath the table, keeping the display clean and focused on the visible storage cylinder.

Connecting the Science to the Solution

The PCM mock-up developed by RUBI is more than a demonstration prop — it is a communication tool that translates the underlying science of phase change storage into an experience that stakeholders can see and understand directly. As the HYSTORE project moves towards demonstrating its full-scale solutions in real buildings, tools like this play a vital role in building awareness and confidence in the technology among industry professionals, policymakers, and the wider public.