The HYSTORE PCM ALL-IN-ONE Solution:
 Compact Thermal Storage for Multi-Family Buildings

A New Approach to Domestic Hot Water

What if every apartment in a multi-family building could generate its own domestic hot water (DHW) efficiently, sustainably, and without sacrificing precious living space? That is precisely the challenge the PCM ALL-IN-ONE solution — developed collaboratively by AIT, OCHS, PINK, RAAL, and RUBI within the HYSTORE project — was designed to solve.

The concept is smart: a fully self-contained thermal storage unit, small enough to fit within a bathroom wall, that taps into an existing low-temperature underfloor heating network to deliver high-quality hot water on demand. 

How It Works

The underlying assumption is that each apartment is connected to a low-temperature underfloor heating network operating at approximately 35 °C. Two small-scale propane heat pumps upgrade this heat to 60 °C for DHW generation and are mounted directly onto the storage unit. The refrigerant charge is limited to 150 g per heat pump circuit to address safety requirements.

The storage is designed to be charged once per day, with a heat pump charging power of 2 to 3 kW and a total storage capacity of 6 to 9 kWh. On the discharge side, the system meets the short-term power requirements of the Austrian ÖNORM standard, delivering domestic hot water at 40 °C with a peak thermal output of up to 20 kW.

The ALL-IN-ONE solution uses two small-scale propane (R290) heat pumps as refrigerant due to its low Global Warming Potential (GWP = 3) and suitability for small-charge systems, allowing the solution to avoid the safety classifications that apply to larger refrigerant quantities.

The Core Component: A High-Performance PCM Storage Unit

The central element of the ALL-IN-ONE solution — shown in the photos with Felix Hochwallner from AIT — is a modular Phase Change Material (PCM) storage unit. Unlike conventional water tanks, PCM storage exploits latent heat: the energy exchanged when a material transitions between solid and liquid states. This allows a comparatively large amount of thermal energy to be stored within a compact volume.

The key technical challenge for DHW applications is not storage capacity alone, but the ability to release heat rapidly when needed. This is addressed through an internal Refrigerant-PCM-Water Heat Exchanger (RPW-HEX) built with aluminum Micro/Multi Port Extrusion (MPE) tubes. The MPE geometry provides a high surface area-to-volume ratio, enabling sufficient heat transfer rates between the refrigerant circuit, the PCM, and the domestic water within a single integrated component.

Flat by Design: The Footprint Imperative

One look at the prototype in the photos and a striking feature is immediately apparent: the storage unit is remarkably flat. This is a direct consequence of the installation context.

Given that the ALL-IN-ONE solution is intended for installation within bathroom walls of existing apartments, the footprint area is one of the most critical Key Performance Indicators (KPIs) for the entire project. Wall space in residential bathrooms is limited, and any thermal storage system must fit within the available cavity without compromising the room’s functionality or aesthetics.

The team has therefore optimised the geometry of the storage unit to minimise depth while maximising thermal capacity; a form factor that makes wall integration genuinely feasible in real retrofit scenarios. Beyond thermal performance, the ALL-IN-ONE solution was developed with practical deployment in mind.

The design emphasises ease of installation, the use of replaceable components to support long operational life, and material choices that allow for recycling at the end of life. These considerations are intended to make the system viable not just technically, but in the context of real building stock and maintenance practices.

Built for the Long Term

The ALL-IN-ONE solution was conceived with sustainability beyond energy in mind. The design prioritises:

• Ease of installation, reducing labour costs and disruption in occupied buildings
• Replaceable components, so that individual parts can be serviced or upgraded without replacing the entire unit
• Recyclability, with material choices that support end-of-life recovery

The modular architecture means the system can adapt as technology evolves — an important consideration for infrastructure expected to serve buildings for decades.

From Lab to the Mürz Valley: The Austrian Demonstration Site

Within HYSTORE, the ALL-IN-ONE solution is planned for demonstration at a multi-purpose building near Semmering mountain in the Mürz valley, Austria. The building dates from the 1960s and has undergone limited retrofitting. Its energy system is mixed, combining fan coil heaters, underfloor heating, high-temperature office radiators, and a heated ceiling for peak loads, with thermostatic valves controlling temperatures on daily and weekly schedules.

The site is not connected to a district heating and cooling (DHC) network, which makes it a representative case for the decentralized application scenario the ALL-IN-ONE solution targets. Integration into the existing system will allow the consortium to evaluate performance under real operating conditions.

Key Technical Specifications at a Glance

Parameter	Value
Storage capacity	6–9 kWh
Charging frequency	Once per day
Charging power (heat pump)	2–3 kW
Peak discharge power (DHW)	Up to 20 kW
Refrigerant	Propane R290 (GWP = 3)
Refrigerant charge per circuit	< 150 g
Source temperature (underfloor heating)	~35 °C
DHW delivery temperature	60 °C (40 °C tap)

 

Looking Ahead

The PCM ALL-IN-ONE solution represents a convergence of material science, thermodynamic engineering, and practical design thinking. By combining latent heat storage with natural refrigerant heat pumps and an innovative multi-fluid heat exchanger — all packaged in a form factor that fits within a bathroom wall — the consortium is developing technology that could genuinely transform how multi-family buildings generate domestic hot water.

Stay tuned to the HYSTORE blog for updates as the system moves from the workshop to the Mürz valley, and from demonstration to deployment.