The Austrian Institute of Technology (AIT) and Ochsner Wärmepumpen are developing a domestic heat pump that uses no more than150g (0.33lbs) of propane (R290) per module in a multi-circuit system to provide space heating and domestic hot water in lieu of gas boilers in large multifamily houses.

The development of domestic heat pumps is the focus of AIT’s and Ochsner’sGasthermenersatz” (gas boiler replacement) project and of the European funded project “HYSTORE”, which were explained by Fabrizia Giordano, Junior Research Engineer at AIT during the European Heat Pump Summit, which took place in Nuremberg, Germany, October 24–25. Her presentation was called “Modular low capacity R290 heat pump water heaters enhanced with compact PCM thermal storage.”

AIT collaborated on these projects with Ochsner Wärmepumpen GmbH (Gasthermenersatz) and Pink GmbH, Raal S.A., Rubitherm Technologies GmbH and Ochsner Wärmepumpen again (HYSTORE).

“We are bridging the gap between today’s reliance on boilers and a more sustainable, eco-friendly future,” said Giordano. “We focus on developing a heat pump solution that is not just environmentally conscious but also user friendly, with easy installation, modularity, compactness, use of ecological refrigerants and minimized noise emissions.”

In Vienna alone, around 440,000 households rely on gas-fired boilers for heating and domestic hot water, she noted.

Gasthermenersatz Project

The Gasthermenersatz project is a national-level project in Austria, focusing on developing, building and testing a functional model of a decentralized, sound-optimized heat pump solution with natural refrigerant.

“In this project together with Ochsner, we have been meticulous about the refrigerant charge per module,” Giordano said. “Each module is designed with a maximum refrigerant charge of 150 grams. This specification is limiting the achievable capacity and is critical for the desired efficiency.”

The system meets specific heating demands by connecting these modules in series or parallel, she explained. The series connection of the modules achieves higher temperatures in the distribution system, while the parallel connection increases the system’s output capacity, making it suitable for buildings with higher energy demands.

The basic module comprises two refrigerant circuits and is designed to cover an area of approximately 30m2 (323ft2) with a 2.5kW (0.71TR) capacity, Giordano said. “This compact design enables installers to replace gas boilers swiftly, often within a single day,” she added.

The laboratory validation of the project with the first prototype has been completed, Giordano said. “Although these findings are encouraging, the research will continue to enhance the module’s performance,” she added.

HYSTORE Project

HYSTORE is focused on developing, building, and validating a PCM (phase change materials)-enhanced heat pump for domestic hot water (DHW) generation and cooling purposes.

“The HYSTORE project focuses explicitly on buildings requiring low-temperature heating,” said Giordano.

The project includes a heat exchanger with three passages for the heat transfer fluids. Two channels are used for water and one for the refrigerant, allowing efficient heat transfer. The system design includes a nominal storage capacity of 6 to 9 kWh.

Giordano emphasizes the role of PCM in HYSTORE, illustrating the selection of materials of interest for the project. “PCM’s ability to efficiently release or absorb large quantities of heat is central to our design.”

The system targets a discharge capacity of 20 kW (5.68TR) while maintaining a mass flow rate of 6l (1.58gal) per minute. It uses 8.8 °C (47.84°F) air – the yearly average outside air temperature in the Austrian region where the module will be tested – and two refrigerant circuits, allowing a maximum supply temperature of 65°C (149°F). Moreover, it incorporates a secondary water loop to avoid corrosion problems caused by the direct contact between the domestic hot water and heat exchanger material like aluminum. The secondary loop is composed by a fresh water station which delivers a supply temperature of 55 °C (131°F) for domestic hot water generation.

The HYSTORE project started in 2023 and is expected to be completed in 2026. The laboratory testing and the system validation will be done in an existing multifamily house.

“We are bridging the gap between today’s reliance on boilers and a more sustainable, eco-friendly future. We focus on developing a heat pump solution that is not just environmentally conscious but also user friendly, with easy installation, modularity, compactness, use of ecological refrigerants and minimized noise emissions.”

Fabrizia Giordano, Junior Research Engineer at Austrian Institue of Technology

Author Saroj Thapa